Multiple charged particle beam device with low crosstalk - Patents.com

Abstract

To provide systems and methods of enhancing imaging resolution by reducing crosstalk between detection elements of a secondary charged-particle detector in a multi-beam apparatus.SOLUTION: The multi-beam apparatus comprises an electro-optical system for projecting a plurality of secondary charged-particle beams from a sample onto a charged-particle detector (140). The electro-optical system includes a first pre-limit aperture plate (155P) comprising a first aperture configured to block peripheral charged-particles of the plurality of secondary charged-particle beams, and a beam-limit aperture array (155) comprising a second aperture configured to trim the plurality of secondary charged-particle beams. The charged-particle detector may include a plurality of detection elements (140_1, 140_2, 140_3), where each of the detection elements is associated with a corresponding trimmed beam of the secondary charged-particle beams.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. patent application Ser. No. 62 / 853,677, filed May 28, 2019, which is incorporated by reference in its entirety herein.

[0002]

[0002] The embodiments provided in this specification disclose a multi-beam device, and more specifically, a multi-beam charged particle microscope that improves imaging signal fidelity using a combination of aperture arrays configured to reduce crosstalk. [Background technology]

[0003]

[0003] In the manufacturing process of integrated circuits (ICs), unfinished or finished circuit components are inspected to ensure that they are manufactured according to design and are free of defects. Inspection systems utilizing optical microscopes or charged particle (e.g., electron) beam microscopes, such as scanning electron microscopes (SEMs), can be employed. As the physical size of IC components continues to shrink, accuracy and yield in defect detection become more important. Although multiple electron beams can be used to increase throughput, limitations in the fidelity of the imaging signal received by the charged particle detector limit the imaging resolution desired for reliable defect detection and analysis, making inspection tools unsuitable for their desired purpose. Summary of the Invention

[0004]

[0004] In some embodiments of the present disclosure, an electro-optical system of a multiple charged particle beam device is disclosed. The electro-optical system may include a first pre-limiting aperture plate including a first aperture configured to block peripheral charged particles of a plurality of secondary charged particle beams from a sample, and a beam-limiting aperture array including a second aperture configured to trim the plurality of secondary charged particle beams. The electro-optical system may further include a charged particle detector including a plurality of detection elements, each detection element of the plurality of detection elements being associated with a corresponding trimmed beam of the plurality of secondary charged particle beams.

[0005]

[0005] The distance between the first pre-limiting aperture plate and the beam-limiting aperture array may be 5 mm or less. The first pre-limiting aperture plate may be positioned upstream or downstream of the beam-limiting aperture array. The electro-optical system may include a second pre-limiting aperture plate. The first pre-limiting aperture plate may be positioned upstream of the beam-limiting aperture array, and the second pre-limiting aperture plate may be positioned downstream of the beam-limiting aperture array. The beam-limiting aperture array may include a plurality of apertures of different sizes. At least two of the plurality of apertures may have a similar size. The plurality of apertures may be arranged in a rectangular, circular, or spiral pattern.

[0006]

[0006] The multiple secondary charged particle beams may include at least one of secondary electrons or backscattered electrons generated from the sample in response to interactions between the multiple primary charged particle beams and the sample. The multiple secondary charged particle beams may overlap to create a crossover area on a crossover surface perpendicular to a secondary optical axis of the electro-optical system. The beam-limiting aperture array may be positioned at or within a range of positions of the crossover surface and perpendicular to the secondary optical axis. The range of positions of the crossover surface may be determined based on the landing energies of the multiple primary charged particle beams on the sample. The beam-limiting aperture array may be moved along the secondary optical axis based on the range of positions of the crossover surface.

[0007]

[0007] The second aperture can be centered in the crossover area. The centers of the first and second apertures can be aligned with the secondary optical axis. The beam-limiting aperture array can be moved to align each aperture of the plurality of apertures with the crossover area. The plane of the first pre-limiting aperture plate can be outside the range of positions of the crossover surface, and the planes of the beam-limiting aperture array and the first pre-limiting aperture plate can be within the range of positions of the crossover surface.

[0008]

[0008] In another embodiment of the present disclosure, a multiple charged particle beam device is disclosed. The multiple charged particle beam device may include an electro-optical system for projecting multiple secondary charged particle beams from a sample to a charged particle detector. The electro-optical system may include a first pre-limiting aperture plate including a first aperture configured to block peripheral charged particles of the multiple secondary charged particle beams, and a beam-limiting aperture array including a second aperture configured to reduce the multiple secondary charged particle beams. The charged particle detector may include multiple detection elements, and each detection element of the multiple detection elements may be associated with a corresponding reduction beam of the multiple secondary charged particle beams.

[0009]

[0009] In some embodiments of the present disclosure, a method may be disclosed that is performed by a secondary imaging system to form an image of a sample. The method may include generating a plurality of secondary charged particle beams from the sample, blocking peripheral charged particles of the plurality of secondary charged particle beams using a pre-limiting aperture plate, reducing the plurality of secondary charged particle beams using apertures of a beam-limiting aperture array, and projecting the reduced plurality of secondary charged particle beams onto corresponding detection elements of a charged particle detector.

