Systems and methods for beam alignment in multi charged-particle beam systems

Abstract

Systems and methods of aligning beams with an aperture array for multi-beam inspection of a sample are disclosed. The method comprises the steps of forming a plurality of primary charged-particle beamlets from a charged-particle beam, positioning a movable aperture plate upstream from a first aperture array in a first position, wherein in the first position, apertures of the movable aperture plate are aligned with corresponding apertures of the first aperture array, adjusting a path of the primary charged-particle beamlets, using a charged-particle beam alignment deflector, to enable a portion of charged particles of the primary charged-particle beamlets to pass through the aligned apertures, resulting in an alignment between the beamlets and the corresponding apertures of the first aperture array, and moving the movable aperture plate in a second position to enable the primary charged-particle beamlets to pass through the first aperture array without passing through the movable aperture plate.

Description

SYSTEMS AND METHODS FOR BEAM ALIGNMENT IN MULTI CHARGED-PARTICLE BEAM SYSTEMSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 538,030 which was filed on September 12, 2023 and which is incorporated herein in its entirety by reference.TECHNICAL FIELD

[0002] The description herein relates to the field of charged-particle beam systems, and more particularly to systems and methods for alignment of charged-particle beams in a multi-beam apparatus used for inspection of samples.BACKGROUND

[0003] In manufacturing processes of integrated circuits (ICs), unfinished or finished circuit components are inspected to ensure that they are manufactured according to design and are substantially free of defects. Inspection systems utilizing optical microscope typically have resolution down to a few hundred nanometers; and the resolution is limited by the wavelength of light. As the physical sizes of IC components continue to reduce down to sub- 100 or even sub- 10 nanometers, inspection systems capable of higher resolution than those utilizing optical microscopes are needed.

[0004] A charged-particle (e.g., electron) beam microscope, such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM), capable of resolution down to less than a nanometer, serves as a practicable tool for inspecting IC components having a feature size that is sub- 100 nanometers. With a SEM, electrons of a single primary electron beam, or electrons of a plurality of primary electron beams, can be focused on locations of interest of a semiconductor wafer under inspection. The primary electrons interact with the semiconductor wafer and the layers deposited thereon and may cause the wafer to emit secondary electrons or backscattered electrons, among other things. The intensity of the electron beams comprising the backscattered electrons and the secondary electrons forming a SEM image may vary based on the characteristics of the incident primary electron beamlets, and thereby may indicate whether the inspected region of the wafer has defects. To improve defect detection performance, accurate alignment of the primary electron beamlets in the SEM column is desired.SUMMARY

[0005] Embodiments of the present disclosure provide apparatuses, systems, and methods of aligning charged-particle beams with an aperture array for multi-beam inspection of a sample.

[0006] One aspect of the present disclosure is directed to a multi-beam apparatus comprisinga charged-particle source configured to emit charged particles for forming a plurality of primary charged-particle beamlets, a charged-particle beam alignment deflector configured to influence a path of the plurality of primary charged-particle beamlets, a first aperture array, each aperture of the first aperture array configured to allow a portion of charged particles of a primary charged-particle beamlet to pass through, and a movable aperture plate comprising at least two apertures, the aperture plate located between the charged-particle beam alignment deflector and the first aperture array, wherein each of the at least two apertures of the movable aperture plate is aligned with a corresponding aperture of the first aperture array.

[0007] Another aspect of the present disclosure is directed to a method of aligning charged-particle beams for multi-beam inspection of a sample. The method includes forming a plurality of primary charged-particle beamlets from a charged-particle beam, positioning a movable aperture plate upstream from a first aperture array in a first position, wherein in the first position, apertures of the movable aperture plate are aligned with corresponding apertures of the first aperture array, adjusting a path of the plurality of primary charged-particle beamlets, using a charged-particle beam alignment deflector, to enable a portion of charged particles of the primary charged-particle beamlets to pass through the aligned apertures, resulting in an alignment between the primary charged-particle beamlets and the corresponding apertures of the first aperture array, and moving the movable aperture plate in a second position to enable the plurality of primary charged-particle beamlets to pass through the first aperture array without passing through the movable aperture plate.

[0008] Yet another aspect of the present disclosure is directed to a non-transitory computer readable medium storing a set of instructions that is executable by one or more processors of a charged-particle beam apparatus to cause the charged-particle beam apparatus to perform operations comprising activating a charged-particle source to emit charged particles and forming a plurality of primary charged-particle beamlets from the emitted charged particles, positioning a movable aperture plate upstream from a first aperture array in a first position, wherein in the first position, apertures of the movable aperture plate are aligned with corresponding apertures of the first aperture array, adjusting a path of the plurality of primary charged-particle beamlets, using a charged-particle beam alignment deflector, to enable a portion of charged particles of the primary charged-particle beamlets to pass through the aligned apertures, resulting in an alignment between the primary charged-particle beamlets and the corresponding apertures of the first aperture array, and moving the movable aperture plate in a second position to enable the plurality of primary charged-particle beamlets to pass through the first aperture array without passing through the movable aperture plate.

[0009] Other advantages of the embodiments of the present disclosure will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the present invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Fig. 1 is a schematic diagram illustrating an exemplary electron beam inspection (EBI) system, consistent with embodiments of the present disclosure.

[0011] Fig. 2 is a schematic diagram illustrating an exemplary multi-beam system that is part of the exemplary charged particle beam inspection system of Fig. 1, consistent with embodiments of the present disclosure.

[0012] Fig. 3 is a schematic diagram illustrating an exemplary multi-beam inspection system using an exemplary beam alignment method.

[0013] Fig. 4A is a schematic diagram illustrating alignment between apertures of an aperture array of a source conversion unit and corresponding charged-particle beam spots in a 5x5 array.

[0014] Fig. 4B shows exemplary images of alignment between apertures of a source conversion unit and corresponding charged-particle beamlet profiles in a 5x5 array.

[0015] Fig. 5 illustrates an exemplary apparatus including a movable aperture plate, consistent with embodiments of the present disclosure.

[0016] Figs. 6A-6C are schematic illustrations of an exemplary movable aperture plate of apparatus shown in Fig. 5, consistent with embodiments of the present disclosure.

[0017] Fig. 7 is a schematic illustration of an aperture plate disposed over the apertures of the source conversion unit, consistent with embodiments of the present disclosure.

[0018] Fig. 8 illustrates an exemplary apparatus including a movable aperture plate configured to be rotated in and out of an alignment position, consistent with embodiments of the present disclosure.

[0019] Fig. 9A illustrates an exemplary apparatus including a source conversion unit comprising a plurality of aperture arrays, consistent with embodiments of the present disclosure.

[0020] Fig. 9B illustrates an exemplary aperture plate comprising two aperture arrays, consistent with embodiments of the present disclosure.

[0021] Fig. 10 is a process flowchart for a method of aligning beamlets of a multi-beam inspection system with apertures of a source conversion unit, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0022] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations consistent with the disclosure. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the subject matter 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 beamsmay be similarly applied. Furthermore, other imaging systems may be used, such as optical imaging, photodetection, x-ray detection, extreme ultraviolet inspection, deep ultraviolet inspection, or the like.

[0023] 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.

[0024] Making these extremely small ICs is a complex, time-consuming, and expensive process, often involving hundreds of individual steps. Errors in even one step have the potential to result in defects in the finished IC rendering it useless. Thus, one goal of the manufacturing process is to avoid such defects to maximize the number of functional ICs made in the process, that is, to improve the overall yield of the process.

[0025] 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 may be carried out using a scanning electron microscope (SEM). A SEM can be used to image these extremely small structures, in effect, taking a “picture” of the structures of the wafer. The image can be used to determine if the structure was formed properly and also if it was formed at the proper location. If the structure is defective, then the process can be adjusted so the defect is less likely to recur. Defects may be generated during various stages of semiconductor processing. For the reason stated above, it is important to find defects accurately, efficiently, and as early as possible.

[0026] The working principle of a SEM is similar to a camera. A camera takes a picture by receiving and recording brightness and colors of light reflected or emitted from people or objects. A SEM takes a “picture” by receiving and recording energies or quantities of electrons reflected or emitted from the structures. Before taking such a “picture,” an electron beam may be provided onto the structures, and when the electrons are reflected or emitted (“exiting”) from the structures, a detector of the SEM may receive and record the energies or quantities of those electrons to generate an image. To take such a “picture,” some SEMs use a single electron beam (referred to as a “single-beam SEM”), while some SEMs use multiple electron beams (referred to as a “multi-beam SEM”) to take multiple “pictures” of the wafer. By using multiple electron beams, the SEM may provide more electron beams onto the structures for obtaining these multiple “pictures,” resulting in more electrons exiting from the structures. Accordingly, the detector may receive more exiting electrons simultaneously, and generate images of the structures of the wafer with a higher efficiency and a faster speed.

[0027] SEM images are often relied upon for physical defect inspection of a semiconductor wafer during fabrication of IC components. In some applications, high-resolution SEM images are desirable to allow detection of sub-micron or nano-sized defects (less than a few nanometers). In normaloperation of a SEM, a primary beam or a cloud of electrons is generated by an electron source and the generated beam is directed towards the sample through the SEM column. In a multi-beam SEM, the primary beam may be split into multiple beamlets using an aperture plate (e.g., a Coulomb aperture array), each beamlet having a characteristic beam current and traversing along a path in the SEM column. Each of the primary electron beamlets may pass through a number of electrostatic or electromagnetic lenses and apertures in the SEM column before impinging on a desired region of the wafer with the desired beam current. The primary electron beamlets having beam characteristics such as, but not limited to, appropriate beam current, uniform beam diameter, desired beam shape, beam focus, or beam energy, may be referred to as “probing beamlets.” The interaction of probing beamlets with the sample generates secondary electrons and backscattered electrons, which upon detection by a charged-particle detector (not illustrated in Figs. 3-8), provide information associated with the interaction volume of the sample, including defect information, if present.

