Multi-beam charged-particle apparatus for uniform probe current

Abstract

A charged-particle beam apparatus comprises a first aperture array including a plurality of elongated apertures configured to generate a plurality of primary beamlets from a primary charged- particle beam and project the plurality of primary beamlets on a second aperture array. The second aperture array may include a plurality of apertures configured to generate a plurality of probing beams from the plurality of primary beamlets. A first aperture located at a corner of the plurality of elongated apertures of the first aperture array may be vertically aligned with a second aperture located at a corner of the plurality of apertures of the second aperture array, and a size of the first aperture in a first direction may be smaller than the size of the second aperture in the first direction.

Description

MULTI-BEAM CHARGED-PARTICLE APPARATUS FOR UNIFORM PROBE CURRENTCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 612,290 which was filed on 19 December 2023 and which is incorporated herein in its entirety by reference.TECHNICAL FIELD

[0002] The embodiments provided herein disclose a multi-beam apparatus, and more particularly a multi-beam charged-particle microscope including aperture arrays with apertures configured to provide a relatively uniform probe current.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 free of defects. Inspection systems utilizing optical microscopes or charged-particle (e.g., electron) beam microscopes, such as a scanning electron microscope (SEM) can be employed. As the physical sizes of IC components continue to shrink, accuracy and yield in defect detection become more important. Although multiple electron beams may be used to increase throughput, variation of probe current between the beams may result in non-uniform defect detection.SUMMARY

[0004] In one aspect of the present disclosure, a charged-particle beam apparatus is disclosed. The apparatus may include a first aperture array including a plurality of elongated apertures. The first aperture array may be configured to generate a plurality of primary beamlets from a primary charged- particle beam and project the plurality of primary beamlets on a second aperture array. The second aperture array may include a plurality of apertures configured to generate a plurality of probing beams from the plurality of primary beamlets. A first aperture located at a corner of the plurality of elongated apertures of the first aperture array may be vertically aligned with a second aperture located at a corner of the plurality of apertures of the second aperture array. Wherein a size of the first aperture in a first direction may be smaller than the size of the second aperture in the first direction.

[0005] In another aspect of the present disclosure, a method for generating probing beams using a charged-particle beam apparatus is disclosed. The method may comprise generating a plurality of beamlets by passing a primary charged-particle beam through a plurality of first apertures of a first aperture array, and generating a plurality of probing beams by passing the plurality of beamlets through a plurality of second apertures of a second aperture array. The method may also include adjusting a size of a selected first aperture of the plurality of first apertures to reduce a beam current of a selected probing beam of the plurality of probing beams.

[0006] In yet another aspect of the present disclosure, a non-transitory computer readable medium that stores a set of instructions that is executable by at least on processor of a computing device to cause the computing device to perform operations for generating probing beams using a charged-particle beam apparatus is disclosed. The operations may comprise generating a charged-particle beam and directing the charged-particle beam to pass through a plurality of first apertures of a first aperture array to produce a plurality of beamlets to pass through a plurality of second apertures of a second aperture array to produce a plurality of probing beams. The method may also comprise adjusting a size of a selected first aperture of the plurality of first apertures to reduce a beam current of a selected probing beam of the plurality of probing beams.

[0007] In yet another aspect of the present disclosure, a non-transitory computer readable medium that stores a set of instructions that is executable by at least on processor of a computing device to cause the computing device to perform operations for generating probing beams using a charged-particle beam apparatus is disclosed. The operations may comprise generating a charged-particle beam and directing the charged-particle beam to pass through a plurality of first apertures of a first aperture array to produce a plurality of beamlets for passing through a plurality of second apertures of a second aperture array to produce a plurality of second beamlets for passing through a plurality of third apertures of a third aperture array to produce a plurality of probing beams. The operations may also comprise adjusting a size of a selected second aperture of the plurality of second apertures to reduce a beam current of a selected probing beam of the plurality of probing beams.

[0008] Other advantages of the embodiments of the present disclosure will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the present disclosure.BRIEF DESCRIPTION OF FIGURES

[0009] Fig. 1 is a schematic diagram illustrating an exemplary electron beam inspection (EBI) system, consistent with embodiments of the present disclosure.

[0010] Fig. 2 is a schematic diagram illustrating an exemplary electron beam tool that can be a part of the exemplary electron beam inspection system of Fig. 1 , consistent with embodiments of the present disclosure.

[0011] Fig. 3 is a schematic diagram illustrating an exemplary electron beam tool in a multi-beam apparatus, consistent with embodiments of the present disclosure.

[0012] Fig. 4 is a schematic illustration of electron beams emanating from an exemplary aperture array projected on another exemplary aperture array in the electron beam tool of Fig. 3, consistent with embodiments of the present disclosure.

[0013] Figs. 5A-5C illustrate non-uniformity in beam currents in exemplary embodiments of the electron beam tool of Fig. 3, consistent with embodiments of the present disclosure.

[0014] Fig. 6 schematically illustrates the relationship between electron beam intensity and half angle of emission in the electron beam tool of Fig. 3, consistent with embodiments of the present disclosure.

[0015] Figs. 7A-7B are schematic diagrams illustrating the projection of exemplary curved apertures of one aperture array on the exemplary apertures of another aperture array in the electron beam tool of Fig. 3, consistent with embodiments of the present disclosure.

[0016] Fig. 7C is a schematic cross-sectional illustration of an electron beam passing through an aperture of one aperture array and an aperture of another aperture array in the electron beam tool of Fig. 3, consistent with embodiments of the present disclosure.

[0017] Figs. 8A-8B are schematic diagrams illustrating the projection of exemplary curved apertures of one aperture array on the exemplary apertures of another aperture array in the electron beam tool of Fig. 3, consistent with embodiments of the present disclosure.

[0018] Fig. 9 is a schematic diagram illustrating a movable aperture array placed above another aperture array of the electron beam tool of Fig. 3, consistent with embodiments of the present disclosure.

[0019] Fig. 10 is a process flowchart representing an exemplary method of decreasing beam current non-uniformity in a multi-beam inspection tool, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0020] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the disclosed embodiments as recited in the appended claims. For example, although some embodiments are described in the context of utilizing electron beams, the disclosure is not so limited. Other types of charged-particle beams may be similarly applied. Furthermore, other imaging systems may be used, such as optical imaging, photo detection, x-ray detection, etc.

[0021] 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 l / 1000th the size of a human hair. Making these extremely small ICs is a complex, time-consuming, and expensive process, often involving hundreds of individual steps. Errors in even one step have the potential to result in defects in the finished IC 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.

[0022] One component of improving yield is monitoring the chip making process to ensure that it is producing a sufficient number of functional integrated circuits. One way to monitor the process is to inspect the chip circuit structures at various stages of their formation. Inspection can be carried out using a scanning electron microscope (SEM). An SEM can be used to image these extremely small structures, in effect, taking a “picture” of the structures. The image can be used to determine if the structure was formed properly and also if it was formed in the proper location. If the structure is defective, then the process can be adjusted so the defect is less likely to recur.

[0023] Although a multiple charged-beam particle imaging system, such as a multi-beam SEM, may be useful in increasing the wafer inspection throughput, the imaging resolution of multi-beam SEM may be negatively affected by the Coulomb interaction effects. In order to achieve high throughput, it is desirable that the beam contains as many electrons as possible. However, due to the repulsive Coulomb interaction between electrons, it is difficult to confine a large number of electrons in a very small volume. Moreover, these interactions may broaden the width of the beam and change the direction of the flight of electrons. As a result, the probe spot will be larger, thus negatively impacting the overall resolution of the SEM. Therefore, it is desirable to mitigate the Coulomb interaction effects for maintaining high resolution of multi-beam SEMs.

[0024] To alleviate the Coulomb interaction effects, an aperture array may be placed close to the electron source to cut-off peripheral electrons or split the primary electron beam into multiple beamlets. However, adjusting the probing beam current to adjust resolution may result in a change of the beam path through the aperture array. One of the several problems that may be encountered is that the apertures of the aperture array may not be configured to allow beams of various sizes and positions to pass through while maintaining a small enough area to mitigate Coulomb interaction effects.

