Patent Publication Number: US-2023137186-A1

Title: Systems and methods for signal electron detection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. application 63/008,639 which was filed on Apr. 10, 2020, and which is incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     The embodiments provided herein disclose a charged-particle beam apparatus, and more particularly improved systems and methods for signal electron detection. 
     BACKGROUND 
     When manufacturing semiconductor integrated circuit (IC) chips, undesired pattern defects, as a consequence of, for example, optical effects and incidental particles, inevitably occur on a substrate (i.e., wafer) or a mask during the fabrication processes, thereby reducing the yield. Monitoring the extent of the undesired pattern defects is therefore an important process in the manufacture of IC chips. More generally, the inspection or measurement of a surface of a substrate, or other object/material, is an important process during and after its manufacture. 
     Pattern inspection tools with a charged particle beam have been used to inspect objects, for example to detect pattern defects. These tools typically use electron microscopy techniques, such as a scanning electron microscope (SEM). In a SEM, a primary electron beam of electrons at a relatively high energy is targeted with a final deceleration step in order to land on a sample at a relatively low landing energy. The beam of electrons is focused as a probing spot on the sample. The interactions between the material structure at the probing spot and the landing electrons from the beam of electrons cause electrons to be emitted from the surface, such as secondary electrons, backscattered electrons, or Auger electrons. The generated secondary electrons may be emitted from the material structure of the sample. By scanning the primary electron beam as the probing spot over the sample surface, secondary electrons can be emitted across the surface of the sample. By collecting these emitted secondary electrons from the sample surface, a pattern inspection tool may obtain an image representing characteristics of the material structure of the surface of the sample. 
     SUMMARY 
     The embodiments provided herein disclose a charged-particle beam apparatus, and more particularly improved systems and methods for signal electron detection. 
     In some embodiments, an electron detector for detecting a plurality of signal electrons generated from a sample is provided. The detector includes a first semiconductor layer having a first portion and a second portion, a second semiconductor layer adjacent to the first semiconductor layer, and a third semiconductor layer adjacent to the second semiconductor layer. The detector also includes a PIN region formed by the first, second, and third semiconductor layers. The detector may also include a power supply configured to apply a reverse bias between the first and the third semiconductor layers. The detector further includes a depletion region formed within the PIN region by the reverse bias, the depletion region comprising a portion of the second semiconductor layer, and the depletion region configured to generate a detector signal based on a first subset of the plurality of signal electrons captured within the depletion region, wherein the second portion of the first semiconductor layer is not depleted and is configured to provide an energy barrier to block a second subset of the plurality of signal electrons and to allow the first subset of the plurality of signal electrons to pass through to reach the depletion region. 
     In some embodiments, an electron detector for detecting a plurality of signal electrons generated from a sample is provided. The detector includes a first semiconductor layer having a first portion and a second portion, a second semiconductor layer adjacent to the first semiconductor layer. The detector also includes multiple segments of a third semiconductor layer, each of the multiple segments being adjacent to the second semiconductor layer. The detector also includes a PIN region formed by the first, second, and third semiconductor layers and a power supply configured to apply a reverse bias between the first and the third semiconductor layers. The detector further includes a depletion region formed within the PIN region by the reverse bias, the depletion region comprising a portion of the second semiconductor layer, and the depletion region configured to generate a plurality of detector signals based on a first subset of the plurality of signal electrons captured within the depletion region, wherein the second portion of the first semiconductor layer is not depleted and is configured to provide an energy barrier to block a second subset of the plurality of signal electrons and to allow the first subset of the plurality of signal electrons to pass through to reach the depletion region. 
     A method for manufacturing an electron detector having an energy barrier that filters out electrons based on the electrons&#39; energy is provided. The method includes providing a semiconductor substrate having a first portion, a second portion adjacent to the first portion, and a third portion adjacent to the second portion. The method also includes forming a first semiconductor layer by doping the first portion of the substrate with a first type of dopant, forming a third semiconductor layer by doping the third portion of the substrate with a second type of dopant, and forming a second semiconductor layer in the second portion of the substrate. A doping concentration of the first type of dopant in the first semiconductor layer is determined to configure the energy barrier of the electron detector, and a thickness of the first semiconductor layer is determined to further configure the energy barrier of the electron detector. 
     In some embodiments, a charged particle beam apparatus for inspecting a sample is provided. The apparatus includes a charge particle beam source configured to emit a charged particle beam along a primary optical axis, an objective lens configured to focus the charged particle beam onto the sample, and an electron detector according to the embodiments described above. The electron detector is configured to detect a plurality of signal electrons generated from incidence of the charged particle beam onto the sample. 
     In some embodiments, a charged particle beam apparatus for inspecting a sample is provided. The apparatus includes a charge particle beam source configured to emit a charged particle beam along a primary optical axis, an objective lens configured to focus the charged particle beam onto the sample, an electron detector configured to detect a plurality of signal electrons generated from incidence of the charged particle beam onto the sample, and a passive energy filter between the electron detector and the sample. 
     In some embodiments, an electron detector for detecting a plurality of signal electrons generated from a sample is provided. The detector includes a first semiconductor layer having a first portion and a second portion, a second semiconductor layer adjacent to the first semiconductor layer, and a third semiconductor layer adjacent to the second semiconductor layer. The detector also includes a PIN region formed by the first, second, and third semiconductor layers. The detector further includes a depletion region formed by a reverse bias applied to the PIN region, the depletion region comprising a portion of the second semiconductor layer, and the depletion region configured to generate a detector signal based on a first subset of the plurality of signal electrons captured within the depletion region, wherein the second portion of the first semiconductor layer is not depleted and is configured to provide an energy barrier to block a second subset of the plurality of signal electrons and to allow the first subset of the plurality of signal electrons to pass through to reach the depletion region. 
     In some embodiments, an electron detector for detecting a plurality of signal electrons generated from a sample is provided. The detector includes a first semiconductor layer having a first portion and a second portion, a second semiconductor layer adjacent to the first semiconductor layer. The detector also includes multiple segments of a third semiconductor layer, each of the multiple segments being adjacent to the second semiconductor layer. The detector also includes a PIN region formed by the first, second, and third semiconductor layers. The detector further a depletion region formed by a reverse bias applied to the PIN region, the depletion region comprising a portion of the second semiconductor layer, and the depletion region configured to generate a plurality of detector signals based on a first subset of the plurality of signal electrons captured within the depletion region, wherein the second portion of the first semiconductor layer is not depleted and is configured to provide an energy barrier to block a second subset of the plurality of signal electrons and to allow the first subset of the plurality of signal electrons to pass through to reach the depletion region. 
     Other advantages of the embodiments of the present disclosure will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG.  1    is a schematic diagram illustrating a charged-particle beam inspection system, consistent with embodiments of the present disclosure. 
         FIG.  2    is a schematic diagram illustrating an exemplary configuration of an electron beam tool that can be a part of the charged-particle beam inspection system of  FIG.  1   , consistent with embodiments of the present disclosure. 
         FIG.  3 A  is a schematic diagram shows a charged-particle beam apparatus comprising a plurality of signal electron detectors, consistent with embodiments of the present disclosure. 
         FIGS.  3 B and  3 C  are schematic diagrams of a charged-particle beam apparatus comprising a signal electron detector with an active energy filter. 
         FIG.  4 A  is a schematic diagram of an exemplary signal electron detector, consistent with embodiments of the present disclosure. 
         FIGS.  4 B and  4 C  are illustrations showing exemplary operations of the signal electron detector of  FIG.  4 A , consistent with embodiments of the present disclosure. 
         FIGS.  5 A and  5 B  are schematic diagrams of an exemplary signal electron detector with an external passive energy filter, consistent with embodiments of the present disclosure. 
         FIGS.  6 A- 6 F  are schematic diagrams of exemplary charged-particle beam apparatuses comprising the signal electron detector and the external passive energy filter according to  FIGS.  5 A and  5 B , consistent with embodiments of the present disclosure. 
         FIG.  7    illustrates an exemplary method of forming the signal electron detector of  FIG.  4 A , consistent with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the disclosed embodiments as recited in the appended claims. For example, although some embodiments are described in the context of utilizing electron beams, the disclosure is not so limited. Other types of charged particle beams may be similarly applied. Furthermore, other imaging systems may be used, such as optical imaging, photo detection, x-ray detection, etc. 
     Electronic devices are constructed of circuits formed on a piece of silicon called a substrate. Many circuits may be formed together on the same piece of silicon and are called integrated circuits or ICs. The size of these circuits has decreased dramatically so that many more of them can fit on the substrate. For example, an IC chip in a smart phone can be as small as a thumbnail and yet may include over 2 billion transistors, the size of each transistor being less than 1/1000th the size of a human hair. 
     Making these extremely small ICs is a complex, time-consuming, and expensive process, often involving hundreds of individual steps. Errors in even one step have the potential to result in defects in the finished IC, thereby rendering it useless. Thus, one goal of the manufacturing process is to avoid such defects to maximize the number of functional ICs made in the process, that is, to improve the overall yield of the process. 
     One component of improving yield is monitoring the chip making process to ensure that it is producing a sufficient number of functional integrated circuits. One way to monitor the process is to inspect the chip circuit structures at various stages of their formation. Inspection can be carried out using a scanning electron microscope (SEM). An SEM can be used to image these extremely small structures, in effect, taking a “picture” of the structures. The image can be used to determine if the structure was formed properly and also if it was formed in the proper location. If the structure is defective, then the process can be adjusted so the defect is less likely to recur. 