[0028] As previously described, high-resolution SEM images are desirable for detecting defects with high detection efficiency or high defect-capture rate, and in particular, detection of sub-micron or sub- nano sized defects. In multi-beam inspection, one of several features useful in obtaining high- resolution SEM images is a constant beam current of all the probing beamlets. In images generated by signals received from probe spots irradiated by probing beamlets having a constant beam current, similar features may be represented by pixels having similar gray scale values across the image. As a result, the difference of gray scale values of a defect feature and a non-defect feature in the generated image may be substantially consistent, rendering the defect detection technique (e.g., manual, machine-assisted, or a combination thereof) more efficient. However, producing multiple probing beamlets from corresponding primary electron beamlets such that all probing beamlets have a constant beam current is challenging.

[0029] In some existing multi-beam SEM tools, primary electron beamlets, after being condensed and collimated by a condenser lens (e.g., condenser lens 310 of Fig. 3), may pass through a currentlimiting aperture array comprising apertures configured to define the size, and thereby the current of the beamlets passing through to form the probing beamlets having a desired beam current. The probing beamlets exiting the current-limiting aperture array may pass through one or more aperture arrays such as an image-forming means (e.g., micro-deflector array, a micro-compensator array) and a beamlet-limiting means, before being focused on a sample surface by an objective lens (e.g., objective lens 331 of Fig. 3). It is therefore desirable to establish an accurate alignment between the primary beamlets and the image-forming means and the beamlet-limiting means to maintain the desired beam current of the probing beamlets while passing therethrough. Minor misalignments can cause obstruction of electrons passing through, resulting in a reduction of beam current from the desired beam current, or non-uniformity in beam currents of the multiple probing beamlets, thus negatively impacting the obtainable image resolution and defect detection. Therefore, it may be desirable toprovide systems and methods for alignment of the primary beamlets or the electron source with the apertures to at least improve the image resolution and defect detectability using SEM images.

[0030] Existing procedures include a beam alignment step in which the primary electron beamlets in a multi-beam SEM tool are scanned across a stack of micro-deflector arrays and micro-lens arrays (e.g., a microelectromechanical systems (MEMS) stack) to align the beamlets with the apertures of the stack. The alignment between the beamlets and their corresponding apertures may be determined based on the images obtained from each of the primary beamlet impinging on the sample. In practice, however, because the beam diameter of primary electron beamlets is substantially similar to the size of the apertures through which they are configured to pass, the alignment inspection images are blur, and cannot be relied on by an operator to establish proper alignment, rendering the alignment process inefficient and misleading. Further, in the existing beam-alignment procedure, alignment inspection images from each beamlet are obtained sequentially and not in parallel, rendering the process slow and time-consuming. As an example, for a 5x5 array of primary electron beamlets, a total of 25 alignment inspection images are captured sequentially by an operator. Furthermore, even if an acceptable alignment is established, there may be several challenges in maintaining the alignment for extended periods of time due to beam drift issues caused due to electrical instability, charging, mechanical vibrations, among other things. Therefore, it may be desirable to provide systems and methods for improving the alignment accuracy between the primary electron beamlets and the apertures and lenses of the SEM column by, for example, providing an aperture plate upstream from the MEMS apertures with apertures smaller in size than the MEMS apertures.

[0031] Some embodiments of the present disclosure are directed to systems and methods for improving beam alignment accuracy between the electron beams and a source conversion unit configured to generate probing beamlets from primary electron beamlets. High alignment accuracy allows a user to obtain high-resolution images for defect inspection, for example. The method includes positioning a movable aperture plate in position such that apertures of the movable aperture plate and underlying apertures of an aperture array of a source conversion unit are aligned. The apertures of the movable aperture plate are smaller than and concentric with the corresponding apertures of the aperture array of the source conversion unit. A beam scanning deflector is used to scan the primary electron beamlets across the aligned apertures of the aperture plate and the aperture array. The source conversion unit is configured to generate probing beamlets from corresponding primary electron beamlets. The probing beamlets impinge on the sample, resulting in secondary electrons being emitted from the sample, and detected by a detector. The method further includes determining a setting of the beam scan deflector that aligns the primary beamlets to the apertures of the movable aperture plate based on the secondary electrons detected by a charged-particle detector. Because the beamlets are aligned with the small apertures of the aperture plate, more accurate alignment of the apertures with the source conversion unit can be accomplished, leading to images with high resolution. The movable aperture plate can also be used to prevent particles from fallingonto the source conversion unit during servicing, transportation, shipment, etc. of the SEM, and preventing electrical failures such as, charging, electrical shorts. After the beams are aligned, the movable aperture plate is retracted or pushed away from the path of the primary electron beamlets such that all the primary electron beamlets may enter the source conversion unit and impinge on the sample.

[0032] 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.

[0033] Fig. 1 illustrates an exemplary electron beam inspection (EBI) system 100 consistent with embodiments of the present disclosure. EBI system 100 may be used for imaging. As shown in Fig. 1, EBI system 100 includes a main chamber 101, a load / lock chamber 102, an electron beam tool 104, and an equipment front end module (EFEM) 106. Electron beam tool 104 is located within main chamber 101. EFEM 106 includes a first loading port 106a and a second loading port 106b. EFEM 106 may include additional loading port(s). First loading port 106a and second loading port 106b receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other material(s)) or samples to be inspected (wafers and samples may be used interchangeably). A “lot” is a plurality of wafers that may be loaded for processing as a batch.

[0034] One or more robotic arms (not shown) in EFEM 106 may transport the wafers to load / lock chamber 102. Load / lock chamber 102 is connected to a load / lock vacuum pump system (not shown) which removes gas molecules in load / lock chamber 102 to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robotic arms (not shown) may transport the wafer from load / lock chamber 102 to main chamber 101. Main chamber 101 is connected to a main chamber vacuum pump system (not shown) which removes gas molecules in main chamber 101 to reach a second pressure below the first pressure. After reaching the second pressure, the wafer is subject to inspection by electron beam tool 104. Electron beam tool 104 may be a single-beam system or a multi-beam system.

[0035] A controller 109 is electronically connected to electron beam tool 104. Controller 109 may be a computer configured to execute various controls of EBI system 100. While controller 109 is shown in Fig. 1 as being outside of the structure that includes main chamber 101, load / lock chamber 102, and EFEM 106, it is appreciated that controller 109 may be a part of the structure.

[0036] In some embodiments, controller 109 may include one or more processors (not shown). A processor may be a generic or specific electronic device capable of manipulating or processing information. For example, the processor may include any combination of any number of a central processing unit (or “CPU”), a graphics processing unit (or “GPU”), an optical processor, a programmable logic controllers, a microcontroller, a microprocessor, a digital signal processor, an intellectual property (IP) core, a Programmable Logic Array (PLA), a Programmable Array Logic (PAL), a Generic Array Logic (GAL), a Complex Programmable Logic Device (CPLD), a Field- Programmable Gate Array (FPGA), a System On Chip (SoC), an Application-Specific Integrated Circuit (ASIC), and any type circuit capable of data processing. The processor may also be a virtual processor that includes one or more processors distributed across multiple machines or devices coupled via a network.

[0037] In some embodiments, controller 109 may further include one or more memories (not shown). A memory may be a generic or specific electronic device capable of storing codes and data accessible by the processor (e.g., via a bus). For example, the memory may include any combination of any number of a random-access memory (RAM), a read-only memory (ROM), an optical disc, a magnetic disk, a hard drive, a solid-state drive, a flash drive, a security digital (SD) card, a memory stick, a compact flash (CF) card, or any type of storage device. The codes may include an operating system (OS) and one or more application programs (or “apps”) for specific tasks. The memory may also be a virtual memory that includes one or more memories distributed across multiple machines or devices coupled via a network.

[0038] Reference is now made to Fig. 2, which is a schematic diagram illustrating an exemplary electron beam tool 104 including a multi-beam inspection tool that is part of the EBI system 100 of Fig. 1, consistent with embodiments of the present disclosure. In some embodiments, electron beam tool 104 may be operated as a single-beam inspection tool that is part of EBI system 100 of Fig. 1. Multi-beam electron beam tool 104 (also referred to herein as apparatus 104) comprises an electron source 201, a Coulomb aperture plate (or “gun aperture plate”) 271, a condenser lens 210, a source conversion unit 220, a primary projection system 230, a motorized stage 209, and a sample holder 207 supported by motorized stage 209 to hold a sample 208 (e.g., a wafer or a photomask) to be inspected. Multi-beam electron beam tool 104 may further comprise a secondary projection system 250 and an electron detection device 240. Primary projection system 230 may comprise an objective lens 231. Electron detection device 240 may comprise a plurality of detection elements 241, 242, and 243. A beam separator 233 and a deflection scanning unit 232 may be positioned inside primary projection system 230.

[0039] Electron source 201, Coulomb aperture plate 271, condenser lens 210, source conversion unit 220, beam separator 233, deflection scanning unit 232, and primary projection system 230 may be aligned with a primary optical axis 204 of apparatus 104. Secondary projection system 250 and electron detection device 240 may be aligned with a secondary optical axis 251 of apparatus 104.

[0040] Electron source 201 may comprise a cathode (not shown) and an extractor or anode (not shown), in which, during operation, electron source 201 is configured to emit primary electrons from the cathode and the primary electrons are extracted or accelerated by the extractor and / or the anode to form a primary electron beam 202 that form a primary beam crossover (virtual or real) 203. Primary electron beam 202 may be visualized as being emitted from primary beam crossover 203.