[0025] Furthermore, because the imaging resolution may be adversely affected by Coulomb interaction effects, in some cases, it may be desirable to maintain uniformity of overall areas of the apertures, despite their non-uniform shapes, to accommodate variations in probing beam currents. Variations in Coulomb interaction effects between apertures of the aperture array may result in non-uniformity of resolution of regions within the same image, affecting the user’s ability to detect and identify defects, for example.

[0026] In conventional SEMs and multi-beam SEMs, a size of the probing electron beam or the probe spot size may determine the imaging resolution. For example, a large probe spot having high beam current may result in poor resolution, and a small probe spot having low beam current may result in better resolution. Poor imaging resolution from large beam spot sizes may partly be attributed to enhanced Coulomb interaction effects in high current beams.

[0027] A large range of beam currents may be desired in a multibeam SEM tool to perform various operations during wafer inspection. For example, high current beams may be used to perform low resolution, macroscopic inspection of large areas on a wafer, and low current, high resolution scans may be desired to perform thorough defect investigation on a microscopic or a nanoscopic level. One of theseveral ways to generate a large range of beam currents from a single source of electrons in a multibeam SEM may include increasing the size or the length of the apertures through which the electrons may pass. However, multiple large apertures may not only increase the Coulomb interaction effects but may also negatively affect the mechanical integrity of the aperture array. Therefore, it may be desirable to have an imaging tool, for example, a multibeam SEM that enables providing a large range of uniform beam currents while reducing Coulomb interaction effects and maintaining the mechanical integrity of the aperture array.

[0028] In some embodiments of the present disclosure, a multi-beam apparatus may include a first aperture array comprising a plurality of elongated curved apertures configured to generate a plurality of beamlets associated with the primary charged-particle beam. The apparatus may also comprise a condenser lens on an adjustable plane along a primary optical axis, and a second aperture array comprising a plurality of apertures configured to generate probing beamlets. The shapes of the first plurality of apertures are based on paths of corresponding beamlets associated with adjustable probe currents of the probing beamlets, on the adjustability of the plane of the condenser lens, and on characteristics of the second aperture array such as sizes, shapes and positioning of the apertures. The shapes or sizes of the first and second plurality of apertures may be adapted to generate probing beamlets having uniform intensity.

[0029] In some embodiments of the present disclosure, a multibeam apparatus may include a first aperture array comprising a plurality of sets of apertures configured to form a plurality of primary beamlets from the primary charged-particle beam. The apparatus may also include a second aperture array comprising a plurality of apertures configured to generate a plurality of probing beamlets. A probing beamlet may be associated with each set of first and second aperture array and the shape or size of the first and second aperture arrays may be configured to generate probing beamlets with uniform intensity.

[0030] 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.

[0031] All relative terms such as “about,” “substantially,” “approximately,” etc., indicate a possible variation of ±10% (unless noted otherwise or another variation is specified). For example, a feature disclosed as being about “w” units wide (or long, thick, etc.) may vary in width from (w - O.lw) to (w + 0.1 w) units. Similarly, a width within a range of about A-B units can have a width between (A - 10%)and (B + 10%). Further, a width varying from, or between, A-B units includes the endpoints (i.e., A and B). In some cases, the specification and / of figures provide context to some of the relative terms used.

[0032] Reference is now made to Fig. 1 , which illustrates an exemplary electron beam inspection (EBI) system 100 consistent with embodiments of the present disclosure. As shown in Fig. 1, charged-particle beam inspection system 100 includes a main chamber 10, a load-lock chamber 20, an electron beam tool 40, and an equipment front end module (EFEM) 30. Electron beam tool 40 is located within main chamber 10. While the description and drawings are directed to an electron beam, it is appreciated that the embodiments are not used to limit the present disclosure to specific charged-particles.

[0033] EFEM 30 includes a first loading port 30a and a second loading port 30b. EFEM 30 may include additional loading port(s). First loading port 30a and second loading port 30b receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other material(s)) or samples to be inspected (wafers and samples are collectively referred to as “wafers” hereafter). One or more robot arms (not shown) in EFEM 30 transport the wafers to load-lock chamber 20.

[0034] Load-lock chamber 20 is connected to a load / lock vacuum pump system (not shown), which removes gas molecules in load-lock chamber 20 to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robot arms (not shown) transport the wafer from loadlock chamber 20 to main chamber 10. Main chamber 10 is connected to a main chamber vacuum pump system (not shown), which removes gas molecules in main chamber 10 to reach a second pressure below the first pressure. After reaching the second pressure, the wafer is subject to inspection by electron beam tool 40. In some embodiments, electron beam tool 40 may comprise a single-beam inspection tool. In other embodiments, electron beam tool 40 may comprise a multi-beam inspection tool.

[0035] Controller 50 may be electronically connected to electron beam tool 40 and may be electronically connected to other components as well. The controller may be a device or a system that is configured for regulating, managing, and coordinating the operation of EBI system 100. It may receive input from sensors or other sources (e.g., an operator), processes this information, and then makes decisions or adjustments based on predefined parameters or algorithms. In some embodiments, controller 50 may serve as the central unit that coordinates and controls the functions and parameters of EBI system 100. Controller 50 may assist in ensuring precise and accurate performance during electron beam microscopy or other applications. Non-limiting functions of controller 50 may include: beam control (e.g., managing the generation, intensity, and focus of the electron beam by, for example, regulating parameters like beam current, accelerating voltage, and spot size); stage movement and positioning (e.g., by allowing for precise manipulation and alignment of the sample position under the electron beam); image acquisition and processing (e.g., by overseeing the acquisition of images and allowing for adjustments of, for example, contrast, brightness, and other imaging parameters); vacuum system operation (e.g., by ensuring that the appropriate level of vacuum is maintained for suitable electron beam interactions with the sample); detector management (e.g., by coordinating the operationand signal processing of secondary electron detectors, backscatter detectors, etc.); temperature control (e.g., by assisting in the management of cooling systems, etc. to regulate the temperature within the electron beam chamber, etc.); safety and interlocks (e.g., by incorporating or managing safety features and interlocks to ensure that the instrument operates within safe parameters and to prevent potential damage to the system or samples, etc.); data logging (e.g., record and store data related to instrument settings, operating conditions, and acquired images for future reference or analysis); calibration and alignment (e.g., it may include tools or functions for instrument calibration and alignment to ensure accurate and reliable performance); user interface (e.g., a user interface through which operators can interact with the electron beam apparatus). In some embodiments, controller 50 may serve only a portion of the above-described functions, and in some embodiments, controller 50 may serve other or additional functions.

[0036] Controller 50 may be a physical system or device that includes some or all of: one or more microprocessors or microcontrollers; one or more memories (e.g., random access memory (RAM); read-only memory (ROM), etc.); input / output (I / O) ports (e.g., to interface with sensors, actuators, and other components of EBI system 100); analog and digital inputs; analog and digital outputs; user interface (e.g., a graphical user interface (GUI), a touch screen, buttons, knobs, or a combination of these); display (e.g., a display screen to provide feedback, status updates, and information to the operator); control buttons and keypad (e.g., to allow an operator to input commands or set parameters for EBI system 100); communication ports (e.g., to enable the controller to communicate with external devices or networks); power supply; control algorithms and software (e.g., programs and algorithms that dictate how the controller processes input data and makes decisions to control EBI system 100); safety interlocks (e.g., components designed to ensure safe operation by monitoring critical conditions and shutting down EBI system 100 if necessary to prevent damage or hazards); diagnostic and monitoring systems (e.g., sensors and monitoring circuits that provide feedback on the status and performance of various components of EBI system 100); enclosure and housing (e.g., the components that make up the controller may be housed in a protective enclosure to shield it from environmental factors and potential physical damage).

[0037] In some embodiments, controller 50 may be a computer configured to execute various controls of charged-particle beam inspection system 100. Controller 50 may also include processing circuitry configured to execute various signal and image processing functions. While controller 50 is shown in Fig. 1 as being outside of the structure that includes main chamber 10, load-lock chamber 20, and EFEM 30, it is appreciated that controller 50 can be part of the structure.

[0038] While the present disclosure provides examples of main chamber 10 housing an electron beam inspection system, it should be noted that aspects of the disclosure in their broadest sense, are not limited to a chamber housing an electron beam inspection system. Rather, it is appreciated that the foregoing principles may be applied to other chambers as well.

