Patent Publication Number: US-2022216029-A1

Title: Multi-beam inspection apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. application 62/787,157 which was filed on Dec. 31, 2018, and which is incorporated herein in its entirety by reference. 
    
    
     FIELD 
     The embodiments provided herein generally relate to a multi-beam inspection apparatus, and more particularly, a multi-beam inspection apparatus including an improved source conversion unit. 
     BACKGROUND 
     When manufacturing semiconductor integrated circuit (IC) chips, pattern defects or uninvited particles (residuals) inevitably appear on a wafer or a mask during fabrication processes, thereby reducing the yield. For example, uninvited particles may be troublesome for patterns with smaller critical feature dimensions, which have been adopted to meet the increasingly more advanced performance requirements of IC chips. 
     Pattern inspection tools with a charged particle beam have been used to detect the defects or uninvited particles. These tools typically employ a scanning electron microscope (SEM). In a SEM, a beam of primary electrons having a relatively high energy is decelerated to land on a sample at a relatively low landing energy and is focused to form a probe spot thereon. Due to this focused probe spot of primary electrons, secondary electrons will be generated from the surface. The secondary electrons may comprise backscattered electrons, secondary electrons, or Auger electrons, resulting from the interactions of the primary electrons with the sample. By scanning the probe spot over the sample surface and collecting the secondary electrons, pattern inspection tools may obtain an image of the sample surface. 
     SUMMARY 
     The embodiments provided herein disclose a particle beam inspection apparatus, and more particularly, an inspection apparatus using a plurality of charged particle beams. 
     In some embodiments, a micro-structure deflector array in the inspection apparatus includes a plurality of multipole structures, each multipole structure comprising a plurality of pole electrodes. The micro-deflector array includes a first multipole structure of the plurality of multipole structures, which has a first radial shift from a central axis of the array, and a second multipole structure of the plurality of multipole structures, which has a second radial shift from the central axis of the array. The first radial shift is larger than the second radial shift. Furthermore, the first multipole structure comprises a greater number of pole electrodes than the second multipole structure to reduce deflection aberrations when the plurality of multipole structures deflects a plurality of charged particle beams. 
     In some embodiments, the micro-structure deflector array may include one or more layers of multipole structures. A first layer of the plurality of multipole structures comprises a first multipole structure having a first radial shift from a central axis of the array and a second multipole structure having a second radial shift from the central axis of the array. The first radial shift is larger than the second radial shift. Furthermore, the first multipole structure comprises a greater number of pole electrodes than the second multipole structure to reduce deflection aberrations of the corresponding charge particle beams. The micro-structure deflector array also includes a second layer of multipole structures of the plurality of multipole structures, which comprises a third multipole structure having a third radial shift from the central axis of the array. 
     In some embodiments, a method of manufacturing the micro-structure deflector array is provided. The micro-structure deflector array includes a plurality of multipole structures and each multipole structure comprising a plurality of pole electrodes. The method comprises forming the first multipole structure to have a first radial shift from a central axis of the array. The method further comprises forming the second multipole structure to have a second radial shift from the central axis of the array, wherein the first radial shift is larger than the second radial shift and the first multipole structure has a different number of pole electrodes from the second multipole structure. 
     Other advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       The above and other aspects of the present disclosure will become more apparent from the description of exemplary embodiments, taken in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic diagram illustrating an exemplary charged particle beam inspection system, consistent with embodiments of the present disclosure. 
         FIG. 2  is a schematic diagram illustrating an exemplary multi-beam apparatus that is part of the exemplary charged particle beam inspection system of  FIG. 1 , consistent with embodiments of the present disclosure. 
         FIG. 3A  is a schematic diagram of exemplary multi-beam apparatus illustrating an exemplary configuration of source conversion unit of the exemplary charged particle beam inspection system of  FIG. 1 , consistent with embodiments of the present disclosure. 
         FIG. 3B  is a schematic diagram of exemplary multipole structure array with a 3×3 configuration that is part of exemplary source conversion unit of  FIG. 3A . 
         FIG. 4  is an illustration of distributions of radial and tangential electrostatic fields within a multipole structure. 
         FIGS. 5A, 5B, 5C, 5D, and 5E  are schematic diagrams of exemplary multipole structures. 
         FIG. 6A  is a schematic diagram of exemplary multipole structure array, consistent with embodiments of the present disclosure. 
         FIG. 6B  schematically illustrates grouping of multipole structures of the exemplary multipole structure array of  FIG. 6A , consistent with embodiments of the present disclosure. 
         FIG. 6C  schematically illustrates subgrouping of a group shown in  FIG. 6B , consistent with embodiments of the present disclosure. 
         FIG. 7A  is a schematic diagram of exemplary multipole structure array with multiple layers, consistent with embodiments of the present disclosure. 
         FIG. 7B  is a schematic diagram of an exemplary layer of multipole structure array of  FIG. 7A , consistent with embodiments of the present disclosure. 
         FIGS. 8A and 8B  are schematic diagrams of an exemplary multipole structure array with multiple layers, consistent with embodiments of the present disclosure. 
         FIGS. 8C, 8D and 8E  are schematic diagrams of exemplary layers of multipole structure array of  FIG. 8A , consistent with embodiments of the present disclosure. 
         FIG. 9  is a flow chart illustrating an exemplary method of manufacturing an exemplary configuration of a multipole structure array, 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. 
     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, an IC chip of a smart phone, 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. Thus, it is not surprising that semiconductor IC manufacturing is a complex and time-consuming 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 to a 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%. 
     While high process yield is desirable in an IC chip manufacturing facility, maintaining a high wafer throughput, defined as the number of wafers processed per hour, is also essential. High process yield and high wafer throughput can be impacted by the presence of defects, especially if operator intervention is required for reviewing the defects. Thus, high throughput detection and identification of micro and nano-sized defects by inspection tools (such as a SEM) is essential for maintaining high yield and low cost. 
     A SEM scans the surface of a sample with a focused beam of primary electrons. The primary electrons interact with the sample and generate secondary electrons. By scanning the sample with the focused beam and capturing the secondary electrons with a detector, the SEM creates an image of the scanned area of the sample. For high throughput inspection, some of the inspection systems use multiple focused beams of primary electrons. As the multiple focused beams can scan different parts of a wafer at the same time, multi-beam inspection system can inspect a wafer at a much higher speed than a single-beam inspection system. 
     In a conventional multi-beam inspection system, however; increasing the number of focused beams means that more off-axis (not on a primary optical axis of the system) focused beams are employed. An off-axis focused beam has aberrations that increase with its radial shift from the primary optical axis, and therefore degrades the quality of images that are produced for inspection. This aberration increase is, in some cases, a consequence of the directions of some of the electron beams needing to be changed substantially to scan the surface of the wafer. When the number of electron beams are increased, some of the electron beams need to be routed away from the central axis of the scanning device. To ensure all electron beams arrives at the surface of the wafer at the right angle, these off-center electron beams are manipulated more than the other electron beams around the central axis. This higher level of manipulation may cause blurry and out-of-focus images of the sample wafer. One aspect of the present disclosure relates to a system and a method of reducing aberrations of off-axis focused beams to minimize degradation of image quality. This can be achieved by using inherently small-aberration source-conversion unit. 
     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 can include A or B, then, unless specifically stated otherwise or infeasible, the database can include A, or B, or A and B. As a second example, if it is stated that a database can include A, B, or C, then, unless specifically stated otherwise or infeasible, the database can include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C. 
     Reference is now made to  FIG. 1 , which is a schematic diagram illustrating an exemplary charged particle beam inspection system  100 , consistent with embodiments of the present disclosure. As shown in  FIG. 1 , charged particle beam inspection system  100  includes a main chamber  10 , a load lock chamber  20 , an electron beam tool  40 , and an equipment front end module (EFEM)  30 . Electron beam tool  40  is located within main chamber  10 . While the description and drawings are directed to an electron beam, it is appreciated that the embodiments are not used to limit the present disclosure to specific charged particles. 
     EFEM  30  includes a first loading port  30   a  and a second loading port  30   b . EFEM  30  may include additional loading port(s). First loading port  30   a  and second loading port  30   b  may, for example, receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other material(s)) or samples to be inspected (wafers and samples are collectively referred to as “wafers” hereafter). One or more robot arms (not shown) in EFEM  30  transport the wafers to load lock chamber  20 . 
     Load lock chamber  20  may be connected to a load lock vacuum pump system (not shown), which removes gas molecules in load lock chamber  20  to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robot arms (not shown) transport the wafer from load lock chamber  20  to main chamber  10 . Main chamber  10  is connected to a main chamber vacuum pump system (not shown), which removes gas molecules in main chamber  10  to reach a second pressure below the first pressure. After reaching the second pressure, the wafer is subject to inspection by electron beam tool  40 . In some embodiments, electron beam tool  40  may comprise a single-beam inspection tool. In other embodiments, electron beam tool  40  may comprise a multi-beam inspection tool. 
     A controller  50  is electronically connected to electron beam tool  40 . Controller  50  may be a computer configured to execute various controls of charged particle beam inspection system  100 . Controller  50  may also include a processing circuitry configured to execute various signal and image processing functions. While controller  50  is shown in  FIG. 1  as being outside of the structure that includes main chamber  10 , load lock chamber  20 , and EFEM  30 , it is appreciated that controller  50  may be part of the structure. While the present disclosure provides examples of main chamber  10  housing an electron beam inspection tool, it should be noted that aspects of the disclosure in their broadest sense are not limited to a chamber housing an electron beam inspection tool. Rather, it is appreciated that the foregoing principles may also be applied to other tools that operate under the second pressure. 
     Reference is now made to  FIG. 2 , which is a schematic diagram illustrating an exemplary electron beam tool  40  including a multi-beam inspection tool that is part of the exemplary charged particle beam inspection system  100  of  FIG. 1 , consistent with embodiments of the present disclosure. Multi-beam electron beam tool  40  (also referred to herein as apparatus  40 ) comprises an electron source  201 , a gun aperture plate  271 , a condenser lens  210 , a source conversion unit  220 , a primary projection system  230 , a motorized stage  209 , and a sample holder  207  supported by motorized stage  209  to hold a sample  208  (e.g., a wafer or a photomask) to be inspected. Multi-beam electron beam tool  40  may further comprise a secondary projection system  250  and an electron detection device  240 . Primary projection system  230  may comprise an objective lens  231 . Electron detection device  240  may comprise a plurality of detection elements  241 ,  242 , and  243 . A beam separator  233  and a deflection scanning unit  232  may be positioned inside primary projection system  230 . 
     Electron source  201 , gun aperture plate  271 , condenser lens  210 , source conversion unit  220 , beam separator  233 , deflection scanning unit  232 , and primary projection system  230  may be aligned with a primary optical axis  204  of apparatus  40 . Secondary projection system  250  and electron detection device  240  may be aligned with a secondary optical axis  251  of apparatus  40 . 
     Electron source  201  may comprise a cathode (not shown) and an extractor or anode (not shown), in which, during operation, electron source  201  is configured to emit primary electrons from the cathode and the primary electrons are extracted or accelerated by the extractor and/or the anode to form a primary electron beam  202  that form a primary beam crossover (virtual or real)  203 . Primary electron beam  202  may be visualized as being emitted from primary beam crossover  203 . 