[0010]

[0010] Other advantages of the embodiments of the present disclosure will become apparent from the following description taken in conjunction with the accompanying drawings, in which:

[0010] In the following description, by way of illustration and example, certain embodiments of the invention are set forth. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram illustrating an exemplary electron beam inspection (EBI) system consistent with an embodiment of the present disclosure. [Diagram 2]

[0012] 2 is a schematic diagram illustrating an example electron beam tool that may be part of the example electron beam inspection system of FIG. 1 consistent with an embodiment of the present disclosure. [Diagram 3]

[0013] 1 is a schematic diagram illustrating an example configuration of a secondary imaging system of a multi-beam device consistent with an embodiment of the present disclosure. [Figure 4]

[0014] 4 is a schematic diagram illustrating an example arrangement of apertures in an aperture array of the secondary imaging system of FIG. 3, consistent with an embodiment of the present disclosure. [Diagram 5]

[0015] 1 is a schematic diagram illustrating an example configuration of a secondary imaging system of a multi-beam device consistent with an embodiment of the present disclosure. [Figure 6A]

[0016] 6 is a schematic diagram illustrating an example arrangement of apertures in an aperture array of the secondary imaging system of FIG. 5, consistent with an embodiment of the present disclosure. [Figure 6B]FIG. 6 is a schematic diagram illustrating an example arrangement of apertures in an aperture array of the secondary imaging system of FIG. 5, consistent with an embodiment of the present disclosure. [Figure 6C] FIG. 6 is a schematic diagram illustrating an example arrangement of apertures in an aperture array of the secondary imaging system of FIG. 5, consistent with an embodiment of the present disclosure. [Figure 6D] FIG. 6 is a schematic diagram illustrating an example arrangement of apertures in an aperture array of the secondary imaging system of FIG. 5, consistent with an embodiment of the present disclosure. [Figure 7]

[0017] 1 is a schematic diagram illustrating an example configuration of a secondary imaging system of a multi-beam device consistent with an embodiment of the present disclosure. [Figure 8]

[0018] 1 is a schematic diagram illustrating an example configuration of a secondary imaging system of a multi-beam device consistent with an embodiment of the present disclosure. [Figure 9]

[0019] 6 is a process flow diagram depicting an exemplary method of detecting secondary charged particles from a sample using the secondary imaging system of FIG. 5, consistent with an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012]

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

[0013]

[0021] Electronic devices are built with circuits formed on a piece of silicon called a substrate. Many circuits can be formed together on the same piece of silicon and are called integrated circuits, or ICs. The size of these circuits has decreased dramatically, so that many more can fit on a substrate. For example, an IC chip in a smartphone is only about the size of a thumbnail, yet it can contain over 2 billion transistors, each less than 1 / 1000 the size of a human hair.

[0014]

[0022] Creating these tiny ICs is a time-consuming, complex, and expensive process that often involves hundreds of individual steps. An error in even one step can introduce defects into the finished IC, rendering it useless. Thus, one of the goals of a manufacturing process is to avoid such defects in order to maximize the number of functioning ICs produced by the process, i.e., to increase the overall yield of the process.

[0015]

[0023] One factor in improving yield is monitoring the chip creation process to ensure that it produces a sufficient number of functioning integrated circuits. One way to monitor the process is to inspect the chip circuit structures at various stages of their formation. Inspection can be done using a scanning electron microscope (SEM). An SEM can be used to image these extremely small structures, in effect taking a "picture" of the structure. The image can be used to determine if the structure was formed correctly, and if it was formed in the correct location. If the structure is defective, the process can be adjusted to make it less likely that the defect will reoccur.

[0016]

[0024] Although multiple charged particle beam imaging systems such as multi-beam SEMs can be useful in increasing wafer inspection throughput, the imaging resolution of a multi-beam SEM can be limited by the quality of the imaging signal received and detected by a secondary electron detection system. A secondary charged particle beam, such as an electron beam, generated by the interaction of a primary beamlet on a sample surface can contain secondary electrons with a large energy spread of about 50 eV and a large emission angle range of about 90° relative to the normal of the sample surface. Such a divergent electron beam can have a large incidence spot on the secondary electron detector. In a conventional multi-beam SEM, the divergent electron beam can be incident on multiple detection elements of a secondary electron detector. In other words, each of the multiple detection elements can receive secondary electrons from a corresponding secondary electron beam and other adjacent beams. As a result, the imaging signal of one detection element can include a main component arising from the corresponding secondary electron beam and a crosstalk component arising from adjacent electron beams. The crosstalk component, among other things, can deteriorate the fidelity of the imaging signal. Therefore, it is desirable to minimize the crosstalk between multiple detection elements to improve the imaging resolution.

[0017]

[0025] To mitigate the occurrence of crosstalk, an aperture mechanism can be employed in the secondary imaging system to block peripheral secondary electrons, but if the secondary electron beam radius is larger than the distance between two apertures of the aperture mechanism, some of the electrons in the secondary electron beam may leak through the adjacent apertures, resulting in crosstalk between the detection elements of the secondary electron detector.

[0018]

[0026] To accommodate the large radius of the secondary electron beam, the apertures can be spaced farther apart and the aperture mechanism can be moved in the x- and y-directions relative to the optical axis (z-axis) of the secondary imaging system. However, the movement of the aperture mechanism in the x- and y-directions can be limited due to space limitations within the secondary imaging system, and thus the number of apertures in the aperture array can be limited. One of several problems that can be encountered is that the aperture mechanism may not be able to mitigate crosstalk while still allowing the passage of a maximum number of secondary electrons to improve imaging resolution.

[0019]

[0027] In some embodiments of the present disclosure, the multi-beam device may include an electro-optical system for projecting a plurality of secondary electrons from the sample to a charged particle detector. The electro-optical system may include a pre-limiting aperture plate configured to block peripheral secondary electrons and a beam-limiting aperture array including apertures configured to reduce the plurality of secondary electrons. The pre-limiting aperture plate may be positioned upstream of the beam-limiting aperture array to prevent peripheral secondary electrons from being projected onto unintended apertures of the beam-limiting aperture array, which may further prevent peripheral secondary electrons from being projected onto detection elements of the electron detector. The combination of the pre-limiting aperture plate and the beam-limiting aperture array may reduce crosstalk between detection elements, thus improving imaging resolution.