[0041] Source conversion unit 220 may comprise an image-forming element array (not shown), an aberration compensator array (not shown), a beam-limit aperture array (not shown), and a pre-bending micro-deflector array (not shown). In some embodiments, the pre-bending micro-deflector array deflects a plurality of primary beamlets 211, 212, 213 of primary electron beam 202 to normally enter the beam-limit aperture array, the image-forming element array, and an aberration compensator array. In some embodiments, apparatus 104 may be operated as a single -beam system such that a single primary beamlet is generated. In some embodiments, condenser lens 210 is designed to focus primary electron beam 202 to become a parallel beam and be normally incident onto source conversion unit 220. The image-forming element array may comprise a plurality of micro-deflectors or micro-lenses to influence the plurality of primary beamlets 211, 212, 213 of primary electron beam 202 and to form a plurality of parallel images (virtual or real) of primary beam crossover 203, one for each of the primary beamlets 211, 212, and 213. In some embodiments, the aberration compensator array may comprise a field curvature compensator array (not shown) and an astigmatism compensator array (not shown). The field curvature compensator array may comprise a plurality of micro-lenses to compensate field curvature aberrations of the primary beamlets 211, 212, and 213. The astigmatism compensator array may comprise a plurality of micro-stigmators to compensate astigmatism aberrations of the primary beamlets 211, 212, and 213. The beam-limit aperture array may be configured to limit diameters of individual primary beamlets 211, 212, and 213. Fig. 2 shows three primary beamlets 211, 212, and 213 as an example, and it is appreciated that source conversion unit 220 may be configured to form any number of primary beamlets. Controller 109 may be connected to various parts of EBI system 100 of Fig. 1, such as source conversion unit 220, electron detection device 240, primary projection system 230, or motorized stage 209. In some embodiments, as explained in further details below, controller 109 may perform various image and signal processing functions. Controller 109 may also generate various control signals to govern operations of the charged particle beam inspection system.

[0042] Condenser lens 210 is configured to focus primary electron beam 202. Condenser lens 210 may further be configured to adjust electric currents of primary beamlets 211, 212, and 213 downstream of source conversion unit 220 by varying the focusing power of condenser lens 210. Alternatively, the electric currents may be changed by altering the radial sizes of beam-limit apertures within the beam-limit aperture array corresponding to the individual primary beamlets. The electric currents may be changed by both altering the radial sizes of beam-limit apertures and the focusing power of condenser lens 210. Condenser lens 210 may be an adjustable condenser lens that may beconfigured so that the position of its first principal plane is movable. The adjustable condenser lens may be configured to be magnetic, which may result in off-axis beamlets 212 and 213 illuminating source conversion unit 220 with rotation angles. The rotation angles change with the focusing power or the position of the first principal plane of the adjustable condenser lens. Condenser lens 210 may be an anti-rotation condenser lens that may be configured to keep the rotation angles unchanged while the focusing power of condenser lens 210 is changed. In some embodiments, condenser lens 210 may be an adjustable anti-rotation condenser lens, in which the rotation angles do not change when its focusing power and the position of its first principal plane are varied.

[0043] Objective lens 231 may be configured to focus beamlets 211, 212, and 213 onto a sample 208 for inspection and may form, in the current embodiments, three probe spots 221, 222, and 223 on the surface of sample 208. Coulomb aperture plate 271, in operation, is configured to block off peripheral electrons of primary electron beam 202 to reduce Coulomb effect. The Coulomb effect may enlarge the size of each of probe spots 221, 222, and 223 of primary beamlets 211, 212, 213, and therefore deteriorate inspection resolution.

[0044] Beam separator 233 may, for example, be a Wien filter comprising an electrostatic deflector generating an electrostatic dipole field and a magnetic dipole field (not shown in Fig. 2). In operation, beam separator 233 may be configured to exert an electrostatic force by electrostatic dipole field on individual electrons of primary beamlets 211, 212, and 213. The electrostatic force is equal in magnitude but opposite in direction to the magnetic force exerted by magnetic dipole field of beam separator 233 on the individual electrons. Primary beamlets 211, 212, and 213 may therefore pass at least substantially straight through beam separator 233 with at least substantially zero deflection angles.

[0045] Deflection scanning unit 232, in operation, is configured to deflect primary beamlets 211, 212, and 213 to scan probe spots 221, 222, and 223 across individual scanning areas in a section of the surface of sample 208. In response to incidence of primary beamlets 211, 212, and 213 or probe spots 221, 222, and 223 on sample 208, electrons emerge from sample 208 and generate three secondary electron beams 261, 262, and 263. Each of secondary electron beams 261, 262, and 263 typically comprise secondary electrons (having electron energy < 50eV) and backscattered electrons (having electron energy between 50eV and the landing energy of primary beamlets 211, 212, and 213). Beam separator 233 is configured to deflect secondary electron beams 261, 262, and 263 towards secondary projection system 250. Secondary projection system 250 subsequently focuses secondary electron beams 261, 262, and 263 onto detection elements 241, 242, and 243 of electron detection device 240. Detection elements 241, 242, and 243 are arranged to detect corresponding secondary electron beams 261, 262, and 263 and generate corresponding signals which are sent to controller 109 or a signal processing system (not shown), e.g., to construct images of the corresponding scanned areas of sample 208.

[0046] In some embodiments, detection elements 241, 242, and 243 detect corresponding secondary electron beams 261, 262, and 263, respectively, and generate corresponding intensity signal outputs (not shown) to an image processing system (e.g., controller 109). In some embodiments, each detection element, 241, 242, and 243 may comprise one or more pixels. The intensity signal output of a detection element may be a sum of signals generated by all the pixels within the detection element.

[0047] In some embodiments, controller 109 may comprise image processing system that includes an image acquirer (not shown), a storage (not shown). The image acquirer may comprise one or more processors. For example, the image acquirer may comprise a computer, a server, a mainframe host, terminals, a personal computer, any kind of mobile computing devices, and the like, or a combination thereof. The image acquirer may be communicatively coupled to electron detection device 240 of apparatus 104 through a medium such as an electrical conductor, an optical fiber cable, a portable storage media, IR, Bluetooth, internet, a wireless network, a wireless radio, among others, or a combination thereof. In some embodiments, the image acquirer may receive a signal from electron detection device 240 and may construct an image. The image acquirer may thus acquire images of sample 208. The image acquirer may also perform various post-processing functions, such as generating contours, superimposing indicators on an acquired image, and the like. The image acquirer may be configured to perform adjustments of brightness and contrast, etc. of acquired images. In some embodiments, the storage may be a storage medium such as a hard disk, flash drive, cloud storage, random access memory (RAM), other types of computer readable memory, and the like. The storage may be coupled with the image acquirer and may be used for saving scanned raw image data as original images, and post-processed images.

[0048] In some embodiments, the image acquirer may acquire one or more images of a sample based on an imaging signal received from electron detection device 240. An imaging signal may correspond to a scanning operation for conducting charged particle imaging. An acquired image may be a single image comprising a plurality of imaging areas. The single image may be stored in the storage. The single image may be an original image that may be divided into a plurality of regions. Each of the regions may comprise one imaging area containing a feature of sample 208. The acquired images may comprise multiple images of a single imaging area of sample 208 sampled multiple times over a time sequence. The multiple images may be stored in the storage. In some embodiments, controller 109 may be configured to perform image processing steps with the multiple images of the same location of sample 208.

[0049] In some embodiments, controller 109 may include measurement circuitries (e.g., analog-to- digital converters) to obtain a distribution of the detected secondary electrons. The electron distribution data collected during a detection time window, in combination with corresponding scan path data of each of primary beamlets 211, 212, and 213 incident on the wafer surface, can be used to reconstruct images of the wafer structures under inspection. The reconstructed images can be used toreveal various features of the internal or external structures of sample 208, and thereby can be used to reveal any defects that may exist in the wafer.

[0050] In some embodiments, controller 109 may control motorized stage 209 to move sample 208 during inspection of sample 208. In some embodiments, controller 109 may enable motorized stage 209 to move sample 208 in a direction continuously at a constant speed. In other embodiments, controller 109 may enable motorized stage 209 to change the speed of the movement of sample 208 overtime depending on the steps of scanning process.

[0051] Although Fig. 2 shows that apparatus 104 uses three primary electron beams, it is appreciated that apparatus 104 may use a greater number of primary electron beams. The present disclosure does not limit the number of primary electron beams used in apparatus 104. For example, apparatus 104 may use a 3x3 array, a 5x5 array, or a m x n array of primary electron beams or beamlets. In some embodiments, apparatus 104 may be a SEM used for lithography, defect inspection, or a combination thereof.

[0052] Compared with a single charged-particle beam imaging system (“single-beam system”), a multiple charged-particle beam imaging system (“multi-beam system”) may be designed to optimize throughput for different scan modes. Embodiments of this disclosure provide a multi-beam system with the capability of optimizing throughput for different scan modes by using beam arrays with different geometries, adapting to different throughputs and resolution requirements. It is to be appreciated that a single -beam system may be used as well to acquire one or more images of a substrate or a region of interest on a semiconductor wafer during fabrication of devices, as appropriate.

[0053] A non-transitory computer readable medium may be provided that stores instructions for a processor (e.g., processor of controller 109 of Figs. 1-2) to carry out image processing, data processing, beamlet scanning, database management, graphical display, operations of a charged particle beam apparatus, or another imaging device, or the like. 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 CD-ROM, any other optical data storage medium, any physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM or any other flash memory, NVRAM, a cache, a register, any other memory chip or cartridge, and networked versions of the same.

[0054] Reference is now made to Fig. 3, which is a schematic illustration of an exemplary charged- particle beam (e.g., electron beam) tool 300. Electron beam tool 300 (also referred to herein as apparatus 300) may be used as a metrology, an inspection, or a review tool configured to detect the presence of physical defects such as micro-bridges between features, discontinuous lines, contact-hole mergers, missing contact holes, line edge roughness, among other defects.