     The accuracy and reliability of inspection of high-density IC chips using SEMs may depend on the imaging resolution of the system, among other things. One of the several ways to obtain and maintain high imaging resolution is to maximize the collection efficiency of signal electrons, such as secondary electrons (SE) and backscattered electrons (BSEs). When a primary electron strikes the surface of a sample, it interacts with a volume of the sample based on the landing energy, sample material, and spot size, among other things, and generates a plurality of signal electrons. SEs, which are produced from the emission of the valence electrons (e.g., outer shell electrons) of the constituent atoms of the sample, have ≤50 eV emission energies and originate from the surface or the near-surface region of the sample. BSEs, which result predominantly from an elastic collision of an electron of the electron beam with the nucleus of a constituent atom, have higher emission energies, e.g., in a range of 50 eV to the landing energy of primary electrons on a sample (as high as 1,000 to 10,000 eV or more), and often originate from deeper areas within the interaction volume of the sample, and thus can provide information associated with material composition and distribution of the sample. In some embodiments, it may be desired to have a mechanism to collect only a certain type of signal electrons, such as BSEs, to enhance the quality of obtained images. For example, maximum detection of backscattered electrons may be desirable to obtain high resolution images of underlying defects or structures from a deeper subsurface region of the sample. 
     In conventional SEMs, one way to enable selective collection of a certain type of signal electron may include placing an active energy filter on the path of the signal electrons between the sample and the electron detector, so that an unwanted type of signal electrons can be filtered out before reaching the surface of the electron detector. For example, the active energy filter may include an electrode (biased negatively with respect to the sample when the charged particle beam is an electron beam) that generates an electric field to block off SEs while allowing BSEs to pass through. In some embodiments, however, the electric field generated by the active energy filter may disturb the primary electrons and increase the aberrations of the objective lens, resulting in an increase the size of a probe spot on the sample, and resultantly negatively affecting the imaging resolution. Accordingly, it may be desirable to detect only BSEs without using an active filter. Some embodiments of the present disclosure are directed to charged-particle beam apparatuses and methods of forming an image of a sample. The apparatus may include an electron detector with a passive filter that provides the selective collection capability of BSEs without a need to generate an electric field. 
     Relative dimensions of components in drawings may be exaggerated for clarity. Within the following description of drawings, the same or like reference numbers refer to the same or like components or entities, and only the differences with respect to the individual embodiments are described. As used herein, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, if it is stated that a component may include A or B, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or A and B. As a second example, if it is stated that a component may include A, B, or C, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C. 
     Reference is now made to  FIG.  1   , which illustrates an exemplary charged particle beam inspection system  100  such as an electron beam inspection (EBI) system, consistent with embodiments of the present disclosure. As shown in  FIG.  1   , charged particle beam inspection system  100  includes a main chamber  10 , a load-lock chamber  20 , an electron beam tool  40 , and an equipment front end module (EFEM)  30 . Electron beam tool  40  is located within main chamber  10 . While the description and drawings are directed to an electron beam, it is appreciated that the embodiments are not used to limit the present disclosure to specific charged particles. 
     EFEM  30  includes a first loading port  30   a  and a second loading port  30   b.  EFEM  30  may include additional loading port(s). First loading port  30   a  and second loading port  30   b  receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other material(s)) or samples to be inspected (wafers and samples are collectively referred to as “wafers” hereafter). One or more robot arms (not shown) in EFEM  30  transport the wafers to load-lock chamber  20 . 
     Load-lock chamber  20  is connected to a load/lock vacuum pump system (not shown), which removes gas molecules in load-lock chamber  20  to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robot arms (not shown) transport the wafer from load-lock chamber  20  to main chamber  10 . Main chamber  10  is connected to a main chamber vacuum pump system (not shown), which removes gas molecules in main chamber  10  to reach a second pressure below the first pressure. After reaching the second pressure, the wafer is subject to inspection by electron beam tool  40 . In some embodiments, electron beam tool  40  may comprise a single-beam inspection tool. 
     Controller  50  may be electronically connected to electron beam tool  40  and may be electronically connected to other components as well. Controller  50  may be a computer configured to execute various controls of charged particle beam inspection system  100 . Controller  50  may also include processing circuitry configured to execute various signal and image processing functions. While controller  50  is shown in  FIG.  1    as being outside of the structure that includes main chamber  10 , load-lock chamber  20 , and EFEM  30 , it is appreciated that controller  50  can be part of the structure. 
     While the present disclosure provides examples of main chamber  10  housing an electron beam inspection system, it should be noted that aspects of the disclosure in their broadest sense, are not limited to a chamber housing an electron beam inspection system. Rather, it is appreciated that the foregoing principles may be applied to other chambers as well. 
     Reference is now made to  FIG.  2   , which is a schematic diagram illustrating an exemplary configuration of an electron beam tool  40  that can be a part of the charged particle beam inspection system  100  of  FIG.  1   , consistent with embodiments of the present disclosure. Electron beam tool  40  (also referred to herein as apparatus  40 ) may comprise an electron emitter, which may comprise a cathode  203 , an anode  220 , and a gun aperture  222 . Electron beam tool  40  may further include a Coulomb aperture array  224 , a condenser lens  226 , a beam-limiting aperture array  235 , an objective lens assembly  232 , and an electron detector  244 . Electron beam tool  40  may further include a sample holder  236  supported by motorized stage  234  to hold a sample  250  to be inspected. It is to be appreciated that other relevant components may be added or omitted, as needed. 
     In some embodiments, electron emitter may include cathode  203 , an extractor anode  220 , wherein primary electrons can be emitted from the cathode and extracted or accelerated to form a primary electron beam  204  that forms a primary beam crossover  202  (virtual or real). Primary electron beam  204  can be visualized as being emitted from primary beam crossover  202 . 
     In some embodiments, the electron emitter, condenser lens  226 , objective lens assembly  232 , beam-limiting aperture array  235 , and electron detector  244  may be aligned with a primary optical axis  201  of apparatus  40 . In some embodiments, electron detector  244  may be placed off primary optical axis  201 , along a secondary optical axis (not shown). 
     Objective lens assembly  232 , in some embodiments, may comprise a modified swing objective retarding immersion lens (SORIL), which includes a pole piece  232   a,  a control electrode  232   b,  a deflector  232   c  (or more than one deflectors), and an exciting coil  232   d.  In a general imaging process, primary electron beam  204  emanating from the tip of cathode  203  is accelerated by an accelerating voltage applied to anode  220 . A portion of primary electron beam  204  passes through gun aperture  222 , and an aperture of Coulomb aperture array  224 , and is focused by condenser lens  226  so as to fully or partially pass through an aperture of beam-limiting aperture array  235 . The electrons passing through the aperture of beam-limiting aperture array  235  may be focused to form a probe spot on the surface of sample  250  by the modified SORIL lens and deflected to scan the surface of sample  250  by deflector  232   c.  Secondary electrons emanated from the sample surface may be collected by electron detector  244  to form an image of the scanned area of interest. 
     In objective lens assembly  232 , exciting coil  232   d  and pole piece  232   a  may generate a magnetic field that is leaked out through the gap between two ends of pole piece  232   a  and distributed in the area surrounding optical axis  201 . A part of sample  250  being scanned by primary electron beam  204  can be immersed in the magnetic field and can be electrically charged, which, in turn, creates an electric field. The electric field may reduce the energy of impinging primary electron beam  204  near and on the surface of sample  250 . Control electrode  232   b,  being electrically isolated from pole piece  232   a,  controls the electric field above and on sample  250  to reduce aberrations of objective lens assembly  232  and control focusing situation of signal electron beams for high detection efficiency. Deflector  232   c  may deflect primary electron beam  204  to facilitate beam scanning on the wafer. For example, in a scanning process, deflector  232   c  can be controlled to deflect primary electron beam  204 , onto different locations of top surface of sample  250  at different time points, to provide data for image reconstruction for different parts of sample  250 . 
     Backscattered electrons (BSEs) and secondary electrons (SEs) can be emitted from the part of sample  250  upon receiving primary electron beam  204 . Electron detector  244  may capture the BSEs and SEs and generate image of the sample based on the information collected from the captured signal electrons. If electron detector  244  is positioned off primary optical axis  201 , a beam separator (not shown) can direct the BSEs and SEs to a sensor surface of electron detector  244 . The detected signal electron beams can form corresponding secondary electron beam spots on the sensor surface of electron detector  244 . Electron detector  244  can generate signals (e.g., voltages, currents) that represent the intensities of the received signal electron beam spots, and provide the signals to a processing system, such as controller  50 . The intensity of secondary or backscattered electron beams, and the resultant beam spots, can vary according to the external or internal structure of sample  250 . Moreover, as discussed above, primary electron beam  204  can be deflected onto different locations of the top surface of sample  250  to generate secondary or backscattered signal electron beams (and the resultant beam spots) of different intensities. Therefore, by mapping the intensities of the signal electron beam spots with the locations of primary electron beam  204  on sample  250 , the processing system can reconstruct an image of sample  250  that reflects the internal or external structures of sample  250 . 
     In some embodiments, controller  50  may comprise an image processing system that includes an image acquirer (not shown) and a storage (not shown). The image acquirer may comprise one or more processors. For example, the image acquirer may comprise a computer, server, mainframe host, terminals, personal computer, any kind of mobile computing devices, and the like, or a combination thereof. The image acquirer may be communicatively coupled to electron detector  244  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 detector  244  and may construct an image. The image acquirer may thus acquire images of regions of sample  250 . 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, 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 a primary beam  204  incident on the sample (e.g., a 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  250 , and thereby can be used to reveal any defects that may exist in sample  250  (such as wafer). 
     In some embodiments, controller  50  may control motorized stage  234  to move sample  250  during inspection. In some embodiments, controller  50  may enable motorized stage  234  to move sample  250  in a direction continuously at a constant speed. In other embodiments, controller  50  may enable motorized stage  234  to change the speed of the movement of sample  250  over time depending on the steps of scanning process. 
     Reference is now made to  FIG.  3 A , which is a schematic diagram shows an embodiment of a charged-particle beam apparatus  300 A comprising a plurality of signal electron detectors, consistent with embodiments of the present disclosure. In some conventional SEMs, apparatus  300 A may comprise an electron source  302  configured to emit primary electrons from a cathode (e.g., cathode  203  of  FIG.  2   ) and form a primary electron beam  304  that emanates from a primary beam crossover  303  (virtual or real) along a primary optical axis  301 . Apparatus  300 A may further comprise a condenser lens  321 , a beam-limiting aperture array  312 , an in-lens electron detector  331 , a backscattered electron detector  341 , a scanning deflection unit  350 , and an objective lens assembly  322 . In the context of this disclosure, an in-lens electron detector refers to a charged-particle detector (e.g., electron detector) located inside or above objective lens assembly  322  and may be arranged rotationally symmetric around the primary optical axis (e.g., primary optical axis  301 ). In some embodiments, an in-lens electron detector may also be referred to as a through-the lens detector, an immersion lens detector, or an upper detector. It is to be appreciated that relevant components may be added or omitted or reordered, as appropriate. 
