Method and system for fine focusing secondary beam spot onto detector of multi-beam inspection device

Abstract

A measurement system and method for optimizing secondary charged particle collection efficiency 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 a plurality of secondary charged particle beams (236, 238, 240) in response to scanning the sample; and a controller (296) including circuitry communicatively coupled to the multi-beam inspection device and detector communication configured to: a focus lens to adjust a size of a secondary beam spot, where the secondary beam spot is formed by a plurality of secondary charged particle beams on the detector; for each secondary charged particle beam of the plurality of secondary charged particle beams, such that outlier charged particles of each secondary charged particle beam are not detected by the detector; and refocusing the lens to adjust the current of the portion of the plurality of secondary charged particle beams detected by the detector, wherein the outlier charged particles do not contribute to the current.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. application 63 / 368,597, filed on July 15, 2022, and the entire contents of which are incorporated herein by reference. Technical Field

[0003] The description herein relates to the field of image inspection devices, and more particularly to methods and systems for finely focusing a secondary beam spot on a detector of a multi-beam inspection device. Background Art

[0004] An image inspection device (e.g., a charged particle beam device or an optical beam device) is capable of generating a two-dimensional (2D) image of a wafer substrate by detecting particles (e.g., secondary electrons, backscattered electrons, other types of electrons, or photons) from a surface of a wafer substrate while scanning the surface of the wafer substrate with one or more beams (e.g., including a charged particle beam or an optical beam) generated by a source associated with the inspection device. In the semiconductor industry, various image inspection devices are used for semiconductor wafers for various purposes, such as critical dimension measurement (CD-SEM), wafer inspection (e.g., electron beam inspection system), or defect analysis (e.g., defect review SEM (DR-SEM)).

[0005] In semiconductor manufacturing, 2D images of wafers can be recorded and analyzed in order to control the quality of fabricated structures on the wafer substrate and detect potential defects. As the physical size of IC components continues to shrink, the accuracy and yield of defect detection become more and more critical. An important aspect of improving the throughput and accuracy of defect detection is to use multiple beams to scan multiple areas on the wafer surface simultaneously. This concept is implemented in a multi-beam scanning inspection system (e.g., a multi-beam SEM system).

[0006] For multi-beam inspection systems, the collection efficiency of secondary electrons and the crosstalk between secondary electron beamlets are two important parameters. For best performance, the collection efficiency should be maximized while the crosstalk should be minimized. The collection efficiency depends on the method of finely focusing the secondary charged particles to form a spot on the detector. Optimizing the method of finely focusing the secondary charged particles may increase the collection efficiency and ultimately improve the yield of defect detection. Summary of the invention

[0007] Embodiments of the present disclosure provide systems and methods for optimizing the collection efficiency of secondary charged particles. 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 a circuit system communicatively coupled to the multi-beam inspection device and the detector. The controller may be configured to focus the lens to adjust the size of the secondary beam spot, wherein the secondary beam spot is formed on the detector by multiple secondary charged particle beams. The controller may also be configured to, for each of the multiple secondary charged particle beams, cause the stray charged particles of each secondary charged particle beam to be not detected by the detector. The controller may also be configured to refocus 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.

[0008] In some embodiments, a non-transitory computer-readable medium may store an instruction set that can be executed by at least one processor of a multi-beam inspection device to cause the multi-beam inspection device to perform a method. The method may include focusing a lens of the multi-beam inspection device to adjust the size of a secondary beam spot, wherein the secondary beam spot is formed on a detector by a plurality of secondary charged particle beams. The method may also include, for each of the plurality of secondary charged particle beams, causing stray charged particles of each secondary charged particle beam to be not detected by the detector. The method may also include refocusing the lens to adjust the current of a portion of the plurality of secondary charged particle beams detected by the detector, wherein the stray charged particles do not contribute to the current.

[0009] In some embodiments, a method for optimizing the collection efficiency of secondary charged particles may include focusing a lens of a multi-beam inspection device to adjust the size of a secondary beam spot, wherein the secondary beam spot is formed on a detector by a plurality of secondary charged particle beams. The method may also include, for each of the plurality of secondary charged particle beams, making the stray charged particles of each secondary charged particle beam not detected by the detector. The method may also include refocusing the lens to adjust the current of a portion of the plurality of secondary charged particle beams detected by the detector, wherein the stray charged particles do not contribute to the current. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a schematic diagram illustrating an example charged particle beam inspection (CPBI) system consistent with some embodiments of the present disclosure.

[0011] Figure 2 It is shown that some embodiments of the present disclosure can be used as Figure 1 Schematic diagram of an example multi-beam beam tool that is part of an example charged particle beam inspection system.

[0012] Figure 3 It is shown that some embodiments of the present disclosure can be used as Figure 2 Schematic diagram of an example secondary projection system of a portion of an example multi-beam beam tool.

[0013] Figure 4A is a schematic diagram illustrating one example of a method for paraxial focusing of a charged particle beam consistent with certain embodiments of the present disclosure.

[0014] Figure 4B is a schematic diagram illustrating one example of a method for minimum dispersion focusing of a charged particle beam consistent with some embodiments of the present disclosure.

[0015] Figure 5A is a diagram visualizing a cross section of an example charged particle beam at a beam-limiting aperture plane consistent with some embodiments of the present disclosure.

[0016] Figure 5B It is consistent with some embodiments of the present disclosure. Figure 5A A diagram of the profile of an example charged particle beam.

[0017] Figure 6 is a chart illustrating example beam spot size optimization using different beam focusing methods consistent with some embodiments of the present disclosure.

[0018] Figure 7 is a graph illustrating example signal intensity distributions of secondary beam spots corresponding to different beam focusing methods consistent with certain embodiments of the present disclosure.

[0019] Fig. 8A is a graph showing example integrated signal intensities of secondary beam spots corresponding to different beam focusing methods consistent with certain embodiments of the present disclosure.

[0020] Figure 8B is a graph showing the difference between example integrated signal intensities of secondary beam spots corresponding to different beam focusing methods consistent with some embodiments of the present disclosure.

[0021] Fig. 9 is a graph showing profiles of example secondary beam spots on the plane of a beam limiting aperture corresponding to different beam focusing methods consistent with certain embodiments of the present disclosure.

[0022] Fig. 10A is a graph showing example integrated signal intensities of secondary beam spots corresponding to different beam focusing methods consistent with some embodiments of the present disclosure.

[0023] Fig. 10B is a graph illustrating the difference between example integrated signal intensities of secondary beam spots corresponding to different beam focusing methods consistent with certain embodiments of the present disclosure.

[0024] Fig.11 is a flow chart illustrating an example method of optimizing collection efficiency of secondary charged particles consistent with some embodiments of the present disclosure. DETAILED DESCRIPTION

[0025] Reference will now be made in detail to example embodiments, examples of which are shown in the accompanying drawings. The following description refers to the accompanying drawings, wherein, unless otherwise specified, the same numbers in different figures represent the same or similar elements. The implementations set forth in the following description of the example embodiments do not represent all implementations consistent with the present disclosure. Instead, they are merely examples of devices and methods consistent with aspects related to the subject matter described 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 detection methods in systems utilizing electron beams ("e-beams"). However, the present disclosure is not so limited. Other types of charged particle beams (e.g., including protons, ions, or any other particles carrying a charge) may be similarly applied. In addition, systems and methods for detection may be used in other measurement systems, such as optical imaging, photon detection, x-ray detection, ion detection, and the like.

[0026] Microchips consist of circuits formed on a semiconductor material called a substrate. The semiconductor material may include, for example, silicon, gallium arsenide, indium phosphide, or silicon germanium. Many circuits can be formed on the same silicon wafer, called an integrated circuit, or IC. The size of these circuits has been reduced dramatically, so more can fit on a substrate. For example, the IC chip in a smartphone may be as small as a thumbnail, but may contain more than 2 billion transistors, each less than one thousandth the size of a human hair.

[0027] Manufacturing these ICs, with their extremely small components and structures, is a complex, time-consuming, and expensive process that typically involves hundreds of individual steps. An error in even one step can result in a defect in the finished IC, rendering it unusable. Therefore, one goal of the manufacturing process is to avoid such defects in order to maximize the number of functional ICs made in the process; that is, to increase the yield of the overall process.

[0028] One component of improving yield is monitoring the chip manufacturing process. One way to monitor the process is to inspect the chip circuit structures at various stages of their formation. This 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 be used to image these extremely small structures, in effect taking a "photograph" of the wafer structure. This image can be used to determine if the structure was formed correctly in the right location. If the structure is defective, the process can be adjusted so the defect is less likely to occur again.

[0029] In a single beam SEM, a surface image can be created by scanning the inspected area line by line with a focused primary electron beam. When the primary electron beam hits the surface, a spot is formed, which can be called a "probe point" where secondary electrons and backscattered electrons are emitted in response to the primary electron beam. In the present disclosure, unless explicitly described, the term "secondary electrons" may include secondary electrons, or may include secondary electrons and backscattered electrons. The surface image can be reconstructed by collecting secondary electrons emitted from the probe points on the surface. The relationship between the secondary electron intensity and the position of the probe point can be determined. This relationship can also be presented (for example, as a two-dimensional graph). Even if the SEM scan rate is very high, creating a high-resolution image of the surface line by line can be a slow process. Therefore, wafer inspection can be very time-consuming.

[0030] A multi-beam SEM system can increase measurement speed and achieve higher throughput for wafer inspection applications. In a multi-beam SEM, an array of primary electron beams (or "beamlets") can be formed to simultaneously scan multiple sub-areas within an inspection area. Each primary electron beamlet can form a detection point on a sub-area of ​​the inspection area, and the detection points formed can form an array corresponding to the array of primary electron beamlets. Multiple secondary electron beamlets originating from the detection points can be formed and directed to a detector via a column of secondary electronic devices. The detector may include an array of electronic detection elements (referred to herein as "detector elements"). The array of detector elements may be implemented as an array of a single sensor, a two-dimensional pixelated detector, or any other form. If the array of detector elements is implemented as a two-dimensional pixelated detector, each detector element may be implemented as a different group of pixels. In the present disclosure, each group of pixels may be referred to as a detector element (or alternatively referred to as a "detector unit"), and the pixels forming the group that constitute the detector unit may be configurable (e.g., via a switch network between pixels). The secondary electron array may be configured such that each detector element may detect an intensity of a secondary electron beamlet corresponding to one of the detection points, which detection point also corresponds to a sub-area of ​​the inspected area.

[0031] In the present disclosure, the collection efficiency of a single detector element refers to the fraction of secondary electrons emitted from a detection point in one of the sub-regions in the inspection region that are detected by the corresponding detector element, or refers to the ratio of the number of detected electrons to the number of emitted electrons. To characterize the entire detector, the average, minimum, or maximum value of the collection efficiency across all detector elements can be used.

[0032] In the present disclosure, crosstalk refers to the fraction of secondary electrons detected by a single detector element that do not come from its corresponding sub-region of the inspected region but instead come from, for example, one or more adjacent sub-regions thereof, or refers to the ratio between the number of detected electrons that do not come from the corresponding sub-region and the total number of electrons detected by a single detector element.

[0033] High crosstalk values ​​may degrade the performance of a multi-beam charged particle inspection system. To reduce crosstalk, several techniques may be employed. For example, a beam limiting aperture (or "BLA") may be positioned at a point along the secondary electron optical path between the wafer surface and the detector (e.g., directly in front of the detector) to cut off the tail of the secondary electron beam distribution of the beam spot. In the present disclosure, all charged particles outside the beam limiting aperture may be collectively referred to as the "tail" of the beam spot. The tail of the beam spot may have a significant impact on the crosstalk at the detector. As another example, if the detector is a detector array (e.g., a pixel detector) that includes an array of detector elements (e.g., sub-units, units, or groups of pixelated detector elements), the size of the detector element (e.g., the size of the detector cell) may be set to a limited size to reduce crosstalk (e.g., by reducing the size of the detector element to reduce the amount of "tail" electrons collected by the detector element). Increasing the beam limiting aperture or limiting the size of the detector element may be used to reduce crosstalk while reducing collection efficiency. Thus, the collection efficiency may be limited by a predetermined 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 critical to maximize the collection efficiency at a predetermined level of crosstalk rate for any combination of parameters of the multi-beam charged particle inspection system.