[0055] Apparatus 300 comprises an electron source 301 configured to generate electrons upon activation. In some embodiments, controller 109 may activate, deactivate, or control electron source301 or elements thereof to regulate characteristics of electrons generated by electron source 301 by applying or adjusting one or more electrical signals to electron source 301. Characteristics of generated electrons may include number, intensity, acceleration, direction of emission, among other things. Electron source 301 may be analogous to electron source 201 of Fig. 2 and may include a cathode, an extractor electrode, and an anode (not illustrated in Fig. 3).

[0056] As used in the context of this disclosure, “downstream” refers to a direction along the path of a primary electron beam (e.g., primary electron beam 302 of Fig. 3) starting from the electron source (e.g., electron source 301 of Fig. 3) towards the sample (e.g., sample 308 of Fig. 3). With reference to positioning of an element of a charged-particle beam apparatus (e.g., apparatus 300 of Fig. 3), “downstream” may refer to a position of an element located below or after another element, along the path of primary electron beam starting from the electron source, and “immediately downstream” refers to a position of a second element below or after a first element along the path of primary electron beam 302 such that there are no other active elements between the first and the second element. As used in the context of this disclosure, “upstream” may refer to a position of an element located above or before another element, along the path of primary electron beam starting from the electron source, and “immediately upstream” refers to a position of a second element above or before a first element along the path of primary electron beam 302 such that there are no other active elements between the first and the second element. As used herein, “active element” may refer to any element or component, the presence of which may modify the electromagnetic field between the first and the second element, either by generating an electric field, a magnetic field, or an electromagnetic field.

[0057] As used in the context of this disclosure, “on-axis” refers to a position of an element with respect to the primary optical axis such that a geometric center of the element (e.g., an aperture of an aperture array) coincides or is aligned with the primary optical axis (e.g., primary optical axis 304 of Fig. 3). “Off-axis” refers to a position of an element with respect to the primary optical axis such that a geometric center of the element is a radial distance away from the primary optical axis of the inspection apparatus (e.g., apparatus 300 of Fig. 3). As an example, primary electron beamlet 312 is an on-axis beamlet and primary electron beamlets 311 and 313 are off-axis beamlets.

[0058] Apparatus 300 includes Coulomb aperture array 305 located downstream from electron source 301 to block peripheral electrons of primary electron beam 302 and form primary electron beamlets (e.g., primary electron beamlets 311, 312, and 313). Coulomb aperture array 305 comprises a plate including one or more apertures to allow a portion of primary electron beam 302 to pass through such that the coulombic interactions between electrons in primary electron beam 302 may be minimized while maintaining the desired beam characteristics. The apertures of Coulomb aperture array 305 may be uniform or non-uniform in shape, size, pitch, or cross-section, and may be arranged in a regular pattern to form an array or an irregular pattern. The geometric center of an aperture used to form on-axis primary electron beam 312 may be aligned with primary optical axis 304. Theposition of Coulomb aperture array 305 may be adjustable along an axis substantially perpendicular or substantially parallel to primary optical axis 304. Adjusting the position of Coulomb aperture array 305 horizontally along a plane perpendicular to primary optical axis 304 may allow selection of a desired aperture size or shape. Adjusting the position of Coulomb aperture array 305 vertically along primary optical axis 304 may allow adjustment of the number of electrons allowed to pass through an aperture of Coulomb aperture array 305, thereby adjusting the beam size. The position of Coulomb aperture array 305 may be adjusted horizontally, or vertically, or both, as appropriate.

[0059] Apparatus 300 further includes condenser lens 310 configured to condense primary electron beam 302 after exiting Coulomb aperture array 305. Condenser lens 310 may be an electrostatic, an electromagnetic, or a compound lens. Controller 109 may be configured to adjust excitation of condenser lens 310 by adjusting one or more electrical signals applied to condenser lens 310. Adjusting the excitation of condenser lens 310 allows an adjustment of focusing power of condenser lens 310. Condenser lens 310 may be an adjustable condenser lens that may be configured so that the position of its principal plane (not shown) is adjustable. The adjustable condenser lens (e.g., condenser lens 310) may be electromagnetic. Condenser lens 310 may be an anti-rotation condenser lens, the focusing power of which may be changed without influencing a rotation angle of the electron beam (e.g., primary electron beam 302) passing therethrough. Condenser lens 310 may be an adjustable anti-rotation condenser lens, which may include an anti-rotation lens with an adjustable principal plane. An adjustable anti-rotation lens may comprise two or more lenses, for example.Adjustable condenser lens and anti-rotation lens are further described in International Application No. PCT / EP2017 / 084429, which is incorporated herein by reference in its entirety.

[0060] Condenser lens 310 may be further configured to collimate one or more primary electron beamlets exiting Coulomb aperture array 305. Collimated primary electron beamlets 311, 312, and 313 travel downstream towards the sample along a path substantially parallel to primary optical axis 304. It is to be appreciated that although only three primary electron beamlets are illustrated in Fig. 3, there may be a greater number of beamlets, for example, nine beamlets in a 3x3 array, twenty-five beamlets in a 5x5 array, or any number of arrayed or non-arrayed beamlets, as appropriate.

[0061] Apparatus 300 further includes a beam scanning deflector 320 located downstream from condenser lens 310. Beam scanning deflector 320 may be located immediately downstream from condenser lens 310, as illustrated in Fig. 3. Beam scanning deflector 320 may be configured to deflect a path of the primary electron beamlets by generating an electric field based on the applied voltage, such that they may be scanned horizontally over a region. Beam scanning deflector 320 may comprise a static or a dynamic beam scan deflector. Beam scanning deflector 320 may be configured to influence a path of one or more primary electron beamlets during the initial beam alignment procedure, as discussed previously. In some embodiments, beam scanning deflector 320 may be referred to as a beam alignment deflector. Beam alignment deflector 320 may be activated to deflect the path of the primary electron beamlets during beam alignment and may be deactivated or set at astatic deflection value during sample inspection once the beam alignment is established. In some embodiments, beam alignment deflector 320 may be uninstalled or removed from apparatus 300 after the beam alignment and before beam scanning for sample inspection.

[0062] Apparatus 300 further includes a source conversion unit 330. A source conversion unit is used to generate a plurality of parallel real or virtual images of a single electron source (e.g., electron source 301 of Fig. 3) to form a plurality of images of the source. Each image is formed by one part or beamlet of the primary electron beam generated by the single electron source, and therefore can be taken as one sub-source emitting the one beamlet. Accordingly, the single electron source is virtually changed into a plurality of sub-sources forming a real or virtual multi-source array. Within the source conversion unit, the beamlet intervals are at micron level spacing so as to make more beamlets available, and hence the source conversion unit 330 can be made by semiconductor manufacturing processes or MEMS (Micro Electro-Mechanical Systems) processes. In the context of this disclosure, source conversion unit 330 may also be referred to as a MEMS stack.

[0063] Source conversion unit 330 may comprise an image-forming means and a beamlet-limit means with beamlet-limit apertures (or openings). The image-forming means may comprise one or more micro-deflector arrays with micro-deflectors, and one or more micro-compensator arrays with micro-compensators. The beamlet-limit means may be a beamlet-limit-aperture array. The optical axes of the micro-deflectors and the micro-compensators may be parallel to the primary optical axis 304, and each micro-deflector may be aligned with one of the micro-compensators and one of the beamlet-limit openings. In some embodiments, a micro-deflector, a micro-compensator, and a beamlet-limit opening may be aligned with primary optical axis 304, but they can be off the primary optical axis 304 as well.

[0064] As illustrated in Fig. 3, source conversion unit 330 may include an aperture array 325. Aperture array 325 may comprise a plate including plurality of apertures. In some embodiments, apertures of aperture array 325 may be uniform or non-uniform in shape, size, pitch, or cross-section, and may be arranged in a regular pattern to form an array or an irregular pattern. In some embodiments, the number of apertures in aperture array 325 may be equal to the number of apertures of Coulomb aperture array 305 to allow each primary electron beamlet generated from an aperture of Coulomb aperture array 305 to pass through a corresponding aperture of aperture array 325. For example, a 5x5 array of primary electron beamlets may be generated by a 5x5 array of apertures of Coulomb aperture array 305 and each beamlet of the 5x5 array of beamlets may be configured to pass through a corresponding aperture of a 5x5 array of apertures of aperture array 325.

[0065] Apparatus 300 further includes an objective lens 331 configured to focus probing beamlets 311, 312, and 313 onto a sample 308 for inspection and may form, in the current embodiments, three probe spots 321, 322, and 323 on the surface of sample 308.

[0066] In order to achieve the high-resolution images from multi-beam SEM inspection tools, an accurate alignment between the electron source and a source conversion unit (e.g., source conversionunit 330 of Fig. 3) is desirable, among other things. In practice, a beam alignment procedure (also referred to as a source alignment procedure) is performed prior to using an inspection apparatus (e.g., apparatus 300 of Fig. 3) for defect detection and inspection. Primary electron beamlets, after exiting the Coulomb aperture array 305 and the condenser lens 310, impinge on an aperture array (e.g., aperture array 325 of Fig. 3) of the source conversion unit which is configured to generate probing beamlets from the primary electron beamlets. In some embodiments, aperture array 325 may be a current-limiting aperture array, also referred to as a beam-limiting aperture array comprising apertures configured to form beamlets having a desired beam size and a desired beam current. The beam alignment procedure is typically an iterative, lengthy process involving scanning each electron beamlet over the corresponding aperture of aperture array 325, detecting signal electrons generated from the sample after interaction of the scanned beamlet with the sample, obtaining an associated image, analyzing the image manually (usually by an operator of the apparatus), and accordingly, adjusting the beam scan deflector voltage. Apparatus 300 may further comprise a signal electron detector located between source conversion unit 330 and objective lens 331, configured to detect electrons generated from the probe spots formed by probing beamlets. In some embodiments, signal electron detector 350 may be activated during the beam alignment procedure and de-activated or set at a static deflection value during sample inspection. The obtained image may indicate the relative positions of the primary beamlet and the aperture of the aperture array 325 to which it should be aligned. However, when the primary beamlet is incident on aperture array 325, the margin for misalignment is small because the beamlet size is substantially similar to the size of the aperture through which it is configured to pass through. For example, the size of an aperture of aperture array 325 may be approximately 70 pm and the beamlet diameter may be 70 pm ± 3 pm. While a beam scanning deflector (e.g., beam scanning deflector 320 of Fig. 3) may be used to adjust the path of the incident beamlet to align with the aperture, the images obtained may still be blur, rendering the alignment process inadequate, inefficient, or challenging.

