Patent Publication Number: US-2023154722-A1

Title: Pulsed charged-particle beam system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. application 62/786,268 which was filed on Dec. 28, 2018, and U.S. application 62/942,671 which was filed on Dec. 2, 2019, which are incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     The embodiments provided herein disclose a multiple charged-particle beam apparatus, and more particularly a multiple charged-particle beam apparatus including a radio-frequency based source of charged particles. 
     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 and more important. Multiple charged-particle beams may be used to increase the inspection throughput; however, productivity and efficiency of beam current usage may be compromised, rendering the inspection tools inadequate for their desired purpose. 
     Thus, related art systems face limitations in, for example, efficient usage of beam current and maintaining individual beam quality of a multiple charged-particle beam apparatus. Further improvements in the art are desired. 
     SUMMARY 
     In one aspect, the present disclosure is directed to a multi-beam apparatus for observing a sample. The multi-beam apparatus may include a deflector configured to form a plurality of deflected charged-particle beams from a primary charged-particle beam comprising a plurality of charged-particle pulses, and a detector configured to detect a plurality of signals generated from a plurality of probe spots formed by the plurality of deflected charged-particle beams. The multi-beam apparatus may further include a controller configured to obtain a first timing information related with formation of a deflected charged-particle beam of the plurality of charged-particle beams, obtain a second timing information related with detection of a signal of the plurality of signals, and associate the signal with the deflected charged-particle beam based on the obtained first and second timing information. The multi-beam apparatus may further include a charged-particle source, an acceleration cavity, and a bunching cavity. 
     The charged-particle source may comprise a pulsed radio-frequency source having a source frequency in a range of 100 MHz to 10 GHz. The deflector may comprise one or more charged-particle deflectors, each of the one or more charged-particle deflectors forming the plurality of deflected charged-particle beams based on an operating frequency. The deflector and the charged-particle source may be synchronized such that the operating frequency and the source frequency are related by the equation: 
     
       
         
           
             
               v 
               ⁢ 
               1 
             
             = 
             
               
                 1 
                 n 
               
               ⁢ 
               
                 ( 
                 
                   v 
                   ⁢ 
                   2 
                 
                 ) 
               
             
           
         
       
     
     where v1 is the operating frequency, v2 is the source frequency, and n is a positive integer. 
     The multi-beam apparatus may further include an electron optical system and a charged-particle beam scanning system configured to scan each of the plurality of deflected charged-particle beams on the sample. The electron optical system may comprise one of a single-lens system or a multiple-lens system. The controller may be further configured to communicate with at least one of the deflector, the charged-particle beam scanning system, and the detector. The plurality of probe spots formed by the plurality of deflected charged-particle beams comprise one of a one-dimensional or a two-dimensional pattern. The two-dimensional pattern comprises a Lissajous pattern, a matrix, or an array. 
     In another aspect, the present disclosure is directed to a method of observing a sample using a multi-beam apparatus. The method may include forming a plurality of deflected charged-particle beams from a primary charged-particle beam comprising a plurality of charged-particle pulses using a deflector, detecting a plurality of signals generated from a plurality of probe spots formed by the plurality of deflected charged-particle beams using a detector. The method may include obtaining a first timing information related with formation of a deflected charged-particle beam of the plurality of charged-particle beams, obtaining a second timing information related with detection of a signal of the plurality of signals, and associating the signal with the deflected charged-particle beam based on the obtained first and second timing information, using a controller. 
     The method may further include focusing the plurality of deflected charged-particle beams on the sample using an electron optical system, and scanning each of the plurality of deflected charged-particle beams on the sample using a charged-particle beam scanning system. 
     In yet another aspect, the present disclosure is directed to a controller of a multi-beam apparatus. The controller may include a memory storing a set of instructions, and a processor configured to execute the set of instructions to cause the controller to obtain a first timing information related with formation of a deflected charged-particle beam of a plurality of charged-particle beams, obtain a second timing information related with detection of a signal of a plurality of signals, and associate the signal with the deflected charged-particle beam based on the obtained first and second timing information. 
     In yet another aspect, 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 to observe a sample. The method may include forming a plurality of deflected charged-particle beams from a primary charged-particle beam comprising a plurality of charged-particle pulses using a deflector, detecting a plurality of signals generated from a plurality of probe spots formed by the plurality of deflected charged-particle beams using a detector. The method may include obtaining a first timing information related with formation of a deflected charged-particle beam of the plurality of charged-particle beams, obtaining a second timing information related with detection of a signal of the plurality of signals, and associating the signal with the deflected charged-particle beam based on the obtained first and second timing information, using a controller. The method may further include focusing the plurality of deflected charged-particle beams on the sample using an electron optical system and scanning each of the plurality of focused deflected charged-particle beams on the sample using a charged-particle beam scanning system. 
     In yet another aspect, the present disclosure is directed to a multi-beam apparatus. The multi-beam apparatus may include a charged-particle source configured to generate a primary charged-particle beam comprising a plurality of charged-particle pulses, a bunching cavity configured to form a plurality of charged-particle beams from the primary charged-particle beam; and a deflector configured to deflect the plurality of charged-particle beams to form a plurality of probe spots on a sample. 
     In yet another aspect, the present disclosure is directed to a method for observing a sample in a multi-beam apparatus. The method may include generating a primary charged-particle beam comprising a plurality of charged-particle pulses from a charged-particle source, forming a plurality of charged-particle beams from the primary charged-particle beam, and deflecting the plurality of charged-particle beams to form a plurality of probe spots on a sample using a deflector. 
     In yet another aspect, the present disclosure is directed to an apparatus comprising a deflector configured to deflect a plurality of pulses of charged particles to a plurality of probe spots on a sample, a detector configured to detect a plurality of signals from the sample that result from the plurality of pulses interacting with the sample, and a controller configured to correlate a particular detected signal to a particular probe spot on the sample based on a correlation between a time that the particular signal generated from the particular probe spot was detected and a time that a particular charged particle pulse forming the particular probe spot was deflected. 
     In yet another aspect, the present disclosure is directed to a multi-beam apparatus for charged-particle detection. The multi-beam apparatus may include a deflector system configured to direct charged-particle pulses; a detector having a detection element, the detection element configured to detect the charged-particle pulses; and a controller having a circuitry configured to: control the deflector system to direct a first charged-particle pulse and a second charged-particle pulse to the detection element, wherein the first charged-particle pulse is emitted from a first probing spot, and the second charged-particle pulse is emitted from a second probing spot; obtain a first timestamp associated with when the first charged-particle pulse is directed by the deflector system, a second timestamp associated with when the first charged-particle pulse is detected by the detection element, a third timestamp associated with when the second charged-particle pulse is directed by the deflector system, and a fourth timestamp associated with when the second charged-particle pulse is detected by the detection element; and identify a first exiting beam based on the first timestamp and the second timestamp, and a second exiting beam based on the third timestamp and the fourth timestamp. 
     In yet another aspect, the present disclosure is directed to a method for charged-particle detection in a multi-beam apparatus. The method may include: controlling a deflector system to direct a first charged-particle pulse and a second charged-particle pulse to a detection element of a detector; obtaining, using a controller, a first timestamp associated with when the first charged-particle pulse is directed by the deflector system, a second timestamp associated with when the first charged-particle pulse is detected by the detection element, a third timestamp associated with when the second charged-particle pulse is directed by the deflector system, and a fourth timestamp associated with when the second charged-particle pulse is detected by the detection element; and identifying, using a controller, a first exiting beam based on the first timestamp and the second timestamp, and a second exiting beam based on the third timestamp and the fourth timestamp. 
     In yet another aspect, 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 for charged-particle detection. The method may include: controlling a deflector system to direct a first charged-particle pulse and a second charged-particle pulse to a detection element of a detector; obtaining, using a controller, a first timestamp associated with when the first charged-particle pulse is directed by the deflector system, a second timestamp associated with when the first charged-particle pulse is detected by the detection element, a third timestamp associated with when the second charged-particle pulse is directed by the deflector system, and a fourth timestamp associated with when the second charged-particle pulse is detected by the detection element; and identifying, using a controller, a first exiting beam based on the first timestamp and the second timestamp, and a second exiting beam based on the third timestamp and the fourth timestamp. 
     In yet another aspect, the present disclosure is directed to a multi-beam apparatus for reducing interaction of charged particles. The multi-beam apparatus may include a first cluster cavity configured to receive a first set of charged particles to form a first cluster of charged particles; a second cluster cavity configured to receive a second set of charged particles to form a second cluster of charged particles; and a controller having a circuitry configured to cause the first cluster and the second cluster to pass a downstream position in a predetermined time-space order. 
     In yet another aspect, the present disclosure is directed to a method for reducing interaction of charged particles in a charged-particle beam of a multi-beam apparatus. The method may include: receiving a first set of charged particles in a first cluster cavity to form a first cluster of charged particles; receiving a second set of charged particles in a second cluster cavity to form a second cluster of charged particles; and causing the first cluster and the second cluster to pass a downstream position in a predetermined time-space order. 
     In yet another aspect, 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 for reducing interaction of charged particles in a charged-particle beam. The method may include: receiving a first set of charged particles in a first cluster cavity to form a first cluster of charged particles; receiving a second set of charged particles in a second cluster cavity to form a second cluster of charged particles; and causing the first cluster and the second cluster to pass a downstream position in a predetermined time-space order. 
     In yet another aspect, the present disclosure is directed to a multi-beam apparatus. The multi-beam apparatus may include: a first charged-particle source; a second charged-particle source; and at least one deflector system downstream from the first charged-particle source and the second charged-particle source, configured to: receive a first beam of pulsed charged particles from the first charged-particle source and a second beam of pulsed charged particles from the second charged-particle source; and direct the first beam and the second beam in a predetermined time-space order, wherein the first beam and the second beam pass a downstream position in sequence. 
     In yet another aspect, the present disclosure is directed to a method for providing multiple charged-particle beams in a multi-beam apparatus. The method may include: receiving, in at least one deflector system, a first beam of pulsed charged particles from a first charged-particle source and a second beam of pulsed charged particles from a second charged-particle source; and directing the first beam and the second beam in a predetermined time-space order, wherein the first beam and the second beam pass a downstream position in sequence. 
     In yet another aspect, 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 for providing multiple charged-particle beams. The method may include: receiving, in at least one deflector system, a first beam of pulsed charged particles from a first charged-particle source and a second beam of pulsed charged particles from a second charged-particle source; and directing the first beam and the second beam in a predetermined time-space order, wherein the first beam and the second beam pass a downstream position in sequence. 
    
    
     