[0034] Maximizing the collection efficiency is a challenge. In many existing technical solutions, the standard approach to focusing a charged particle beam spot onto a detector is to minimize the size of the beam spot, without considering the detector element size and without using a beam-limiting aperture. The focusing parameters (e.g., the excitation of the focusing lens) are usually fixed after this minimization process. However, after fixing the focusing parameters to minimize the beam spot size, if the crosstalk rate is still too high, the crosstalk can be suppressed by increasing the beam-limiting aperture or limiting the size of the detector elements. In this case, the collection efficiency may not reach its theoretically possible maximum value because the focusing parameters were fixed before increasing the beam-limiting aperture or limiting the size of the detector elements.

[0035] Embodiments of the present disclosure may provide methods, devices, and systems for optimizing the collection efficiency of secondary charged particles. In some disclosed embodiments, when optimizing the focusing parameters of a multi-beam inspection device, one or more lenses may be focused to adjust (e.g., minimize) the spot size of multiple secondary beams on a detector. In addition, stray charged particles of each of the multiple secondary beams may be prevented from being detected by the detector (e.g., by using a beam-limiting aperture, limiting the size of the detector element, etc.). Then, the focusing parameters of the lens may be adjusted to adjust (e.g., maximize) the current of a portion of the multiple secondary beams detected by the detector, wherein the portion of the multiple secondary beams detected by the detector with the maximized current does not include stray charged particles. Compared to the prior art, while suppressing or reducing the crosstalk to below a predetermined level, the collection efficiency corresponding to a predetermined level of crosstalk may be improved or maximized by considering the size of the detector element or whether a beam-limiting aperture is used.

[0036] For the sake of clarity, the relative sizes of the components in the figures may be exaggerated. In the following description of the figures, the same or similar reference numerals denote the same or similar components or entities, and only the differences with respect to the various embodiments are described.

[0037] As used herein, unless expressly stated otherwise, the term "or" encompasses all possible combinations unless not feasible. For example, if a component is specified to include A or B, then unless expressly stated otherwise or not feasible, the component may include A, or B, or A and B. As a second example, if a component is specified to include A, B, or C, then unless expressly stated otherwise or not feasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

[0038] Figure 1 An exemplary charged particle beam inspection (CPBI) system 100 consistent with some embodiments of the present disclosure is shown. The CPBI system 100 can be used for imaging. For example, the CPBI system 100 can use an electron beam for imaging. Figure 1 As shown, the CPBI system 100 includes a main chamber 101, a load / lock chamber 102, a beam tool 104, and an equipment front end module (EFEM) 106. The beam tool 104 is located in the main chamber 101. The EFEM 106 includes a first loading port 106a and a second loading port 106b. The EFEM 106 may include additional loading ports. The first loading port 106a and the second loading port 106b receive a wafer front opening transport box (FOUP) containing a wafer (e.g., a semiconductor wafer or a wafer made of other materials) or a sample to be inspected (wafers and samples can be used interchangeably). A "batch" is a plurality of wafers that can be loaded as a batch for processing.

[0039] One or more robotic arms (not shown) in the EFEM 106 can transport the wafer to the load / lock chamber 102. The load / lock chamber 102 is connected to a load / lock vacuum pump system (not shown), which removes gas molecules in the load / lock chamber 102 to reach a first pressure below atmospheric pressure. After reaching the first pressure, one or more robotic arms (not shown) can transport the wafer from the load / lock chamber 102 to the main chamber 101. The main chamber 101 is connected to a main chamber vacuum pump system (not shown), which removes gas molecules in the main chamber 101 to reach a second pressure below the first pressure. After reaching the second pressure, the wafer will be inspected by the beam tool 104. The beam tool 104 can be a single beam system or a multi-beam system. Typically, in a CPBI system, the wafer is placed on a platform. In the present disclosure, the platform may be referred to as a "stage".

[0040] The controller 109 is electrically connected to the beam tool 104. The controller 109 may be a computer that can perform various controls on the CPBI system 100. Figure 1 The controller 109 is shown as being external to the structure including the main chamber 101 , the load / lock chamber 102 , and the EFEM 106 , but it will be appreciated that the controller 109 may be part of the structure.

[0041] In some embodiments, the controller 109 may include one or more processors (not shown). The processor may be a general or special electronic device capable of manipulating or processing information. For example, the processor may include any number of any combination of the following: a central processing unit (or "CPU"), a graphics processing unit (or "GPU"), an optical processor, a programmable logic controller, a microcontroller, a microprocessor, a digital signal processor, an intellectual property (IP) core, a programmable logic array (PLA), a programmable array logic (PAL), a general array logic (GAL), a complex programmable logic device (CPLD), a field programmable gate array (FPGA), a system on a chip (SoC), an application-specific integrated circuit (ASIC), and any type of circuit capable of data processing. The processor may also be a virtual processor, which includes one or more processors distributed on multiple machines or devices coupled via a network.

[0042] In some embodiments, the controller 109 may also include one or more memories (not shown). The memory may be a general or specific electronic device capable of storing processor-accessible code and data (e.g., via a bus). For example, the memory may include any number of any combination of the following: random access memory (RAM), read-only memory (ROM), optical disk, magnetic disk, hard disk, solid-state drive, flash drive, secure digital (SD) card, memory stick, compact flash (CF) card, or any type of storage device. The code may include an operating system (OS) and one or more applications (or "apps") for specific tasks. The memory may also be a virtual memory, which includes one or more memories distributed on multiple machines or devices coupled via a network.

[0043] In some embodiments, the beam tool 104 may be a multi-beam system. For example, Figure 2 An example multi-beam beam tool 104 (also referred to herein as apparatus 104) and a method for use in conjunction with an apparatus consistent with embodiments of the present disclosure are shown. Figure 1 Schematic diagram of the image processing system 290 of the CPBI system 100 in FIG.

[0044] refer to Figure 2 , the beam tool 104 includes a charged particle source 202, a gun aperture 204, a converging lens 206, a primary charged particle beam 210 emitted from the charged particle source 202, a source conversion unit 212, a plurality of 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, a plurality of 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 splitter 222, a deflection scanning unit 226, and an objective lens 228. The charged particle detector 244 may include detector elements 246, 248, and 250.

[0045] Charged particle source 202, gun aperture 204, converging lens 206, source conversion unit 212, beam splitter 222, deflection scanning unit 226, and objective lens 228 may be aligned with a primary projection axis 260 (e.g., similar to a primary optical axis in an optical system) of device 104. Secondary projection system 242 and charged particle detector 244 may be aligned with a secondary projection axis 252 (e.g., similar to a secondary optical axis in an optical system) of device 104.

[0046] The charged particle source 202 may emit one or more charged particles, such as electrons, protons, ions, or any other particles that carry an electric charge. In some embodiments, the charged particle source 202 may be an electron source. For example, the charged particle source 202 may include a cathode, an extractor, or an anode. Primary electrons may be emitted from the cathode and extracted or accelerated to form a primary charged particle beam 210 (e.g., a primary electron beam) having an intersection point 208 (e.g., virtual or real). The primary charged particle beam 210 may be visualized as a beam from Figure 2 The gun aperture 204 can block the peripheral charged particles of the primary charged particle beam 210 to reduce the Coulomb effect. The Coulomb effect can cause the size of the detection point to increase.

[0047] 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 intersection 208 with multiple primary sub-beams (including primary sub-beams 214, 216, and 218) of the primary charged particle beam 210. The beam limiting aperture array may limit the multiple sub-beams 214, 216, and 218. Although Figure 2 Three primary sub-bundles 214 , 216 , and 218 are shown, but embodiments of the present disclosure are not limited thereto.

[0048] The converging lens 206 can focus the primary charged particle beam 210. By adjusting the optical power (e.g., excitation) of the converging lens 206 or by changing the radial size of the corresponding beam-limiting apertures in the beam-limiting aperture array, the current of the primary sub-beams 214, 216, and 218 downstream of the source conversion unit 212 can be changed. The objective lens 228 can focus the primary sub-beams 214, 216, and 218 onto the sample 230 for imaging, and can form a plurality of detection points (including detection points 270, 272, and 274) on or near the surface of the sample 230 (e.g., a wafer).

[0049] For 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., from a few nanometers to a few microns) and interact with particles of the sample 230. Some electrons of the primary electron beamlets may elastically interact with particles of the sample 230 (e.g., in the form of elastic scattering) and may be reflected or bounced off the surface of the sample 230. Elastic interactions preserve the kinetic energy of the interacting bodies (e.g., electrons of the primary electron beamlets and particles of the sample 230), wherein the kinetic energy of the interacting bodies is not converted into other forms of energy (e.g., heat). Such reflected electrons generated by elastic interactions may be referred to as backscattered electrons (BSE). In addition, some electrons of the primary electron beamlets may inelastically interact with particles of the sample 230 (e.g., in the form of inelastic scattering). Inelastic interactions do not conserve the kinetic energy of the interacting bodies, wherein part or all of the kinetic energy of the interacting bodies may be converted into other forms of energy. For example, inelastic interactions may ionize some particles of sample 230, and the ionized particles may generate additional electrons, which may be referred to as secondary electrons (SE). Secondary electrons may leave the surface of sample 230. The yield or emission rate of BSE and SE may depend on factors such as the electron energy of the primary electron beamlet and the material of sample 230. The number of BSE and SE may be greater than, less than or equal to the injected electrons of the primary electron beamlet. For ease of explanation in an unambiguous context, unless explicitly stated, backscattered electrons and secondary electrons may be referred to as "secondary electrons" hereinafter. In addition, as used herein, a "detection point" (e.g., detection point 270, 272 or 274) refers to an area on or near the surface of the sample under test, wherein the area emits secondary charged particles (e.g., secondary electrons) corresponding to the incident charged particle beam or beamlet.

[0050] Still refer to Figure 2 , the beam splitter 222 may include a beam splitter (e.g., a Wien filter) that generates an electrostatic dipole field and a magnetic dipole field. In some embodiments, the force exerted by the electrostatic dipole field on the charged particles (e.g., electrons) of the primary beamlets 214, 216, and 218 may be substantially equal in magnitude to the force exerted by the magnetic dipole field on the charged particles, but opposite in direction. Thus, the primary beamlets 214, 216, and 218 may pass directly through the beam splitter 222 at a zero deflection angle. In some cases, the total dispersion of the primary beamlets 214, 216, and 218 generated by the beam splitter 222 may also be non-zero.

[0051] The deflection scanning unit 226 may deflect the primary beamlets 214, 216, and 218 to scan the surface area of ​​the sample 230. In response to the incident of the primary beamlets 214, 216, and 218, secondary charged particle beams (including secondary beamlets 236, 238, and 240) may be emitted from the sample 230 at detection points 270, 272, and 274. The secondary beamlets 236, 238, and 240 may include charged particles (e.g., electrons) having an energy distribution and an upward moving direction. When the secondary beamlets (e.g., including the secondary beamlets 236, 238, and 240) enter the primary projection system 220, the beam splitter 222 may separate the secondary beamlets from the primary beamlets (e.g., the primary beamlets 214, 216, and 218), and further guide the secondary beamlets to the secondary projection system 242.