[0067] Reference is now made to Fig. 4A, which is a schematic illustration 400 of primary electron beamlets 415 passing through apertures of an aperture array 425 of a source conversion unit. Primary electron beamlets 415 are formed from a primary electron beam generated by an electron source and passing through Coulomb aperture array (e.g., Coulomb aperture array 305). The apertures of Coulomb aperture array are configured to generate a plurality of beamlets having a large range of beam currents while reducing the Coulomb interaction effects and maintaining the mechanical and structural integrity of the aperture array. To accomplish these features, the apertures of Coulomb aperture array may have an elongated, curved, and tapering shape profile. Coulomb aperture arrays with elongated and curved apertures are further described in U.S. Pat. Pub. 2020 / 0381211, which is incorporated herein by reference in its entirety.

[0068] As can be seen in Fig. 4A, illustration 400 represents an exemplary near-ideal scenario where each aperture of aperture array 425 is substantially aligned with a corresponding primary electronbeamlet 415. Such an alignment is desirable to obtain high-resolution SEM images. In practice, however, aligning the primary electron beamlets and apertures of the aperture array of the source conversion unit is challenging, and often inaccurate. This is because the beamlet diameter is substantially similar to the size of the aperture, and minor misalignment can result in significant reduction in resolution of the images. It is to be appreciated that while Fig. 4A illustrates exemplary curved and elongated beam spots formed by primary electron beamlets 415, the beam spots may be any shape, as appropriate. As an example, the beam spots may be circular, elliptical, or any suitable shape.

[0069] Fig. 4B shows exemplary beam alignment scan images 45O_P1, 450_P5, 450_P21, and 450_P25, obtained during a beam alignment procedure prior to using an inspection apparatus to, for example, inspect a wafer for defects. Alignment scan images are SEM images obtained during the beam alignment procedure of a region on the wafer irradiated by probing electron beamlets after passing through an aperture (e.g., Pl, P5, P21, or P25) of aperture array 425 of a source conversion unit. For example, image 450_P21 is a SEM image of a region of the sample (e.g., a semiconductor wafer) generated by the interaction of a probing beamlet exiting from aperture number 21 of an aperture array comprising 25 apertures (e.g., a 5x5 array such as aperture array 425 of Fig. 4A). There are several issues related with the existing beam alignment procedure using the beam scanning deflector to adjust paths of beamlets to align the beamlets with the apertures of the source conversion unit. Some of the issues include, but are not limited to, blurry alignment scan images which render the process inefficient, alignment procedure is slow and resource-intensive, detecting and adjusting for image drift issues, exposure of the source conversion unit to contaminants or debris generated by operation of the gate valve to regulate the flow of primary electron beams.

[0070] Because the size of the incident primary electron beamlet is substantially similar to the size of apertures of the aperture array in the source conversion unit, when scanning the beamlets across the aperture, the acquired images are inadequate in resolution and are low-quality images, rendering it challenging to determine the concentricity, and therefore the alignment, of the beamlets and the apertures. Therefore, it is challenging and time-consuming to align the electron source and the source conversion unit. Further, an operator or a user may be misled to believe that the alignment is poor, indicating that further adjustments to the beam scanning deflector should be made. Even if the adjustments to the beam path were implemented by the operator, it would be challenging to determine whether the adjustment impacted the alignment because the alignment scan image would still appear blurred. These issues render the alignment procedure extremely inefficient.

[0071] Operating the inspection apparatus (e.g., apparatus 300 of Fig. 3) for extended periods of time (e.g., few hours) may introduce image drift issues due to electrical instability, charging effects, mechanical drift, among other things. One of several ways to troubleshoot the image drift issues include checking the mechanical alignment of the electron source to the apertures of the source conversion unit, for example, by performing the beam alignment procedure. However, due to the poorresolution of images obtained during beam alignment scans, it may be challenging to determine whether the adjustments to compensate the mechanical misalignment result in better alignment of the beamlets and the apertures.

[0072] In existing SEM apparatuses, a gate valve (e.g., gate valve 520 of Fig. 5) between the electron source and source conversion unit is disposed to block or allow the electrons to pass through, as appropriately needed, during tool operation. In a “block” position or the “closed” position, the gate valve may be engaged in a position to block all the primary electron beamlets from entering the source conversion unit. In an “allow” position or “open” position, the gate valve is moved away from the path of electrons of the primary electron beamlets such that there is no hindrance for the beamlets from entering the source conversion unit. Operating the gate valve from an open to a closed position, or vice versa, may generate particles or debris, to which the source conversion unit may be exposed. The generated particles, contaminants, or debris, trapped in the apertures of the source conversion unit may cause issues such as, but not limited to, charging, electrical shorts, or may block the beamlet causing a reduction in beam current, thereby negatively impacting the image resolution. In some cases, for example, electrical failures may be caused due to the particles in the source conversion unit, requiring the tool to be shut down, increasing the machine down-time and impacting the throughput. To overcome one or more of the issues discussed above, it may be desirable to provide a system and a method to improve alignment accuracy of the electron source with the apertures of the aperture array of the source conversion unit to obtain high-resolution image while maintaining the inspection throughput.

[0073] Reference is now made to Fig. 5, which is a schematic illustration of a portion of an exemplary inspection apparatus (SEM) 500 including the source conversion unit and a movable aperture plate, consistent with embodiments of the present disclosure. Apparatus 500 may be analogous to apparatus 300, and may include an electron source, a Coulomb aperture array, a condenser lens, a beam scanning deflector, an objective lens, none of which are shown in Fig. 5 for brevity and clarity purposes only. Apparatus 500 may comprise a chamber 510, a gate valve 520 disposed between aperture plate 524 and electron source (not shown), a source conversion unit 530, and a mechanical structure 540.

[0074] In some embodiments, aperture plate 524 may be disposed upstream from source conversion unit 530 and downstream from a condenser lens (e.g., condenser lens 310 of Fig. 3). In some embodiments, aperture plate 524 may be disposed between a beam scanning deflector (e.g., beam scanning deflector 320 of Fig. 3) and an aperture array of the source conversion unit 530. In some embodiments, aperture plate 524 may be disposed downstream from gate valve 520 such that the particles generated by operation of gate valve 520 from a closed to an open position, or vice versa, may be prevented from falling into one or more apertures of aperture array of source conversion unit 530.

[0075] In some embodiments, aperture plate 524 may be a plate including at least two apertures, or at least three apertures, or at least four apertures, or at least five apertures. In aperture plate 524 comprising at least two apertures, one aperture may be an on-axis aperture and the other one may be an off-axis aperture. In aperture plate 524 comprising at least three apertures, one aperture may be an on-axis aperture and the remaining apertures may be off-axis apertures. On-axis and off-axis apertures of aperture plate 524 may be pre- aligned with apertures of aperture array of source conversion unit 530. In this context, “pre-aligned” refers to the positioning of an aperture on the aperture plate determined with reference to apertures of aperture array of the source conversion unit such that when the aperture plate is extended or moved to a first predetermined location in the path of the primary electron beamlets traversing towards the sample, the apertures of the aperture plate 524 and aperture array of source conversion unit are aligned. At the first predetermined location, on-axis aperture of aperture plate 524 and an on-axis aperture of the aperture array of source conversion unit 530 are aligned with the primary optical axis of apparatus 500.

[0076] Pre-aligning of aperture plate 524 with apertures of an aperture array of source conversion unit 530 may be performed using a stepper motor configured to cause aperture plate 524 to move in incremental unit distance along x-y axes. For example, a stepper motor (not shown) may cause aperture plate 524 to move with a 1 pm step precision. It is to be appreciated that the movement precision of stepper motor may be adjusted to less than 1 pm, less than 2 pm, less than 3 pm, less than 4 pm, or less than 5 pm, or any suitable step precision. In some embodiments, the alignment between aperture plate 524 and aperture array of source conversion unit 530 may be inspected using an optical microscope, or other suitable technique. Alternatively, or additionally, a mechanical stop may be used to stop aperture plate 524 at an aligned position. The mechanical stop may allow moving the aperture plate 524 to a predetermined alignment position without intermittent alignment checks and it may also prevent aperture plate 524 from moving out-of- alignment during the alignment procedure. In some embodiments, aperture plate 524 may be made from a material which is amenable to mechanical fabrication techniques such as drilling, etching, polishing, or semiconductor processing techniques. For example, aperture plate 524 may be made from a metal plate.

[0077] In some embodiments, apertures of aperture plate 524 may be smaller than and concentric with corresponding apertures of aperture array of source conversion unit 530 (discussed with reference to Fig. 7). As an example, for circular apertures, the diameter of apertures of aperture plate 524 may be between 5 pm - 10 pm and the diameter of apertures of source conversion unit 530 may be between 65 pm - 75 pm. It is to be appreciated that although circular apertures are illustrated in Figs. 6A, 6B, 6C, and 7, apertures may be substantially circular, elliptical, or other shapes may be used as well. In some embodiments, apertures of aperture plate 524 may be smaller than the diameter of the primary electron beamlets (e.g., primary electron beamlets 311, 312, and 313) and in some embodiments, apertures of aperture plate 524 may be smaller than both the apertures of source conversion unit 530 and the diameter of the primary electron beamlets.

