Method and system for precisely focusing a secondary beam spot onto a detector for a multi-beam inspection system

Abstract

The system and method for optimizing collection efficiency of secondary charged particles includes a multi-beam inspection device (104) configured to scan (226) a sample (230) and including a lens (242), a detector (244) configured to receive multiple secondary charged particle beams (236, 238, 240) in response to the scanning of the sample, and a controller (296) including circuitry communicatively coupled to the multi-beam inspection device and the detector, wherein the controller is configured to: focus the lens to adjust a size of a secondary beam spot, where the secondary beam spot is formed on the detector by the multiple secondary charged particle beams; for each secondary charged particle beam of the multiple secondary charged particle beams, prevent stray charged particles of each secondary charged particle beam from being detected by the detector; and refocus the lens to adjust a current of a portion of the multiple secondary charged particle beams detected by the detector, where the stray charged particles do not contribute to the current.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Application No. 63 / 368,597, filed July 15, 2022, and incorporated herein by reference in its entirety.

[0002] TECHNICAL FIELD

[0002] The description herein relates to the field of imaging inspection systems, and more particularly to methods and systems for precisely focusing secondary beam spots onto detectors for multi-beam inspection systems. [Background technology]

[0003]

[0003] An imaging inspection device (e.g., a charged particle beam device or an optical beam device) can generate two-dimensional (2D) images of a wafer substrate by detecting particles (e.g., secondary electrons, backscattered electrons, other types of electrons or photons) from the wafer substrate surface as it is scanned with one or more beams (e.g., including a charged particle beam or an optical beam) generated from a source associated with the inspection device. In the semiconductor industry, various imaging inspection devices are used on semiconductor wafers for various purposes, such as critical dimension measurement (CD-SEM), wafer inspection (e.g., electron beam inspection systems), or defect analysis (e.g., defect review SEM (DR-SEM)).

[0004]

[0004] In semiconductor manufacturing, 2D images of wafers can be recorded and analyzed to control the quality of manufactured structures on the wafer substrate and to detect potential defects. As the physical size of IC components continues to shrink, accuracy and yield in defect detection become increasingly important. An essential aspect for increasing throughput and accuracy in defect detection is the use of multiple beams to simultaneously scan multiple areas on the wafer surface. This concept is realized in multi-beam scanning inspection systems, for example, multi-beam SEM systems.

[0005]

[0005] For a multi-beam inspection system, the collection efficiency of secondary electrons and the crosstalk between secondary electron beamlets are two essential parameters. To achieve the best performance, the collection efficiency must be maximized while the crosstalk must be minimized. The collection efficiency depends on how precisely the secondary charged particles are focused to form a spot on the detector. Optimizing how precisely the secondary charged particles are focused can improve the collection efficiency and ultimately improve the yield in defect detection. Summary of the Invention

[0006]

[0006] Embodiments of the present disclosure provide systems and methods for optimizing secondary charged particle collection efficiency. In some embodiments, the system may include a multi-beam inspection device configured to scan a sample and including a lens; a detector configured to receive multiple secondary charged particle beams in response to scanning the sample; and a controller including circuitry communicatively coupled to the multi-beam inspection device and the detector. The controller may be configured to focus the lens to adjust a size of a secondary beam spot, the secondary beam spot being formed on the detector by the multiple secondary charged particle beams. The controller may also be configured, for each of the multiple secondary charged particle beams, to prevent stray charged particles of each secondary charged particle beam from being detected by the detector. The controller may further be configured to refocus the lens to adjust a current of a portion of the multiple secondary charged particle beams detected by the detector, the stray charged particles not contributing to the current.

[0007] In some embodiments, a non-transitory computer-readable medium can store a set of instructions executable by at least one processor of a multi-beam inspection apparatus to cause the multi-beam inspection apparatus to perform a method. The method can include focusing a lens of the multi-beam inspection apparatus to adjust a size of a secondary beam spot, where the secondary beam spot is formed on a detector by the multiple secondary charged particle beams. The method can also include, for each secondary charged particle beam of the multiple secondary charged particle beams, preventing stray charged particles of each secondary charged particle beam from being detected by the detector. The method can further include refocusing a lens to adjust a current of a portion of the multiple secondary charged particle beams detected by the detector, where the stray charged particles do not contribute to the current.

[0008]

[0008] In some embodiments, the method for optimizing the collection efficiency of secondary charged particles may include focusing a lens of a multi-beam inspection apparatus to adjust a size of a secondary beam spot, where the secondary beam spot is formed on a detector by the multiple secondary charged particle beams. The method may also include, for each secondary charged particle beam of the multiple secondary charged particle beams, preventing stray charged particles of each secondary charged particle beam from being detected by the detector. The method may further include refocusing the lens to adjust a current of a portion of the multiple secondary charged particle beams detected by the detector, where the stray charged particles do not contribute to the current. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram illustrating an exemplary charged particle beam inspection (CPBI) system consistent with some embodiments of the present disclosure. [Figure 2]

[0010] 2 is a schematic diagram illustrating an example multi-beam beam tool that may be part of the example charged particle beam inspection system of FIG. 1, consistent with some embodiments of the present disclosure. [Figure 3]

[0011] FIG. 3 is a schematic diagram illustrating an example secondary projection system that may be part of the example multi-beam tool of FIG. 2, consistent with some embodiments of the present disclosure. [Figure 4A]

[0012] FIG. 1 is a schematic diagram illustrating an example of a paraxial focusing technique for a charged particle beam, consistent with certain embodiments of the present disclosure. [Figure 4B]

[0013] FIG. 1 is a schematic diagram illustrating an example of a minimal confusion focusing technique for a charged particle beam, consistent with certain embodiments of the present disclosure. [Figure 5A]

[0014] 1 is a graph visualizing the cross section of an exemplary charged particle beam on the plane of a beam-limiting aperture, consistent with certain embodiments of the present disclosure. [Figure 5B]

[0015] 5B is a graph illustrating a profile of the example charged particle beam of FIG. 5A, consistent with certain embodiments of the present disclosure. [Figure 6]

[0016] 10 is a graph illustrating an exemplary beam spot size optimization using different beam focusing techniques, consistent with certain embodiments of the present disclosure. [Figure 7]

[0017] 10A-10C are graphs illustrating exemplary signal intensity distributions of secondary beam spots corresponding to different beam focusing approaches, consistent with certain embodiments of the present disclosure. [Figure 8A]

[0018] 10 is a graph illustrating exemplary integrated signal intensities of secondary beam spots corresponding to different beam focusing approaches, consistent with certain embodiments of the present disclosure. [Figure 8B]

[0019] 10 is a graph illustrating the difference between exemplary integrated signal intensities of secondary beam spots corresponding to different beam focusing approaches, consistent with certain embodiments of the present disclosure. [Figure 9]

[0020] 10 is a graph illustrating exemplary secondary beam spot profiles on the plane of the beam-limiting aperture corresponding to different beam focusing approaches, consistent with certain embodiments of the present disclosure. [Figure 10A]

[0021] 10 is a graph illustrating exemplary integrated signal intensities of secondary beam spots corresponding to different beam focusing approaches, consistent with certain embodiments of the present disclosure. [Figure 10B]

[0022] 10 is a graph illustrating the difference between exemplary integrated signal intensities of secondary beam spots corresponding to different beam focusing approaches, consistent with certain embodiments of the present disclosure. [Figure 11]

[0023] 1 is a flowchart illustrating an exemplary method for optimizing collection efficiency of secondary charged particles, consistent with certain embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010]

[0024] 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 like numbers in different drawings represent the same or similar elements unless otherwise specified. The implementations described in the following description of example embodiments do not represent all implementations consistent with the present disclosure. Rather, they are merely examples of apparatus and methods consistent with aspects related to the subject matter recited in the appended claims. Without limiting the scope of the present disclosure, some embodiments may be described in the context of providing detection systems and methods in systems utilizing electron beams ("e-beams"). However, the present disclosure is not limited thereto. Other types of charged particle beams (e.g., including protons, ions, or any other particles carrying an electric charge) may be similarly irradiated. Furthermore, the systems and methods for detection may be used in other measurement systems, such as optical imaging, light detection, x-ray detection, ion detection, etc.

[0011]

[0025] A microchip consists of circuits formed on a piece of semiconductor material called a substrate. The semiconductor material can include, for example, silicon, gallium arsenide, indium phosphide, or silicon germanium. Many circuits can be formed together on the same piece of silicon and are called integrated circuits, or ICs. The dimensions of these circuits have been dramatically reduced so that many more circuits can fit on a substrate. For example, the IC chip in a smartphone can be as small as a thumbnail, yet contain over 2 billion transistors, each smaller than 1 / 1000 the size of a human hair.

[0012]

[0026] Fabricating ICs with these tiny components and structures is a complex, time-consuming, and expensive process, often involving hundreds of individual steps. An error in just one step can result in a defect in the finished IC, rendering it unusable. Therefore, one of the goals of a manufacturing process is to avoid such defects and maximize the number of functional ICs produced in the process, i.e., to improve the overall yield of the process.

[0013]

[0027] One component of improving yield is monitoring the chip fabrication process. One way to monitor the process is to inspect the chip circuit structures at various stages in their formation. Inspection can be done using a scanning charged particle microscope ("SCPM"). For example, a scanning charged particle microscope can be a scanning electron microscope (SEM). A scanning charged particle microscope can actually take a "picture" of the structures on the wafer and can be used to image those tiny structures. This image can be used to determine if the structures were properly formed in the correct location. If the structures are defective, the process can be adjusted to make the defect less likely to reoccur.

[0014]

[0028] In a single-beam SEM, a surface image can be created by scanning a focused primary electron beam line-by-line across an inspection area. When the primary electron beam strikes a surface, it forms a spot, which may be referred to as a "probe spot," and secondary electrons and backscattered electrons are emitted in response to the primary beam. In this disclosure, unless explicitly stated, the term "secondary electrons" can encompass both secondary electrons and backscattered electrons. A surface image can be reconstructed by collecting secondary electrons emitted from the probe spot on the surface. A relationship between the intensity of the secondary electrons and the position of the probe spot can be determined. Such a relationship can also be presented (e.g., as a two-dimensional plot). Creating a high-resolution image of a surface line-by-line can be a time-consuming process, even with the SEM's fast scanning speed. As a result, wafer inspection can be very time-consuming.

[0015]

[0029] Multi-beam SEM systems can improve measurement speed and achieve higher throughput in wafer inspection applications. In a multi-beam SEM, an array of primary electron beams (also called "beamlets") can be formed to simultaneously scan multiple subregions within an inspection region. Each primary electron beamlet can form a probe spot on a subregion of the inspection region, and the formed probe spots can form an array corresponding to the array of primary electron beamlets. Multiple secondary electron beamlets can be formed from the probe spots and directed to a detector via a secondary electron column. The detector can include an array of electron detection elements (referred to herein as "detector elements"). The array of detector elements can be implemented as an array of individual sensors, as a two-dimensional pixel detector, or in any other form. When the array of detector elements is implemented as a two-dimensional pixel detector, each of the detector elements can be implemented as a different pixel group. Each pixel group may be referred to in this disclosure as a detector element (or alternatively as a "detector cell"), and the pixels forming the group that forms the detector cell may be configurable (e.g., via a switch network between the pixels). The secondary electron column may be configured such that each detector element detects the intensity of a secondary electron beamlet corresponding to one of the probe spots, which in turn corresponds to a sub-region of the examination region.

[0016]

[0030] In this disclosure, the collection efficiency of a single detector element refers to the proportion of secondary electrons emitted from a probe spot in one sub-region of the examination region and detected by the corresponding detector element, or the ratio of the number of detected electrons to the number of emitted electrons. For characterization of the entire detector, the average, minimum, or maximum collection efficiency across all detector elements can be used.

[0017]

[0031] In this disclosure, crosstalk refers to the proportion of secondary electrons detected by an individual detector element that do not originate from its corresponding sub-region of the examination region, but rather, for example, from one or more of its neighboring sub-regions, or refers to the ratio of the number of detected electrons that do not originate from the corresponding sub-region to the total number of electrons detected by an individual detector element.