       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. 
         FIG.  3    is a schematic diagram illustrating an exemplary multiple electron beam system using a timing control mechanism in a single-lens configuration, consistent with embodiments of the present disclosure. 
         FIG.  4    is a schematic diagram illustrating an exemplary multiple electron beam system using a timing control mechanism in a multiple-lens configuration, consistent with embodiments of the present disclosure. 
         FIG.  5 A  illustrates an exemplary low frequency scan pattern for observing samples using a multiple electron beam system, consistent with embodiments of the present disclosure. 
         FIGS.  5 B and  5 C  illustrate exemplary high frequency scan patterns for observing samples using a multiple electron beam system, consistent with embodiments of the present disclosure. 
         FIGS.  6 A and  6 B  illustrate an adjustable scan frequency pattern for imaging samples using a multiple electron beam system, consistent with embodiments of the present disclosure. 
         FIG.  7    is a flowchart showing an exemplary method of observing a sample using a multiple electron beam system, consistent with embodiments of the present disclosure. 
         FIGS.  8 A- 8 B  illustrate two exemplary methods of controlling charged particles to enter a crossover area in a multi-beam apparatus, consistent with embodiments of the present disclosure. 
         FIGS.  9 A- 9 B  illustrate an exemplary cluster generator for clustering charged particles, consistent with embodiments of the present disclosure. 
         FIG.  10    illustrates an exemplary set of cluster generators releasing charged-particle clusters in a time-space order, consistent with embodiments of the present disclosure. 
         FIG.  11    illustrates an exemplary arrangement of first and second cluster generators for clustering charged particles, consistent with embodiments of the present disclosure. 
         FIG.  12    illustrates an exemplary arrangement of a cluster generator and a filtering system for reducing stray charged particles, consistent with embodiments of the present disclosure. 
         FIG.  13    is a flowchart showing an exemplary method of reducing interaction of charged particles in a charged-particle beam of a multi-beam apparatus, consistent with embodiments of the present disclosure. 
         FIG.  14    illustrates an exemplary multi-beam apparatus for providing multiple charged-particle beams, consistent with embodiments of the present disclosure. 
         FIG.  15    is a flowchart showing an exemplary method of providing multiple charged-particle beams in a multi-beam apparatus, consistent with embodiments of the present disclosure. 
         FIGS.  16 A- 16 C  illustrate exemplary subsystems of a multi-beam apparatus for charged-particle detection, consistent with embodiments of the present disclosure. 
         FIG.  17    is a flowchart showing an exemplary method for charged-particle detection using a multi-beam apparatus, consistent with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations consistent with the invention. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the invention as recited in the appended claims. For example, although some embodiments are described in the context of utilizing charged-particle 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. 
     The enhanced computing power of electronic devices, while reducing the physical size of the devices, can be accomplished by significantly increasing the packing density of circuit components such as, transistors, capacitors, diodes, etc. on an IC chip. For example, in a smart phone, an IC chip (which is the size of a thumbnail) may include over 2 billion transistors, the size of each transistor being less than 1/1000th of a human hair. Not surprisingly, semiconductor IC manufacturing is a complex process, with hundreds of individual steps. Errors in even one step have the potential to dramatically affect the functioning of the final product. Even one “killer defect” can cause device failure. The goal of the manufacturing process is to improve the overall yield of the process. For example, for a 50-step process to get 75% yield, each individual step must have a yield greater than 99.4%, and if the individual step yield is 95%, the overall process yield drops to 7%. 
     As the geometries shrink and the IC chip industry migrates to three-dimensional (3D) architectures (such as, NAND gates, Fin field-effect transistors (FinFETs), and advanced dynamic random-access memory (DRAM)), finding defects is becoming more challenging and expensive at each lower node. While high process yield is desirable in an IC chip manufacturing facility, it is also essential to maintain a high wafer throughput, defined as the number of wafers processed per hour. High process yields, and high wafer throughput can be impacted by the presence of defects, especially when operator intervention is involved. Thus, detection and identification of micro and nano-sized defects by inspection tools (such as, a SEM) may be essential for maintaining high yields, high throughput, and low cost. 
     Semiconductor chips are fabricated in an extremely clean and controlled environment that has a very low level of pollutants such as dust, airborne particles, aerosol particles, and chemical vapors. More specifically, a semiconductor cleanroom is required to have a controlled level of contamination that is specified by the number of particles per cubic foot at a specified particle size. A typical chip manufacturing cleanroom contains 1-10 particles per cubic foot of air, each particle being less than 5 um in diameter. For comparison, the ambient air outside in a typical city environment contains approximately 1.25 billion particles per cubic foot, each particle having an average size of ˜200 um in diameter. A speck of dust as small as 1 um, on a wafer in process may span across thousands of transistors located on the chip, which could potentially render the entire chip useless. In some cases, a speck of dust on a reticle or a photomask that is used to create repeating patterns on the wafer may cause recurring physical or electrical defects. For example, one or more metal wires connecting transistors in a single chip may overlap or may be undesirably connected through the dust particle, causing a short in the circuit throughout the entire chip. Identifying and characterizing each defect or defect type while maintaining high throughput may improve process yield and product reliability. 
     One component of improving yield is monitoring the chip making process to ensure that it is producing a sufficient number of functional integrated circuits. One way to monitor the process is to inspect the chip circuit structures at various stages of their formation. Inspection can be carried out using a scanning electron microscope (SEM). A SEM can be used to image these extremely small structures, in effect, taking a “picture” of the structures of the wafer. 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. 
     The working principle of the SEM is similar to a camera. The camera takes a picture by receiving and recording brightness and colors of light reflected or emitted from people or objects. The SEM takes a “picture” by receiving and recording energies of electrons reflected or emitted from the structures. Before taking such a “picture,” an electron beam may be provided onto the structures, and when the electrons are reflected or emitted (“exiting”) from the structures, a detector of the SEM may receive and record the energies of those electrons to generate an image. To take such a “picture,” some SEMs use a single electron beam (referred to as a “single-beam SEM”), while some SEMs use multiple electron beams (referred to as a “multi-beam SEM”) to take multiple “pictures” of the wafer. By using multiple electron beams, the SEM may provide more electron beams onto the structures for obtaining these multiple “pictures,” resulting in more electrons exiting from the structures. Accordingly, the detector may receive more exiting electrons simultaneously, and generate the image of the structures with a higher efficiency and a faster speed. 
     Although a multiple charged-particle beam imaging system, such as a multi-beam SEM, may be useful in detecting micro and nano-sized defects while maintaining the high throughput; the overall impact on efficiency of beam usage, structural complexity of the system, and productivity cannot be overlooked. For example, splitting an electron beam to form multiple beamlets using micro-electromechanical systems (MEMS) aperture arrays may result in an inefficient usage of beam electrons because only a portion of the electrons may be allowed to pass through the apertures, while the remaining electrons are blocked off and unutilized. Generating multiple probe spots from multiple electron beams may require additional lenses and components, adding to the structural and operational complexity of the multi-beam apparatus. In particular, the additional lenses may have to be aligned with the system and noise-shielded as well. In addition, maintaining the beam quality for each of the individual beams or beamlets, expressed at least by high peak current, small beam cross-section, and a low beam energy spread within the beam may be challenging. 
     A routine defect detection and identification operation during wafer inspection using SEM may require dynamic adjustment of magnification and resolution. For example, a low-resolution scan may be performed to locate a defect, and a high-resolution scan may then be performed to identify or characterize the located defect. Such an operation requires manually adjusting scan parameters such as, for example, magnification, focus, field-of-view, beam energy, among other things. Besides manual interference from a skilled operator, the operation may be time consuming and susceptible to judgment error, adversely affecting the throughput and inspection yield. 
     The charged-particle beam imaging system (e.g., a single-beam SEM or a multi-beam SEM) may use a continuous or pulsed electron beam for scanning. A continuous beam is a continuous stream of electrons, similar to a water stream coming out of a hose. For multi-beam imaging systems that may generate a plurality of beamlets, continuous beamlets are continuous streamlets of electrons, similar to the many water streamlets coming out of a shower head. In contrast, a pulsed beam includes electrons clustered into bunches or pulses, with no electron between each pulse, similar to bullets out of a machine gun. For multi-beam imaging systems that may generate a plurality of beamlets, pulsed beamlets include, in each beamlet, electrons clustered into pulses, similar to bullets coming out of multiple machine guns arrange as an array and all firing at the same time. The charged-particle beam imaging system may use a pulsed charged-particle source to generate a pulsed beam. 
     In one aspect of the present disclosure, a multi-beam apparatus including a deflector, a detector, and a controller may be used to form multiple charged-particle beams for observing a sample. A high frequency pulsed charged-particle source may generate multiple charged-particle pulses that may be clustered to form bunches, also referred to herein as beams of charged-particle pulses. A deflector may be configured to receive and deflect the beams of high frequency charged-particle pulses and each of the deflected beams may form a probing spot on the sample. Upon interacting with the sample, each charged-particle beam may generate a signal comprising information related with the sample. A detector may be configured to detect the signals generated from the multiple probe spots. The multi-beam apparatus may comprise a controller configured to obtain timing information related with the formation of the deflected charged-particle beams and detection of the signal generated by the corresponding beam. The controller may then associate the detected signal with the deflected charged-particle beam based on the obtained timing information. 
     Some of the advantages of some embodiments of the claimed multi-beam apparatus include, but are not limited to, efficient beam usage, higher productivity, fast switching between low-resolution and high-resolution inspection, high beam quality, high wafer processing and inspection throughput. In addition, by forming the beams of charged-particle pulses, the number of the beamlets in a unit area may be increased without incurring significant Coulomb effect (explained below) because the pulses of different charged-particle beams may be staggered, thus allowing more efficient utilization of the high frequency pulsed charged-particle source. 
     The Coulomb effect is an electric interaction between charged particles. In the electric interaction, particles with like charges repel each other, while particles with opposite charges attract each other. In a multi-beam apparatus, the Coulomb effect may occur between beamlets of charged particles. For example, when the beamlets are electron beams, electrons in the beamlets may repel each other when they are too close, thus affecting the travel speeds and directions of each other. That may cause deterioration of the performance of the multi-beam apparatus. 
     Typically, in a multi-beam apparatus, beamlets may be converged to cross each other and form a common crossover (or “crossover area”) before reaching the sample. As demands for imaging throughput increases, the multi-beam apparatus may generate and use more beamlets for scanning. As the number of beamlets increase, more beamlets may cross the common crossover, and the Coulomb effect may become more significant. In a multi-beam apparatus capable of providing a large number of beamlets, the Coulomb effect may become a dominant factor that limits the imaging resolution. When the required energy of the beamlets is fixed, it is extremely challenging to control the Coulomb effect. 
     In another aspect of the present disclosure, a multi-beam apparatus including multiple cluster generators may be used for reducing the Coulomb effect. The cluster generators may be used to generate clusters of charged particles, such as by compressing the pulsed beamlets using cavities having electric fields along the moving direction of the beamlets. Each cluster generator may receive and cluster a beamlet. The cluster generators may be coordinated to release or eject the generated clusters in a predetermined time-space order, such that the clusters of different beamlets may enter the common crossover in sequence (e.g., one by one, with only one or zero cluster being in the common crossover at any point in time), as shown in  FIG.  8 B . That is, at a given time, there is at most one cluster passing through the common crossover. Such an arrangement may reduce the possibilities of incurring the Coulomb effect and reduce the strength of the Coulomb effect. In an ideal scenario, the strength of the Coulomb effect in a multi-beam apparatus may be reduced to the level of a single-beam apparatus. In addition, such an arrangement may accommodate more beams (e.g., generated from more charged-particle sources) in the multi-beam apparatus, thus significantly increasing the imaging throughput. 
     Moreover, as more beams are added to pursue faster inspection speeds and better inspection image quality, more challenges may arise. Typically, a single charged-particle source is used for generating charged particles (e.g., electrons), and an array of apertures is used for generating the beams of charged particles. The charged-particle source generates charged particles that are accelerated and shed onto the array of apertures, and the array of apertures may split them into beams. However, several problems would occur if this existing design is used to generate a higher number of beams. Due to the limited capability of the single charged-particle source, the current of the emission of the charged-particles may have an upper limit, which may limit the brightness of the generated inspection image of each beam. To accommodate the increasing demand of high throughputs of the multi-beam apparatus, more beams are needed. 
     And to generate more beams, the number of the apertures may have to be increased. In that case, more electric connections are needed for those apertures, and electric routing for those apertures becomes significantly more complex. Also, the apertures typically work under high voltages (e.g., 100 V). Because more connections are confined within a limited space, a risk of electric breakdowns between electric connections of the electric routing may greatly increase. Further, as the distances between the apertures shrink, the distance between the generated beams also shrink, which may cause more significant Coulomb effect between the beams. 
     In another aspect of the present disclosure, a multi-beam apparatus including multiple charged-particle sources (e.g., electron sources), and multiple deflectors may be used for providing multiple charged-particle beams. The charged-particle sources may produce beams of charged particles, and the deflectors may receive, deflect and release them as parallel beams. The deflectors may deflect the received beams in a manner such that the released beams may be more concentrated (e.g., distances between the beams are shortened) yet remain parallel. Because the deflectors may concentrate beams, the charged-particle sources may be spaced more distantly, reducing the complexity of the electric routing and risk of electric breakdowns. No aperture plate is used for producing the beams, further reducing the complexity of the electric routing and risk of electric breakdowns. In such a way, the beams of clusters may be deflected by the deflectors to become more condensed to enable a high and scalable brightness and a uniform current. The multiple deflectors may create a scalable approach to mitigate the Coulomb effect. In some cases, the deflectors may also cluster the beams and release them such that the clusters pass through the crossover area in sequence (e.g., one by one). In such a way, the Coulomb effect may be mitigated because there is at most one cluster passing through the crossover area. 
     Moreover, in a multi-beam apparatus, more challenges may result from a “cross-talk” problem, in which charged-particles of one beamlet may reach a destination corresponding to another beamlet, as explained below. In a multi-beam apparatus, multiple beamlets may be incident onto an inspected sample, each beamlet forming a spot with a sized cross section (or “spot size”). The beamlets interact with materials of the sample within a region with a depth (referred to as “interaction volume”). An interaction volume of a beamlet may be larger than a spot size of the beamlet. That is, the interaction volumes of the beamlets may overlap. Exiting electrons corresponding to a beamlet may exit from anywhere in the corresponding interaction volume. Due to the overlap between the interaction volumes, the exiting charged-particle beams may have overlaps. Typically, the detector detecting the exiting charged-particle beams may include multiple detection elements (e.g., sub-detectors). Typically, each detection element may detect secondary electrons generated based on a particular beamlet. Because the exiting charged-particle beams may have overlaps, charged-particles of one exiting beam may reach a detection element corresponding to another beamlet, hence incurring the cross-talk problem. This is only one example as to how crosstalk may occur, and crosstalk may occur in several other ways. To accommodate the increasing high throughput demand of the multi-beam apparatus, more beamlets are needed. However, as the number of beamlets increase, the number of corresponding detection elements may also increase, which may cause not only increasing complexity and cost of building the detector system, but also a more significant cross-talk problem. 
     In another aspect of the present disclosure, a multi-beam apparatus including a first deflector, a second deflector, multiple detectors, and a controller may be used for charged-particle detection. The first deflector may be used for forming a first number of beams of charged-particle pulses. Because the charged-particle pulses are deflected by the first deflector at different times, the pulses of the formed charged-particle beams may reach the inspected sample at different times. Charged-particle beams may exit from the probing spots, which may be deflected by the second deflector to form a second number of exiting charged-particle beams. If the detector can differentiate fine temporal details, the detector may then differentiate which pulse comes from which probing spot based on the times of the pulses of the exiting charged-particle beams arriving at the detector, the times of the pulses entering the second deflector, and the times of the pulses entering the first deflector. By doing so, the needed detection elements may be reduced, the complexity and cost of building the detector may be lowered, and the cross-talk problem may be alleviated. Essentially, this approach converts a spatial problem into a temporal problem. 
     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 database may include A or B, then, unless specifically stated otherwise or infeasible, the database may include A, or B, or A and B. As a second example, if it is stated that a database may include A, B, or C, then, unless specifically stated otherwise or infeasible, the database 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. 
     While this disclosure uses electrons to describe some embodiments and examples, it is to be understood that any description related to the electrons may be equivalently applicable to any type of charged particles (e.g., ions, protons, subatomic particles, or the like). 
       FIG.  1    illustrates an exemplary electron beam inspection (EBI) system  1  consistent with embodiments of the present disclosure. EBI system  1  may be used for imaging. As shown in  FIG.  1   , EBI system  1  includes a main chamber  10 , a load/lock chamber  20 , an electron beam tool  100 , and an equipment front end module (EFEM)  30 . Electron beam tool  100  is located within main chamber  10 . 
     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 may be used interchangeably). A “lot” is a plurality of wafers that may be loaded for processing as a batch. 
     One or more robotic arms (not shown) in EFEM  30  may 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 robotic arms (not shown) may 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  100 . Electron beam tool  100  may be a single-beam system or a multi-beam system. 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 is a schematic diagram illustrating an exemplary electron beam tool  100  including a multi-beam inspection tool that is part of the EBI system  1  of  FIG.  1   , consistent with embodiments of the present disclosure. Multi-beam electron beam tool  100  (also referred to herein as apparatus  100 ) comprises an electron source  101 , a gun aperture plate  171  with a gun aperture  103 , a condenser lens  110 , a source conversion unit  120 , a primary projection system  130 , a motorized stage  109 , and a sample holder  107  supported by motorized stage  109  to hold a sample  190  (e.g., a wafer or a photomask) to be inspected. Multi-beam electron beam tool  100  may further comprise a secondary projection system  150  and an electron detection device  140 . Primary projection system  130  may comprise an objective lens  131 . Electron detection device  140  may comprise a plurality of detection elements  140 _ 1 ,  140 _ 2 , and  140 _ 3 . A beam separator  160  and a deflection scanning unit  132  may be positioned inside primary projection system  130 . 
     Electron source  101 , gun aperture plate  171 , condenser lens  110 , source conversion unit  120 , beam separator  160 , deflection scanning unit  132 , and primary projection system  130  may be aligned with a primary optical axis  100 _ 1  of apparatus  100 . Secondary projection system  150  and electron detection device  140  may be aligned with a secondary optical axis  150 _ 1  of apparatus  100 . 
     Electron source  101  may comprise a cathode (not shown) and an extractor or anode (not shown), in which, during operation, electron source  101  is configured to emit primary electrons from the cathode and the primary electrons are extracted or accelerated by the extractor or the anode to form a primary electron beam  102  that forms a primary beam crossover (virtual or real)  101   s . Primary electron beam  102  may be visualized as being emitted from primary beam crossover  101   s.    
     Source conversion unit  120  may comprise an image-forming element array (not shown), an aberration compensator array (not shown), a beam-limit aperture array (not shown), and a pre-bending micro-deflector array (not shown). In some embodiments, the pre-bending micro-deflector array deflects a plurality of primary beamlets  102 _ 1 ,  102 _ 2 ,  102 _ 3  of primary electron beam  102  to normally enter the beam-limit aperture array, the image-forming element array, and an aberration compensator array. In some embodiment, condenser lens  110  is designed to focus primary electron beam  102  to become a parallel beam and be normally incident onto source conversion unit  120 . The image-forming element array may comprise a plurality of micro-deflectors or micro-lenses to influence the plurality of primary beamlets  102 _ 1 ,  102 _ 2 ,  1023  of primary electron beam  102  and to form a plurality of parallel images (virtual or real) of primary beam crossover  101   s , one for each of the primary beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3 . In some embodiments, the aberration compensator array may comprise a field curvature compensator array (not shown) and an astigmatism compensator array (not shown). The field curvature compensator array may comprise a plurality of micro-lenses to compensate field curvature aberrations of the primary beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3 . The astigmatism compensator array may comprise a plurality of micro-stigmators to compensate astigmatism aberrations of the primary beamlets  1021 ,  102 _ 2 , and  102 _ 3 . The beam-limit aperture array may be configured to limit diameters of individual primary beamlets  1021 ,  102 _ 2 , and  102 _ 3 .  FIG.  2    shows three primary beamlets  1021 ,  102 _ 2 , and  102 _ 3  as an example, and it is appreciated that source conversion unit  120  may be configured to form any number of primary beamlets. 
     Condenser lens  110  is configured to focus primary electron beam  102 . Condenser lens  110  may further be configured to adjust electric currents of primary beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  downstream from source conversion unit  120  by varying the focusing power of condenser lens  110 . Alternatively, the electric currents may be changed by altering the radial sizes of beam-limit apertures within the beam-limit aperture array corresponding to the individual primary beamlets. The electric currents may be changed by both altering the radial sizes of beam-limit apertures and the focusing power of condenser lens  110 . Condenser lens  110  may be an adjustable condenser lens that may be configured so that the position of its first principle plane is adjustable. The adjustable condenser lens may be configured to be magnetic, which may result in off-axis beamlets  102 _ 2  and  102 _ 3  illuminating source conversion unit  120  with rotation angles. The rotation angles change with the focusing power or the position of the first principal plane of the adjustable condenser lens. Condenser lens  110  may be an anti-rotation condenser lens that may be configured to keep the rotation angles unchanged while the focusing power of condenser lens  110  is changed. In some embodiments, condenser lens  110  may be an adjustable anti-rotation condenser lens, in which the rotation angles do not change when a focusing power and a position of a first principal plane of condenser lens  110  are varied. 
     Objective lens  131  may be configured to focus beamlets  1021 ,  102 _ 2 , and  102 _ 3  onto a sample  190  for inspection and may form, in the current embodiments, three probe spots  102 _ 1   s ,  102 _ 2   s , and  102 _ 3   s  on the surface of sample  190 . Gun aperture plate  171 , in operation, is configured to block off peripheral electrons of primary electron beam  102  to reduce Coulomb effect. The Coulomb effect may enlarge the size of each of probe spots  102 _ 1   s ,  102 _ 2   s , and  102 _ 3   s  of primary beamlets  102 _ 1 ,  102 _ 2 ,  102 _ 3 , and therefore deteriorate inspection resolution. 
     Beam separator  160  may, for example, be a Wien filter comprising an electrostatic deflector generating an electrostatic dipole field and a magnetic dipole field (not shown in  FIG.  2   ). In operation, beam separator  160  may be configured to exert an electrostatic force by electrostatic dipole field on individual electrons of primary beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3 . The electrostatic force is equal in magnitude but opposite in direction to the magnetic force exerted by magnetic dipole field of beam separator  160  on the individual electrons. Primary beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  may therefore pass at least substantially straight through beam separator  160  with at least substantially zero deflection angles. 
     Deflection scanning unit  132 , in operation, is configured to deflect primary beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  to scan probe spots  102 _ 1   s ,  102 _ 2   s , and  102 _ 3   s  across individual scanning areas in a section of the surface of sample  190 . In response to incidence of primary beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  on probe spots  102 _ 1   s ,  102 _ 2   s , and  102 _ 3   s  on sample  190 , electrons emerge from sample  190  and generate three secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se . Each of secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se  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  102 _ 1 ,  102 _ 2 , and  102 _ 3 ). Beam separator  160  is configured to deflect secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se  towards secondary projection system  150 . Secondary projection system  150  subsequently focuses secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se  onto detection elements  140 _ 1 ,  140 _ 2 , and  140 _ 3  of electron detection device  140 . Detection elements  140 _ 1 ,  140 _ 2 , and  140 _ 3  are arranged to detect corresponding secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se  and generate corresponding signals which are sent to a controller or a signal processing system (not shown), e.g., to construct images of the corresponding scanned areas of sample  190 . 
     In some embodiments, detection elements  140 _ 1 ,  140 _ 2 , and  140 _ 3  detect corresponding secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se , respectively, and generate corresponding intensity signal outputs (not shown) to an image processing system (not shown). In some embodiments, each of detection elements  140 _ 1 ,  140 _ 2 , and  140 _ 3  may comprise one or more pixels. The intensity signal output of a detection element may be a sum of signals generated by all the pixels within the detection element. 
     Although  FIG.  2    shows that apparatus  100  uses three primary electron beams, it is appreciated that apparatus  100  may use two or more number of primary electron beams. The present disclosure does not limit the number of primary electron beams used in apparatus  100 . 
     Reference is now made to  FIG.  3   , which illustrates a multi-beam apparatus  300  for observing a sample using a timing control mechanism, consistent with embodiments of the present disclosure. In some embodiments, the multi-beam apparatus  300  may be electron beam tool  100  as shown in  FIG.  2   . As shown in  FIG.  3   , multi-beam apparatus  300  may comprise a primary electron source  301 , deflected electron beams  305 _ 1  and  305 _ 2 , an acceleration cavity  310 , a bunching cavity  320 , a deflection cavity  330 , a condenser lens  340 , a detector  350 , a primary projection system  360 , a beam raster system  370 , a controller  380 , and a sample  390 . 
     As illustrated in  FIG.  3   , primary electron source  301 , acceleration cavity  310 , bunching cavity  320 , deflection cavity  330 , condenser lens  340 , primary projection system  360 , beam raster system  370 , and sample  390  may be aligned with a primary optical axis  302  of multi-beam apparatus  300 . 
     Primary electron source  301  may be a continuous electron source or a pulsed electron source. In some embodiments, primary electron source  301  of multi-beam apparatus  300  may be electron source  101  of electron beam tool  100 , which may be used to generate a continuous electron beam. In some embodiments, primary electron source  301  may be a pulsed electron source, which may be used to generate a pulsed electron beam. Primary electron source  301  can generate charged particle pulses in any of several ways. In some embodiments, primary electron source  301  may be a laser induced pulsed source, such as one that uses a pulsed laser to excite electrons in an emitter and resultantly generate electron pulses. In some embodiments, primary electron source  301  may be a voltage controlled pulsed source, such as one that uses a pulsed voltage to excite electrons and generate electron pulses. In some embodiments, primary electron source  301  may comprise a superconducting radio-frequency (SCRF) photoinjector, a normal conducting radio-frequency (NCRF) photoinjector, or a high voltage direct current photoemission gun (e.g., electron source  101  in  FIG.  2   ) followed by an RF accelerating module photoinjector. Further, primary electron source  301  may be any combination of the above discussed components. In some embodiments, a photoinjector may include a photocathode, an electron gun powered by radio-frequency (RF) or biased at a high voltage, a laser and optical system producing a desired pulse structure, an RF source, and a timing and synchronization system. 
     High quantum efficiency photocathodes may be useful for the operation of photoinjector driven electron accelerators with high average current and high brightness beams. In some embodiments, the photocathodes for conventional NCRF photoinjectors may comprise metallic photocathodes such as copper (Cu), lead (Pb), or magnesium (Mg). The NCRF photoinjectors may comprise a low duty factor gun (0.1%), or a high duty factor gun (25%). The photocathodes for SCRF photoinjectors may comprise semiconductor photocathodes made of alkali antimonides, bi-alkali antimonides, multi-alkali antimonides, gallium arsenide (GaAs), gallium nitride (GaN), cesium iodide (CsI), cesium telluride (Cs2Te), CsK2Sb, Na2KSb, Cs3Sb, or the like. 
     In some embodiments, the source frequency, defined herein as the frequency of the RF signal applied to the photoinjector to generate electron pulses may be in the range of 100 MHz to 10 GHz. It is to be appreciated that source frequency may be 1.3 GHz or 3 GHz. 
     In some embodiments, primary electron source  301  may be a pulsed electron source, and acceleration cavity  310  may receive charged-particle pulses generated by primary electron source  301  and accelerate them to eject a pulsed primary electron beam. In some embodiments, primary electron source  301  may be a continuous electron source, and acceleration cavity  310  may receive a charged-particle stream generated by primary electron source  301  and accelerate them to eject a continuous primary electron beam. For example, acceleration cavity  310  may comprise a linear particle accelerator configured to accelerate charged particles of the received electrons (e.g., electron pulses or a continuous electron stream) to a high speed by subjecting them to a series of oscillating electric potentials along a linear beamline. In a linear accelerator such as acceleration cavity  310 , electrons may be accelerated by the action of RF electromagnetic waves. Relatively low energy electrons are injected into acceleration cavity  310  and gain energy as they travel down the structure. In some embodiments, acceleration cavity  310  may include corrugations, diaphragms, or baffles, causing the RF waves to travel at a velocity determined by the corrugations and accelerator dimensions. In most electron linear accelerators, very high frequency waves, usually of wavelength of around 10 cm (i.e. around 3 GHz) are made to propagate down the accelerating structure in the same direction as the electrons. In some embodiments, when primary electron source  301  is a pulsed electron source, the RF wave propagating within acceleration cavity  310  may be matched with the source frequency of the incoming electron pulses from primary electron source  301 . 
     In some embodiments, the electric field component in the RF wave may act on the injected electrons, initially by forming them into a bunch, then accelerating them down the structure of acceleration cavity  310 . Each cycle of the RF electric field may increase the energy of the particles so that when they emerge out of acceleration cavity  310 , the effect may be the same as if they were accelerated by a static electric field. In some embodiments, acceleration cavity  310  may additionally function as a lens. For example, besides accelerating charged particles or forming them into bunches, acceleration cavity may also divert traveling directions of the charged particles, such as converging them into a downstream component (e.g., bunching cavity  320 ). 
     In some embodiments, multi-beam apparatus  300  may comprise bunching cavity  320  (also referred to herein as an RF chopper system) configured to receive and modify the primary electron beam from acceleration cavity  310 . For example, when primary electron source  301  is a pulsed electron source, bunching cavity  320  may modify the period of electron pulses. The electron pulses generated by acceleration cavity  310  may be reduced (“compressed”). For example, the electron pulses may be compressed to 100 fs (femtoseconds), or a frequency of 10 THz (terahertz) to allow for fast electron beam imaging. Other electron pulse periods or frequencies may be used, as appropriate. For another example, when primary electron source  301  is a continuous electron source, bunching cavity  320  may generate a pulsed primary electron beam from the received continuous primary electron beam. It should be noted that, when primary electron source  301  is a pulsed electron source, bunching cavity  320  may be optional in multi-beam apparatus  300 . 
     In some embodiments, when primary electron source  301  is a pulsed electron source, an RF chopper system of bunching cavity  320  may “chop” out a fraction of the pulses such that the pulses are at the proper repetition frequency or sub-harmonics thereof. Although  FIG.  3    illustrates bunching cavity  320  disposed downstream from acceleration cavity  310 , it is to be appreciated that bunching cavity  320  may be disposed upstream from acceleration cavity  310  such that the period of the electron pulses generated by primary electron source  301  may be adjusted to match the frequency of RF waves in acceleration cavity  310 . In some embodiments, one or more bunching cavities may be used, as appropriate. 
     In some embodiments, when primary electron source  301  is a pulsed electron source, bunching cavity  320  may additionally function as a cluster generator. That is, bunching cavity  320  may further group charged-particle pulses into clusters. Each cluster may include one or more consecutive pulses such that intra-cluster distances may be shorter than inter-cluster distances. 
     In some embodiments, multi-beam apparatus  300  may comprise deflection cavity  330  configured to receive a beam of electron pulses from bunching cavity  320  and deflect individual pulses of the beam to different directions to form a plurality of deflected charged-particle beams, such as deflected electron beams  305 _ 1  and  305 _ 2 . Deflected electron beams  305 _ 1  and  305 _ 2  may be similar to one or more of primary beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  in  FIG.  2   . An electron beam, as used herein, may comprise a group or a “bunch” of electron pulses having a frequency (e.g., in a range of 100 MHz to 10 GHz). Deflection cavity  330  may comprise one or more beam deflectors configured to direct the bunches of electron pulses generated by bunching cavity  320 . In some embodiments, deflection cavity  330  may receive electron pulses from bunching cavity  320  and deflect individual pulses into a plurality of directions to form deflected electron beams  305 _ 1  and  305 _ 2 . It should be noted that any deflector described herein, including deflectors in deflection cavity  330 , may direct charged particles (e.g., electrons) to either change their moving directions (via deflection) or keep their moving directions unchanged. That is, the deflectors may apply a “neutral” effect on charged particles when directing them. In some embodiments, the deflectors described herein, including deflectors in deflection cavity  330 , may be RF cavities or implementations other than RF cavities (e.g., MEMS deflectors). 
     In some embodiments, deflection cavity  330  may operate in synchronization with primary electron source  301 . In particular, deflection cavity  330  may be operated at an operating frequency synchronous with the source frequency, such that the operating frequency and the source frequency are related based on the equation 1 below: 
     
       
         
           
             
               
                 
                   
                     v 
                     ⁢ 
                     1 
                   
                   = 
                   
                     
                       1 
                       n 
                     
                     ⁢ 
                     
                       ( 
                       
                         v 
                         ⁢ 
                         2 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                       
                   1 
                 
               
             
           
         