[0052] The secondary projection system 242 may focus the secondary beamlets 236, 238, and 240 onto the detector elements 246, 248, and 250 of the charged particle detector 244. A single detector element (e.g., detector element 246, 248, or 250) may be designed to detect a single corresponding secondary beamlet (e.g., secondary beamlet 236, 238, or 240, respectively) originating from a single detection point (e.g., detection point 270, 272, or 274) and generate a corresponding signal (e.g., voltage, current, etc.) to reconstruct an image of the scanned surface of the sample 230. In some embodiments, the charged particle detector 244 may include an array of single sensors, wherein a single detector element (e.g., detector element 246, 248, or 250) may be a single sensor. In some embodiments, the charged particle detector 244 may include a 2D pixelated detector, which includes an array of detector cells, wherein a single detector element (e.g., detector element 246, 248, or 250) may be implemented as a group of pixels (e.g., each pixel represents a single detector cell).

[0053] 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 ( ) in communication with the charged particle detector 244, the primary projection system 220, and the motorized wafer stage 280. Figure 2 230). The intensities of the secondary beamlets 236, 238, and 240 may vary depending on the external or internal structure of the sample 230, and thus may indicate whether the sample 230 includes a defect. In addition, the primary beamlets 214, 216, and 218 may be projected onto different locations on the top surface of the sample 230 to generate secondary beamlets 236, 238, and 240 of different intensities. Therefore, by mapping the intensities of the secondary beamlets 236, 238, and 240 to the area of ​​the sample 230, the image processing system 290 may reconstruct an image reflecting the internal or external structural features of the sample 230.

[0054] In some embodiments, the image processing system 290 may include an image acquirer 292, a memory 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, a terminal, a personal computer, any type of mobile computing device, etc., 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, radio, etc. In some embodiments, the image acquirer 292 may receive a signal from the charged particle detector 244 and may construct an image. Thus, 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, etc. The image acquirer 292 may be configured to adjust the brightness and contrast of the acquired image. In some embodiments, the memory 294 may be a storage medium such as a hard disk, a flash drive, a cloud storage, a random access memory (RAM), other types of computer-readable memory, etc. The memory 294 may be coupled to the image acquisition device 292 and may be used to save the scanned raw image data as a raw image and a post-processed image. The image acquisition device 292 and the memory 294 may be connected to a controller 296. In some embodiments, the image acquisition device 292, the memory 294 and the controller 296 may be integrated together as a control unit.

[0055] In some embodiments, the image acquirer 292 may acquire one or more images of the wafer based on the imaging signal received from the charged particle detector 244. The imaging signal may correspond to a scanning operation for performing charged particle imaging. The acquired image may be a single image including multiple imaging regions. The single image may be stored in the memory 294. The single image may be an original image that may be divided into multiple regions. Each region may include an imaging region that includes a feature of the sample 230. The acquired image may include multiple images of a single imaging region of the sample 230 sampled multiple times in a time series. The multiple images may be stored in the memory 294. In some embodiments, the image processing system 290 may be configured to perform an image processing step using multiple images of the same position of the sample 230.

[0056] For example, Figure 3 is a schematic diagram showing an exemplary secondary device array 300 consistent with some embodiments of the present disclosure, which may be Figure 2The secondary array 300 may include a portion of the beam tool 104. For example, the secondary array 300 may include a charged particle detector 244. The secondary array 300 may also include an anti-scan deflection system 302 and a secondary projection system 242. In some embodiments, the secondary array 300 may also include one or more focusing lenses (including lens 308 and lens 310) and a beam splitter ( Figure 3 One or more focusing lenses may be objective lenses, such as Figure 2 The objective lens 228 is the same as or similar to the objective lens 228. One or more focusing lenses can collect the secondary charged particles emitted from the detection point and form secondary sub-beams (including secondary sub-beams 236, 238 and 240). Figure 3 As shown, the charged particle detector 244 further includes detector elements 246, 248, and 250. The detector elements 246, 248, and 250 may be designed to detect corresponding secondary beamlets 236, 238, and 240, respectively. Figure 3 The secondary beamlets 236, 238 and 240 in the embodiment may be derived from Figure 2 Detection points 270 , 272 or 274 are shown. Components of secondary device array 300 may be aligned with secondary projection axis 252 .

[0057] In some embodiments, lens 308 may be used to set the magnification and lens 310 may be used to focus the image. Either lens 308 or lens 310 may be referred to as a "focusing lens," meaning that both contribute to focusing the secondary beamlets. A beam splitter (e.g., similar or identical to beam splitter 222) may direct the secondary beamlets toward secondary projection system 242. Anti-scan deflection system 302 may direct all secondary beamlets toward secondary projection axis 252 to minimize image displacement on charged particle detector 244. For example, referring to Figure 2 , image displacements may originate from detection points (e.g., including Figure 2 The detection points 270, 272 and 274 in the sample 230 are moved synchronously with the deflection scanning unit 226 across the inspection area of ​​the sample 230.

[0058] The secondary projection system 242 can project the secondary beamlets (e.g., including the secondary beamlets 236, 238, and 240) onto the charged particle detector 244 while maintaining their characteristics (e.g., including focus, size, and rotation). The secondary projection system 242 can maintain the characteristics of the secondary beamlets to be nearly constant and consistent with the secondary projection system 242. Figure 2 The imaging conditions of the beam tool 104 are irrelevant. Figure 3 As shown, the secondary projection system 242 includes a beam limiting aperture 304. The beam limiting aperture 304 has a radius, and the radius may determine which portion of the secondary beamlets is allowed to reach the charged particle detector 244.

[0059] Similar to optical systems, the electron optical elements of the secondary device array 300 may have aberrations. Such aberrations may blur the image projected by the secondary projection system 242 onto the charged particle detector 244 and limit the detection performance. Aberrations may also induce or increase the crosstalk described herein and limit the collection efficiency of the charged particle detector 244.

[0060] In order to suppress crosstalk, Figure 3 As shown, the beam limiting aperture 304 can be positioned at or near the intersection 306 of the secondary projection axis 252. The beam limiting aperture 304 can cut off stray electrons of the secondary beamlets, and only the central portion of the secondary beamlets can reach the charged particle detector 244. By using the beam limiting aperture 304, the edges of the spots of the secondary beamlets formed on the detector elements (e.g., including the detector elements 244, 246, and 248) can be cut off. Therefore, the size of the spots of the secondary beamlets formed on the detector elements can be limited, thereby reducing crosstalk. It should be noted that the use of the beam limiting aperture 304 may also limit the collection efficiency.

[0061] Consistent with some embodiments of the present disclosure, a method for optimizing the collection efficiency of secondary charged particles may include focusing a lens (e.g., one or more focusing lenses) of a multi-beam inspection device to adjust (e.g., minimize) the size of a secondary beam spot. The secondary beam spot may be formed by a plurality of secondary charged particle beams (e.g., secondary electron beams) on a detector. In some embodiments, the multi-beam inspection device 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).

[0062] As used herein, a lens may refer to a focusing lens or a focusing lens group or any functionally equivalent component of an electronic projection imaging system (e.g., a secondary electronic projection imaging system). As used herein, a focusing lens may refer to any operation (e.g., under the control of a controller or processor) that increases, decreases, or maintains the optical power (e.g., refractive power) of a lens. For example, if the lens is an electrostatic lens (or a magnetic lens or a composite lens), the excitation of the lens may be set to increase, decrease, or maintain the optical power of the lens.

[0063] As used herein, the secondary charged particle beam may refer to a beam formed by secondary charged particles that leave a detection point on or near a surface of a sample (e.g., a wafer) inspected by a multi-beam inspection apparatus in response to the primary charged particle beam being incident on the detection point. 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 that leave the detection point in response to the primary electron beam being incident on the detection point.

[0064] The secondary beam spot in the present disclosure refers to the image of the wafer detection point (i.e., the detection point on the wafer) formed by the secondary electron beam on the detector. The size of the secondary beam spot depends on many factors, such as the optical power of the lens, the incident angle distribution of the secondary charged particle beam, etc.

[0065] For example, refer to Figure 2-Figure 3 and its associated description, the multi-beam inspection device may be a multi-beam inspection tool (e.g., Figure 2 The focusing lens may be, for example, a secondary device array before the detector (e.g., Figure 3 The last lens in the secondary device array 300 in FIG. Figure 3 The plurality of 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 detection point formed on the charged particle detector 244 by the plurality of secondary charged particle beams.

[0066] In some embodiments, in order to focus the lens of the multi-beam inspection device (e.g., Figure 3 Various methods can be performed to adjust the size of the secondary beam spot by focusing the lens 310 in the detector. For example, if the detector is a pixelated detector, in order to prevent stray charged particles from being detected by the detector, the number of pixels of the detector unit of the detector can be adjusted, wherein the pixels of the detector unit can be used to detect multiple secondary charged particle beams. When the secondary charged particle beam is projected onto the surface of the pixelated detector by the lens and forms a beam spot, the pixels covered by the beam spot can respond and generate a signal representing the intensity of the secondary electron beam. The number of responding pixels can be proportional to the size of the beam spot. In order to adjust the size of the beam spot, the lens can be focused to adjust the number of pixels of the pixelated detector that respond to the beam spot.

[0067] For another example, in order to adjust the size of the secondary beam spot, the lens can be focused so that the paraxial rays of multiple secondary charged particle beams are focused on the plane of the detector. The paraxial rays of the charged particle beam used herein may refer to charged particle rays that belong to the charged particle beam and are close to the projection axis of the lens so that the approximation sinθ≈θ is effective, wherein the charged particle beam is refracted by the lens, and θ represents the refraction angle of the charged particle ray. This focusing method may be referred to as a "paraxial focusing method" in the present disclosure.

[0068] For example, Figure 4A is a schematic diagram illustrating one example of a method for paraxial focusing of a charged particle beam consistent with some embodiments of the present disclosure. Figure 4AA focusing lens 402 (indicated by a thick double arrow) and a focal plane 404 (indicated by a thick line) are depicted. The focusing lens 402 has a projection axis 406 (similar to the optical axis of an optical lens) and a focal point 408 (indicated by a black dot). The focal point 408 is located on the focal plane 404. The focal plane 404 can be placed (e.g., by adjusting the optical power of the objective lens 410) at the detector ( Figure 4A The focal plane 404 may be on or near a surface of a detector (not shown), in which case the focal plane 404 may also be referred to as an image plane. It should be noted that the focal plane may only coincide with the image plane for focusing of a collimated beam (e.g., a parallel beam). For a converging or diverging beam, the image plane may only coincide with the plane of the detector (referred to herein as the "detector plane").

[0069] like Figure 4A As shown, a charged particle beam 410 (e.g., an electron beam) is projected onto the focusing lens 402 at an incident angle of 0° and is focused onto a focal point 408 by the focusing lens 402. The charged particle beam 410 includes a plurality of charged particle rays (represented by solid lines). Figure 4A ), if the charged particle beam 410 is collimated (e.g., all incident charged particle rays are parallel to each other) and the focusing lens 402 is a perfect lens (e.g., without aberrations), all charged particle rays of the charged particle beam 410 can be focused at or near the focal point 408 (e.g., pointing toward the focal point 408). Figure 4A As shown in FIG. 4 , the focusing lens 402 is not a perfect lens (eg, has aberrations, such as spherical aberration), some charged particle rays of the charged particle beam 410 may not be focused at the focal point 408 .

[0070] like Figure 4A As shown, the charged particle beam 410 includes a paraxial ray 412 and an edge ray 414. As used herein, an edge ray of a charged particle beam may refer to a charged particle ray that belongs to the charged particle beam and is at a distance from the projection axis of the lens such that the approximation sinθ≈θ of the charged particle beam being refracted by the lens is violated, wherein the charged particle beam is refracted by the lens, and θ represents the refraction angle of the charged particle ray. Figure 4A , the paraxial rays 412 are focused at the focal point 408, and the marginal rays 414 are focused at various points (including points 416 and 418) different from the focal point 408. On the focal plane 404, the focused charged particle beam 410 forms a beam spot 420. It should be noted that although Figure 4A The charged particle beam 410 is shown as a collimated beam, but in the case where the charged particle beam 410 is not collimated (eg, converging or diverging) before the focusing lens 402 ( Figure 4A408), the charged particle beam 410 will be focused before or after the focal point 408. In this case, the paraxial rays 412 and the edge rays 414 are still focused at different positions. For example, the edge rays 412 may be focused at a point along the projection axis 406 before the point at which the paraxial rays 412 are focused.