[0018]

[0032] High crosstalk values can degrade the performance of a multi-beam charged particle inspection system. To reduce crosstalk, several techniques can be employed. For example, a beam-limiting aperture (or "BLA") can be positioned at a point along the secondary electron optical path between the wafer surface and the detector (e.g., just before the detector) to eliminate the tail of the secondary electron beam distribution at the beam spot. All charged particles outside the beam-limiting aperture may be collectively referred to as the "tail" of the beam spot in this disclosure. The tail of the beam spot may strongly contribute to crosstalk at the detector. As another example, if the detector is a detector array (e.g., a pixel detector) including an array of detector elements (e.g., subunits, cells, or groups of pixel detector elements), the size of the detector elements (e.g., detector cell size) can be set to a limited size to reduce crosstalk (e.g., by reducing the size of the detector elements to reduce the amount of "tail" electrons collected by the detector elements). Adding a beam-limiting aperture or limiting the size of the detector elements can reduce crosstalk, but collection efficiency may also be reduced. As such, collection efficiency may be limited by a given level of crosstalk. To maximize the throughput and defect detection accuracy of a multi-beam charged particle inspection system, in some embodiments, it may be paramount to maximize collection efficiency at a given level of crosstalk ratio for any combination of parameters of the multi-beam charged particle inspection system.

[0019]

[0033] Maximizing collection efficiency is extremely challenging. In many existing technical solutions, the standard approach for focusing a charged particle beam spot on a detector is to minimize the size of the beam spot without considering the size of the detector elements and without using a beam-limiting aperture. Focus parameters (e.g., excitation of the condenser lens) are typically fixed after such a minimization process. However, if the crosstalk ratio is excessively high even after fixing the focus parameters and minimizing the beam spot size, the crosstalk may be suppressed by adding a beam-limiting aperture or limiting the size of the detector elements. In such cases, because the focus parameters are fixed before adding the beam-limiting aperture or limiting the size of the detector elements, the collection efficiency may not reach its theoretically possible maximum value.

[0020]

[0034] Embodiments of the present disclosure may provide methods, devices, and systems for optimizing the collection efficiency of secondary charged particles. In some disclosed embodiments, during optimization of focus parameters of a multi-beam inspection apparatus, one or more lenses may be focused to adjust (e.g., minimize) the spot size of multiple secondary beams on a detector. Also, stray charged particles in each of the multiple secondary beams may be prevented from being detected by the detector (e.g., by using a beam-limiting aperture, by limiting the size of the detector element, etc.). The focus parameters of the lenses may then be adjusted to adjust (e.g., maximize) the current of a portion of the multiple secondary beams detected by the detector, such that the portion of the multiple secondary beams with the maximized current detected by the detector does not contain stray charged particles. Compared to existing techniques, crosstalk may be suppressed or reduced to below a predetermined level, while the collection efficiency corresponding to a predetermined level of crosstalk may be improved or maximized, taking into account the size of the detector element or the use or non-use of a beam-limiting aperture.

[0021]

[0035] The relative dimensions of components in the drawings may be exaggerated for clarity. In the following description of the drawings, the same or similar reference numerals refer to the same or similar components or entities, and only the differences with respect to the individual embodiments are described.

[0022]

[0036] As used herein, unless otherwise stated, the term "or" includes all possible combinations unless impractical. For example, if a component is described as being able to include A or B, the component can include A, or B, or A and B, unless otherwise stated or impractical. As a second example, if a component is described as being able to include A, B, or C, the component can include A, or B, or C, or A and B, or A and C, or B and C, or A, B, and C, unless otherwise stated or impractical.

[0023]

[0037] FIG. 1 illustrates an exemplary charged particle beam inspection (CPBI) system 100 consistent with some embodiments of the present disclosure. The CPBI system 100 can be used for imaging. For example, the CPBI system 100 can use an electron beam for imaging. As shown in FIG. 1 , the CPBI system 100 includes a main chamber 101, a load / lock chamber 102, a beam tool 104, and a front-end equipment module (EFEM) 106. The beam tool 104 is disposed inside the main chamber 101. The EFEM 106 includes a first load port 106a and a second load port 106b. The EFEM 106 may include additional load ports. The first load port 106a and the second load port 106b receive wafer FOUPs (front opening unified pods) containing wafers (e.g., semiconductor wafers or wafers made of other materials) or samples (wafer and sample may be used interchangeably) to be inspected. A "lot" is a number of wafers that can be loaded for processing as a batch.

[0024]

[0038] One or more robot arms (not shown) in the EFEM 106 can transfer wafers to the load / lock chamber 102. The load / lock chamber 102 is connected to a load / lock vacuum pumping system (not shown), which removes gas molecules from the load / lock chamber 102 to reach a first pressure lower than atmospheric pressure. After the first pressure is reached, one or more robot arms (not shown) can transfer the wafers from the load / lock chamber 102 to the main chamber 101. The main chamber 101 is connected to a main chamber vacuum pumping system (not shown), which removes gas molecules from the main chamber 101 to reach a second pressure lower than the first pressure. After the second pressure is reached, the wafers are subjected to inspection by the beam tool 104. The beam tool 104 can be a single beam system or a multi-beam system.

[0025]

[0039] A controller 109 is electronically connected to the beam tool 104. The controller 109 may be a computer that may perform various controls of the CPBI system 100. While the controller 109 is illustrated in Figure 1 as being external to the structure including the main chamber 101, the load / lock chamber 102, and the EFEM 106, it will be understood that the controller 109 may be part of the structure.

[0026]

[0040] In some embodiments, the controller 109 can include one or more processors (not shown). A processor can be a general-purpose or specific electronic device capable of manipulating or processing information. For example, a processor can include any number and combination of central processing units (i.e., "CPUs"), graphics processing units (i.e., "GPUs"), optical processors, programmable logic controllers, microcontrollers, microprocessors, digital signal processors, intellectual property (IP) cores, programmable logic arrays (PLAs), programmable array logic (PALs), general array logic (GALs), complex programmable logic devices (CPLDs), field programmable gate arrays (FPGAs), systems-on-chips (SoCs), application-specific integrated circuits (ASICs), and any type of circuitry capable of processing data. A processor can also be a virtual processor, including one or more processors distributed across multiple machines or devices coupled via a network.

[0027]

[0041] In some embodiments, the controller 109 may further include one or more memories (not shown). Memory may be a general-purpose or specialized electronic device capable of storing code and data accessible to a processor (e.g., via a bus). For example, memory may include any combination of any number of random access memories (RAMs), read-only memories (ROMs), optical disks, magnetic disks, hard drives, solid-state drives, flash drives, security digital (SD) cards, memory sticks, compact flash (CF) cards, or any type of storage device. Code may include an operating system (OS) and one or more application programs (i.e., "apps") for specific tasks. Memory may also be virtual memory, including one or more memories distributed across multiple machines or devices coupled via a network.

[0028]

[0042] In some embodiments, the beam tool 104 may be a multi-beam system. By way of example, Figure 2 shows a schematic diagram of an exemplary multi-beam beam tool 104 (also referred to herein as apparatus 104) and image processing system 290 that may be used in the CPBI system 100 of Figure 1, consistent with embodiments of the present disclosure.

[0029]

[0043] 2 , the beam tool 104 includes a charged particle source 202, a gun aperture 204, a condenser lens 206, a primary charged particle beam 210 emitted from the charged particle source 202, a source conversion unit 212, multiple primary beamlets of the primary charged particle beam 210 (including primary beamlets 214, 216, and 218), a primary projection system 220, a motorized wafer stage 280, a wafer holder 282, multiple secondary beamlets 236, 238, and 240, a secondary projection system 242, and a charged particle detector 244. The primary projection system 220 may include a beam separator 222, a deflection scanning unit 226, and an objective lens 228. The charged particle detector 244 may include detector elements 246, 248, and 250.

[0030]

[0044] The charged particle source 202, the gun aperture 204, the condenser lens 206, the source conversion unit 212, the beam separator 222, the deflection scanning unit 226, and the objective lens 228 can be aligned with a primary projection axis 260 (e.g., similar to a primary optical axis in an optical system) of the apparatus 104. The secondary projection system 242 and the charged particle detector 244 can be aligned with a secondary projection axis 252 (e.g., similar to a secondary optical axis in an optical system) of the apparatus 104.

[0031]

[0045] The charged particle source 202 can emit one or more charged particles, such as electrons, protons, ions, or any other particles having an electric charge. In some embodiments, the charged particle source 202 can be an electron source. For example, the charged particle source 202 can include a cathode, an extractor, or an anode. Primary electrons can be emitted from the cathode and then extracted or accelerated to form a primary charged particle beam 210 (e.g., a primary electron beam) with a crossover (e.g., a virtual or real image) 208. The primary charged particle beam 210 can be visualized in FIG. 2 as being emitted from the crossover 208. The gun aperture 204 can block charged particles around the primary charged particle beam 210 to reduce the Coulomb effect. The Coulomb effect can result in an increase in the size of the probe spot.

[0032]

[0046] The source conversion unit 212 may include an image-forming element array and a beam-limiting aperture array. The image-forming element array may include a micro-deflector or a micro-lens array. The image-forming element array may form multiple parallel images (virtual or real) of the crossover 208 using multiple primary beamlets (including primary beamlets 214, 216, and 218) of the primary charged particle beam 210. The beam-limiting aperture array may limit the multiple beamlets 214, 216, and 218. Although three primary beamlets 214, 216, and 218 are shown in FIG. 2 , embodiments of the present disclosure are not so limited.

[0033]

[0047] The condenser lens 206 can focus the primary charged particle beam 210. The currents of the primary beamlets 214, 216, 218 downstream of the source conversion unit 212 can be varied by adjusting the focusing power (e.g., excitation) of the condenser lens 206 or by changing the radial size of the corresponding beam-limiting aperture in the beam-limiting aperture array. The objective lens 228 can focus the primary beamlets 214, 216, 218 onto the sample 230 for imaging and can form multiple probe spots (including probe spots 270, 272, 274) on or near the surface of the sample 230 (e.g., a wafer).

[0034]

[0048] As an example, the charged particle source 202 may be an electron source, and the primary beamlets 214, 216, and 218 may be electron beamlets. The primary electron beamlets may penetrate the surface of the sample 230 to a certain depth (e.g., several nanometers to several micrometers) and interact with particles of the sample 230. Some electrons of the primary electron beamlets may interact with particles of the sample 230 elastically (e.g., in the form of elastic scattering) and be reflected or recoiled from the surface of the sample 230. The elastic interaction preserves the kinetic energy of the interaction bodies (e.g., electrons of the primary electron beamlets and particles of the sample 230), and the kinetic energy of the interaction bodies is not converted to other forms of energy (e.g., heat). Such reflected electrons resulting from the elastic interaction may be referred to as backscattered electrons (BSE). Some electrons of the primary electron beamlets may also interact with particles of the sample 230 inelastically (e.g., in the form of inelastic scattering). Inelastic interactions do not conserve the kinetic energy of the interacting bodies; some or all of the kinetic energy of the interacting bodies may be converted to other forms of energy. For example, inelastic interactions may ionize some particles of the sample 230, and the ionized particles may generate additional electrons, which may be called secondary electrons (SEs). Secondary electrons may be emitted from the surface of the sample 230. The yield or emission rate of BSEs and SEs may depend, among other things, on the energy of the electrons of the primary electron beamlet and the material of the sample 230. The number of BSEs and SEs may be greater than, less than, or equal to the injected electrons of the primary electron beamlet. To facilitate unambiguous context, hereinafter, backscattered electrons and secondary electrons may be referred to as "secondary electrons" unless explicitly stated otherwise. Also, as used herein, a "probe spot" (e.g., probe spot 270, 272, or 274) refers to a region on or near the surface of the sample under examination, which region emits secondary charged particles (e.g., secondary electrons) corresponding to the incident charged particle beam or beamlet.

[0035]

[0049] Still referring to FIG. 2 , the beam separator 222 may include a beam separator (e.g., a Wien filter type) that generates an electrostatic dipole field and a magnetic dipole field. In some embodiments, the force exerted on the charged particles (e.g., electrons) of the primary beamlets 214, 216, 218 by the electrostatic dipole field may be substantially equal to the force exerted on the charged particles by the magnetic dipole field, with the same magnitude and opposite direction. Accordingly, the primary beamlets 214, 216, 218 may pass straight through the beam separator 222 with zero deflection angle. In some cases, the total dispersion of the primary beamlets 214, 216, 218 generated by the beam separator 222 may be non-zero.

[0036]

[0050] The deflection scanning unit 226 can deflect the primary beamlets 214, 216, 218 to scan across a surface region of the sample 230. In response to the incidence of the primary beamlets 214, 216, 218, secondary charged particle beams (including secondary beamlets 236, 238, 240) can be emitted from probe spots 270, 272, 274 on the sample 230. The secondary beamlets 236, 238, 240 can include charged particles (e.g., electrons) having an energy distribution and an upward traveling direction. When the secondary beamlets (e.g., including the secondary beamlets 236, 238, 240) are incident on the primary projection system 220, the beam separator 222 can separate the secondary beamlets from the primary beamlets (e.g., the primary beamlets 214, 216, 218) and further direct the secondary beamlets towards the secondary projection system 242.