     In currently existing SEMs, as shown in  FIG.  3 A , primary electron beam  304  may be emitted from electron source  302  and accelerated to a higher energy by an anode (e.g., anode  220  of  FIG.  2   ). A gun aperture (e.g., gun aperture  222  of  FIG.  2   ) may limit the current of primary electron beam  304  to a desired initial value, and may work in conjunction with beam-limiting aperture array  312  to obtain a final beam current. Primary electron beam  304  may be focused by condenser lens  321  and objective lens assembly  322  to form a small probe spot  306  on the surface of a sample  371 . In some embodiments, the focusing power of condenser lens  321  and the opening size of an aperture of beam-limiting aperture array  312  may be selected to get a desired probe current and make the probe spot size as small as desired. 
     To obtain small spot sizes over a large range of probe current, beam-limiting aperture array  312  may comprise multiple apertures having various sizes. The beam-limiting aperture array  312  may be configured to move so that, based on a desired probe current or a probe spot size, one of the apertures of the aperture array  312  can be aligned with the primary optical axis  301 . For example, as shown in  FIG.  3 A , one of the apertures of the aperture array  312  may be configured to generate primary electron beamlet  304 - 1  by blocking peripheral electrons of primary electron beam  304 . In some embodiments, scanning deflection unit  350  may include one or more deflectors configured to deflect primary electron beamlet  304 - 1  to scan a desired area on the surface of sample  371 . 
     As described earlier with respect to  FIG.  2   , interaction of electrons of primary electron beamlet  304 - 1  with sample  371  may generate SEs and BSEs. As is commonly known in the art, the emission of SEs and BSEs obeys Lambert&#39;s law and has a large energy spread—the electrons emerging from different depths of sample  371  have different emission energies. For example, SEs originate from the surface or the near-surface region of the sample  371  and have lower emission energies (e.g., lower than 50 eV). SEs may be useful in providing information about surface or near-surface features and geometries. On the other hand, BSEs may be generated by elastic scattering events of the incident electrons from deeper subsurface regions of sample  371 , and may have higher emission energies in comparison to SEs, in a range from 50 eV to approximately the landing energy of the incident electrons. BSEs may provide compositional information of the material being inspected. The number of BSEs generated may depend on factors such as the atomic number of the material in the sample or the landing energy of primary electron beam, among other things. 
     In addition to focusing primary electron beam  304  on the surface of sample  371 , objective lens assembly  322  may be further configured to focus the signal electrons on the surface of detector  331 . As described earlier with respect to sample  250  of  FIG.  2   , sample  371  may be immersed in a magnetic field of objective lens assembly  322 , and the magnetic field may focus the signal electrons with lower energies faster than the signal electrons with higher energies. For example, because of SE&#39;s low emission energy, objective lens assembly  322  may be able to strongly focus the SEs (such as along electron paths  381  and  382 ) so that a large portion of the SEs land on a detection layer of in-lens detector  331 . In contrast to SEs, objective lens assembly  322  may only be able to weakly focus BSEs due to their high emission energies. Accordingly, although some BSEs with small emission angles may travel along electron paths  391  and  392  and be detected by in-lens electron detector  331 , the BSEs with large emission angles, for example electrons on path  393 , may not be able to be detected by in-lens electron detector  331 . 
     In some embodiments, an additional electron detector, such as backscattered electron detector  341 , can be used to detect those BSEs with large emission angles (e.g., electrons travelling on path  393 ). In the context of this disclosure, an emission polar angle is measured with reference to primary optical axis  301 , which is substantially perpendicular to sample  371 . As shown in  FIG.  3 A , the emission polar angle of secondary electrons in path  382  is smaller than the emission polar angles of backscattered electrons in path  391 ,  392 , and  393 . Backscattered electron detector  341  may be placed between objective lens assembly  322  and sample  371 , and in-lens electron detector  331  may be placed between objective lens assembly  322  and condenser lens  321 , allowing the detection of SEs as well as BSEs. 
     Based on the difference in emission energy, or emission angle, among other things, SEs and BSEs may be separately detected using separate electron detectors, segmented electron detectors, energy filters, and the like. For example, as shown in  FIG.  3 A , in-lens electron detector  331  may be configured as a segmented detector (discussed later in reference to  FIG.  4 C ) comprising multiple detection segments arranged in a two-dimensional or a three-dimensional arrangement. In some cases, the detection segments of in-lens electron detector  331  may be arranged, e.g., radially, circumferentially, or azimuthally around primary optical axis  301 . 
     Apparatus  300 A may comprise condenser lens  321  configured to focus primary electron beam  304  so that a portion  304 - 1  thereof may pass through an on-axis aperture of beam-limiting aperture array  312 . Condenser lens  321  may be substantially similar to condenser lens  226  of  FIG.  2    and may perform similar functions. Condenser lens  321  may comprise an electrostatic, a magnetic, or a compound electromagnetic lens, among others. Condenser lens  321  may be electrically or communicatively coupled with a controller, such as controller  50  illustrated in  FIG.  2   . Controller  50  may apply an electrical excitation signal to condenser lens  321  to adjust the focusing power of condenser lens  321  based on factors such as the operation mode, application, desired analysis, or sample material being inspected, among other things. 
     Apparatus  300 A may further comprise scanning deflection unit  350  configured to dynamically deflect primary electron beam  304  or primary electron beamlet  304 - 1  on surface of sample  371 . The dynamic deflection of primary electron beamlet  304 - 1  may enable a desired area or a desired region of interest to be scanned, for example in a raster scan pattern, to generate SEs and BSEs for sample inspection. Scanning deflection unit  350  may comprise one or more deflectors (not shown) configured to deflect primary electron beamlet  304 - 1  in the X-axis or Y-axis. As used herein, X-axis and Y-axis form Cartesian coordinates, and primary electron beam  304  propagates along primary optical axis  301  which is aligned with Z-axis. X-axis refers to the horizontal axis or the lateral axis extending along the width of the paper, and Y-axis refers to the vertical axis extending in-and-out of the plane of the paper. 
     Reference is now made to  FIG.  3 B , which illustrates a schematic diagram of an embodiment of a charged-particle beam apparatus  300 B comprising a charged-particle detector and an active energy filter. Apparatus  300 B may comprise a magnetic objective lens assembly  322 . In some embodiments, objective lens assembly  322  may comprise a compound electromagnetic lens including a magnetic lens  322 M and an electrostatic lens formed by an inner pole piece  322 A (similar to pole piece  232   a  of  FIG.  2   ), and a control electrode  322 B (similar to control electron  232   b  of  FIG.  2   ), which work in conjunction to focus beam  304  at sample  371 . 
     One of the ways to selectively detect signal electrons from sample  371 , for example SEs versus BSEs, is to filter out a certain type of electrons based on their emission energies with an active energy filter. As shown in  FIG.  3 B , in some embodiments, control electrode  322 B may be placed to form an energy filter between sample  371  and in-lens electron detector  331 . In some embodiments, control electrode  322 B may be disposed between sample  371  and magnetic lens  322 M of objective lens assembly  322 . When control electrode  322 B is biased to a voltage by power supply  375  with reference to sample  371 , an electric field is generated between control electrode  322 B and sample  371 , resulting in an electrostatic potential barrier for the signal electrons. The electrostatic potential barrier blocks off signal electrons that have emission energies lower than a threshold energy level of the barrier. It is appreciated that an “active filter” means an electron filter using active components, such as an electrode generating an “active” electric field—in contrast to a “passive filter” that uses only passive elements. 
     In an example, control electrode  322 B is biased negatively with reference to sample  371  such that the negatively charged signal electrons (e.g., the SEs on path  381 ) are reflected back to sample  371  because the SEs on path  381  do not have enough energy to pass through the energy barrier. On the other hand, signal electrons which have emission energies higher than the threshold energy level of the barrier (e.g., the BSEs on path  391 ) can overcome the energy barrier formed by control electrode  322 B and propagate towards in-lens electron detector  331 . Accordingly, in-lens electron detector  331  may be configured as a backscattered electron detector. It is appreciated that  381  and  391  indicate paths of example SEs and BSEs generated from sample  371 , respectively. 
     Reference is now made to  FIG.  3 C , which illustrates a schematic diagram of an embodiment of a charged-particle beam apparatus  300 C comprising a charged-particle detector and an active energy filter. In comparison to apparatus  300 B of  FIG.  3 B , apparatus  300 C comprises an energy filter disposed near in-lens electron detector  331 . The active energy filter, as shown in  FIG.  3 C , may comprise a mesh-type electrode  331 E configured to reflect signal electrons with low emission energies (e.g., the SEs on path  381 ) back towards sample  371  or objective lens assembly  322 , and allow signal electrons with high emission energies (e.g., the BSEs on path  391 ) to be incident on a detection layer of in-lens electron detector  331 . In some embodiments, mesh-type electrode  331 E may comprise a mesh-like structure fabricated from an electrically conducting material, such as a metal, an alloy, a semiconductor, or a composite, among other things. Mesh-type electrode  331 E may be disposed between objective lens assembly  322  and in-lens electron detector  331 . In some embodiments, mesh-type electrode  331 E may be disposed closer to in-lens electron detector  331  than objective lens assembly  322 . 