[0071] Figure 4A An exemplary paraxial focusing method is shown for minimizing the size of the beam spot 420. Figure 4A In FIG. 4 , focusing lens 402 is focused (eg, by setting its excitation) so that paraxial rays 412 are focused on the plane of the detector.

[0072] In some embodiments, in order to focus the lens of the multi-beam inspection device to adjust the size of the secondary beam spot, as another example, the lens can be focused to the minimum dispersion ellipse position of multiple secondary charged particle beams on the detector plane to form a secondary beam spot. The minimum dispersion ellipse of the focused charged particle beam used herein refers to a spot of any shape (e.g., elliptical or circular) formed on the image plane, which is the smallest spot among all spots formed on all possible image planes. The ellipse in the present disclosure includes any ellipse with two foci or a circle with a single focus (e.g., a circle). When the first outer edge of the projected edge ray of the charged particle beam overlaps with the second outer edge of the projected paraxial ray of the charged particle beam, the minimum dispersion ellipse can be determined. The first outer edge and the second outer edge can be measured from the projection axis on the image plane. This focusing method may be referred to as a "minimum dispersion focusing method" in the present disclosure.

[0073] For example, Figure 4B is a schematic diagram illustrating one example of a method for minimum dispersion focusing of a charged particle beam consistent with some embodiments of the present disclosure. Figure 4B 4 and 4. The image plane 422 formed by the projecting of the charged particle beam 410 onto the image plane 422 by the focusing lens 402 and the minimum dispersion ellipse 424 (indicated by the thick line covering the image plane 422) are shown. It should be noted that although Figure 4B The charged particle beam 410 is shown as a collimated beam, but in the case where the charged particle beam 410 is not collimated (eg, converging or diverging) before the focusing lens 402 ( Figure 4B 408 ), the charged particle beam 410 will be focused before or after the focal point 408. In this case, the paraxial rays 412 and the edge rays 414 are still focused at different positions. For example, the edge rays 412 may be focused at a point along the projection axis 406 before the point at which the paraxial rays 412 are focused.

[0074] like Figure 4BAs shown, on the image plane 422, the minimum dispersion ellipse 424 is the smallest spot among all possible spots formed on the image plane. When the outer edge of the projected edge ray 414 overlaps with the outer edge of the projected paraxial ray 412, the minimum dispersion ellipse 424 is determined. Figure 4B In the illustrated minimum dispersion focusing method, the focusing lens 402 is focused (e.g., by setting its excitation) so that the minimum dispersion ellipse 424 is positioned at the detector ( Figure 4B In other words, Figure 4B , image plane 422 is the detector plane, and minimum dispersion ellipse 424 is the beam spot on the detector plane. Figure 4B In order to position the minimum dispersion ellipse 424 on the detector plane (i.e., to the left of the detector plane) when the distance between the focusing lens 402 and the detector is fixed, the optical power (e.g., refractive power) of the focusing lens 402 can be reduced so that the minimum dispersion ellipse 424 moves to the right toward the detector until the minimum dispersion ellipse 424 is located on the detector plane.

[0075] In some embodiments, in order to focus the lens of the multi-beam inspection device to adjust the size of the secondary beam spot, as another example, the lens can be focused to minimize the rising edge width of the secondary beam spot. In order to determine the rising edge width of the beam spot, the signal intensity of the beam spot can be integrated along the direction from the first side of the beam spot to the second side of the beam spot to form a step function (e.g., a one-dimensional step function), wherein the integrated signal intensity represents the detected charge intensity. The rising edge of the step function used herein refers to a portion of the step function, in which the level of the integrated signal intensity (which represents the detected charge intensity) increases rapidly to form a step. The rising edge width of the beam spot used herein refers to the length of the rising edge of the step function. For example, the rising edge can be a portion of the step function, in which the level of the relative signal intensity (expressed as a percentage) increases, such as from 15% to 85% of the maximum level of the step function (e.g., 100% intensity level), or from 20% to 80%, or from 25% to 75%. In the case where the charged particle beam is well focused, the rising edge width of its beam spot may be smaller. When the charged particle beam is poorly focused, the width of the rising edge of its beam spot may be large.

[0076] For example, in order to minimize the width of the rising edge of the secondary beam spot, the focusing lens (e.g., Figure 4A-4BThe focal point 408 is moved forward, backward or at the detector plane by adjusting the focal power (e.g., refractive power) of the focusing lens 402 in the detector. During this process, the rising edge width of the secondary beam spot can be measured and recorded, and when the rising edge width of the secondary beam spot reaches the minimum value among all recorded values ​​within a predetermined time interval, such adjustment can be stopped. This focusing method may be referred to as a "minimum rising edge focusing method" in the present disclosure.

[0077] In some embodiments, in order to find an excitation for focusing a lens of a multi-beam inspection device to adjust the size of a secondary beam spot, a simulation process may be performed to optimize the distribution of the secondary beam spot on a detector. For example, such a simulation process may be performed by projecting a set of test rays (e.g., including principal rays, paraxial rays, marginal rays, or any combination thereof) onto a lens in a simulation model, and adjusting parameters of the simulation model to adjust (e.g., minimize) the value of one or more predetermined merit functions. As an example, the maximum radius of the secondary beam spot may be used as a merit function.

[0078] Consistent with some embodiments of the present disclosure, the method may further include: for each secondary charged particle beam in a plurality of secondary charged particle beams, making the stray charged particles of each secondary charged particle beam not detected by the detector. The stray charged particles of the charged particle beam used herein may refer to charged particles belonging to the charged particle beam and whose distance from the projection axis of the lens exceeds a predetermined value, or charged particles belonging to the charged particle beam and whose signal intensity is lower than a predetermined level. For example, the stray charged particles may belong to the tail of the cross section of the secondary electron beamlet or to a secondary electron point formed on the detector. In some embodiments, in order to prevent the stray charged particles from being detected by the detector, one or more operations may be performed to prevent the stray charged particles from reaching the surface of the detector, or if the stray charged particles reach the surface of the detector, the detector is made not to respond to the stray charged particles (e.g., by using a control signal).

[0079] In some embodiments, in order to prevent the stray charged particles of each secondary charged particle beam from being detected by the detector, the beam-limiting aperture configuration can be adjusted so that the beam-limiting aperture is positioned upstream of the detector (e.g., upstream of the lens) and filters the stray charged particles (e.g., allowing portions of multiple secondary charged particle beams to pass through the beam-limiting aperture). The beam-limiting aperture configuration in the present disclosure may refer to the arrangement, combination, grouping, placement or any spatial positioning of one or more beam-limiting apertures. For example, the aperture plate may include one or more beam-limiting apertures (e.g., with different sizes) arranged in a linear manner, a circular manner, a rectangular manner or any geometric manner. As used herein, adjusting the beam-limiting aperture configuration may refer to any operation of moving, rotating, switching, inserting, adding, activating or changing the positioning of one or more beam-limiting apertures in the beam-limiting aperture configuration. For example, if the aperture plate includes a plurality of beam-limiting apertures with different sizes, the aperture plate may be moved (e.g., linearly) or rotated to switch between one or more beam-limiting apertures so that one of the beam-limiting apertures in the beam-limiting aperture configuration is active (e.g., by positioning the beam-limiting aperture on the projection axis of the lens). In some embodiments, in the absence of a beam-limiting aperture positioned upstream of a detector (e.g., upstream of a lens), the beam-limiting aperture configuration can be adjusted by inserting (e.g., moving by a mechanical activator) an aperture plate including one or more beam-limiting apertures to position the beam-limiting aperture of the aperture plate on the projection axis of the lens (e.g., upstream of the detector or upstream of the lens), such that the beam-limiting aperture filters multiple secondary charged particle beams. In some embodiments, in the absence of a beam-limiting aperture positioned upstream of a detector (e.g., upstream of a lens), the beam-limiting aperture configuration can be adjusted by switching (e.g., by moving or rotating the aperture plate) from the first beam-limiting aperture to a second beam-limiting aperture of a second size, 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., Figure 2 The controller 109 in the embodiment of the present invention 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 indicating that the beam-limiting aperture is enabled) from an operator (e.g., a person). In some embodiments, the controller can monitor a predetermined condition (e.g., a timing condition, etc.) and automatically (e.g., without manual operation or intervention) adjust the beam-limiting aperture configuration if the predetermined condition is met.

[0080] For example, refer to Figure 3, the beam-limiting aperture 304 may be enabled to filter the plurality of secondary beamlets (including the secondary beamlets 236 , 238 , and 240 ). In this case, one or more of the plurality of secondary beamlets may pass through the beam-limiting aperture 304 .

[0081] Figure 5A 3 is a diagram that visualizes a cross-section of an example charged particle beam at a beam-limiting aperture plane consistent with some embodiments of the present disclosure. The beam-limiting aperture can be positioned before the detector plane along the projected path of the charged particle beam. For example, the beam-limiting aperture can be the beam-limiting aperture 304, as shown in conjunction with Figure 3 Shown and described.

[0082] like Figure 5A As shown in Figure 1, a charged particle beam is represented by a set of black dots, each of which represents a charged particle ray belonging to the charged particle beam. The outline of the beam-limiting aperture is Figure 5A Indicated by white dotted circle. Figure 5A The X and Y axes in are located in the plane of the beam-limiting aperture and are expressed in arbitrary units. The center of the cross section of the charged particle beam is located at coordinates (0, 0), as Figure 5A As shown. The beam limiting aperture is positioned in such a way that the center of the beam limiting aperture is also positioned at the coordinate (0, 0). The radius of the beam limiting aperture is r (r is a number in arbitrary units).

[0083] refer to Figure 5A When the beam limiting aperture is enabled, the charged particles in the charged particle rays outside the beam limiting aperture (i.e., represented by the white dotted circle) may be stray charged particles and are filtered by the beam limiting aperture, so that the charged particle rays outside the beam limiting aperture cannot reach the detector and thus cannot be detected by the detector; in contrast, the charged particle rays within the beam limiting aperture can pass through the beam limiting aperture to reach the detector to be detected.

[0084] like Figure 5A As shown, the charged particle rays outside the beam-limiting aperture may be edge rays (e.g., Figure 4A-4B Edge ray 414 shown). Figure 5B It is shown that some embodiments of the present disclosure are consistent with Figure 5A Figure 2. A diagram of the profile of an example charged particle beam. Figure 5B The X-axis and Figure 5A The X-axis is the same as in . Figure 5B The Y axis in represents the detector signal intensity (eg, represents the count of detected charged particles). The center of the charged particle beam is located at position 0 on the X axis, as shown in FIG. Figure 5B As shown in . The beam limiting aperture is positioned in such a way that the center of the beam limiting aperture is also positioned at position 0 on the X-axis. The radius of the beam limiting aperture is r, and the boundaries of the beam limiting aperture are represented by two vertical dashed lines at positions +r and -r on the X-axis. Figure 5BAs shown, when the beam limiting aperture is enabled, the tail of the beam spot will be cut off (eg, blocked or filtered) by the beam limiting aperture, and only the central portion of the charged particle beam can pass through the beam limiting aperture to reach the detector to be detected.