[0037]

[0051] The secondary projection system 242 can focus the secondary beamlets 236, 238, 240 onto detector elements 246, 248, 250 of the charged particle detector 244. A single detector element (e.g., detector element 246, 248, or 250) can be designed to detect a single corresponding secondary beamlet (e.g., each of secondary beamlets 236, 238, or 240) emanating from a single probe spot (e.g., each of probe spots 270, 272, or 274) and generate a corresponding signal (e.g., voltage, current, etc.) for reconstructing an image of the scanned surface of the sample 230. In some embodiments, the charged particle detector 244 can include an array of individual sensors, and a single detector element (e.g., detector element 246, 248, or 250) can be a single sensor. In some embodiments, the charged particle detector 244 may include a 2D pixel detector including a detector cell array in which a single detector element (e.g., detector element 246, 248, or 250) may be implemented as a pixel group (e.g., each pixel represents a single detector cell).

[0038]

[0052] The generated signals may represent the intensities of the secondary beamlets 236, 238, and 240 and may be provided to an image processing system 290 (represented by dotted lines in FIG. 2 ) in communication with the charged particle detector 244, the primary projection system 220, and the motorized wafer stage 280. The intensities of the secondary beamlets 236, 238, and 240 may vary according to the external or internal structure of the sample 230 and may therefore indicate whether the sample 230 contains defects. Furthermore, the primary beamlets 214, 216, and 218 may be projected onto different locations on the top surface of the sample 230, thereby generating secondary beamlets 236, 238, and 240 with different intensities. Thus, by correlating the intensities of the secondary beamlets 236, 238, and 240 with regions of the sample 230, the image processing system 290 can reconstruct an image reflecting characteristics of the internal or external structure of the sample 230.

[0039]

[0053] In some embodiments, the image processing system 290 may include an image acquirer 292, a storage 294, and a controller 296. The image acquirer 292 may include one or more processors. For example, the image acquirer 292 may include a computer, a server, a mainframe host, a terminal, a personal computer, any type of mobile computing device, or the like, or a combination thereof. The image acquirer 292 may be communicatively coupled to the charged particle detector 244 of the beam tool 104 through a medium such as an electrical conductor, a fiber optic cable, a portable storage medium, IR, Bluetooth, the Internet, a wireless network, a wireless radio, or a combination thereof. In some embodiments, the image acquirer 292 may receive signals from the charged particle detector 244 and construct an image. In this manner, the image acquirer 292 may acquire an image of the sample 230. The image acquirer 292 may also perform various post-processing functions, such as generating contours, superimposing indicators on the acquired image, or the like. The image acquirer 292 may be configured to perform brightness and contrast adjustments on the acquired image. In some embodiments, storage 294 may be a storage medium such as a hard disk, a flash drive, cloud storage, random access memory (RAM), other types of computer-readable memory, or the like. Storage 294 may be coupled to image acquirer 292 and may be used to store raw scanned image data as original images or to store post-processed images. Image acquirer 292 and storage 294 may be connected to controller 296. In some embodiments, image acquirer 292, storage 294, and controller 296 may be integrated together as a single control unit.

[0040]

[0054] In some embodiments, the image acquirer 292 can acquire one or more images of the wafer based on an imaging signal received from the charged particle detector 244. The imaging signal can correspond to a scanning motion to perform charged particle imaging. The acquired image can be a single image including multiple imaging regions. The single image can be stored in the storage 294. The single image can be an original image that can be divided into multiple regions. Each region can include an imaging region that includes a feature of the sample 230. The acquired image can include multiple images of a single imaging region of the sample 230 sampled multiple times over a time series. The multiple images can be stored in the storage 294. In some embodiments, the image processing system 290 can be configured to perform image processing steps using multiple images of the same location of the sample 230.

[0041]

[0055] By way of example, FIG. 3 is a schematic diagram illustrating an exemplary secondary column 300 that may be part of the beam tool 104 of FIG. 2 , consistent with some embodiments of the present disclosure. For example, the secondary column 300 may include a charged particle detector 244. The secondary column 300 may also include an anti-scan deflection system 302 and a secondary projection system 242. In some embodiments, the secondary column 300 may further include one or more focusing lenses (including lenses 308 and 310) and a beam separator (not shown in FIG. 3 ). The one or more focusing lenses may be objective lenses, such as the same or similar objective lens as the objective lens 228 of FIG. 2 . The one or more focusing lenses may collect secondary charged particles emitted from the probe spot and form secondary beamlets (including secondary beamlets 236, 238, and 240). As shown in FIG. 3 , the charged particle detector 244 further includes detector elements 246, 248, and 250. Detector elements 246, 248, 250 can be designed to detect corresponding secondary beamlets 236, 238, 240, respectively. Secondary beamlets 236, 238, 240 in Figure 3 can originate from probe spots 270, 272, or 274, respectively, shown in Figure 2. Components of secondary column 300 can be aligned with secondary projection axis 252.

[0042]

[0056] In some embodiments, lens 308 can be used to set the magnification, and lens 310 can be used to focus the image. Lens 308 or lens 310 may be referred to as a "focusing lens," meaning that both lenses contribute to focusing the secondary beamlets. A beam separator (e.g., similar to or the same as beam separator 222) can direct the secondary beamlets toward secondary projection system 242. Anti-scan deflection system 302 can direct all secondary beamlets toward secondary projection axis 252 to minimize image displacement on charged particle detector 244. For example, referring to FIG. 2 , image displacement can result from movement of probe spots (e.g., including probe spots 270, 272, and 274 in FIG. 2 ) across the inspection region of sample 230 in coordination with deflection scanning unit 226.

[0043]

[0057] The secondary projection system 242 can project the secondary beamlets (including, for example, secondary beamlets 236, 238, 240) onto the charged particle detector 244 while maintaining their properties (including, for example, focus, size, and rotation). The secondary projection system 242 can maintain the properties of the secondary beamlets substantially constant and independent of the imaging conditions of the beam tool 104 of FIG. 3. As shown in FIG. 3, the secondary projection system 242 includes a beam-limiting aperture 304. The beam-limiting aperture 304 has a radius that can determine what portion of the secondary beamlets are allowed to reach the charged particle detector 244.

[0044]

[0058] Like the optical system, the electron-optical elements of the secondary column 300 may also have aberrations. Such aberrations can blur the image projected by the secondary projection system 242 onto the charged particle detector 244, limiting detection performance. Aberrations can also introduce or increase crosstalk as described herein, limiting the collection efficiency of the charged particle detector 244.

[0045]

[0059] To suppress crosstalk, as shown in FIG. 3 , a beam-limiting aperture 304 can be positioned at or near the crossover 306 of the secondary projection axis 252. The beam-limiting aperture 304 eliminates stray electrons of the secondary beamlets, and only the central portion of the secondary beamlets reaches the charged particle detector 244. Use of the beam-limiting aperture 304 can eliminate the edges of the spots of the secondary beamlets formed on the detector elements (e.g., including detector elements 244, 246, and 248). Accordingly, the size of the spots of the secondary beamlets formed on the detector elements is limited, which can reduce crosstalk. It should be noted that use of the beam-limiting aperture 304 can also limit the collection efficiency.

[0046]

[0060] Consistent with some embodiments of the present disclosure, a method for optimizing collection efficiency of secondary charged particles may include focusing a lens (e.g., one or more focusing lenses) of a multi-beam inspection apparatus to adjust (e.g., minimize) a size of a secondary beam spot. The secondary beam spot may be formed on a detector by multiple secondary charged particle beams (e.g., secondary electron beams). In some embodiments, the multi-beam inspection apparatus may include a multi-beam scanning electron microscope (SEM). In some embodiments, the detector may include a charged particle detector (e.g., an electron detector).

[0047]

[0061] Lens, as used herein, may refer to a focusing lens or set of focusing lenses or any functionally equivalent component of an electron projection imaging system (e.g., a secondary electron projection imaging system). Lens focus, as used herein, may refer to any action (e.g., under the control of a controller or processor) to increase, decrease, or maintain the focusing power (e.g., refractive power) of the lens. For example, if the lens is an electrostatic lens (or a magnetic lens or a compound lens), the excitation of the lens can be set to increase, decrease, or maintain the focusing power of the lens.

[0048]

[0062] A secondary charged particle beam, as used herein, may refer to a beam formed by secondary charged particles emerging from a probe spot on or near the surface of a sample (e.g., a wafer) during inspection by a multi-beam inspection apparatus in response to a primary charged particle beam being incident on the probe spot. For example, if the multi-beam inspection apparatus is a multi-beam SEM, the secondary charged particle beam may be a secondary electron beam formed by secondary electrons and backscattered electrons emerging from the probe spot in response to a primary electron beam being incident on the probe spot.

[0049]

[0063] The secondary beam spot in this disclosure may refer to the image of the wafer probe spot (i.e., the probe spot on the wafer) formed on the detector by the secondary electron beam. The size of the secondary beam spot may depend on various factors, such as the focusing power of the lens, the incident angle distribution of the secondary charged particle beam, or the like.

[0050]

[0064] 2 and 3 and their associated description, the multi-beam inspection apparatus may be a multi-beam beam tool (e.g., beam tool 104 of FIG. 2). The focusing lens may be, for example, the last lens (e.g., lens 310 of FIG. 3) of a secondary column (e.g., secondary column 300 of FIG. 3) before the detector. The multiple secondary charged particle beams may include secondary beamlets 236, 238, and 240. The detector may be a charged particle detector 244. The secondary beam spot may be an image of a wafer probe spot formed by the multiple secondary charged particle beams on the charged particle detector 244.

[0051]

[0065] In some embodiments, various techniques can be implemented to focus a lens (e.g., lens 310 in FIG. 3 ) of a multi-beam inspection apparatus to adjust the size of the secondary beam spot. For example, if the detector is a pixel detector, the number of pixels in the detector's detector cells can be adjusted to prevent stray charged particles from being detected by the detector, and the pixels in the detector cells can then be used to detect multiple secondary charged particle beams. When the secondary charged particle beam is projected by the lens onto the surface of the pixel detector to form a beam spot, the pixels covered by the beam spot respond and a signal representing the secondary electron beam intensity can be generated. The number of responding pixels can be proportional to the size of the beam spot. To adjust the size of the beam spot, the lens can be focused to adjust the number of pixels in the pixel detector that respond to the beam spot.

[0052]

[0066] As another example, to adjust the size of the secondary beam spot, a lens can be focused to converge paraxial rays of multiple secondary charged particle beams onto the detector plane. Paraxial rays of a charged particle beam, as used herein, may refer to charged particle rays belonging to a charged particle beam and close to the projection axis of the lens such that the approximation sinθ≒θ (where the charged particle beam is refracted by the lens, and θ represents the refraction angle of the charged particle rays) is valid. Such focusing techniques may be referred to as "paraxial focusing techniques" in this disclosure.

[0053]

[0067] By way of example, FIG. 4A is a schematic diagram illustrating an example of a paraxial focusing technique for a charged particle beam, consistent with some embodiments of the present disclosure. FIG. 4A shows a focusing lens 402 (represented by a thick double arrow) and a focal plane 404 (represented by a thick line). The focusing lens 402 has a projection axis 406 (similar to the optical axis of an optical lens) and a focal point 408 (represented by a black dot). The focal point 408 lies on the focal plane 404. The focal plane 404 can be located on or near the surface of a detector (not shown in FIG. 4A ) (e.g., by adjusting the focusing power of an objective lens 410), in which case the focal plane 404 is sometimes referred to as an image plane. It should be noted that the focal plane can only coincide with the image plane when focusing a collimated beam (e.g., a parallel beam). In the case of a converging or diverging beam, the image plane can only coincide with the plane of the detector (referred to herein as the “detector plane”).

[0054]

[0068] 4A, a charged particle beam 410 (e.g., an electron beam) is projected onto a condenser lens 402 at an angle of incidence of 0° and focused by the condenser lens 402 toward a focal point 408. The charged particle beam 410 includes multiple charged particle rays (represented by solid lines). In an ideal scenario (not shown in FIG. 4A), if the charged particle beam 410 were collimated (e.g., all incident charged particle rays are parallel to each other) and the condenser lens 402 were a perfect lens (e.g., with no aberrations), all charged particle rays of the charged particle beam 410 would be focused at or near (e.g., directed to) the focal point 408. In a real scenario (as shown in FIG. 4A), the focusing lens 402 is not a perfect lens (e.g., it has aberrations such as spherical aberration), so some charged particle rays of the charged particle beam 410 may not be focused at the focal point 408.