[0039] Reference is now made to Fig. 2, which illustrates a schematic diagram illustrating an exemplary electron beam tool 40 that can be a part of the exemplary charged-particle beam inspection system 100 of Fig. 1, consistent with embodiments of the present disclosure. An electron beam tool 40 (also referred to herein as apparatus 40) comprises an electron source 101, a gun aperture plate 171 with a gun aperture 103, a condenser lens 110, a source conversion unit 120, a primary projection optical system 130, a sample stage (not shown in Fig. 2), a secondary optical system 150, and an electron detection device 140. Primary projection optical system 130 can comprise an objective lens 131. Electron detection device 140 can comprise a plurality of detection elements 140_l, 140_2, and 140_3. Beam separator 160 and deflection scanning unit 132 can be placed inside primary projection optical system 130. It may be appreciated that other commonly known components of apparatus 40 may be added / omitted as appropriate.

[0040] Electron source 101, gun aperture plate 171, condenser lens 110, source conversion unit 120, beam separator 160, deflection scanning unit 132, and primary projection optical system 130 can be aligned with a primary optical axis 100_l of apparatus 100. Secondary optical system 150 and electron detection device 140 can be aligned with a secondary optical axis 150_l of apparatus 40. Electron source 101 can comprise a cathode, an extractor or an anode, wherein primary electrons can be emitted from the cathode and extracted or accelerated to form a primary electron beam 102 that forms a crossover (virtual or real) 101s. Primary electron beam 102 can be visualized as being emitted from crossover 101s.

[0041] Source conversion unit 120 may comprise an image-forming element array (not shown in Fig. 2), an aberration compensator array (not shown), a beam-limit aperture array (not shown in Fig. 2), and a pre -bending micro-deflector array (not shown). The image-forming element array can comprise a plurality of micro-deflectors or micro-lenses to form a plurality of parallel images (virtual or real) of crossover 101s with a plurality of beamlets of primary electron beam 102. Fig. 2 shows three beamlets 102_l, 102_2, and 102_3 as an example, and it is appreciated that the source conversion unit 120 can handle any number of beamlets.

[0042] Condenser lens 110 is configured to focus primary electron beam 102. The electric currents of beamlets 102_l, 102_2, and 102_3 downstream of source conversion unit 120 can be varied by adjusting the focusing power of condenser lens 110 or by changing the radial sizes of the corresponding beam-limit apertures within the beam-limit aperture array. Typically, the electric currents may be changed by both, altering the radial sizes of beam-limit apertures and the focusing power of condenser lens 110. Condenser lens 110 may be an adjustable condenser lens that may be configured so that the position of its first principle plane is movable. The adjustable condenser lens may be configured to be magnetic, which may result in off-axis beamlets 102_2 and 102_3 illuminating source conversion unit 120 with rotation angles. The rotation angles may change with the focusing power or the position of the first principal plane of the adjustable condenser lens. Accordingly, condenser lens 110 may be an antirotation condenser lens that may be configured to keep the rotation angles unchanged while the focusingpower of condenser lens 110 is changed. In some embodiments, condenser lens 110 may be an adjustable anti-rotation condenser lens, in which the rotation angles do not change when the focusing power and the position of the first principal plane of condenser lens 110 are varied.

[0043] Objective lens 131 may be configured to focus beamlets 102_l, 102_2, and 102_3 onto a sample 190 for inspection and can form three probe spots 102_ls, 102_2s, and 102_3s on surface of sample 190. Gun aperture plate 171 can block off peripheral electrons of primary electron beam 102 not in use to reduce Coulomb interaction effects. Coulomb interaction effects can enlarge the size of each of probe spots 102_ls, 102_2s, and 102_3s, and therefore deteriorate inspection resolution.

[0044] Beam separator 160 may be a beam separator of Wien filter type comprising an electrostatic deflector generating an electrostatic dipole field El and a magnetic dipole field Bl (both of which are not shown in Fig. 2). If they are applied, the force exerted by electrostatic dipole field El on an electron of beamlets 102_l , 102_2, and 102_3 is equal in magnitude and opposite in direction to the force exerted on the electron by magnetic dipole field Bl. Beamlets 102_l, 102_2, and 102_3 can therefore pass straight through beam separator 160 with zero deflection angles.

[0045] Deflection scanning unit 132 can deflect beamlets 102_l, 102_2, and 102_3 to scan probe spots 102_ls, 102_2s, and 102_3s over three scanned areas in a section of the surface of sample 190. In response to incidence of beamlets 102_l, 102_2, and 102_3 at probe spots 102_ls, 102_2s, and 102_3s, three secondary electron beams 102_lse, 102_2se, and 102_3se can be emitted from sample 190. Each of secondary electron beams 102_lse, 102_2se, and 102_3se can comprise electrons with a distribution of energies including secondary electrons (e.g., with energies < 50 eV) and backscattered electrons (e.g., with energies between 50 eV and landing energies of beamlets 102_l, 102_2, and 102_3). Beam separator 160 can direct secondary electron beams 102_lse, 102_2se, and 102_3se towards secondary optical system 150. Secondary optical system 150 can focus secondary electron beams 102_lse, 102_2se, and 102_3se onto detection elements 140_l, 140_2, and 140_3 of electron detection device 140. Detection elements 140_l, 140_2, and 140_3 can detect corresponding secondary electron beams 102_lse, 102_2se, and 102_3se and generate corresponding signals used to construct images of the corresponding scanned areas of sample 190.

[0046] In some embodiments, controller 50 may comprise an image processing system that includes an image acquirer (not shown) and a storage (not shown). The image acquirer may comprise one or more processors. For example, the image acquirer may comprise a computer, server, mainframe host, terminals, personal computer, any kind of mobile computing devices, and the like, or a combination thereof. The image acquirer may be communicatively coupled to electron detection device 140 of apparatus 40 through a medium such as an electrical conductor, optical fiber cable, portable storage media, IR, Bluetooth, internet, wireless network, wireless radio, among others, or a combination thereof. In some embodiments, the image acquirer may receive a signal from electron detection device 140 and may construct an image. The image acquirer may thus acquire images of sample 190. 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 postprocessed images.

[0047] 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 140. 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 190. The acquired images may comprise multiple images of a single imaging area of sample 190 sampled multiple times over a time sequence. The multiple images may be stored in the storage. In some embodiments, controller 50 may be configured to perform image processing steps with the multiple images of the same location of sample 190.

[0048] In some embodiments, controller 50 may include measurement circuitries (e.g., analog-to- digital converters) to obtain a distribution of the detected secondary electrons. The electron distribution data collected during a detection time window, in combination with corresponding scan path data of each of primary beamlets 102_l, 102_2, and 102_3 incident on the wafer surface, can be used to reconstruct images of the wafer structures under inspection. The reconstructed images can be used to reveal various features of the internal or external structures of sample 190, and thereby can be used to reveal any defects that may exist in the wafer.

[0049] In some embodiments, controller 50 may control a motorized stage (not shown) to move sample 190 during inspection. In some embodiments, controller 50 may enable the motorized stage to move sample 190 in a direction continuously at a constant speed. In other embodiments, controller 50 may enable the motorized stage to change the speed of the movement of sample 190 over time depending on the steps of scanning process. In some embodiments, controller 50 may adjust a configuration of primary projection system 130 or secondary projection system 150 based on images of secondary electron beams 102_lse, 102_2se, and 102_3se. Although Fig. 2 shows that electron beam tool 40 uses three primary electron beams, it is appreciated that electron beam tool 40 may use two or more number of primary electron beams. The present disclosure does not limit the number of primary electron beams used in apparatus 40.

[0050] Reference is now made to Fig. 3, which is a schematic diagram of multi-beam apparatus 300 illustrating an exemplary adjustable condenser lens 110. It is appreciated that multi-beam apparatus 300 may be part of charged-particle beam inspection system (e.g., electron beam inspection system 100 of Fig. 1). Multi-beam apparatus 300 may comprise an electron source 101, a pre-beamlet forming aperturemechanism or a first aperture array (referred to herein as a coulomb aperture array (CAA) 172, a condenser lens 110, an objective lens 131, and a source conversion unit 120 comprising a second aperture array (referred to herein as a beam-limiting aperture array (BLAA) 121, and an image-forming element array 122. In some embodiments, electron source 101 may be configured to emit primary electrons and form a primary electron beam 102. In some embodiments, a gun aperture plate (not shown) may be configured to block off peripheral electons of primary electron beam 102 to reduce the Coulomb effect. In some embodiments, the first aperture array (e.g., coulomb aperture array 172) further cuts the peripheral electrons of primary electron beam 102 to reduce the Coulomb effect. Primaryelectron beam 102 may be trimmed into three primary electron beamlets 102_l, 102_2, and 102_3 (or any other number of beamlets) after passing through apertures of coulomb aperture array 172. Electron source 101, gun aperture plate 171, coulomb aperture array 172, and condenser lens 110 may be aligned with a primary optical axis 100_l of multi-beam electron beam tool 300.