[0085] In some embodiments, when the detector is a pixelated detector, in order to prevent the stray charged particles of each secondary charged particle beam from being detected by the detector, the method may include selecting a subset of pixels from the pixels covered by each secondary charged particle beam. For example, the pixels covered by the secondary charged particle beam may include pixels of a detector unit. The detection results of the pixels of the detector unit outside the subset may be ignored. For example, without enabling the beam-limiting aperture, the stray charged particles of the secondary charged particle beam may fall on the pixels of the pixelated detector and affect the size of the beam spot (e.g., forming the outside of the beam spot). The center of the beam spot can be determined, and a subset of pixels can be selected based on a predetermined radius (e.g., the subset is a specific pixel within a predetermined radius). Based on the pixel subset, for any pixel that responds to the beam spot and is located outside the subset, their detection results can be ignored. As another example, in a scenario where the beam-limiting aperture is enabled as described herein, the above operations may be additionally performed to further filter any stray charged particles that are not filtered by the beam-limiting aperture.

[0086] 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 plurality of secondary charged particle beams detected by the detector. Stray charged particles do not affect the current. Refocusing the lens as used herein may refer to any operation that adjusts (e.g., increases, decreases, or maintains) the optical power (e.g., diopter) of the lens after the optical power is set. For example, if the lens is an electromagnetic lens, and if the optical power of the electromagnetic lens has been set in a previous operation, the excitation of the electromagnetic lens may be adjusted to increase, decrease, or maintain its optical power.

[0087] The beam current used herein 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 that pass through the detector plane and are recorded by the detector within a unit time interval (e.g., within one second). In the present disclosure, the contribution of charged particles to the beam current refers to the charge carried by the charged particles that pass through the detector plane and are recorded by the detector. In some embodiments, if the charged particle belongs to the charged particle beam but does not reach the surface of the detector (e.g., 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 pixelated detector, and if the charged particle belongs to the charged particle beam and reaches a pixel of the pixelated detector but is not recorded by the detector (e.g., ignoring the detection result of the pixel), the charged particle does not contribute to the current of the charged particle beam.

[0088] Figure 6is a diagram illustrating example beam spot size optimization using different beam focusing methods consistent with some embodiments of the present disclosure. Figure 6 In the example, the X-axis represents the Figure 2 The emission angle measured at the surface of the sample 230 shown in FIG. 2 is in degrees. The emission angle used herein refers to the angle between the trajectory of the secondary charged particles and the normal to the sample surface. Figure 6 The Y-axis in represents the radius of the secondary beam spot measured at the detector plane, in arbitrary units. Figure 6 Each curve in may represent a change in the radius of a secondary beam spot (eg, an offset or distance measured from a projection axis) in response to a change in the emission angle of a secondary charged particle beam forming the secondary beam spot. Figure 6 The different curves in show different secondary beam spot sizes achievable under different beam focusing methods.

[0089] Figure 6 Three curves corresponding to three beam focusing methods are shown. Figure 6 In FIG. 1 , the first curve (indicated by a dotted line) corresponds to the paraxial focusing method, the second curve (indicated by a solid line) corresponds to the minimum dispersion focusing method, and the third curve (indicated by a dotted line) corresponds to the minimum rising edge focusing method. Figure 6 As shown in the first curve in FIG, when the emission angle corresponding to the secondary beam spot (e.g., formed by the paraxial ray) is substantially zero (e.g., not exceeding d1, d1 is a number with units of degrees), its radius is small (e.g., less than r1, r1 is a number with arbitrary units). As the emission angle increases, the radius of the secondary beam spot may increase rapidly. When the emission angle corresponding to it is large (e.g., exceeding d2, d2 is a number with units of degrees), the radius of the secondary beam spot (e.g., formed by the edge ray) may be large (e.g., exceeding r2, r2 is a number with arbitrary units).

[0090] like Figure 6 As shown in the second curve in , corresponding to an emission angle of d2, the maximum radius of the marginal ray projected onto the detector plane is r3 (r3 is a number with arbitrary units), and the maximum radius of the paraxial ray projected onto the detector plane is also r3 (corresponding to an emission angle between d1 and d3, d3 is a number with degrees). Without filtering any stray charged particles of the secondary charged particle beam (e.g., by limiting the beam aperture or by deactivating the detector pixels described herein), Figure 6 The second curve in shows that the minimum dispersion focusing method can produce the smallest maximum radius of the secondary beam spot on the detector plane (eg, not exceeding r3).

[0091] like Figure 6As shown in the third curve, for an emission angle less than 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 method is smaller than the maximum radius of the paraxial ray under the paraxial focusing method or the minimum dispersion focusing method; however, for an emission angle greater than or equal to d4, the maximum radius of the paraxial ray projected onto the detector plane under the minimum rising edge focusing method is greater than the maximum radius of the paraxial ray under the minimum dispersion focusing method, and is smaller than the maximum radius of the paraxial ray under the paraxial focusing method.

[0092] from Figure 6 It can be seen that no beam focusing method can constantly produce the minimum and maximum radius of the secondary beam spot on the detector plane. Instead, the minimum and 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 the stray charged particles of the secondary charged particle beam are not detected by the detector (for example, by enabling a beam-limiting aperture or limiting the detector cell size of a pixelated detector), the tail of the secondary beam spot may be cut off, and only part of the secondary beam spot contributes to the final detection signal. In this case, the minimum and maximum radius of the secondary beam spot may also depend on which part of the secondary beam spot contributes to the final detection signal. In some embodiments, the selected beam focusing method may emphasize focusing the central portion of the secondary charged particle beam and relax the requirements for focusing the edge portion of the secondary charged particle beam.

[0093] In some embodiments, the method may include focusing the lens using a first beam focusing method (e.g., one of a paraxial focusing method, a minimum dispersion focusing method, or a minimum rising edge focusing method) to adjust the size of the secondary beam spot. The method may include preventing stray 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 the lens using a second beam focusing method different from the first beam focusing method (e.g., one of a paraxial focusing method, a minimum dispersion focusing method, or a minimum rising edge focusing method) to adjust the current of a portion of a plurality of secondary charged particle beams detected by the detector. For example, the first beam focusing method may be a minimum dispersion focusing method, and the second focusing method may be a paraxial focusing method.

[0094] In some embodiments, in order to refocus the lens to adjust the current, the method may include refocusing the lens to move the focus of the lens toward (or away from) the plane of the detector by a first step. The method may also include determining whether the value of the collection efficiency increases and whether the value of the crosstalk rate is below a predetermined threshold. The method may also include, based on the determination that the value of the collection efficiency increases and the determination that the value of the crosstalk rate is below the predetermined threshold, refocusing the lens to move the focus of the lens toward (or away from) the plane of the detector by a second step. In some embodiments, based on the determination that the value of the collection efficiency decreases, the method may also include changing the scanning direction of the multi-beam inspection device.

[0095] For example, the lens may be focused using a minimum dispersion focusing method to adjust the size of the secondary beam spot, and stray charged particles of each secondary charged particle beam may not be detected by the detector (e.g., by enabling a beam-limiting aperture or limiting the detector cell size of a pixelated detector). At this stage, since the minimum dispersion ellipse is placed on the detector plane, the focus of the lens may be located behind the detector plane in the projection direction. Afterwards, the lens may be refocused using a method different from the minimum dispersion focusing method.

[0096] For example, the lens can be refocused using a paraxial focusing method. In such an example, the optical power (e.g., diopter) of the lens can be adjusted (e.g., increased) to move the focus of the lens toward or away from the detector plane by a first step. After moving the focus by the first step, the focus may still be located behind the detector plane along the projection direction. At this stage, it can be determined whether the value of the collection efficiency increases, and it can also be determined whether the value of the crosstalk rate is below a predetermined threshold. If the value of the collection efficiency increases and the value of the crosstalk rate is below a predetermined threshold, the optical power of the lens can be further adjusted (e.g., further increased) to move the focus of the lens toward or away from the detector plane by a second step. The first step can be the same as or different from the second step.

[0097] As another example, the minimum step edge focusing method can be used to refocus the lens. In such an example, the optical power (e.g., diopter) of the lens can be adjusted (e.g., increased or decreased) to reduce the rising edge width of the secondary beam spot to a first value. After reducing the rising edge width, it can be determined whether the value of the collection efficiency increases, and it can also be determined whether the value of the crosstalk rate is lower than a predetermined threshold. If the value of the collection efficiency increases and the value of the crosstalk rate is lower than the predetermined threshold, the optical power of the lens can be further adjusted (e.g., further increased or further decreased) to reduce the rising edge width to a second value. The first value can be the same as or different from the second value.

[0098] 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 a value of the collection efficiency reaches a maximum value or a value of the crosstalk rate is not lower than a predetermined threshold.

[0099] In some embodiments, when the detector is a pixelated detector, the lens can be refocused to adjust the current according to the detector cell size of the pixelated detector. Figure 7 is a graph showing example signal intensity distributions of secondary beam spots corresponding to different beam focusing methods consistent with some embodiments of the present disclosure. Figure 7 In FIG. 1 , the X-axis represents the spatial position on the detector surface with arbitrary units. Position 0 at the X-axis may represent the center of the secondary beam spot. Figure 7 The Y-axis in represents the signal intensity value with arbitrary units at the corresponding spatial position (eg, representing the count of charged particles). Figure 7 A top subgraph and a bottom subgraph are included which are stacked vertically, wherein the bottom subgraph is an enlarged view of the lower portion of the top subgraph (eg, the portion where the signal strength is from 0 to s1, s1 being a number with arbitrary units) and shares the same X-axis with the top subgraph. Figure 7 Each curve in can represent the distribution of the signal intensity of the secondary beam spot along the direction represented by the X-axis.

[0100] Figure 7 Three curves corresponding to three beam focusing methods are plotted. Figure 7 In FIG. 1 , the first curve (represented by the dotted line) corresponds to the paraxial focusing method, the second curve (represented by the dotted line) corresponds to the minimum dispersion focusing method, and the third curve (represented by the solid line) corresponds to the minimum rising edge focusing method. Figure 7 As shown in the three curves in FIG. 1 , the first curve has the highest intensity at position 0, the smallest full width at half maximum (FWHM), and the largest beam tail (e.g., represented by the highest signal intensity at a position far from position 0). The second curve has the lowest intensity at position 0, the largest FWHM, and the smallest beam tail (e.g., represented by the lowest signal intensity at a position far from position 0). The third curve has a medium intensity at position 0, a medium FWHM, and a medium beam tail. Figure 7 As shown, depending on the detector cell size, an optimized secondary beam focus (eg, represented by highest intensity at the beam spot center, smallest FWHW, or smallest beam spot tail) is not always produced by the same beam focusing method.

[0101] In some embodiments, when the detector is a pixelated detector, in order to refocus the lens to adjust the current, the method may include refocusing the lens to adjust the current of the portion of the plurality of secondary charged particle beams detected by the detector based on a detector cell size (e.g., the size of a group of pixels) of the pixelated detector. For example, a beam focusing method (e.g., a paraxial focusing method or a minimum rising edge focusing method) different from a beam focusing method (e.g., a minimum dispersion focusing method) used to focus the lens to adjust the secondary beam spot size may be used to refocus the lens, wherein the detector cell size may be a factor for determining which beam focusing method to use to refocus the lens.

[0102] For example, Fig. 8A is a graph showing example integrated signal intensities of secondary beam spots corresponding to different beam focusing methods consistent with some embodiments of the present disclosure. Fig. 8A , the X-axis represents the radius of the detector cell size with arbitrary units (eg, the detector cell radius). Fig. 8A The Y-axis in represents the integrated signal intensity value (eg, representing the total count of charged particles within the detector cell), which has arbitrary units corresponding to the size of the detector cell. Fig. 8A Each curve in can represent the relationship between the integrated signal intensity of the secondary beam spot and the size of the detector unit. Fig. 8A The curve in shows that the integrated signal intensity increases with increasing detector cell size.