[0020]

[0028] In the drawings, the relative dimensions of the components may be exaggerated for clarity. Within the following description of the drawings, the same or similar reference numbers refer to the same or similar components or entities and only describe the differences for the individual embodiments. As used herein, unless specifically stated otherwise, the term "or" encompasses all possible combinations unless impracticable. For example, if it is stated that a component may include A or B, the component may include A or B, or A and B, unless specifically stated otherwise or impracticable. As a second example, if it is stated that a component may include A, B, or C, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A, B, and C, unless specifically stated otherwise or impracticable.

[0021]

[0029] Reference is now made to Figure 1, which illustrates an exemplary electron beam inspection (EBI) system 100 consistent with embodiments of the present disclosure. As shown in Figure 1, the charged particle beam inspection system 100 includes a main chamber 10, a load-lock chamber 20, an electron beam tool 40, and a front-end equipment module (EFEM) 30. The electron beam tool 40 is located within the main chamber 10. Although the description and drawings are directed to an electron beam, it is understood that the embodiments are not used to limit the present disclosure to any particular charged particle.

[0022]

[0030] The EFEM 30 includes a first load port 30a and a second load port 30b. The EFEM 30 may include additional load ports. The first load port 30a and the second load port 30b receive wafer front-opening integrated pods (FOUPs) containing wafers (e.g., semiconductor wafers or wafers made of other materials) or samples to be inspected (hereinafter, wafers and samples are collectively referred to as "wafers"). One or more robotic arms (not shown) of the EFEM 30 transfer the wafers to the load-lock chamber 20.

[0023]

[0031] The load-lock chamber 20 is connected to a load / lock vacuum pumping system (not shown) that removes gas molecules in the load-lock chamber 20 to reach a first pressure below atmospheric pressure. After the first pressure is reached, one or more robot arms (not shown) transfer the wafer from the load-lock chamber 20 to the main chamber 10. The main chamber 10 is connected to a main chamber vacuum pumping system (not shown) that removes gas molecules in the main chamber 10 to reach a second pressure below the first pressure. After the second pressure is reached, the wafer is inspected by the electron beam tool 40. In some embodiments, the electron beam tool 40 may include a single beam inspection tool. In other embodiments, the electron beam tool 40 may include a multi-beam inspection tool.

[0024]

[0032] The controller 50 may be electronically connected to the electron beam tool 40 and may also be electronically connected to other components. The controller 50 may be a computer configured to perform various controls of the charged particle beam inspection system 100. The controller 50 may also include processing circuitry configured to perform various signal and image processing functions. In FIG. 1, the controller 50 is shown as being external to the structure including the main chamber 10, the load lock chamber 20, and the EFEM 30, but it is understood that the controller 50 may also be part of the structure.

[0025]

[0033] It should be noted that while the present disclosure provides an example of a main chamber 10 housing an electron beam inspection system, aspects of the present disclosure, in its broadest sense, are not limited to chambers housing an electron beam inspection system, rather, it is understood that the principles described above may be applied to other chambers as well.

[0026]

[0034] Referring now to FIG. 2, FIG. 2 shows a schematic diagram illustrating an exemplary electron beam tool 40 that may be part of the exemplary charged particle beam inspection system 100 of FIG. 1 consistent with an embodiment of the present disclosure. The electron beam tool 40 (also referred to herein as the apparatus 40) includes an electron source 101, a gun aperture plate 171 having a gun aperture 103, a pre-beamlet forming mechanism 172, a condenser lens 110, a source conversion unit 120, a primary projection optics 130, a sample stage (not shown in FIG. 2), a secondary imaging system 150, and an electron detection device 140. The primary projection optics 130 may include an objective lens 131. The electron detection device 140 may include a plurality of detection elements 140_1, 140_2, 140_3. The beam separator 160 and the deflection scanning unit 132 may be disposed within the primary projection optics 130. It may be understood that other commonly known components of the apparatus 40 may be appropriately added / omitted as necessary.

[0027]

[0035] The electron source 101, the gun aperture plate 171, the condenser lens 110, the source conversion unit 120, the beam separator 160, the deflection scanning unit 132 and the primary projection optics 130 may be aligned with a main optical axis 100_1 of the apparatus 40. The secondary imaging system 150 and the electron detection device 140 may be aligned with a secondary optical axis 150_1 of the apparatus 40.

[0028]

[0036] The electron source 101 may include a cathode, extractor, or anode, and primary electrons may be emitted from the cathode and then extracted or accelerated to form a primary electron beam 102 that forms a crossover (virtual or real image) 101s. The primary electron beam 102 may be visualized as emitting from the crossover 101s.

[0029]

[0037] The source conversion unit 120 may include an image forming element array (not shown in FIG. 2), an aberration compensator array (not shown), a beam limiting aperture array (not shown), and a pre-bent micro-deflector array (not shown). The image forming element array may include multiple micro-deflectors or micro-lenses to form multiple parallel images (virtual or real images) of the crossover 101s with multiple beamlets of the primary electron beam 102. FIG. 2 shows three beamlets 102_1, 102_2, 102_3 as an example, it being understood that the source conversion unit 120 can handle any number of beamlets.