       
     
     where v1 is the operating frequency, v2 is the source frequency, and n is a positive integer. As used herein, source frequency is the frequency of the RF source or primary electron source  301 , operating frequency is the frequency of deflection cavity  330 , and n indicates the number of deflected charged-particle beams to be generated. For example, for a source frequency of 4 GHz and n=2, the operating frequency of deflection cavity  330  may be 2 GHz. In other words, if the source frequency is 4 GHz and deflection cavity  330  is operated at 2 GHz, deflection cavity  330  may generate two deflected electron beams, such as  305 _ 1  and  305 _ 2 , as illustrated in  FIG.  3   . The frequency of each of deflected electron beams  305 _ 1  and  305 _ 2  may be 2 GHz. It is to be appreciated that any number of deflected electron beams may be generated, as desired. 
     In some embodiments, deflected electron beams  305 _ 1  and  305 _ 2  may be deflected symmetrically-off primary optical axis  302 , as shown in  FIG.  3   . For example,  305 _ 1  and  3052  may be deflected such that the angle subtended by the primary axis of  305 _ 1  and primary axis of  305 _ 2  may be 180° and primary optical axis  302  bisects the subtended angle. 
     In some embodiments, according to Equation 1, for a source frequency of 4 GHz and the operating frequency of deflection cavity  330  of 1 GHz, four deflected electron beams may be generated, each having a frequency of 1 GHz. In other words, the operating frequency of deflection cavity  330  may be selected based on the number of deflected electron beams desired. Each deflected electron beam (e.g., deflected electron beams  305 _ 1  and  305 _ 2 ) may travel downstream from deflection cavity  330  towards sample  390  to generate a corresponding probe spot on sample  390 . 
     In some embodiments, multi-beam apparatus  300  may comprise more than one deflection cavities. For example, a second deflection cavity (not shown) configured to generate deflected electron beams may be disposed perpendicular to deflection cavity  330 . In such a configuration, the probe spots formed by deflected electron beams generated by second deflection cavity may be perpendicular to probe spots formed by deflected electron beams such as  305 _ 1  and  305 _ 2 , generated by deflection cavity  330 . Such an arrangement may result in a two-dimensional pattern of probe spots formed on sample  390 . 
     In some embodiments, the number and pattern of probe spots on sample  390  may be determined by, but not limited to, the number of deflectors within deflection cavity  330 , the number of deflection cavities, the operating frequency of deflection cavity  330 , the spatial arrangement of deflectors or deflection cavities, etc. 
     In some embodiments, a two-dimensional pattern of probe spots may comprise a square matrix, a rectangular matrix, an array, or a Lissajous pattern. A Lissajous pattern or a Lissajous curve, as used herein, may be defined as the pattern produced by the intersection of two sinusoidal curves, the axes of which are perpendicular to each other. In some embodiments, a one-dimensional pattern of probe spots may be generated, for example, a number of linearly arranged probe spots on sample  390 . 
     In some embodiments, a de-bunching cavity (not illustrated) may be used to de-compress the electron pulses, as appropriate. One or more de-bunching cavities may be employed in multi-beam apparatus  300  to suit the application or sample being investigated. 
     Referring back to  FIG.  3   , multi-beam apparatus  300  may comprise condenser lens  340 . Condenser lens  340  of multi-beam apparatus  300  is substantially similar to condenser lens  110  of electron beam tool  100  illustrated in  FIG.  2   . Condenser lens  340  may be positioned downstream from deflection cavity  330  and configured to converge deflected electron beams  305 _ 1  and  305 _ 2 , such that they cross-over along primary optical axis  302 , on a plane perpendicular to primary optical axis  302 . Condenser lens  340  may be configured to focus each of deflected electron beams  305 _ 1  and  305 _ 2 . It is appreciated that condenser lens  340  may be configured to focus and converge any number of deflected charged-particle beams as appropriate. 
     Multi-beam apparatus  300  may comprise detector  350 . In some embodiments, detector  350  is similar to detection device  140  of  FIG.  2   . Detector  350  may be configured to detect, but not limited to, secondary electrons, back-scattered electrons, transmitted electron, X-rays, auger electrons, etc. depending on factors such as the accelerating voltage, sample density, among other things. Secondary electrons, for example, may be generated from the probe spots on sample  390  formed by deflected electron beams  305 _ 1  and  305 _ 2 . 
     In some embodiments, detector  350  may comprise solid-state detectors containing p-n junctions for detecting back-scattered electrons, or an Everhart-Thornley detector for secondary electrons. A secondary electron detector, such as Everhart-Thornley detector may include a scintillator inside a Faraday cage, which is positively charged and configured to attract the secondary electrons. The scintillator may then be used to accelerate the electrons and convert them to light signals before reaching a photomultiplier for amplification. Detector  350  may be positioned at an angle to increase the detection efficiency of detecting secondary electrons. 
     In some embodiments, multi-beam apparatus  300  may comprise primary projection system  360 . Primary projection system  360 , also referred to as an electron optical system, may comprise a single-lens system or a multi-lens system. Multi-beam apparatus  300  of  FIG.  3    comprises a single-lens system, whereas multi-beam apparatus  400  of  FIG.  4    describes a multi-lens system. Primary projection system  360  may include objective lens  131  of  FIG.  2   . In some embodiments, primary projection system  360  comprising objective lens  131  may be configured to focus each deflected electron beams  305 _ 1  and  305 _ 2  onto sample  390 . Primary projection system  360  may comprise more than one objective lens  131 , as illustrated in  FIG.  4    (described later). 
     To obtain a higher resolution of images formed by a charged-particle beam (such as, primary electron beam  102  of  FIG.  2   ) objective lens  131  may be an electromagnetic compound lens in which the sample may be immersed in the magnetic field of objective lens  131 . In some embodiments, objective lens  131  may include a magnetic lens and an electrostatic lens (not illustrated). The magnetic lens may be configured to focus the charged-particle beam, or each primary beamlet in a multi-beam apparatus (such as, electron beam tool  100  of  FIG.  2   ), at relatively low aberrations to generate relatively small probe spots on a sample. The electrostatic lens may be configured to influence the landing energy of the charged-particle beam or each primary beamlet to ensure that the primary charged-particles land on the sample at a relatively low kinetic energy and pass through the apparatus with a relatively high kinetic energy. In some embodiments, objective lens  131  may be configured to be an “immersion lens.” As a result, the sample may be immersed both in an electrostatic field E (electrostatic immersion) of the electrostatic lens and a magnetic field B (magnetic immersion) of the magnetic lens. Electrostatic immersion and magnetic immersion may reduce aberrations of objective lens  131 . As electrostatic and magnetic fields get stronger, the aberrations of objective lens  131  may become smaller. Electrostatic field E, however, should be limited to within a safe range in order to avoid discharging or arcing on the sample. Due to this limitation of the field strength of electrostatic field E, further enhancement of the magnetic field strength in an immersion configuration may allow a further reduction in the aberrations of objective lens  131 , and thereby improve image resolution. 
     Referring back to  FIG.  3   , in some embodiments, deflected electron beams  305 _ 1  and  305 _ 2  may arrive on surface of sample  390 , in an at least a substantially perpendicular direction. Magnetic immersion, however, may influence the landing angles of all primary modified beamlets landing on sample  390 . In particular, magnetic field B may cause each electron in a modified beamlet to obtain an angular velocity θ (1) , as shown in equation (2) below: 
     
       
         
           
             
               
                 
                   
                     
                       r 
                       2 
                     
                     ⁢ 
                     
                       θ 
                       
                         ( 
                         1 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         1 
                         2 
                       
                       ⁢ 
                       
                         e 
                         m 
                       
                       ⁢ 
                       
                         r 
                         2 
                       
                       ⁢ 
                       B 
                     
                     + 
                     C 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                       
                   2 
                 
               
             
           
         
       
     
     wherein C is a constant related to an initial angular velocity of the electron, r is a position shift from optical axis of objective lens  131 , and e and m are the charge and the mass of the electron, respectively. For the electron to land on sample  390  in a perpendicular manner, angular velocity θ (1)  must be zero on sample  390 . 
     In some embodiments, magnetic lens may be configured to operate in a non-magnetic immersion mode, and magnetic field B is zero (or substantially zero) or below the preset ratio value on sample  390 . If an electron enters magnetic field B along a meridional path, its corresponding constant C is zero and its angular velocity θ (1)  will be zero or substantially zero on sample  390 . Objective lens  131  may have a real front focal point on its front focal plane. When the chief rays (or center rays) of off-axis deflected electron beams  305 _ 1  and  305 _ 2  enter objective lens  131  along some specific meridional paths, the chief rays can pass through the real front focal point and off-axis deflected electron beams  305 _ 1  and  305 _ 2  can land perpendicular on sample  390 . Accordingly, deflected electron beams  305 _ 1  and  305 _ 2  may overlap together on the front focal plane and form a relatively sharp beamlet crossover centering at the real front focal point. 
     In other embodiments, magnetic lens may be configured to operate in magnetic immersion mode in which magnetic field B is not zero on sample  390 . Therefore, angular velocity θ (1)  of an electron may be zero (or substantially zero) on sample  390  if its corresponding constant C is not zero when the electron enters magnetic field B and complies with the condition in equation (3): 
     
       
         
           
             
               
                 
                   C 
                   = 
                   
                     
                       - 
                       
                         1 
                         2 
                       
                     
                     ⁢ 
                     
                       e 
                       m 
                     
                     ⁢ 
                     
                       r 
                       2 
                     
                     ⁢ 
                     B 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                       
                   3 
                 
               
             
           
         
       
     