[0051] In some embodiments, source conversion unit 120 may be provided with a second or the beamlimiting aperture array (BLAA) 121, and image-forming element array 122. Beam-limiting aperture array 121 may comprise beam-limit apertures (or BLAs) 121_1, 121_2, and 121_3. Although, Fig. 3 illustrates only three beam-limit apertures (e.g., BLA 121_1, 121_2, and 121_3), it is appreciated that beam-limiting aperture array 121 may include any number of beam-limit apertures (e.g., a 5X5 array of apertures, a 7X7 array of apertures, etc.), as appropriate. Beam limit apertures 121_1, 121_2, and 121_3 may limit sizes of beamlets 102_l, 102_2, and 102_3 of primary-electron beam 102. Image-forming element array 122 may comprise image-forming deflectors 122_1, 122_2, and 122_3. Deflectors 122_1, 122_2, and 122_3 may be configured to deflect beamlets 102_l, 102_2, and 102_3 by varying angles towards primary optical axis 100_l. In some embodiments, deflectors further away from primary optical axis 100_l may deflect beamlets to a greater extent. Furthermore, image-forming element array 122 may comprise multiple layers (not illustrated), and deflectors 122_1, 122_2, and 122_3 may be provided in separate layers. Deflectors 122_1, 122_2, and 122_3 may be individually controlled independent from one another. In some embodiments, a deflector may be controlled to adjust a pitch of probe spots (e.g., 1O2_1S, 102_2S, and 102_3S) formed on a surface of sample 190. As referred to herein, pitch of the probe spots may be defined as the distance between two immediately adjacent probe spots on the surface of sample 190.

[0052] A centrally located deflector of image-forming element array 122 may be aligned with primary optical axis 100_l of multi-beam apparatus 300. Thus, in some embodiments, a central deflector may be configured to maintain the trajectory of beamlet 102_l to be straight. In some embodiments, the central deflector may be omitted. However, in some embodiments, primary electron source 101 may not necessarily be aligned with the center of source conversion unit 120. Furthermore, it is appreciated that while Fig. 3 shows a side view of apparatus 300 where beamlet 102_l is on primary optical axis 100_l, beamlet 102_l may be off primary optical axis 100_l when viewed from a different side. Thatis, in some embodiments, all of beamlets 102_l, 102_2, and 102_3 may be off-axis. An off-axis component may be offset relative to primary optical axis 100_l.

[0053] The deflection angles of the deflected beamlets may be set based on one or more criteria. As shown in Fig. 3, deflectors 122_2 and 122_3 may deflect off-axis beamlets radially outward or away (not illustrated) from primary optical axis 100_l. In some embodiments, deflectors 122_2 and 122_3 may deflect off-axis beamlets radially inward or towards primary optical axis 100_l. Deflection angles of the beamlets may be set so that beamlets 102_l, 102_2, and 102_3 land perpendicularly on sample 190. Off-axis aberrations of images due to lenses, such as objective lens 131, may be reduced by adjusting paths of the beamlets passing through the lenses. Therefore, deflection angles of off-axis beamlets 102_2 and 102_3 generated by deflectors 122_2 and 122_3 may be set so that probe spots 102_2S and 102_3S have small aberrations. Beamlets may be deflected so as to pass through or close to the front focal point of objective lens 131 to decrease aberrations of off-axis probe spots 102_2S and 102_3S. In some embodiments, deflectors may be set to make beamlets 102_l, 102_2, and 102_3 land perpendicularly on sample 1 while probe spots 1O2_1S, 102_2S, and 102_3S have small aberrations.

[0054] As explained previously, pre-beamlet forming aperture mechanism may comprise a coulomb aperture array 172 having a plurality of apertures (referred to herein as coulomb apertures). A central aperture (also referred to herein as the on-axis aperture) of coulomb aperture array 172 and a central deflector of source conversion unit 120 may be aligned with primary optical axis 100_l of multi-beam apparatus 300. Coulomb aperture array 172 may be provided with a plurality of pre-trimming apertures 172_1 , 172_2, and 172_3. Central aperture 172_1 (of coulomb aperture array 172) may be aligned with primary optical axis 100_l and may be provided above condenser lens 110 and close to electron source 101. In Fig. 3, the three beamlets 102_l, 102_2 and 102_3 are generated when primary electron beam 102 passes through the three pre-trimming apertures 172_1, 172_2, and 172_3, and much of the remaining part of primary electron beam 102 is cut off. That is, coulomb aperture array 172 may trim much or block most of the electrons from primary electron beam 102 that do not form the three beamlets 102_l, 102_2 and 102_3. In other words, coulomb aperture array 172 may cut off electrons that will ultimately not be used to form probe spots 1O2_1S, 102_2S and 102_3S before primary electron beam 102 enters source conversion unit 120. In this way, the Coulomb effect above beam-limiting aperture array 121 may be reduced to a great degree. As used in this disclosure, “pre-beamlet” refers to the primary beamlet having an intermediate beam current value before entering the beam-limit aperture. In some embodiments, a gun aperture plate (not illustrated) may be provided close to electron source 101 to cut off electrons at an early stage, while coulomb aperture array 172 may also be provided to further cut off electrons around a plurality of beamlets. Although Fig. 3 illustrates three apertures (172_1, 172_2, and 172_3) in coulomb aperture array 172, it should be appreciated that coulomb aperture array 172 may include any number of apertures, as appropriate. U.S. Patent Publication No. US 2020 / 0381211 Al, the disclosure of which is incorporated by reference herein in its entirety, discloses exemplary aperture arrays (e.g., pre-beamlet forming aperture mechanisms or Coulomb aperture arrays172 and beam-limiting aperture arrays 121 that may be used with a multi-beam apparatus 300 of the current disclosure.

[0055] In some embodiments, coulomb aperture array 172 may be placed below condenser lens 110. Placing coulomb aperture array 172 closer to electron source 101 may more effectively reduce the Coulomb effect. In some embodiments, gun aperture plate may be omitted when coulomb aperture array 172 is able to be located sufficiently close to source 101 while still being manufacturable. In some embodiments, CAA 172 may be placed along a plane 172P normal to primary optical axis 100_l.

[0056] Condenser lens 110 may be configured to be adjustable for adjusting electric currents of beamlets 102_l, 102_2, and 102_3. Adjustable condenser lens 110 may have a principal plane 110_2 normal to primary optical axis 100_l, as illustrated in Fig. 3. Principal plane 110_2 may be moved along primary optical axis 100_l of multi-beam apparatus 300. For example, principal plane 110_2 may be moved further away from electron source 101 to reduce the electric currents of beamlets 102_l, 102_2 and 102_3, and principal plane 110_2 may be moved closer to electron source 101 to increase the currents of beamlets 102_l, 102_2 and 102_3. Additionally, currents of beamlets 102_l, 102_2 and 102_3 may be changed by changing the sizes of the apertures (172_1 , 172_2, and 172_3) of coulomb aperture array 172 or the sizes of the apertures (121_1, 121_2, and 121_3) of beam-limiting aperture array 121. In some embodiments, the multiple beamlets (e.g., beamlets 102_l, 102_2 and 102_3) from coulomb aperture array 172 that fall or impinge on (or illuminate) beam-limiting aperture array 121 may be slightly larger than the beam-limit apertures 121_1, 121_2, and 121_3. In some such embodiments, the beam-limit apertures may trim these beamlets and the beam current may be determined by the size of the beam-limit apertures.