     Detection and inspection of some defects in semiconductor fabrication processes, such as buried particles resulting from photolithography, metal deposition, dry etching, or wet etching, among other things, may benefit from inspection of sample surface features as well as compositional analysis of features below the sample surface. Accordingly, a user may utilize information obtained from electron detectors which can selectively detect SEs or BSEs to identify the defect(s), analyze the composition of the defect(s), and adjust process parameters based on the obtained information. In a charged particle beam apparatus (such as a SEM), the collection efficiency for BSEs may be improved by using an energy filter or additional electron detectors, as discussed in reference to  FIGS.  3 A- 3 C . For example, as illustrated in  FIGS.  3 B and  3 C , an active energy filter utilizing an electric field may be used to separate SEs from BSEs, and thus improve individual collection efficiencies. 
     However, in some embodiments, an active energy filter may pose some drawbacks to the overall performance of the inspection system. For example, placing the negatively biased energy filter closer to the sample (as illustrated in  FIG.  3 B ) may increase the aberrations of the objective lens assembly and increase the size of probe spot  306 , thereby adversely impacting the imaging resolution. As an alternative, the active energy filter may be placed closer to the in-lens electron detector (e.g., mesh-type electrode  331 E place near the detector  331  as illustrated in  FIG.  3 C ) to minimize the impact on the aberrations of the objective lens assembly. In such configuration, however, the primary electron beam  304  may be directly influenced by the energy filter, thereby enlarging the size of probe spot  306 . To avoid the electric field influence on the primary electron beam, a shielding mesh or box (not shown) may be used to enclose detector  331  and electrode  331 E. But the shielding box may limit the detector shape (e.g., requiring a large center hole of detector  331 ), which may hinder the detection of signal electrons with small emission angles. In some configurations, to reduce the influence on the primary electron beam and improve the detection rate of signal electrons with small emission angles, a beam separator (not shown) may be used to deflect the signal electrons away from primary optical axis  301  towards a detector placed on a secondary optical axis (not shown). But, even in this configuration, an active energy filter may still be needed for SE filtering, and accordingly the shielding mesh or box may need to be implemented. Furthermore, the beam separator may add undesirable aberrations to the incident primary electron beam, thereby negatively affecting the imaging resolution. 
     Reference is now made to  FIG.  4 A , which illustrates a diagram of an exemplary structure of a signal electron detector  400  taken along a cross section in the thickness direction of the signal electron detector, consistent with embodiments of the present disclosure. Signal electron detector  400  may be a part of a charged-particle beam apparatus, such as apparatus  300 A of  FIG.  3 A . The detector  400  may be aligned with a primary optical axis  301  of the charged-particle beam apparatus. A primary electron beam  304  travels in the +Z direction (from the top to the bottom of  FIG.  4 A ). Signal electrons  490  generated from a sample (not shown) travel in the −Z direction (from the bottom to the top of  FIG.  4 A ) to enter the detector  400  from a first surface  401   s  of the detector  400 . 
     In some embodiments, signal electron detector  400  may be based on a PIN diode structure comprising an intrinsic semiconductor layer between a p-type semiconductor layer and an n-type semiconductor layer, hence creating a P-I-N structure.  FIG.  4 A  shows a five-layer PIN electron detector including a first metal layer  410 , a first semiconductor layer  420 , a second semiconductor layer  430 , a third semiconductor layer  440 , and a second metal layer  450  along the thickness direction (the −Z direction) of the detector  400 . The five layers (layers  410 - 450 ) have thicknesses of  412 ,  422 ,  432 ,  442 , and  452 , respectively. 
     The first metal layer  410  and the second metal layer  450  at the bottom and the top of the detector  400  may form electrodes that are configured to apply a bias voltage to the detector  400 . For example, first metal layer  410  may function as an anode and second metal layer  450  may function as a cathode of the detector  400 . In addition, the two metal layers may protect the internal semiconductor layers. Although  FIG.  4 A  illustrates embodiments in which the signal electrons enter from the anode side, it is appreciated that, in different embodiments, the signal electrons may enter from the cathode side. 
     First metal layer  410  may be configured to receive signal electrons  490  incident on the surface  401   s  of the electron detector  400 . First metal layer  410  may be thin (e.g., in a range of 10 to 200 nm) and made of light metal to reduce scattering and energy loss of the incoming electrons. For example, a material of first metal layer  410  may be aluminum or other metal that is highly conductive and easily penetrable by signal electrons. The thickness  412  and the material of first metal layer  410  may be determined based on a consideration of blocking of particles other than incident electrons to reduce noise or filtering out some signal electrons based on their emission energy (e.g., filtering out SEs with very low emission energy). 
     First semiconductor layer  420  is formed adjacent to first metal layer  410 . In some embodiments, first semiconductor layer  420  may comprise a p-type semiconductor. For example, first semiconductor layer  420  may be doped with trivalent impurities, such as boron, aluminum, gallium, etc., so as to create free holes. First semiconductor layer  420  may be a heavily doped region, such as a P+ region. A portion of the first semiconductor layer  420  may form an energy barrier that selectively filters out a certain type of incoming signal electrons. The doping concentration and the thickness  422  of first semiconductor layer  420  may be determined based on desired characteristics of the energy filter, such as the threshold or cutoff energy level of the filter. Further details on the operation of the energy filter are provided below with respect to  FIGS.  4 B and  4 C . First metal layer  410  may be deposited on top of first semiconductor layer  420 . Thus, first semiconductor layer  420  may be coated and protected by first metal layer  410 . 
     Second semiconductor layer  430  is formed adjacent to first semiconductor layer  420 . In some embodiments, second semiconductor layer  430  may comprise an intrinsic semiconductor region. For example, second semiconductor layer  430  may be an undoped pure semiconductor, or slightly n-doped or p-doped without any significant dopant species present. Second semiconductor layer  430  may have a doping concentration lower than doping concentrations of other layers of the electron detector  400 . Second semiconductor layer  430  may have a doping concentration that is set so that it has a high resistance as a result of being lightly doped. In some embodiments, signal electron detector  400  may be formed from a silicon wafer in which case second semiconductor layer  430  may be an N− region. Thickness  432  of second semiconductor layer  430  between first semiconductor layer  420  and third semiconductor layer  440  may be determined based on the range of expected emission energy levels of received signal electrons. 
     Third semiconductor layer  440  is formed adjacent to second semiconductor layer  430 . In some embodiments, third semiconductor layer  440  may comprise an n-type semiconductor region. For example, third semiconductor layer  440  may be doped with pentavalent impurities, such as phosphorous, antimony, arsenic, etc., so as to create free electrons. Similar to first semiconductor layer  420 , third semiconductor layer  440  may be a heavily doped region, such as a N+ region. 
     Second metal layer  450  may be deposited on third semiconductor layer  440 . A material of second metal layer  450  may be metal with high surface conductivity, such as aluminum or copper. Unlike first metal layer  410 , second metal layer  450  may not need to be highly electron penetrable as, in some embodiments, signal electrons are not entering through second metal layer  450 . 
       FIGS.  4 B and  4 C  are illustrations showing exemplary operations of the signal electron detector of  FIG.  4 A , consistent with embodiments of the present disclosure. 
     First metal layer  410  and second metal layer  450  may be connected to power supply  467 . As two metal layers  410  and  450  are formed directly on the adjacent semiconductor layers ( 420  and  440 ), an electrical connection may be formed between the three semiconductor layers ( 420 ,  430 ,  440 ) and power supply  467  through the metal layers. Power supply  467  may be configured to provide a reverse bias to the PIN region formed by the first, second, and third semiconductor layers-connecting a negative and positive terminal of power supply  467  to first metal layer  410  (anode) and second metal layer  450  (cathode), respectively. The resultant electric potential difference between first metal layer  410  and second metal layer  450  may create an internal electric field through the PIN region. In some embodiments, power supply  467  may be directly connected to first semiconductor layer  420  and third semiconductor layer  440 , as those semiconductor layers have low resistance due to the high doping concentration. 
     Under a reverse bias condition in normal operation, free charge carriers (e.g., free electrons and holes) are removed away by the electric field, and therefore a depletion region  437  may be formed within the volume of the detector body, specifically within the PIN region. As a result, under reverse bias, there may be substantially no bias current flowing through the detector (except a very small leakage current). In some embodiments, depletion region  437  may exist almost completely within second semiconductor layer  430  (the intrinsic semiconductor region). In some embodiments, depletion region  437  may expand beyond second semiconductor layer  430  forming a depleted portion ( 420   a ) within first semiconductor layer  420  and a depleted portion ( 440   a ) within third semiconductor layer  440 . The other portion of first semiconductor layer  420 —a nondepleted portion ( 420   b )—may remain having free holes. Similarly, the other portion of third semiconductor layer  440 —a nondepleted portion ( 440   b )—may remain having free electrons. 
     As described earlier, incoming signal electrons may have a different emission energy. As some signal electrons or charged particles may have very low emission energy (e.g., electrons  495 ), they may be blocked off or scattered by the first metal layer. Some signal electrons (e.g., electrons  496  and  497 ) may have higher emission energy to reach beyond first metal layer  410 . 
     Anytime when a signal electron (such as electrons  496  and  497 ) enters the detector body after passing first metal layer  410 , the signal electron may start interacting with the semiconductor material and generating electron-hole pairs (e.g.,  496   er - 496   hr ,  497   er - 497   hr ,  497   e - 497   h ). The signal electrons keep losing energy as they interact with the detector to form electron-hole pairs. 
     As shown in  FIG.  4 B , some signal electrons (e.g., electron  496 ) may lose all their energy while generating electron-hole pairs (such as electron-hole pair  496   er - 496   hr ) in the nondepleted portion  420   b  of first semiconductor region  420 , thereby failing to reach depletion region  437 . In some embodiments, some of the generated electrons (e.g., electrons  496   er ) may themselves contribute to generate other electron-hole pairs (not shown). These generated electron-hole pairs (e.g., pair  496   er - 496   hr  and other pairs generated by the electron  496   er ) outside the depletion region  437  may be drifted slowly because the electric field outside of the depletion region  437  is relatively weak. Therefore, there is high probability that the generated electrons and holes are recombined with each other or with any other nearby free opposite carriers (e.g., as shown by the arrow  496   r ). Due to this quick recombination, the generated electron-hole pairs (e.g., pairs  496   er - 496   hr ) may not be able to contribute to generate a drift current, and, resultantly, no detector signal may be generated by the signal detecting unit  468 . 