[0055]

[0069] 4A , the charged particle beam 410 includes a paraxial ray 412 and a marginal ray 414. As used herein, a marginal ray of a charged particle beam may refer to a charged particle ray belonging to the charged particle beam that is at a certain distance from the projection axis of the lens, violating the approximation sinθ≒θ (where the charged particle beam is refracted by the lens, and θ represents the refraction angle of the charged particle ray). Referring to FIG. 4A , the paraxial ray 412 is focused at a focal point 408, and the marginal ray 414 is focused at various points (including points 416 and 418) different from the focal point 408. A beam spot 420 is formed on the focal plane 404 by the focused charged particle beam 410. 4A depicts the charged particle beam 410 as a collimated beam, it should be noted that if the charged particle beam 410 is not collimated (e.g., converging or diverging) before the focusing lens 402 (not shown in FIG. 4A ), the charged particle beam 410 may be focused before or after the focal point 408. In such a case, the paraxial ray 412 and the marginal ray 414 will still be focused at different positions. For example, the marginal ray 412 may be focused at a point along the projection axis 406 before the point at which the paraxial ray 412 is focused.

[0056]

[0070] Figure 4A shows an exemplary paraxial focusing approach to minimize the size of the beam spot 420. In Figure 4A, the collection lens 402 is focused (e.g., by setting its excitation) to converge the paraxial light beam 412 onto the plane of the detector.

[0057]

[0071] In some embodiments, the lenses of a multi-beam inspection apparatus can be focused to adjust the size of the secondary beam spots. As another example, the lenses can be focused to position the ellipse of least confusion of multiple secondary charged particle beams on the plane of the detector to form the secondary beam spots. The ellipse of least confusion of a focused charged particle beam, as used herein, refers to a spot of any shape (e.g., elliptical or circular) formed on the image plane, which is the smallest of all spots formed on all possible image planes. In this disclosure, ellipse includes any elliptical shape with two foci or a circular shape (e.g., circle) with a single focus. The ellipse of least confusion can be determined when a first outer edge of a projected marginal ray of the charged particle beam and a second outer edge of a projected paraxial ray of the charged particle beam overlap. The first outer edge and the second outer edge can be measured from the projection axis on the image plane. Such a focusing technique may be referred to as a "minimum confusion focusing technique" in this disclosure.

[0058]

[0072] By way of example, FIG. 4B is a schematic diagram illustrating an example of a minimum-confusion focusing technique for a charged particle beam consistent with certain embodiments of the present disclosure. FIG. 4B illustrates an image plane 422 and a minimum-confusion ellipse 424 (represented by a thick, bold line overlapping the image plane 422) formed by a focusing lens 402 projecting a charged particle beam 410 onto the image plane 422. It should be noted that while FIG. 4B depicts the charged particle beam 410 as a collimated beam, if the charged particle beam 410 is not collimated (e.g., converging or diverging) before the focusing lens 402 (not shown in FIG. 4B ), the charged particle beam 410 may be focused before or after the focal point 408. In such a case, the paraxial ray 412 and the marginal ray 414 will still be focused at different positions. For example, the marginal ray 412 may be focused at a point along the projection axis 406 before the point at which the paraxial ray 412 is focused.

[0059]

[0073] As shown in Figure 4B, on image plane 422, minimum confusion ellipse 424 is the spot that is smallest among all spots formed on all possible image planes. Minimum confusion ellipse 424 is determined when the outer edges of projected marginal rays 414 and projected paraxial rays 412 overlap. In the minimum confusion focusing approach shown in Figure 4B, focusing lens 402 is focused (e.g., by setting its excitation) so that the minimum confusion ellipse 424 is positioned on the plane of a detector (not shown in Figure 4B) to form a beam spot. That is, in Figure 4B, image plane 422 is the detector plane, and minimum confusion ellipse 424 is the beam spot on the detector plane. For example, referring to FIG. 4B , in a scenario where the distance between the focusing lens 402 and the detector is fixed, to position the least confusion ellipse 424 (i.e., to the left of the detector plane) on the detector plane, the focusing power (e.g., refractive power) of the focusing lens 402 can be reduced such that the least confusion ellipse 424 shifts to move to the right toward the detector until it reaches the detector plane.

[0060]

[0074] In some embodiments, the lenses of the multi-beam inspection apparatus can be focused to adjust the size of the secondary beam spot. As yet another example, the lenses can be focused to minimize the rising edge width of the secondary beam spot. To determine the rising edge width of the beam spot, the signal intensity of the beam spot can be integrated along a direction from a first side of the beam spot to a second side of the beam spot to form a step function (e.g., a one-dimensional step function), where the integrated signal intensity represents the detected charge intensity. The rising edge of the step function, as used herein, refers to the portion of the step function where the level of the integrated signal intensity (representing the detected charge intensity) increases abruptly to form a step. The rising edge width of the beam spot, as used herein, refers to the length of the rising edge of the step function. For example, the rising edge can be the portion of the step function where the level of the relative signal intensity (expressed as a percentage) increases from 15% to 85%, 20% to 80%, or 25% to 75% of the maximum level of the step function (e.g., 100% intensity level). In a scenario where the charged particle beam is well focused, the rising width of the beam spot can be small. In scenarios where the charged particle beam is not well focused, the rising width of the beam spot can be large.

[0061]

[0075] As an example, to minimize the rising edge width of the secondary beam spot, the focusing power (e.g., refractive power) of a collecting lens (e.g., collecting lens 402 in FIGS. 4A and 4B ) can be adjusted (e.g., increased or decreased) to shift the focal point 408 forward, backward, or toward the detector plane. During this process, the rising edge width of the secondary beam spot can be measured and recorded, and such adjustments can be stopped when the rising edge width of the secondary beam spot reaches a value that is the smallest of all values recorded over a predetermined time interval. Such a focusing technique is sometimes referred to in this disclosure as a "minimum rising edge focusing technique."

[0062]

[0076] In some embodiments, to find an excitation for focusing the lens of the multi-beam inspection apparatus to adjust the size of the secondary beam spot, a simulation process can be performed to optimize the distribution of the secondary beam spot on the detector. For example, such a simulation process can be performed by projecting a set of test rays (e.g., including a chief ray, a paraxial ray, a marginal ray, or any combination thereof) onto the lens in a simulation model and adjusting parameters of the simulation model to adjust (e.g., minimize) the value of one or more pre-defined merit functions. As an example, the maximum radius of the secondary beam spot can be used as the merit function.

[0063]

[0077] Consistent with some embodiments of the present disclosure, the method may also include, for each secondary charged particle beam of the multiple secondary charged particle beams, preventing stray charged particles of each secondary charged particle beam from being detected by the detector. As used herein, a stray charged particle of a charged particle beam may refer to a charged particle belonging to the charged particle beam whose distance to the projection axis of the lens exceeds a predetermined value or a charged particle belonging to the charged particle beam whose signal intensity is below a predetermined level. For example, the stray charged particle may belong to a secondary electron beamlet or to the tail of a cross section of a secondary electron spot formed on the detector. In some embodiments, to prevent the stray charged particle from being detected by the detector, one or more actions may be performed to prevent the stray charged particle from reaching the surface of the detector or to prevent the detector from responding to the stray charged particle if the stray charged particle reaches the surface of the detector (e.g., by using a control signal).

[0064]

[0078] In some embodiments, to prevent stray charged particles of each secondary charged particle beam from being detected by the detector, the beam-limiting aperture configuration may be adjusted to position the beam-limiting aperture upstream of the detector (e.g., upstream of a lens) and filter the stray charged particles (e.g., allow some of the multiple secondary charged particle beams to pass through the beam-limiting aperture). In this disclosure, a beam-limiting aperture configuration may refer to an array, combination, grouping, arrangement, or any spatial positioning of one or more beam-limiting apertures. For example, an aperture plate may include one or more beam-limiting apertures (e.g., having different sizes) arranged in a linear, circular, rectangular, or any geometric shape. Adjusting a beam-limiting aperture configuration, as used herein, may refer to moving, rotating, switching, inserting, adding, activating, or any position-changing operation of one or more beam-limiting apertures in the beam-limiting aperture configuration. For example, if the aperture plate includes multiple beam-limiting apertures of different sizes, the aperture plate can be moved (e.g., linearly) or rotated to activate one beam-limiting aperture of the beam-limiting aperture configuration (e.g., by positioning the beam-limiting aperture on the projection axis of the lens) or to switch between one or more beam-limiting apertures. In some embodiments, in scenarios where the beam-limiting aperture is not positioned upstream of the detector (e.g., upstream of the lens), the beam-limiting aperture configuration can be adjusted by inserting (e.g., moved by a mechanical actuator) an aperture plate including one or more beam-limiting apertures and positioning the beam-limiting apertures of the aperture plate on the projection axis of the lens (e.g., upstream of the detector or upstream of the lens) so that the beam-limiting aperture filters multiple secondary charged particle beams.In some embodiments, in a scenario where an aperture plate having one or more beam-limiting apertures of different sizes is positioned on the projection axis of a lens such that a first beam-limiting aperture of a first size is positioned upstream of the detector (e.g., upstream of the lens), the beam-limiting aperture configuration can be adjusted by switching from the first beam-limiting aperture to a second beam-limiting aperture of a second size (e.g., moving or rotating the aperture plate) such that the second beam-limiting aperture is positioned upstream of the detector (e.g., upstream of the lens). The second beam-limiting aperture can then filter multiple secondary charged particle beams. In some embodiments, a controller (e.g., controller 109 in FIG. 2 ) can adjust the beam-limiting aperture configuration in response to receiving an input signal (e.g., from a graphical user interface communicatively coupled to the controller). For example, the graphical user interface can be implemented on a computer and configured to receive input data (e.g., data instructing the enabling of a beam-limiting aperture) from an operator (e.g., an individual). In some embodiments, the controller may monitor predetermined conditions (e.g., timing conditions or the like) and adjust the beam-limiting aperture configuration automatically (e.g., without human operation or intervention) when the predetermined conditions are met.

[0065]

[0079] 3, the beam-limiting aperture 304 may be enabled to filter multiple secondary beamlets, including secondary beamlets 236, 238, and 240. In such a case, one or more of the multiple secondary beamlets may pass through the beam-limiting aperture 304.

[0066]

[0080] 5A is a graph visualizing the cross section of an exemplary charged particle beam in the plane of a beam-limiting aperture, consistent with certain embodiments of the present disclosure. The beam-limiting aperture can be positioned in front of the detector plane along the projection path of the charged particle beam. For example, the beam-limiting aperture can be the beam-limiting aperture 304, as shown and described in connection with FIG. 3.

[0067]

[0081] As shown in FIG. 5A, the charged particle beam is represented by a collection of black dots, each of which may represent a charged particle ray belonging to the charged particle beam. In FIG. 5A, the outline of the beam aperture is represented by a white dashed circle. The X and Y axes in FIG. 5A are on the plane of the beam-limiting aperture and are shown in arbitrary units. The center of the cross section of the charged particle beam is at coordinate (0,0) as shown in FIG. 5A. The beam-limiting aperture is positioned so that its center is also positioned at coordinate (0,0). The radius of the beam-limiting aperture is r (r is a numerical value in arbitrary units).

[0068]

[0082] 5A, when the beam-limiting aperture is effective, the charged particles included in the charged particle beam outside the beam-limiting aperture (i.e., represented by the white dashed circle) may be stray charged particles, which can be filtered by the beam-limiting aperture so that the charged particle beam outside the beam-limiting aperture does not reach the detector, and therefore the stray charged particles are not detected by the detector. In comparison, the charged particle beam inside the beam-limiting aperture can pass through the beam-limiting aperture and reach the detector to be detected.

[0069]

[0083] As shown in FIG. 5A, charged particle rays outside the beam-limiting aperture may be marginal rays (e.g., marginal rays 414 shown in FIGS. 4A and 4B). FIG. 5B is a graph showing a profile of the exemplary charged particle beam of FIG. 5A, consistent with some embodiments of the present disclosure. The X-axis of FIG. 5B is the same as the X-axis of FIG. 5A. The Y-axis of FIG. 5B represents the detector signal intensity (e.g., representing the number of detected charged particle counts). The center of the charged particle beam is located at 0 on the X-axis as shown in FIG. 5B. The beam-limiting aperture is positioned so that its center is also located at 0 on the X-axis. The radius of the beam-limiting aperture is r, and the boundaries of the beam aperture are represented by two vertical dashed lines located at +r and −r on the X-axis. As shown in FIG. 5B, when the beam-limiting aperture is enabled, the tail of the beam spot is eliminated (e.g., blocked or filtered) by the beam-limiting aperture, and only the central portion of the charged particle beam passes through the beam-limiting aperture and reaches the detector so that it can be detected.