     When C is not equal to zero, the electron enters magnetic field B along a skew path and cannot cross primary optical axis before entering magnetic field B. Hence, an electron can perpendicularly land on sample  390  only if entering magnetic field B along a specific skew path, and the electron cannot really cross primary optical axis during passing through magnetic field B. Accordingly, objective lens  131  may have a virtual front focal point. When the chief rays (or center rays) of off-axis deflected electron beams  305 _ 1  and  305 _ 2  enter objective lens  131  along some specific skew paths, they can virtually pass through virtual front focal point and land perpendicular on sample  390 . Under this scenario, off-axis deflected electron beams  305 _ 1  and  305 _ 2  are closest to each other on principal plane of objective lens  131 , and each off-axis deflected electron beams  305 _ 1  and  305 _ 2  has a radial shift from primary optical axis  302 . The deflected electron beams  305 _ 1  and  305 _ 2 , therefore only partially overlap with each other on principal plane and form a partial overlap beamlet crossover on principal plane. Moreover, radial shift increases as magnetic field B on sample  390  increases. Current density is lower in the partial overlap beamlet crossover than in the foregoing sharp beamlet crossover. Therefore, the Coulomb interaction effect between deflected electron beams  305 _ 1  and  305 _ 2  in magnetic immersion mode is relatively low, thereby further contributing to the small sizes of probe spots. 
     In some embodiments, multi-beam apparatus  300  may include beam raster system  370  configured to scan deflected electron beams  305 _ 1  and  305 _ 2  on sample  390 . Beam raster system  370  may be positioned downstream from primary projection system  360 , as shown in  FIG.  3   . Alternatively, beam raster system  370  may be positioned between condenser lens  340  and primary projection system  360 . Beam raster system  370  may be configured to raster or scan the received deflected electron beams  305 _ 1  and  305 _ 2  on sample  390  in a predefined pattern, as appropriate. In some embodiments, beam raster system  370  may be aligned with primary optical axis  302 . 
     Referring back to  FIG.  3   , multi-beam apparatus  300  may comprise controller  380  configured to synchronize the relevant components of apparatus  300 , such as, but not limited to, RF cavity including acceleration cavity  310 , bunching cavity  320 , and deflection cavity  330 , detection systems including detector  350 , and beam raster system  370 , etc. In some embodiments, controller  380  may be electronically connected to electron beam tool  100 . Controller  380  may be a computer configured to execute various controls of EBI system  1 . For example, controller  380  may be connected to various parts of EBI system  1  of  FIG.  1   , such as source conversion unit  120 , electron detection device  140 , primary projection system  130 , or motorized stage  109 . In some embodiments, as explained in further details below, controller  380  may perform various image and signal processing functions. Controller  380  may also generate various control signals to govern operations of the charged particle beam inspection system. 
     In some embodiments, controller  380  may include one or more processors (not shown). A processor may be an electronic device capable of manipulating or processing information. For example, the processor may include any combination of any number of a central processing unit (or “CPU”), a graphics processing unit (or “GPU”), an optical processor, a programmable logic controllers, a microcontroller, a microprocessor, a digital signal processor, an intellectual property (IP) core, a Programmable Logic Array (PLA), a Programmable Array Logic (PAL), a Generic Array Logic (GAL), a Complex Programmable Logic Device (CPLD), a Field-Programmable Gate Array (FPGA), a System On Chip (SoC), an Application-Specific Integrated Circuit (ASIC), and any type circuit capable of data processing. The processor may also be a virtual processor that includes one or more processors distributed across multiple machines or devices coupled via a network. 
     In some embodiments, controller  380  may further include one or more memories (not shown). A memory may be a generic or specific electronic device capable of storing codes and data accessible by the processor (e.g., via a bus). For example, the memory may include any combination of any number of a random-access memory (RAM), a read-only memory (ROM), an optical disc, a magnetic disk, a hard drive, a solid-state drive, a flash drive, a security digital (SD) card, a memory stick, a compact flash (CF) card, or any type of storage device. The codes may include an operating system (OS) and one or more application programs (or “apps”) for specific tasks. The memory may also be a virtual memory that includes one or more memories distributed across multiple machines or devices coupled via a network. 
     In some embodiments, controller  380  may comprise image processing system that includes an image acquirer (not shown), 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 detector  350  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 detector  350  and may construct an image. The image acquirer may thus acquire images of sample  390 . 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 detector  350 . 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  390 . The acquired images may comprise multiple images of a single imaging area of sample  390  sampled multiple times over a time sequence. The multiple images may be stored in the storage. In some embodiments, controller  380  may be configured to perform image processing steps with the multiple images of the same location of sample  390 . 
     In some embodiments, controller  380  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 deflected electron beams  305 _ 1  and  305 _ 2  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  390 , and thereby can be used to reveal any defects that may exist in the wafer. 
     In some embodiments, sample  390  may be placed in multi-beam apparatus  300  in a way similar to sample  190  in electron beam tool  100 . For example, controller  380  may control motorized stage (e.g., such motorized stage  109  of  FIG.  2   ) to move sample  390  during inspection of sample  390 . In some embodiments, controller  380  may enable motorized stage  109  to move sample  390  in a direction continuously at a constant speed. In other embodiments, controller  380  may enable motorized stage  109  to change the speed of the movement of sample  390  overtime depending on the steps of scanning process. 
     In some embodiments, controller  380  may be configured to control injection by triggering primary electron source  301  and activating acceleration cavity  310  and bunching cavity  320  at appropriate times. Also, beam diagnostic components (not illustrated) including, but not limited to, beam position monitors, current transformers, profile monitors, etc. may be synchronized to the passage of the charged-particle beam with high precision and fine time resolution, defined as “fast timing”. 
     In some embodiments, controller  380  may be a timing controller configured to timestamp control system processes such as, but not limited to, formation of a plurality of deflected charged-particle beams, detection of multiple signals comprising secondary electrons generated from the probe spots, correlating measurements and timing information of formation of deflected electron beams and detection of signals, etc. 
     In some embodiments, as illustrated in  FIG.  3   , controller  380  may be configured to communicate with each of acceleration cavity  310 , bunching cavity  320 , deflection cavity  330 , detector  350 , and beam raster system  370 . In some embodiments, controller  380  may be configured to communicate with and control operation of other components, such as beam optical components, electromagnetic lens, beam collimators, sample stage, position monitors, etc. 
     In some embodiments, controller  380  may include, but is not limited to, timing circuit components comprising digital and analog circuits, microprocessors, data storage module, communications module including data ports, display module, sequencing circuits, etc. Controller  380  may communicate with an external computer or a processor as well. 
     Reference is now made to  FIG.  4   , which illustrates a multi-beam apparatus  400  using a multi-lens system, consistent with embodiments of the present disclosure. Multi-beam apparatus  400  may comprise primary electron source  301 , deflected electron beams  305 _ 1  and  3052 , acceleration cavity  310 , bunching cavity  320 , deflection cavity  330 , condenser lens  340 , detector  350 , primary projection system  360 , beam raster system  370 , controller  380 , and sample  390 . Multi-beam apparatus  400  may include a beam aperture array  410  comprising a plurality of apertures or holes. 
     In some embodiments, as shown in  FIG.  4   , beam aperture array  410  may be placed between condenser lens  340  and detector  350 . Beam aperture array  410  may comprise a matrix of uniform apertures, for example, each of the apertures of beam aperture array  410  may be uniform in cross-section, shape, or size. In some embodiments, the apertures may be arranged in a linear, circular, rectangular, spiral, zig-zag, serpentine, triangular pattern, or combinations thereof. In some embodiments, the apertures within beam aperture array  410  may be non-uniform in shape, size or cross-section. It is appreciated that apertures of beam aperture array  410  may be laid out randomly across the array. Other suitable layouts and configurations of the apertures may be used as well. 
     In some embodiments, beam aperture array  410  may comprise a metal, a ceramic, a plastic, an alloy, a composite, a semiconductor, or any suitable material that is vacuum-compatible and can be processed to form apertures. The apertures of beam aperture array  410  may be fabricated using photolithography, embossing, ultraprecision laser machining, injection molding, mechanical drilling, etc. or any suitable technique. 
     In some embodiments, the pattern of apertures of beam aperture array  410  may be predefined and the information related with the aperture pattern such as, but not limited to, number, pitch, size, location, arrangement, cross-section, etc. may be retrievably stored in a database or data storage module of controller  380 , or the like. 
     In some embodiments, controller  380  may be configured to synchronize timing and deflection of electron beams based on the aperture pattern. In some embodiments, the operating frequency of deflection cavity  330  may also be adjusted based on the number or pitch of apertures arranged in beam aperture array  410 . 
     As illustrated in  FIG.  4   , primary projection system  360  may include a plurality of objective lens  131  for each deflected electron beam. Each objective lens  131  may be aligned with primary axis of a corresponding deflected electron beam  305 _ 1  or  305 _ 2 . Such a configuration may allow each deflected electron beam (e.g., deflected electron beams  305 _ 1  and  305 _ 2 ) to be focused and manipulated independently. 
     The chromatic and spherical aberrations of round lenses may be the main factors limiting resolution in charged-particle beam systems such as multi-beam apparatus  300  of  FIG.  3    or multi-beam apparatus of  FIG.  4   . Astigmatism may result from misalignment or from limitations in manufacturing tolerances and can be compensated by electric or magnetic stigmators. However, in contrast to light optics, the chromatic and spherical aberrations cannot be corrected by lens combinations. Independent of type and geometry of a round lens, the spherical aberration coefficient and the chromatic aberration coefficient are always positive. This fundamental property of electron-optical round lenses is referred to as Scherzer&#39;s theorem. As a consequence, the electron beam paths in multi-beam apparatus  300  may be restricted by very small aperture diaphragms. It is to be appreciated that the preconditions for validity of Scherzer&#39;s theorem or occurrence of spherical aberration are: round lenses, real images, static fields (time-independent), and no space charge. 
     Typically, chromatic aberration correction may be obtained using either a monochromator, an electron mirror, or crossed electric and magnetic quadrupoles acting as Wien filters. For spherical aberration correction, it is necessary to break the rotational symmetry. This can be accomplished by using electromagnetic multipole lenses such as, but not limited to, dipoles, quadrupoles, sextupoles, octopoles, etc. 
     In some embodiments, multi-beam apparatus  400  may use time-varying electric fields (time-dependent) to manipulate and focus each deflected electron beam, thus allowing for correction of spherical aberration. This may enable larger opening angles and larger beam currents to be used, also providing larger field-of-view in electron microscopes, such as multi-beam apparatus  300  of  FIG.  3   . 
       FIG.  5 A  illustrates a low frequency scan pattern  510  for observing samples using multi-beam apparatus  300  or  400 , consistent with embodiments of the present disclosure. Low frequency scan pattern  510  may comprise a one-dimensional pattern or a two-dimensional pattern. In some embodiments, two-dimensional patterns may include a matrix, an array, or a Lissajous pattern (as illustrated in  FIG.  5 A ), or combinations thereof. 
     In some embodiments, a low frequency scan as shown in  FIG.  5 A , comprises multiple probe spots arranged in a pattern forming two sinusoidal curves, the axes of which are mutually perpendicular. Such a scan comprising a plurality of probe spots may be useful in large-area scanning of a sample, such as sample  390 . 
       FIGS.  5 B and  5 C  illustrate high frequency scan patterns  520  and  530 , for observing samples using multi-beam apparatus  300  or  400 , consistent with embodiments of the present disclosure. Although high frequency scan patterns  520  and  530  are one-dimensional, two-dimensional patterns may also be used. The frequency for high frequency scans may be adjusted by adjusting either the source frequency of accelerated electron pulses, the operating frequency of deflection cavity  330 , or both. 
     Reference is now made to  FIGS.  6 A and  6 B , which illustrate high frequency scan pattern  610  and low frequency scan pattern  620 , respectively, for observing samples using multi-beam apparatus  300  or  400 , consistent with embodiments of the present disclosure.  FIG.  6 A  illustrates a one-dimensional high frequency scan pattern  610  with two probe points.  FIG.  6 B  illustrates a two-dimensional low frequency scan pattern  620  performed on each of probe points shown in high frequency scan pattern  610  of  FIG.  6 A . 
     In some embodiments, the frequency of the scan may be adjusted by changing the operating frequency of deflection cavity  330 , or changing source frequency using RF choppers, or using a de-bunching cavity, or other appropriate means. The scan frequency can be lowered, increased, or quickly switched between low and high, based on the requirement and application. For example, a user may perform a large area scan of sample  390  using low scan frequency to locate a target region of interest or a feature on a wafer, and upon locating the feature, may switch to a high frequency scan for a deeper analysis of the identified feature. 
     In some embodiments, for example, a user may perform a high frequency scan with fewer points (e.g., high frequency scan pattern  610 ) to locate a feature on sample  390 . The user may add a two-dimensional low frequency scan pattern  620  to locally inspect the identified feature. In some embodiments, low frequency scan pattern  620  may be, but is not limited to, serpentine, circular spirals, rectangular spirals, concentric circles, etc. 
     The tunability between fast low-resolution inspection and target localization or high-resolution inspection, efficient beam usage, higher productivity, simple modification of existing technology, and high inspection throughput may be some of the advantages of the embodiments of this disclosure. 
     An image manipulation software may be used to generate an image from the Lissajous pattern to a representative x-y coordinate system, enabled by the timing information obtained by controller  380 . In other words, the timing information allows for interpretation of the plurality of signals received from a Lissajous pattern of probe spots corresponding to the region of sample  390  investigated. It is to be appreciated that commonly known and available image manipulation or image processing software/application may be used. 
       FIG.  7    is a process flowchart of an exemplary method  700  of observing a sample using a multi-beam apparatus (e.g., multi-beam apparatus  300  of  FIG.  3   ), consistent with embodiments of the present disclosure. The method  700  may include forming a plurality of deflected charged-particle beams; detecting a plurality of signals generated by the plurality of deflected charged-particle beams; and associating timing information related with formation of deflected charged-particle beams and signals. 
     In step  710 , a deflection cavity (e.g., deflection cavity  330  of  FIG.  3   ), also referred to herein as a deflector, may be configured to form a plurality of deflected charged-particle beams from a primary charged-particle beam comprising a plurality of charged-particle pulses. For example, the primary charged-particle beam may be a pulsed electron beam exiting bunching cavity  320  in  FIG.  3   , and the deflected charged-particle beams may be deflected electron beams  305 _ 1  and  305 _ 2  of  FIG.  3   . In the context of this disclosure, as described earlier, the deflected charged-particle beams comprise bunches of charged-particle pulses, rather than a continuous beam of charged-particles. The charged-particle pulses may be generated from a primary charged-particle source (e.g., primary electron source  301  of  FIG.  3   ). The primary electron source may comprise, among others, a superconducting radio-frequency photoinjector, a normal conducting radio-frequency photoinjector, a high voltage direct current photoemission gun followed by an RF accelerating module photoinjector, or combinations thereof. The multi-beam apparatus  300  may include an RF cavity or a laser-trigger based cavity including, but not limited to, an acceleration cavity (e.g., acceleration cavity  310  of  FIG.  3   ), a bunching cavity (e.g., bunching cavity  320  of  FIG.  3   ), and the deflection cavity. The RF cavity may include one or more of each of the acceleration, bunching and deflection cavities. 
     An electron beam formed by the deflection cavity may be characterized as a beam comprising a “bunch” of electron pulses having high frequency. The frequency of the electron pulses of an electron beam may be based on the operating frequency of the deflectors and the source frequency of the RF powered electron source. The electron pulse frequency may be in the range of 100 MHz to 10 GHz. 
     In some embodiments, a plurality of deflection cavities may be used to generate a plurality of deflected electron beams. For example, a first and a second deflection cavity, each generating two deflected electron beams, may be disposed perpendicular to each other such that a rectangular matrix of four probe spots may be formed on sample  390 . It is to be appreciated that a number of combinations of deflection cavities, their arrangement, and operating frequency may be possible as well. 
     In step  720 , a detector (e.g., detector  350  of  FIG.  3   ) may be configured to detect a plurality of signals generated from electrons exiting from a plurality of probe spots formed by the plurality of deflected electron beams. In some embodiments, in response to incidence of primary beamlets (e.g., primary beamlets  1021 ,  1022 , and  102 _ 3  in  FIG.  2   ) on the plurality of probe spots (e.g., probe spots  102 _ 1   s ,  102 _ 2   s , and  102 _ 3   s  in  FIG.  2   ) on a sample (e.g., sample  190  in  FIG.  2   ), electrons exit from the sample and form exiting electron beams (e.g., secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se  in  FIG.  2   ). The exiting electrons may include, for example, secondary electrons, back-scattered electrons, transmitted electrons, auger electrons, or the like. The exiting electron beams may be deflected (e.g., by beam separator  160  in  FIG.  2   ) towards a detection device (e.g., electron detection device  140  in  FIG.  2   ) through a secondary projection system (e.g., secondary projection system  150  in  FIG.  2   ). Secondary projection system  150  may focus the exiting beams onto the detection device. The detection device may include one or more detection elements (e.g., detection elements  140 _ 1 ,  140 _ 2 , and  140 _ 3  in  FIG.  2   ), which may be arranged to detect corresponding exiting beams and generate corresponding signals that are sent to a controller or a signal processing system (not shown) to construct images of the corresponding scanned areas of the sample. 
     In step  730 , a controller (e.g., controller  380  of  FIG.  3   ) may be configured to obtain a first timing information related with formation of the deflected electron beam of the plurality of deflected electron beams. The controller may be configured to communicate with the deflection cavity to obtain the first timing information. The first timing information may include, but is not limited to, a time of deflection of a charged-particle pulse of the deflected beam or time of deflection, a time of formation of a charged-particle pulse of the primary charged-particle beam, a frequency or a period of the electron beam, or an average number of electron pulses in the beam, etc. For example, the time of deflection of the charged-particle pulse of the deflected charged-particle beam may include a timestamp when a deflector (e.g., deflection cavity  330  in  FIG.  2   ) directs a charged-particle pulse of the deflected charged-particle beam (e.g., a compressed beam exiting bunching cavity  320 ) into a direction to form a portion of the deflected beam (e.g., deflected electron beam  305 _ 1  in  FIG.  2   ). The first timing information may also include timestamps when the pulses of the pulsed primary electron beam are generated (e.g., by bunching cavity  320  in  FIG.  3   ). If the electron source is a laser induced pulsed source, the first timing information may also include timestamps when a laser illuminates the pulsed source for boosting electron emission. For another example, if a primary electron beam is a continuous beam, the first timing information may include timestamps when an RF cavity converts a portion of the continuous beam into a pulse of electrons. The first timing information may be obtained by the controller controlling the deflector. In some embodiments, the first timing information may be retrievably stored in a data storage module of the controller. It is appreciated that step  730  may occur during or after the forming of a plurality of deflected charged particle beams at step  710 . 
     In step  740 , the controller may be configured to obtain a second timing information related with detection of a signal of the plurality of signals from the plurality of probe spots formed by the plurality of deflected electron beams. The controller may be configured to communicate with the detector to obtain timing information including, but not limited to, time of detection of a signal, energy of the electron, etc. The second timing information related with the detection of a signal may be retrievably stored in the data storage module of the controller. The first and the second timing information may be stored in a remote accessible location such as a server, computer, processor, external memory modules, etc. as well, in communication with the controller. It is appreciated that step  740  may occur during or after the detecting the plurality of signals at step  720 . 
     In step  750 , the controller may be configured to associate the detected signal and the corresponding deflected electron beam based on the obtained first and second timing information. The timing information related with formation of the deflected electron beam and the detection of the corresponding signal stored in the controller may enable conversion of the plurality of detection signals to a visual representation of the region of sample  390  being investigated, by an image manipulation tool such as, but not limited to an image processing software, an image manipulation application, etc. 
     In some embodiments, the controller may be configured to obtain timing information through timing signals. A timing signal, as referred to herein, may be an electric pulse, a voltage signal, a current signal, etc. based on the component generating the signal or generated by the clock of the processor of the controller. 
     The controller may be further configured to synchronize the timing of relevant components of the multi-beam apparatus including, but not limited to, primary electron source, acceleration cavity, deflection cavity, bunching cavity, detector, beam raster system, etc. The timing of injection of pulses into the acceleration cavity, bunching of electron pulses by the bunching cavity, deflection of the formed bunches, etc. may have to be synchronized for an efficient operation of the multi-beam apparatus based on RF powered beams. 
     Reference is now made to  FIGS.  8 A- 8 B , which illustrate two exemplary methods of controlling charged particles to enter a crossover area (or “common area”) in a multi-beam apparatus, consistent with embodiments of the present disclosure.  FIG.  8 A  shows a first method of controlling charged particles (e.g., electrons) of beams  802 ,  804 , and  806  to enter a crossover area  808  near an objective lens  810 . In some embodiments, objective lens  810  may be objective lens  131  in  FIG.  2   .  FIG.  8 A  shows beams  802 ,  804 , and  806  as pulsed beams as an example. In some embodiments, beams  802 ,  804 , and  806  may also be continuous beams, in which electrons travel like a stream. For ease of explanation without causing ambiguity, beams  802 ,  804 , and  806  will be described as pulsed beams hereinafter unless specified otherwise. However, it should be noted that any description related to beams  802 ,  804 , and  806  may be equivalently applicable to pulsed beams and continuous beams. The pulses of charged particles are shown as round dots along the direction of beams  802 ,  804 , and  806 . Beams  802 ,  804 , and  806  cross each other at crossover area  808 . Crossover area  808  is a position in space. In some embodiments, crossover area  808  may be near, on, or in objective lens  810 . As can be seen in  FIG.  8 A , at a given time, there may be more than one beam that crosses crossover area  808 , in which charged particles from different beams may come across each other. That is, when beams  802 ,  804 , and  806  are pulsed beams (as shown in  FIG.  8 A ), there is time when more than one pulse of charged particles cross at or are near crossover area  808 . When beams  802 ,  804 , and  806  are continuous beams (not shown), there is time when electrons from at least two of beams  802 ,  804 , and  806  cross at or are near crossover area  808 . This may lead to significant Coulomb effect. 
     The Coulomb effect may occur between the beamlets in two directions: a longitudinal direction (i.e., along the travel direction of the charged particles) and a transversal direction (i.e., perpendicular to the travel direction of the charged particles). In a longitudinal direction, the Coulomb effect may occur between charged particles of the same beamlet if they are too close. For example, if a beamlet is a pulsed beamlet in which charged particles are bunched into pulses, the longitudinal Coulomb effect may occur if the pulses are too compressed. In a transversal direction, the Coulomb effect may occur between charged particles of neighboring beamlets if they are too close. For example, if the neighboring beamlets are too close to each other, the transversal Coulomb effect may occur. The longitudinal Coulomb effect may cause the energy of the same-beam charged particles to become less uniform, which may further cause chromatic aberration in generated inspection images. The transversal Coulomb effect may enlarge the cross-sectional size of the beamlet (e.g., due to repelling of the charged particles with like charges). Both the longitudinal and the transversal Coulomb effect may enlarge the sizes of the probing spots on the sample and deteriorate imaging resolution. Typically, the major contribution of the deterioration comes from the transversal Coulomb effect. 
     For example, in some embodiments, in  FIG.  8 A , when the multi-beam apparatus provides  50  beams and each beam has a current of 10 nano-Ampere (nA), the Coulomb effect at crossover area  808  may contribute to an enlargement of a spot size for 12.1 nanometer (nm) on a surface of a sample (e.g., a silicon wafer). The spot size may be determined as a line of 50% signal strength enclosing a portion of full width of detected signals (“FW50 method”). Without the Coulomb effect, a beam spot may be of a size 10 nm. With the Coulomb effect, the beam spot may be of a size 15.7 nm. That amounts to about a 60% increase of the spot size. Also, in this example, the Coulomb effect at crossover area  808  may contribute to an increase of energy spread of the pulses for about 0.1 electron volt (eV), as compared with a typical energy spread of 0.5-0.7 eV for a beam in a high-resolution multi-beam SEM. That amounts to 14-20% increase of the energy spread. Due to the Coulomb effect, it could set limits for the multi-beam apparatus in this example to achieve a higher imaging resolution. 
       FIG.  8 B  illustrates another method of controlling charged particles of beams  812 ,  814 , and  816  to enter crossover area  808  near objective lens  810 . In beams  812 ,  814 , and  816 , pulses of charged particles are grouped into clusters (shown as triplet round dots), such as, for example, by acceleration cavity  310  or bunching cavity  320  in  FIG.  3   . Each cluster may include one or more consecutive pulses such that intra-cluster distances may be shorter than inter-cluster distances. The clusters are released in a predetermined time-space order (e.g., staggering) such that they may enter crossover area  808  in sequence (e.g., one by one, with only one or zero cluster being in crossover area  808  at any point in time). For example, near crossover area  808 , a cluster  818  from beam  812  may enter crossover area  808  first, followed by a cluster  820  from beam  816 , and then followed by a cluster  822  from beam  814 . That is, the number of clusters that simultaneously cross crossover area  808  may be limited. In some embodiments, at a given time, there may be at most N (an integer) cluster crossing crossover area  808 . For example, N may be 1. In such cases, the Coulomb effect may be significantly reduced, especially the transversal Coulomb effect. 
       FIGS.  9 A- 9 B  illustrate an exemplary cluster generator for clustering charged particles, consistent with embodiments of the present disclosure. In some embodiments, the cluster generator in  FIGS.  9 A- 9 B  may be acceleration cavity  310  or bunching cavity  320  in  FIG.  3   . In  FIG.  9 A , the cluster generator is a cluster cavity  902 . It should be noted that the cluster generator may be implemented as forms other than cavities. A dynamic electric field  904  is provided inside cluster cavity  902 , the direction of which is represented by the black arrows. Electric field  904  may have a direction parallel to (e.g., along or against) the moving direction of a beam (e.g., beam  812 ,  814 , or  816 ). That is, electric field  904  may accelerate or decelerate charged particles when it is non-zero. In some embodiments, electric field  904  may change periodically. For example, electric field  904  may be sinusoidal, and its direction may change to be up and down in cycles. It is noted that electric field  904  may vary in any suitable manner for clustering charged particles, and this disclosure does not have limitations on that aspect. 
       FIG.  9 B  shows how two charged particles are clustered by cluster cavity  902 . In  FIG.  9 B , vertical sections of cluster cavity  902  are shown along a timeline at three timestamps t1, t2, and t3. Before t1, charged pulses  908  and  906  move downward toward cluster cavity  902 , roughly at the same speed. A charged pulse may include one or more charged particles. At t1, charged pulse  906  enters cluster cavity  902 , while charged pulse  908  is outside cluster cavity  902 . Electric field  904  may be provided to decelerate charged pulse  906 , indicated by the solid arrow attached to charged pulse  906 . At t2, charged pulse  906  exits or is about to exit cluster cavity  902  with decreased speed, and charged pulse  908  enters cluster cavity  902 . At t 2 , electric field  904  may be provided to accelerate (e.g., by reversing the direction of electric field  904  at t 1 ) or not decelerate (e.g., by setting a zero amplitude for electric field  904 ) charged pulse  908 , indicated by the dashed arrow. By doing so, charged pulse  908  may move faster than charged pulse  906  and close the distance between them. At t 3 , both charged pulses  906  and  908  exit cluster cavity  902 , and charged pulse  908  catches up with charged pulse  906 . That is, a cluster of charged pulses  906  and  908  is formed. 
     In some embodiments, to ensure that the charged pulses in the formed cluster have substantially similar speed, before charged pulse  908  exits cluster cavity  902 , electric field  904  may decelerate charged pulse  908  such that when the deceleration stops, charged pulse  908  not only catches up with charged pulse  906 , but also has substantially similar speed with charged pulse  906 . 
     In some embodiments, electric field  904  may be provided dynamically (e.g., changing in cycles) such that more than two charged particles may form a cluster. For example, when multiple charged particles enter cluster cavity  902  in sequence, electric field  904  may alternate between a deceleration field and a neutral field (e.g., with a zero amplitude), by which the distances between the charged pulses may decrease and form a cluster when they exit cluster cavity  902 . 
     In some embodiments, a cluster generator (e.g., cluster cavity  902 ) may receive one beam (e.g., beam  812 ,  814 , or  816 ) of charged pulses. When the electric field of the cluster generator varies in cycles, the frequency of such cycles may depend on a scan frequency of the multi-beam apparatus and the number of the beams provided by the multi-beam apparatus. For example, when the scan frequency is 10 MHz, and the number of provided beams is 10, the frequency of the electric field of the cluster generator may be set to be higher than 100 MHz (e.g., to the order of 1 GHz). 
     To achieve the time-space order of clusters as shown in  FIG.  8 B , multiple cluster generators may be used for clustering multiple beams.  FIG.  10    illustrates an exemplary set of cluster generators releasing charged-particle clusters in a time-space order, consistent with embodiments of the present disclosure. In  FIG.  10   , the cluster generators may be cluster cavities  1002 ,  1004 , and  1006  similar to cluster cavity  902 . Cluster cavities  1002 ,  1004 , and  1006  receive charged-particle beams  1008 ,  1010 , and  1012  (shown as discrete black dots), respectively. Each of cluster cavities  1002 ,  1004 , and  1006  may turn its received beam into clusters (shown as triplet black dots) and release or eject the clusters in accordance with a time-space order. For example,  FIG.  10    shows six timestamps t1 to t6, with t1 being the earliest one and t6 being the latest one. In some embodiments, the intervals between t1 to t6 may be equal. In some embodiments, cluster cavities  1002 ,  1004 , and  1006  may release clusters in an alternate way. For example, at t1, cluster cavity  1006  releases a cluster. At t2, cluster cavity  1002  releases a cluster. At t3, cluster cavity  1004  releases a cluster. At t4, cluster cavity  1006  releases a cluster. At t5, cluster cavity  1002  releases a cluster. At t6, cluster cavity  1004  releases a cluster. In addition, the longitudinal distance between the released clusters may also be equal. The longitudinal distance is the difference of positions along the moving direction of the clusters. For example, the longitudinal distances between the clusters released from t1 to t6 may be equal, as indicated by the equidistant dashed boxes enclosing each cluster in  FIG.  10   . In some embodiments, the time-space order may be the temporal and spatial relationship between the released clusters as shown and described in  FIG.  10   . 