     Only those incoming signal electrons with sufficiently high emission energy (e.g., electrons  497 ) can go through and reach beyond nondepleted portion  420   b  of first semiconductor layer  420 . When passing nondepleted portion  420   b,  signal electrons  497  may lose some of the energy for generating electron-hole pairs (e.g.,  497   er - 497   hr ). But the initial emission energy (before entering the detector body) of signal electrons  497  may be high enough so that the electrons may reach the depletion region  437  with certain energy left to generate more electron-hole pairs  497   e - 497   h  within depletion region  437 . Furthermore, some of the generated electrons (e.g., electrons  497   er ) may have enough energy to pass through the nondepleted portion  420   b  and reach the depletion region  437 . 
     These generated electrons may also contribute to generate other electron-hole pairs within the depletion region  437 . 
     Electron-hole pairs  497   e - 497   h  generated within the depletion region  437  may be separated by the electric field (formed by the reverse bias as described above), rather than being recombined. For example, electrons  497   e  may be directed toward third semiconductor layer  440  (N+ region) as illustrated by arrow  497   em , while holes  497   h  may be directed toward first semiconductor layer  420  (P+ region) as illustrated by arrow  497   hm . Accordingly, those electrons  497   e  and holes  497   h  may eventually reach the electrodes at the top and bottom of the detector (e.g., the cathode—second metal layer  450 ; the anode—first metal layer  410 ) respectively and generate an electric current. In some embodiments, signal detecting unit  468  may measure this electric current and generate a corresponding detector signal. In some embodiments, signal detecting unit  468  may comprise a transimpedance amplifier (TIA) connected between the power supply  467  and the detector to process the current detector signals. 
     As described above, metal layer  410  and first semiconductor layer  420  in combination provide an energy barrier that filters out incoming signal electrons with emission energies lower than the energy barrier—the amount of initial energy needed to reach depletion region  437 . In this way, incoming signal electrons with different energies can be separated and selectively detected without using an active energy filter, as described earlier with respect to  FIGS.  3 B and  3 C . 
     In some embodiments, the energy barrier may be increased as the thickness of nondepleted region  420   b  increases. For example, the energy barrier in logarithmic scale may be proportional to the thickness of nondepleted region  420   b  in logarithmic scale. In other words, as the thickness of nondepleted region  420   b  increases, the energy barrier may also increase so that signal electrons with higher emission energy will be filtered out. The thickness of nondepleted region  420   b  may be determined based on multiple factors, such as the bias voltage, the material used to for the detector body (e.g., silicon), the doping profile of the semiconductor layers, or the thickness of the first, second, and third semiconductor layers ( 420 ,  430 ,  440 ), among others. For example, under the same bias voltage, the same device structure, and the same doping concentrations, to a certain extent, making first semiconductor layer  420  thicker may result in a thicker nondepleted region  420   b.  In some embodiments, a doping concentration of first semiconductor layer  420  may be changed to configure the thickness of nondepleted region  420   b.    
     In some embodiments, a device simulation based on the known detector structure may be performed to determine an appropriate thickness of nondepleted region  420   b  given a predefined desired level of the energy barrier. For example, to separate BSEs from SEs, a detector may be desired to have an energy barrier that is approximately equal to the sum of 50 eV and the energy that the signal electron acquired from the acceleration voltage in the column of the electron beam apparatus. To configure the signal electron detector to have such energy barrier, a device simulation can be performed to determine various manufacturing process knobs (such as doping profiles, thicknesses of semiconductor layers, the bias voltage, etc.) which, when set properly, can provide the appropriate thickness of nondepleted region  420   b  that corresponds to the desired level of the energy barrier. 
       FIG.  4 C  shows a segmented signal electron detector which operates in the same way as the signal electron detector shown in  FIG.  4 B , except the third semiconductor layer and the second metal layer are segmented to generate multiple detector signals based on the relative positions where the incoming signal electrons are captured. In some embodiments, the electron detector may comprise multiple segments of a third semiconductor layer (e.g.,  440 - 1 ,  440 - 2 ,  440 - 3 ). The materials or doping profiles of those segments  440 - 1 ,  440 - 2 ,  440 - 3  may be substantially similar to the third semiconductor layer  440  of  FIG.  4 B . The electron detector may further comprise multiple segments of a second metal layer (e.g.,  450 - 1 ,  450 - 2 ,  450 - 3 ), each of which is formed directly on the adjacent third semiconductor layer segment ( 440 - 1 ,  440 - 2 , or  440 - 3 ). When signal electron  498  reaches depletion region  437 , electron-hole pairs (e.g.,  498   e - 498   h ) are generated, and the majority of those electrons  498   e  may move towards the closest segment  440 - 2  of the third semiconductor layer. Accordingly, a detector signal corresponding to electrons  498  may be collected by signal detecting unit  468 - 2  which is electrically connected to metal layer segment  450 - 2  and semiconductor layer segment  440 - 2  Similarly, a detector signal generated by incoming signal electron  497  may be collected by signal detecting unit  468 - 3 . 
     Reference is now made to  FIGS.  5 A and  5 B , which are schematic diagrams of an exemplary signal electron detector  531  with an external passive energy filter  532 , consistent with embodiments of the present disclosure. In some embodiments, signal electron detector  531  may be a signal electron detector having a built-in passive energy filter as described in  FIGS.  4 A- 4 C . In some embodiments, signal electron detector  531  may be a conventional electron detector without a built-in passive energy filter. 
     As shown in  FIGS.  5 A and  5 B , signal electron detector  531  may comprise a PIN diode structure similar to the signal detector shown in  FIGS.  4 A- 4 C . For example, signal electron detector  531  may be reverse biased to create depletion region  537 . A nondepleted region (not shown) for detector  531  may provide a built-in energy barrier to filter out signal electrons having energy lower than a predefined threshold energy level. 
     In some embodiments, external passive energy filter  532  may be used to provide an extra energy barrier in addition to the built-in energy barrier of the detector  531 . External passive energy filter  532  may be a plate made of materials capable of attenuating the energy of the incoming signal electrons. For example, external passive energy filter  532  may comprise a semiconductor material (such as silicon nitride) or electric conduction material (such as aluminum film), which provide an attenuation capability while also providing a certain level of electric conductivity to discharge any charge that may build up within energy filter  532  from the incidence of the signal electrons. External passive filter  532  may have a center opening aligned with the primary optical axis of the inspection apparatus to enable a primary beam  304  to pass through. The energy barrier of external passive energy filter  532  may be determined, e.g, by adjusting the thickness or material of the plate. 
     In some embodiments, external passive energy filter  532  may be movable between a first position for a high filtering mode (as shown in  FIG.  5 A ) and a second position for a low filtering mode (as shown in  FIG.  5 B ). When the inspection apparatus operates in a high filtering mode, external passive energy filter  532  may be positioned in the first position between a sample (not shown) and signal electron detector  531 . In the high filtering mode, external passive energy filter  532  may be configured to provide an extra energy barrier in addition to the built-in energy barrier of the detector  531 . The effective total energy barrier for the incoming signal electrons may become the sum of the built-in energy barrier of the detector  531  and the extra energy barrier of external passive energy filter  532 . For example,  FIGS.  5 A and  5 B  shows three exemplary incoming signal electrons  595 ,  596 , and  597 . The emission energy of electrons  595  may be lower than the extra energy barrier of external passive energy filter  532 . The emission energy of electrons  596  may be higher than the extra energy barrier of external passive energy filter  532  but lower than the effective total energy barrier. The emission energy of electrons  597  may be higher than the effective total energy barrier. With external passive energy filter  532  positioned in the first position, signal electrons  595  and  596  are filtered out as their energies are lower than the effective total energy barrier, and only signal electrons  597  may reach depletion region  537  to be detected. 
     When the inspection apparatus operates in the low filtering mode, external passive energy filter  532  may be positioned in the second position away from the detector  531  so that external passive energy filter  532  would not affect any incoming signal electrons. Accordingly, in the low filtering mode, the incoming signal electrons are filtered only by the built-in energy barrier of the detection  531 . For example, as shown in  FIG.  5 B , with external passive energy filter  532  positioned in the second position, only signal electron  595  may be filtered out, while signal electrons  596  and  597  may reach depletion region  537  to be detected. 
     Reference is now made to  FIGS.  6 A- 6 F , which are schematic diagrams of exemplary charged-particle beam apparatuses comprising the signal electron detector and the external passive energy filter according to  FIGS.  5 A and  5 B , consistent with embodiments of the present disclosure. 
       FIG.  6 A  shows an exemplary charged-particle beam apparatus  600 A, similar to the charged-particle beam apparatus  300 A of  FIG.  3 A , consistent with some embodiments of the present disclosure. A movable external passive energy filter  632  (similar to the movable external passive energy filter  532  shown in  FIGS.  5 A and  5 B ) may be used within the charged-particle beam apparatus  600 A. Apparatus  600 A may comprise a signal electron detector  631 , which may be a signal electron detector having a built-in passive energy filter as described in  FIGS.  4 A- 4 C , or a conventional electron detector without a built-in passive energy filter. 
     Similar to  FIG.  5 A , when apparatus  600 A operates in a high filtering mode, external passive energy filter  632  may be positioned in a first position between a sample  371  and signal electron detector  631 . In the high filtering mode, external passive energy filter  632  may be configured to provide an extra energy barrier in addition to the built-in energy barrier of the detector  631 . The effective total energy barrier for the incoming signal electrons may become the sum of both energy barriers of the detector  631  and the filter  632 . For example, as shown in  FIG.  6 A , with external passive energy filter  632  positioned in the first position, signal electrons  381  (e.g., electrons with low emission energies, such as SEs) may be filtered out because their energy is lower than the effective total energy barrier, and only signal electrons  391  (comprising electrons with emission energies higher than the effective total energy barrier, such as BSEs) may reach a depletion region of the detector  631  to be detected. 