[0057] Fig. 4 is a schematic illustration of beamlets 412 from the coulomb aperture array 172 (e.g., of Fig. 3) projected on beam-limit apertures 422 of an exemplary beam-limiting aperture array 420 consistent with some embodiments of the current disclosure. As explained previously, the coulomb aperture array 172 may include a plurality of elongated curved apertures that each generate a corresponding beamlet 412 of charged-particles that impinge on the beam-limiting aperture array 420. The beamlets than emanate from the beam-limit apertures 422 of the beam-limiting aperture array 420 impinge on the sample 190 (see Fig. 3) to form probe spots (e.g., probe spots 1O2_1S, 102_2S, and 102_3S of Fig. 3). Note that although beam-limiting aperture array 420 of Fig. 4 is shown with a 5X5 array of twenty five (25) beam-limit apertures, this is only exemplary. In general, a beam-limiting aperture array may include any suitable number of apertures. In Fig. 4, the beam-limit apertures 422 are numbered 1-25 to aid in the description below.

[0058] Fig. 5A illustrates the beam current of the beams emanating through each beam-limit aperture 422 of beam-limiting aperture array 420. Note that the values plotted in Fig. 5A were obtained by simulations. Experimentally obtained values indicate a similar trend in beam current. In Fig. 5A, the Y- axis represents the intensity or beam current of a beam emanating from a bean-limit aperture (in arbitrary units) and the X-axis represents the beam-limit aperture number (marked in Fig. 4). As evidentfrom Fig. 5A, the currents of the beams emanating from the four corner beam-limit apertures (BLAs marked 1, 5, 21, and 25 in Fig. 4) are significantly higher than the adjacent beams. The beam current increases radially from the center to the corner of the beam-limiting aperture array 420.

[0059] Fig. 5B is a bar graph illustrating the non-uniformity in beam current from the center to the corner of beam-limiting aperture array 420 in the embodiment of Fig. 5A. Figs. 5A and 5B illustrate the non-uniformity in beam current emanating from the beam-limiting aperture array 420 for an exemplary electron beam current of 15 nanoamps (nA). In the example of Fig. 5B, the non-uniformity in beam current from the center to the corner of beam-limiting aperture array 420 is about 40%. The non-uniformity in beam current may decrease with decreasing electron beam current. For example, Fig. 5C illustrates the non-uniformity in beam current for an application with an exemplary electron beam current of 4 nA. In Fig. 5C, the non-uniformity in beam currents is about 10%.

[0060] A multi-beam apparatus (such as, for e.g., multi-beam apparatus 300 of Fig. 3) typically requires a large half angle of emission and a high angular intensity to enable multiple beamlets and high-throughput inspection. With the increase of emission half angle, the emission beam current increases and causes the corner beams to have a higher beam current compared to adjacent beams as illustrated in Figs. 5A-5C. Fig. 6 schematically illustrates the electron beam intensity at different emission half angles. A smaller half angle of emission (e.g., half angle 01 ) is used to generate an electron beam of lower current (e.g., 4 nA), and a larger half angle of emission (e.g., half angle 62) is used to generate an electron beam of higher energy (e.g., 15 nA). Consequently, the beam current non- uniformity associated with lower electron beam current (e.g., 4 nA) is expected to be lower than that associated with higher electron beam current (e.g., 15 nA).

[0061] The beams emanating from the beam-limit apertures 422 of beam-limiting aperture array 420 impinge on and interact with the sample to generate a voltage contrast that assist in imaging and defect detection. The generated voltage contrast is highly dependent on beam current. Therefore, non- uniformity in beam current across the sample surface may cause variations in imaging different regions of the sample. For example, the image of a region of the sample probed by a corner beam having higher current (e.g., emanating from beam-limit apertures numbered 1, 5, 21, or 25) may appear different from the image of the region of the sample probed by the center beam with a relatively lower current (e.g., emanating from mean-limit aperture numbered 13). To reduce this non-uniformity in imaging different regions of the sample, in embodiments of the current disclosure, the shape or size of the apertures of the coulomb aperture array 172 or the beam-limiting aperture array 420 (or beam-limiting aperture array 121 of Fig. 3) are adjusted (e.g., increased, decreased, etc.) to make the beam current associated with the beams emanating from the different apertures of the beam-limiting aperture array uniform.

[0062] In some embodiments of the current disclosure, the apertures of the coulomb aperture array (in other words, the coulomb apertures) may be configured to limit the beam current at the corner locations. For example, in the example illustrated in Figs. 5A and 5B, because the four corner beams (e.g., the beams emanating from the beam-limit apertures 422 numbered 1, 5, 21, 25 in Fig. 4) have a significantlyhigher beam current than the other beams (e.g., when the electron beam current is 15 nA), the size of the coulomb apertures above the four corner beam-limit apertures 422 (e.g., the beam-limit apertures 422 numbered 1, 5, 21, 25 in Fig. 4) may be shrunk or reduced in size to limit (e.g., reduce) the size of the beams that exit the four corner beam-limit apertures. In other words, the smaller-sized coulomb apertures above the four corner beam-limit apertures 422 may act to trim and limit the beam size and thereby reduce the beam current.

[0063] In some embodiments, the size of the coulomb apertures of a coulomb aperture array may be changed by replacing one coulomb aperture array with another coulomb aperture array with smaller- sized coulomb apertures above the four corner beam-limit apertures 422 (or at other locations). In some embodiments, the size of the coulomb apertures may be automatically changed by a controller (e.g., controller 50 of EBI system 100 as shown in Fig. 1). For example, in some exemplary embodiments, the coulomb aperture array may be formed by moveable overlapping plates with openings that form the coulomb apertures, and the controller may adjust the size of the apertures by moving (e.g., sliding) a plate with respect to another. It should be noted that this is only exemplary and the size of the coulomb apertures may be automatically adjusted in any manner. For example, in some embodiments, the coulomb aperture array may be formed by a MEMS array and micro-mechanical actuators (or other devices) in the MEMS array may adjust the size of the apertures by physically changing the shape or position of the components that form the aperture. In some embodiments, the MEMS array may include piezoelectric elements that respond to electrical signals and cause mechanical movements that result in changes in aperture size.

[0064] Figs. 7A and 7B schematically illustrate reducing the size of the coulomb aperture 772 above a corner beam-limit aperture 422 (e.g., the beam-limit aperture numbered 25 in Fig. 4) to reduce the size of the corner beam in some exemplary embodiments. Figs. 7A and 7B show the projection of a coulomb aperture 722 on a corner beam-limit aperture 422 (e.g., a coulomb aperture 722 overlaid on a beamlimit aperture 422). In Figs. 7A and 7B, the images on the left side (or left-side images) represent the configuration of an exemplary coulomb aperture 722, before modification or adjustment of the aperture size, and the images on the right side (or right-side images) represent its configuration after adjustment (e.g., reduction in size). As illustrated in these figures (e.g., in the left-side images of Figs. 7A and 7B), in some embodiments, before the adjustment, the coulomb aperture 722 may circumscribe the corner beam-limit aperture 422. In other words, the boundary or periphery of the coulomb aperture 722 may surround or contain the periphery (or perimeter) of the corner beam-limit aperture 422 such that the beam-limit aperture 422 is enclosed within the (projection of) the coulomb aperture 722. It should be noted that although the boundary of the beam-limit aperture 422 is shown to be completely within, and spaced apart from, the boundary of the coulomb aperture 722 (in the left-side images of Figs. 7A and 7B), this is only exemplary. In some embodiments, at least some portions of the boundaries of the coulomb aperture 722 and the beam-limit aperture 422 may overlap. As illustrated in the right-side images of Figs. 7A and 7B, the coulomb aperture 722 (e.g., above the corner beam-limit aperture 422)may be shrunk such that at least a portion of the corner beam-limit aperture 422 is not enclosed within the (e.g., projection of) the overlying coulomb aperture 722. In other words, the size (e.g., the diameter “d”) of beam-limit aperture 422 may be greater than the corresponding size (e.g., the size in the same direction or the width “w”) of the coulomb aperture 722 overlying the beam-limit aperture 422. It should be noted that the size of the coulomb aperture 722 above the corner beam-limit aperture 422 may be reduced in any manner. For example, in some exemplary embodiments, as illustrated in right-side image of Fig. 7A, the length and width of the reduced-size coulomb aperture 722A may be such that one end of the coulomb aperture 722A is positioned over the beam-limit aperture 422. Fig. 7B illustrates another example where only the width of a coulomb aperture 722B in the corner is reduced while its length remains the same or is increased, for example, for ease of alignment.