     Source conversion unit  220  may comprise an image-forming element array (e.g., image-forming element array  322  of  FIG. 3A ), an aberration compensator array (e.g., aberration compensator array  324  of  FIG. 3A ), a beam-limit aperture array (e.g., beam-limit aperture array  321  of  FIG. 3A ), and a pre-bending micro-deflector array (e.g., pre-bending micro-deflector array  323  of  FIG. 3A ). In some embodiments, the pre-bending micro-deflector array deflects a plurality of primary beamlets  211 ,  212 ,  213  of primary electron beam  202  to normally enter the beam-limit aperture array, the image-forming element array, and an aberration compensator array. In some embodiment, condenser lens  210  is designed to focus primary electron beam  202  to become a parallel beam and be normally incident onto source conversion unit  220 . The image-forming element array may comprise a plurality of micro-deflectors or micro-lenses to influence the plurality of primary beamlets  211 ,  212 ,  213  of primary electron beam  202  and to form a plurality of parallel images (virtual or real) of primary beam crossover  203 , one for each of the primary beamlets  211 ,  212 , and  213 . 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  211 ,  212 , and  213 . The astigmatism compensator array may comprise a plurality of micro-stigmators to compensate astigmatism aberrations of the primary beamlets  211 ,  212 , and  213 . The beam-limit aperture array may be configured to limit diameters of individual primary beamlets  211 ,  212 , and  213 .  FIG. 2  shows three primary beamlets  211 ,  212 , and  213  as an example, and it is appreciated that source conversion unit  220  may be configured to form any number of primary beamlets. Controller  50  may be connected to various parts of charged particle beam inspection system  100  of  FIG. 1 , such as source conversion unit  220 , electron detection device  240 , primary projection system  230 , or motorized stage  209 . In some embodiments, as explained in further details below, controller  50  may perform various image and signal processing functions. Controller  50  may also generate various control signals to govern operations of the charged particle beam inspection system. 
     Condenser lens  210  is configured to focus primary electron beam  202 . Condenser lens  210  may further be configured to adjust electric currents of primary beamlets  211 ,  212 , and  213  downstream of source conversion unit  220  by varying the focusing power of condenser lens  210 . 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  210 . Condenser lens  210  may be a movable condenser lens that may be configured so that the position of its first principle plane is movable. The movable condenser lens may be configured to be magnetic, which may result in off-axis beamlets  212  and  213  illuminating source conversion unit  220  with rotation angles. The rotation angles change with the focusing power or the position of the first principal plane of the movable condenser lens. Condenser lens  210  may be an anti-rotation condenser lens that may be configured to keep the rotation angles unchanged while the focusing power of condenser lens  210  is changed. In some embodiments, condenser lens  210  may be a movable anti-rotation condenser lens, in which the rotation angles do not change when its focusing power and the position of its first principal plane are varied. 
     Objective lens  231  may be configured to focus beamlets  211 ,  212 , and  213  onto a sample  208  for inspection and may form, in the current embodiments, three probe spots  221 ,  222 , and  223  on the surface of sample  208 . Gun aperture plate  271 , in operation, is configured to block off peripheral electrons of primary electron beam  202  to reduce Coulomb effect. The Coulomb effect may enlarge the size of each of probe spots  221 ,  222 , and  223  of primary beamlets  211 ,  212 ,  213 , and therefore deteriorate inspection resolution. 
     Beam separator  233  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  233  may be configured to exert an electrostatic force by electrostatic dipole field on individual electrons of primary beamlets  211 ,  212 , and  213 . The electrostatic force is equal in magnitude but opposite in direction to the magnetic force exerted by magnetic dipole field of beam separator  233  on the individual electrons. Primary beamlets  211 ,  212 , and  213  may therefore pass at least substantially straight through beam separator  233  with at least substantially zero deflection angles. 
     Deflection scanning unit  232 , in operation, is configured to deflect primary beamlets  211 ,  212 , and  213  to scan probe spots  221 ,  222 , and  223  across individual scanning areas in a section of the surface of sample  208 . In response to incidence of primary beamlets  211 ,  212 , and  213  or probe spots  221 ,  222 , and  223  on sample  208 , electrons emerge from sample  208  and generate three secondary electron beams  261 ,  262 , and  263 . Each of secondary electron beams  261 ,  262 , and  263  typically comprise secondary electrons (having electron energy ≤50 eV) and backscattered electrons (having electron energy between 50 eV and the landing energy of primary beamlets  211 ,  212 , and  213 ). Beam separator  233  is configured to deflect secondary electron beams  261 ,  262 , and  263  towards secondary projection system  250 . Secondary projection system  250  subsequently focuses secondary electron beams  261 ,  262 , and  263  onto detection elements  241 ,  242 , and  243  of electron detection device  240 . Detection elements  241 ,  242 , and  243  are arranged to detect corresponding secondary electron beams  261 ,  262 , and  263  and generate corresponding signals which are sent to controller  50  or a signal processing system (not shown), e.g. to construct images of the corresponding scanned areas of sample  208 . 
     In some embodiments, detection elements  241 ,  242 , and  243  detect corresponding secondary electron beams  261 ,  262 , and  263 , respectively, and generate corresponding intensity signal outputs (not shown) to an image processing system (e.g., controller  50 ). In some embodiments, each detection element  241 ,  242 , and  243  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. 
     In some embodiments, controller  50  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 electron detection device  240  of apparatus  40  through a medium such as an electrical conductor, optical fiber cable, portable storage media, IR, Bluetooth, internet, wireless network, wireless radio, among others, or a combination thereof. In some embodiments, the image acquirer may receive a signal from electron detection device  240  and may construct an image. The image acquirer may thus acquire images of sample  208 . The image acquirer may also perform various post-processing functions, such as generating contours, superimposing indicators on an acquired image, and the like. The image acquirer may be configured to perform adjustments of brightness and contrast, etc. of acquired images. In some embodiments, the storage may be a storage medium such as a hard disk, flash drive, cloud storage, random access memory (RAM), other types of computer readable memory, and the like. The storage may be coupled with the image acquirer and may be used for saving scanned raw image data as original images, and post-processed images. 
     In some embodiments, the image acquirer may acquire one or more images of a sample based on an imaging signal received from electron detection device  240 . An imaging signal may correspond to a scanning operation for conducting charged particle imaging. An acquired image may be a single image comprising a plurality of imaging areas. The single image may be stored in the storage. The single image may be an original image that may be divided into a plurality of regions. Each of the regions may comprise one imaging area containing a feature of sample  208 . The acquired images may comprise multiple images of a single imaging area of sample  208  sampled multiple times over a time sequence. The multiple images may be stored in the storage. In some embodiments, controller  50  may be configured to perform image processing steps with the multiple images of the same location of sample  208 . 
     In some embodiments, controller  50  may include measurement circuitries (e.g., analog-to-digital converters) to obtain a distribution of the detected secondary electrons. The electron distribution data collected during a detection time window, in combination with corresponding scan path data of each of primary beamlets  211 ,  212 , and  213  incident on the wafer surface, can be used to reconstruct images of the wafer structures under inspection. The reconstructed images can be used to reveal various features of the internal or external structures of sample  208 , and thereby can be used to reveal any defects that may exist in the wafer. 
     In some embodiments, controller  50  may control motorized stage  209  to move sample  208  during inspection of sample  208 . In some embodiments, controller  50  may enable motorized stage  209  to move sample  208  in a direction continuously at a constant speed. In other embodiments, controller  50  may enable motorized stage  209  to change the speed of the movement of sample  208  overtime depending on the steps of scanning process. 
     Although  FIG. 2  shows that apparatus  40  uses three primary electron beams, it is appreciated that apparatus  40  may use two or more number of primary electron beams. The present disclosure does not limit the number of primary electron beams used in apparatus  40 . 
     Reference is now made to  FIG. 3A , which is a schematic diagram of exemplary multi-beam apparatus illustrating an exemplary configuration of source conversion unit of the exemplary charged particle beam inspection system of  FIG. 1 , consistent with embodiments of the present disclosure. In some embodiments, apparatus  300  may comprise an election source  301 , a pre-beamlet-forming aperture array  372 , a condenser lens  310  (similar to condenser lens  210  of  FIG. 2 ), a source conversion unit  320  (similar to source conversion unit  120  of  FIG. 2 ), an objective lens  331  (similar to objective lens  231  of  FIG. 2 ), and a sample  308  (similar to sample  208  of  FIG. 2 ). Election source  301 , pre-beamlet-forming aperture array  372 , condenser lens  310 , source conversion unit  320 , and objective lens  331  are aligned with a primary optical axis  304  of the apparatus. Electron source  301  generates a primary-electron beam  302  along primary optical axis  304  and with a source crossover (virtual or real)  301 S. Pre-beamlet-forming aperture array  372  cuts the peripheral electrons of primary electron beam  302  to reduce the Coulomb Effect. Primary-electron beam  302  may be trimmed into three beamlets  311 ,  312  and  313  by pre-beamlet-forming aperture array  372  of a pre-beamlet-forming mechanism. 
     In some embodiments, source conversion unit  320  may include a beamlet-limit aperture array  321  with beam-limit apertures configured to limit beamlets  311 ,  312 , and  313  of primary electron beam  302 . Source conversion unit  320  may also include an image-forming element array  322  with image-forming micro-deflectors,  322 _ 1 ,  322 _ 2 , and  322 _ 3 , which are configured to deflect beamlets  311 ,  312 , and  313  towards optical axis  304  to form virtual images of source crossover  301 S. The virtual images are projected onto sample  308  by objective lens  331  and form probe spots,  391 ,  392 , and  393  thereon. Source conversion unit  320  may further comprise an aberration compensator array  324  configured to compensate aberrations of probe spots,  391 ,  392 , and  393 . In some embodiments, aberration compensator array  324  may include a field curvature compensator array (not shown) with micro-lenses which are configured to compensate field curvature aberrations of probe spots,  391 ,  392 , and  393 , respectively. In some embodiments, aberration compensator array  324  may include an astigmatism compensator array (not shown) with micro-stigmators which are configured to compensate astigmatism aberrations of probe spots,  391 ,  392 , and  393 , respectively. 
     In some embodiments, source conversion unit  320  may further comprise a pre-bending micro-deflector array  323  with pre-bending micro-deflectors  323 _ 1 ,  323 _ 2 , and  323 _ 3  to bend beamlets  311 ,  312 , and  313  respectively to be normally incident onto beamlet-limit aperture array  321 . In some embodiments, condenser lens  310  may focus three beamlets  311 ,  313 , and  313  to become a parallel beam along primary optical axis  304  and perpendicularly incident onto source conversion unit  320 . 
     In some embodiments, image-forming element array  322 , aberration compensator array  324 , and pre-bending micro-deflector array  323  may comprise multiple layers of micro-deflectors, micro-lenses, or micro-stigmators. 
     In source conversion unit  320 , beamlets  311 ,  312  and  313  of primary electron beam  302  are respectively deflected by micro-deflectors  322 _ 1 ,  322 _ 2  and  322 _ 3  of image-forming element array  322  towards the primary optical axis  304 . It is appreciated that beamlet  311  may already be on optical axis  304  prior to reaching micro-deflector  322 _ 1 ; accordingly, beamlet  311  may not be deflected by micro-deflector  322 _ 1 . 
     Objective lens  331  focuses beamlets onto the surface of sample  308 , i.e., projecting the three virtual images onto the sample surface. The three images formed by three beamlets  311 - 313  on the sample surface form three probe spots  391 ,  392  and  393  thereon. The deflection angles of beamlets  311 - 313  are adjusted to reduce the off-axis aberrations of three probe spots  391 - 393  due to objective lens  331 , and the three deflected beamlets consequently pass through or approach the front focal point of objective lens  331 . 
     A deflection angle of a beamlet deflected by a micro-deflector (e.g., a micro-deflector in image-forming element array  322 ) corresponds with a radial shift of the beamlet (i.e., distance from optical axis  304  to the corresponding beamlet). The deflection angle increases as the radial shift increases. Beamlets having the same radial shifts have the same or substantially the same deflection angles. For example, as shown in an exemplary multipole structure array with a 3×3 array configuration in  FIG. 3B , the deflection angle of micro-deflector  322 _ 2  may be equal to the deflection angle of micro-deflector  322 _ 3 , if their radial shifts  328  and  329  are the same. Moreover, the deflection directions of beamlets are related to their corresponding radial shift directions. Furthermore, the aberrations of beamlet (e.g., field curvature aberrations and astigmatism aberrations) increases as the radial shift increases. The aberrations of beamlet having the same or substantially same radial shifts are same or substantially same, and the directions of astigmatism aberrations are related to the directions of their radial shifts. 
       FIG. 3B  shows a 3×3 image-forming micro-deflector array configuration that can deflect total nine beamlets simultaneously. As the number of beamlets increases, the size of the array increases as well. In a large image-forming micro-deflector array, therefore, some of beamlets would be located further away from the optical axis (e.g., optical axis  304  of  FIG. 3A ) of the apparatus, and the deflection angles thereof increase accordingly. Deflection aberrations generated by a micro-deflector increase with deflection angle thereof. Therefore, non-uniformity of the corresponding probe spots increases with the number of beamlets. While  FIG. 3B  shows image-forming micro-deflector array as an example, it is appreciated that a similar relationship between the deflection aberrations and the size of array may exist in other types of micro-deflector arrays. 
     Reference is now made to  FIG. 4 , which is an illustration of distributions of radial and tangential electrostatic fields of first order electric field E l  within a deflector. The radial and tangential components E r  and E θ  of electric field E at a spatial point (r, θ) inside a deflector with a center axis may be represented as equations (1) and (2), respectively: 
     