[0030]

[0038] In some embodiments, the source conversion unit 120 can be provided with a beam-limiting aperture array and an image forming element array (both not shown). The beam-limiting aperture array can include a beam-limiting aperture. It is understood that any number of apertures can be used as appropriate as needed. The beam-limiting aperture can be configured to limit the size of the beamlets 102_1, 102_2, 102_3 of the primary electron beam 102. The image forming element array can include an image forming deflector (not shown) configured to deflect the beamlets 102_1, 102_2, 102_3 by changing the angle toward the main optical axis 100_1. In some embodiments, the deflector deflects the beamlets more the farther away from the main optical axis 100_1. Moreover, the image forming element array can include multiple layers (not shown) and the deflectors can be provided in separate layers. The deflectors can be configured to be individually controlled independent of each other. In some embodiments, the deflector can be controlled to adjust the pitch of the probe spots (e.g., 102_1S, 102_2S, 102_3S) formed on the surface of the sample 1. As referred to herein, the pitch of the probe spots can be defined as the distance between two directly adjacent probe spots on the surface of the sample 1.

[0031]

[0039] The deflector located at the center of the image forming element array can be aligned with the main optical axis 100_1 of the electron beam tool 40. Thus, in some embodiments, the central deflector can be configured to keep the trajectory of the beamlet 102_1 straight. In some embodiments, the central deflector can be omitted. However, in some embodiments, the primary electron source 101 is not necessarily aligned with the center of the source conversion unit 120. Moreover, although FIG. 2 shows a side view of the apparatus 40 with the beamlet 102_1 located on the main optical axis 100_1, it is understood that the beamlet 102_1 may be located outside the main optical axis 100_1 when viewed from a different side. That is, in some embodiments, the beamlets 102_1, 102_2, 102_3 may all be off-axis. The off-axis components may have an offset with respect to the main optical axis 100_1.

[0032]

[0040] The deflection angles of the deflected beamlets can be set based on one or more criteria. In some embodiments, the deflector can deflect the off-axis beamlets radially outward or away from the main optical axis 100_1 (not shown). In some embodiments, the deflector can be configured to deflect the off-axis beamlets radially inward or towards the main optical axis 100_1. The deflection angles of the beamlets can be set such that the beamlets 102_1, 102_2, 102_3 land perpendicularly on the sample 1. Off-axis aberrations of the image caused by lenses (such as the objective lens 131) can be reduced by adjusting the path of the beamlets through the lenses. Thus, the deflection angles of the off-axis beamlets 102_2 and 102_3 can be set such that the probe spots 102_2S and 102_3S have small aberrations. The beamlets can be deflected to pass through or near the front focus of the objective lens 131 to reduce aberrations in the off-axis probe spots 102_2S and 102_3S. In some embodiments, the deflectors can be set such that the beamlets 102_1, 102_2, 102_3 land perpendicularly on the sample 1 while the probe spots 102_1S, 102_2S, 102_3S have small aberrations.

[0033]

[0041] The condenser lens 110 is configured to focus the primary electron beam 102. The current of the beamlets 102_1, 102_2, 102_3 downstream of the source transformation unit 120 can be varied by adjusting the focusing power of the condenser lens 110 or by changing the radial size of the corresponding beam-limiting aperture in the beam-limiting aperture array. The current can be changed by changing both the radial size of the beam-limiting aperture and the focusing power of the condenser lens 110. The condenser lens 110 can be an adjustable condenser lens that can be configured to move the position of its first principal plane. The adjustable condenser lens can be configured to be magnetic, whereby the off-axis beamlets 102_2 and 102_3 are irradiated to the source transformation unit 120 with a rotation angle. The rotation angle can be changed together with the focusing power or the position of the first principal plane of the adjustable condenser lens. Accordingly, the focusing lens 110 may be an anti-rotation focusing lens that can be configured to keep the rotation angle unchanged while changing the focusing power of the focusing lens 110. In some embodiments, the focusing lens 110 may be an adjustable anti-rotation focusing lens that does not change the rotation angle when changing the focusing power and the position of the first principal plane of the focusing lens 110.

[0034]

[0042] The electron beam tool 40 may include a pre-beamlet forming mechanism 172. In some embodiments, the electron source 101 may be configured to emit primary electrons and form the primary electron beam 102. In some embodiments, the gun aperture plate 171 may be configured to block electrons on the periphery of the primary electron beam 102 to reduce the Coulomb effect. In some embodiments, the pre-beamlet forming mechanism 172 further removes electrons on the periphery of the primary electron beam 102 to further reduce the Coulomb effect. The primary electron beam 102 may be reduced to three primary electron beamlets 102_1, 102_2, 102_3 (or any other number of beamlets) after passing through the pre-beamlet forming mechanism 172. The electron source 101, the gun aperture plate 171, the pre-beamlet forming mechanism 172, and the condenser lens 110 may be aligned with a main optical axis 100_1 of the electron beam tool 40.

[0035]

[0043] The pre-beamlet forming mechanism 172 may include a Coulomb aperture array. A central aperture (also referred to herein as an on-axis aperture) of the pre-beamlet forming mechanism 172 and a central deflector of the source conversion unit 120 may be aligned with the main optical axis 100_1 of the electron beam tool 40. The pre-beamlet forming mechanism 172 may be provided with a plurality of pre-reduction apertures (e.g., a Coulomb aperture array). In FIG. 2, when the primary electron beam 102 passes through the three pre-reduction apertures, three beamlets 102_1, 102_2, 102_3 are generated, and most of the remaining part of the primary electron beam 102 is deleted. That is, the pre-beamlet forming mechanism 172 may reduce most or most of the electrons from the primary electron beam 102 that do not form the three beamlets 102_1, 102_2, 102_3. The pre-beamlet forming mechanism 172 can remove electrons that are not ultimately used to form the probe spots 102_1S, 102_2S, 102_3S before the primary electron beam 102 enters the source conversion unit 120. In some embodiments, a gun aperture plate 171 can be provided near the electron source 101 to remove electrons at an early stage, while a pre-beamlet forming mechanism 172 can also be provided to further remove electrons around multiple beamlets. Although FIG. 2 shows three apertures in the pre-beamlet forming mechanism 172, it is understood that there can be any number of apertures as needed and appropriate.