[0078] Aperture plate 524 may be a movable aperture plate such that the position of the aperture plate is adjustable along a horizontal plane perpendicular to the primary optical axis. In some embodiments, aperture plate 524 may be movable along a horizontal plane along the x-axis in “in” and “out” positions. An “in” position refers to the position of aperture plate such that the on-axis aperture of aperture plate is aligned with the primary optical axis or an on-axis aperture of source conversion unit. An “out” position refers to the position of aperture plate such that aperture plate is away from the path of electrons of primary electron beamlets and configured to allow all the electrons to enter source conversion unit. In some embodiments, the “in” and “out” positions of aperture plate 524 may be referred to as first and second positions, respectively. In some embodiments, aperture plate 524 may be moved between the “in” and “out” positions by an actuation mechanism including, but not limited to, manual, electrical, hydraulic, pneumatic, or mechanical. In some embodiments, the actuation mechanism may include a piezoelectric actuator, a servo motor actuator, or a rack and pinion actuator. In apparatus 500 shown in Fig. 5, aperture plate 524 is in the “in” position, gate valve 520 is in the “closed” position, and the on-axis aperture of aperture plate 524 and on-axis aperture of aperture array of source conversion unit 530 are aligned with the primary optical axis.

[0079] Some of the advantages of a movable aperture plate are as follows: i) High-resolution image and short alignment cycle time - The smaller size of the apertures of movable aperture plate enable a high-quality SEM image, which will provide a high accuracy of alignment between the electron source and the source conversion unit, leading to high resolution alignment scan SEM images and shorter cycle time for mechanical alignment using a beam or a source alignment procedure. ii) Easy detectability of alignment drift - The high-resolution, high-quality SEM images are helpful in identifying minor drift related issues because differences in high- resolution images are easier to identify. iii) Debris and particle shielding - The small holes in aperture plate act as a shield or a cover to protect the underlying source conversion unit (lenses and deflector arrays) from debris, contaminants, particles, etc. falling from the moving parts in the SEM column, for example, moving operation of gate valve may generate debris or particles.

[0080] In some embodiments, controller 109 may be configured to apply electrical signals to an actuator (not shown) associated with aperture plate 524 to enable the movement of aperture plate 524 along the horizontal axis to “in” and “out” positions. In some embodiments, the applied electrical signals may be adjusted to adjust the speed of movement of aperture plate 524. In some embodiments, controller 109 may be further configured to apply electrical signals to enable movement of gate valve 520 to “open” and “closed” positions, as appropriate.

[0081] Reference is now made to Figs. 6A, 6B, and 6C, which are schematic illustrations of an exemplary aperture plate 624A, 624B, and 624C, respectively, consistent with embodiments of the present disclosure. Aperture plates 624A-624C are analogous to aperture plate 524 of Fig. 5 and mayperform substantially similar functions. As illustrated in Fig. 6A, aperture plate 624A includes one on-axis aperture 626a and one off-axis aperture 626b. On-axis aperture 626a and off-axis aperture 626b may be pre-aligned with corresponding apertures of aperture array of source conversion unit (e.g., source conversion unit 530 of Fig. 5). As illustrated in Fig. 6B, aperture plate 624B includes one on-axis aperture 626a and two off-axis apertures 626b. On-axis aperture 626a and off-axis apertures 626b may be pre-aligned with corresponding apertures of aperture array of source conversion unit (e.g., source conversion unit 530 of Fig. 5). As illustrated in Fig. 6C, aperture plate 624C includes one on-axis aperture 626a and four off-axis apertures 626b. On-axis aperture 626a and off-axis apertures 626b may be pre-aligned with corresponding apertures of aperture array of source conversion unit (e.g., source conversion unit 530 of Fig. 5). It is to be appreciated that although off- axis apertures 626b are illustrated to be in a specific configuration (e.g., equidistant from on-axis aperture 626a), it is not so limited. For example, two off-axis apertures may be diagonally opposite from each other.

[0082] Reference is now made to Fig. 7, which is a schematic illustration 700 of an aperture plate disposed over the apertures of the source conversion unit, consistent with embodiments of the present disclosure. Illustration 700 represents the relative positioning of aperture plate 724 (analogous to aperture plate 624C of Fig. 6C) and aperture array 730 of source conversion unit located downstream from aperture plate 724 during the beam alignment procedure.

[0083] Aperture plate 724 may include an on-axis aperture (i.e., center aperture, shown but not labeled) and four off-axis apertures 726b located on the corners of aperture plate 724. Aperture array 730 of source conversion unit may include an on-axis aperture concentric with and larger than on-axis aperture of aperture plate 724 and twenty-four off-axis apertures. Four off-axis apertures of aperture array 730 are concentric with and larger than the four off-axis apertures of aperture plate 724. In some embodiments, as shown in illustration 700, apertures of aperture array 730 may be arranged in a 5x5 grid pattern.

[0084] Reference is now made to Fig. 8, which is a schematic illustration of a portion of an exemplary inspection apparatus (SEM) 800 including the source conversion unit and a movable aperture plate, consistent with embodiments of the present disclosure. Apparatus 800 may comprise a chamber 810, a gate valve 820 disposed between aperture plate 824 and electron source (not shown), a source conversion unit 830, and a mechanical structure 840.

[0085] In some embodiments, aperture plate 824 may be substantially similar to and may perform substantially similar functions as aperture plate 524 of Fig. 5. Aperture plate 824 may be disposed between a beam scanning deflector (e.g., beam scanning deflector 320 of Fig. 3) and an aperture array of the source conversion unit 830. In some embodiments, aperture plate 824 may be disposed downstream from gate valve 820 such that the particles generated by operation of gate valve 820 from a closed to an open position, or vice versa, may be prevented from falling into one or more apertures of aperture array of source conversion unit 830.

[0086] Over an extended period of operation and maintenance routines, alignment of the aperture plate (e.g., aperture plate 524 of Fig. 5) with the apertures of the source conversion unit (e.g., source conversion unit 530 of Fig. 5) may require re-calibration and adjustment of the positioning or movement of the aperture plate. In addition to being time-consuming, the re-calibration of aperture plate may raise other issues such as generating particles, high machine down-time, among other challenges. Therefore, it may be desirable to provide an arrangement of aperture plate with respect to the source conversion unit to address one or more issues associated with repeated alignment of the aperture plate with the apertures of the source conversion unit.

[0087] Aperture plate 824 may be mechanically coupled with one or more aperture arrays of source conversion unit 830 such that aperture plate 824 is hinged to allow a rotational motion along a horizontal plane (as denoted by the dashed arrow curve) perpendicular to the primary optical axis. In some embodiments, aperture plate 824 may be rotated in and out of the alignment position during alignment and scanning, respectively. An “in” position refers to the position of aperture plate such that the on-axis aperture of aperture plate is aligned with the primary optical axis or an on-axis aperture of source conversion unit. An “out” position refers to the position of aperture plate such that aperture plate is away from the path of electrons of primary electron beamlets and configured to allow all the electrons to enter source conversion unit. In some embodiments, aperture plate 824 may be moved between the “in” and “out” positions by an actuation mechanism including, but not limited to, manual, electrical, hydraulic, pneumatic, or mechanical. In some embodiments, the actuation mechanism may include a piezoelectric actuator, a servo motor actuator, or a rack and pinion actuator. In apparatus 800 shown in Fig. 8, aperture plate 824 is in the “in” position, gate valve 820 is in the “closed” position, and the on-axis aperture of aperture plate 824 is aligned with the on-axis aperture of aperture array of source conversion unit 830 and with the primary optical axis. The mechanical coupling of aperture plate 824 with aperture array(s) of source conversion unit 830 may minimize the relative change in position between the aperture plate and the source conversion unit, thereby maintaining the alignment accuracy.

[0088] In some embodiments, aperture plate 824 may be a plate including at least two apertures, or at least three apertures, or at least four apertures, or at least five apertures. In aperture plate 824 comprising at least two apertures, one aperture may be an on-axis aperture and the other one may be an off-axis aperture. In aperture plate 824 comprising at least three apertures, one aperture may be an on-axis aperture and the remaining apertures may be off-axis apertures. On-axis and off-axis apertures of aperture plate 824 may be pre-aligned with apertures of aperture array of source conversion unit 830. At the first predetermined location, on-axis aperture of aperture plate 824 and an on-axis aperture of the aperture array of source conversion unit 830 are aligned with the primary optical axis of apparatus 800. In some embodiments, aperture plate 824 may be made from a metal by mechanical fabrication techniques such as drilling, etching, polishing, or semiconductor processing techniques.

[0089] Reference is now made to Fig. 9A, which is a schematic illustration of a portion of an exemplary inspection apparatus (SEM) 900 including a source conversion unit 930 comprising a plurality of aperture arrays, consistent with embodiments of the present disclosure. Apparatus 900 may be analogous to apparatus 300, and may include an electron source, a Coulomb aperture array, a condenser lens, a beam scanning deflector, an objective lens, none of which are shown in Fig. 9A for brevity and clarity purposes only. Apparatus 900 comprises a chamber 910, a gate valve 920 disposed between source conversion unit 930 and electron source (not shown), and a mechanical structure 940.

[0090] Source conversion unit 930 may comprise vertically stacked aperture plates 930a, 930b, and 930c. An aperture plate (e.g., aperture plate 930a) of source conversion unit 930 may be implemented by a plate comprising a plurality of apertures. In some embodiments, vertically stacked aperture plates of source conversion unit 930 may be aligned with respect to each other or with respect to the primary optical axis such that at least one aperture of an aperture plate aligns with the corresponding aperture of the upstream or downstream aperture plate.