       
         
           
             
               
                 
                   
                     
                       
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                   ) 
                 
               
             
           
         
       
     
     The strength and direction angle α k  of the K th  order electric field E k  are E k =k·r k-1 ·d k , and α k =kθ. d k  is the K th  order component on the center axis or called as Kth order on-axis component. Accordingly, E l  shown in  FIG. 4  is, E l =d 1 , and α k =θ. The strength and direction of the first order electric field E l  does not change with r or θ. Therefore, the first order electric field (i.e., E l  field or d l  component) is desired to deflect an electron beam, and the other higher order on-axis components (e.g., d 2n+1 ) need to be reduced or even eliminated. The larger the deflection angle is, the stronger the E l  field or d l  component will be needed. 
     A multi-equal-pole deflector may be defined as a multipole structure deflector with a center axis and even number of pole electrodes (e.g., 2, 4, 6, 8, 10, so on). In a section normal to the center axis of the deflector, inner outlines of all pole electrodes are in a circle having a radius R and equally segmented with segment angle β. For example,  FIGS. 5A, 5B, 5C, 5D, and 5E  show deflectors with four, six, eight, ten, and twelve pole electrodes, and 90°, 60°, 45°, 36°, and 30° segment angles, β respectively. 
     Numbers P, division angles β, and potentials of pole electrodes em in a multi-equal-pole deflector may be configured to generate d1 while making d2n+1 as small as possible. The pole electrodes em are counted from the X-axis anticlockwise. For example, in  FIG. 5A , electrodes  511 ,  512 ,  513  and  514  are e 1 , e 2 , e 3  and e 4  respectively. Table 1 shows an excitation setting of relative excitation voltages that can be applied to each electrode e n , to generate E 1  parallel to the X axis. In accordance with Table 1, Table 2 shows the on-axis components of electric fields with respect to the number of pole electrodes. By rotating the setting of relative excitation voltages in Table 1 through one or more pole electrodes, as shown in Table 3, E 1  will accordingly rotate one or more times of segment angle β. In Table 3, the excitation setting in Table 1 is rotated one time of segment angle for 4-pole deflector and 6-pole deflector, two times of segment angle for 8-pole deflector and 10-pole deflector, three times of segment angle for 12-pole deflector. Combining the excitation settings in Table 1 and Table 3 and adjusting base voltage V1 for Table 1 and base voltage V2 for Table 3, as shown in Table 4, E 1  in any direction and with any strength can be generated, and on-axis components of electric fields with respect to the number of pole electrodes are the same as Table 2. Furthermore, as shown in Table 2, some of the higher order components of the electric fields may be eliminated by using the deflectors with higher numbers of pole electrodes. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 e1 
                 e2 
                 e3 
                 e4 
                 e5 
                 e6 
                 e7 
                 e8 
                 e9 
                 e10 
                 e11 
                 e12 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 4- 
                 1 
                 0 
                 −1 
                 0 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 pole 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 6- 
                 1 
                 0.5 
                 −0.5 
                 −1 
                 −0.5 
                 0.5 
                   