[0036]

[0044] In some embodiments, the pre-beamlet forming mechanism 172 can be located below the collecting lens 110. The closer the pre-beamlet forming mechanism 172 is located to the electron source 101, the more effectively the Coulomb effect can be reduced. In some embodiments, the gun aperture plate 171 can be omitted when the pre-beamlet forming mechanism 172 can be positioned close enough to the electron source 101 while still being manufacturable.

[0037]

[0045] The objective lens 131 can be configured to focus the beamlets 102_1, 102_2, 102_3 onto the sample 1 for inspection, forming three probe spots 102_1s, 102_2s, 102_3s on the surface of the sample 1. The gun aperture plate 171 can block unused peripheral electrons of the primary electron beam 102 to reduce Coulomb interaction effects, which can enlarge the size of each of the probe spots 102_1s, 102_2s, 102_3s and thus degrade the inspection resolution.

[0038]

[0046] The beam separator 160 may be a Wien filter type beam separator that includes an electrostatic deflector that generates an electrostatic dipole field E1 and a magnetic dipole field B1 (both not shown in FIG. 2). When these fields are applied, the force exerted on the electrons of the beamlets 102_1, 102_2, 102_3 by the electrostatic dipole field E1 is equal in magnitude and opposite in direction to the force exerted on the electrons by the magnetic dipole field B1. Thus, the beamlets 102_1, 102_2, 102_3 can pass straight through the beam separator 160 with zero deflection angle.

[0039]

[0047] The deflection scanning unit 132 can deflect the beamlets 102_1, 102_2, 102_3 to scan the probe spots 102_1s, 102_2s, 102_3s over three small scanning areas of a section of the surface of the sample 1. In response to the beamlets 102_1, 102_2, 102_3 being incident on the probe spots 102_1s, 102_2s, 102_3s, three secondary electron beams 102_1se, 102_2se, 102_3se can be emitted from the sample 1. Each of the secondary electron beams 102_1se, 102_2se, 102_3se can include electrons with distributed energies, including secondary electrons (energy ≦50 eV) and backscattered electrons (energy between 50 eV and the landing energy of the beamlets 102_1, 102_2, 102_3). The beam separator 160 can direct the secondary electron beams 102_1se, 102_2se, 102_3se towards the secondary imaging system 150. The secondary imaging system 150 can focus the secondary electron beams 102_1se, 102_2se, 102_3se onto the detection elements 140_1, 140_2, 140_3 of the electron detection device 140. The detection elements 140_1, 140_2, 140_3 can detect the corresponding secondary electron beams 102_1se, 102_2se, 102_3se and generate corresponding signals that are used to construct an image of a corresponding scanned area of ​​the sample 1.

[0040]

[0048] In Fig. 2, three secondary electron beams 102_1se, 102_2se, 102_3se generated by three probe spots 102_1S, 102_2S, 102_3S, respectively, move upwards along a main optical axis 100_1 toward the electron source 101 and pass through an objective lens 131 and a deflection scanning unit 132 in succession. The three secondary electron beams 102_1se, 102_2se, 102_3se are redirected by a beam separator 160 (such as a Wien filter) to enter a secondary imaging system 150 along its secondary optical axis 150_1. The secondary imaging system 150 focuses the three secondary electron beams 102_1se-102_3se on an electron detection device 140 that includes three detection elements 140_1, 140_2, 140_3. Therefore, the electronic detection device 140 can simultaneously generate images of the three scanning regions scanned by the three probe spots 102_1S, 102_2S, 102_3S, respectively. In some embodiments, the electronic detection device 140 and the secondary imaging system 150 form one detection unit (not shown). In some embodiments, the electronic optical elements on the path of the secondary electron beam, such as but not limited to the objective lens 131, the deflection scanning unit 132, the beam separator 160, the secondary imaging system 150, and the electronic detection device 140, can form one detection system.

[0041]

[0049] In some embodiments, the controller 50 may include an image processing system, which includes an image acquirer (not shown) and storage (not shown). The image acquirer may include one or more processors. For example, the image acquirer may include a computer, a server, a mainframe host, a terminal, a personal computer, any type of mobile computing device, and the like, or a combination thereof. The image acquirer may be communicatively coupled to the electronic detection device 140 of the apparatus 40 through a medium such as electrical conductors, fiber optic cables, portable storage media, IR, Bluetooth, the Internet, a wireless network, a wireless radio, or a combination thereof, among others. In some embodiments, the image acquirer may receive signals from the electronic detection device 140 and construct an image. Thus, the image acquirer may acquire an image of the sample 1. The image acquirer may also perform various post-processing functions such as generating contours, superimposing indicators on the acquired image, and the like. The image acquirer may be configured to perform adjustments such as brightness and contrast of the acquired image. In some embodiments, the storage may be a storage medium such as a hard disk, a flash drive, cloud storage, random access memory (RAM), other types of computer readable memory, and the like. The storage may be coupled with the image acquirer and may be used to store raw scanned image data as original images or to store post-processed images.

[0042]

[0050] In some embodiments, the image acquirer can acquire one or more images of the sample based on the imaging signal received from the electronic detection device 140. The imaging signal can correspond to a scanning operation to perform charged particle imaging. The acquired image can be a single image including multiple imaging areas. The single image can be stored in the storage. The single image can be an original image that can be divided into multiple regions. Each of the regions can include an imaging area that includes a feature of the sample 1. The acquired image can include multiple images of a single imaging area of ​​the sample 1 sampled multiple times over a time series. The multiple images can be stored in the storage. In some embodiments, the controller 50 can be configured to perform an image processing step using multiple images of the same location of the sample 1.