     Although  FIG.  10    shows that cluster cavities  1002 ,  1004 , and  1006  are positioned in a line, it should be noted that this is an example embodiment only, and other positional or spatial arrangements of the cluster cavities are possible. For example, they may be positioned along a line, in a triangle, have different relative longitudinal distances (e.g., not positioned on the same plane), or any spatial configuration in three-dimensional space. It should also be noted that the number of the cluster cavities may depend on the number of beams provided by the multi-beam apparatus and not be limited to three, as shown in  FIG.  10   . 
     In some embodiments, besides the implementations of  FIGS.  9 A- 10   , the charged particles may be grouped into clusters using different manners.  FIG.  11    illustrates an exemplary arrangement of first and second cluster generators for clustering charged particles, consistent with embodiments of the present disclosure.  FIG.  11    shows a cluster cavity  1102  and a cluster cavity  1104  as the first cluster generator and the second cluster generator, respectively. Cluster cavity  1102  is provided with a dynamic electric field  1106 . Cluster cavity  1104  is provided with a dynamic electric field  1108 . In some embodiments, cluster cavities  1102  and  1104  may be implemented similar to cluster cavity  902  in  FIGS.  9 A- 9 B , and electric fields  1106  and  1108  may be implemented similar to electric field  904 . In  FIG.  11   , cluster cavity  1104  may be arranged as downstream from cluster cavity  1102 . In some embodiments, cluster cavities  1102  and  1104  may be arranged coaxially about an axis  1110 , such that a beam may enter and exit them along its path. Cluster cavities  1102  and  1104  may have the same or different features or configurations (e.g., sizes, height, diameters, geometries, or the like). For example, maximum amplitudes and frequencies of electric fields  1106  and  1108  may be the same or different. 
     In  FIG.  11   , a charged pulse  1112  and a charged pulse  1114  may be clustered in the following way. Vertical sections of cluster cavity  1102  are shown along a timeline at three timestamps t1, t2, and t3. Vertical sections of cluster cavity  1104  are shown along a timeline at three timestamps t4, t5, and t6, with t4 being later than t3. Before t1, charged pulses  1112  and  1114  move downward toward cluster cavity  1102 , roughly at the same speed. At t1, charged pulse  1112  enters cluster cavity  1102 , while charged pulse  1114  is outside cluster cavity  1102 . Electric field  1106  may be provided to decelerate charged pulse  1112 , indicated by the solid arrow attached to charged pulse  1112 . At t2, charged pulse  1112  exits or is about to exit cluster cavity  1102  with decreased speed, and charged pulse  1114  enters cluster cavity  1102 . At t2, electric field  1106  may be provided to decelerate or not accelerate charged pulse  1114 , indicated by the dashed arrow. For example, at t2, the direction of electric field  1106  may be the same as at t1, and its amplitude may be smaller than that at t1. By doing so, charged pulse  1114  may move faster than charged pulse  1112  and close the distance between them. At t3, both charged pulses  1112  and  1114  exit cluster cavity  1102 , and charged pulse  1114  catches up with charged pulse  1112 . That is, a cluster of charged pulses  1112  and  1114  is formed. 
     In some embodiments, at t3, the speed of charged pulse  1114  may be still higher than that of charged pulse  1112 , and the formed cluster may disperse again after a certain amount of time. In those cases, the energy spread (e.g., longitudinal energy spread) of the cluster may be enlarged. For reducing the energy spread, cluster cavity  1104  may be used to equalize the speeds of charged pulses  1112  and  1114  after the cluster is formed. In some embodiments, cluster cavity  1104  may be arranged at a position such that, before t (when the cluster reaches cluster cavity  1104 ), second charged pulse  1114  has surpassed charged pulse  1112  along the path of the cluster. At t 4 , charged pulse  1114  enters cluster cavity  1104 , while charged pulse  1112  is outside cluster cavity  1104 . Electric field  1108  may be provided to decelerate charged pulse  1114 , indicated by the dashed arrow attached to charged pulse  1114 . At t 5 , charged pulse  1114  exits or is about to exit cluster cavity  1104  with decreased speed, and charged pulse  1112  enters cluster cavity  1104 . At t 5 , electric field  1108  may be provided to decelerate or not accelerate charged pulse  1112 , indicated by the solid arrow. For example, at t 5 , the direction of electric field  1108  may be the same as at t 4 , and its amplitude may be smaller than that at t 4 . By doing so, charged pulse  1112  may move faster than charged pulse  1114  such that the distance between them become close, and the speeds of them become substantially similar when electric field  1108  stops decelerating charged pulse  1112 . At t 6 , both charged pulses  1112  and  1114  exit cluster cavity  1104 , and charged pulse  1114  not only catches up with charged pulse  1112 , but also has substantially similar speed with charged pulse  1112 . That is, a cluster of charged pulses  1112  and  1114  having substantially similar speed is formed. The charged pulses in a cluster may be deemed as having substantially similar speed when the relative speed between the charged pulses is zero or within a predetermined percentage (e.g., 1%, 3%, 5%, 10%, or any reasonable percentage). 
     In some embodiments, electric fields  1106  and  1108  may be provided dynamically (e.g., changing in cycles) such that more than two charged particles may form a cluster. For example, when multiple charged particles enter first and second cluster cavities  1102  and  1104  in sequence, electric fields  1106  and  1108  may alternate between a deceleration field and a neutral field (e.g., with a zero amplitude), by which the distances between the charged pulses may decrease and form a cluster when they exit first and second cluster cavities  1102  and  1104 . The frequencies of electric fields  1106  and  1108  may depend on a scan frequency of the multi-beam apparatus and the number of the beams provided by the multi-beam apparatus. It should be noted that, electric fields  1106  and  1108  may also have other implementations, such as applying an acceleration field for accelerating charged pulse  1114  in cluster cavity  1102  or accelerating charged pulse  1112  in second cluster cavity  1114 , similar to cluster cavity  902 . This disclosure does not limit on that aspect as long as the speeds of the charged pulses of the generated clusters may be adjusted to be substantially similar. 
     In some embodiments, stray charged particles may exist near the formed cluster, which may enlarge the energy spread (e.g., transversal energy spread) of the cluster. Stray charged particles are charged particles not clustered by the cluster generators. In some embodiments, a filtering system may be used to remove or reduce the stray charged particles. 
       FIG.  12    illustrate an exemplary arrangement of a cluster generator  1202  and a filtering system for reducing stray charged particles, consistent with embodiments of the present disclosure. The cluster generator  1202  may be implemented similar to cluster cavities  902 ,  1002 ,  1004 , and  1006 , or  1102  and  1104  in  FIGS.  9 A- 11   . Cluster generator  1202  may be provided with a dynamic electric field  1204  for clustering charged particles. Electric field  1204  may be implemented similar to electric fields  904 ,  1106 , or  1108 . Cluster generator  1202  may receive beams of charged particles and generate clusters  1206 , indicated by round dots downstream from cluster generator  1202 . Stray charged particles  1208  may exist around the generated clusters, indicated by a grayscale band. 
     The filtering system may include a filter cavity  1210  arranged downstream from cluster generator  1202 . In some embodiments, cluster generator  1202  and filter cavity  1210  may be arranged coaxially about the path of clusters  1206 . Filter cavity  1210  may be provided with a dynamic electromagnetic field  1212  for deflecting stray charged particles  1208 . An “electromagnetic field” may include a pure electric field, a pure magnetic field, or a combination of an electric field and a magnetic field. Electromagnetic field  1212  may be configured to not affect the moving direction of clusters  1206 . In some embodiments, electromagnetic field  1212  may be a dynamic electric field, a dynamic magnetic field, or a combination of them. For example, electromagnetic field  1212  may be configured such that, when a cluster enters filter cavity  1210 , the moving direction of the cluster may be kept unchanged (e.g., by setting a zero amplitude of electromagnetic field  1212 ). When stray charged particles  1208  enters filter cavity  1210 , electromagnetic field  1212  may be set as non-zero and deflect the moving directions of the stray charged particles  1208 . For example, as shown in  FIG.  12   , clusters  1206  may move along a downward direction without being affected by electromagnetic field  1212  when passing through filter cavity  1210 . However, stray charged particles  1208  may be deflected to the left or right by electromagnetic field  1212  when passing through filter cavity  1210 , and may be blocked or absorbed by a wall of filter cavity  1210 . 
     In some embodiments, the direction of electromagnetic field  1212  may change perpendicular to the moving direction of cluster  1206 . For example, electromagnetic field  1212  may be a dynamic electric field, the direction of which changes perpendicular to the moving direction of clusters  1206 . For another example, electromagnetic field  1212  may be a dynamic magnetic field, the direction of which changes perpendicular to the moving direction of clusters. In some embodiments, the frequency of electromagnetic field  1212  may depend on distances between clusters  1206 , a frequency of cluster generator  1202  (which further depend on the scan frequency of the multi-beam apparatus and the number of beams provided by the multi-beam apparatus), strength or amplitude of electromagnetic field  1212 , the speeds of clusters  1206  and stray charged particles  1208 , the number or density of stray charged particles  1208 , or the like. 
     In some embodiments, to further remove the deflected charged particles  1208 , the filtering system may further include an aperture plate  1214  downstream from filter cavity  1210 . In some embodiments, aperture plate  1214  may include an aperture  1216  on the path of clusters  1206 . Cluster generator  1202 , filter cavity  1210 , and aperture  1216  may be arranged coaxially about the path of clusters  1206 . Aperture  1216  may have a size S that allows clusters  1206  to pass and block all or majority of the stray charged particles  1208 . As shown in  FIG.  12   , the deflected stray charged particles  1208  are blocked by aperture plate  1214 , and cluster  1206  may pass through aperture  1216 . By doing so, transversal energy spread of clusters  1206  may be significantly reduced. In some embodiments, by applying the filtering system, the energy spread of the clusters introduced by the cluster generator (e.g., cluster generator  1202 ) may be reduced to about 0.1 eV. 
     It should be noted that embodiments in  FIGS.  10 - 12    may be modified, combined, or rearranged such that elements of them may be combined in any manner for actual applications. For example, additional cluster cavities (e.g., cavity  1104  in  FIG.  11   ) may be arranged downstream from cluster cavities (e.g., cluster cavities  1002 ,  1004 , and  1006  in  FIG.  10   ), respectively. For another example, filtering systems (e.g., including at least one of filter cavity  1210  and aperture plate  1214 ) may be arranged downstream from cluster cavities  1002 ,  1004 , and  1006  in  FIG.  10    or cluster cavity  1104  in  FIG.  11   , respectively. For another example, filter cavity  1210  may be arranged between cluster cavity  1102  and cluster cavity  1104 , and aperture plate  1214  may be arranged as downstream from cluster cavity  1104  in  FIG.  11   . Other variations and modifications are also possible, and this disclosure does not limit on that aspect. 
       FIG.  13    is a flowchart showing an exemplary method  1300  of reducing interaction of charged particles in a charged-particle beam of a multi-beam apparatus, consistent with embodiments of the present disclosure. Method  1300  may be performed by a controller that may be coupled with a charged particle beam apparatus (e.g., EBI system  1 ). For example, the controller may be controller  380  in  FIGS.  3 - 4   . The controller may be programmed to implement method  1300 . 
     At step  1302 , a first cluster cavity receives a first set of charged particles to form a first cluster of charged particles. At step  1304 , a second cluster cavity receives a second set of charged particles to form a second cluster of charged particles. In some embodiments, the first set of charged particles and the second set of charged particles may be charged-particle pulses. In some embodiments, the first set of charged particles and the second set of charged particles may be continuous charged-particle streams. 
     In some embodiments, the first cluster cavity may be provided with a first dynamic electric field, and the second cluster cavity may be provided with a second dynamic electric field. In some embodiments, the first cluster cavity may be one of cluster cavities  1002 ,  1004 , and  1006  in  FIG.  10    (e.g., cluster cavity  1002 ), and the first beam may be a corresponding beam of beams  1008 ,  1010 , and  1012  (e.g., beam  1008 ). The second cluster cavity may be another one of cluster cavities  1002 ,  1004 , and  1006  in  FIG.  10    (e.g., cluster cavity  1004 ), and the second beam may be a corresponding beam of beams  1008 ,  1010 , and  1012  (e.g., beam  1010 ). 
     In some embodiments, the controller may control a direction of the first dynamic electric field to change parallel to a direction of the first beam, and a direction of the second dynamic electric field to change parallel to a direction of the second beam. For example, the first and second dynamic electric fields may change in a direction as shown and described in  FIGS.  9 A- 9 B . In some embodiments, the controller may control at least one of the direction of the first dynamic electric field or the direction of the second dynamic electric field to change in a first cycle. For example, the direction of the first dynamic electric field, the direction of the second dynamic electric field, or both may be changed periodically. In some embodiments, the first cycle may be determined based on at least one of a scan frequency of the multi-beam apparatus or a number of the beams. 
     Still referring to  FIG.  13   , at step  1304 , the first cluster cavity forms a first cluster of charged particles using at least two charged particles in the first beam and at least the first dynamic electric field, and the second cluster cavity forms a second cluster of charged particles using at least two charged particles in the second beam and at least the second dynamic electric field. In some embodiments, the first cluster may be the released cluster at t2 in  FIG.  10   , and the second cluster may be the released cluster at t3 in  FIG.  10   . The first and second clusters may be formed in a way as shown and described in  FIGS.  9 A- 12   . 
     In some embodiments, the at least two charged particles may include the first charged particle (e.g., charged particle  906  in  FIG.  9 B ) and the second charged particle (e.g., charged particle  908  in  FIG.  9 B ). The controller may cause the first dynamic electric field (e.g., electric field  904  in  FIG.  9 B ) to decelerate a first charged particle before a second charged particle enters the first cluster cavity (e.g., as what occurs at t1 in  FIG.  9 B ). When the second charged particle enters the first cluster cavity, the controller may control the first dynamic electric field for causing the second charged particle to move faster than the first charged particle. For example, the controller may control the first dynamic electric field to accelerate the second charged particle, as what occurs at t2 in  FIG.  9 B . For another example, the controller may control the first dynamic electric field to neither accelerate nor decelerate the second charged particle, such as by setting an amplitude of the first dynamic electric field to be zero. For another example, the controller may control the first dynamic electric field to decelerate the second charged particle in a lesser degree than decelerating the first charged particle, as what occurs at t2 in  FIG.  11   . 
     In some embodiments, to reduce energy spread of the formed clusters, additional cluster cavities may be used to uniformize the moving speeds of the charged particles in the clusters. For example, the moving speeds of the charged particles of the same cluster may be uniformized in accordance with a manner shown and described in  FIG.  11   . In some embodiments, when forming the first cluster, the controller may receive the first cluster in a third cluster cavity (e.g., cluster cavity  1104  in  FIG.  11   ) downstream from the first cluster cavity (e.g., cluster cavity  1102  in  FIG.  11   ). The first cluster may be formed by the first cluster cavity. The controller may cause the first charged particle (e.g., charged particle  1112  in  FIG.  11   ) and the second charged particle (e.g., charged particle  1114  in  FIG.  11   ) to move in a substantially similar speed (e.g., an equal speed or different speeds within a predetermined range) using a third dynamic electric field (e.g., electric field  1108  in  FIG.  11   ) in the third cluster cavity, as what occurs at t4-t6 shown and described in  FIG.  11   . For example, the controller may cause the third dynamic electric field to decelerate the second charged particle (e.g., charged particle  1114 ) to move at the substantially similar speed, as what occurs at t4 in  FIG.  11   . In some embodiments, the controller may coordinate the third dynamic electric field and the first dynamic electric field to change. For example, the controller may control a direction of the third dynamic electric field (e.g., electric field  1108  in  FIG.  11   ) to change parallel to a direction of the first beam (e.g., a downward direction along axis  1110  in  FIG.  11   ). In some embodiments, the controller may change the direction of the third dynamic electric field in a second cycle. For example, the second cycle (e.g., a cycle of electric field  1108 ) may be different from or the same as the first cycle (e.g., a cycle of electric field  1106 ), as shown and described in  FIG.  11   . In some embodiments, the controller may determine the second cycle based on at least one of the scan frequency of the multi-beam apparatus, the number of the beams, or the first cycle. It should be noted that the uniformization approach of the moving speeds of the charged particles in the first cluster is also applicable to the second cluster for uniformizing the moving speeds of the charged particles in the second cluster. 
     In some embodiments, to reduce energy spread of the formed clusters, a filtering system may be used to remove or reduce stray charged particles near the clusters. For example, the stray charged particles may be removed or reduced in accordance with a manner as shown and described in  FIG.  12   . In some embodiments, a filter cavity (e.g., filter cavity  1210 ) downstream from the first cluster cavity (e.g., cluster generator  1202 ) may receive the cluster (e.g., one of clusters  1206 ) and a first stray charged particle (e.g., one of stray charged particles  1208 ). A dynamic electromagnetic field (e.g., electromagnetic field  1212 ) in the filter cavity may filter the first stray charged particle. In some embodiments, the dynamic electromagnetic field may include at least one of a third dynamic electric field or a dynamic magnetic field. The third dynamic electric field may be different from the first and second dynamic electric fields (e.g., electric fields  1106  and  1108 , respectively). In some embodiments, the controller may cause a direction of the dynamic electromagnetic field (e.g., electromagnetic field  1212 ) to change perpendicular to the direction of the first beam. For example, for filtering the first stray charged particle, when the first cluster is in the filter cavity (e.g., when the cluster is passing through the filter cavity), the controller may cause the dynamic electromagnetic field not to direct the cluster, such as by setting its amplitude to be zero. When the cluster is not in the filter cavity (e.g., before the cluster entering the filter cavity or after the cluster exiting the filter cavity), the controller may cause the dynamic electromagnetic field to direct the first stray charged particle away from the direction of the first beam, as shown and described in  FIG.  12   . In some embodiments, the controller may coordinate the dynamic electromagnetic field (e.g., electromagnetic field  1212 ) and the first dynamic electric field (e.g., electric field  1204 ) to change. For example, the controller may change the direction of the dynamic electromagnetic field in a third cycle. The third cycle may be different from or the same as the first and second cycles. In some embodiments, the controller may determine the third cycle based on at least one of the scan frequency of the multi-beam apparatus, the number of the beams, the first cycle, or the second cycle. 
     In some embodiments, for further reducing the energy spread of the formed clusters, the filtering system may further include an aperture plate to remove more stray charged particles. For example, as shown and described in  FIG.  12   , an aperture plate (e.g., aperture plate  1214 ) having an aperture (e.g., aperture  1216 ) downstream from the filter cavity (e.g., filter cavity  1210 ) may filter a second stray charged particle. The second stray charged particle may be different from the first stray charged particle. In some embodiments, the filter cavity and the aperture plate may be upstream or downstream from the third cluster cavity. For example, the filter cavity (e.g., filter cavity  1210 ) and the aperture plate (e.g., aperture plate  1214 ) may be positioned between the first cluster cavity (e.g., cluster cavity  1102  in  FIG.  11   ) and the third cluster cavity (e.g., cluster cavity  1104  in  FIG.  11   ). For another example, both the filter cavity (e.g., filter cavity  1210 ) and the aperture plate (e.g., aperture plate  1214 ) may be positioned downstream from the third cluster cavity (e.g., cluster cavity  1104  in  FIG.  11   ). In some embodiments, the third cluster cavity may be between the filter cavity and the aperture plate. For example, the third cluster cavity (e.g., cluster cavity  1104  in  FIG.  11   ) may be positioned downstream from the filter cavity (e.g., filter cavity  1210 ) and upstream from the aperture plate (e.g., aperture plate  1214 ). It should be noted that the filtering approach for the stray charged particles (e.g., the first and second stray charged particles) is also applicable to the second cluster for removing or reducing the stray charged particles in the second cluster. 
     Still referring to  FIG.  13   , at step  1306 , the controller causes the first cluster and the second cluster to pass a downstream position in a predetermined time-space order. The downstream position may be a crossover area (e.g., crossover area  808  in  FIGS.  8 A- 8 B ). In some embodiments, the crossover area may be determined as a position that may achieve small (e.g., as small as possible within condition constraints) aberrations of an objective lens (e.g., objective lens  810  in  FIGS.  8 A- 8 B ). For example, the controller may control the first and second cluster cavities to release the first and the second clusters in the predetermined time-space order. In some embodiments, the predetermined time-space order may include that the first cluster and the second cluster pass the downstream position in sequence. In some embodiments, the predetermined time-space order may be that at most one of the first cluster and the second cluster passes the downstream position at any time. In some embodiments, the downstream position may be near or in an objective lens in the multi-beam apparatus. For example, the downstream position may be crossover area  808  near objective lens  810  in  FIGS.  8 A- 8 B . In some embodiments, the downstream position may be deemed as “near” the objective lens when it is located outside of the objective lens in the order of millimeters (e.g., smaller than or equal to 20 millimeters). 
     In some embodiments, at steps  1302 - 1304 , a plurality of cluster cavities may receive respective sets of charged particles in each cluster cavity to form a plurality of beams, and each beam may include clusters of charged particles. Further, at step  1306 , the controller may cause the clusters of the plurality of beams to pass the downstream position in the predetermined time-space order. Moreover, in some cases, the predetermined time-space order at step  1306  may be that the clusters of the plurality of beams pass the downstream position in non-overlapping sequence. For example, the predetermined time-space order may be that at most one of the clusters of the plurality of beams passes the downstream position at any time. 
     In some embodiments, the controller may coordinate the first dynamic electric field and the second dynamic electric field to change. For example, the controller may coordinate the electric field of cluster cavity  1002  and the electric field of cluster cavity  1004  to change such that the first cluster released at t2 and the second cluster released at t3 may be in a time-space order shown in  FIG.  10   . In some embodiments, the controller may cause the first cluster to exit the first cluster cavity and the second cluster to exit the second cluster cavity in an alternate manner, and the first cluster and the second cluster pass the downstream position in the alternate manner. For example, as shown and described in  FIG.  10   , the first cluster exits the first cluster cavity (e.g., cluster cavity  1002 ) at t2 before the second cluster exits the second cluster cavity (e.g., cluster cavity  1104 ) at t3, in an alternate manner. That is, the first and second cluster cavities may release the first and second clusters in sequence (e.g., one by one). The first cluster and the second cluster may pass the downstream position (e.g., crossover area  808  in  FIG.  8 B ) in the same alternate manner. That is, if the second cluster follows the first cluster when exiting the first and second cluster cavities, the second cluster would also follow the first cluster when passing the downstream position. 
     Reference is now made to  FIG.  14   .  FIG.  14    illustrates an exemplary multi-beam apparatus  1400  for providing multiple charged-particle beams, consistent with embodiments of the present disclosure. In  FIG.  14   , charged-particle sources  1402 ,  1404 , and  1406  are configured to generate pulsed bunches of charged particles (e.g., electrons), forming beams  1414 ,  1416 , and  1418 . In some embodiments, beams  1414 ,  1416 , and  1418  may be pulsed, such as beams  802 ,  804 , and  806  in  FIG.  8 A . In some embodiments, beams  1414 ,  1416 , and  1418  may be clustered, such as beams  812 ,  814 , and  816  in  FIG.  8 B . Each beam is indicated by circles with different shades in  FIG.  11   . In some embodiments, each of charged-particle sources  1402 ,  1404 , and  1406  may be primary electron source  101  in  FIG.  2    or primary electron source  301  in  FIGS.  3 - 4   . Charged-particle sources  1402 ,  1404 , and  1406  may match with deflectors  1408 ,  1410 , and  1412 , respectively. Deflectors  1408 ,  1410 , and  1412  may deflect beams  1414 ,  1416 , and  1418  to make them converged as they approach charged-particle accelerator  1420 . 
     A charged-particle accelerator  1420  (e.g., acceleration cavity  310  in  FIGS.  3 - 4   ) receives beams  1414 ,  1416 , and  1418  and accelerates the charged particles to increase their energy, and eject them towards beam concentrator  1422 . In some embodiments, beam concentrator  1422  may function as a combination of acceleration cavity  310 , bunching cavity  320 , and deflection cavity  330  in  FIGS.  3 - 4   , capable of accelerating the charged-particle beams (e.g., beams  1414 ,  1416 , and  1418 ) and converging each beam toward beam concentrator  1422 . Beam concentrator  1422  may include multiple deflectors (e.g., radio-frequency cavities or MEMS deflectors) implemented similar to deflection cavity  330 , including deflectors  1424 ,  1426 , and  1428 . In some embodiments, the deflectors may be arranged to form an array. In some embodiments, each deflector may receive and deflect a beam in a manner that the beams exiting beam concentrator  1422  become parallel again. For example, in  FIG.  14   , deflector  1424 ,  1426 , and  1428  can receive and deflect beams  1414 ,  1416 , and  1418 , respectively, although it is appreciated that a deflector (e.g., deflector  1426 ) on the optical axis of multi-beam apparatus  1400  may not perform any deflection. In some embodiments, a deflector may receive more than one beam. In some embodiments, deflector  1424 ,  1426 , and  1428  may be MEMS deflectors. In some embodiments, deflector  1424 ,  1426 , and  1428  may be electric cavities (e.g., RF cavities) that are provided with dynamic electric fields. For example, the dynamic electric fields may be similar to electromagnetic field  1212  in  FIG.  12    that change perpendicular to the moving direction of a beam. The dynamic electric fields may change deflection angles of beams  1414 ,  1416 , and  1418 . Beams  1414 ,  1416 , and  1418  may eventually reach a surface of a sample  1436  after being projected by projection lenses  1432 . 
     Deflectors  1424 ,  1426 , and  1428  may deflect beams  1414 ,  1416 , and  1418  such that the outgoing beams  1414 ,  1416 , and  1418  from deflectors  1424 ,  1426 , and  1428  may be parallel, and distances between them may be shorter than distances between charged-particle sources  1402 ,  1404 , and  1406 . For example, as shown in  FIG.  14   , the distances between charged-particle sources  1402 ,  1404 , and  1406  are greater than the distances between beams  1414 ,  1416 , and  1418  exiting deflectors  1424 ,  1426 , and  1428 . Such a design may integrate more charged-particle sources into a multi-beam apparatus for providing more beams to accommodate the increasing demands of imaging throughput and brightness, yet avoiding the problems of complex designs of electric routing for an aperture plate (functioning as a beam splitter) because no aperture plate may be used (replaced by the deflectors), and the problems of increasing risks of electric breakdowns because the electric routing of the charged-particle sources now have greater distances in between. 
     Also, deflectors  1424 ,  1426 , and  1428  may release beams  1414 ,  1416 , and  1418  in a predetermined time-space order to reduce the Coulomb effect. For example, deflectors  1424 ,  1426 , and  1428  may release pulses or clusters in an alternate manner (e.g., one by one), such that a pulse or cluster may exit deflectors  1424 ,  1426 , and  1428  in sequence (e.g., one at a time). The predetermined time-space order may be in a manner as shown and described in  FIG.  10   . Beams  1414 ,  1416 , and  1418  may be converged downstream from at a crossover area  1434  near projection lenses  1432 . Crossover area  1434  may be similar to crossover area  808  in  FIGS.  8 A- 8 B . Projection lenses  1432  may be similar to objective lens  810  in  FIGS.  8 A- 8 B . For example, a controller (e.g., controller  380  in  FIGS.  3 - 4   ) may coordinate deflectors  1424 ,  1426 , and  1428  (e.g., RF cavities or MEMS deflectors) and determine one of deflectors  1424 ,  1426 , and  1428  to deflect a beam towards crossover area  1434 . The controller may control deflectors  1424 ,  1426 , and  1428  to release pulses or clusters in sequence (e.g., one pulse or cluster being released at a time), and the pulses or clusters may pass crossover area  1434  in sequence (e.g., one by one, with only one or zero cluster being in crossover area  1434  at any point in time). As shown in  FIG.  14   , the distances between beams  1414 ,  1416 , and  1418  after exiting deflectors  1424 ,  1426 , and  1428  are more concentrated than distances between them before entering accelerator  1420 , in which more concentrated beams may contribute to higher brightness of the beams and higher throughput of the multi-beam apparatus  1400 . Also, by deflecting beams  1414 ,  1416 , and  1418  such that pulses or clusters of beams  1414 ,  1416 , and  1418  pass through crossover area  1434  in sequence (e.g., one by one), the Coulomb effect may be significantly reduced because there is at most one pulse or cluster passing crossover area  1434 . 
     In some embodiments, deflectors  1424 ,  1426 , and  1428  may be created out of a slab or module. For example, deflectors  1424 ,  1426 , and  1428  may be created as cavities on the slab or module. Power supply may be connected to deflectors  1424 ,  1426 , and  1428  for providing a dynamic electric field in each deflector. For example, an AC voltage may be provided to each of the deflectors in beam concentrator  1422 . The dynamic electric field may change in cycles, such as sinusoidally. In some embodiments, the cycle may depend on symmetry of beam concentrator  1422 , symmetry of the deflectors, or material choices of the deflectors. Although deflectors  1424 ,  1426 , and  1428  are shown as equidistant and placed in a line in  FIG.  14   , it should be noted that any number of deflectors may be configured at any position in three-dimensional space. For example, four deflectors may be arranged in beam concentrator  1422  if there are four charged-particle sources. For another example, the deflectors may be placed at different planes (i.e., they are at different longitudinal distances along the direction of the beams). 
     In some embodiments, beam concentrator  1422  may further include cluster generators (not shown in  FIG.  14   ) for clustering beams  1414 ,  1416 , and  1418 . For example, the cluster generators may be separate components implemented similar to cluster cavities  1002 ,  1004 , and  1006  in  FIG.  10   . The cluster generators may be arranged upstream or downstream from deflectors  1424 ,  1426 , and  1428 . That is, the beams may be clustered before being deflected or be deflected before being clustered. In some embodiments, a cluster generator and a deflector (e.g., one of deflectors  1424 ,  1426 , and  1428 ) may be implemented as a single component, such as a single electric cavity provided with two different dynamic electric fields (e.g., a longitudinal dynamic electric field for clustering and a transversal dynamic electric field for deflecting), in which the single component may generate clusters and deflect them into different paths to form different beams of clusters, effectively making multiple beams out of a single component. In some embodiments, the number of beams and their paths may depend on parameters (e.g., frequencies) of at least one of the deflectors (e.g., deflectors  1424 ,  1426 , and  1428 ) or the cluster generators. 
     In some embodiments, accelerator  1420  may be optional for multi-beam apparatus  1400 . In some embodiments, the relative positions of charged-particle accelerator  1420  and deflector and concentrator  1422  may be switched. That is, the clusters may be formed before being accelerated. 