                   
                   
                   
                   
                   
               
               
                 pole 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 8- 
                 1 
                 0.41 
                 — 
                 −1 
                 −1 
                 — 
                 0.41 
                 1 
                   
                   
                   
                   
               
               
                 pole 
                   
                 42 
                 0.41 
                   
                   
                 0.41 
                 42 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 42 
                   
                   
                 42 
                   
                   
                   
                   
                   
                   
               
               
                 10- 
                 1 
                 0.80 
                 0.30 
                 — 
                 — 
                 −1 
                 — 
                 — 
                 0.30 
                 0.80 
                   
                   
               
               
                 pole 
                   
                 90 
                 90 
                 0.30 
                 0.80 
                   
                 0.80 
                 0.30 
                 90 
                 90 
                   
                   
               
               
                   
                   
                   
                   
                 90 
                 90 
                   
                 90 
                 90 
                   
                   
                   
                   
               
               
                 12- 
                 1 
                 0.73 
                 0.26 
                 — 
                 — 
                 −1 
                 −1 
                 — 
                 — 
                 0.26 
                 0.73 
                 1 
               
               
                 pole 
                   
                 21 
                 79 
                 0.26 
                 0.73 
                   
                   
                 0.73 
                 0.26 
                 79 
                 21 
                   
               
               
                   
                   
                   
                   
                 79 
                 21 
                   
                   
                 21 
                 79 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 d1 
                 d3 
                 d5 
                 d7 
                 d9 
                 d11 
                 d13 
                 d15 
                 d17 
               
               
                   
                 *(πR) 
                 *(3R 3 ) 
                 *(5R 4 ) 
                 *(7R 6 ) 
                 *(9R 8 ) 
                 *(11R 10 ) 
                 *(13R 12 ) 
                 *(15R 14 ) 
                 *(17R 16 ) 
               
               
                   
               
             
            
               
                 4-pole 
                 −2.8284 
                 d1 
                 −d1 
                 −d1 
                 d1 
                 d1 
                 −d1 
                 −d1 
                 d1 
               
               
                 6-pole 
                 −3 
                 0 
                 d1 
                 −d1 
                 0 
                 −d1 
                 d1 
                 0 
                 d1 
               
               
                 8-pole 
                 −3.3137 
                 0 
                 0 
                 −d1 
                 d1 
                 0 
                 0 
                 −d1 
                 d1 
               
               
                 10-pole 
                 −3.0902 
                 0 
                 0 
                 0 
                 d1 
                 −d1 
                 0 
                 0 
                 0 
               
               
                 12-pole 
                 −3.2154 
                 0 
                 0 
                 0 
                 0 
                 −d1 
                 d1 
                 0 
                 0 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 e1 
                 e2 
                 e3 
                 e4 
                 e5 
                 e6 
                 e7 
                 e8 
                 e9 
                 e10 
                 e11 
                 e12 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 4- 
                 0 
                 1 
                 0 
                 −1 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 pole 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 6- 
                 0.5 
                 1 
                 0.5 
                 −0.5 
                 −1 
                 −0.5 
                   
                   
                   
                   
                   
                   
               
               
                 pole 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 8- 
                 0.41 
                 1 
                 1 
                 0.41 
                 — 
                 −1 
                 −1 
                 — 
                   
                   
                   
                   
               
               
                 pole 
                 42 
                   
                   
                 42 
                 0.41 
                   
                   
                 0.41 
                   
                   
                   
                   
               
               
                   
                   
                   
                   
                   
                 42 
                   
                   
                 42 
                   
                   
                   
                   
               
               
                 10- 
                 0.30 
                 0.80 
                 1 
                 0.80 
                 0.30 
                 — 
                   
                 −1 
                 — 
                 — 
                   
                   
               
               
                 pole 
                 90 
                 90 
                   
                 90 
                 90 
                 0.30 
                 0.80 
                   
                 0.80 
                 0.30 
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 90 
                 90 
                   
                 90 
                 90 
                   
                   
               
               
                 12- 
                 0.26 
                 0.73 
                 1 
                 1 
                 0.73 
                 0.26 
                 — 
                 — 
                 −1 
                 −1 
                 — 
                 — 
               
               
                 pole 
                 79 
                 21 
                   
                   
                 21 
                 79 
                 0.26 
                 0.73 
                   
                   
                 0.73 
                 0.26 
               
               
                   
                   
                   
                   
                   
                   
                   
                 79 
                 21 
                   
                   
                 21 
                 79 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                 4-pole 
                 6-pole 
                 8-pole 
                 10-pole 
                 12-pole 
               
               
                   
               
             
            
               
                 e1 
                   
                   
                 0.4142 * 
                 0.3090 * 
                 (0.2679 * 
               
               
                 e2 
                 (0 * V1) + 
                 (0.5 * V 1 ) + 
                 (0.4142 * 
                 (0.8090 * 
                 (0.7321 * 
               
               
                   
                 (1 *V 2 ) 
                 (l *) 
                   
                 (0.8090 * 
                 (0.7321 * 
               
               
                 e3 
                 (−1 * V 1 ) + 
                 (−0.5 * V 1 ) + 
                 (−0.4142 * V 1 ) + 
                 (0.3090 * V 1 ) + 
                 (0.2679 * V 1 ) + 
               
               
                   
                 (0 * V 2 ) 
                 (0.5 * V 2 ) 
                 (1 * V 2 ) 
                 (1 * V 2 ) 
                 (1 * V 2 ) 
               
               
                 e4 
                 (0 * V1) + 
                 (−1 * V 1 ) + 
                 (−1 * V 1 ) + 
                 (−0.3090 * V 1 ) + 
                 (−0.2679 * V 1 ) + 
               
               
                   
                 (−1 * V 2 ) 
                 (−0.5* V 2 ) 
                 (0.4142 * V 2 ) 
                 (0.8090 * V 2 ) 
                 (1 * V 2 ) 
               
               
                 e5 
                   
                 (−0.5 * V 1 ) + 
                 (−1 * V 1 ) + 
                 (−0.8090 * V 1 ) + 
                 (−0.7321 * V 1 ) + 
               
               
                   
                   
                 (−1 * V 2 ) 
                 (−0.4142 * V 2 ) 
                 (0.3090 * V 2 ) 
                 (0.7321 * V 2 ) 
               
               
                 e6 
                   
                 (0.5 * V 1 ) + 
                 (−0.4142 * V 1 ) + 
                 (−1 * V 1 ) + 
                 (−1 * V 1 ) + 
               
               
                   
                   
                 (−0.5 * V 2 ) 
                 (−1 * V 2 ) 
                 (−0.3090 * V 2 ) 
                 (0.2679 * V 2 ) 
               
               
                 e7 
                   
                   
                 (0.4142 * V 1 ) + 
                 (−0.8090 * V 1 ) + 
                 (−1 * V 1 ) + 
               
               
                   
                   
                   
                 (−1 * V 2 ) 
                 (−0.8090 * V 2 ) 
                 (−0.2679 * V 2 ) 
               
               
                 e8 
                   
                   
                 (1 * V 1 ) +  
                 (−0.3090 * V 1 ) + 
                 (−0.7321 * V 1 ) + 
               
               
                   
                   
                   
                 (−0.4142 * V 2 ) 
                 (−1 * V 2 ) 
                 (−0.7321 * V 2 ) 
               
               
                 e9 
                   
                   
                   
                 (0.3090 * V 1 ) + 
                 (−0.2679 * V 1 ) + 
               
               
                   
                   
                   
                   
                 (−0.8090 * V 2 ) 
                 (−1 * V 2 ) 
               
               
                 e10 
                   
                   
                   
                 (0.8090 * V 1 ) + 
                 (0.2679 * V 1 ) + 
               
               
                   
                   
                   
                   
                 (−0.3090 * V 2 ) 
                 (−1 * V 2 ) 
               
               
                 e11 
                   
                   
                   
                   
                 (0.7321 * V 1 ) + 
               
               
                   
                   
                   
                   
                   
                 (−0.7321 * V 2 ) 
               
               
                 e12 
                   
                   
                   
                   
                 (1 * V 1 ) + 
               
               
                   
                   
                   
                   
                   
                 (−0.2679 * V 2 ) 
               
               
                   
               
            
           
         
       
     