[0043]

[0051] In some embodiments, the controller 50 may include measurement circuitry (e.g., an analog-to-digital converter) to obtain a distribution of detected secondary electrons. The electron distribution data collected during the detection time window, in combination with the corresponding scan path data of each of the primary beamlets 102_1, 102_2, 102_3 incident on the wafer surface, may be used to reconstruct an image of the wafer structure under inspection. The reconstructed image may be used to reveal various features of the internal or external structure of the sample 1, and thus any defects that may be present in the wafer.

[0044]

[0052] In some embodiments, the controller 50 can control a motorized stage (not shown) to move the sample 1 during inspection. In some embodiments, the controller 50 can cause the motorized stage to move the sample 1 continuously at a constant speed in one direction. In other embodiments, the controller 50 can cause the motorized stage to change the speed at which the sample 1 moves over time depending on the step of the scanning process. In some embodiments, the controller 50 can adjust the configuration of the primary projection optics 130 or the secondary imaging system 150 based on the images of the secondary electron beams 102_1se, 102_2se, 102_3se.

[0045]

[0053] 2 illustrates that the electron beam tool 40 uses three primary electron beams, it is understood that the electron beam tool 40 can use two or more primary electron beams, and the present disclosure does not limit the number of primary electron beams used in the apparatus 40.

[0046]

[0054] Reference is now made to Figure 3, which is a schematic diagram of an example configuration of a secondary imaging system of a multi-beam device consistent with embodiments of the present disclosure, it being understood that the secondary imaging system 150 may be part of a charged particle beam inspection system (e.g., the electron beam inspection system 100 of Figure 1).

[0047]

[0055] In some embodiments, the secondary imaging system 150 is shown and described in conjunction with the overall detection system as shown in FIG. 3. With reference to FIG. 3, only three secondary electron beams for three probe spots are shown, but there can be any number of secondary electron beams. Within the overall detection system, starting from the sample 1, a first portion is along the primary optical axis 100_1 and a second portion is along the secondary optical axis 150_1. For illustrative purposes only, the first portion is rotated along the secondary optical axis 150_1 so that the entire detection system is shown along one straight optical axis, although this is nothing like the actual configuration.

[0048]

[0056] In some embodiments, as shown in FIG. 3, the secondary imaging system 150 may include a zoom lens 151, a projection lens 152, a secondary beam limiting aperture array 155, and an anti-scan deflection unit (not shown), all of which are aligned with the secondary optical axis 150_1. The detection elements 140_1, 140_2, 140_3 of the electron detection device 140 may be disposed on a detection plane SP3 perpendicular to the secondary optical axis 150_1. The zoom lens 151, the projection lens 152, and the objective lens 131 collectively project the surface of the sample 1 onto the detection plane SP3 (i.e., focus the secondary electron beams 102_1se-102_3se to form secondary electron spots on the detection elements 140_1, 140_2, 140_3, respectively) when the deflection scanning unit 132 is off.

[0049]

[0057] In some embodiments, the zoom lens 151 may include two electrostatic lenses 151_11 and 151_12. The image plane of the zoom lens 151 may be at the transfer plane SP2 as shown in FIG. 3. The projection lens 152 may include one electrostatic lens and one magnetic lens (both not shown), and its image plane may be at the detection plane SP3. A first imaging magnification M1 from the surface of the sample 1 to the transfer plane SP2 may be realized by the objective lens 131 and the zoom lens 151, a second imaging magnification M2 from the transfer plane SP2 to the detection plane SP3 may be realized by the projection lens 152, and a total imaging magnification M from the surface of the sample 1 to the detection plane SP3 may be determined based on M1 and M2. Specifically, the total imaging magnification M may be determined based on M1. * It may be based on M2.

[0050]

[0058] In some embodiments, the zoom lens 151 can be configured to perform a zoom function. By adjusting the focusing power of the two electrostatic lenses 151_11 and 151_12, the first imaging magnification Ml can be changed to achieve a desired value of the total imaging magnification M. The projection lens 152 can be configured to perform an anti-rotation function. By adjusting the magnetic field of the magnetic lens and the focusing power of the electrostatic lens, the total image rotation and the second imaging magnification M2 at the detection plane SP3 can remain the same. The anti-scan deflection unit (not shown) can be configured to perform an anti-scan function. By deflecting the secondary electron beam synchronously with the deflection scanning unit 132, the displacement of the three secondary electron spots at the detection plane SP3 can be substantially canceled. As a result, the corresponding relationship between the multiple probe spots and the multiple detection elements can be always maintained. In order to reduce the additional aberrations of the zoom lens 151 and the projection lens 152 resulting from the deflection of the secondary electron beam generated by the deflection scanning unit 132, the anti-scan deflection unit is better placed before the zoom lens 151, so that the secondary electron beam passes through the zoom lens 151 and the projection lens 152 as if the deflection scanning unit 132 is off. However, in this case, the zoom lens 151 is placed far away from the beam separator 160, and as a result, large initial aberrations may be generated.

[0051]

[0059] As is commonly known in the art, the emission of secondary electrons follows Lambert's law and has a large energy spread. The energy of secondary electrons can be up to 50 eV, but most have energies of about 5 eV, depending on the sample material, among other things. The landing energy of the primary electron beamlets (such as the energy of beamlet 102_1 when it lands on the sample) can range from 0.1 keV to 5 keV. The landing energy can be adjusted by varying either or both the bias of the primary electron source 101 and the bias of the sample 1. Thus, the excitation of the objective lens 131 can be adjusted to provide a corresponding focusing power for the three beamlets. Furthermore, to reduce aberrations, the objective lens 131 can be a magnetic or electromagnetic compound lens, configured to rotate the beamlets and affect the landing energy. The size, position or magnification of the secondary electron spot formed by the secondary electron beams 102_1se, 102_2se, 102_3se on the detector elements 140_1, 140_2, 140_3 may vary, so that the secondary electron spot may partially enter the detector element adjacent to the corresponding detector element. The secondary electrons detected by the adjacent detector elements may result in image overlap, e.g., degrading image resolution. The image signal from one detector element may contain information from two or more scanned areas of the sample 1, resulting in loss of resolution due to crosstalk.