     When apparatus  600 A operates in the low filtering mode, external passive energy filter  632  may be positioned in a second position away from the detector  631  so that the effective total energy barrier may be decreased, thereby allowing signal electrons with lower energy (e.g., SEs on path  381 ) to also be detected by detector  631 . 
       FIG.  6 B  shows another example ( 600 B) of a charged-particle beam apparatus with a movable passive energy filter, consistent with some embodiments of the present disclosure Similar to the apparatus  600 A in  FIG.  6 A , the apparatus  600 B may comprise a signal electron detector  631 , which may be a signal electron detector having a built-in passive energy filter as described in  FIGS.  4 A- 4 C , or an electron detector without a built-in passive energy filter. In some embodiments, the movable external passive energy filter  632  may comprise multiple filtering zones (e.g., filtering zones  632   a  and  632   b ). In some embodiments, as shown in  FIG.  6 C , the movable external passive energy filter  632  may comprise a filtering plate with multiple filtering zones, with each filtering zone having a center hole allowing a primary beam  304  to pass through. Each filtering zone  632   a  or  632   b  may provide a different level of energy barrier. As described earlier, the energy barrier of external passive energy filter zones  632   a  and  632   b  may be determined by adjusting the thickness or material of each filter zone plate. 
     In some embodiments, apparatus  600 B may operate in various filtering modes, such as a high/medium/low filtering modes. When the apparatus  600 B operates in the high filtering mode, a filtering zone with the higher energy barrier (e.g.,  632   a ) may be positioned in front of the electron detector  631 , thereby providing the maximum level of electron filtering. When the apparatus operates in the medium filtering mode, a filtering zone with the lower energy barrier (e.g.,  632   b ) may be positioned in front of the electron detector  631 , thereby providing a medium level of electron filtering. When the apparatus  600 B operates in the low filtering mode, external passive energy filter  632  may be positioned away from the electron detector  631  (no filtering zone in front of the detector  631 ), thereby providing the minimum level of electron filtering. Although  FIGS.  6 B and  6 C  illustrate the filter  632  having two filtering zones  632   a  and  632   b,  it is appreciated that any number of filtering zones can be implemented in the filter  632 . 
       FIG.  6 D  shows another example of the external passive energy filter  632  having multiple filtering segments. In some embodiments, the external passive energy filter  632  may comprise a center hole allowing a primary beam  304  to pass through and multiple filtering segments (e.g., filtering segments  632   c,    632   d,  and  632   e ) positioned around the center hole. Each filtering segment  632   c,    632   d,  or  632   e  may provide a different level of energy barrier. This enables the electron detector (such as electron detector  631  in  FIGS.  6 A and  6 B ) to detect signals electrons with different energy levels (e.g., SEs vs. BSEs) in terms of emission radial angles (emission angle with respect to surface normal) thereof and is helpful for defect inspection of some types of samples. 
     As described above with respect to  FIGS.  6 A and  6 B , the external passive energy filter  632  may be used with the signal electron detector  631 , which may comprise an electron detector with a built-in passive energy filter as described in  FIGS.  4 A- 4 C , or an electron detector without a built-in passive energy filter. 
     As shown in  FIG.  6 E , in some embodiments, a movable passive energy filter  632  (such as movable passive energy filter  632  used in apparatus  600 B of  FIG.  6 B ) may also comprise multiple filtering segments within each filtering zones  632   a  and  632   b.  This enables signal electrons to be detected in terms of various emission angles and emission energy levels. Although  FIG.  6 E  illustrates the filter  632  having two filtering zones  632   a  and  632   b,  it is appreciated that any number of filtering zones can be implemented in the filter  632 . Similarly, although  FIG.  6 E  illustrates three segments in filtering zone  632   a  and two segments in filtering zone  632   b,  it is appreciated that any number of segments can be implemented within each filtering zone. 
       FIG.  6 F  shows another example ( 600 F) of a charged-particle beam apparatus with a movable passive energy filter  632 , consistent with some embodiments of the present disclosure. The movable passive energy filter  632  may have the configuration shown in  FIGS.  6 C- 6 E . As described earlier with respect to  FIG.  3 A , some BSEs (e.g., electrons on path  393 ) may have large emission angles, so that the objective lens assembly  322  may not able to focus the BSEs  393  onto the electron detector  631 . In some embodiments, apparatus  600 F may include an additional electron detector  641  to detect those BSEs with large emission angles (e.g., electrons travelling on path  393 ). Although  FIG.  6 F  shows an apparatus with a movable passive energy filter, the passive energy filter  632  can be implemented as a fixed filter (similar to a passive energy filter  632  shown in  FIG.  6 A ). 
     Reference is now made to  FIG.  7   , which illustrates an exemplary method of forming the signal electron detector of  FIGS.  4 A- 4 C , consistent with embodiments of the present disclosure. 
     In step A 1 , a substrate  700  is provided. The substrate  700  may be a part of a semiconductor wafer having a first surface  701   s  and a second surface  702   s.  The substrate  700  may be made of silicon, germanium, or other appropriate semiconductor materials. Although  FIG.  7    shows an example process where a lightly doped N− silicon wafer is used as substrate  700 , it is appreciated that a different material, e.g., P− doped semiconductor, may be used. 
     In step A 2 , a first semiconductor layer  720  is formed in a portion of substrate  700  having the first surface  701   s.  In some embodiments, first semiconductor layer  720  may comprise a p-type semiconductor. For example, to create first semiconductor layer  720 , substrate  700  may be doped with trivalent impurities, such as boron, aluminum, gallium, etc., so as to create free holes. The doping impurities, e.g. boron, may be implanted from the first surface  701   s  of substrate  700 . In some embodiments, first semiconductor layer  720  may be heavily doped, such as a P+ region as shown in  FIG.  7   . 
     As described earlier with respect to  FIGS.  4 A- 4 C , when the detector is in a normal operation, a nondepleted region (such as nondepleted region  420   b  of  FIG.  4 B ) of the first semiconductor layer  720  may form an energy barrier that selectively filters out a certain type of incoming signal electrons based on their emission energies. The energy barrier may be increased as the thickness of the nondepleted region increases, and therefore the energy barrier can be varied by changing the thickness of nondepleted region. Among other things, the thickness of nondepleted region may depend on the doping concentration or thickness of first semiconductor layer  720 . Accordingly, multiple manufacturing knobs, including the doping concentration and the thickness of first semiconductor layer  720  may be determined before the step A 2  based on desired characteristics of the energy filter, such as the level of the energy barrier. 
     In step A 3 , a second semiconductor layer  730  and a third semiconductor layer  740  are formed within the body of substrate  700 . The third semiconductor layer  740  is formed in a portion of substrate  700  having the second surface  702   s.  In some embodiments, third semiconductor layer  740  may comprise an n-type semiconductor region. For example, to create third semiconductor layer  740 , substrate  700  may be doped with pentavalent impurities, such as phosphorous, antimony, arsenic, etc., so as to create free electrons. The doping impurities, e.g. phosphorous, may be implanted from the second surface  702   s  of substrate  700 . Similar to first semiconductor layer  720 , third semiconductor layer  740  may be a heavily doped region, such as a N+ region as shown in  FIG.  7   . 
     After the first and third semiconductor layers ( 720 ,  740 ) are formed, the remaining portion between those two layers may become the second semiconductor layer  730 . As second semiconductor layer  730  remains undoped or slightly n-doped or p-doped without any significant dopant species present, the layer is referred as an intrinsic semiconductor region. Along with the neighboring P+ region (first semiconductor layer  720 ) and N+ region (third semiconductor layer  740 ), the intrinsic semiconductor region (second semiconductor layer  730 ) forms a PIN region. 
     When a reverse bias is applied to the detector, a depletion region (such as depletion region  437  of  FIG.  4 B ) may be formed within the PIN region. In some embodiments, the depletion region may exist almost completely within second semiconductor layer  730 . Accordingly, the thickness of second semiconductor layer  730 , which is one of the factors controlling the thickness of the depletion region, may be determined before the steps A 1  and A 2  based on the characteristics of incoming signal electrons, e.g., the range of emission energy of the incoming electrons. 
     In step A 4 , a first metal layer  710  is formed on the first surface  701   s  of substrate  700  and adjacent to the first semiconductor layer  720 . For example, first metal layer  710  may be deposited on top of first semiconductor layer  720  (the P+ region) after doping impurities are introduced into substrate  700 . 
     First metal layer  710  may be configured to receive signal electrons (not shown). Accordingly, first metal layer  710  may be thin and made of light metal to reduce scattering and the energy loss of the incoming electrons. For example, a material of first metal layer  710  may be aluminum or other metal that is highly conductive and easily penetrable by signal electrons. The thickness and the material of first metal layer  710  may be determined based on a consideration of blocking of particles other than incident electrons to reduce noise or filtering out some signal electrons based on their emission energy (e.g., filtering out SEs with very low emission energy). 
     In step A 5 , a second metal layer  750  is formed on the second surface  702   s  of substrate  700  and adjacent to the third semiconductor layer  740 . For example, second metal layer  750  may be deposited on top of second semiconductor layer  740  (the N+ region) after doping impurities are introduced into substrate  700 . A material of second metal layer  750  may be metal with high surface conductivity, such as aluminum or copper. Unlike first metal layer  710 , in some embodiments, second metal layer  750  may not need to be highly electron penetrable. 
     Although  FIG.  7    illustrate a method describing an exemplary order of manufacturing process forming an electron detector, it is appreciated that some steps could be reordered. For example, the third semiconductor layer  740  may be formed before the first semiconductor layer  720 . The first metal layer  710  may be formed before the second metal layer  750 . 