     For example, as shown in Table 2, a 4-equal-pole deflector has all the higher order components d 2n+1 . In contrast, a 6-equal-pole deflector does not have some of the higher order components (e.g., d 3 , d 9 , and d 15 ). With a 12-equal-pole deflector, many higher order components disappear (e.g., d 3 , d 5 , d 7 , d 9 , d 15 , and d 17 ). In general, the higher order components become zero in a period dependent on the pole number of the deflector. For example, order k of zero component depends on pole number (P=4+2p) as shown in equation (3) below: 
         K= 1+2 i+n (4+2 p )  (3);
 
     where p, n, and i are integers, i=1, 2, . . . p, and n=0, 1, 2, . . . ∞. In some embodiments, the micro-deflectors further away from the optical axis of the apparatus (e.g., optical axis  304  of  FIG. 3A ) may be configured to have a higher number of pole electrodes than the micro-deflectors close to the optical axis to reduce more high order on-axis components. 
     For the non-zero component d k , the corresponding electric field E k (r, θ) changes with k−1 power number of the ratio of radial position r and inner radius R of the deflector as well as the first component d1. Therefore, Ek can be reduced by reducing the ratio. In some embodiments, the micro-deflectors further away from the optical axis of the apparatus (e.g., optical axis  304  of  FIG. 3A ) may be configured to have a larger inner radius than the micro-deflectors close to the axis to reduce the non-zero electric fields Ek. 
     Reference is now made to  FIGS. 6A, 6B, and 6C , which are schematic diagrams of exemplary multipole structure array  622 , consistent with embodiments of the present disclosure. Multipole structure array  622  may be the part of a source conversion unit (such as source conversion unit  320  of  FIG. 3A ). In particular, multipole structure array  622  may be an image-forming element array (such as image-forming element array  322  of  FIG. 3A ) or a pre-bending micro-deflector array (such as pre-bending micro-deflector array  323  of  FIG. 3A ). In some embodiments, multipole structure array  622  may comprise a plurality of micro-deflectors  622 _ 1 - 622 _ 49 . With this 7×7 array configuration, forty-nine charged particle beams (e.g., electron beams) may be simultaneously deflected to form probe spots on the sample surface. In some embodiments, the center axis of the micro-deflector that is located in the middle of the array (e.g., micro-deflector  622 _ 1 ) may be aligned with optical axis  604  of the inspection apparatus (such as optical axis  304  of  FIG. 3A ). While  FIGS. 6A, 6B, and 6C  show an embodiment of multipole structure array with a 7×7 configuration, it is appreciated that the array may be any size. 
     Some of forty-nine micro-deflectors  622 _ 1 - 622 _ 49  are located on the outer portion of the array structure and are further away from optical axis  604  than others, thereby having larger radial shifts. For example, the micro-deflectors at the four corners (i.e., micro-deflectors  622 _ 29 ,  622 _ 35 ,  622 _ 41 , and  622 _ 47 ) are located the farthest away from optical axis  604  and may have to generate the largest deflection angles. Moreover, when moving right along the X axis from micro-deflector  622 _ 1  to micro-deflector  622 _ 26 , the corresponding radial shifts (distance from optical axis  604 ) increase. 
     To reduce the higher order components of electric fields generated by these outer micro-deflectors, and thus to reduce the resulting deflection aberrations and non-uniformity of the corresponding probe spots, micro-deflectors with a higher number of pole electrodes may be used to deflect the corresponding beams. In addition, the inner radii R of micro-deflectors with larger radial shifts may be larger than those with smaller radial shifts. Accordingly, moving along the X axis as shown in the  FIG. 6A , micro-deflector  622 _ 1  may comprise six poles, micro-deflector  622 _ 2  may comprise eight poles, micro-deflector  622 _ 10  may comprise ten poles, and micro-deflector  622 _ 26  may comprise twelve poles. In some embodiments, the inner radius R of micro-deflector  622 _ 2  may be larger than micro-deflector  622 _ 1 , the inner radius R of micro-deflector  622 _ 10  may be larger than micro-deflector  622 _ 2 , and the inner radius R of micro-deflector  622 _ 26  may be larger than micro-deflector  622 _ 10 . 
       FIG. 6B  illustrates an exemplary grouping of micro-deflectors based on proximity of radial shift from the optical axis. For example, the micro-deflectors having radial shift differences within a &lt;50% range may be classified into one group. In some embodiments, all micro-deflectors of a group may use the same type of micro-deflectors. For example, group 1 (annotated as G1 in  FIG. 6B ) comprises the micro-deflector in the middle of the array (e.g., micro-deflector  622 _ 1 ), which may comprise a 6-pole micro-deflector. Group 2 (annotated as G2 in  FIG. 6B ) comprises eight micro-deflectors surrounding group 1 (e.g., micro-deflectors  622 _ 2 - 622 _ 9 ), which may comprise 8-pole micro-deflectors. Group 3 (annotated as G3 in  FIG. 6B ) comprises sixteen micro-deflectors surrounding group 2 (e.g., micro-deflectors  622 _ 10 - 622 _ 25 ), which may use 10-pole micro-deflectors. Group 4 (annotated as G4 in  FIG. 6B ) comprises twenty-four micro-deflectors surrounding group 4 (e.g., micro-deflectors  622 _ 26 - 622 _ 49 ), which may comprise 12-pole micro-deflectors. 
     Even though, in  FIG. 6B , different groups comprise micro-deflectors with different number of poles, it is appreciated that some differing groups may comprise micro-deflectors with the same number of poles. For example, in some embodiments, micro-deflectors in group 1 and group 2 (e.g., micro-deflectors  622 _ 1 - 622 _ 9 ) may all use 6-pole micro-deflectors, if the differences of deflection aberrations are within an acceptable range between group 1 and group 2. In such embodiment, group 3 and group 4 may use micro-deflectors with higher number of poles, such as 8 poles, 10 poles, 12 poles, or higher. 
     As described earlier with respect to  FIG. 6A , for those micro-deflectors with the same radial shifts from optical axis  604  of the apparatus, such as the micro-deflectors at the four corners (i.e., micro-deflectors  622 _ 29 ,  622 - 35 ,  622 _ 41 , and  622 _ 47 ), the deflection angles for the corresponding beams may be equal or substantially equal. Furthermore, those micro-deflectors with the same deflection angles may be configured to have the same orientation angles. Accordingly, in multipole structure array  622 , a group of multi-pole deflectors having same or substantially same radial shifts and same or substantially same orientation angles may be grouped to share a common driver (which performs various control functions, e.g., generating excitation voltages for each electrode, controlling deflection characteristics, and driving the control signals to micro-deflectors). By sharing a common driver for a plurality of micro-deflectors that are configured to deflect the corresponding beams with the same deflection angles, the number of connecting circuits in the array configuration may be reduced because a common set of voltages can be routed to all micro-deflectors in the group. Examples of the driver sharing technique can be found in U.S. Application No. 62/665,451, which is incorporated by reference in its entirety. 
       FIG. 6C  illustrates an exemplary subgrouping of a group of micro-deflectors based on the desired deflection angles. Among the micro-deflectors of group 4, the micro-deflectors having same or substantially same radial shift may be sub-grouped together and share a common driver. For example, four micro-deflectors annotated as G4_SG1 (the ones at the corners) may be grouped together as sub-group 1. Similarly, eight micro-deflectors annotated as G4_SG2 (neighbors of sub-group 1) may be grouped together as sub-group 2; eight micro-deflectors annotated as G4_SG3 (neighbors of sub-group 2) may be grouped together as sub-group 3; and four micro-deflectors annotated as G4_SG4 (the ones on the X axis or the Y axis) may be grouped together as sub-group 4. Because the micro-deflectors in each sub-group have the same or substantially the same radial shift (and thus having the same deflection angles) and have the same number of poles, those micro-deflectors in the sub-group may be connected to a common driver. 
     Reference is now made to  FIG. 7A , which is a schematic diagram of an exemplary multipole structure array  722  with multiple layers, consistent with embodiments of the present disclosure. Multipole structure array  722  may be the part of a source conversion unit (such as source conversion unit  320  of  FIG. 3A ). In particular, multipole structure array  722  may function as an image-forming element array (such as image-forming element array  322  of  FIG. 3A ) or a pre-bending micro-deflector array (such as pre-bending micro-deflector array  323  of  FIG. 3A ). 
     In some embodiments, multipole structure array  722  may comprise a plurality of layers of multipole structures, such as layer  722   a  and  722   b , and each layer may comprise a plurality of multipole structures (e.g., micro-deflectors). For example, layer  722   a  may comprise micro-deflectors  722   a _ 1 - 722   a _ 5 . Similarly, layer  722   b  may comprise micro-deflectors  722   b _ 1 - 722   b _ 5 . In some embodiments, the center of the layers may be aligned with an optical axis  704  of the apparatus. The center of the micro-deflector in the middle of each layer (e.g.,  722   a _ 1  and  722   b _ 1 ) may be aligned with optical axis  704 . 
     In some embodiments, a pair of micro-deflectors, one from each layer, may be aligned together and deflect a corresponding beam. For example, both  722   a _ 1  and  722   b _ 1  may deflect beam  711 . Similarly, both  722   a _ 2  and  722   b _ 2  may deflect beam  712 ; both  722   a _ 3  and  722   b _ 3  may deflect beam  713 ; both  722   a _ 4  and  722   b _ 4  may deflect beam  714 ; and both  722   a _ 5  and  722   b _ 5  may deflect beam  715 . In a multi-layer configuration, because a pair of micro-deflectors deflect a single beam in series, the desired deflection angle for each micro-deflector may be smaller than in a single-layer configuration. 
     In some embodiments, the pair of micro-deflectors may use the same type of micro-deflector having the same number of poles. For example, micro-deflectors  722   a _ 1  and  722   b _ 1  may both comprise 8-pole micro-deflectors. In other embodiments, the pair of micro-deflectors may use different types of micro-deflectors. For example, micro-deflector  722   a _ 4  may use a 12-pole micro-deflector, while micro-deflector  722   b _ 4  may use a 10-pole micro-deflector. 
     Reference is now made to  FIG. 7B , which is a schematic diagram of an exemplary layer of multipole structure array of  FIG. 7A , consistent with embodiments of the present disclosure. As the number of beams increases, the size of the multipole structure array increases as well. In a large array of micro-deflectors, therefore, some of beams would be located further away from the optical axis (e.g., optical axis  304  of  FIG. 3A ) of the apparatus. The deflection angles of micro-deflectors located at the outer edge of the array also increase accordingly. Because of the large deflection angles, the beams deflected by these micro-deflectors located at the outer edge may suffer with higher deflection aberrations, thereby increasing the size and non-uniformity of the corresponding probe spots. As described with respect to  FIG. 6A , in order to reduce the higher order components of electric fields generated by these micro-deflectors (thereby reducing the resulting deflection aberration and non-uniformity of the corresponding probe spots), micro-deflectors with a higher number of pole electrodes may be used to deflect the corresponding beams. The same approach may be utilized for a multi-layer micro-deflector array as well. 
       FIG. 7B  shows an example of a 3×3 micro-deflector array, consistent with embodiments of the present disclosure. The array can be a layer of a multi-layer micro-deflector array, such as layer  722   a  or  722   b  of array  722  of  FIG. 7A . Nine micro-deflectors  750 _ 1 - 750 _ 9  may be grouped based on radial shift, as described with respect to  FIGS. 6A and 6B . For example, a first group may comprise micro-deflector  750 _ 1 , which has the lowest radial shift among nine micro-deflectors. A second group may include the micro-deflectors on the X and Y axis (e.g., micro-deflectors  750 _ 2 ,  750 _ 4 ,  750 _ 6 ,  750 _ 8 ), which have higher radial shifts than the first group (e.g., micro-deflector  750 _ 1 ). A third group may include the micro-deflectors at the four corners (e.g., micro-deflectors  750 _ 3 ,  750 _ 5 ,  750 _ 7 ,  750 _ 9 ), which have the largest radial shifts from optical axis  704 . Accordingly, the first group (micro-deflector  751 _ 1 ), the second group (micro-deflectors  750 _ 2 ,  750 _ 4 ,  750 _ 6 ,  750 _ 8 ), and the third group (micro-deflectors  750 _ 3 ,  750 _ 5 ,  750 _ 7 ,  750 _ 9 ) may comprise 6-pole micro-deflector, 8-pole micro-deflectors, and 12-pole micro-deflectors, respectively, to reduce higher order components of electric fields, thereby reducing the resulting deflection aberrations and non-uniformity of the corresponding probe spots. While  FIG. 7B  shows a 3×3 array configuration, it is appreciated that the array may be any size. Furthermore, while  FIG. 7B  shows three groups each having different types of micro-deflectors, it is appreciated that the array may comprise any combination of groups and types of micro-deflectors. 