[0065] Fig. 7C schematically illustrates a cross-sectional view of a coulomb aperture 722 (of a coulomb aperture array 720) overlying a corner beam-limit aperture 422 (of a beam-limiting aperture array 420). Consistent with Figs. 7A and 7B, the left-side image of Fig. 7C represents the configuration of the coulomb aperture 722 before adjustment (e.g., reduction in size) and the right-side image represents its configuration after adjustment. As illustrated in the left-side image of Fig. 7C, before adjustment of the coulomb aperture 722, an electron beam 780 that passes through coulomb aperture 722 is trimmed and reduced in size as it passes through the beam-limit aperture 422. And as illustrated in the right-side image of Fig. 7C, the coulomb aperture 722A (above the corner beam-limit aperture 422) may be shrunk or reduced in size such that, instead of the beam-limit aperture 422, the overlying coulomb aperture 722 trims and limits the size of the electron beam 780 (in the plane shown in Fig. 7C) that passes through the corner beam-limit aperture 422. Note that since the coulomb aperture 722 has an elongated shape and the beam-limit aperture 422 is circular (see, e.g., the right-side images of Figs. 7A and 7B), even after the reduction in size of the coulomb aperture 722A (as illustrated in the right-side image of Fig. 7C), the beam-limit aperture 422 will continue to trim the beam in the direction into the plane of the paper.

[0066] Fig. 8A illustrates a portion of an exemplary coulomb aperture array 720 with its apertures 722 positioned over the beam-limit apertures 422 of an underlying beam-limiting aperture array 420. Only one quarter of the coulomb aperture array 720 is illustrated in Fig. 8A and the beam-limit apertures 422 in Fig. 8A are numbered consistent with Fig. 4. As illustrated in Fig. 8A, in some embodiments, only the size of the coulomb apertures 722 overlying the corner beam-limit apertures (e.g., the beam-limit apertures 422 numbered 1, 5, 21, 25 in Fig. 4) may be shrunk or reduced in size to selectively reduce the size of the beams from the corner beam-limit apertures. As explained with reference to Figs. 5A and 5B, when the electron beam current is high (e.g., 15 nA), the beam current of the corner beams are substantially higher than that of the other beams. Therefore, selectively reducing the size of the corner beams will enable a substantial reduction in the non-uniformity of the beam currents. Meanwhile, since a smaller half angle of emission (e.g., angle 0i of Fig. 6) is used for a lower electron beam current (e.g.,4 nA) application, the reduced size of the corner coulomb apertures 722 may not significantly affect the beam currents in a low electron energy application.

[0067] With reference to Figs. 5A and 5B, in addition to the beam current of the corner beams being significantly higher than the inner beams, the beam currents vary (e.g., increase) radially outwards from the center beam-limit aperture (e.g., beam-limit aperture numbered 13 in Fig. 4) to the corner beamlimit aperture (e.g., the beam-limit apertures 422 numbered 1, 5, 21, 25 in Fig. 4). To reduce this nonuniformity in beam currents, in some embodiments, as illustrated in Fig. 8B, the size of the coulomb apertures 722 may be continuously or progressively varied (e.g., reduced) from the center aperture to the corner apertures. In some such embodiments, the reduction in size (e.g., width “w”) of the coulomb apertures 722 may increase radially from the center aperture to the corner aperture such that W2 > Wi (e.g., as shown in FIG. 8B). It should be noted that the configuration of the coulomb apertures 722 illustrated in Figs. 8A and 8B are only exemplary and these apertures may have any suitable configuration. Furthermore, it should also be noted that, the apertures need not necessarily reduce in size radially from the center to the corner. For example, in some embodiments, the size of the apertures may radially vary in another manner.

[0068] In some exemplary embodiments, the size of the beam-limit apertures 422 of the beam-limiting aperture array 420 may be uniform. In other words, the non-uniformity in beam currents may be reduced by varying the configuration of the coulomb apertures. However, this is only exemplary and in some embodiments, the size of the beam-limit apertures 422 may additionally or alternatively be varied based on the actual distribution of emission density to improve beam current uniformity. For example, as illustrated in Figs. 5B and 5C, for both higher and lower electron beam current applications (e.g., 15 nA and 4 nA), the beam current associated with beam-limit apertures numbered 7 and 8 are smaller than the beam current associated with the center beam-limit aperture numbered 13. In some embodiments, in addition to reducing the size of the coulomb apertures 722 overlying the corner beam-limit apertures 422 (to reduce corner beam currents as described previously), the size of the beam-limit apertures may also be adjusted (e.g., increased or decreased) to reduce such observed non-uniformities. For example, in some such embodiments, the size (e.g., diameter) of the beam-limit apertures numbered 7 and 8 may be increased with respect to the center beam-limit aperture to make the corresponding beam currents more uniform. In general, the size of the beam-limit apertures 422 may be varied in any manner (e.g., increased, decreased, etc.) to reduce the non-uniformity in beam currents. For example, in some embodiments (in addition to or as an alternative to reducing the size of the corner coulomb apertures), the size (e.g., diameter) of the corner beam-limit apertures 422 may be made smaller than the other beam-limit apertures to reduce the beam current of the corner beams. In some embodiments, the size of the beam-limit apertures 422 may be reduced continuously from the center to the corner. In some embodiments, the size of selected beam-limit apertures (or coulomb apertures) may be adjusted (e.g., increased, decreased, etc.) based on the current density distributions predicted by simulations. The size of the beam-limit apertures 422 may be varied in a manner similar to the coulomb limit apertures (e.g.,by replacing the beam-limiting aperture array 420 with another beam limit aperture array with openings of a suitable size, automatically by controller, etc.).

[0069] In some embodiments, a movable aperture array may be positioned above the beam-limiting aperture array 420 (e.g., between the coulomb aperture array and the beam-limiting aperture array, immediately above the beam-limiting aperture array, etc.) to reduce the beam size of selected beams, and thereby make the beam currents more uniform. It should be noted that such a moveable aperture array may be used in addition to or as an alternative to varying the size of the coulomb apertures 722 or the beam-limit apertures 422 (as discussed previously). Fig. 9 illustrates an exemplary embodiment where a moveable aperture array 920 with apertures 922 is used to reduce the beam current associated with selected beams and thereby reduce the non-uniformity of beam currents. The size of apertures 922 (of the moveable aperture array 920) may be varied in any manner (e.g., increased, decreased, etc.) to reduce the non-uniformity in beam currents. For example, in some embodiments, the size (e.g., diameter) of the corner apertures 922 may be made smaller than the other apertures to reduce the beam current of the corner beams. In some embodiments, the size of the apertures 922 may be reduced continuously from the center to the corner. In some embodiments, the size of selected apertures 922 may be adjusted (e.g., increased, decreased, etc.) based on the current density distributions predicted by simulations. In general, calculations or simulations may be used to predict the expected distribution of beam currents (or emission density) in an application, and the sizes of one or more of the coulomb apertures 722, the beam-limit apertures 422, and the apertures 922 of the moveable aperture array 902 may be varied to improve beam current uniformity.

[0070] In some embodiments, different sets of aperture arrays 920 (e.g., with different aperture sizes) may be used for different electron beam current applications (e.g., 4 nA, 15 nA, etc.). For example, when the apparatus is used in a 4 nA application, a first aperture plate with apertures sized to make the beams currents uniform in such an application (see, e.g., Fig. 5C) may be positioned above the beamlimiting aperture array 420 and used to trim the beams (e.g., function as a beam limit aperture) and reduce beam current non-uniformity. And when the apparatus is used in a 15 nA application, a second aperture plate with apertures sized to make the beam current uniform in the 15 nA application (see, e.g., Fig. 5B) may be positioned above the beam-limiting aperture array 420 to trim and size the beams and reduce non-uniformity.

[0071] Thus, in embodiments of the current disclosure, one or a combination of (i) a coulomb aperture array with apertures sized to reduce beam current non-uniformity (e.g., with narrower apertures at the corners), (ii) a beam limit aperture array with apertures sized to reduce non-uniformity (e.g., larger aperture at the center and smaller apertures at the corners), and (iii) an additional aperture plate with apertures sized to reduce beam current non-uniformity may be used to make probe current uniform for the entire current range.