     In some embodiments, an aperture array  1430  may be arranged downstream from beam concentrator  1422  for further filtering beams  1414 ,  1416 , and  1418  (e.g., by removing stray charged particles in a manner similar to aperture plate  1214  in  FIG.  12   ), by which imaging aberrations may be reduced. In some embodiments, aperture array  1430  may be a MEMS aperture array. In some embodiments, aperture array  1430  may be optional (i.e., may be omitted) for multi-beam apparatus  1400 . 
     By using multiple charged-particle sources matched with multiple deflectors, multi-beam apparatus  1400  may provide a higher number of beams, higher throughput, scalable brightness (e.g., by adding or reducing charged-particle sources as needed), limited Coulomb effect, lower risks of electric breakdowns, and less complexity and costs for building the system. 
       FIG.  15    is a flowchart showing an exemplary method  1500  of providing multiple charged-particle beams in a multi-beam apparatus, consistent with embodiments of the present disclosure. Method  1500  may be performed by a controller that may be coupled with a charged particle beam apparatus (e.g., multi-beam apparatus  1400 ). For example, the controller may be controller  380  in  FIG.  34   . The controller may be programmed to implement method  1500 . 
     At step  1502 , at least one deflector system receives a first beam of pulsed charged particles from a first charged-particle source and a second beam of pulsed charged particles from a second charged-particle source. For example, the at least one deflector system may be one or more of deflectors  1424 ,  1426 , and  1428  in  FIG.  14   . For example, the first and second charged-particle sources may be charged-particle sources  1402  and  1404 , respectively. The first and second beams may be, for example, beams  1414  and  1416 , respectively. In some embodiments, the at least one deflector system may include a radio-frequency cavity deflector. For example, the at least one deflector system may include deflection cavity  330  in  FIGS.  3 - 4   . 
     In some embodiments, the first charged-particle source and the second charged-particle source may be charged-particle pulse generators. For example, a pulse generator may include a charged-particle emitter and a pulse laser generator. When the first charged-particle source and the second charged-particle source are the charged-particle pulse generators, the first beam and the second beam may be pulsed beams of charged particles. In some embodiments, the first beam and the second beam may be clustered, such as in a way as shown and described in  FIGS.  8 B- 13   . In those embodiments, the first beam and the second beam may be beams of clustered charged particles before entering the at least one deflector system. 
     In some embodiments, the at least one deflector system may include multiple deflectors, and each deflector may receive one beam of charged-particles. For example, the at least one deflector system may include a first deflector (e.g., deflector  1424 ) and a second deflector (e.g., deflector  1426 ) and may receive the first beam (e.g., beam  1414 ) in the first deflector and the second beam (e.g., beam  1416 ) in the second deflector. In some embodiments, if the deflector responds sufficiently fast, it may deflect beams from different charged-particle sources. In some embodiments, the at least one deflector system may include one or more deflectors, and each deflector may receive one or more beams of charged particles. For example, the at least one deflector system may include a single deflector, and the single deflector may receive the first beam and the second beam. 
     The at least one deflector system may be arranged in the three-dimensional space in any suitable manner for deflecting the beams. For example, when the at least one deflector system includes the first deflector and the second deflector, the first deflector and the second deflector may be arranged to be on a same plane. The plane may be perpendicular to paths of the beams before entering them, such as shown and described in  FIG.  14   . For another example, when the at least one deflector system includes the first deflector and the second deflector, the first deflector and the second deflector may be arranged to be on different planes. 
     Still referring to  FIG.  15   , at step  1504 , the at least one deflector system deflects the first beam and the second beam in a predetermined time-space order. The first beam and the second beam pass a downstream position in an alternate matter (e.g., in sequence). In some embodiments, the downstream position may be crossover area  1434  in  FIG.  14   . For example, the at least one deflector system may deflect the first beam and the second beam in a way as shown and described in  FIG.  14   . 
     In some embodiments, the predetermined time-space order may include that a distance between the first beam and the second beam decreases after the first beam and the second beam are released. For example, as shown in  FIG.  14   , the first beam and the second beam may be beams  1414  and  1416 , respectively, and the distance between beams  1414  and  1416  before they enter deflectors  1424  and  1426 , respectively, is greater than the distance between them after they exit deflectors  1424  and  1426 , respectively. In some embodiments, the predetermined time-space order may include that the first beam and the second beam are parallel after the first beam and the second beam are released. For example, as shown in  FIG.  14   , the first beam and the second beam may be beams  1414  and  1416 , respectively, they are parallel after exiting deflectors  1424  and  1426 , respectively. 
     In some embodiments, the at least one deflector system may release pulses or clusters from the first beam and the second beam in sequence. For example, as shown in  FIG.  14   , deflectors  1424  and  1426  may release pulses or clusters of beams  1414  and  1416  in sequence, in which deflector  1424  releases a pulse or cluster of beam  1414  first (as indicated by a dense-dotted circle downstream from deflector  1424 ), and deflector  1426  releases a pulse or cluster of beam  1416  second (as indicated by a sparse-dotted circle downstream from deflector  1426 ). In some embodiments, the at least one deflector system may release at most one pulse or cluster at a given time. That is, the pulses or clusters of different beams may be released one by one. In some embodiments, the at least one deflector system may direct the first beam in a first direction and the second beam in a second direction. For example, as shown in  FIG.  14   , deflector  1424  may deflect beam  1414  in a first direction (indicated by a large angle between the directions of beam  1414  before entering and after exiting deflector  1424 ), and deflector  1426  may deflect beam  1416  in a second direction (indicated by a substantially zero angle between the directions of beam  1416  before entering and after exiting deflector  1426 ). 
     In some embodiments, after the first beam and the second beam are released, an aperture plate (e.g., an aperture plate providing aperture array  1430 ) downstream from the at least one deflector system may receive them. The aperture plate may include a first aperture and a second aperture. The first aperture may receive the first beam, and the second aperture may receive the second beam. 
     In some embodiments, the at least one deflector system may not only deflect the received beams, but also cluster them into beams of clusters. For example, the at least one deflector system may be provided with clustering electric fields (e.g., electric field  904 ,  1106 ,  1108 , or  1204  in  FIGS.  9 - 12   ) in addition to deflecting electric fields (e.g., electromagnetic field  1212  in  FIG.  12   ), and the received beams may be clustered while being deflected. For example, the at least one deflector system may form a first cluster of charged particles using the first beam and a second cluster of charged particles using the second beam. The at least one deflector system may then release the first cluster and the second cluster in the predetermined time-space order. In some embodiments, the at least one deflector system may release the first cluster and the second cluster in sequence, and the first cluster and the second cluster may pass the downstream position in sequence. 
     Reference is now made to  FIGS.  16 A- 16 C , which illustrate exemplary subsystems  1600 A- 1600 C of a multi-beam apparatus for charged-particle detection, consistent with embodiments of the present disclosure. The multi-beam apparatus may be, for example, the multiple electron beam system in  FIG.  3    or  FIG.  4   . 
       FIG.  16 A  is a schematic diagram of an exemplary subsystem  1600 A of a multi-beam apparatus. In some embodiments, the charged particles may be electrons. Subsystem  1600 A may include at least primary electron source  1602 , a first deflector  1606 , and a first projection lens system  1614 . Primary electron source  1602  may emit primary electrons. In some embodiments, primary electron source  1602  may be primary electron source  301  in  FIGS.  3 - 4   . In some embodiments, primary electron source  1602  may include multiple charged-particle sources, such as charged-particle sources  1402 ,  1404 , and  1406 . The primary electrons may be bunched (e.g., by bunching cavity  320  in  FIGS.  3 - 4   ) into multiple primary electron pulses, including pulses  1604 . Pulses  1604  may be accelerated and projected along directions indicated by arrows in  FIG.  16 A . In some embodiments, pulses  1604  may be clustered, such as beams  812 ,  814 , and  816  as showed in  FIG.  8 B . For ease of explanation without causing ambiguity, pulses  1604  described hereinafter include clustered electrons or un-clustered electrons unless explicitly specified. [01%] In some embodiments, first deflector  1606  may be a MEMS deflector. In some embodiments, first deflector  1606  may be a radio-frequency cavity deflector. In some embodiments, first deflector  1606  may be deflection cavity  330  in  FIGS.  3 - 4    or one or more of deflectors  1424 ,  1426 , and  1428  in  FIG.  14   . First deflector  1606  may deflect pulses  1604  to form a first number (e.g., an integer N1) of deflected electron pulse beams, including incident beams  1608 ,  1610 , and  1612 , pulses of which are indicated by circles of different shades in  FIG.  16 A . For example, the leading pulse of pulses  1604  is deflected to form incident beam  1612 , the trailing pulse of pulses  1604  is deflected to form incident beam  1608 , and the intermediate pulse of pulses  1604  may be deflected (or allowed to pass through) to form incident beam  1610 . It can be seen from  FIG.  16 A  that pulses  1604  are deflected by first deflector  1606  at different timestamps. Such timestamps (e.g., timestamps of entering first deflector  1606  or timestamps of being directed by first deflector  1606 ) may be recorded by a controller (e.g., controller  380  in  FIGS.  3 - 4   ) as first timing information. The first timing information may further include any information related with formation of deflected electron beams including incident beams  1608 ,  1610 , and  1612 . 
     In some embodiments, first projection lens system  1614  may include at least one of condenser lens  340 , primary projection system  360 , or beam raster system  370  in  FIGS.  3 - 4   . First projection lens system  1614  may project and focus the N1 deflected electron beams onto a sample  1622 , including incident beams  1608 ,  1610 , and  1612 . The cross-sections of the N1 deflected electron beams on the surface of sample  1622  form the N1 probing spots, including probing spots  1616 ,  1618 , and  1620 , corresponding to incident beams  1608 ,  1610 , and  1612 , respectively. 
       FIG.  16 B  is a schematic diagram of an exemplary subsystem  1600 B of the multi-beam apparatus. Subsystem  1600 B may include at least second deflector  1630 , second projection lens system  1638 , and detector  1644 . As shown in  FIG.  16 A , incident beams  1608 ,  1610 , and  1612  may be incident onto sample  1622  at probing spots  1616 ,  1618 , and  1620  and interact in corresponding interaction volumes (not shown) under the surface of sample  1622 . Reflected or emitted electrons (“exiting electrons”), including secondary electrons (“SEs”) and backscattered electrons (“BSEs”), may exit from the interaction volumes along directions indicated by arrows in  FIG.  16 B  and form initial exiting beams  1624 ,  1626 , and  1628 . Initial exiting beams  1624 ,  1626 , and  1628  exit from probing spots  1616 ,  1618 , and  1620 , respectively. 
     As shown in  FIG.  16 B , the pulses (indicated as ellipses of different shades) of initial exiting beams  1624 ,  1626 , and  1628  have larger cross-section than pulses  1604 , indicating that the exiting electrons may exit from anywhere in the interaction volumes that are larger than the probing spots. This may lead to the cross-talk problem. 
     In some embodiments, second deflector  1630  may be a MEMS deflector. In some embodiments, second deflector  1630  may be a radio-frequency cavity deflector. In some embodiments, second deflector  1630  may be a deflection cavity similar to deflection cavity  330  in  FIGS.  3 - 4    or one or more of deflectors  1424 ,  1426 , and  1428  in  FIG.  14   . Second deflector  1630  may deflect the N1 initial exiting beams (including initial exiting beams  1624 ,  1626 , and  1628 ) to form a second number (e.g., an integer N2) of deflected exiting beams, including deflected exiting beams  1632 ,  1634 , and  1636 , indicated as ellipses of different shades. Initial exiting beams  1624 ,  1626 , and  1628  may be deflected to form deflected exiting beams  1632 ,  1634 , and  1636 , respectively. For example, the pulses in initial exiting beam  1624  may be deflected to form deflected exiting beam  1632 , the pulses in initial exiting beam  1626  may be deflected to form deflected exiting beam  1634 , and the pulses in initial exiting beam  1628  may be deflected to form deflected exiting beam  1636 . In some embodiments, the N2 deflected exiting beams may be smaller than or equal to the N1 deflected exiting beams. In other words, pulses of different initial exiting beams may be deflected into the same deflected exiting beam. For example, the pulses in initial exiting beam  1628  and at least one pulse of another initial exiting beam (not shown) may be deflected to form deflected exiting beam  1636 . 
     It can be seen from  FIGS.  16 A- 16 B  that the pulses of initial exiting beams  1624 ,  1626 , and  1628  leave the surface of sample  1622  in a sequence the same as pulses  1604 . For example, the leading pulse of pulses  1604  firstly reaches sample  1622  at probing spot  1620 , from which a pulse of initial exiting beam  1628  firstly leaves sample  1622 . The intermediate pulse of pulses  1604  secondly reaches sample  1622  at probing spot  1618 , from which a pulse of initial exiting beam  1626  secondly leaves sample  1622 . The trailing pulse of pulses  1604  thirdly reaches sample  1622  at probing spot  1616 , from which a pulse of initial exiting beam  1624  thirdly leaves sample  1622 . That is, the pulses of initial exiting beams  1624 ,  1626 , and  1628  enter second deflector  1630  at different timestamps, in the same order as pulses of the respectively corresponding incident beams that hit sample  1622 . Such timestamps (e.g., timestamps of entering second deflector  1630  or timestamps of being directed by second deflector  1630 ) may be recorded by the controller (e.g., controller  380  in  FIGS.  3 - 4   ) as second timing information. The second timing information may further include any information related with arrival of the initial exiting beams (including initial exiting beams  1624 ,  1626 , and  1628 ) at second deflector  1630 . 
     In some embodiments, second projection lens system  1638  may include at least one of a condenser lens  1640  or a secondary projection system  1642 . In some embodiments, condenser lens  1640  may be similar to condenser lens  340 . In some embodiments, secondary projection system  1642  may be similar to first projection lens system  1614 . In some embodiments, condenser lens  1640  may be omitted. In some embodiments, second projection lens system  1638  may include components beyond condenser lens  1640  and secondary projection system  1642 . In some embodiments, second projection lens system  1638  may share one or more components with first projection lens system  1614 . In some embodiments, second projection lens system  1638  may be first projection lens system  1614  itself. 
     Second projection lens system  1638  may project and focus the N2 deflected exiting beams (including deflected exiting beams  1632 ,  1634 , and  1636 ) onto a surface of detector  1644 . Detector  1644  may include a third number (e.g., an integer N3) of detection elements corresponding to the N2 exiting beams, including detection elements  1646 ,  1648 , and  1650 . Detection elements may be units or sub-detectors of the same detector, or individual detectors of a detector array. In some embodiments, detector  1644  may be similar to electron detection device  140  in  FIG.  2    or detector  350  in  FIGS.  3 - 4   . In some embodiments, detector  1644  may be a detector array. In some embodiments, detection elements  1646 ,  1648 , and  1650  may be detection elements  140 _ 2 ,  140 _ 1 , and  140 _ 3 , respectively. 
     Each detection element may detect a deflected exiting beam. For example, detection elements  1646 ,  1648 , and  1650  may detect deflected exiting beams  1632 ,  1634 , and  1636 , respectively. Because the pulses of the deflected exiting beams do not arrive at detector  1644  simultaneously, in some embodiments, the number (e.g., N3) of the detection elements may be smaller than or equal to the number (e.g., N2) of the deflected exiting beams. That is, one detection element may detect pulses of different deflected exiting beams. For example, detection element  1650  may detect pulses of deflected exiting beam  1636  and pulses of another deflected exiting beam (not shown). By reducing the number of detection elements, the complexity and cost of building the multi-beam apparatus may be further lowered. In an embodiment, N3 may be as small as 1, as shown in  FIG.  16 C . That is, all deflected exiting beams may be detected by a single detection element. 
     It can be seen from  FIGS.  16 A and  16 B  that the pulses of deflected exiting beams  1632 ,  1634 , and  1636  arrive at the surface of detector  1644  in a sequence the same as the pulses of incident beams reaching sample  1622  and the pulses of initial exiting beams reaching second deflector  1630 . For example, the leading pulse of pulses  1604  firstly reaches sample  1622  at probing spot  1620 , from which a pulse of initial exiting beam  1628  firstly leaves sample  1622 , and a pulse of deflected exiting beam  1636  firstly reaches detection element  1650 . The intermediate pulse of pulses  1604  secondly reaches sample  1622  at probing spot  1618 , from which a pulse of initial exiting beam  1626  secondly leaves sample  1622 , and a pulse of deflected exiting beam  1634  secondly reaches detection element  1648 . The trailing pulse of pulses  1604  thirdly reaches sample  1622  at probing spot  1616 , from which a pulse of initial exiting beam  1624  thirdly leaves sample  1622 , and a pulse of deflected beam  1632  thirdly reaches detection element  1646 . That is, the pulses of deflected exiting beams  1632 ,  1634 , and  1636  enter detector  1644  at different timestamps. Such timestamps (e.g., timestamps of entering detector  1644  or timestamps of being detected by any detection element of detector  1644 ) may be recorded by the controller (e.g., controller  380  in  FIGS.  3 - 4   ) as third timing information. The third timing information may further include any information related with arrival of the deflected exiting beams (including deflected exiting beams  1632 ,  1634 , and  1636 ) at detector  1644 . 
     Because the pulses of the deflected exiting beams arrive at detector  1644  at different timestamps, although the cross-section of them overlaps, they do not interfere with each other, and thus the issues related to cross-talk may be greatly alleviated. For example, when a pulse reaches detector  1644 , there may be more than one responding detection elements that generate signals, similar to a cross-talk situation. However, based on the priori knowledge that the pulses reach detector  1644  one at a time no matter which exiting beam they are from, the controller may determine deposited charge or energy of the responding detection elements, and further identify the detection element (“arrival detection element”) with the most deposited charge or energy as the actual arrival position where the pulse reaches. After identifying the arrival detection element that the pulse reaches, the controller may further determine a travel distance of the pulse between second deflector  1630  and the arrival detection element by, for example, inquiring a database storing predetermined distances between second deflector  1630  and the detection elements of detector  1644 . 
     Based on the first timing information (e.g., the timestamps of pulses  1604  arriving at first deflector system  1606 ), the second timing information (e.g., the timestamps of the pulses of initial exiting beams  1624 ,  1626 , and  1628  arriving at second deflector  1630 ), and the third timing information (e.g., the timestamps of deflected exiting beams  1632 ,  1634 , and  1636  arriving at detector  1644 ), the controller may determine which pulse comes from which deflected exiting beam, which initial exiting beam, and which incident beam. 
     For example, the multi-beam apparatus (including subsystems  1600 A and  1600 B) may operate in a vacuum, in which the travel speed of electrons may be substantially constant. Also, the relative distances between different components of the multi-beam apparatus may be predetermined or measured, such as the distance between first deflector system  1606  and the surface of sample  1622 , the distance between the surface of sample  1622  and second deflector  1630 , and the distances between second deflector  1630  and surfaces of the detection elements of detector  1644 . Because of the substantial constant travel speed of the electrons and the known distances, for a pulse of electrons, its first timing information, second timing information, and third timing information may be related. For example, for a pulse from incident beam  1608 , initial exiting beam  1624 , and deflected exiting beam  1632 , the difference between its arrival time at second deflector  1630  and its arrival time at first deflector system  1606  may be a quotient of a sum of the distance between first deflector system  1606  and the surface of sample  1622  (e.g., probing spot  1616 ) and the distance between the surface of sample  1622  (e.g., probing spot  1616 ) and second deflector  1630  divided by the travel speed of electrons. For another example, for the same pulse in the previous example, the difference between its arrival time at detector  1644  and its arrival time at second deflector  1630  may be a quotient of a sum of the distance between the surface of sample  1622  (e.g., probing spot  1616 ) and second deflector  1630  and the distance between second deflector  1630  and the surface of detector  1644  divided by the travel speed of electrons. By finding arrival times related by the above relationships among the first, second, and third timing information, the controller may differentiate different initial exiting beams, and thus may be able to generate SEM images respectively associated with the differentiated initial exiting beams. It should be noted that the above-mentioned relationships between the travel speed, the known distances between components of the multi-beam apparatus, and the first, second, and third timing information are examples only, and this disclosure does not limit that aspect. 
     In some embodiments, the number (e.g., N3) of the detection elements and the number (e.g., N2) of the deflected exiting beams may depend on temporal resolution of detector  1644 . It should be noted that the longitudinal time spread of the pulses (i.e., the time consumed for all electrons in a pulse to reach a surface perpendicular to a path of the pulse) may be so small (e.g., smaller than 0.5 picosecond) that the longitudinal time spread of the same pulse may be deemed as substantially zero compared with a difference between arrival times of any two consecutive pulses. 
       FIG.  16 C  is a schematic diagram of an exemplary subsystem  1600 C of the multi-beam apparatus. Subsystem  1600 C is the same as subsystem  1600 B except that subsystem  1600 C includes a detector  1652 , in which one detection element may receive pulses from more than one deflected exiting beams. In some embodiments, the number (e.g., N3) of detection elements of detector  1652  may be one. For example, if detector  1652  has only one detection element with sufficiently high temporal resolution, even though second deflector  1630  deflects all deflected exiting beams to the detection element, detector  1652  may still be able to differentiate arrival times of all the pulses, and further differentiate which pulse comes from which initial exiting beam. 
     In some embodiments, electrons having different energies may be further directed by second deflector  1630  into different detectors or detection elements. Typically, SEs have lower energy (e.g., smaller than 50 eV) than BSEs, and may result better SEM image resolution to show fine surface structures. Though BSEs have higher energies and may result worse SEM image resolution, it may reflect information of deeper structures under the surface. Therefore, it may be beneficial to differentiate SEs and BSEs for SEM image generation. In a substantial vacuum environment, the energy of electrons may be dominantly in the form of kinetic energy. As long as an electron has substantially no energy dissipation (e.g., due to collision with other particles, electric accelerations, electric decelerations, or the like), the electron may travel at a substantially constant speed. The higher the energy of the electron, the higher the travel speed the electron may have. That is, for the same pulse (including SEs and BSEs) of an initial exiting beam, the arrival times of SEs may lag the arrival times of BSEs, the difference of which may be represented as an “intra-pulse” temporal gap. The intra-pulse temporal gap may be smaller than an “inter-pulse” temporal gap between arrival times of different pulses, based on which the controller may determine whether two arrival times are associated with electrons of the same pulse or different pulses. If the controller determines that a temporal gap of arrival times is smaller enough (e.g., smaller than or equal to a predetermined threshold) to be deem as an intra-pulse gap, the controller may further determine that the first arrival electrons are BSEs and the second arrival electrons are SEs. In some embodiments, based on such determination, the controller may control the second deflector  1630  to deflect the SEs and the BSEs within the same pulse into different detection elements of detector  1644 . Based on the separately detected SEs and BSEs, SEM images for different purposes may be generated accordingly. 
     In some embodiments, a Wien filter may also be used for separating SEs and BSEs such that SEs and BSEs may reach different detection elements. For example, the Wien filter may be positioned downstream from second deflector  1630  and act as a stand-alone SE/BSE separator. That is, second deflector  1630  does not deflect SEs and BSEs in this example. For another example, the Wien filter may be positioned upstream or downstream from second deflector  1630  and cooperate with second deflector  1630  for further separating the SEs and BSEs. That is, SEs and BSEs may be deflected or separated by second deflector  1630  and the Wien filter in this example. 
       FIG.  17    is a flowchart showing an exemplary method  1700  for charged-particle detection using a multi-beam apparatus, consistent with embodiments of the present disclosure. Method  1700  may be performed by a controller that may be coupled with a charged particle beam apparatus (e.g., EBI system  1 ). For example, the controller may be controller  380  in  FIG.  34   . The controller may be programmed to implement method  1700 . 
     At step  1702 , the controller controls a deflector system to direct a first charged-particle pulse and a second charged-particle pulse to a detection element of a detector. The first charged-particle pulse may be emitted from a first probing spot. The second charged-particle pulse may be emitted from a second probing spot. In some embodiments, the charged particles may be electrons. For example, the deflector system may be second deflector  1630  in  FIG.  16 B . The first and second charged-particle pulses may be any two pulses of deflected exiting beams (e.g., deflected exiting beams  1632 ,  1634 , and  1636  in  FIG.  16 B ). For example, the first charged-particle pulse may be a pulse of deflected exiting beam  1632 , and the second charged-particle pulse may be a pulse of deflected exiting beam  1634 . The detector may be detector  1644  or detector  1652  in  FIGS.  16 B- 16 C . The detection element may be any of detection elements  1646 ,  1648 , and  1650 . The first and second probing spots may be any two of probing spots  1616 ,  1618 , and  1620  in  FIGS.  16 A- 16 C , such as, for example, probing spots  1616  and  1618 , respectively. 
     In some embodiments, the controller may control the deflector system to direct a first number (e.g., an integer N1) of charged-particle pulses (e.g., including the first and second charged-particle pulses) to a second number (e.g., an integer N2) of detection elements of the detector. For example, as shown in  FIGS.  16 B- 16 C , the controller may control second deflector  1630  to direct N1 charged particle pulses from deflected exiting beams  1632 ,  1634 , and  1636  to N2 detection elements of detector  1644 . In some embodiments, N1 may be greater than or equal to N2. In some embodiments, N2 may be greater than or equal to one. For example, as shown in  FIG.  16 C , N2 may be 1. 
     At step  1704 , the controller obtains first timestamp associated with when the first charged-particle pulse is directed by the deflector system, second timestamp associated with when the first charged-particle pulse is detected by the detection element, third timestamp associated with when the second charged-particle pulse is directed by the deflector system, and fourth timestamp associated with when the second charged-particle pulse is detected by the detection element. For example, the first and third time stamps may be included in the second timing information described in association with  FIGS.  16 A- 16 C . The second and fourth timestamps may be included in the third timing information described in association with  FIGS.  16 A- 16 C . 
     In some embodiments, any one of the first or second charged-particle pulses may include at least one of SEs or BSEs. In some embodiments, using the deflector system (e.g., second deflector  1630 ), at least one of a pulse of SEs or a pulse of BSEs may be formed based on time of the SEs and the BSEs arriving at the deflector system. For example, second deflector  1630  may deflect SEs and BSEs based on the in-pulse temporal gap as described in parts associated with  FIG.  16 B . In some embodiments, the controller may obtain at least one of a first signal generated from the pulse of SEs or a second signal generated from the pulse of BSEs. In some embodiments, the first signal and the second signal may be generated by different detection elements (e.g., detection elements  1646 ,  1648 , and  1650 ). For example, the detector may include a first detection element (e.g., detection element  1646 ) generating the first signal and a second detection element (e.g., detection element  1648 ) generating the second signal. In some embodiments, the first signal and the second signal may be generated by the same detection element (e.g., detector  1652 ). For example, the first signal and the second signal are generated at different timestamps by the detector  1652 . 
     Still referring to  FIG.  17   , at step  1706 , the controller identifies a first exiting beam based on associating the first timestamp and the second timestamp, and a second exiting beam based on associating the third timestamp and the fourth timestamp. For example, the controller may identify the first exiting beam starting from probing spot  1616 , passing through second deflector  1630 , and arriving at detection element  1646  based on at least the first and second timestamps. The controller may also identify the second exiting beam starting from probing spot  1618 , passing through second deflector  1630 , and arriving at detection element  1648  based on at least the third and fourth timestamps. 
     In some embodiments, before controlling the deflector system to direct the first and second charged-particle pulses, the controller may control an incident deflector system to form a first incident charged-particle pulse and a second incident charged-particle pulse. The controller may further obtain fifth timestamp associated with when the first incident charged-particle pulse is formed, and a sixth timestamp associated with when the second incident charged-particle pulse is formed. The controller may then identify the first exiting beam based on associating the first timestamp, the second timestamp, and the fifth timestamp, and the second exiting beam based on associating the third timestamp, the fourth timestamp, and the sixth timestamp. The first and second incident charged-particle pulses may be incident at the first and second probing spots (e.g., probing sports  1616  and  1618 ), respectively, on a surface of a sample. For example, the incident deflector system may be first deflector system  1606  in  FIG.  16 A . The first and second incident charged-particle pulses may be any two incident charged-particle pulses of incident beams, such as incident beams  1608 ,  1610 , and  1612  in  FIG.  16 A . 
     In some embodiments, the incident deflector system may be the deflector system. In some embodiments, the incident deflector system may not be the deflector system. In some embodiments, the controller may communicate with at least one of the deflector system, the incident deflector system, or the detector. In some embodiments, at least one of the deflector system (e.g., second deflector  1630 ) or the incident deflector system (e.g., first deflector system  1606 ) may include a radio-frequency cavity deflector. 
     In some embodiments, using an electron optical system, the first and second charged-particle pulses may be focused on the detection element. For example, the electron optical system may be second projection lens system  1638  in  FIGS.  16 B- 16 C . In some embodiments, the deflector system (e.g., second deflector  1630 ) may be upstream from the electron optical system. In some embodiments, the deflector system may be downstream from the electron optical system. In some embodiments, the deflector system may be in the electron optical system. For example, as shown in  FIGS.  16 B- 16 C , second projection lens system  1638  may include second deflector  1630  and secondary projection system  1642 , and second deflector  1630  is between condenser lens  1640  and secondary projection system  1642 . 
     The embodiments may further be described using the following clauses: 
     1. A multi-beam apparatus for observing a sample, comprising: 
     a deflector configured to form a plurality of deflected charged-particle beams from a primary charged-particle beam comprising a plurality of charged-particle pulses; 
     a detector configured to detect a plurality of signals generated from a plurality of probe spots formed by the plurality of deflected charged-particle beams; and 
     a controller configured to: 
     obtain a first timing information related with formation of a deflected charged-particle beam of the plurality of charged-particle beams; 
     obtain a second timing information related with detection of a signal of the plurality of signals; and 
     associate the signal with the deflected charged-particle beam based on the obtained first and second timing information. 
     2. The multi-beam apparatus of clause 1, further comprising a charged-particle source, an acceleration cavity, and a bunching cavity.
 