[0052]

[0060] For example, crosstalk in a multi-beam SEM such as the EBI system 100 can be mitigated by using a secondary beam limiting aperture array 155 to eliminate secondary electrons on the periphery of a secondary electron beam such as 102_1se. The secondary beam limiting aperture array 155 can include multiple apertures. Although FIG. 3 shows only two apertures 155_1 and 155_2, any number of apertures can be used as needed and appropriate. For example, the secondary beam limiting aperture array 155 shown in FIG. 4 (described below) includes six apertures, 155_1, 155_2, 155_3, 155_4, 155_5, and 155_6.

[0053]

[0061] In general, when the size of the aperture (e.g., aperture 155_1 in FIG. 3) of the secondary beam limiting aperture array 155 is increased, the overall collection efficiency of the secondary electron detector may be increased. However, the difference in collection efficiency of each detection element of the detector may also be increased, and the crosstalk between the detection elements 140_1, 140_2, 140_3 may also be increased. Although the increase in the overall collection efficiency of the electron detection device 140 increases the throughput, the difference in collection efficiency of each detection element 140_1, 140_2, 140_3 may cause further differences in gray levels of the image formed by the secondary electron beams 102_1se-102_3se. To remove the inspection error caused by the difference in gray levels, one or more additional processes may be performed, which may result in a decrease in the inspection throughput and a deterioration in the resolution of the MBI device. When the crosstalk between the secondary electron beams 102_1se-102_3se is increased, the image formed by the secondary electron beams 102_1se-102_3se may be degraded. That is, large crosstalk deteriorates the examination resolution of the MBI apparatus.

[0054]

[0062] In some embodiments, each of the apertures of the secondary beam limiting aperture array 155 may have a uniform or non-uniform size, shape, or cross section. The smaller the radial size of the aperture, the lower the collection efficiency and generally there is crosstalk of the imaging signals of the detection elements 140_1, 140_2, 140_3. Therefore, the radial size of the aperture may depend on the application or the desired result.

[0055]

[0063] In some embodiments where the objective lens 131 functions in a non-magnetic field immersion mode, the angular velocity of the newly emerged secondary electrons may be zero on the sample surface. In such embodiments, the chief rays of the off-axis secondary electron beams 102_2se and 102_3se may still be in a meridional direction after leaving the objective lens 131 and may intersect with the secondary optical axis 150_1 of the secondary imaging system 150. Moreover, the chief rays may intersect with the secondary optical axis 150_1 at the same location (when aberrations are not considered) of the secondary imaging system 150. Thus, the secondary electron beams 102_1se to 102_3se may be configured to overlap at a common area of ​​intersection, and thus a relatively sharp secondary beam crossover may be formed. The plane where the common area of ​​intersection or secondary beam crossover is located is called the intersection plane or secondary beam crossover plane.

[0056]

[0064] Although FIG. 3 shows an exemplary relatively sharp secondary beam crossover surface formed by completely overlapping the secondary beams 102_1se-102_3se on one crossing plane, it is understood that one or more of the secondary electron beams may have an offset with the other secondary electron beams on the crossing plane, such that the secondary beam crossover is not as sharp, and a range of secondary beam crossover surfaces may be formed along the secondary optical axis 150_1. The location of the secondary beam crossover surface may depend, among other things, on the landing energy of the primary beamlets or the excitation of the objective lens 131. In some embodiments, the secondary beam limiting aperture array 155 may be positioned at the secondary beam crossover surface. Or, in other words, the plane of the secondary beam limiting aperture array 155 may coincide with the secondary beam crossover surface. In some embodiments, the plane of the secondary beam limiting aperture array 155 may be within a movement range of the position of the secondary beam crossover surface. The secondary beam limiting aperture array 155 can be moved along the secondary beam crossover plane so that a desired aperture or aperture size can be used to block ambient secondary electrons that are directed towards the electron detection device 140. In some embodiments, the secondary beam limiting aperture array 155 can be positioned at an optimal location within a range of positions of the secondary beam crossover plane.

[0057]

[0065]

[0036] Referring now to Figure 4, Figure 4 shows a schematic diagram of an exemplary arrangement of apertures in a secondary beam-limiting aperture array 155, consistent with embodiments of the present disclosure. In Figure 4, beam-limiting aperture array 155 is shown to include six apertures of varying sizes, although any number of apertures may be used.

[0058]

[0066] As shown in Fig. 4, the cross section 102se represents the cross section of the overlapping secondary electron beams 102_1se to 102_3se incident on the secondary beam limiting aperture array 155. In some embodiments, the secondary electron beams 102_1se to 102_3se may not overlap at the same crossing plane to form a sharp secondary beam crossover, but may instead be offset along the secondary optical axis 150_1 to form a range of crossing planes. In such cases, the cross sections 102se of the overlapping secondary electron beams 102_1se to 102_3se are not sharp.

[0059]

[0067] In some embodiments, to minimize the occurrence of crosstalk, the distance between two adjacent apertures, such as 155_2 and 155_3, can be greater than the sum of the radius R of the overlapped secondary electron beam 102se and the radius of the larger of the two apertures. In some embodiments, the secondary beam limiting aperture array 155 can include at least two apertures of equal size, and in such a case, the distance between two adjacent apertures, such as 155_2 and 155_3, can be greater than the sum of the radius R of the cross section 102se of the overlapped secondary electron beams 102_1se to 102_3se and the radius of one of the two apertures.