[0072] Reference is now made to Fig. 10, which illustrates a process flowchart representing an exemplary method 1000 of decreasing the non-uniformity of beam currents while imaging a sampleusing a multi-beam inspection tool, consistent with embodiments of the present disclosure. Method 1000 may be performed by controller 50 of EBI system 100, as shown in Fig. 1 , for example. Controller 50 may be programmed to implement one or more steps of method 1000. For example, controller 50 may instruct a module of a charged particle beam apparatus to activate a charged-particle source to generate charged particle beam and carry out other functions described below.

[0073] In step 1010, a charged-particle beam (e.g., primary electron beam 102 of Fig. 2) may be generated by activating a charged-particle source (e.g., electron source 101 of Fig. 2). For example, electron source 101 may be powered on to emit the primary electron beam that is formed along a primary optical axis. 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 be trimmed by a gun aperture plate (e.g., gun aperture plate 171 of Fig. 2) or an aperture array (e.g., coulomb aperture array 172 of Fig. 2), or both.

[0074] In step 1020, a plurality of beamlets (e.g., beamlets 102_l, 102_2, and 102_3 of Fig. 3) may be generated after passing through the apertures of the coulomb aperture array (e.g., coulomb aperture array 172 of Fig. 3, coulomb aperture array 720 of Fig. 7C, etc.). The number of beamlets generated depends on the number of apertures of the coulomb aperture array. In general, the coulomb aperture array may have any number of apertures. The beamlets generated after the primary electron beam passes through the coulomb aperture array may include an on-axis beamlet and at least one off-axis beamlet. The coulomb aperture array may comprise an on-axis aperture and at least one off-axis aperture that generate the on-axis beamlet and the at least one off-axis beamlet, respectively. In some embodiments, the coulomb aperture array may include a plurality of off-axis aperture and a plurality of off-axis beamlets may be generated. The on-axis aperture may be circular or substantially circular in shape or cross-section. The off-axis apertures may be elongated apertures having rounded ends. In some embodiments, the off-axis apertures may have a tapering width. In some embodiments, the off-axis apertures may be elongated and curved. In some embodiments, the beamlet generated by each aperture of the coulomb aperture array may have a shape substantially corresponding to the shape of the aperture (see, e.g., Fig. 4).

[0075] In step 1030, the plurality of beamlets generated by the coulomb aperture array (in step 1020) may be projected on the beam-limiting aperture array (or BEAA). (see, e.g., Fig. 4). The beam-limiting aperture array may also include a plurality of apertures. For example, in some embodiments, the beamlimiting aperture array may include a 5X5 array of apertures as illustrated, for example, in Fig. 4. In some embodiments, the number of apertures of the coulomb aperture array and the beam-limiting aperture array may be the same. In some embodiments, each aperture of the beam-limiting aperture array may be located directly below an aperture of the coulomb aperture array such that a portion of each beamlet (projected on the beam-limiting aperture array from the coulomb aperture array) lands on an aperture of the beam-limiting aperture array. At least a portion of a beamlet that lands on a beamlimit aperture may pass through the aperture and land on the sample. In some embodiments, the size(e.g., width) of each beamlet that lands on a beam-limit aperture may be greater than the size (e.g., diameter) of the aperture such that the size of the beam that lands on the sample is based on the size of the beam-limit aperture (see, e.g., left-side image of Fig. 7C). The beam current associated with the electron beams that land on the sample may be a function of the beam size. In some embodiments, due to the distribution of electron emission density (see, e.g., Fig. 6), the beam current associated with the beams that emanate from the corner beam-limit apertures and land on the sample may be higher than that of the other beams (see, e.g., Figs. 5A-5C).

[0076] In step 1040, the size of the coulomb apertures overlying the corner beam-limit apertures (or corner coulomb apertures) may be adjusted to reduce the beam current associated with the electron beams that emanate from the corner beam-limit apertures. In general, the adjustment of the corner coulomb apertures may be adapted to limit the beam current of the beams emanating from the corner beam-limit apertures. In some embodiments, the size of the corner coulomb apertures may be reduced to trim and thereby reduce the size of the beams that emanate through the corner beam-limit apertures. For example, as described previously (e.g., with reference to Figs. 7A-7C), before adjusting the size of a corner coulomb aperture, when viewed from the top, a corner coulomb aperture may circumscribe a corner beam-limit aperture. In other words, the periphery (perimeter, boundary, etc.) of a corner coulomb aperture may surround or contain a corner beam-limit aperture such that the beam-limit aperture is enclosed within a projection of the coulomb aperture on the beam-limit aperture (see, e.g., left-side images of Figs. 7A-7B). Each corner coulomb aperture may be adjusted such that at least a portion of the corner beam-limit aperture is not enclosed within the projection of the overlying corner coulomb aperture (see, e.g., right-side images of Figs. 7A-7B). In some embodiments, before adjustment of a corner coulomb aperture, an electron beam (or another charged-particle beam) that passes through this coulomb aperture is trimmed and reduced in size as it passes through the corner beam-limit aperture located below this coulomb aperture (see, e.g., the left-side image of Fig. 7C, 422). In contrast, after the size of a corner coulomb aperture is reduced, instead of the corner beam-limit aperture below this coulomb aperture, the coulomb aperture may trim the beam in at least some directions and limit the size of the electron beam that passes through the corner beam-limit aperture.

[0077] In some embodiments, in addition to adjusting the size of the corner coulomb apertures (step 1040) to reduce the beam current of the beams emanating from the corner beam-limit apertures, the size of the coulomb apertures may be progressively adjusted from the center to the corner to produce corresponding changes in the beam current of the beams emanating from the other beam-limit apertures. Such adjustments to the size of the coulomb apertures may be made to make the beam currents more uniform. In some embodiments, in addition to or as an alternative to adjusting the size of the coulomb apertures, adjustments may be made to the size of the beam-limit apertures to make the beam current uniform. For example, in some embodiments, the size of the corner beam-limit apertures may be made smaller than the other beam-limit apertures. In some embodiments, in addition to or as an alternative to adjusting the size of the coulomb apertures or beam-limit apertures, a movable aperture array may bepositioned upstream (e.g., above) of the beam-limit apertures to reduce the beam size of selected beams and thereby make the beam currents more uniform. In some embodiments, numerical simulations (or calculations) may be used to predict the expected distribution of beam currents, and the sizes of one or more of the coulomb apertures, the beam-limit apertures, and the apertures of the moveable aperture array may be adjusted (e.g., increased, decreased, etc.) to improve beam current uniformity.