     Aspects of the present disclosure are set out in the following numbered clauses:
         1. An electron detector for detecting a plurality of signal electrons generated from a sample, comprising:   a first semiconductor layer having a first portion and a second portion;   a second semiconductor layer adjacent to the first semiconductor layer;   a third semiconductor layer adjacent to the second semiconductor layer;   a PIN region formed by the first, second, and third semiconductor layers;   a power supply configured to apply a reverse bias between the first and the third semiconductor layers; and   a depletion region formed within the PIN region by the reverse bias, the depletion region comprising a portion of the second semiconductor layer, and the depletion region configured to generate a detector signal based on a first subset of the plurality of signal electrons captured within the depletion region,   wherein the second portion of the first semiconductor layer is not depleted and is configured to provide an energy barrier to block a second subset of the plurality of signal electrons and to allow the first subset of the plurality of signal electrons to pass through to reach the depletion region.   2. The detector of clause 1, wherein the depletion region further comprises the first portion of the first semiconductor layer.   3. The detector of any one of clauses 1 and 2, wherein the depletion region further comprises a portion of the third semiconductor layer.   4. The detector of any one of clauses 1-3, wherein the portion of the second semiconductor layer comprises an entirety of the second semiconductor layer.   5. The detector of any one of clauses 1-4, wherein the first subset of the plurality of signal electrons comprises electrons having sufficiently high energy to pass through the energy barrier.   6. The detector of any one of clauses 1-5, wherein the second subset of the plurality of signal electrons comprises electrons having insufficient energy to pass through the energy barrier.   7. The detector of any one of clauses 1-6, wherein the detector signal is further influenced by a first set of internal electrons that are generated by interactions between the plurality of signal electrons and the second portion of the first semiconductor layer.   8. The detector of clause 7, wherein the first set of internal electrons comprises electrons having sufficiently high energy to pass through the energy barrier.   9. The detector of any one of clauses 7 and 8, wherein the second portion of the first semiconductor layer is further configured to prevent a second set of internal electrons from reaching the depletion region, wherein the second set of internal electrons are generated by interactions between the plurality of signal electrons and the second portion of the first semiconductor layer and have insufficient energy to pass through the energy barrier.   10. The detector of any one of clauses 1-9, wherein the first semiconductor layer is doped with a dopant, and a cutoff energy level of the energy barrier of the first semiconductor layer is determined by a doping concentration in the first semiconductor layer, a thickness of the first semiconductor layer, or a reverse bias voltage applied by the power supply.   11. The detector of any one of clauses 1-10, wherein the detector signal is generated based on an electron-hole pair produced within the depletion region by the first subset of the plurality of signal electrons or the first set of internal electrons.   12. The detector of any one of clauses 1-11, wherein the first semiconductor layer is doped with a p-type dopant, and the second and third semiconductor layers are doped with an n-type dopant.   13. The detector of clause 12, wherein an electric potential near the first semiconductor layer is lower than an electric potential near the third semiconductor layer.   14. The detector of any one of clauses 1-11, wherein the first semiconductor layer is doped with an n-type dopant, and the second and third semiconductor layers are doped with a p-type dopant.   15. The detector of clause 14, wherein an electric potential near the first semiconductor layer is higher than an electric potential near the third semiconductor layer.   16. The detector of any one of clauses 1-15, further comprising:   a first electrode adjacent to the first semiconductor layer and coupled to a first terminal of the power supply; and   a second electrode adjacent to the third semiconductor layer and coupled to a second terminal of the power supply.   17. The detector of clause 16, wherein the first electrode is a part of a first metal layer adjacent to the first semiconductor layer.   18. The detector of any one of clauses 16 and 17, wherein the second electrode is a part of a second metal layer adjacent to the third semiconductor layer.   19. An electron detector for detecting a plurality of signal electrons generated from a sample, comprising:   a first semiconductor layer having a first portion and a second portion;   a second semiconductor layer adjacent to the first semiconductor layer;   multiple segments of a third semiconductor layer, each of the multiple segments being adjacent to the second semiconductor layer,   a PIN region formed by the first, second, and third semiconductor layers;   a power supply configured to apply a reverse bias between the first and the third semiconductor layers; and   a depletion region formed within the PIN region by the reverse bias, the depletion region comprising a portion of the second semiconductor layer, and the depletion region configured to generate a plurality of detector signals based on a first subset of the plurality of signal electrons captured within the depletion region,   wherein the second portion of the first semiconductor layer is not depleted and is configured to provide an energy barrier to block a second subset of the plurality of signal electrons and to allow the first subset of the plurality of signal electrons to pass through to reach the depletion region.   20. The detector of clause 19, wherein the depletion region further comprises the first portion of the first semiconductor layer.   21. The detector of any one of clauses 19 and 20, wherein the depletion region further comprises a portion of the third semiconductor layer.   22. The detector of any one of clauses 19-21, wherein the portion of the second semiconductor layer comprises an entirety of the second semiconductor layer.   23. The detector of any one of clauses 19-22, wherein the first subset of the plurality of signal electrons comprises electrons having sufficiently high energy to pass through the energy barrier.   24. The detector of any one of clauses 19-23, wherein the second subset of the plurality of signal electrons comprises electrons having insufficient energy to pass through the energy barrier   25. The detector of any one of clauses 19-24, wherein the plurality of detector signals is further influenced by a first set of internal electrons that are generated by interactions between the plurality of signal electrons and the second portion of the first semiconductor layer.   26. The detector of clause 25, wherein the first set of internal electrons comprises electrons having sufficiently high energy to pass through the energy barrier.   27. The detector of any one of clauses 25 and 26, wherein the second portion of the first semiconductor layer is further configured to prevent a second set of internal electrons from reaching the depletion region, wherein the second set of internal electrons are generated by interactions between the plurality of signal electrons and the second portion of the first semiconductor layer and have insufficient energy to pass through the energy barrier.   28. The detector of any one of clauses 19-27, wherein the first semiconductor layer is doped with a dopant, and a cutoff energy level of the energy barrier of the first semiconductor layer is determined by a doping concentration in the first semiconductor layer, a thickness of the first semiconductor layer, or a reverse bias voltage applied by the power supply.   29. The detector of any one of clauses 19-28, wherein the plurality of detector signals is generated based on electron-hole pairs produced within the depletion region by the first subset of the plurality of signal electrons or the first set of internal electrons.   30. The detector of clause 29, wherein one of the plurality of detector signals is generated based on a subset of the electron-hole pairs captured by a corresponding segment of the multiple segments of third semiconductor layer.   31. The detector of clause 30, wherein the corresponding segment is the closest one from a position where the subset of the electron-hole pairs are generated.   32. The detector of clause 30, wherein the corresponding segment is determined by an electric field generated by the reverse bias within the PIN region.   33. The detector of any one of clauses 19-32, wherein the first semiconductor layer is doped with a p-type dopant, and the second semiconductor layer and the multiple segments of third semiconductor layer are doped with an n-type dopant.   34. The detector of clause 33, wherein an electric potential near the first semiconductor layer is lower than an electric potential near the multiple segments of third semiconductor layer.   35. The detector of any one of clauses 19-32, wherein the first semiconductor layer is doped with an n-type dopant, and the second semiconductor layer and the multiple segment of third semiconductor layer are doped with a p-type dopant.   36. The detector of clause 35, wherein an electric potential near the first semiconductor layer is higher than an electric potential near the multiple segments of third semiconductor layer.   37. The detector of any one of clauses 19-36, further comprising:   a first electrode adjacent to the first semiconductor layer and coupled to a first terminal of the power supply;   a second electrode adjacent to one of the multiple segments of the third semiconductor layer and coupled to a second terminal of the power supply; and   a third electrode adjacent to another one of the multiple segments of the third semiconductor layer and coupled to the second terminal of the power supply.   38. The detector of clause 37, wherein the first electrode comprises is a part of a first metal layer adjacent to the first semiconductor layer.   39. The detector of any one of clauses 37 and 38, wherein the second electrode and the third electrode are parts of a second metal layer adjacent to the third semiconductor layer.   40. A method for manufacturing an electron detector having an energy barrier that filters out electrons based on the electrons&#39; energy, the method comprising:   providing a semiconductor substrate having:
           a first portion;   a second portion adjacent to the first portion; and   a third portion adjacent to the second portion;   
           forming a first semiconductor layer by doping the first portion of the substrate with a first type of dopant;   forming a third semiconductor layer by doping the third portion of the substrate with a second type of dopant; and   forming a second semiconductor layer in the second portion of the substrate,   wherein a doping concentration of the first type of dopant in the first semiconductor layer is determined to configure the energy barrier of the electron detector, and   a thickness of the first semiconductor layer is determined to further configure the energy barrier of the electron detector.   41. The method of clause 40, wherein the second semiconductor layer is formed in the second portion of the substrate after the first and third portion of the substrate are doped.   42. The method of any one of clauses 40 and 41, wherein the third semiconductor layer has a doping concentration that is higher than a doping concentration in the second semiconductor layer.   43. The method of any one of clauses 40-42, further comprising forming a first metal layer adjacent to the first semiconductor layer.   44. The method of clause 43, wherein the first metal layer is configured to accept a connection from a first terminal of a power supply.   45. The method of clause 44, further comprising forming a second metal layer adjacent to the third semiconductor layer.   46. The method of clause 45, wherein the second metal layer is configured to accept a connection from a second terminal of the power supply.   47. The method of any one of clauses 43-46, wherein the first metal layer comprises a first electrode configured to function as an anode or a cathode.   48. The method of any one of clauses 43-47, wherein the second metal layer comprises a second electrode configured to function as a cathode or an anode.   49. A charged particle beam apparatus for inspecting a sample, comprising:   a charge particle beam source configured to emit a charged particle beam along a primary optical axis;   an objective lens configured to focus the charged particle beam onto the sample; and   an electron detector, according to any one of clauses 1-39, configured to detect a plurality of signal electrons generated from incidence of the charged particle beam onto the sample.   50. The apparatus of clause 49, further comprising a passive energy filter between the electron detector and the sample.   51. The apparatus of clause 49, further comprising a passive energy filter movable between a first position and a second position, wherein:   when the apparatus operates in a high filtering mode, the passive energy filter is positioned in the first position between the sample and the electron detector and is configured to filter out a first subset of the plurality of signal electrons, and   when the apparatus operates in a low filtering mode, the passive energy filter is positioned in the second position away from the electron detector and is configured to allow a second subset of the plurality of signal electrons to pass through to the electron detector, wherein the second subset of the plurality of signal electrons comprises electrons having a similar energy level as the first subset of the plurality of signal electrons.   