     Furthermore, the driver sharing technique described with respect to  FIG. 6C  may also apply to multi-layer micro-deflector array. For example, the first group (micro-deflector  750 _ 1 ) may be connected and driven by a first driver. Similarly, all micro-deflectors in the second group (micro-deflectors  750 _ 2 ,  750 _ 4 ,  750 _ 6 ,  750 _ 8 ) may be connected and driven by a second driver, because those micro-deflectors have the same deflection angles. Similarly, all micro-deflectors in the third group (micro-deflectors  750 _ 3 ,  750 _ 5 ,  750 _ 7 ,  750 _ 9 ) may be connected and driven by a third driver. 
     Reference is now made to  FIGS. 8A and 8B , which are schematic diagrams of an exemplary multipole structure array with multiple layers, consistent with embodiments of the present disclosure. Multipole structure array  822  may be the part of a source conversion unit (such as source conversion unit  320  of  FIG. 3A ). In particular, multipole structure array  822  may function as an image-forming element array (such as image-forming element array  322  of  FIG. 3A ) or a pre-bending micro-deflector array (such as pre-bending micro-deflector array  323  of  FIG. 3A ). 
     In some embodiments with a plurality of micro-deflectors, some of the particle beams may be deflected by micro-deflectors in one layer, while the other particle beams may be deflected by micro-deflectors in another layer. For example, beams  811 ,  814 , and  815  may be deflected by micro-deflector  822   a _ 1 ,  822   a _ 4 , and  822   a _ 5  of layer  822   a , while beams  812  and  813  may be deflected by micro-deflectors  822   b _ 2  and  822   b _ 3  of layer  822   b . By placing some of the micro-deflectors in one layer and the other micro-deflectors in another layer, circuits connecting the poles in each layer may be reduced in comparison with packing the full set of micro-deflectors into one layer. This, therefore, may improve electrical safety and also reduce complexity of design and manufacturing process of the multipole structure array. 
     In some embodiments, layers  822   a  and  822   b  may include beam path holes  822   a _ 2 ,  822   a _ 3 ,  822   b _ 1 ,  822   b _ 4 , and  822   b _ 5 , which let beams pass through without deflection. As shown in  FIG. 8B , because beam path holes are narrower than micro-deflectors (e.g., the width of beam path hole  822   a _ 3  is narrower than the width of micro-deflector  822   a _ 1 ), the overall width of array  822  may be reduced by placing micro-deflectors in alternating fashion as shown in  FIGS. 8A and 8B . 
     Reference is now made to  FIGS. 8C, 8D, and 8E , which illustrate schematic diagrams of exemplary layers that can be used within multipole structure array  822  of  FIG. 8A , consistent with embodiments of the present disclosure. As the number of beams increases, the size of the multipole structure array increases as well. In a large array of micro-deflectors, therefore, some beams are located further away from the optical axis (e.g., optical axis  304  of  FIG. 3A ) of the apparatus. The deflection angles of micro-deflectors located at the outer edge of structure array  822  also increases accordingly. Because of the large deflection angles, the beams deflected by these micro-deflectors located at the outer edge may suffer with higher deflection aberrations, thereby increasing the size and non-uniformity of the corresponding probe spots. As described with respect to  FIG. 6A , in order to reduce higher order components of electric fields generated by these micro-deflectors, thereby reducing the resulting deflection aberrations and non-uniformity of the corresponding probe spots, micro-deflectors with a higher number of pole electrodes may be used to deflect the corresponding beams. The same approach may be utilized for multi-layer micro-deflector array, like array  822  of  FIG. 8A , as well. 
       FIG. 8C  shows an exemplary pair of micro-deflector array layers that can simultaneously deflect  25  particle beams (5×5 configuration). In this embodiment, micro-deflectors (such as micro-deflector  822   a _ 1 ,  822   a _ 4 ,  822   a _ 5  of  FIG. 8B ) and beam path holes (such as beam path holes  822   a _ 2  and  822   a _ 3  of  FIG. 8B ) are arranged alternatively in each layer, in such a way that one particle beam is deflected by a micro-deflector in only one layer. For example, layer  822   a  comprises micro-deflectors  822   a _ 1 ,  822   a _ 3 ,  822   a _ 5 ,  822   a _ 7 ,  822   a _ 9 ,  822   a _ 10 ,  822   a _ 12 ,  822   a _ 14 ,  822   a _ 16 ,  822   a _ 18 ,  822   a _ 20 ,  822   a _ 22 ,  822   a _ 24 , and beam path holes  822   a _ 2 ,  822   a _ 4 ,  822   a _ 6 ,  822   a _ 8 ,  822   a _ 11 ,  822   a _ 13 ,  822   a _ 15 ,  822   a _ 17 ,  822   a _ 19 ,  822   a _ 21 ,  822   a _ 23 ,  822   a _ 25 . Similarly, layer  822   b  comprises micro-deflectors  822   b _ 2 ,  822   b _ 4 ,  822   b _ 6 ,  822   b _ 8 ,  822   b _ 11 ,  822   b _ 13 ,  822   b _ 15 ,  822   b _ 17 ,  822   b _ 19 ,  822   b _ 21 ,  822   b _ 23 ,  822   b _ 25 , and beam path holes  822   b _ 1 ,  822   b _ 3 ,  822   b _ 5 ,  822   b _ 7 ,  822   b _ 9 ,  822   b _ 10 ,  822   b _ 12 ,  822   b _ 14 ,  822   b _ 16 ,  822   b _ 18 ,  822   b _ 20 ,  822   b _ 22 ,  822   b _ 24 . Accordingly, 13 of 25 beams are deflected by micro-deflectors of layer  822   a , while the remaining 12 beams are deflected by micro-deflectors of layer  822   h.    
     Like previous embodiments, micro-deflectors with the same or similar radial shifts (e.g., radial shift differences within a &lt;50% range may be grouped together and have a certain number of pole electrodes to reduce the deflection aberrations. For example, in layer  822   a , micro-deflector  822   a _ 1  is a 6-pole micro-deflector, micro-deflectors  822   a _ 3 ,  822   a _ 5 ,  822   a _ 7 , and  822   a _ 9  are 8-pole micro-deflectors, and micro-deflectors  822   a _ 10 ,  822   a _ 12 ,  822   a _ 14 ,  822   a _ 16 ,  822   a _ 18 ,  822   a _ 20 ,  822   a _ 22 , and  822   a _ 24  are 10-pole micro-deflectors. Similarly, in layer  822   h , micro-deflectors  822   b _ 2 ,  822   b _ 4 ,  822   b _ 6 , and  822   b _ 8  are 8-pole micro-deflectors, and micro-deflectors  822   b _ 11 ,  822   b _ 13 ,  822   b _ 15 ,  822   b _ 17 ,  822   b _ 19 ,  822   b _ 21 ,  822   b _ 23 , and  822   b _ 25  are 10-pole micro-deflectors. While  FIG. 8C  shows a 5×5 array configuration, it is appreciated that the array may be any size. Also, it is appreciated that the array configuration may comprise any combination of groups and types of micro-deflectors. 
     Furthermore, the driver sharing technique described with respect to  FIGS. 6C and 7B  may also apply to this embodiment. In layer  822   b , for example, all micro-deflectors in a first group (micro-deflector  822   b _ 2 ,  822   b _ 4 ,  822   b _ 6 ,  822   b _ 8 ) may be connected and driven by a first driver, because those micro-deflectors have the same deflection angle and the same numbers of pole electrodes. Similarly, all micro-deflectors in a second group (micro-deflectors  822   b _ 11 ,  822   b _ 13 ,  822   b _ 15 ,  822   b _ 17 ,  822   b _ 19 ,  822   b _ 21 ,  822   b _ 23 , and  822   b _ 25 ) may be connected and driven by a second driver. 
       FIGS. 8D and 8E  show another embodiment of micro-deflector array layer that can simultaneously deflect forty-nine particle beams (7×7 configuration). In this embodiment, micro-deflectors with a certain number of pole electrodes are placed in one layer. For example, all micro-deflector with six poles or ten poles are placed in layer  822   a  of  FIG. 8D , while all micro-deflectors with eight poles and twelve poles are placed in layer  822   b  of  FIG. 8E . Among the total of forty-nine beams, seventeen beams are deflected by layer  822   a , and the remaining thirty-two beams are deflected by layer  822   b . In layer  822   a , a first group includes micro-deflector  822   a _ 1  in the middle, which may comprise six poles. A second group includes micro-deflectors  822   a _ 10 - 822   a _ 25 , which may comprise ten poles. Similarly, in layer  822   b , a third group includes micro-deflectors  822   b _ 2 - 822   b _ 9 , which may comprise eight poles. A fourth group includes micro-deflectors  822   b - 26 - 822   b _ 49 , which may comprise twelve poles. Furthermore, the driver sharing technique described with respect to  FIGS. 6C and 7B  may also apply to this embodiment. For example, in the layer  822   b  of  FIG. 8E , the four micro-deflectors in the third group that are positioned on one of the X- or Y-axis (e.g., micro-deflector  822   b _ 2 ,  822   b _ 4 ,  822   b _ 6 ,  822   b _ 8 ) may be connected and driven by one common driver, because those micro-deflectors have the same deflection angles, same orientation angles, and the same numbers of pole electrodes. Similarly, the other four micro-deflectors in the third group that are positioned on the corners (e.g., micro-deflector  822   b _ 3 ,  822   b _ 5 ,  822   b _ 7 ,  822   b _ 9 ) may be connected and driven by another common driver. 
     Reference is now made to  FIG. 9 , which is a flow chart illustrating an exemplary method of manufacturing an exemplary configuration of a multipole structure array, consistent with embodiments of the present disclosure. In some embodiments, the multipole structure array may be manufactured using a semiconductor fabrication process. In some embodiments, the multipole structure array may comprise a micro-deflector array, such as micro-deflector array  622  of  FIG. 6A . In some embodiments, the multipole structure array may comprise a plurality of micro-deflectors, such as micro-deflectors  622 _ 1 - 622 _ 49  of  FIG. 6A . To reduce the higher order components of electric fields generated by the micro-deflectors, thereby reducing the resulting deflection aberrations and non-uniformity of the corresponding probe spots, micro-deflectors with higher number of pole electrodes may be used to deflect the corresponding beams that are further away from an optical axis of an inspection apparatus. For example, a first multipole structure (such as micro-deflector  622 _ 29  of  FIG. 6A ) having a higher radial shift (i.e., further away from the optical axis) than a second multipole structure (such as micro-deflector  622 _ 3  of  FIG. 6A ), may comprise a micro-deflector with a higher number of pole electrodes than the second micro-deflector. 
     In step  910 , the number of pole electrodes of the first multipole structure is configured based on deflection aberration characteristic of the first multipole structure. In step  920 , the number of pole electrodes of the second multipole structure is configured based on deflection aberration characteristic of the second multipole structure. The number of pole electrodes selected for the second multipole structure in step  920  is less than the number of pole electrodes selected for the first multipole structure in step  910 . 
     In step  930 , the first multipole structure is formed at a location with a first radial shift from a central axis of the array. In step  940 , the second multipole structure is formed at a location with a second radial shift from a central axis of the array. The distance between the optical axis and the location of the first multipole structure is greater than the distance between the optical axis and the location of the second multipole structure. Accordingly, the first multipole structure has a larger radial shift than the second multipole structure. 
     It is appreciated that the first and second multipole structures can be part of separate groups of multipole structures as explained above with respect to; for example,  FIG. 6B . Moreover, it is appreciated that the first and second multipole structures can be located on separate layers as explained above with respect to, for example,  FIG. 8C . 
     The embodiments may further be described using the following clauses: 
     1. A micro-structure deflector array including a plurality of multipole structures, each multipole structure comprising a plurality of pole electrodes, the array comprising: 
     a first multipole structure of the plurality of multipole structures, the first multipole structure having a first radial shift from a central axis of the array; and 
     a second multipole structure of the plurality of multipole structures, the second multipole structure having a second radial shift from the central axis of the array, 
     wherein the first radial shift is larger than the second radial shift, and the first multipole structure comprises a greater number of pole electrodes than the second multipole structure. 
     2. The array of clause 1, wherein the first multipole structure comprises a greater number of pole electrodes than the second multipole structure to reduce deflection aberrations when the plurality of multipole structures deflects a plurality of charged particle beams.
 