[0078] The embodiments may be further described using the following clauses:1. A charged-particle beam apparatus comprising: a first aperture array including a plurality of elongated apertures, the first aperture array being configured to generate a plurality of primary beamlets from a primary charged-particle beam and project the plurality of primary beamlets on a second aperture array; and the second aperture array including a plurality of apertures configured to generate a plurality of probing beams from the plurality of primary beamlets, wherein a first aperture located at a corner of the plurality of elongated apertures of the first aperture array is vertically aligned with a second aperture located at a corner of the plurality of apertures of the second aperture array, and wherein a size of the first aperture in a first direction is smaller than the size of the second aperture in the first direction.2. The apparatus of clause 1, wherein the first aperture array is a coulomb aperture array and the second aperture array is a beam-limiting aperture array, and wherein a number of elongated apertures of the first aperture array is same as a number of apertures of the second aperture array.3. The apparatus of clause 2, wherein each aperture of the second aperture array is vertically aligned with an elongated aperture of the first aperture array.4. The apparatus of clause 3, wherein a size of each elongated aperture of the first aperture array in the first direction is smaller than the size of a vertically aligned aperture of the second aperture array in the first direction.5. The apparatus of any of clauses 1-4, wherein a size of the second aperture in the first direction is different from the size of another aperture of the plurality of apertures of the second aperture array.6. A method for generating probing beams using a charged-particle beam apparatus, comprising: generating a plurality of beamlets by passing a primary charged-particle beam through a plurality of first apertures of a first aperture array; generating a plurality of probing beams by passing the plurality of beamlets through a plurality of second apertures of a second aperture array; and adjusting a size of a selected first aperture of the plurality of first apertures to reduce a beam current of a selected probing beam of the plurality of probing beams.7. The method of clause 6, wherein the selected probing beam is a probing beam generated by a selected second aperture located at a corner of the plurality of second apertures, and the selected first aperture is a first aperture vertically aligned with the selected second aperture.8. The method of any of clauses 6-7, wherein adjusting the size of the selected first aperture includes reducing the size of the selected first aperture.9. The method of clause 7, wherein adjusting the size of the selected first aperture includes adjusting the size of the selected first aperture in a first direction to be smaller than the size of the selected second aperture in the first direction.10. The method of any of clauses 7 or 9, further comprising reducing the size of the selected second aperture.11. The method of clause 6, further comprising adjusting the size of additional first apertures of the plurality of first apertures to reduce the beam current of additional probing beams of the plurality of probing beams.12. The method of any of clauses 6, 7, 9, or 11, further comprising reducing the size of one or more second apertures of the plurality of second apertures to reduce the beam current of one or more probing beams of the plurality of probing beams.13. The method of clause 6, further comprising passing the plurality of beamlets generated by the first aperture array through a plurality of third apertures of a third aperture array before passing the plurality of beamlets through the plurality of second apertures of the second aperture array, and adjusting a size of a third aperture of the plurality of third apertures to reduce the beam current of a probing beam of the plurality of probing beams.14. The method of clause 6, wherein adjusting the size of the selected first aperture includes replacing the plurality of first apertures with another plurality of first apertures having one or more first apertures of a different size.15. The method of clause 6, wherein adjusting the size of the selected first aperture includes automatically adjusting the size of the selected first aperture.16. A non-transitory computer readable medium that stores a set of instructions that is executable by at least on processor of a computing device to cause the computing device to perform operations for generating probing beams using a charged-particle beam apparatus, the operations comprising: generating a charged-particle beam and directing the charged-particle beam to pass through a plurality of first apertures of a first aperture array to produce a plurality of beamlets to pass through a plurality of second apertures of a second aperture array to produce a plurality of probing beams; and adjusting a size of a selected first aperture of the plurality of first apertures to reduce a beam current of a selected probing beam of the plurality of probing beams.17. The non-transitory computer readable medium of clause 16, wherein the selected probing beam is a probing beam generated by a selected second aperture located at a corner of the plurality of second apertures, and the selected first aperture is a first aperture vertically aligned with the selected second aperture.18. The non-transitory computer readable medium of any of clauses 16-17, wherein adjusting the size of the selected first aperture includes reducing the size of the selected first aperture.19. The non-transitory computer readable medium of clause 17, wherein adjusting the size of the selected first aperture includes adjusting the size of the selected first aperture in a first direction to be smaller than the size of the selected second aperture in the first direction.20. The non-transitory computer readable medium of clauses 17 or 19, further comprising reducing the size of the selected second aperture.21. The non-transitory computer readable medium of clause 16, wherein the operations further comprise adjusting the size of additional first apertures of the plurality of first apertures to reduce the beam current of additional probing beams of the plurality of probing beams.22. The non-transitory computer readable medium of clause 16, wherein adjusting the size of the selected first aperture includes automatically adjusting the size of the selected first aperture.23. The non-transitory computer readable medium of any of clauses 16, 17, 19, or 21, wherein the operations further comprise reducing the size of one or more second apertures of the plurality of second apertures to reduce beam currents of one or more probing beams of the plurality of probing beams.24. A non-transitory computer readable medium that stores a set of instructions that is executable by at least on processor of a computing device to cause the computing device to perform operations for generating probing beams using a charged-particle beam apparatus, the operations comprising: generating a charged-particle beam and directing the charged-particle beam to pass through a plurality of first apertures of a first aperture array to produce a plurality of beamlets for passing through a plurality of second apertures of a second aperture array to produce a plurality of second beamlets for passing through a plurality of third apertures of a third aperture array to produce a plurality of probing beams; and adjusting a size of a selected second aperture of the plurality of second apertures to reduce a beam current of a selected probing beam of the plurality of probing beams.25. The non-transitory computer readable medium of clause 24, wherein the selected probing beam is a probing beam generated by a selected third aperture located at a corner of the plurality of third apertures, and the selected second aperture is a second aperture vertically aligned with the selected third aperture.26. The non-transitory computer readable medium of any of clauses 24-25, wherein adjusting the size of the selected second aperture includes reducing the size of the selected second aperture.27. The non-transitory computer readable medium of clause 25, wherein adjusting the size of the selected second aperture includes adjusting the size of the selected second aperture in a first direction to be smaller than the size of the selected third aperture in the first direction.28. The non-transitory computer readable medium of clauses 24 or 27, wherein the operations further comprise reducing the size of the selected third aperture.29. The non-transitory computer readable medium of clause 24, wherein the operations further comprise adjusting the size of additional second apertures of the plurality of second apertures to reduce the beam current of additional probing beams of the plurality of probing beams.30. The non-transitory computer readable medium of any of clauses 24, 25, 27, or 29, wherein the operations further comprise reducing the size of one or more third apertures of the plurality of third apertures to reduce beam currents of one or more probing beams of the plurality of probing beams.31. The non-transitory computer readable medium of any of clauses 24, 25, 27, or 29, wherein adjusting the size of the selected second aperture includes automatically adjusting the size of the selected second aperture.

[0079] 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 disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the disclosure 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 charged-particle beam apparatus comprising: a first aperture array including a plurality of elongated apertures, the first aperture array being configured to generate a plurality of primary beamlets from a primary charged-particle beam and project the plurality of primary beamlets on a second aperture array; and the second aperture array including a plurality of apertures configured to generate a plurality of probing beams from the plurality of primary beamlets, wherein a first aperture located at a corner of the plurality of elongated apertures of the first aperture array is vertically aligned with a second aperture located at a corner of the plurality of apertures of the second aperture array, and wherein a size of the first aperture in a first direction is smaller than the size of the second aperture in the first direction.

2. The apparatus of claim 1 , wherein the first aperture array is a coulomb aperture array and the second aperture array is a beam-limiting aperture array, and wherein a number of elongated apertures of the first aperture array is same as a number of apertures of the second aperture array.

3. The apparatus of claim 2, wherein each aperture of the second aperture array is vertically aligned with an elongated aperture of the first aperture array.

4. The apparatus of claim 3, wherein a size of each elongated aperture of the first aperture array in the first direction is smaller than the size of a vertically aligned aperture of the second aperture array in the first direction.

5. The apparatus of claim 1, wherein a size of the second aperture in the first direction is different from the size of another aperture of the plurality of apertures of the second aperture array.

6. A method for generating probing beams using a charged-particle beam apparatus, comprising: generating a plurality of beamlets by passing a primary charged-particle beam through a plurality of first apertures of a first aperture array; generating a plurality of probing beams by passing the plurality of beamlets through a plurality of second apertures of a second aperture array; and adjusting a size of a selected first aperture of the plurality of first apertures to reduce a beam current of a selected probing beam of the plurality of probing beams.

7. The method of claim 6, wherein the selected probing beam is a probing beam generated by a selected second aperture located at a corner of the plurality of second apertures, and the selected first aperture is a first aperture vertically aligned with the selected second aperture.

8. The method of claim 6, wherein adjusting the size of the selected first aperture includes reducing the size of the selected first aperture.

9. The method of claim 7, wherein adjusting the size of the selected first aperture includes adjusting the size of the selected first aperture in a first direction to be smaller than the size of the selected second aperture in the first direction.

10. The method of claim 7, further comprising reducing the size of the selected second aperture.

11. A non-transitory computer readable medium that stores a set of instructions that is executable by at least on processor of a computing device to cause the computing device to perform operations for generating probing beams using a charged-particle beam apparatus, the operations comprising: generating a charged-particle beam and directing the charged-particle beam to pass through a plurality of first apertures of a first aperture array to produce a plurality of beamlets to pass through a plurality of second apertures of a second aperture array to produce a plurality of probing beams; and adjusting a size of a selected first aperture of the plurality of first apertures to reduce a beam current of a selected probing beam of the plurality of probing beams.

12. The non-transitory computer readable medium of claim 11, wherein the selected probing beam is a probing beam generated by a selected second aperture located at a corner of the plurality of second apertures, and the selected first aperture is a first aperture vertically aligned with the selected second aperture.

13. The non-transitory computer readable medium of claim 11, wherein adjusting the size of the selected first aperture includes reducing the size of the selected first aperture.

14. The non-transitory computer readable medium of claim 12, wherein adjusting the size of the selected first aperture includes adjusting the size of the selected first aperture in a first direction to be smaller than the size of the selected second aperture in the first direction.

15. The non-transitory computer readable medium of claim 12, further comprising reducing the size of the selected second aperture.

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