3. The multi-beam apparatus of clause 2, wherein the charged-particle source comprises a pulsed radio-frequency source having a source frequency in a range of 100 MHz to 10 GHz.
 
4. The multi-beam apparatus of any one of clauses 1-3, wherein the deflector comprises one or more charged-particle deflectors, each of the one or more charged-particle deflectors forming the plurality of deflected charged-particle beams based on an operating frequency.
 
5. The multi-beam apparatus of clause 4, wherein the deflector is synchronized with the charged-particle source such that the operating frequency and the source frequency are related by the equation:
 
     
       
         
           
             
               v 
               ⁢ 
               1 
             
             = 
             
               
                 1 
                 n 
               
               ⁢ 
               
                 ( 
                 
                   v 
                   ⁢ 
                   2 
                 
                 ) 
               
             
           
         
       
     
     where v1 is the operating frequency, v2 is the source frequency, and n is a positive integer.
 
6. The multi-beam apparatus of any one of clauses 1-5, further comprising:
 
     an electron optical system; and 
     a charged-particle beam scanning system configured to scan each of the plurality of deflected charged-particle beams on the sample. 
     7. The multi-beam apparatus of clause 6, wherein the electron optical system comprises one of a single-lens system or a multiple-lens system.
 
8. The multi-beam apparatus of any one of clauses 6 and 7, wherein the controller is further configured to communicate with at least one of the deflector, the charged-particle beam scanning system, and the detector.
 
9. The multi-beam apparatus of any one of clauses 1-8, wherein the plurality of probe spots formed by the plurality of deflected charged-particle beams comprise one of a one-dimensional or a two-dimensional pattern.
 
10. The multi-beam apparatus of clause 9, wherein the two-dimensional pattern comprises a Lissajous pattern, a matrix, or an array.
 
11. The multi-beam apparatus of any of preceding clauses, wherein the first time information comprises at least one of a time of deflection of a charged-particle pulse of the deflected charged-particle beam, a time of formation of a charged-particle pulse of the primary charged-particle beam, a frequency or a period of the deflected charged-particle beam, or an average number of electron pulses in the deflected charged-particle beam.
 
12. The multi-beam apparatus of clause 11, wherein the time of deflection of the charged-particle pulse of the deflected charged-particle beam comprises a timestamp when the deflector directs the charged-particle pulse of the deflected charged-particle beam into a direction to form a portion of the deflected charged-particle beam.
 
13. A method for observing a sample in a multi-beam apparatus, the method comprising:
 
     forming, using a deflector, a plurality of deflected charged-particle beams from a primary charged-particle beam comprising a plurality of charged-particle pulses; 
     detecting, using a detector, a plurality of signals generated from a plurality of probe spots formed by the plurality of deflected charged-particle beams; 
     obtaining, using a controller, a first timing information related with formation of a deflected charged-particle beam of the plurality of charged-particle beams, and a second timing information related with detection of a signal of the plurality of signals; and 
     associating, using the controller, the signal with the deflected charged-particle beam based on the obtained first and second timing information. 
     14. The method of clause 13, wherein a pulsed radio-frequency charged-particle source is configured to generate the plurality of charged-particle pulses having a source frequency in a range of 100 MHz to 10 GHz.
 
15. The method of any one of clauses 13 and 14, wherein the deflector comprises one or more charged-particle deflectors, each of the one or more charged-particle deflectors forming the plurality of deflected charged-particle beams based on an operating frequency.
 
16. The method of clause 11, wherein the deflector is synchronized with the charged-particle source such that the operating frequency and the source frequency are related by the equation:
 
     
       
         
           
             
               v 
               ⁢ 
               1 
             
             = 
             
               
                 1 
                 n 
               
               ⁢ 
               
                 ( 
                 
                   v 
                   ⁢ 
                   2 
                 
                 ) 
               
             
           
         
       
     
     where v1 is the operating frequency, v2 is the source frequency, and n is a positive integer.
 
17. The method of any one of clauses 13-16, further comprising focusing the plurality of deflected charged-particle beams on the sample using an electron optical system.
 
18. The method of any one of clauses 13-17, further comprising scanning each of the plurality of deflected charged-particle beams on the sample using a charged-particle beam scanning system.
 
19. The method of any one of clauses 18, further comprising communicating, via the controller, with at least one of the deflector, the charged-particle beam scanning system, and the detector.
 
20. The method of any one of clauses 13-19, wherein the plurality of probe spots formed by the plurality of deflected charged-particle beams comprise one of a one-dimensional or a two-dimensional pattern.
 
21. The method of clause 20, wherein the two-dimensional pattern comprises a Lissajous pattern, a matrix, or an array.
 
22. The method of any of clauses 13-21, wherein the first time information comprises at least one of time of formation of the deflected charged-particle beam, a frequency or a period of the deflected charged-particle beam, or an average number of electron pulses in the deflected charged-particle beam.
 
23. The method of clause 22, wherein the time of formation of the deflected charged-particle beam comprises a timestamp when the deflector directs a charged-particle pulse into a direction to form the deflected charged-particle beam.
 
24. A controller of a multi-beam apparatus, comprising:
 
     a memory storing a set of instructions; and 
     a processor configured to execute the set of instructions to cause the controller to: 
     obtain a first timing information related with formation of a deflected charged-particle beam of a plurality of charged-particle beams; 
     obtain a second timing information related with detection of a signal of a plurality of signals; and 
     associate the signal with the deflected charged-particle beam based on the obtained first and second timing information. 
     25. 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 to observe a sample, the method comprising: 
     forming a plurality of deflected charged-particle beams from a primary charged-particle beam comprising a plurality of charged-particle pulses; 
     detecting a plurality of signals generated from a plurality of probe spots formed by the plurality of deflected charged-particle beams; 
     obtaining a first timing information related with formation of a deflected charged-particle beam of the plurality of charged-particle beams, and a second timing information related with detection of a signal of the plurality of signals; and 
     associating the signal with the deflected charged-particle beam based on the obtained first and second timing information. 
     26. The non-transitory computer readable medium of clause 25, wherein the set of instructions that is executable by one or more processors of a multi-beam apparatus cause the multi-beam apparatus to further perform: 
     focusing the plurality of deflected charged-particle beams on the sample using an electron optical system; and 
     scanning each of the plurality of focused deflected charged-particle beams on the sample using a charged-particle beam scanning system. 
     modify the associated beam current of each of the plurality of beamlets; and 
     focus each of the plurality of beamlets on a focal plane. 
     27. A multi-beam apparatus comprising: 
     a charged-particle source configured to generate a primary charged-particle beam comprising a plurality of charged-particle pulses; 
     a bunching cavity configured to form a plurality of charged-particle beams from the primary charged-particle beam; and 
     a deflector configured to deflect the plurality of charged-particle beams to form a plurality of probe spots on a sample. 
     28. The apparatus of clause 27, further comprising a detector configured to detect a plurality of signals generated from the plurality of probe spots formed by the plurality of deflected charged-particle beams.
 
29. The apparatus of any one of clauses 27 and 28, further comprising a controller configured to:
 
     obtain a first timing information related with formation of the deflected charged-particle beam of the plurality of charged-particle beams; 
     obtain a second timing information related with detection of a signal of the plurality of signals; and 
     associate the signal with the deflected charged-particle beam based on the obtained first and second timing information. 
     30. The apparatus of clause 29, wherein the first time information comprises at least one of time of formation of the deflected charged-particle beam, a frequency or a period of the deflected charged-particle beam, or an average number of electron pulses in the deflected charged-particle beam.
 
31. The apparatus of clause 30, wherein the time of formation of the deflected charged-particle beam comprises a timestamp when the deflector directs a charged-particle pulse into a direction to form the deflected charged-particle beam.
 
32. The apparatus of any one of clauses 27-29, wherein the charged-particle source comprises a pulsed radio-frequency source having a source frequency in a range of 100 MHz to 10 GHz.
 
33. The apparatus of any one of clauses 27-32, wherein the deflector comprises one or more charged-particle deflectors, each of the one or more charged-particle deflectors deflecting the plurality of charged-particle beams based on an operating frequency.
 
34. A method for observing a sample in a multi-beam apparatus, the method comprising:
 
     generating a primary charged-particle beam comprising a plurality of charged-particle pulses from a charged-particle source; 
     forming, using a bunching cavity, a plurality of charged-particle beams from the primary charged-particle beam; and 
     deflecting, using a deflector, the plurality of charged-particle beams to form a plurality of probe spots on a sample. 
     35. The method of clause 34, further comprising detecting the plurality of signals generated from the plurality of probe spots formed by the plurality of deflected charged-particle beams.
 
36. The method of any one of clauses 34 and 35, further comprising:
 
     obtaining, using a controller, a first timing information related with formation of the deflected charged-particle beam of the plurality of charged-particle beams; 
     obtaining, using the controller, a second timing information related with detection of a signal of the plurality of signals; and 
     associating, using the controller, the signal with the deflected charged-particle beam based on the obtained first and second timing information. 
     37. The method of clause 36, wherein the first time information comprises at least one of time of formation of the deflected charged-particle beam, a frequency or a period of the deflected charged-particle beam, or an average number of electron pulses in the deflected charged-particle beam.
 
38. The method of clause 37, wherein the time of formation of the deflected charged-particle beam comprises a timestamp when the deflector directs a charged-particle pulse into a direction to form the deflected charged-particle beam.
 
39. The method of any one of clauses 34-36, wherein the charged-particle source comprises a pulsed radio-frequency source configured to generate the plurality of charged-particle pulses having a source frequency in a range of 100 MHz to 10 GHz
 
40. The method of any one of clauses 34-39, wherein the deflector comprises one or more charged-particle deflectors, each of the one or more charged-particle deflectors deflecting the plurality of charged-particle beams based on an operating frequency.
 
41. An apparatus for observing a sample, comprising:
 
     a deflector configured to deflect a plurality of pulses of charged particles to a plurality of probe spots on a sample; 
     a detector configured to detect a plurality of signals from the sample that result from the plurality of pulses interacting with the sample; and 
     a controller configured to correlate a particular detected signal to a particular probe spot on the sample based on a correlation between a time that the particular signal generated from the particular probe spot was detected and a time that a particular charged particle pulse forming the particular probe spot was deflected. 
     42. The apparatus of clause 41, further comprising a charged particle source, an acceleration cavity, and a bunching cavity.
 
43. The apparatus of clause 42, wherein the charged-particle source comprises a pulsed radio-frequency source having a source frequency in a range of 100 MHz to 10 GHz.
 
44. The apparatus of any one of clauses 41-43, wherein the deflector comprises one or more charged-particle deflectors, each of the one or more charged-particle deflectors deflecting the plurality of pulses of charged particles based on an operating frequency.
 
45. The apparatus of clause 44, the deflector is synchronized with the charged-particle source such that the operating frequency and the source frequency are related by the equation:
 
     
       
         
           
             
               v 
               ⁢ 
               1 
             
             = 
             
               
                 1 
                 n 
               
               ⁢ 
               
                 ( 
                 
                   v 
                   ⁢ 
                   2 
                 
                 ) 
               
             
           
         
       
     
     where v1 is the operating frequency, v2 is the source frequency, and n is a positive integer.
 
46. The apparatus of any one of clauses 41-45, further comprising:
 
     an electron optical system; and 
     a charged particle beam scanning system configured to scan each of the plurality of deflected pulses of charged particles on the sample. 
     47. The apparatus of clause 46, wherein the electron optical system comprises one of a single-lens system or a multiple-lens system.
 
48. The apparatus of any one of clauses 46 and 47, wherein the controller is further configured to communicate with at least one of the deflector, the charged-particle beam scanning system, and the detector.
 
49. A multi-beam apparatus for charged-particle detection, comprising:
 
     a deflector system configured to direct charged-particle pulses; 
     a detector having a detection element, the detection element configured to detect the charged-particle pulses; and 
     a controller having a circuitry configured to:
         control the deflector system to direct a first charged-particle pulse and a second charged-particle pulse to the detection element, wherein the first charged-particle pulse is emitted from a first probing spot, and the second charged-particle pulse is emitted from a second probing spot;   obtain a first timestamp associated with when the first charged-particle pulse is directed by the deflector system, a second timestamp associated with when the first charged-particle pulse is detected by the detection element, a third timestamp associated with when the second charged-particle pulse is directed by the deflector system, and a fourth timestamp associated with when the second charged-particle pulse is detected by the detection element; and   identify a first exiting beam based on the first timestamp and the second timestamp, and a second exiting beam based on the third timestamp and the fourth timestamp.
 
50. The multi-beam apparatus of clause 49, wherein the circuitry configured to control the deflector system to direct the first charged-particle pulse and the second charged-particle pulse is further configured to:
       

     control the deflector system to direct N charged-particle pulses to M detection elements of the detector, wherein the N charged-particle pulses comprise the first charged-particle pulse and the second charged-particle pulse, and wherein N and M are integers. 
     51. The multi-beam apparatus of clause 50, wherein the N is greater than or equal to M.
 
52. The multi-beam apparatus of any one of clauses 50-51, wherein M is greater than or equal to one.
 
53. The multi-beam apparatus of clause 51, wherein N is equal to one.
 
54. The multi-beam apparatus of any one of clauses 50-52, wherein the circuitry is further configured to:
 
     before controlling the deflector system to direct the first charged-particle pulse and the second charged-particle pulse, control an incident deflector system to form a first incident charged-particle pulse and a second incident charged-particle pulse, wherein the first incident charged-particle pulse is configured to be incident at the first probing spot on a surface of a sample, and the second incident charged-particle pulse is configured to be incident at the second probing spot on the surface of the sample; 
     obtain fifth timestamp associated with forming of the first incident charged-particle pulse, and a sixth timestamp associated with forming of the second incident charged-particle pulse; and 
     identify the first exiting beam based on the first timestamp, the second timestamp, and the fifth timestamp, and the second exiting beam based on the third timestamp, the fourth timestamp, and the sixth timestamp. 
     55. The multi-beam apparatus of clause 54, wherein the circuitry is further configured to communicate with at least one of the deflector system, the incident deflector system, or the detector.
 
56. The multi-beam apparatus of any one of clauses 54-55, wherein at least one of the deflector system or the deflector system comprises a radio-frequency cavity deflector.
 
57. The multi-beam apparatus of any one of clauses 49-56, wherein any one of the first charged-particle pulse and the second charged-particle pulse comprises at least one of secondary electrons or backscattered electrons.
 
58. The multi-beam apparatus of clause 57, wherein the deflector system is further configured to form at least one of a pulse of secondary electrons or a pulse of backscattered electrons based on time of the secondary electrons and the backscattered electrons arriving at the deflector system.
 
59. The multi-beam apparatus of clause 58, wherein the circuitry is further configured to obtain at least one of a first signal generated from the pulse of secondary electrons or a second signal generated from the pulse of backscattered electrons.
 
60. The multi-beam apparatus of clause 59, wherein the detector comprises a first detection element configured to generate the first signal and a second detection element configured to generate the second signal.
 
61. The multi-beam apparatus of clause 59, wherein the detection element is configured to generate the first signal and the second signal are generated at different timestamps.
 
62. The multi-beam apparatus of any one of clauses 49-63, further comprising:
 
     an electron optical system configured to focus the first charged-particle pulse and the second charged-particle pulse on the detection element. 
     63. The multi-beam apparatus of clause 62, wherein the deflector system is upstream from the electron optical system.
 
64. The multi-beam apparatus of clause 62, wherein the deflector system is downstream from the electron optical system.
 
65. The multi-beam apparatus of clause 62, wherein the deflector system is in the electron optical system.
 
66. A method for charged-particle detection in a multi-beam apparatus, the method comprising:
 
     controlling a deflector system to direct a first charged-particle pulse and a second charged-particle pulse to a detection element of a detector; 
     obtaining, using a controller, a first timestamp associated with when the first charged-particle pulse is directed by the deflector system, a second timestamp associated with when the first charged-particle pulse is detected by the detection element, a third timestamp associated with when the second charged-particle pulse is directed by the deflector system, and a fourth timestamp associated with when the second charged-particle pulse is detected by the detection element; and 
     identifying, using a controller, a first exiting beam based on the first timestamp and the second timestamp, and a second exiting beam based on the third timestamp and the fourth timestamp. 
     67. The method of clause 66, wherein the first charged-particle is emitted from a first probing spot, and the second charged-particle is emitted from a second probing spot.
 
68. The method of any of clauses 66-67, wherein controlling the deflector system to direct the first charged-particle pulse and the second charged-particle pulse comprises:
 
     controlling the deflector system to direct N charged-particle pulses to M detection elements of the detector, wherein the N charged-particle pulses comprise the first charged-particle pulse and the second charged-particle pulse, and wherein N and M are integers. 
     69. The method of clause 68, wherein N is greater than or equal to M.
 