[0103] Fig. 8A Three curves corresponding to three beam focusing methods are plotted. Fig. 8A In FIG. 1 , the first curve (indicated by a dotted line) corresponds to the paraxial focusing method, the second curve (indicated by a solid line) corresponds to the minimum dispersion focusing method, and the third curve (indicated by a dotted line) corresponds to the minimum rising edge focusing method. Fig. 8A As shown in the three curves in , the second curve has the highest integrated intensity value for large detector units (e.g., detector unit size is larger than r4, r4 is a number in arbitrary units), and the third curve has the highest integrated intensity value for small detector units (e.g., detector unit size is smaller than r4). In addition, for detector units with a size smaller than r5 (r5 is a number in arbitrary units), the integrated intensity value of the first curve is higher than the integrated intensity value of the second curve. Fig. 8A As shown, depending on the detector cell size, the optimal secondary beam focus (eg represented by the highest integrated intensity value) is not always produced by the same beam focusing method.

[0104] Figure 8B is a graph showing the difference between example integrated signal intensities of secondary beam spots corresponding to different beam focusing methods consistent with certain embodiments of the present disclosure. Fig. 8Asimilar, Figure 8B The x-axis in represents the radius of the detector cell size in arbitrary units. Figure 8B The Y-axis in represents the difference between two integrated signal intensity values, with arbitrary units corresponding to the detector cell size.

[0105] Figure 8B Two curves are plotted. Figure 8B The first difference curve (indicated by a dotted line) represents Fig. 8A The difference between the first curve and the second curve shown and described in FIG. 1 , the second difference curve (shown as a dashed line) represents Fig. 8A The difference between the third curve and the second curve shown and described in FIG. Figure 8B As shown, the first difference curve is positive for detector cell sizes below r4, with a maximum value of m1 (e.g., when the detector cell size is r5), where m1 is a number with arbitrary units. The second difference curve is positive for detector cell sizes below r6 (r6 is a number with arbitrary units), with a maximum value of m2 (e.g., at a detector cell size of r7, r7 is a number with arbitrary units), where m2 is a number with arbitrary units. Figure 8B As shown, if the detector unit size is set to a specific value, switching from the minimum dispersion focusing method to the paraxial focusing method or the minimum rising edge focusing method can increase the maximum integrated signal intensity from m2 to m1.

[0106] For example, refer to Figure 8A-B , when the detector is a pixelated detector, the minimum dispersion focusing method can be used to focus the lens to adjust (e.g., minimize) the size of the secondary beam spot. Thereafter, if the detector cell size of the pixelated detector exceeds r4, there is no need to refocus the lens because the current is maximized in this case. If the detector cell size is between r5 and r4, in order to refocus the lens to adjust the current, the minimum rising edge focusing method can be used to refocus the lens. If the detector cell size is less than r5, the minimum rising edge focusing method or the paraxial focusing method can be used to refocus the lens.

[0107] In some embodiments, if a beam-limiting aperture is enabled, the lens can be refocused to adjust the current according to the aperture size of the beam-limiting aperture. Fig. 9 is a diagram showing the profiles (represented by curves) of example secondary beam spots on a beam-limiting aperture plane corresponding to different beam focusing methods consistent with some embodiments of the present disclosure. Fig. 9 In the example, the x-axis represents the spatial position in arbitrary units on the plane of the beam-limiting aperture (e.g., similar to the combination of Figure 5A-Figure 5B The position 0 on the x-axis may represent the center of the secondary beam spot. Fig. 9The Y-axis in represents the signal intensity value with arbitrary units at the corresponding spatial position (eg, representing the count of charged particles). Fig. 9 Each curve in can represent the distribution of the signal intensity of the secondary beam spot along the direction represented by the X-axis.

[0108] Fig. 9 Three curves are plotted, corresponding to three beam focusing methods. Fig. 9 In FIG. 1 , the first curve (indicated by a dotted line) corresponds to the paraxial focusing method, the second curve (indicated by a solid line) corresponds to the minimum dispersion focusing method, and the third curve (indicated by a dotted line) corresponds to the minimum rising edge focusing method. Fig. 9 It can be seen from the three curves in FIG. 1 that the second curve has the largest FWHM and the first curve has the highest signal intensity at the center of the beam spot.

[0109] In some embodiments, to refocus the lens to adjust the current, the method may include refocusing the lens to adjust the current of the plurality of secondary charged particle beams based on an aperture size (e.g., radius) of the beam-limiting aperture. For example, a beam focusing method (e.g., a paraxial focusing method or a minimum rising edge focusing method) different from a beam focusing method (e.g., a minimum dispersion focusing method) used to focus the lens to adjust the secondary beam spot size may be used to refocus the lens, wherein the aperture size may be a factor for determining which beam focusing method to use to refocus the lens.

[0110] For example, Fig. 10A is a graph showing example integrated signal intensities of secondary beam spots corresponding to different beam focusing methods consistent with some embodiments of the present disclosure. Fig. 10A , the x-axis represents the aperture size (eg, aperture radius) in arbitrary units. Fig. 10A The Y-axis in represents the integrated signal intensity value (eg, representing the total count of charged particles passing through the beam-limiting aperture), which has arbitrary units corresponding to the aperture size. Fig. 10A Each curve in FIG. 1 can represent the relationship between the integrated signal intensity of the secondary beam spot and the aperture size. Fig. 10A The curve in shows that the integrated signal intensity increases with increasing aperture size.

[0111] Fig. 10A Three curves corresponding to three beam focusing methods are plotted. Fig. 10A In FIG. 1 , the first curve (indicated by a dotted line) corresponds to the paraxial focusing method, the second curve (indicated by a solid line) corresponds to the minimum dispersion focusing method, and the third curve (indicated by a dotted line) corresponds to the minimum rising edge focusing method. Fig. 10AAs shown in the three curves in FIG. 1 , for a wide range of aperture sizes (e.g., between the sizes of a1 and a2, where a1 and a2 are numbers with 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. Fig. 10A As shown, depending on the aperture size, the optimal secondary beam focusing (eg, represented by the highest integrated intensity value) is not always produced by the same beam focusing method.

[0112] Fig. 10B is a graph showing the difference between example integrated signal intensities of secondary beam spots corresponding to different beam focusing methods consistent with certain embodiments of the present disclosure. Fig. 10A similar, Fig. 10B The x-axis in represents the pore size with arbitrary units. Fig. 10B The Y-axis in represents the difference between two integrated signal intensity values, which has arbitrary units corresponding to the aperture size.

[0113] Fig. 10B Two curves are plotted. Fig. 10B The first difference curve (indicated by a dotted line) represents Fig. 10A The difference between the first curve and the second curve shown and described in FIG. 1 , and the second difference curve (shown in dashed line) represents Fig. 10A The difference between the third curve and the second curve shown and described in FIG. Fig. 10B As shown, both the first difference curve and the second difference curve have positive values ​​in a wide range of aperture sizes (e.g., aperture sizes between a3 and a4, where a3 and a4 are numbers with arbitrary units). In addition, in the aperture size range between a3 and a5 (a5 is a number with arbitrary units), the value of the first difference curve is higher than the value of the second difference curve. Fig. 10B As shown, if the aperture size is set to a specific value, switching from the minimum dispersion focusing method to the paraxial focusing method or the minimum rising edge focusing method can increase the maximum integrated signal intensity.

[0114] For example, refer to Figure 10A-10B , the minimum dispersion focusing method can be used to focus the lens to adjust the size of the secondary beam spot. Then, the beam limiting aperture can be enabled as described in this article. Afterwards, if the aperture size of the beam limiting aperture is less than a4, the minimum rising edge focusing method or the paraxial focusing method can be used to refocus the lens. If the aperture size is less than a5, in order to refocus the lens to adjust the current, the paraxial focusing method can be used to refocus the lens.

[0115] It should be noted that if the detector is a pixelated detector and if a beam-limiting aperture is enabled, the lens can be refocused to adjust the current according to the detector cell size and the aperture size of the beam-limiting aperture. For example, the lens can be refocused according to the detector cell size of the pixelated detector and the aperture size of the beam-limiting aperture to maximize the current of the portion of the plurality of secondary charged particle beams detected by the detector.

[0116] In some embodiments, if the beam limiting aperture is enabled, the lens can be refocused to adjust the current according to the position of the beam limiting aperture along the lens projection axis. For example, to refocus the lens to adjust the current, the method may include: refocusing the lens to adjust the current of the plurality of secondary charged particle beams based on the position of the beam limiting aperture along the lens projection axis.

[0117] 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). For example, in an experimental process, assuming that the detector of the multi-beam inspection device is a pixelated detector, the pixelated detector can be switched to operate in an imaging mode that can generate a secondary beam spot image. In the imaging mode, the lens of the multi-beam inspection device can be focused (e.g., using a minimum dispersion focusing method) to adjust the size of the secondary beam spot formed on the detector by the multiple secondary charged particle beams. In some embodiments, the lens can be focused without enabling any beam-limiting aperture in the projection path of the multiple secondary charged particle beams. Then, the pixelated detector can be switched to operate in a measurement mode that can generate a detection result that does not include a secondary beam spot image. In the measurement mode, a detector unit (e.g., a group of pixels) can receive the secondary beam spot, and the intensity value of the secondary beam spot can be determined (e.g., by integrating the signal values ​​of all pixels in the detector unit). Thereafter, the beam limiting aperture configuration may be adjusted so that the beam limiting aperture is positioned upstream of the detector (e.g., upstream of the lens) and stray charged particles are filtered. By enabling the beam limiting aperture, crosstalk may be suppressed (e.g., so that the crosstalk rate is lower than a predetermined threshold). After enabling the beam limiting aperture, the lens may be refocused (e.g., using a paraxial focusing method or a minimum rising edge focusing method) to adjust the current of a plurality of secondary charged particle beams detected by the detector, wherein the stray charged particles do not contribute to the current. For example, in order to refocus the lens, the optical power of the lens may be adjusted (e.g., increased). As another example, in order to refocus the lens, the optical power of the lens may be adjusted (e.g., increased or decreased) to reduce the rising edge width of the secondary beam spot.

[0118] It should also be noted that although the present disclosure only describes three beam focusing methods (e.g., the paraxial focusing method, the minimum dispersion focusing method, and the minimum rising edge focusing method), they are only exemplary embodiments and are only used for illustration purposes. Other beam focusing methods are not excluded and may also be used in the embodiments of the present disclosure.

[0119] Fig.11 1 is a flow chart illustrating an example method for optimizing the collection efficiency of secondary charged particles consistent with some embodiments of the present disclosure. The method 1100 may be performed by a charged particle beam inspection device (e.g., Figure 1-Figure 3 The controller may be coupled to the beam tool 104 described in the related description. For example, the controller may be Figure 2 The controller 109 in the embodiment of the present invention can be programmed to implement the method 1100.

[0120] At step 1102, the controller may focus the multi-beam inspection device (e.g., Figure 2 The lens of the beam tool 104 (e.g., Figure 3 The secondary beam spot can be composed of multiple secondary charged particle beams (for example, including Figure 3 The secondary beamlets 236, 238 and 240) are detected at a detector (e.g., Figure 3 is formed on a charged particle detector 244).

[0121] In some embodiments, the controller may focus the lens using a first beam focusing method. The first beam focusing method may include one of the following: focusing the lens so that the paraxial rays of the plurality of secondary charged particle beams are focused on the plane of the detector; focusing the lens so that the minimum dispersion ellipse of the plurality of secondary charged particle beams is positioned on the plane of the detector to form a secondary beam spot; or focusing the lens so that the rising edge width of the secondary beam spot is minimized.

[0122] At step 1104, for each of the plurality of secondary charged particle beams, the controller may prevent stray charged particles of each secondary charged particle beam from being detected by the detector. In some embodiments, in order to prevent stray charged particles from being detected by the detector, the controller may adjust the beam limiting aperture configuration so that the beam limiting aperture (e.g., Figure 3 The beam-limiting aperture 304 is positioned at the detector (e.g., Figure 3 The lens 310) is located upstream of the lens 310 and filters out stray charged particles. Figure 5A-Figure 5B The beam-limiting aperture may block stray charged particles of the secondary charged particle beam (eg, charged particles at the tail of the secondary beam spot) to prevent them from reaching the detector.