[0070]

[0084] In some embodiments, if the detector is a pixel detector, the method may include selecting a subset of pixels from the pixels covered by each secondary charged particle beam to prevent stray charged particles of each secondary charged particle beam from being detected by the detector. For example, the pixels covered by the secondary charged particle beam may include pixels of the detector cell. Detection results of pixels of the detector cell outside the subset can be ignored. For example, in a scenario where a beam-limiting aperture is not enabled, stray charged particles of the secondary charged particle beam may land on pixels of the pixel detector and contribute to the size of the beam spot (e.g., form an outer portion of the beam spot). The center of the beam spot may be determined, and a subset of pixels may be selected based on a predetermined radius (e.g., the subset is certain pixels inside the predetermined radius). Based on the subset of pixels, detection results of pixels that are sensitive to the beam spot and located outside the subset can be ignored. As another example, in a scenario where a beam-limiting aperture is enabled as described herein, the above-mentioned operations may be additionally performed to further filter stray charged particles not filtered by the beam-limiting aperture.

[0071]

[0085] Consistent with some embodiments of the present disclosure, the method may further include refocusing the lens to adjust (e.g., maximize) the current of a portion of the multiple secondary charged particle beams detected by the detector. Stray charged particles do not contribute to the current. Lens refocusing, as used herein, may refer to any action to adjust (e.g., increase, decrease, or maintain) the focusing power (e.g., refractive power) of the lens after the focusing power has been set. For example, if the lens is an electromagnetic lens and the focusing power of the electromagnetic lens has been set in a previous action, the excitation of the electromagnetic lens may be adjusted to increase, decrease, or maintain the focusing power.

[0072]

[0086] As used herein, beam current may refer to the current of a charged particle beam. The beam current may represent the total charge carried by the charged particles of the charged particle beam, passing through the detector plane in a unit time interval (e.g., 1 second), and recorded by the detector. In this disclosure, the charged particle contribution to the beam current refers to the charge carried by the charged particles, passing through the detector plane, and recorded by the detector. In some embodiments, if a charged particle belongs to the charged particle beam but does not reach the surface of the detector (e.g., is filtered by a beam-limiting aperture), the charged particle does not contribute to the current of the charged particle beam. In some embodiments, if the detector is a pixel detector and a charged particle belongs to the charged particle beam and reaches a pixel of the pixel detector but is not recorded by the detector (e.g., the detection result of the pixel is ignored), the charged particle does not contribute to the current of the charged particle beam.

[0073]

[0087] FIG. 6 is a graph illustrating an exemplary beam spot size optimization using different beam focusing techniques, consistent with some embodiments of the present disclosure. In FIG. 6, the X-axis represents the emission angle in degrees measured at the surface of a sample (e.g., sample 230 shown in FIG. 2). Emission angle, as used herein, refers to the angle between the trajectory of the secondary charged particles and the normal to the sample surface. The Y-axis in FIG. 6 represents the radius of the secondary beam spot measured on the detector plane in arbitrary units. Each curve in FIG. 6 may represent the change in the radius of the secondary beam spot (e.g., the offset or distance measured from the projection axis) as a function of the emission angle of the secondary charged particle beam that forms the secondary beam spot. The different curves in FIG. 6 illustrate different secondary beam spot sizes achievable under different beam focusing techniques.

[0074]

[0088] FIG. 6 shows three curves corresponding to three beam focusing techniques. In FIG. 6, the first curve (represented by the dashed curve) corresponds to the paraxial focusing technique, the second curve (represented by the solid curve) corresponds to the minimum-scatter focusing technique, and the third curve (represented by the dashed curve) corresponds to the minimum-rising-edge focusing technique. As shown by the first curve in FIG. 6, the radius of the secondary beam spot (e.g., formed by paraxial rays) is small (e.g., less than r1, where r1 is a numerical value in arbitrary units) when its corresponding emission angle is substantially zero (e.g., not exceeding d1, where d1 is a numerical value in degrees). As the emission angle increases, the radius of the secondary beam spot can rapidly increase. When its corresponding emission angle is large (e.g., greater than d2, where d2 is a numerical value in degrees), the radius of the secondary beam spot (e.g., formed by marginal rays) can become very large (e.g., greater than r2, where r2 is a numerical value in arbitrary units).

[0075]

[0089] As shown by the second curve in Figure 6, the maximum radius of the marginal ray projected onto the detector plane is r3 (r3 is a numerical value in arbitrary units), corresponding to an emission angle of d2, and the maximum radius of the paraxial ray projected onto the detector plane is also r3 (d3 is a numerical value in degrees, corresponding to an emission angle between d1 and d3). If any secondary charged particle beam stray charged particles are not filtered (e.g., by a beam-limiting aperture or by detector pixel disabling, as described herein), the second curve in Figure 6 shows that the minimum-scatter focusing technique can result in a minimum value for the maximum radius of the secondary beam spot on the detector plane (e.g., not exceeding r3).

[0076]

[0090] As shown by the third curve in Figure 6, for emission angles below d4 (d4 is a number in degrees), the maximum radius of the paraxial ray projected onto the detector plane under the minimum rising-edge focusing technique is smaller than the maximum radius of the paraxial ray under either the paraxial focusing technique or the minimum confusion focusing technique. However, for emission angles equal to or greater than d4, the maximum radius of the paraxial ray projected onto the detector plane under the minimum rising-edge focusing technique is larger than the maximum radius of the paraxial ray under the minimum confusion focusing technique and smaller than the maximum radius of the paraxial ray under the paraxial focusing technique.

[0077]

[0091] As can be seen from FIG. 6 , there is no beam focusing technique that always minimizes the maximum radius of the secondary beam spot on the detector plane. Rather, the minimum value of the maximum radius of the secondary beam spot may depend on various factors, such as one or more emission angles of the secondary charged particle beam. In some embodiments, when outlying charged particles of the secondary charged particle beam are prevented from being detected by the detector (e.g., by enabling a beam-limiting aperture or limiting the detector cell size of a pixel detector), the tail of the secondary beam spot is eliminated, and only a portion of the secondary beam spot contributes to the final detection signal. In such a situation, the minimum value of the maximum radius of the secondary beam spot may also depend on which portion of the secondary beam spot contributes to the final detection signal. In some embodiments, the selected beam focusing technique may emphasize focusing the central portion of the secondary charged particle beam and relax the requirements on focusing the marginal portions of the secondary charged particle beam.

[0078]

[0092] In some embodiments, the method may include focusing a lens to adjust a size of a secondary beam spot using a first beam focusing technique (e.g., one of a paraxial focusing technique, a minimum confusion focusing technique, and a minimum rising edge focusing technique). The method may include preventing outlying charged particles of the secondary charged particle beam from being detected by the detector (e.g., when a beam-limiting aperture is enabled). The method may also include refocusing a lens to adjust a current of a portion of the multiple secondary charged particle beams detected by the detector using a second beam focusing technique (e.g., one of a paraxial focusing technique, a minimum confusion focusing technique, and a minimum rising edge focusing technique) different from the first beam focusing technique. For example, the first beam focusing technique may be a minimum confusion focusing technique, and the second focusing technique may be a paraxial focusing technique.

[0079]

[0093] In some embodiments, to refocus the lens to adjust the current, the method may include refocusing the lens to move the focal point of the lens a first step distance toward (or away from) the plane of the detector. The method may also include determining whether the collection efficiency value increases and whether the crosstalk ratio value is below a predetermined threshold. The method may further include refocusing the lens to move the focal point of the lens a second step distance toward (or away from) the plane of the detector based on a determination that the collection efficiency value increases and a determination that the crosstalk ratio value is below a predetermined threshold. In some embodiments, based on a determination that the collection efficiency value decreases, the method may further include changing a scan direction of the multi-beam inspection device.

[0080]

[0094] For example, the lens can be focused to adjust the size of the secondary beam spot using a minimum confusion focusing technique, and outlying charged particles of each secondary charged particle beam can be prevented from being detected by the detector (e.g., by enabling a beam-limiting aperture or limiting the detector cell size of a pixel detector). At this stage, the minimum confusion ellipse is located on the detector plane, so the focal point of the lens can be behind the detector plane along the projection direction. The lens can then be refocused using a technique different from the minimum confusion focusing technique.

[0081]

[0095] For example, the lens can be refocused using a paraxial focusing technique. In such an example, the focusing power (e.g., refractive power) of the lens can be adjusted (e.g., increased) to move the focal point of the lens a first step distance toward or away from the detector plane. After moving the focal point the first step distance, the focal point can still be behind the detector plane along the projection direction. At this stage, it can be determined whether the collection efficiency value increases and whether the crosstalk ratio value is below a predetermined threshold. If the collection efficiency value increases and if the crosstalk ratio value is below a predetermined threshold, the focusing power of the lens can be further adjusted (e.g., further increased) to move the focal point of the lens a second step distance toward or away from the detector plane. The first step distance can be the same as or different from the second step distance.

[0082]

[0096] As another example, a lens can be refocused using a minimum step-edge focusing technique. In such an example, the focusing power (e.g., refractive power) of the lens can be adjusted (e.g., increased or decreased) to decrease the rising edge width of the secondary beam spot to a first value. After decreasing the rising edge width, it can be determined whether the collection efficiency value increases, and it can also be determined whether the crosstalk ratio value falls below a predetermined threshold. If the collection efficiency value increases and if the crosstalk ratio value falls below a predetermined threshold, the focusing power of the lens can be further adjusted (e.g., further increased or further decreased) to decrease the rising edge width to a second value. The first value can be the same as or different from the second value.

[0083]

[0097] In some embodiments, after refocusing the lens to adjust the current, the method may further include stopping refocusing the lens based on a determination that the collection efficiency value reaches a maximum value or that the crosstalk ratio value does not fall below a predetermined threshold.

[0084]

[0098] In some embodiments, if the detector is a pixel detector, the lens can be refocused to adjust the current according to the detector cell size of the pixel detector. As an example, FIG. 7 is a graph showing exemplary signal intensity distributions of secondary beam spots corresponding to different beam focusing techniques, consistent with some embodiments of the present disclosure. In FIG. 7, the X-axis represents a spatial position on the detector surface in arbitrary units. The position of 0 on the X-axis may represent the center of the secondary beam spot. The Y-axis of FIG. 7 represents a signal intensity value in arbitrary units (e.g., representing the number of charged particle counts) at the corresponding spatial position. FIG. 7 includes an upper subgraph and a lower subgraph stacked vertically, where the lower subgraph is an enlarged view of the lower portion of the upper subgraph (e.g., the portion of signal intensity from 0 to s1, where s1 is a numerical value in arbitrary units) and shares the same X-axis as the upper subgraph. Each curve in FIG. 7 may represent the distribution of signal intensity of the secondary beam spot along the direction represented by the X-axis.

[0085]

[0099] FIG. 7 shows three curves corresponding to three beam focusing techniques. In FIG. 7, the first curve (represented by the dotted curve) corresponds to the paraxial focusing technique, the second curve (represented by the dashed curve) corresponds to the minimum confusion focusing technique, and the third curve (represented by the solid curve) corresponds to the minimum rising edge focusing technique. As shown by the three curves in FIG. 7, the first curve has the highest intensity at the zero position, the smallest full width at half maximum (FWHM), and the largest beam spot tail (e.g., represented by the highest signal intensity at a position far from the zero position). The second curve has the lowest intensity at the zero position, the largest FWHM, and the smallest beam spot tail (e.g., represented by the lowest signal intensity at a position far from the zero position). The third curve has an intermediate intensity at the zero position, an intermediate FWHM, and an intermediate beam spot tail. As shown by FIG. 7, depending on the detector cell size, optimized secondary beam focusing (e.g., represented by highest intensity at the beam spot center, smallest FWHW, or smallest beam spot tail) may not always result from the same beam focusing technique.

[0086]

[0100] In some embodiments, if the detector is a pixel detector, the method may include refocusing the lens to adjust the current of a portion of the multiple secondary charged particle beams detected by the detector based on a detector cell size (e.g., a pixel group size) of the pixel detector. For example, the lens may be refocused using a beam focusing technique (e.g., a paraxial focusing technique or a minimum rising edge focusing technique) different from the beam focusing technique (e.g., a minimum clutter focusing technique) used to focus the lens to adjust the size of the secondary beam spot, in which case the detector cell size may be a factor in determining which beam focusing technique to use to refocus the lens.