70. The method of any one of clauses 68-69, wherein M is greater than or equal to one.
 
71. The method of clause 69, wherein N is equal to one.
 
72. The method of any one of clauses 66-70, further comprising:
 
     before controlling the deflector system to direct the first charged-particle pulse and the second charged-particle pulse, controlling an incident deflector system to form a first incident charged-particle pulse and a second incident charged-particle pulse, wherein the first incident charged-particle pulse is configured to be incident at the first probing spot on a surface of a sample, and the second incident charged-particle pulse is configured to be incident at the second probing spot on the surface of the sample; 
     obtaining fifth timestamp associated with when the first incident charged-particle pulse is formed, and a sixth timestamp associated with when the second incident charged-particle pulse is formed; and 
     identifying the first exiting beam based on the first timestamp, the second timestamp, and the fifth timestamp, and the second exiting beam based on the third timestamp, the fourth timestamp, and the sixth timestamp. 
     73. The method of clause 72, further comprising: 
     communicating, by the controller, with at least one of the deflector system, the incident deflector system, or the detector. 
     74. The method of any one of clauses 72-73, wherein at least one of the deflector system or the incident deflector system comprises a radio-frequency cavity deflector.
 
75. The method of any one of clauses 66-74, wherein any one of the first charged-particle pulse and the second charged-particle pulse comprises at least one of secondary electrons or backscattered electrons.
 
76. The method of clause 75, further comprising:
 
     forming, using the deflector system, at least one of a pulse of secondary electrons or a pulse of backscattered electrons based on time of the secondary electrons and the backscattered electrons arriving at the deflector system. 
     77. The method of clause 76, further comprising: 
     obtaining at least one of a first signal generated from the pulse of secondary electrons or a second signal generated from the pulse of backscattered electrons. 
     78. The method of clause 77, wherein the detector comprises a first detection element generating the first signal and a second detection element generating the second signal.
 
79. The method of clause 77, wherein the first signal and the second signal are generated at different timestamps by the detection element.
 
80. The method of any one of clauses 66-79, further comprising:
 
     focusing the first charged-particle pulse and the second charged-particle pulse on the detection element using an electron optical system. 
     81. The method of any one of clauses 80, wherein the deflector system is upstream from the electron optical system.
 
82. The method of any one of clauses 80, wherein the deflector system is downstream from the electron optical system.
 
83. The method of any one of clauses 80, wherein the deflector system is in the electron optical system.
 
84. 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 for charged-particle detection, the method comprising:
 
     controlling a deflector system to direct a first charged-particle pulse and a second charged-particle pulse to a detection element of a detector; 
     obtaining, using a controller, first timestamp associated with when the first charged-particle pulse is directed by the deflector system, second timestamp associated with when the first charged-particle pulse is detected by the detection element, third timestamp associated with when the second charged-particle pulse is directed by the deflector system, and fourth timestamp associated with when the second charged-particle pulse is detected by the detection element; and 
     identifying, using a controller, a first exiting beam based on associating the first timestamp and the second timestamp, and a second exiting beam based on associating the third timestamp and the fourth timestamp. 
     85. A multi-beam apparatus for reducing interaction of charged particles, the apparatus comprising: 
     a first cluster cavity configured to receive a first set of charged particles to form a first cluster of charged particles; 
     a second cluster cavity configured to receive a second set of charged particles to form a second cluster of charged particles; and 
     a controller having a circuitry configured to cause the first cluster and the second cluster to pass a downstream position in a predetermined time-space order. 
     86. The multi-beam apparatus of clause 85, wherein the first set of charged particles and the second set of charged particles comprise charged-particle pulses.
 
87. The multi-beam apparatus of clause 85, wherein the first set of charged particles and the second set of charged particles comprise a continuous charged-particle stream.
 
88. The multi-beam apparatus of any of clauses 85-87, wherein the predetermined time-space order comprises that the first cluster and the second cluster pass the downstream position in sequence.
 
89. The multi-beam apparatus of clause 88, wherein the predetermined time-space order comprises that at most one of the first cluster and the second cluster passes the downstream position at any time.
 
90. The multi-beam apparatus of any of clauses 85-89, further comprising:
 
     a plurality of cluster cavities configured to receive respective sets of charged particles by each cluster cavity to form a plurality of beams, wherein
         each beam comprises clusters of charged particles, and   the controller is configured to cause the clusters of the plurality of beams to pass the downstream position in the predetermined time-space order.
 
91. The multi-beam apparatus of clause 90, wherein the predetermined time-space order comprises that the clusters of the plurality of beams pass the downstream position in non-overlapping sequence.
 
92. The multi-beam apparatus of clause 91, wherein the predetermined time-space order comprises that at most one of the clusters of the plurality of beams passes the downstream position at any time.
 
93. The multi-beam apparatus of any one of clauses 85-92, further comprising an objective lens, wherein the downstream position is near or in the objective lens.
 
94. The multi-beam apparatus of any one of clauses 85-93, wherein the first cluster cavity is provided with a first dynamic electric field, and the second cluster cavity is provided with a second dynamic electric field.
 
95. The multi-beam apparatus of clause 94, wherein the controller is further configured to:
       

     coordinate the first dynamic electric field and the second dynamic electric field to change. 
     96. The multi-beam apparatus of any one of clauses 94-95, wherein the controller is further configured to: 
     change a direction of the first dynamic electric field parallel to a direction of the first cluster, and a direction of the second dynamic electric field parallel to a direction of the second cluster. 
     97. The multi-beam apparatus of clause 96, wherein the controller is further configured to: 
     change at least one of the direction of the first dynamic electric field or the direction of the second dynamic electric field in a first cycle. 
     98. The multi-beam apparatus of clause 96, wherein the controller is further configured to: 
     determine the first cycle based on at least one of a scan frequency of the multi-beam apparatus or a number of beams. 
     99. The multi-beam apparatus of any one of clauses 94-98, wherein the controller is further configured to: 
     cause the first dynamic electric field to decelerate a first charged particle before a second charged particle enters the first cluster cavity, wherein the at least two charged particles comprise the first charged particle and the second charged particle; and 
     when the second charged particle enters the first cluster cavity, control the first dynamic electric field for causing the second charged particle to move faster than the first charged particle. 
     100. The multi-beam apparatus of clause 99, wherein the controller configured to control the first dynamic electric field is further configured to perform one of: 
     accelerating the second charged particle; 
     neither accelerating nor decelerating the second charged particle; or 
     decelerating the second charged particle in a lesser degree than decelerating the first charged particle. 
     101. The multi-beam apparatus of any one of clauses 99-100, further comprising: 
     a third cluster cavity downstream from the first cluster cavity and being provided with a third dynamic electric field, configured to receive the first cluster, wherein 
     the controller is further configured to cause, using the third dynamic electric field, the first charged particle and the second charged particle to move in a substantially similar speed. 
     102. The multi-beam apparatus of clause 101 wherein the controller is further configured to: 
     cause the third dynamic electric field to decelerate the second charged particle to move at the substantially similar speed. 
     103. The multi-beam apparatus of any one of clauses 101-102, wherein the controller is further configured to: 
     coordinate the third dynamic electric field and the first dynamic electric field to change. 
     104. The multi-beam apparatus of any one of clauses 101-103, wherein the controller is further configured to: 
     change a direction of the third dynamic electric field parallel to a direction of the first cluster. 
     105. The multi-beam apparatus of clause 104 wherein the controller is further configured to: 
     change the direction of the third dynamic electric field in a second cycle. 
     106. The multi-beam apparatus of clause 105, wherein the controller is further configured to: 
     determine the second cycle based on at least one of the scan frequency of the multi-beam apparatus, the number of the beams, or the first cycle. 
     107. The multi-beam apparatus of any one of clauses 94-106, further comprising: 
     a filter cavity downstream from the first cluster cavity and being provided with a dynamic electromagnetic field, wherein the filter cavity is configured to receive the first cluster and a first stray charged particle, the dynamic electromagnetic field comprising at least one of a third dynamic electric field or a dynamic magnetic field; wherein 
     the controller is further configured to control the filter cavity to filter the first stray charged particle using the dynamic electromagnetic field. 
     108. The multi-beam apparatus of clause 107, wherein the controller is further configured to: 
     change a direction of the dynamic electromagnetic field perpendicular to the direction of the first cluster. 
     109. The multi-beam apparatus of any one of clauses 107-108, wherein the controller is further configured to: 
     when the first cluster is in the filter cavity, cause the dynamic electromagnetic field not to direct the first cluster; and 
     when the first cluster is not in the filter cavity, cause the dynamic electromagnetic field to direct the first stray charged particle away from the direction of the first cluster. 
     110. The multi-beam apparatus of any one of clauses 107-109, wherein the controller is further configured to: 
     coordinate the dynamic electromagnetic field and the first dynamic electric field to change. 
     111. The multi-beam apparatus of any one of clauses 108-110, wherein the controller is further configured to: 
     change the direction of the dynamic electromagnetic field in a third cycle. 
     112. The multi-beam apparatus of clause 111, wherein the controller is further configured to: 
     determine the third cycle based on at least one of the scan frequency of the multi-beam apparatus, the number of the beams, the first cycle, or the second cycle. 
     113. The multi-beam apparatus of any one of clauses 107-112, further comprising: 
     an aperture plate downstream from the filter cavity and configured to filter a second stray charged particle. 
     114. The multi-beam apparatus of any one of clauses 111-113, wherein the filter cavity and the aperture plate are upstream from or downstream from the third cluster cavity.
 
115. The multi-beam apparatus of any one of clauses 111-113, wherein the third cluster cavity is between the filter cavity and the aperture plate.
 
116. The multi-beam apparatus of any one of clauses 85-115, wherein the controller is further configured to:
 
     cause the first cluster to exit the first cluster cavity and the second cluster to exit the second cluster cavity in an alternate manner, wherein the first cluster and the second cluster pass the downstream position in the alternate manner. 
     117. A method for reducing interaction of charged particles in a charged-particle beam of a multi-beam apparatus, the method comprising: 
     receiving a first set of charged particles in a first cluster cavity to form a first cluster of charged particles; 
     receiving a second set of charged particles in a second cluster cavity to form a second cluster of charged particles; and 
     causing the first cluster and the second cluster to pass a downstream position in a predetermined time-space order. 
     118. The method of clause 117, wherein the first set of charged particles and the second set of charged particles comprise charged-particle pulses.
 
119. The method of clause 117 wherein the first set of charged particles and the second set of charged particles comprise a continuous charged-particle stream.
 
120. The method of any of clauses 117-119, wherein the predetermined time-space order comprises that the first cluster and the second cluster pass the downstream position in sequence.
 
121. The method of clause 120 wherein the predetermined time-space order comprises that at most one of the first cluster and the second cluster passes the downstream position at any time.
 
122. The method of any of clauses 117-121, further comprising:
 
     receiving respective sets of charged particles in each cluster cavity of a plurality of cluster cavities to form a plurality of beams, each beam comprising clusters of charged particles; and 
     causing the clusters of the plurality of beams to pass the downstream position in the predetermined time-space order. 
     123. The method of clause 122, wherein the predetermined time-space order comprises that the clusters of the plurality of beams pass the downstream position in non-overlapping sequence.
 
124. The method of clause 123, wherein the predetermined time-space order comprises that at most one of the clusters of the plurality of beams passes the downstream position at any time.
 
125. The method of any one of clauses 117-124, wherein the downstream position is near or in an objective lens in the multi-beam apparatus.
 
126. The method of any one of clauses 117-125, wherein the first cluster cavity is provided with a first dynamic electric field, and the second cluster cavity is provided with a second dynamic electric field.
 
127. The method of clause 126, further comprising:
 
     coordinating the first dynamic electric field and the second dynamic electric field to change. 
     128. The method of any one of clauses 126-127, further comprising: 
     changing a direction of the first dynamic electric field parallel to a direction of the first cluster, and a direction of the second dynamic electric field parallel to a direction of the second cluster. 
     129. The method of clause 128, further comprising: 
     changing at least one of the direction of the first dynamic electric field or the direction of the second dynamic electric field in a first cycle. 
     130. The method of clause 129, further comprising: 
     determining the first cycle based on at least one of a scan frequency of the multi-beam apparatus or a number of beams. 
     131. The method of any one of clauses 126-130, wherein the receiving the first set of pulses of charged particles in the first cluster cavity to form the first cluster of charged particles comprises: 
     causing the first dynamic electric field to decelerate a first charged particle before a second charged particle enters the first cluster cavity, wherein the at least two charged particles comprise the first charged particle and the second charged particle; and 
     when the second charged particle enters the first cluster cavity, controlling the first dynamic electric field for causing the second charged particle to move faster than the first charged particle. 
     132. The method of clause 131, wherein controlling the first dynamic electric field for causing the second charged particle to move faster than the first charged particle comprises one of: 
     accelerating the second charged particle; 
     neither accelerating nor decelerating the second charged particle; or 
     decelerating the second charged particle in a lesser degree than decelerating the first charged particle. 
     133. The method of any one of clauses 131-132, wherein the forming the first cluster of charged particles comprises: 
     receiving the first cluster in a third cluster cavity downstream from the first cluster cavity; and 
     causing, using a third dynamic electric field in the third cluster cavity, the first charged particle and the second charged particle to move in a substantially similar speed. 
     134. The method of clause 133, wherein the causing the first charged particle and the second charged particle to move in the substantially similar speed comprises: 
     causing the third dynamic electric field to decelerate the second charged particle to move at the substantially similar speed. 
     135. The method of any one of clauses 133-134, further comprising: 
     coordinating the third dynamic electric field and the first dynamic electric field to change. 
     136. The method of any one of clauses 133-135, further comprising: 
     changing a direction of the third dynamic electric field parallel to a direction of the first cluster. 
     137. The method of clause 136, further comprising: 
     changing the direction of the third dynamic electric field in a second cycle. 
     138. The method of clause 137, further comprising: 
     determining the second cycle based on at least one of the scan frequency of the multi-beam apparatus, the number of the beams, or the first cycle. 
     139. The method of any one of clauses 126-138, wherein the receiving the first set of pulses of charged particles in the first cluster cavity to form the first cluster of charged particles further comprises: 
     receiving the first cluster and a first stray charged particle in a filter cavity downstream from the first cluster cavity; and 
     filtering the first stray charged particle using a dynamic electromagnetic field in the filter cavity, the dynamic electromagnetic field comprising at least one of a third dynamic electric field or a dynamic magnetic field. 
     140. The method of clause 139, further comprising: 
     changing a direction of the dynamic electromagnetic field perpendicular to the direction of the first cluster. 
     141. The method of any one of clauses 139-140, wherein the filtering the first stray charged particle comprises: 
     when the first cluster is in the filter cavity, causing the dynamic electromagnetic field not to direct the first cluster; and 
     when the first cluster is not in the filter cavity, causing the dynamic electromagnetic field to direct the first stray charged particle away from the direction of the first cluster. 
     142. The method of any one of clauses 139-141, further comprising: 
     coordinating the dynamic electromagnetic field and the first dynamic electric field to change. 
     143. The method of any one of clauses 140-142, further comprising: 
     changing the direction of the dynamic electromagnetic field in a third cycle. 
     144. The method of clause 143, further comprising: 
     determining the third cycle based on at least one of the scan frequency of the multi-beam apparatus, the number of the beams, the first cycle, or the second cycle. 
     145. The method of any one of clauses 139-144, wherein the forming the first cluster of charged particles further comprises: 
     filtering a second stray charged particle using an aperture plate downstream from the filter cavity. 
     146. The method of any one of clauses 133-145, wherein the filter cavity and the aperture plate are upstream from or downstream from the third cluster cavity.
 
147. The method of any one of clauses 133-145, wherein the third cluster cavity is between the filter cavity and the aperture plate.
 
148. The method of any one of clauses 117-147, wherein the causing the first cluster and the second cluster to pass the downstream position comprises:
 
     causing the first cluster to exit the first cluster cavity and the second cluster to exit the second cluster cavity in an alternate manner, wherein the first cluster and the second cluster pass the downstream position in the alternate manner. 
     149. 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 for reducing interaction of charged particles in a charged-particle beam, the method comprising: 
     receiving a first set of charged particles in a first cluster cavity to form a first cluster of charged particles; 
     receiving a second set of charged particles in a second cluster cavity to form a second cluster of charged particles; and 
     causing the first cluster and the second cluster to pass a downstream position in a predetermined time-space order. 
     150. A multi-beam apparatus, the multi-beam apparatus comprising: 
     a first charged-particle source; 
     a second charged-particle source; and 
     at least one deflector system downstream from the first charged-particle source and the second charged-particle source, configured to:
         receive a first beam of pulsed charged particles from the first charged-particle source and a second beam of pulsed charged particles from the second charged-particle source; and   direct the first beam and the second beam in a predetermined time-space order,       

     wherein the first beam and the second beam pass a downstream position in sequence. 
     151. The multi-beam apparatus of clause 150, wherein the first charged-particle source and the second charged-particle source are charged-particle pulse generators.
 
152. The multi-beam apparatus of clause 151, wherein a pulse generator comprises a charged-particle emitter and a pulse laser generator.
 
153. The multi-beam apparatus of any one of clauses 150-152, wherein the first beam and the second beam are beams of clustered charged particles.
 
154. The multi-beam apparatus of any one of clauses 150-153, wherein the at least one deflector system comprises a radio-frequency cavity deflector.
 
155. The multi-beam apparatus of any one of clauses 150-154, wherein the at least one deflector system comprises a first deflector receiving the first beam and a second deflector receiving the second beam.
 
156. The multi-beam apparatus of any one of clauses 150-154, wherein the at least one deflector system comprises a deflector receiving the first beam and the second beam.
 
157. The multi-beam apparatus of any one of clauses 155-156, wherein the first deflector and the second deflector are arranged to be on a same plane.
 
158. The multi-beam apparatus of any one of clauses 155-157, wherein the first deflector and the second deflector are arranged to be on different planes.
 
159. The multi-beam apparatus of any one of clauses 150-158, wherein the at least one deflector system is further configured to:
 
     release the first beam and the second beam in sequence. 
     160. The multi-beam apparatus of any one of clauses 150-159, wherein the at least one deflector system is further configured to: 
     direct the first beam in a first direction and the second beam in a second direction. 
     161. The multi-beam apparatus of any one of clauses 150-160, wherein the predetermined time-space order comprises that a distance between the first beam and the second beam decreases after the at least one deflector system releases the first beam and the second beam.
 
162. The multi-beam apparatus of any one of clauses 150-161, wherein the predetermined time-space order comprises that the first beam and the second beam are parallel after the at least one deflector system releases the first beam and the second beam.
 
163. The multi-beam apparatus of any one of clauses 150-162, further comprising:
 
     an aperture plate downstream from the at least one deflector system, comprising a first aperture and a second aperture, wherein the first aperture receives the first beam, and the second aperture receives the second beam. 
     164. The multi-beam apparatus of any one of clauses 150-163, wherein the at least one deflector system is further configured to: 
     form a first cluster of charged particles using the first beam and a second cluster of charged particles using the second beam; and 
     release the first cluster and the second cluster in the predetermined time-space order. 
     165. The multi-beam apparatus of clause 164, wherein the at least one deflector system is further configured to: 
     release the first cluster and the second cluster in sequence, wherein the first cluster and the second cluster pass the downstream position in sequence. 
     166. A method for providing multiple charged-particle beams in a multi-beam apparatus, the method comprises: 
     receiving, in at least one deflector system, a first beam of pulsed charged particles from a first charged-particle source and a second beam of pulsed charged particles from a second charged-particle source; and 
     directing the first beam and the second beam in a predetermined time-space order, wherein the first beam and the second beam pass a downstream position in sequence. 
     167. The method of clause 166, wherein the first charged-particle source and the second charged-particle source are charged-particle pulse generators.
 
168. The method of clause 167, wherein a pulse generator comprises a charged-particle emitter and a pulse laser generator.
 
169. The method of any one of clauses 166-168, wherein the first beam and the second beam are beams of clustered charged particles.
 
170. The method of any one of clauses 166-169, wherein the at least one deflector system comprises a radio-frequency cavity deflector.
 
171. The method of any one of clauses 166-170, wherein the receiving the first beam and the second beam comprises:
 
     receiving the first beam in a first deflector and the second beam in a second deflector, wherein the at least one deflector system comprises the first deflector and the second deflector. 
     172. The method of any one of clauses 166-170, wherein the receiving the first beam and the second beam comprises: 
     receiving the first beam and the second beam in a deflector, wherein the at least one deflector system comprises the deflector. 
     173. The method of any one of clauses 171-172, wherein the first deflector and the second deflector are arranged to be on a same plane.
 
174. The method of any one of clauses 171-173, wherein the first deflector and the second deflector are arranged to be on different planes.
 
175. The method of any one of clauses 166-174, wherein the directing the first beam and the second beam in the predetermined time-space order comprises:
 
     releasing the first beam and the second beam in sequence. 
     176. The method of any one of clauses 166-175, wherein the directing the first beam and the second beam in the predetermined time-space order comprises: 
     directing the first beam in a first direction and the second beam in a second direction. 
     177. The method of any one of clauses 166-176, wherein the predetermined time-space order comprises that a distance between the first beam and the second beam decreases after the at least one deflector system releases the first beam and the second beam.
 
178. The method of any one of clauses 166-177, wherein the predetermined time-space order comprises that the first beam and the second beam are parallel after the first beam and the second beam are released.
 
179. The method of any one of clauses 166-178, further comprising:
 
     receiving the first beam by a first aperture and the second beam by a second aperture, wherein the first aperture and the second aperture are in an aperture plate downstream from the at least one deflector system. 
     180. The method of any one of clauses 166-179, wherein the directing the first beam and the second beam in the predetermined time-space order comprises: 
     forming a first cluster of charged particles using the first beam and a second cluster of charged particles using the second beam; and 
     releasing the first cluster and the second cluster in the predetermined time-space order. 
     181. The method of clause 180, wherein the releasing the first cluster and the second cluster in the predetermined time-space order comprises: 
     releasing the first cluster and the second cluster in sequence, wherein the first cluster and the second cluster pass the downstream position in sequence. 
     182. 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 for providing multiple charged-particle beams, the method comprising: 
     receiving, in at least one deflector system, a first beam of pulsed charged particles from a first charged-particle source and a second beam of pulsed charged particles from a second charged-particle source; and 
     directing the first beam and the second beam in a predetermined time-space order, wherein the first beam and the second beam pass a downstream position in sequence. 
     The controller may comprise switching circuits, timing control circuits, processors, data storage modules, analog and digital circuit components, input and output ports, a communication module, etc. 
     A non-transitory computer readable medium may be provided that stores instructions for a processor to carry out pulse generation, pulse detection, image inspection, image acquisition, stage positioning, beam focusing, electric field adjustment, beam bending, 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 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.