3. The array of any one of clauses 1 and 2, wherein:
 
     the plurality of pole electrodes of the first multipole structure are electrically connected and driven by a first driver, and the plurality of pole electrodes of the second multipole structure are electrically connected and driven by a second driver. 
     4. The array of clause 3, wherein the first driver and the second driver are configured to enable the first multipole structure and the second multipole structure to function as image-forming elements or pre-bending micro-deflectors in a multi-beam apparatus to deflect the plurality of charged particle beams.
 
5. The array of any one of clauses 1-4, wherein the first multipole structure has an inner diameter larger than the second multipole structure.
 
6. A micro-structure deflector array including a plurality of multipole structures, each multipole structure comprising a plurality of pole electrodes, the array comprising:
 
     a first group of multipole structures of the plurality of multipole structures, the first group of multipole structures having a first set of radial shifts from a central axis of the array, wherein each multipole structure of the first group comprises a same number of corresponding pole electrodes; and 
     a second group of multipole structures of the plurality of multipole structures, the second group of multipole structures having a second set of radial shifts from the central axis of the array, wherein each multipole structure of the second group comprises a same number of corresponding pole electrodes, 
     wherein the lowest value of radial shift of the first set of radial shifts are higher than the highest value of radial shift of the second set of radial shifts, and a multipole structure of the first group comprises a greater number of pole electrodes than a multipole structure of the second group. 
     7. The array of clause 6, wherein the multipole structure of the first group comprises a greater number of pole electrodes than the multipole structure of the second group to reduce deflection aberrations when the plurality of multipole structures deflects a plurality of charged particle beams.
 
8. The array of any one of clauses 6 and 7, wherein the first group or the second group may only comprise one multipole structure.
 
9. The array of any one of clauses 6-8, wherein the first group of multipole structures of the plurality of multipole structures comprises:
 
     a first sub-group of multipole structures that are electrically connected and driven by a first driver, wherein the radial shifts and orientation angles of the first sub-group of multipole structures are equal or substantially equal. 
     10. The array of any one of clauses 6-9, wherein the second group of multipole structures of the plurality of multipole structures comprises: 
     a second sub-group of multipole structures that are electrically connected and driven by a second driver, wherein the radial shifts and orientation angles of the second sub-group of multipole structures are equal or substantially equal. 
     11. The array of any one of clauses 6-10, wherein at least one of the first and second drivers is configured to enable the corresponding multipole structures to function as image-forming elements or pre-bending micro-deflectors in a multi-beam apparatus to deflect the plurality of charged particle beams.
 
12. The array any one of clauses 6-11, wherein one multipole structure of the first group has an inner diameter larger than one multipole structure of the second group.
 
13. A micro-structure deflector array including a plurality of multipole structures configured to deflect a plurality of charged particle beams, each multipole structure comprising a plurality of pole electrodes, the array comprising:
 
     a first layer of multipole structures of the plurality of multipole structures, the first layer comprising a first multipole structure having a first radial shift from a central axis of the array and a second multipole structure having a second radial shift from the central axis of the array, wherein the first radial shift is larger than the second radial shift, and the first multipole structure comprises a greater number of pole electrodes than the second multipole structure; and 
     a second layer of multipole structures of the plurality of multipole structures, the second layer comprising a third multipole structure having a third radial shift from the central axis of the array. 
     14. The array of clause 13, wherein the first multipole structure comprises a greater number of pole electrodes than the second multipole structure to reduce deflection aberrations of the corresponding charge particle beams.
 