[0087]

[0101] As an example, FIG. 8A is a graph showing exemplary integrated signal intensities of secondary beam spots corresponding to different beam focusing techniques, consistent with some embodiments of the present disclosure. In FIG. 8A, the X-axis represents the radius of the detector cell size (e.g., detector cell radius) with arbitrary units. The Y-axis of FIG. 8A represents the integrated signal intensity value (e.g., representing the total number of charged particle counts in the detector cell) with arbitrary units corresponding to the detector cell size. Each curve in FIG. 8A may represent the integrated signal intensity of the secondary beam spot as a function of detector cell size. The curves in FIG. 8A show that the integrated signal intensity increases as the detector cell size increases.

[0088]

[0102] FIG. 8A shows three curves corresponding to three beam focusing techniques. In FIG. 8A, the first curve (represented by the dashed curve) corresponds to the paraxial focusing technique, the second curve (represented by the solid curve) corresponds to the minimum-scatter focusing technique, and the third curve (represented by the dashed curve) corresponds to the minimum-rising-edge focusing technique. As shown by the three curves in FIG. 8A, the second curve has the highest integrated intensity value for large detector cells (e.g., detector cell sizes greater than r4, where r4 is a numerical value in arbitrary units), and the third curve has the highest integrated intensity value for small detector cells (e.g., detector cell sizes less than r4). Also, for detector cell sizes less than r5 (r5 is a numerical value in arbitrary units), the integrated intensity value of the first curve is higher than the integrated intensity value of the second curve. As shown in FIG. 8A, depending on the detector cell size, optimized secondary beam focusing (eg, as represented by the highest integrated intensity value) may not always result from the same beam focusing technique.

[0089]

[0103] 8B is a graph showing the difference between exemplary integrated signal intensities of secondary beam spots corresponding to different beam focusing techniques, consistent with some embodiments of the present disclosure. As in FIG. 8A, the X-axis of FIG. 8B represents the radius of the detector cell size in arbitrary units. The Y-axis of FIG. 8B represents the difference between the two integrated signal intensity values in arbitrary units corresponding to the detector cell size.

[0090]

[0104] FIG. 8B shows two curves. In FIG. 8B, a first difference curve (represented by a dashed curve) represents the difference value between the first curve and the second curve shown and described in FIG. 8A, and a second difference curve (represented by a dashed curve) represents the difference value between the third curve and the second curve shown and described in FIG. 8A. As shown in FIG. 8B, the first difference curve is positive for detector cell sizes less than r4 and has a maximum value of m1 (e.g., for a detector cell size of r5), where m1 is a numerical value in arbitrary units. The second difference curve is positive for detector cell sizes less than r6 (r6 is a numerical value in arbitrary units) and has a maximum value of m2 (e.g., for a detector cell size of r7, where r7 is a numerical value in arbitrary units), where m2 is a numerical value in arbitrary units. As shown in FIG. 8B, when the detector cell size is set to a certain value, switching from the minimum confusion focusing technique to the paraxial focusing technique or the minimum rising edge focusing technique can increase the maximum integrated signal intensity from m2 to m1.

[0091]

[0105] 8A and 8B, for example, if the detector is a pixel detector, a minimum-scatter focusing technique can be used to focus the lens to adjust (e.g., minimize) the size of the secondary beam spot. Then, if the detector cell size of the pixel detector is greater than r4, the current is maximized and the lens is not refocused. If the detector cell size is between r5 and r4, a minimum-rising-edge focusing technique can be used to refocus the lens to adjust the current. If the detector cell size is smaller than r5, a minimum-rising-edge focusing technique or a paraxial focusing technique can be used to refocus the lens.

[0092]

[0106] In some embodiments, if a beam-limiting aperture is enabled, the lens can be refocused to adjust the current depending on the aperture size of the beam-limiting aperture. As an example, FIG. 9 is a graph showing exemplary secondary beam spot profiles (represented by curves) on the plane of the beam-limiting aperture corresponding to different beam focusing techniques, consistent with some embodiments of the present disclosure. In FIG. 9, the X-axis represents a spatial position having arbitrary units on the plane of the beam-limiting aperture (e.g., similar to the beam-limiting aperture described in connection with FIGS. 5A and 5B). The position of 0 on the X-axis may represent the center of the secondary beam spot. The Y-axis in FIG. 9 represents a signal intensity value (e.g., representing the number of charged particle counts) having arbitrary units at the corresponding spatial position. Each curve in FIG. 9 may represent the distribution of signal intensity of the secondary beam spot along the direction represented by the X-axis.

[0093]

[0107] 9 shows three curves corresponding to three beam focusing techniques. In FIG. 9, the first curve (represented by the dashed curve) corresponds to the paraxial focusing technique, the second curve (represented by the solid curve) corresponds to the minimum confusion focusing technique, and the third curve (represented by the dashed curve) corresponds to the minimum rising edge focusing technique. As shown by the three curves in FIG. 9, the second curve has the largest FWHM, and the first curve has the highest signal intensity at the center of the beam spot.

[0094]

[0108] In some embodiments, to refocus the lens to adjust the current, the method may include refocusing the lens to adjust the current of the multiple secondary charged particle beams based on an aperture size (e.g., radius) of the beam-limiting aperture. For example, the lens may be refocused using a beam focusing technique (e.g., paraxial focusing technique or minimum rising edge focusing technique) that is different from the beam focusing technique (e.g., minimum confusion focusing technique) used to focus the lens to adjust the size of the secondary beam spot, in which case the aperture size may be a factor in determining which beam focusing technique to use to refocus the lens.

[0095]

[0109] As an example, FIG. 10A is a graph showing exemplary integrated signal intensities of secondary beam spots corresponding to different beam focusing techniques, consistent with some embodiments of the present disclosure. In FIG. 10A, the X-axis represents aperture size (e.g., aperture radius) with arbitrary units. The Y-axis of FIG. 10A represents integrated signal intensity values (e.g., representing the total count number of charged particles passing through the beam-limiting aperture) with arbitrary units corresponding to the aperture size. Each curve in FIG. 10A may represent the integrated signal intensity of a secondary beam spot as a function of aperture size. The curves in FIG. 10A show that the integrated signal intensity increases as the aperture size increases.

[0096]

[0110] FIG. 10A shows three curves corresponding to three beam focusing techniques. In FIG. 10A, the first curve (represented by the dashed curve) corresponds to the paraxial focusing technique, the second curve (represented by the solid curve) corresponds to the minimum-scatter focusing technique, and the third curve (represented by the dashed curve) corresponds to the minimum-rising-edge focusing technique. As shown by the three curves in FIG. 10A, for a wide range of aperture sizes (e.g., between sizes a1 and a2, where a1 and a2 are numerical values in arbitrary units), the integrated signal intensity values of the first and third curves are higher than the integrated signal intensity value of the second curve. As shown by FIG. 10A, depending on the aperture size, the optimized secondary beam focusing (e.g., represented by the highest integrated intensity value) may not always be produced by the same beam focusing technique.

[0097]

[0111] 10B is a graph showing the difference between exemplary integrated signal intensities of secondary beam spots corresponding to different beam focusing techniques, consistent with some embodiments of the present disclosure. As with FIG. 10A, the X-axis of FIG. 10B represents aperture size in arbitrary units. The Y-axis of FIG. 10B represents the difference between two integrated signal intensity values in arbitrary units corresponding to aperture size.

[0098]

[0112] FIG. 10B shows two curves. In FIG. 10B, a first difference curve (represented by a dashed curve) represents the difference between the first and second curves shown and described in FIG. 10A, and a second difference curve (represented by a dashed curve) represents the difference between the third and second curves shown and described in FIG. 10A. As shown in FIG. 10B, both the first and second difference curves have positive values over a wide range of aperture sizes (e.g., between aperture sizes a3 and a4, where a3 and a4 are numerical values in arbitrary units). Also, the value of the first difference curve is higher than the value of the second difference curve in the range of aperture sizes between a3 and a5 (a5 is a numerical value in arbitrary units). As shown in FIG. 10B, when the aperture size is set to a particular value, the maximum integrated signal intensity can be increased by switching from a minimum-scatter focusing technique to a paraxial focusing technique or a minimum-rising-edge focusing technique.

[0099]

[0113] 10A and 10B, for example, a minimum-scatter focusing technique can be used to focus the lens to adjust the size of the secondary beam spot. Then, as described herein, a beam-limiting aperture can be enabled. Thereafter, if the aperture size of the beam-limiting aperture is less than a4, the lens can be refocused using a minimum-rising-edge focusing technique or a paraxial focusing technique. If the aperture size is less than a5, the lens can be refocused using a paraxial focusing technique to adjust the current.

[0100]

[0114] It should be noted that if the detector is a pixel detector and a beam-limiting aperture is enabled, the lens can be refocused to adjust the current depending on both the detector cell size and the aperture size of the beam-limiting aperture. For example, the lens can be refocused to maximize the current of a portion of the multiple secondary charged particle beams detected by the detector based on both the detector cell size of the pixel detector and the aperture size of the beam-limiting aperture.

[0101]

[0115] In some embodiments, if the beam-limiting aperture is enabled, the lens can be refocused to adjust the current depending on the position of the beam-limiting aperture along the projection axis of the lens. For example, to refocus the lens to adjust the current, the method can include refocusing the lens to adjust the current of the multiple secondary charged particle beams based on the position of the beam-limiting aperture along the projection axis of the lens.

[0102]

[0116] It should also be noted that the method for optimizing the collection efficiency of secondary charged particles described herein can be applied to a theoretical optimization process (e.g., a simulation process for optimizing the performance of an imaging system) or an experimental process (e.g., an actual measurement process). As an example, in the experimental process, assuming that the detector of the multi-beam inspection apparatus is a pixel detector, the pixel detector can be switched to operate in an imaging mode that can generate an image of a secondary beam spot. Under the imaging mode, the lens of the multi-beam inspection apparatus can be focused (e.g., using a minimum-scatter focusing technique) to adjust the size of the secondary beam spot, and the secondary beam spot is formed on the detector by multiple secondary charged particle beams. In some embodiments, the lens can be focused without enabling a beam-limiting aperture in the projection path of the multiple secondary charged particle beams. The pixel detector can then be switched to operate in a measurement mode that can generate detection results that do not include an image of the secondary beam spot. Under the measurement mode, a detector cell (e.g., a group of pixels) can receive the secondary beam spot, and an intensity value of the secondary beam spot can be determined (e.g., by integrating the signal values of all pixels in the detector cell). The beam-limiting aperture can then be positioned upstream of the detector (e.g., upstream of the lens), and the beam-limiting aperture configuration can be adjusted so that stray charged particles are filtered. Enabling the beam-limiting aperture can suppress crosstalk (e.g., so that the crosstalk ratio is below a predetermined threshold). After enabling the beam-limiting aperture, the lens can be refocused (e.g., using a paraxial focusing technique or a minimum leading edge focusing technique) to adjust the current of the multiple secondary charged particle beams detected by the detector, in which case the stray charged particles do not contribute to the current. For example, to refocus the lens, the focusing power of the lens can be adjusted (e.g., increased). As another example, to refocus the lens, the focusing power of the lens can be adjusted (e.g., increased or decreased) to reduce the leading edge width of the secondary beam spot.

[0103]

[0117] It should further be noted that although only three beam focusing techniques (e.g., paraxial focusing technique, minimum confusion focusing technique, and minimum rising edge focusing technique) are described in this disclosure, these are merely example embodiments for illustrative purposes, and other beam focusing techniques are not excluded and may be used in embodiments of the present disclosure.

[0104]

[0118] 11 is a flowchart illustrating an exemplary method for optimizing collection efficiency of secondary charged particles, consistent with certain embodiments of the present disclosure. Method 1100 can be performed by a controller that can be coupled to a charged particle beam inspection apparatus (e.g., beam tool 104 described in connection with FIGS. 1-3). For example, the controller can be controller 109 of FIG. 2. The controller can be programmed to implement method 1100.

[0105]

[0119] In step 1102, the controller can focus a lens (e.g., the objective lens of the secondary column 300 in FIG. 3) of the multi-beam inspection apparatus (e.g., the beam tool 104 in FIG. 2) to adjust the size of the secondary beam spot. The secondary beam spot can be formed by multiple secondary charged particle beams (e.g., including the secondary beamlets 236, 238, 240 in FIG. 3) on a detector (e.g., the charged particle detector 244 in FIG. 3).