[0091] In some embodiments, an aperture plate (e.g., aperture plate 930a or 930b or 930c) may comprise a first aperture array 938, each aperture of the first aperture array configured to generate a probing beamlet from a primary beamlet passing through. In an exemplary source conversion unit 930, a first aperture array 938 may include an array of apertures arranged in a 5x5 array. The first aperture array 938 may comprise an on-axis aperture and twenty-four off-axis apertures. The on-axis aperture of the first aperture array 938 may be aligned with primary optical axis 901. In some embodiments, size of the apertures of the first aperture array 938 may be substantially similar to the size of the desired probing beamlet or the desired beam current. In some embodiments, the on-axis aperture, and off-axis apertures of first aperture array 938 are similar in size.

[0092] Aperture plate (e.g., 930a or 930b or 930c) may further comprise a second aperture array 936. In some embodiments, second aperture array 936 may include at least two or more apertures, three or more apertures, or four or more apertures. Apertures of second aperture array 936 may be located radially outwards from first aperture array 938 and closer to the edge of the aperture plate. In some embodiments, apertures of second aperture array 936 may be significantly smaller in size than apertures of first aperture array 938. As an example, for circular apertures, the diameter of apertures of second aperture array 936 may be between 5 pm - 10 pm and the diameter of apertures of first aperture array 938 may be between 65 pm - 75 pm. It is to be appreciated that apertures may be substantially circular, elliptical, or other shapes may be used as well. In some embodiments, apertures of second aperture array 936 may be smaller than the diameter of the primary electron beamlets (e.g., primary electron beamlets 311, 312, and 313) and in some embodiments, apertures of second aperture array 936 may be smaller than both the apertures of source conversion unit 530 and the diameter of the primary electron beamlets.

[0093] Fig. 9B illustrates an exemplary aperture plate 930a comprising two aperture arrays 936 and 938 of source conversion unit 930, consistent with some disclosed embodiments. Aperture array 938may comprise an on-axis aperture aligned with primary optical axis 901. Aperture array 936 may comprise apertures smaller in size than any aperture of aperture array 938. Although aperture plate 930a in Fig. 9B is illustrated to include six apertures in aperture array 936, aperture plate 930a may include M number of apertures, where M > 2.

[0094] Reference is now made to Fig. 10, which is a process flowchart for a method 1000 of aligning beamlets of a multi-beam inspection system with apertures of a source conversion unit, consistent with embodiments of the present disclosure. One or more steps of method 1000 may be performed by controller 109 of EBI system 100, as shown in Fig. 2, for example. For example, controller 109 may instruct a module of a charged particle beam apparatus to activate a charged-particle source (e.g., electron source) to generate a charged particle beam (e.g., electron beam), apply electrical signals to beam scanning deflector, and carry out other functions.

[0095] In step 1010, a plurality of primary charged-particle beamlets are formed from a charged- particle beam along a primary optical axis. A charged-particle source (e.g., electron source 301 of Fig. 3) may be activated to emit charged particles (e.g., electrons). The electron source may be activated by a controller (e.g., controller 109 of Fig. 2). For example, the electron source may be controlled to emit primary electrons to form an electron beam along a primary optical axis (e.g., primary optical axis 304 of Fig. 3). The electron source may be activated remotely, for example, by using a software, an application, or a set of instructions for a processor of a controller to power the electron source through a control circuitry. The primary electron beam may pass through a Coulomb aperture array (e.g., Coulomb aperture array 305 of Fig. 3) to split the primary electron beam into a plurality of beamlets.

[0096] Step 1020 includes positioning an aperture plate (e.g., aperture plate 524 of Fig. 5) upstream from a first aperture array of a source conversion unit (e.g., source conversion unit 330 of Fig. 3 or 530 of Fig. 5) in a first position, wherein in the first position, apertures of the aperture plate are aligned with corresponding apertures of the first aperture array. Controller 109 may be configured to apply an electrical signal to move the aperture plate from an “out” position to an “in” position such that the apertures of the aperture plate are aligned with the corresponding apertures of the aperture array of source conversion unit. Aperture of the aperture plate are pre-aligned with the apertures of the aperture array of the source conversion unit such that when the aperture plate is in the position, the apertures of the aperture plate and the aperture array of the source conversion unit are aligned.

[0097] Step 1030 includes adjusting a path of the plurality of primary charged-particle beamlets using a charged-particle beam scan deflector (e.g., beam scanning deflector 320 of Fig. 3) to enable a portion of charged particles of the primary charged-particle beamlets to pass through the aligned apertures. Doing so results in an alignment between the primary charged-particle beamlets and the corresponding apertures of the aperture array of the source conversion unit. Controller 109 may be configured to apply an electrical signal (e.g., a voltage) to the beam scanning deflector to deflect the path of one primary electron beamlet.

[0098] Step 1040 includes moving the aperture plate in a second position (e.g., “out” position) to enable the plurality of primary charged-particle beamlets to pass through the aperture array of source conversion unit without passing through the aperture plate. Once the beam alignment procedure is complete (i.e., step 1030), the aperture plate may be pulled or moved out of the path of the primary electron beamlets such that the primary electron beamlets may enter the source conversion unit, which is configured to generate probing beamlets from corresponding primary electron beamlets. The probing beamlets impinge on the surface of the sample (e.g., semiconductor wafer) at desired spots, generating secondary electrons or backscattered electrons, which are detected by a charged-particle detector.

[0099] A non-transitory computer readable medium may be provided that stores instructions for a processor of a controller (e.g., controller 109 of Fig. 1) for controlling the electron beam tool and for aligning charged-particle beams with a source conversion unit, consistent with embodiments in the present disclosure. For example, instructions may include activating an electron source, applying an electrical signal from controller to a movable aperture plate, applying an electrical signal from controller to a gate valve, applying an electrical signal to a beam scanning deflector, 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.