[0123] In some embodiments, the detector may be a pixelated detector. In this case, in order to prevent stray charged particles from being detected by the detector, the controller may adjust the number of pixels of a detector unit of the detector, wherein the pixels of the detector unit may be used to detect multiple secondary charged particle beams. For example, a group of pixels (e.g., a detector unit) may receive and detect secondary charged particle beams, and the detector may include multiple detector units. The controller may adjust the number of pixels of each detector unit (e.g., by reducing the number of pixels in the detector unit) to detect multiple secondary charged particle beams.

[0124] In some embodiments, if the detector is a pixelated detector, in order to prevent stray charged particles from being detected by the detector, the controller can select a subset of pixels from the pixels covered by each secondary charged particle beam. For example, a detector unit (including a group of pixels) can receive and detect a secondary charged particle beam. The pixels of the detector unit can be covered by the secondary charged particle beam. The controller can obtain a predetermined size of the detector unit and select a subset of pixels based on the predetermined size. For all pixels outside the subset, the controller can ignore their detection results (for example, by disabling their functions or not processing their signals) even if charged particles of the secondary charged particle beam fall on them.

[0125] In step 1106, the controller may refocus the lens to adjust (e.g., maximize) the current of the portion of the plurality of secondary charged particle beams detected by the detector. Stray charged particles do not affect the current. In some embodiments, if the controller has focused the lens using the first beam focusing method described in association 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 so that the paraxial rays of the plurality of secondary charged particle beams are focused on the plane of the detector; focusing the lens so that the minimum dispersion ellipse of the plurality of secondary charged particle beams is positioned on the plane of the detector to form a secondary beam spot; or focusing the lens so that the rising edge width of the secondary beam spot is minimized.

[0126] In some embodiments, if the controller has enabled the beam limiting aperture 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 may refocus the lens to adjust the current of the plurality of secondary charged particle beams detected by the detector. Figure 10A-10B In connection with the described relationship between integrated signal intensity and aperture size, the controller may select a beam focusing method to refocus the lens to adjust the current.

[0127] In some embodiments, if the controller has enabled the beam-limiting aperture described in association with step 1104, the controller may further configure the image based on the beam-limiting aperture along the projection axis of the lens (e.g., Figure 2-Figure 3 The controller may refocus the lens to adjust the current of the portion of the plurality of secondary charged particle beams detected by the detector based on the position of the projection axis 252 in the projection axis. For example, based on the relationship between the integrated signal intensity and the position of the beam-limiting aperture along the projection axis, the controller may select a beam focusing method to refocus the lens to adjust the current.

[0128] In some embodiments, if the detector is a pixelated detector, in order to refocus the lens to adjust the current, based on the detector cell size of the pixelated detector, the controller can refocus the lens to adjust the current of the portion of the plurality of secondary charged particle beams detected by the detector. Figure 8A-B In conjunction with the described relationship between integrated signal intensity and detector cell size, the controller can select a beam focusing method to refocus the lens to adjust the current.

[0129] In some embodiments, to refocus the lens to adjust the current, the controller may refocus the lens so that the focal point of the lens (e.g., with Figure 4A-4B The controller may further determine whether the value of the collection efficiency increases and whether the value of the crosstalk rate is below a predetermined threshold based on the determination that the value of the collection efficiency increases and the determination that the value of the crosstalk rate is below a predetermined threshold, and the controller may refocus the lens to move the focus of the lens toward or away from the plane of the detector by a second step. The second step may be the same as or different from the first step. In some embodiments, based on the determination that the value of the collection efficiency decreases, the controller may change the scanning direction of the multi-beam inspection device. In some embodiments, based on the determination that the value of the collection efficiency reaches a maximum value or the determination that the value of the crosstalk rate is not below a predetermined threshold, the controller may stop refocusing the lens.

[0130] A non-transitory computer-readable medium may be provided that stores instructions for a processor (e.g., Figure 1 The processor of the controller 109 of the embodiment of the present invention executes a method for optimizing the collection efficiency of secondary charged particles (e.g., Fig.11 Method 1100), data processing, database management, graphic display, operation of image inspection devices or other imaging devices, detection of defects on samples, etc. Common forms of non-transitory media include, for example, floppy disks, flexible disks, hard disks, solid-state drives, magnetic tapes or any other magnetic data storage media, CD-ROMs, any other optical data storage media, any physical media with a pattern of holes, RAM, PROM and EPROM, FLASH-EPROM or any other flash memory, NVRAM, cache, registers, any other memory chip or cartridge memory, and network versions thereof.

[0131] The embodiments may be further described using the following terms:

[0132] 1. A system comprising:

[0133] a multi-beam inspection device configured to scan a sample and comprising a lens;

[0134] a detector configured to receive the plurality of secondary charged particle beams in response to scanning the sample; and

[0135] a controller comprising circuitry communicatively coupled to the multi-beam inspection device and the detector, the controller being configured to:

[0136] focusing the lens to adjust a size of a secondary beam spot formed by the plurality of secondary charged particle beams on the detector;

[0137] For each of the plurality of secondary charged particle beams,

[0138] stray charged particles of the primary charged particle beam are not detected by the detector; and

[0139] The lens is refocused to adjust a current of a portion of the plurality of secondary charged particle beams detected by the detector, wherein the stray charged particles do not contribute to the current.

[0140] 2. The system according to clause 1, wherein the controller is configured to focus the lens to adjust the size of the secondary beam spot so as to minimize the size of the secondary beam spot.

[0141] 3. The system of any one of clauses 1 to 2, wherein the controller is configured to refocus the lens to adjust the current so as to maximize the current.

[0142] 4. A system according to any one of clauses 1 to 3, wherein the detector is a pixelated detector and the controller is configured to prevent the stray charged particles of each secondary charged particle beam from being detected by adjusting the number of pixels of a detector unit of the detector, wherein the pixels of the detector unit are configured to detect the multiple secondary charged particle beams.

[0143] 5. The system of any one of clauses 1 to 4, wherein the controller is configured to focus the lens using a first beam focusing method to adjust the size of the secondary beam spot, and the first beam focusing method comprises one of:

[0144] focusing the lens so that paraxial rays of the plurality of secondary charged particle beams are focused on a plane of the detector;

[0145] focusing the lens to position a minimum dispersion ellipse of the plurality of secondary charged particle beams on the plane of the detector to form a secondary beam spot; or

[0146] The lens is focused to minimize the width of the rising edge of the secondary beam spot.

[0147] 6. The system of clause 5, wherein 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, and the second beam focusing method comprises one of:

[0148] focusing the lens so that the paraxial rays of the plurality of secondary charged particle beams are focused on the plane of the detector;

[0149] focusing the lens so that the minimum dispersion ellipses of the plurality of secondary charged particle beams are positioned on the plane of the detector to form the secondary beam spots; or

[0150] The lens is focused to minimize the rising edge width of the secondary beam spot.

[0151] 7. A system according to any one of clauses 1 to 6, wherein the controller is configured to prevent the 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 filters the stray charged particles.

[0152] 8. A system according to claim 7, wherein the controller is configured to prevent the 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 lens and filters the stray charged particles.

[0153] 9. A system according to any one of clauses 7 to 8, wherein the controller is configured to refocus the lens to adjust the current, thereby enabling the lens to be refocused to adjust the current based on the aperture size of the beam-limiting aperture.

[0154] 10. A system according to any one of clauses 7 to 9, wherein the controller is configured to refocus the lens to adjust the current, thereby being able to refocus the lens to adjust the current based on the position of the beam-limiting aperture along the projection axis of the lens.

[0155] 11. A system according to any one of clauses 1 to 10, wherein the detector is a pixelated detector and the controller is configured to select a subset of pixels from the pixels covered by each secondary charged particle beam so that the stray charged particles of each secondary charged particle beam are not detected by the detector.

[0156] 12. A system according to any one of clauses 1 to 11, wherein the detector is a pixelated detector and the controller is configured to refocus the lens to adjust the current, thereby being able to refocus the lens to adjust the current based on the detector cell size of the pixelated detector.

[0157] 13. A system according to any one of clauses 1 to 12, wherein the controller is configured to refocus the lens to adjust the current to achieve refocusing of the lens so that the focal point of the lens moves a first step towards or away from the plane of the detector;

[0158] determining whether the value of the collection efficiency increases and whether the value of the crosstalk ratio is below a predetermined threshold;

[0159] refocusing the lens to move the focal point of the lens toward or away from the plane of the detector by a second step distance based on a determination that the value of the collection efficiency has increased and a determination that the value of the crosstalk ratio is below the predetermined threshold;

[0160] Based on the determination that the value of the collection efficiency has decreased, changing the scanning direction of the multi-beam inspection device; and

[0161] Based on a determination that the value of the collection efficiency reaches a maximum value or a determination that the value of the crosstalk rate is not lower than the predetermined threshold, refocusing the lens is stopped.

[0162] 14. A non-transitory computer readable medium storing a set of instructions executable by at least one processor of a multi-beam inspection device to cause the multi-beam inspection device to perform a method comprising:

[0163] focusing a lens of the multi-beam inspection device to adjust a size of a secondary beam spot formed by a plurality of secondary charged particle beams on a detector;

[0164] For each of the plurality of secondary charged particle beams, stray charged particles of the each secondary charged particle beam are not detected by the detector; and

[0165] The lens is refocused to adjust a current of a portion of the plurality of secondary charged particle beams detected by the detector, wherein the stray charged particles do not contribute to the current.

[0166] 15. The non-transitory computer readable medium of clause 14, wherein focusing the lens of the multi-beam inspection device to adjust the size of the secondary beam spot comprises:

[0167] The size of the secondary beam spot is minimized.

[0168] 16. The non-transitory computer readable medium of any of clauses 14-15, wherein refocusing the lens to adjust the current comprises:

[0169] The partial current of the plurality of secondary charged particle beams detected by the detector is maximized.

[0170] 17. The non-transitory computer readable medium of any of clauses 14-16, wherein the detector is a pixelated detector, and wherein causing the stray charged particles of each secondary charged particle beam to be undetected by the detector comprises:

[0171] The number of pixels of a detector cell of the detector is adjusted, wherein the pixels of the detector cell are configured to detect the plurality of secondary charged particle beams.

[0172] 18. The non-transitory computer readable medium of any of clauses 14-17, wherein focusing the lens of the multi-beam inspection device to adjust the size of the secondary beam spot comprises a first beam focusing method, and the first beam focusing method comprises one of:

[0173] focusing the lens so that the paraxial rays of the plurality of secondary charged particle beams are focused on a plane of the detector;

[0174] focusing the lens to position the minimum dispersion ellipse of the plurality of secondary charged particle beams on the plane of the detector to form the secondary beam spot; or

[0175] The lens is focused to minimize the width of the rising edge of the secondary beam spot.

[0176] 19. The non-transitory computer-readable medium of any of clauses 14-17, wherein refocusing the lens to adjust the current comprises a second beam focusing method different from the first beam focusing method, and the second beam focusing method comprises one of:

[0177] focusing the lens so that the paraxial rays of the plurality of secondary charged particle beams are located on a plane of the detector;

[0178] focusing the lens so that the minimum dispersion ellipses of the plurality of secondary charged particle beams are positioned on the plane of the detector to form the secondary beam spots; or

[0179] The lens is focused to minimize the rising edge width of the secondary beam spot.

[0180] 20. The non-transitory computer readable medium of any of clauses 14-19, wherein causing the stray charged particles of the each secondary charged particle beam to be undetected by the detector comprises:

[0181] A beam-limiting aperture configuration is adjusted so that a beam-limiting aperture of the beam-limiting aperture configuration is positioned upstream of the detector and filters the stray charged particles.