52. The apparatus of clause 51, wherein the passive energy filter is configured to provide a first external energy barrier in addition to the energy barrier of the electron detector to filter out the subset of the plurality of signal electrons.   53. The apparatus of clause 52, wherein the apparatus, when operating in a high filtering mode, is configured to filter out the subset of the plurality of signal electrons having energies lower than a sum of the energy barrier of the electron detector and the a first externa energy barrier of the passive energy filter.   54. The apparatus of clause 49, further comprising a passive energy filter movable between a first position, a second position, and a third position, the passive energy filter comprising a first filtering zone with a first external energy barrier and a second filtering zone with a second external energy barrier.   55. The apparatus of clause 54, wherein:   when the apparatus operates in a first filtering mode, the passive energy filter is positioned in the first position so that the first filtering zone is positioned between the sample and the electron detector, and is configured to provide the first external energy barrier in addition to the energy barrier of the electron detector,   when the apparatus operates in a second filtering mode, the passive energy filter is positioned in the second position so that the second filtering zone is positioned between the sample and the electron detector, and is configured to provide the second external energy barrier in addition to the energy barrier of the electron detector, and   when the apparatus operates in a third filtering mode, the passive energy filter is positioned in the third position away from the electron detector.   56. The apparatus of any one of clauses 54 and 55, the first external energy barrier is higher than the second external energy barrier.   57. The apparatus of any one of clauses 50-56, wherein the passive energy filter comprises any material capable of attenuating an energy of the plurality of signal electrons.   58. The apparatus of any one of clauses 50-57, wherein the passive energy filter comprises semiconductor materials or conductor materials.   59. The apparatus of clause 49, further comprising a passive energy filter comprising a plurality of filtering segments, each of the plurality of filtering segments is configured to provide a different level of energy barriers.   60. The apparatus of clause 59, wherein the passive energy filter is movable.   61. A charged particle beam apparatus for inspecting a sample, comprising:   a charge particle beam source configured to emit a charged particle beam along a primary optical axis;   an objective lens configured to focus the charged particle beam onto the sample;   an electron detector configured to detect a plurality of signal electrons generated from incidence of the charged particle beam onto the sample; and   a passive energy filter between the electron detector and the sample.   62. The apparatus of clause 61, wherein the passive energy filter is movable between a first position and a second position, wherein:   when the apparatus operates in a high filtering mode, the passive energy filter is positioned in the first position between the sample and the electron detector and is configured to filter out a subset of the plurality of signal electrons, and   when the apparatus operates in a low filtering mode, the passive energy filter is positioned in the second position away from the electron detector and is configured to allow the subset of the plurality of signal electrons to pass through to the electron detector.   63. The apparatus of any one of clauses 61 and 62, wherein the electron detector comprises an electron detector according to any one of clauses 1-39.   64. The apparatus of any one of clauses 61-63, wherein the passive energy filter is configured to provide a first external energy barrier to filter out the subset of the plurality of signal electrons   65. The apparatus of clause 64, wherein the apparatus, when operating in a high filtering mode, is configured to filter out the subset of the plurality of signal electrons having energies lower than the first external energy barrier of the passive energy filter.   66. The apparatus of any one of clauses 61-65, wherein the passive energy filter comprises a plurality of filtering segments, each of the plurality of filtering segments is configured to provide a different level of energy barrier.   67. The apparatus of any one of clauses 61-66, wherein the passive energy filter comprises material capable of attenuating an energy of the plurality of signal electrons.   68. The apparatus of any one of clauses 61-67, wherein the passive energy filter comprises semiconductor materials or conductor materials.   69. An electron detector for detecting a plurality of signal electrons generated from a sample, comprising:   a first semiconductor layer having a first portion and a second portion;   a second semiconductor layer adjacent to the first semiconductor layer;   a third semiconductor layer adjacent to the second semiconductor layer;   a PIN region formed by the first, second, and third semiconductor layers; and   a depletion region formed by a reverse bias applied to the PIN region, the depletion region comprising a portion of the second semiconductor layer, and the depletion region configured to generate a detector signal based on a first subset of the plurality of signal electrons captured within the depletion region,   wherein the second portion of the first semiconductor layer is not depleted and is configured to provide an energy barrier to block a second subset of the plurality of signal electrons and to allow the first subset of the plurality of signal electrons to pass through to reach the depletion region.   70. The detector of clause 69, wherein the depletion region further comprises the first portion of the first semiconductor layer.   71. The detector of any one of clauses 69 and 70, wherein the depletion region further comprises a portion of the third semiconductor layer.   72. The detector of any one of clauses 69-71, wherein the portion of the second semiconductor layer comprises an entirety of the second semiconductor layer.   73. The detector of any one of clauses 69-72, wherein the first subset of the plurality of signal electrons comprises electrons having sufficiently high energy to pass through the energy barrier.   74. The detector of any one of clauses 69-73, wherein the second subset of the plurality of signal electrons comprises electrons having insufficient energy to pass through the energy barrier.   75. The detector of any one of clauses 69-74, wherein the detector signal is generated based on a first set of internal electrons that are generated by interactions between the plurality of signal electrons and the second portion of the first semiconductor layer.   76. The detector of clause 75, wherein the first set of internal electrons comprises electrons having sufficiently high energy to pass through the energy barrier.   77. The detector of any one of clauses 75 and 76, wherein the second portion of the first semiconductor layer is further configured to prevent a second set of internal electrons from reaching the depletion region, wherein the second set of internal electrons are generated by interactions between the plurality of signal electrons and the second portion of the first semiconductor layer and have insufficient energy to pass through the energy barrier.   78. The detector of any one of clauses 69-77, wherein the first semiconductor layer is doped with a dopant, and a cutoff energy level of the energy barrier of the first semiconductor layer is determined by a doping concentration in the first semiconductor layer or a thickness of the first semiconductor layer.   79. The detector of any one of clauses 69-78, wherein the detector signal is generated based on an electron-hole pair produced within the depletion region by the first subset of the plurality of signal electrons.   80. An electron detector for detecting a plurality of signal electrons generated from a sample, comprising:   a first semiconductor layer having a first portion and a second portion;   a second semiconductor layer adjacent to the first semiconductor layer;   multiple segments of a third semiconductor layer, each of the multiple segments being adjacent to the second semiconductor layer,   a PIN region formed by the first, second, and third semiconductor layers; and   a depletion region formed by a reverse bias applied to the PIN region, the depletion region comprising a portion of the second semiconductor layer, and the depletion region configured to generate a plurality of detector signals based on a first subset of the plurality of signal electrons captured within the depletion region,   wherein the second portion of the first semiconductor layer is not depleted and is configured to provide an energy barrier to block a second subset of the plurality of signal electrons and to allow the first subset of the plurality of signal electrons to pass through to reach the depletion region.   81. The detector of clause 80, wherein the depletion region further comprises the first portion of the first semiconductor layer.   82. The detector of any one of clauses 80 and 81, wherein the depletion region further comprises a portion of the third semiconductor layer.   83. The detector of any one of clauses 80-82, wherein the portion of the second semiconductor layer comprises an entirety of the second semiconductor layer.   84. The detector of any one of clauses 80-83, wherein the first subset of the plurality of signal electrons comprises electrons having sufficiently high energy to pass through the energy barrier.   85. The detector of any one of clauses 80-84, wherein the second subset of the plurality of signal electrons comprises electrons having insufficient energy to pass through the energy barrier.   86. The detector of any one of clauses 80-85, wherein the plurality of detector signals is generated based on a first set of internal electrons that are generated by interactions between the plurality of signal electrons and the second portion of the first semiconductor layer.   87. The detector of clause 86, wherein the first set of internal electrons comprises electrons having sufficiently high energy to pass through the energy barrier.   88. The detector of any one of clauses 86 and 87, wherein the second portion of the first semiconductor layer is further configured to prevent a second set of internal electrons from reaching the depletion region, wherein the second set of internal electrons are generated by interactions between the plurality of signal electrons and the second portion of the first semiconductor layer and have insufficient energy to pass through the energy barrier.   89. The detector of any one of clauses 80-88, wherein the first semiconductor layer is doped with a dopant, and a cutoff energy level of the energy barrier of the first semiconductor layer is determined by a doping concentration in the first semiconductor layer or a thickness of the first semiconductor layer.   90. The detector of any one of clauses 80-89, wherein the plurality of detector signals is generated based on electron-hole pairs produced within the depletion region by the first subset of the plurality of signal electrons.   91. The detector of clause 90, wherein one of the plurality of detector signals is generated based on a subset of the electron-hole pairs captured by a corresponding segment of the multiple segments of third semiconductor layer.   92. The detector of clause 91, wherein the corresponding segment is the closest one from a position where the subset of the electron-hole pairs are generated.   93. The detector of clause 91, wherein the corresponding segment is determined by an electric field generated by the reverse bias within the PIN region.       

     A non-transitory computer readable medium may be provided that stores instructions for a processor of a controller (e.g., controller  50  of  FIG.  1   ) to carry out charged particle beam detection, image processing, adjusting bias voltage, switching between various filtering modes, moving an external passive energy filter (such as filter  632  of  FIGS.  6 A- 6 E ) or other functions and methods consistent with the present disclosure, etc. Common forms of non-transitory media include, for example, a floppy disk, a flexible disk, hard disk, solid state drive, magnetic tape, or any other magnetic data storage medium, a Compact Disc Read Only Memory (CD-ROM), any other optical data storage medium, any physical medium with patterns of holes, a Random Access Memory (RAM), a Programmable Read Only Memory (PROM), and Erasable Programmable Read Only Memory (EPROM), a FLASH-EPROM or any other flash memory, Non-Volatile Random Access Memory (NVRAM), a cache, a register, any other memory chip or cartridge, and networked versions of the same. 
     It will be appreciated that the embodiments of the present disclosure are not limited to the exact construction that has been described above and illustrated in the accompanying drawings, and that various modifications and changes may be made without departing from the scope thereof. The present disclosure has been described in connection with various embodiments, other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 
     The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made as described without departing from the scope of the claims set out below.