[0100] The embodiments may further be described using the following clauses:1. A multi-beam apparatus, comprising: a charged-particle source configured to emit charged particles for forming a plurality of primary charged-particle beamlets; a charged-particle beam alignment deflector configured to influence a path of the plurality of primary charged-particle beamlets; a first aperture array, each aperture of the first aperture array configured to allow a portion of charged particles of a primary charged-particle beamlet to pass through; and a movable aperture plate comprising at least two apertures, the aperture plate located between the charged-particle beam alignment deflector and the first aperture array, wherein each of the at least two apertures of the movable aperture plate is aligned with a corresponding aperture of the first aperture array.2. The multi-beam apparatus of clause 1, wherein each of the at least two apertures of the movable aperture plate has an opening that is smaller than an opening of a corresponding aperture of the first aperture array.3. The multi-beam apparatus of any one of clauses 1 and 2, wherein each of the at least two apertures of the movable aperture plate has an opening that is smaller than a beamlet diameter of a corresponding primary charged-particle beamlet configured to pass through.4. The multi-beam apparatus of any one of clauses 1-3, wherein each of the at least two apertures of the movable aperture plate is concentric with the corresponding aperture of the first aperture array.5. The multi-beam apparatus of any one of clauses 1-4, wherein a position of the movable aperture plate is adjustable along a horizontal plane perpendicular to the primary optical axis.6. The multi-beam apparatus of clause 5, wherein at a first position of the movable aperture plate, each of the at least two apertures of the movable aperture plate is aligned with the corresponding aperture of the first aperture array to enable a portion of charged particles of the primary charged-particle beamlets to pass through the aligned apertures.7. The multi-beam apparatus of clause 6, wherein the charged-particle beam scan deflector is configured to receive a first electrical signal to adjust a path of a primary charged- particle beamlet such that a portion of charged particles of the primary charged-particle beamlet passes through the aligned apertures, resulting in an alignment between the primary charged-particle beamlet and the corresponding aperture of the first aperture array.8. The multi-beam apparatus of any one of clauses 5-7, wherein in a second position of the movable aperture plate, the movable aperture plate is positioned to enable the plurality of primary charged-particle beamlets to pass through the first aperture array without passing through the at least two apertures of the movable aperture plate.9. The multi-beam apparatus of clause 8, wherein the movable aperture plate is movable between the first and the second positions.10. The multi-beam apparatus of any one of clauses 1-9, wherein the charged-particle beam alignment deflector comprises a static or a dynamic scanning deflector.11. The multi-beam apparatus of any one of clauses 1-10, further comprising a second aperture array located between the charged-particle source and the charged-particle beam alignment deflector and configured to form the plurality of primary charged-particle beamlets.12. The multi-beam apparatus of clause 11, further comprising a condenser lens located between the second aperture array and the charged-particle beam alignment deflector.13. The multi-beam apparatus of any one of clauses 1-12, further comprising a charged- particle detector configured to detect signal charged particles originating from a sample, wherein a setting of the charged-particle beam alignment deflector that causes the primary charged-particle beamlet to align with the corresponding aperture of the first aperture array is determined based on the detected signal charged particles.14. The multi-beam apparatus of clause 13, wherein the setting of the charged-particle beam alignment deflector comprises information associated with a first electrical signal to adjust a path of a primary charged-particle beamlet.15. The multi-beam apparatus of any one of clauses 1-14, wherein the first aperture array comprises a current-limiting aperture array or a source conversion unit.16. A method of aligning charged-particle beams with an aperture array for multi-beam inspection of a sample, the method comprising: forming a plurality of primary charged-particle beamlets from a charged-particle beam; positioning a movable aperture plate upstream from a first aperture array in a first position, wherein in the first position, apertures of the movable aperture plate are aligned with corresponding apertures of the first aperture array; adjusting a path of the plurality of primary charged-particle beamlets, using a charged-particle beam alignment deflector, to enable a portion of charged particles of the primary charged-particle beamlets to pass through the aligned apertures, resulting in an alignment between the primary charged-particle beamlets and the corresponding apertures of the first aperture array; and moving the movable aperture plate in a second position to enable the plurality of primary charged-particle beamlets to pass through the first aperture array without passing through the movable aperture plate.17. The method of clause 16, wherein the movable aperture plate comprises at least two apertures, and wherein each of the at least two apertures has an opening that is smaller than an opening of a corresponding aperture of the first aperture array.18. The method of any one of clauses 16 and 17, wherein the movable aperture plate comprises an on-axis aperture and at least one off-axis aperture.19. The method of any one of clauses 16-18, wherein the first aperture array comprises an on-axis aperture and at least one off-axis aperture.20. The method of clause 19, wherein at the first position, the on-axis aperture of the aperture plate and the on-axis aperture of the first aperture array are aligned with a primary optical axis.21. The method of any one of clauses 16-20, wherein each of the at least two apertures has an opening that is smaller than a beamlet diameter of a corresponding primary charged- particle beamlet passing through.22. The method of any one of clauses 16-21, wherein each of the at least two apertures is concentric with the corresponding aperture of the first aperture array.23. The method of any one of clauses 16-22, wherein the plurality of primary charged- particle beamlets are formed from the primary charged-particle beam using a second aperturearray located between a charged-particle source and the charged-particle beam alignment deflector.24. The method of any one of clauses 16-23, wherein adjusting the path of the plurality of primary charged-particle beamlets comprises applying an electrical signal to the charged- particle beam alignment deflector.25. The method of clause 24, further comprising: detecting signal charged particles, using a charged-particle detector, originating from the sample; and determining a setting of the charged-particle beam alignment deflector that causes the primary charged-particle beamlets to align with the corresponding apertures of the first aperture array based on the detected signal charged particles.26. The method of clause 25, wherein the setting comprises information associated with the applied electrical signal.27. The method of any one of clauses 16-26, wherein the charged-particle beam alignment deflector comprises a static or a dynamic scanning deflector.28. The method of any one of clauses 23-27, further comprising collimating the plurality of primary charged-particle beamlets using a condenser lens located between the second aperture array and the charged-particle beam alignment deflector.29. The method of any one of clauses 16-28, wherein the first aperture array comprises a current-limiting aperture or a source conversion unit.30. A non-transitory computer readable medium storing a set of instructions that is executable by one or more processors of a charged-particle beam apparatus to cause the charged-particle beam apparatus to perform operations comprising: activating a charged-particle source to emit charged particles and forming a plurality of primary charged-particle beamlets from the emitted charged particles; positioning a movable aperture plate upstream from a first aperture array in a first position, wherein in the first position, apertures of the movable aperture plate are aligned with corresponding apertures of the first aperture array; adjusting a path of the plurality of primary charged-particle beamlets, using a charged-particle beam alignment deflector, to enable a portion of charged particles of the primary charged-particle beamlets to pass through the aligned apertures, resulting in an alignment between the primary charged-particle beamlets and the corresponding apertures of the first aperture array; and moving the movable aperture plate in a second position to enable the plurality of primary charged-particle beamlets to pass through the first aperture array without passing through the movable aperture plate.31. The non-transitory computer readable medium of clause 30, wherein the movable aperture plate comprises at least two apertures, and wherein each of the at least two apertures has an opening that is smaller than an opening of a corresponding aperture of the first aperture array.32. The non-transitory computer readable medium of any one of clauses 30 and 31, wherein the movable aperture plate comprises an on-axis aperture and at least one off-axis aperture.33. The non-transitory computer readable medium of any one of clauses 30-32, wherein the first aperture array comprises an on-axis aperture and at least one off-axis aperture.34. The non-transitory computer readable medium of clause 33, wherein at the first position, the on-axis aperture of the aperture plate and the on-axis aperture of the first aperture array are aligned with a primary optical axis.35. The non-transitory computer readable medium of any one of clauses 30-34, wherein each of the at least two apertures has an opening that is smaller than a beamlet diameter of a corresponding primary charged-particle beamlet passing through.36. The non-transitory computer readable medium of any one of clauses 30-35, wherein each of the at least two apertures is concentric with the corresponding aperture of the first aperture array.37. The non-transitory computer readable medium of any one of clauses 30-36, wherein the plurality of primary charged-particle beamlets are formed from the primary charged- particle beam using a second aperture array located between a charged-particle source and the charged-particle beam alignment deflector.38. The non-transitory computer readable medium of any one of clauses 30-37, wherein adjusting the path of the plurality of primary charged-particle beamlets comprises applying an electrical signal to the charged-particle beam alignment deflector.39. The non-transitory computer readable medium of clause 38, wherein the operations further comprise: detecting signal charged particles, using a charged-particle detector, originating from the sample; and determining a setting of the charged-particle beam alignment deflector that causes the primary charged-particle beamlets to align with the corresponding apertures of the first aperture array based on the detected signal charged particles.40. The non-transitory computer readable medium of clause 39, wherein the setting comprises information associated with the applied electrical signal.41. The non-transitory computer readable medium of any one of clauses 30-40, wherein the charged-particle beam alignment deflector comprises a static or a dynamic scanning deflector.42. The non-transitory computer readable medium of any one of clauses 37-41, wherein the operations further comprise collimating the plurality of primary charged-particle beamlets using a condenser lens located between the second aperture array and the charged-particle beam alignment deflector.43. The non-transitory computer readable medium of any one of clauses 30-42, wherein the first aperture array comprises a current-limiting aperture or a source conversion unit.

[0101] Block diagrams in the figures may illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer hardware or software products according to various exemplary embodiments of the present disclosure. In this regard, each block in a schematic diagram may represent certain arithmetical or logical operation processing that may be implemented using hardware such as an electronic circuit. Blocks may also represent a module, segment, or portion of code that comprises one or more executable instructions for implementing the specified logical functions. It should be understood that in some alternative implementations, functions indicated in a block may occur out of the order noted in the figures. For example, two blocks shown in succession may be executed or implemented substantially concurrently, or two blocks may sometimes be executed in reverse order, depending upon the functionality involved.Some blocks may also be omitted. It should also be understood that each block of the block diagrams, and combination of the blocks, may be implemented by special purpose hardware-based systems that perform the specified functions or acts, or by combinations of special purpose hardware and computer instructions.

[0102] 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.

Claims

CLAIMS1. A multi-beam apparatus, comprising: a charged-particle source configured to emit charged particles for forming a plurality of primary charged-particle beamlets; a charged-particle beam alignment deflector configured to influence a path of the plurality of primary charged-particle beamlets; a first aperture array, each aperture of the first aperture array configured to allow a portion of charged particles of a primary charged-particle beamlet to pass through; and a movable aperture plate comprising at least two apertures, the aperture plate located between the charged-particle beam alignment deflector and the first aperture array, wherein each of the at least two apertures of the movable aperture plate is aligned with a corresponding aperture of the first aperture array.

2. The multi-beam apparatus of claim 1, wherein each of the at least two apertures of the movable aperture plate has an opening that is smaller than an opening of a corresponding aperture of the first aperture array.

3. The multi-beam apparatus of claim 1, wherein each of the at least two apertures of the movable aperture plate has an opening that is smaller than a beamlet diameter of a corresponding primary charged-particle beamlet configured to pass through.

4. The multi-beam apparatus of claim 1, wherein each of the at least two apertures of the movable aperture plate is concentric with the corresponding aperture of the first aperture array.

5. The multi-beam apparatus of claim 1, wherein a position of the movable aperture plate is adjustable along a horizontal plane perpendicular to the primary optical axis.

6. The multi-beam apparatus of claim 5, wherein at a first position of the movable aperture plate, each of the at least two apertures of the movable aperture plate is aligned with the corresponding aperture of the first aperture array to enable a portion of charged particles of the primary charged-particle beamlets to pass through the aligned apertures.

7. The multi-beam apparatus of claim 6, wherein the charged-particle beam scan deflector is configured to receive a first electrical signal to adjust a path of a primary charged-particle beamlet such that a portion of charged particles of the primary charged-particle beamlet passes through the aligned apertures, resulting in an alignment between the primary charged- particle beamlet and the corresponding aperture of the first aperture array.

8. The multi-beam apparatus of claim 5, wherein in a second position of the movable aperture plate, the movable aperture plate is positioned to enable the plurality of primary charged- particle beamlets to pass through the first aperture array without passing through the at least two apertures of the movable aperture plate.

9. The multi-beam apparatus of claim 8, wherein the movable aperture plate is movable between the first and the second positions.

10. The multi-beam apparatus of claim 1, wherein the charged-particle beam alignment deflector comprises a static or a dynamic scanning deflector.

11. The multi-beam apparatus of claim 1, further comprising a second aperture array located between the charged-particle source and the charged-particle beam alignment deflector and configured to form the plurality of primary charged-particle beamlets.

12. The multi-beam apparatus of claim 11, further comprising a condenser lens located between the second aperture array and the charged-particle beam alignment deflector.

13. The multi-beam apparatus of claim 1, further comprising a charged-particle detector configured to detect signal charged particles originating from a sample, wherein a setting of the charged-particle beam alignment deflector that causes the primary charged-particle beamlet to align with the corresponding aperture of the first aperture array is determined based on the detected signal charged particles.

14. The multi-beam apparatus of claim 13, wherein the setting of the charged-particle beam alignment deflector comprises information associated with a first electrical signal to adjust a path of a primary charged-particle beamlet.

15. A non- transitory computer readable medium storing a set of instructions that is executable by one or more processors of a charged-particle beam apparatus to cause the charged-particle beam apparatus to perform operations comprising:activating a charged-particle source to emit charged particles and forming a plurality of primary charged-particle beamlets from the emitted charged particles; positioning a movable aperture plate upstream from a first aperture array in a first position, wherein in the first position, apertures of the movable aperture plate are aligned with corresponding apertures of the first aperture array; adjusting a path of the plurality of primary charged-particle beamlets, using a charged-particle beam alignment deflector, to enable a portion of charged particles of the primary charged-particle beamlets to pass through the aligned apertures, resulting in an alignment between the primary charged-particle beamlets and the corresponding apertures of the first aperture array; and moving the movable aperture plate in a second position to enable the plurality of primary charged-particle beamlets to pass through the first aperture array without passing through the movable aperture plate.

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