[0106]

[0120] In some embodiments, the controller can focus the lens using a first beam focusing method, which can include one of: focusing the lens to focus paraxial rays of the multiple secondary charged particle beams onto a plane of the detector; focusing the lens to position an ellipse of least confusion of the multiple secondary charged particle beams onto the plane of the detector to form a secondary beam spot; and focusing the lens to minimize a leading edge width of the secondary beam spot.

[0107]

[0121] In step 1104, for each secondary charged particle beam of the multiple secondary charged particle beams, the controller can prevent stray charged particles of each secondary charged particle beam from being detected by the detector. In some embodiments, to prevent stray charged particles from being detected by the detector, the controller can adjust a beam-limiting aperture configuration (e.g., beam-limiting aperture 304 in FIG. 3 ) so that the beam-limiting aperture is positioned upstream of the detector (e.g., lens 310 in FIG. 3 ) and stray charged particles are filtered. For example, with reference to FIGS. 5A and 5B , the beam-limiting aperture can block stray charged particles of the secondary charged particle beam (e.g., charged particles at the tail of the secondary beam spot) from reaching the detector.

[0108]

[0122] In some embodiments, the detector may be a pixel detector. In such a scenario, to prevent stray charged particles from being detected by the detector, the controller may adjust the number of pixels in a detector cell of the detector, in which case the pixels in the detector cell may be used to detect multiple secondary charged particle beams. For example, a group of pixels (e.g., a detector cell) may receive and detect a secondary charged particle beam, and the detector may include multiple detector cells. The controller may adjust the number of pixels in each detector cell (e.g., by reducing the number of pixels in the detector cell) to detect multiple secondary charged particle beams.

[0109]

[0123] In some embodiments, if the detector is a pixel detector, the controller can select a subset of pixels from the pixels covered by each secondary charged particle beam to prevent stray charged particles from being detected by the detector. For example, a detector cell (including a pixel group) can receive and detect a secondary charged particle beam. The pixels of the detector cell can be covered by the secondary charged particle beam. The controller can obtain a predetermined size of the detector cell and select a subset of pixels based on the predetermined size. For all pixels located outside the subset, the controller can ignore the detection results (e.g., by disabling their functions or not processing their signals) even if charged particles of the secondary charged particle beam land on those pixels.

[0110]

[0124] In step 1106, the controller may refocus the lens to adjust (e.g., maximize) the current of a portion of the multiple secondary charged particle beams detected by the detector. Stray charged particles do not contribute to the current. In some embodiments, if the controller focused the lens using a first beam-focusing method as described in connection with step 1102, the controller may refocus the lens to adjust the current using a second beam-focusing method different from the first beam-focusing method. The second beam-focusing method may include one of focusing the lens to focus paraxial rays of the multiple secondary charged particle beams onto the plane of the detector, focusing the lens to position an ellipse of least confusion of the multiple secondary charged particle beams onto the plane of the detector to form a secondary beam spot, and focusing the lens to minimize a leading edge width of the secondary beam spot.

[0111]

[0125] In some embodiments, if the controller enables a beam-limiting aperture as described in association with step 1104 to refocus the lens to adjust the current, then based on the aperture size of the beam-limiting aperture, the controller can refocus the lens to adjust the current of the multiple secondary charged particle beams detected by the detector. As an example, based on the relationship between integrated signal intensity and aperture size as described in association with FIGS. 10A and 10B , the controller can select a beam focusing technique to refocus the lens to adjust the current.

[0112]

[0126] In some embodiments, if the controller enables a beam-limiting aperture as described in connection with step 1104 to refocus the lens to adjust the current, then based on the position of the beam-limiting aperture along the projection axis of the lens (e.g., projection axis 252 in FIGS. 2 and 3 ), the controller can refocus the lens to adjust the current of the portion of the multiple secondary charged particle beams detected by the detector. As an example, based on the relationship between the integrated signal intensity and the position of the beam-limiting aperture along the projection axis, the controller can select a beam focusing technique for refocusing the lens to adjust the current.

[0113]

[0127] In some embodiments, if the detector is a pixel detector, the controller can refocus the lens to adjust the current of a portion of the multiple secondary charged particle beams detected by the detector based on the detector cell size of the pixel detector. For example, the controller can select a beam focusing technique for refocusing the lens to adjust the current based on the relationship between integrated signal strength and detector cell size as described in connection with FIGS. 8A and 8B .

[0114]

[0128] In some embodiments, to refocus the lens to adjust the current, the controller can refocus the lens to move the focal point of the lens (e.g., focal point 408 described in connection with FIGS. 4A and 4B ) a first step distance toward or away from the plane of the detector. The controller can also determine whether the collection efficiency value increases and whether the crosstalk ratio value is below a predetermined threshold based on a determination that the collection efficiency value increases and a determination that the crosstalk ratio value is below a predetermined threshold, and the controller can refocus the lens to move the focal point of the lens a second step distance toward or away from the plane of the detector. The second step distance may be the same as or different from the first step distance. In some embodiments, based on a determination that the collection efficiency value decreases, the controller can change the scan direction of the multi-beam inspection device. In some embodiments, based on a determination that the collection efficiency value reaches a maximum value or that the crosstalk ratio value is not below a predetermined threshold, the controller can stop refocusing the lens.

[0115]

[0129] A non-transitory computer-readable medium can be provided that stores instructions for a processor (e.g., the processor of controller 109 of FIG. 1 ) to perform a method for optimizing collection efficiency of secondary charged particles, such as method 1100 of FIG. 11 , data processing, database management, graphic display, operation of an image inspection machine or other imaging device, detection of defects on a sample, or the like. Common forms of non-transitory medium include, for example, a floppy disk, a flexible disk, a hard disk, a 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 a pattern of holes, RAM, PROM, EPROM, FLASH-EPROM or any other flash memory, NVRAM, cache, registers, any other memory chip or cartridge, and networked versions thereof.

[0116]

[0130] The embodiments can be further described using the following clauses. 1. A multi-beam inspection device configured to scan a sample and including a lens; a detector configured to receive a number of secondary charged particle beams in response to scanning of the sample; a controller including circuitry communicatively coupled to the multi-beam inspection device and the detector; 1. A system comprising: a controller focusing a lens to adjust a size of a secondary beam spot, the secondary beam spot being formed on the detector by the multiple secondary charged particle beams; For each secondary charged particle beam of the multiple secondary charged particle beams, preventing outlying charged particles of each secondary charged particle beam from being detected by a detector; refocusing the lens to adjust the current of a portion of the multiple secondary charged particle beam detected by the detector, where stray charged particles do not contribute to the current; A system configured to: 2. The system of clause 1, wherein the controller is configured to focus the lens to adjust the size of the secondary beam spot so as to enable the size of the secondary beam spot to be minimized. 3. The system of clause 1 or 2, wherein the controller is configured to refocus the lens to adjust the current so as to maximize the current. 4. A system described in any one of clauses 1 to 3, wherein the detector is a pixel detector, and the controller is configured to adjust the number of pixels in a detector cell of the detector so that stray charged particles of each secondary charged particle beam are not detected, and the pixels of the detector cell are configured to detect multiple secondary charged particle beams. 5. The controller is configured to focus the lens to adjust the size of the secondary beam spot using a first beam focusing method, the first beam focusing method comprising: focusing the lens to converge paraxial rays of the multiple secondary charged particle beams onto the plane of the detector; focusing the lens to position an ellipse of least confusion of the multiple secondary charged particle beams on the plane of the detector to form a secondary beam spot; and Focusing the lens to minimize the rising edge width of the secondary beam spot The system of any one of clauses 1 to 4, including one of: 6. The controller is configured to refocus the lens to adjust the current using a second beam focusing method different from the first beam focusing method, the second beam focusing method comprising: focusing the lens to converge paraxial rays of the multiple secondary charged particle beams onto the plane of the detector; focusing the lens to position an ellipse of least confusion of the multiple secondary charged particle beams on the plane of the detector to form a secondary beam spot; and Focusing the lens to minimize the rising edge width of the secondary beam spot 10. The system of claim 5, comprising one of: 7. A system described in any one of clauses 1 to 6, wherein the controller is configured to prevent stray charged particles of each secondary charged particle beam from being detected by the detector by adjusting the beam-limiting aperture configuration so that the beam-limiting aperture of the beam-limiting aperture configuration is positioned upstream of the detector and stray charged particles are filtered. 8. The system of clause 7, wherein the controller is configured to prevent stray charged particles of each secondary charged particle beam from being detected by the detector by adjusting the beam-limiting aperture configuration such that a beam-limiting aperture of the beam-limiting aperture configuration is positioned upstream of the lens and stray charged particles are filtered. 9. The system of clause 7 or 8, wherein the controller is configured to refocus the lens to adjust the current so that the lens can be refocused to adjust the current based on the aperture size of the beam-limiting aperture. 10. The system of any one of clauses 7 to 9, wherein the controller is configured to refocus the lens to adjust the current so that the lens can be refocused to adjust the current based on the position of the beam-limiting aperture along the projection axis of the lens. 11. A system described in any one of clauses 1 to 10, wherein the detector is a pixel detector and the controller is configured to prevent stray charged particles of each secondary charged particle beam from being detected by the detector by selecting a subset of pixels from the pixels covered by each secondary charged particle beam. 12. The system of any one of clauses 1 to 11, wherein the detector is a pixel detector and the controller is configured to refocus the lens to adjust the current so that the lens can be refocused to adjust the current based on a detector cell size of the pixel detector. 13. The controller refocuses the lens to move the focal point of the lens a first step distance toward or away from the plane of the detector; determining whether the collection efficiency value increases and whether the crosstalk ratio value falls below a predetermined threshold; refocusing the lens to move the focal point of the lens a second step distance toward or away from the plane of the detector based on the determination that the collection efficiency value increases and the determination that the crosstalk ratio value is below a predetermined threshold; changing a scan direction of the multi-beam inspection device based on a determination that the collection efficiency value decreases; Stopping the refocusing of the lens based on a determination that the collection efficiency value reaches a maximum value or that the crosstalk ratio value does not fall below a predetermined threshold. 13. The system of any one of clauses 1 to 12, configured to refocus the lens to adjust the current to enable 14. A non-transitory computer-readable medium storing a set of instructions executable by at least one processor of a multi-beam inspection apparatus to cause the multi-beam inspection apparatus to perform a method, the method comprising: focusing a lens of the multi-beam inspection apparatus to adjust a size of a secondary beam spot, where the secondary beam spot is formed on the detector by multiple secondary charged particle beams; For each secondary charged particle beam of the multiple secondary charged particle beams, preventing outlying charged particles of each secondary charged particle beam from being detected by a detector; refocusing the lens to adjust the current of a portion of the multiple secondary charged particle beam detected by the detector, where stray charged particles do not contribute to the current; 1. A non-transitory computer-readable medium, comprising: 15. Focusing the lens of the multi-beam inspection device to adjust the size of the secondary beam spot; Minimizing the size of the secondary beam spot 14. A non-transitory computer-readable medium as set forth in clause 14, comprising: 16. Refocusing the lens to adjust the current Maximizing the current of a portion of the multiple secondary charged particle beams detected by the detector 16. A non-transitory computer-readable medium according to clause 14 or 15, comprising: 17. The detector is a pixel detector, and stray charged particles of each secondary charged particle beam are prevented from being detected by the detector. adjusting the number of pixels of a detector cell of a detector, the pixels of the detector cell being configured to detect multiple secondary charged particle beams; 17. The non-transitory computer-readable medium of any one of clauses 14 to 16, comprising: 18. Focusing a lens of a multi-beam inspection apparatus to adjust the size of a secondary beam spot includes a first beam focusing method, the first beam focusing method comprising: focusing the lens to converge paraxial rays of the multiple secondary charged particle beams onto the plane of the detector; focusing the lens to position an ellipse of least confusion of the multiple secondary charged particle beams on the plane of the detector to form a secondary beam spot; and Focusing the lens to minimize the rising edge width of the secondary beam spot 18. The non-transitory computer-readable medium of any one of clauses 14 to 17, including one of: 19. Refocusing the lens to adjust the current includes a second beam focusing method different from the first beam focusing method, the second beam focusing method comprising: focusing the lens to converge paraxial rays of the multiple secondary charged particle beams onto the plane of the detector; focusing the lens to position an ellipse of least confusion of the multiple secondary charged particle beams on the plane of the detector to form a secondary beam spot; and Focusing the lens to minimize the rising edge width of the secondary beam spot 18. The non-transitory computer-readable medium of any one of clauses 14 to 17, including one of: 20. Preventing stray charged particles of each secondary charged particle beam from being detected by a detector adjusting the beam-limiting aperture arrangement so that the beam-limiting aperture of the beam-limiting aperture arrangement is positioned upstream of the detector and stray charged particles are filtered out; 20. The non-transitory computer-readable medium of any one of clauses 14 to 19, comprising: 21. Adjusting the beam limiting aperture configuration adjusting the beam-limiting aperture arrangement so that the beam-limiting aperture of the beam-limiting aperture arrangement is positioned upstream of the lens and stray charged particles are filtered out; 20. A non-transitory computer-readable medium as set forth in clause 20, comprising: 22. Refocusing the lens to adjust the current detector Refocusing the lens to adjust the current based on the aperture size of the beam-limiting aperture 22. A non-transitory computer-readable medium according to clause 20 or 21, comprising: 23. Refocusing the lens to adjust the current Refocusing the lens to adjust the current based on the position of the beam-limiting aperture along the projection axis of the lens. 23. The non-transitory computer-readable medium of any one of clauses 20 to 22, comprising: 24. The detector is a pixel detector, and stray charged particles of each secondary charged particle beam are prevented from being detected by the detector. selecting a subset of pixels from the pixels covered by each secondary charged particle beam; 24. The non-transitory computer-readable medium of any one of clauses 14 to 23, comprising: 25. The detector is a pixel detector, and refocusing the lens to adjust the current is Refocusing the lens to adjust the current based on the detector cell size of the pixel detector. 25. The non-transitory computer-readable medium of any one of clauses 14 to 24, comprising: 26. Refocusing the lens to adjust the current refocusing the lens to move the focal point of the lens a first step distance toward or away from the plane of the detector; determining whether the collection efficiency value increases and whether the crosstalk ratio value falls below a predetermined threshold; refocusing the lens to move the focal point of the lens a second step distance toward or away from the plane of the detector based on the determination that the collection efficiency value increases and the determination that the crosstalk ratio value is below a predetermined threshold; changing a scan direction of the multi-beam inspection device based on a determination that the collection efficiency value decreases; Stopping the refocusing of the lens based on a determination that the collection efficiency value reaches a maximum value or that the crosstalk ratio value does not fall below a predetermined threshold. 26. The non-transitory computer-readable medium of any one of clauses 14 to 25, comprising: 27. A method for optimizing collection efficiency of secondary charged particles, comprising: focusing a lens of the multi-beam inspection apparatus to adjust a size of a secondary beam spot, where the secondary beam spot is formed on the detector by multiple secondary charged particle beams; For each secondary charged particle beam of the multiple secondary charged particle beams, preventing outlying charged particles of each secondary charged particle beam from being detected by a detector; refocusing the lens to adjust the current of a portion of the multiple secondary charged particle beam detected by the detector, where stray charged particles do not contribute to the current; A method comprising: 28. Focusing a lens of a multi-beam inspection device to adjust the size of a secondary beam spot Minimizing the size of the secondary beam spot 28. The method according to clause 27, comprising: 29. Refocusing the lens to adjust the current Maximizing the current of a portion of the multiple secondary charged particle beams detected by the detector 29. The method according to clause 27 or 28, comprising: 30. The detector is a pixel detector, and stray charged particles of each secondary charged particle beam are prevented from being detected by the detector. adjusting the number of pixels of a detector cell of a detector, the pixels of the detector cell being configured to detect multiple secondary charged particle beams; 30. The method of any one of clauses 27 to 29, comprising: 31. Focusing a lens of a multi-beam inspection apparatus to adjust the size of a secondary beam spot includes a first beam focusing method, the first beam focusing method comprising: focusing the lens to converge paraxial rays of the multiple secondary charged particle beams onto the plane of the detector; focusing the lens to position an ellipse of least confusion of the multiple secondary charged particle beams on the plane of the detector to form a secondary beam spot; and Focusing the lens to minimize the rising edge width of the secondary beam spot 31. The method of any one of clauses 27 to 30, including one of: 32. Refocusing the lens to adjust the current includes a second beam focusing method different from the first beam focusing method, the second beam focusing method comprising: focusing the lens to converge paraxial rays of the multiple secondary charged particle beams onto the plane of the detector; focusing the lens to position an ellipse of least confusion of the multiple secondary charged particle beams on the plane of the detector to form a secondary beam spot; and Focusing the lens to minimize the rising edge width of the secondary beam spot 32. The method of any one of clauses 27 to 31, including one of: 33. Preventing stray charged particles in each secondary charged particle beam from being detected by a detector adjusting the beam-limiting aperture arrangement so that the beam-limiting aperture of the beam-limiting aperture arrangement is positioned upstream of the detector and stray charged particles are filtered out; 33. The method of any one of clauses 27 to 32, comprising: 34. Adjusting the beam limiting aperture configuration adjusting the beam-limiting aperture arrangement so that the beam-limiting aperture of the beam-limiting aperture arrangement is positioned upstream of the lens and stray charged particles are filtered out; 34. The method of claim 33, comprising: 35. Refocusing the lens to adjust the current Refocusing the lens to adjust the current based on the aperture size of the beam-limiting aperture 35. The method of claim 33 or 34, comprising: 36. Refocusing the lens to adjust the current Refocusing the lens to adjust the current based on the position of the beam-limiting aperture along the projection axis of the lens. 36. The method of any one of clauses 33 to 35, comprising: 37. The detector is a pixel detector, and stray charged particles of each secondary charged particle beam are prevented from being detected by the detector. selecting a subset of pixels from the pixels covered by each secondary charged particle beam; 37. The method of any one of clauses 27 to 36, comprising: 38. The detector is a pixel detector, and refocusing the lens to adjust the current of multiple secondary charged particle beams detected by the detector is Refocusing the lens to adjust the current based on the detector cell size of the pixel detector. 38. The method of any one of clauses 27 to 37, comprising: 39. Refocusing the lens to adjust the current refocusing the lens to move the focal point of the lens a first step distance toward or away from the plane of the detector; determining whether the collection efficiency value increases and whether the crosstalk ratio value falls below a predetermined threshold; refocusing the lens to move the focal point of the lens a second step distance toward or away from the plane of the detector based on the determination that the collection efficiency value increases and the determination that the crosstalk ratio value is below a predetermined threshold; 29. The method according to clause 27 or 28, comprising: 40. Changing the scanning direction of a multi-beam inspection device based on a determination that the collection efficiency value is decreasing. 39. The method of claim 38, further comprising: 41. Stopping lens refocusing based on a determination that the collection efficiency value does not increase or the crosstalk ratio value does not fall below a predetermined threshold. 39. The method of claim 38, further comprising:

[0117]

[0131] The block diagrams in the figures 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 flowchart or block diagram may represent a module, segment, or portion of code, including one or more executable instructions for implementing the specified logical function(s). It should be understood that in some alternative implementations, the functions shown in the blocks may occur in an order different from that shown in the figures. For example, depending on the functionality involved, two blocks shown in succession may be executed or implemented substantially simultaneously, or the two blocks may be executed in the reverse order. Some blocks may also be omitted. It should also be understood that each block and combination of blocks in the block diagrams may be implemented by a dedicated hardware-based system that performs the specified function or act, or by a combination of dedicated hardware and computer instructions.

[0118]

[0132] It will be understood that the embodiments of the present disclosure are not limited to the exact configurations described above and illustrated in the accompanying drawings, and that various modifications and changes can be made without departing from the scope of the present invention.

Claims

1. a multi-beam inspection device configured to scan the sample and including a lens; a detector configured to receive a number of secondary charged particle beams in response to scanning of the sample; a controller including circuitry communicatively coupled to the multi-beam inspection device and the detector; 1. A system comprising: focusing the lens to adjust a size of a secondary beam spot, the secondary beam spot being formed on the detector by the multiple secondary charged particle beams; For each secondary charged particle beam of the multiple secondary charged particle beams, preventing outlying charged particles of each secondary charged particle beam from being detected by the detector; refocusing the lens to adjust the current of a portion of the multiple secondary charged particle beams detected by the detector, wherein the stray charged particles do not contribute to the current; A system configured to:

2. The system of claim 1 , wherein the controller is configured to focus the lens to adjust the size of the secondary beam spot to enable minimizing the size of the secondary beam spot.

3. 2. The system of claim 1, wherein the controller is configured to refocus the lens to adjust the current to enable maximizing the current of the portion of the multiple secondary charged particle beams detected by the detector.

4. 2. The system of claim 1, wherein the detector is a pixel detector, and the controller is configured to adjust a number of pixels in a detector cell of the detector so that the stray charged particles of each of the secondary charged particle beams are not detected, and the pixels of the detector cell are configured to detect the multiple secondary charged particle beams.

5. The controller is configured to focus the lens to adjust the size of the secondary beam spot using a first beam focusing method, the first beam focusing method comprising: focusing the lens to converge paraxial rays of the multiple secondary charged particle beams onto the plane of the detector; focusing the lens to position an ellipse of least confusion of the multiple secondary charged particle beams on the plane of the detector to form the secondary beam spots; and focusing the lens to minimize the rising edge width of the secondary beam spot; The system of claim 1 , comprising one of:

6. The controller is configured to refocus the lens to adjust the current using a second beam focusing method different from the first beam focusing method, the second beam focusing method comprising: focusing the lens to converge the paraxial rays of the multiple secondary charged particle beams onto the plane of the detector; focusing the lens to position the ellipse of least confusion of the multiple secondary charged particle beams on the plane of the detector to form the secondary beam spot; and focusing the lens to minimize the rising edge width of the secondary beam spot; The system of claim 5 , comprising one of:

7. 2. The system of claim 1, wherein the controller is configured to prevent the errant charged particles of each of the secondary charged particle beams from being detected by the detector by adjusting a beam-limiting aperture arrangement such that a beam-limiting aperture of the beam-limiting aperture arrangement is positioned upstream of the detector and the errant charged particles are filtered.

8. 8. The system of claim 7, wherein the controller is configured to prevent the stray charged particles of each of the secondary charged particle beams from being detected by the detector by adjusting the beam-limiting aperture arrangement such that the beam-limiting aperture of the beam-limiting aperture arrangement is positioned upstream of the lens and the stray charged particles are filtered.

9. 8. The system of claim 7, wherein the controller is configured to refocus the lens to adjust the current so as to enable the controller to refocus the lens to adjust the current based on an aperture size of the beam-limiting aperture.

10. 8. The system of claim 7, wherein the controller is configured to refocus the lens to adjust the current so as to enable the lens to be refocused to adjust the current based on a position of the beam-limiting aperture along a projection axis of the lens.

11. 2. The system of claim 1, wherein the detector is a pixel detector, and the controller is configured to prevent the stray charged particles of each of the secondary charged particle beams from being detected by the detector by selecting a subset of pixels from those covered by each of the secondary charged particle beams.

12. 2. The system of claim 1, wherein the detector is a pixel detector, and the controller is configured to refocus the lens to adjust the current so as to enable the lens to be refocused to adjust the current based on a detector cell size of the pixel detector.

13. the controller refocusing the lens to move the focal point of the lens a first step distance toward or away from the plane of the detector; determining whether the collection efficiency value increases and whether the crosstalk ratio value falls below a predetermined threshold; refocusing the lens to move the focal point of the lens a second step distance toward or away from the plane of the detector based on a determination that the value of the collection efficiency increases and a determination that the value of the crosstalk ratio is below the predetermined threshold; changing a scan direction of the multi-beam inspection device based on a determination that the value of the collection efficiency is decreasing; and stopping refocusing of the lens based on a determination that the value of the collection efficiency reaches a maximum value or that the value of the crosstalk ratio does not fall below the predetermined threshold; The system of claim 1 , configured to refocus the lens to adjust the current to enable the lens to:

14. 1. A non-transitory computer-readable medium storing a set of instructions executable by at least one processor of a multi-beam inspection apparatus to cause the multi-beam inspection apparatus to perform a method, the method comprising: focusing a lens of the multi-beam inspection apparatus to adjust a size of a secondary beam spot, the secondary beam spot being formed on a detector by multiple secondary charged particle beams; For each secondary charged particle beam of the multiple secondary charged particle beams, preventing outlying charged particles of each secondary charged particle beam from being detected by the detector; refocusing the lens to adjust the current of a portion of the multiple secondary charged particle beams detected by the detector, wherein the stray charged particles do not contribute to the current; 1. A non-transitory computer-readable medium, comprising:

15. focusing the lens of the multi-beam inspection apparatus to adjust the size of the secondary beam spot; minimizing the size of the secondary beam spot; 15. The non-transitory computer-readable medium of claim 14, comprising:

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