15. The array of any one of clauses 13 and 14, wherein the third radial shift is smaller than the first radial shift.
 
16. The array of clause 15, wherein the third radial shift is larger than the second radial shift.
 
17. The array of any one of clauses 13-16, wherein the number of pole electrodes of the third multipole structure is larger than or equal to the second multipole structure.
 
18. The array of any one of clauses 13-16, wherein the number of pole electrodes of the third multipole structure is smaller than or equal to the first multipole structure.
 
19. The array of any one of clauses 13, 14, 16, and 17, wherein the third multipole structure comprises a greater or equal number of pole electrodes than the first multipole structure.
 
20. The array of any one of clauses 13-19, wherein one of the plurality of charged particle beams is deflected by a multipole structure of the first layer, and another one of the plurality of charged particle beams is deflected by a multipole structure of the second layer.
 
21. The array of any one of clauses 13-19, wherein one of the plurality of charged particle beams is deflected by a multipole structure of the first layer and a multipole structure of the second layer in series, and the multipole structure of the first layer and the multipole structure of the second layer are aligned each other.
 
22. The array of any one of clauses 13-19, wherein:
 
     a first beam of the plurality of charged particle beams is deflected by a multipole structure of the first layer, 
     a second beam of the plurality of charged particle beams is deflected by a multipole structure of the second layer, and 
     a third beam of the plurality of charged particle beams is deflected by a multipole structure of the first layer and a multipole structure of the second layer in series. 
     23. The array of any one of clauses 13-22, wherein each multipole structures of the plurality of multipole structures is placed inside an electrically shielding cavity to be electrically shielded from other multipole structures.
 
24. The array of any one of clauses 13-23, wherein the first multipole structure of the first layer has an inner diameter larger than the second multipole structure of the first layer.
 
25. The array of any one of clauses 13-24, wherein two or more of multipole structures of the first layers:
 
     have a same number of pole electrodes, 
     are equal or substantially equal in radial shift and orientation angle, and 
     are electrically connected and driven by a first driver. 
     26. The array of any one of clauses 13-25, wherein two or more of multipole structures of the second layers: 
     have a same number of pole electrodes, 
     are equal or substantially equal in radial shift and orientation angle, and 
     are electrically connected and driven by a second driver. 
     27. A source conversion unit in a charged particle beam system comprising the array of any one of clauses 13-26.
 
28. A method of manufacturing a micro-structure deflector array including a plurality of multipole structures, each multipole structure comprising a plurality of pole electrodes, the method comprising:
 
     forming the first multipole structure to have a first radial shift from a central axis of the array; and 
     forming the second multipole structure to have a second radial shift from the central axis of the array, wherein the first radial shift is larger than the second radial shift and the first multipole structure has a different number of pole electrodes from the second multipole structure. 
     29. The method of clause 28, further comprising selecting the number of pole electrodes of the first multipole structure and the number of pole electrodes of the second multipole structure based on aberration characteristics of the first and second multipole structures.
 
30. The method of clause 29, wherein selecting the number of pole electrodes of the first multipole structure and the number of pole electrodes of the second multipole structure comprises selecting corresponding numbers of pole electrodes to reduce high-order components of electric fields thereof.
 
31. The method of any one of clauses 28-30, wherein the number of pole electrodes of the first multipole structure is larger than the second multipole structure.
 
32. The method of any one of clause 28-31, further comprising placing the plurality of multipole structures in one or more layers.
 
33. The method of clause 32, wherein one multipole structure in a first layer of the one or more layers is aligned with one multipole structure in a second layer of the one or more layers.
 
34. The method of any one of clauses 28-33, further comprising grouping a subset of multipole structures to share one driver, wherein the subset of multipole structures:
 
     have a same number of pole electrodes, and 
     are equal or substantially equal in radial shift and orientation angle. 
     35. The method of any one of clauses 32-34, further comprising grouping a subset of multipole structures in the first layer of the one or more layers to share a first driver, wherein the subset of multipole structures in the first layer: 
     have a same number of pole electrodes, and 
     are equal or substantially equal in radial shift and orientation angle. 
     36. The method of any one of clauses 32-35, further comprising grouping a subset of multipole structures in the second layer of the one or more layers to share a second driver, wherein the subset of multipole structures in the second layer: have a same number of pole electrodes, and are equal or substantially equal in radial shift and orientation angle.
 
37. A micro-structure deflector array including a plurality of multipole structures, each multipole structure comprising a plurality of pole electrodes, the array comprising:
 
     a first group of multipole structures of the plurality of multipole structures, the first group of multipole structures having a first set of radial shifts from a central axis of the array, wherein each multipole structure of the first group comprises a same number of corresponding pole electrodes; and 
     a second group of multipole structures of the plurality of multipole structures, the second group of multipole structures having a second set of radial shifts from the central axis of the array, wherein each multipole structure of the second group comprises a same number of corresponding pole electrodes, 
     wherein a multipole structure of the first group comprises a greater number of pole electrodes than a multipole structure of the second group, and wherein each of the first group and the second group comprises one or more multipole structures. 
     38. The array of clause 37, wherein a lowest value of radial shift of the first set of radial shifts are higher than a highest value of radial shift of the second set of radial shifts.
 
39. The array of clause 37, wherein the plurality of multipole structures are configured to substantially simultaneously deflect a plurality of charged particle beams.
 
40. The array of clause 39, wherein the first group of multipole structures comprises:
 
     a first sub-group of multipole structures that are electrically connected to, and driven by, a first driver, wherein radial shifts and orientation angles of the first sub-group of multipole structures are substantially equal. 
     41. The array of clause 40, wherein the second group of multipole structures comprises:
         a second sub-group of multipole structures that are electrically connected to, and driven by, a second driver, wherein radial shifts and orientation angles of the second sub-group of multipole structures are equal or substantially equal.
 
42. The array of clause 41, wherein one of the first and second drivers is configured to enable corresponding multipole structures to deflect the plurality of charged particle beams in a multi-beam apparatus, and wherein further the plurality of multipole structures are configured as image-forming elements or pre-bending micro-deflectors in the multi-beam apparatus.
 
43. The array of clause 37, wherein a multipole structure of the first group has an inner diameter larger than a multipole structure of the second group.
 
44. The array of clause 39, wherein the first group and the second group are arranged in a first layer of the array, and wherein the array further comprises a second layer that comprises a third group of multipole structures having a third radial shift from the central axis of the array.
 
45. The array of clause 44, wherein the third radial shift is different than the first set of radial shifts or the second set of radial shifts.
 
46. The array of clause 45, wherein a number of pole electrodes of the third multipole structure is different than, or equal to, a number of pole electrodes of a multipole structure of the first group of multipole structures.
 
47. The array of clause 45, wherein one of the plurality of charged particle beams is deflected by a multipole structure of the first layer, and another one of the plurality of charged particle beams is deflected by a multipole structure of the second layer.
 
48. The array of clause 45, wherein one of the plurality of charged particle beams is deflected by a multipole structure of the first layer and a multipole structure of the second layer in series, and the multipole structure of the first layer and the multipole structure of the second layer are aligned to each other.
 
49. The array of clause 45, wherein:
       

     a first beam of the plurality of charged particle beams is deflected by a multipole structure of the first layer and is not deflected by any multipole structure of the second layer, 
     a second beam of the plurality of charged particle beams is deflected by a multipole structure of the second layer and is not deflected by any multipole structure of the second layer and 
     a third beam of the plurality of charged particle beams is deflected by a multipole structure of the first layer and a multipole structure of the second layer in series. 
     50. A source conversion unit in a charged particle beam system, wherein the source conversion unit comprises a micro-structure deflector array including a plurality of multipole structures, each of the multipole structures comprising a plurality of pole electrodes, the array comprising: 
     a first group of multipole structures of the plurality of multipole structures, the first group of multipole structures having a first set of radial shifts from a central axis of the array, wherein each multipole structure of the first group comprises a same number of pole electrodes; and 
     a second group of multipole structures of the plurality of multipole structures, the second group of multipole structures having a second set of radial shifts from the central axis of the array, wherein each multipole structure of the second group comprises a same number of pole electrodes, 
     wherein the first set of radial shifts is different from the second set of radial shifts, wherein a multipole structure of the first group comprises a greater number of pole electrodes than a multipole structure of the second group, and wherein each of the first group and the second group comprises one or more multipole structures. 
     51. The source conversion unit of clause 50, wherein the first group and the second group are arranged in a first layer of the array, the array further comprising a second layer that comprises a third group of multipole structures having a third radial shift from the central axis of the array.
 
52. The array of any one of clauses 6-10, wherein a driver of the first and second drivers is configured to enable the corresponding multipole structures to function as image-forming elements or pre-bending micro-deflectors in a multi-beam apparatus to deflect the plurality of charged particle beams.
 
53. The array of clause 52, wherein the driver of the first and second drivers being configured includes all of the first and second drivers being configured.
 
     While the present invention 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.