[0182] 21. The non-transitory computer-readable medium of clause 20, wherein adjusting the beam-limiting aperture configuration comprises:

[0183] The beam-limiting aperture configuration is adjusted so that the beam-limiting aperture of the beam-limiting aperture configuration is positioned upstream of the lens and filters the stray charged particles.

[0184] 22. The non-transitory computer-readable medium of any of clauses 20-21, wherein refocusing the lens to adjust the current of the detector comprises:

[0185] Based on an aperture size of the beam-limiting aperture, the lens is refocused to adjust the current.

[0186] 23. The non-transitory computer-readable medium of any of clauses 20-22, wherein refocusing the lens to adjust the current comprises:

[0187] The lens is refocused to adjust the current based on the position of the beam-limiting aperture along a projection axis of the lens.

[0188] 24. The non-transitory computer readable medium of any of clauses 14-23, wherein the detector is a pixelated detector, and wherein causing the stray charged particles of each secondary charged particle beam to be undetected by the detector comprises:

[0189] A subset of pixels is selected from the pixels covered by each of the secondary charged-particle beams.

[0190] 25. The non-transitory computer-readable medium of any of clauses 14-24, wherein the detector is a pixelated detector, and wherein refocusing the lens to adjust the current comprises:

[0191] Based on a detector cell size of the pixelated detector, the lens is refocused to adjust the current.

[0192] 26. The non-transitory computer-readable medium of any of clauses 14-25, wherein refocusing the lens to adjust the current comprises:

[0193] refocusing the lens so that the focal point of the lens moves a first step toward or away from the plane of the detector;

[0194] determining whether the value of the collection efficiency increases and whether the value of the crosstalk rate is below a predetermined threshold;

[0195] 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 has increased and a determination that the value of the crosstalk ratio is below the predetermined threshold;

[0196] Based on the determination that the value of the collection efficiency has decreased, changing the scanning direction of the multi-beam inspection device; and

[0197] Based on a determination that the value of the collection efficiency reaches a maximum value or a disadvantage that the value of the crosstalk rate is not lower than the predetermined threshold value, refocusing the lens is stopped.

[0198] 27. A method for optimizing the collection efficiency of secondary charged particles, comprising:

[0199] focusing a lens of a multi-beam inspection device to adjust a size of a secondary beam spot formed by a plurality of secondary charged particle beams on a detector;

[0200] For each of the plurality of secondary charged particle beams, stray charged particles of the each secondary charged particle beam are not detected by the detector; and

[0201] The lens is refocused to adjust a current of a portion of the plurality of secondary charged particle beams detected by the detector, wherein the stray charged particles do not contribute to the current.

[0202] 28. The method of clause 27, wherein focusing the lens of the multi-beam inspection device to adjust the size of the secondary beam spot comprises:

[0203] The size of the secondary beam spot is minimized.

[0204] 29. The method of any of clauses 27-28, wherein refocusing the lens to adjust the current comprises:

[0205] The current of the portion of the plurality of secondary charged-particle beams detected by the detector is maximized.

[0206] 30. A method according to any of clauses 27-29, wherein the detector is a pixelated detector, and wherein causing the stray charged particles of each secondary charged particle beam to be undetected by the detector comprises:

[0207] The number of pixels of a detector cell of the detector is adjusted, wherein the pixels of the detector cell are configured to detect the plurality of secondary charged particle beams.

[0208] 31. The method according to any of clauses 27-30, wherein focusing the lens of the multi-beam inspection device to adjust the size of the secondary beam spot comprises a first beam focusing method, and the first beam focusing method comprises one of the following:

[0209] focusing the lens so that paraxial rays of the plurality of secondary charged particle beams are focused on a plane of the detector;

[0210] focusing the lens to position the minimum dispersion ellipse of the plurality of secondary charged particle beams on the plane of the detector to form the secondary beam spot; or

[0211] The lens is focused to minimize the width of the rising edge of the secondary beam spot.

[0212] 32. The method of any of clauses 27-31, wherein refocusing the lens to adjust the current comprises a second beam focusing method different from the first beam focusing method, and the second beam focusing method comprises one of:

[0213] focusing the lens to focus the paraxial rays of the plurality of secondary charged particle beams on a plane of the detector;

[0214] focusing the lens so that the minimum dispersion ellipses of the plurality of secondary charged particle beams are positioned on the plane of the detector to form the secondary beam spots; or

[0215] The lens is focused to minimize the rising edge width of the secondary beam spot.

[0216] 33. The method of any of clauses 27-32, wherein causing the stray charged particles of each secondary charged particle beam to be undetected by the detector comprises:

[0217] A beam-limiting aperture configuration is adjusted so that a beam-limiting aperture of the beam-limiting aperture configuration is positioned upstream of the detector to filter the stray charged particles.

[0218] 34. The method of clause 33, wherein adjusting the beam-limiting aperture configuration comprises:

[0219] The beam-limiting aperture configuration is adjusted such that the beam-limiting aperture of the beam-limiting aperture configuration is positioned upstream of the lens and filters the stray charged particles.

[0220] 35. The method of any of clauses 33 to 34, wherein refocusing the lens to adjust the current comprises:

[0221] Based on an aperture size of the beam-limiting aperture, the lens is refocused to adjust the current.

[0222] 36. The method of any of clauses 33 to 35, wherein refocusing the lens to adjust the current comprises:

[0223] The lens is refocused to adjust the current based on the position of the beam-limiting aperture along a projection axis of the lens.

[0224] 37. A method according to any of clauses 27 to 36, wherein the detector is a pixelated detector, and wherein causing the stray charged particles of each secondary charged particle beam to be undetected by the detector comprises:

[0225] A subset of pixels is selected from the pixels covered by each of the secondary charged-particle beams.

[0226] 38. A method according to any of clauses 27-37, wherein the detector is a pixelated detector, and wherein refocusing the lens to adjust the current of the plurality of secondary charged particle beams detected by the detector comprises:

[0227] Based on a detector cell size of the pixelated detector, the lens is refocused to adjust the current.

[0228] 39. The method of any of clauses 27 to 28, wherein refocusing the lens to adjust the current comprises:

[0229] refocusing the lens so that the focal point of the lens moves a first step toward or away from the plane of the detector;

[0230] determining whether the value of the collection efficiency increases and whether the value of the crosstalk rate is lower than a predetermined threshold; and

[0231] Based on a determination that the value of the collection efficiency has increased and a determination that the value of the crosstalk ratio is below the predetermined threshold, the lens is refocused to move the focal point of the lens a second step distance toward or away from the plane of the detector.

[0232] 40. The method according to clause 39, further comprising:

[0233] Based on the determination that the value of the collection efficiency has dropped, a scanning direction of the multi-beam inspection device is changed.

[0234] 41. The method according to clause 39, further comprising:

[0235] Based on a determination that the value of the collection efficiency has not increased or a determination that the value of the crosstalk ratio is not lower than the predetermined threshold, refocusing the lens is stopped.

[0236] The block diagram in the figure shows the possible architecture, function and operation of the system, method and computer hardware or software product according to various example embodiments of the present disclosure. In this regard, each box in the flow chart or block diagram can represent a part of a module, segment or code, which includes one or more executable instructions for implementing a specified logical function. It should be understood that in some alternative implementations, the functions indicated in the box may not occur in the order shown in the figure. For example, two boxes displayed in succession can be executed or implemented substantially at the same time, or sometimes two boxes can be executed in reverse order, depending on the functions involved. Some boxes can also be omitted. It should also be understood that each box in the block diagram and the combination of boxes can be implemented by a special hardware system that performs a specified function or action, or by a combination of special hardware and computer instructions.

[0237] It should be understood that the embodiments of the present disclosure are not limited to the exact constructions described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof.

Claims

1. A system comprising: a multi-beam inspection device configured to scan a sample and comprising a lens; a detector configured to receive the plurality of secondary charged particle beams in response to scanning the sample; as well as a controller comprising circuitry communicatively coupled to the multi-beam inspection device and the detector, the controller being configured to: focusing the lens to adjust a size of a secondary beam spot formed by the plurality of secondary charged particle beams on the detector; For each of the plurality of secondary charged particle beams, stray charged particles of the each secondary charged particle beam are not detected by the detector; and The lens is refocused to adjust a current of a portion of the plurality of secondary charged particle beams detected by the detector, wherein the stray charged particles do not contribute to the current.

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

3. The system of claim 1, wherein the controller is configured to refocus the lens to adjust the current so as to maximize the current of the portion of the plurality of secondary charged particle beams detected by the detector.

4. The system of claim 1 , wherein the detector is a pixelated detector, and the controller is configured to prevent the stray charged particles of each secondary charged particle beam from being detected by adjusting the number of pixels of a detector unit of the detector, wherein the pixels of the detector unit are configured to detect the multiple secondary charged particle beams.

5. The system of claim 1 , wherein the controller is configured to focus the lens using a first beam focusing method to adjust the size of the secondary beam spot, and the first beam focusing method comprises one of: focusing the lens so that paraxial rays of the plurality of secondary charged particle beams are focused on a plane of the detector; focusing the lens to position the minimum dispersion ellipse of the plurality of secondary charged particle beams on the plane of the detector to form a secondary beam spot; or The lens is focused to minimize the width of the rising edge of the secondary beam spot.

6. The system of claim 5, wherein 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, and the second beam focusing method comprises one of: focusing the lens so that the paraxial rays of the plurality of secondary charged particle beams are focused on the plane of the detector; focusing the lens so that the minimum dispersion ellipses of the plurality of secondary charged particle beams are positioned on the plane of the detector to form the secondary beam spots; or The lens is focused to minimize the rising edge width of the secondary beam spot.

7. A system according to claim 1, wherein the controller is configured to prevent the 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 filters the stray charged particles.

8. A system according to claim 7, wherein the controller is configured to prevent the 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 lens and filters the stray charged particles.

9. The system of claim 7, wherein the controller is configured to refocus the lens to adjust the current, such that the lens can be refocused to adjust the current based on an aperture size of the beam-limiting aperture.

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

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

12. The system of claim 1, wherein the detector is a pixelated detector and the controller is configured to refocus the lens to adjust the current, thereby enabling the lens to be refocused to adjust the current based on a detector cell size of the pixelated detector.

13. The system of claim 1 , wherein the controller is configured to refocus the lens to adjust the current to effect refocusing of the lens so that the focal point of the lens moves a first step toward or away from the plane of the detector; determining whether the value of the collection efficiency increases and whether the value of the crosstalk ratio is below a predetermined threshold; refocusing the lens to move the focal point of the lens toward or away from the plane of the detector by a second step distance based on a determination that the value of the collection efficiency has increased and a determination that the value of the crosstalk ratio is below the predetermined threshold; changing a scanning direction of the multi-beam inspection device based on a determination that the value of the collection efficiency has decreased; as well as Based on a determination that the value of the collection efficiency reaches a maximum value or a determination that the value of the crosstalk rate is not lower than the predetermined threshold, refocusing the lens is stopped.

14. A non-transitory computer readable medium storing a set of instructions executable by at least one processor of a multi-beam inspection device to cause the multi-beam inspection device to perform a method comprising: focusing a lens of the multi-beam inspection device to adjust a size of a secondary beam spot formed by a plurality of secondary charged particle beams on a detector; For each of the plurality of secondary charged particle beams, stray charged particles of the each secondary charged particle beam are not detected by the detector; and The lens is refocused to adjust a current of a portion of the plurality of secondary charged particle beams detected by the detector, wherein the stray charged particles do not contribute to the current.

15. The non-transitory computer readable medium of claim 14, wherein focusing the lens of the multi-beam inspection device to adjust the size of the secondary beam spot comprises: The size of the secondary beam spot is minimized.

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