[0060]

[0068] In some embodiments, the radius R of the cross section 102se of the overlapping secondary electron beams 102_1se-102_3se may depend on the landing energy of the primary beamlets or the excitation of the objective lens 131. Thus, to accommodate a wide range of radii of the overlapping secondary electron beams 102_1se-102_3se (represented by the cross section 102se in FIG. 4), the distance between two adjacent apertures can be determined based on the maximum value of the radius R.

[0061]

[0069]

[0036] Referring now to Figure 5, Figure 5 illustrates an exemplary configuration of a secondary imaging system 150 of an electron beam tool 500 of a multi-beam apparatus consistent with an embodiment of the present disclosure. In comparison to the secondary imaging system 150 of the electron beam tool 40 of Figure 3, the secondary imaging system 150 of the electron beam tool 500 may include a pre-limiting aperture plate 155P.

[0062]

[0070] 5, the pre-limiting aperture plate 155P may include a plate having an aperture configured to block peripheral electrons while allowing passage of on-axis electrons of the secondary electron beams 102_1se-102_3se. In some embodiments, the pre-limiting aperture plate 155P may be aligned with the secondary beam limiting aperture array 155 and the secondary optical axis 150_1 such that a majority of peripheral electrons of the off-axis secondary electron beams 102_2se and 102_3se are blocked. In some embodiments, the pre-limiting aperture plate 155P may be configured to prevent secondary electrons from being projected into unintended apertures of the secondary beam limiting aperture array 155.

[0063]

[0071] In some embodiments, the pre-limiting aperture plate 155P may be positioned upstream of the secondary beam-limiting aperture array 155, as shown in Figure 5. In the context of this disclosure, "upstream of" refers to a position of the pre-limiting aperture plate 155P such that one or more secondary electron beams 102_1se-102_3se may be incident before being projected onto the secondary beam-limiting aperture array 155.

[0064]

[0072] The pre-limiting aperture plate 155P can be placed in front of the secondary beam-limiting aperture array 155 such that the apertures of the pre-limiting aperture plate 155P are aligned with the desired apertures of the secondary beam-limiting aperture array 155. In such a case, the number of peripheral secondary electrons incident on the secondary beam-limiting aperture array 155 can be reduced or minimized. In some embodiments, the pre-limiting aperture plate 155P can be configured to block peripheral secondary electrons from impinging on other unintended apertures of the secondary beam-limiting aperture array 155, thus reducing crosstalk. Such a configuration also allows the apertures of the secondary beam-limiting aperture array 155 to be positioned closer together, thereby allowing more apertures to be placed in a smaller area of ​​the aperture array 155 while reducing the occurrence of crosstalk.

[0065]

[0073] The size and shape of the apertures of the secondary beam limiting aperture array 155 may change over time based on time of use, material, inspection parameters, etc. For example, exposure to secondary electrons may cause contamination and debris formation at the edges of the apertures, thus reducing the effective size and shape of the aperture through which the secondary electrons can pass. In some embodiments, a pre-limiting aperture plate 155P may be placed in front of the secondary beam limiting aperture array 155 to reduce or minimize the variation in size and shape of the apertures of the secondary beam limiting aperture array 155 by reducing the number of incident peripheral secondary electrons.

[0066]

[0074] 4, the apertures 155_1 and 155_2 of the secondary beam-limiting aperture array 155 can be separated from each other such that secondary electrons passing through the pre-limiting aperture plate 155P pass through only one intended aperture of the secondary beam-limiting aperture array 155. In some embodiments, the pre-limiting aperture plate 155P can be configured to block peripheral electrons so that they do not pass through apertures adjacent to the intended aperture of the secondary beam-limiting aperture array 155, thereby positioning the apertures of the secondary beam-limiting aperture array 155 closer together. This allows the design of an aperture array that includes more aperture sizes in a smaller area of ​​the aperture array 155 while reducing the occurrence of crosstalk.

[0067]

[0075] In some embodiments, the pre-limiting aperture plate 155P may be positioned downstream of the secondary beam-limiting aperture array 155 (e.g., between the secondary beam-limiting aperture array 155 and the electron detection device 140) (this embodiment is not shown). In the context of this disclosure, "downstream of" refers to a position of the pre-limiting aperture plate 155P such that one or more secondary electron beams 102_1se-102_3se may be incident on it after being projected onto the secondary beam-limiting aperture array 155. The pre-limiting aperture plate 155P may be positioned closer to the secondary beam-limiting aperture array 155 than the electron detection device 140.

[0068]

[0076] Although not shown in FIG. 5, in some embodiments, the secondary imaging system 150 may include two or more pre-limiting aperture plates 155P. For example, a primary pre-limiting aperture plate 155P_1 (not shown) may be positioned upstream of the secondary beam-limiting aperture array 155, and a secondary pre-limiting aperture plate 155P_2 (not shown) may be positioned downstream of the secondary beam-limiting aperture array 155. In such a configuration, the primary pre-limiting aperture plate 155P_1 may be configured to block a majority of peripheral secondary electrons from being projected into unintended apertures of the secondary beam-limiting aperture array 155, and the secondary pre-limiting aperture plate 155P_2 may be configured to block stray peripheral secondary electrons that could not be blocked by the primary pre-limiting aperture plate 155P_1, thus reducing the occurrence of crosstalk. It is understood that other combinations of the number of pre-limiting aperture plates and their arrangements are also possible.

[0069]

[0077] The pre-limiting aperture plate 155P and the secondary beam limiting aperture array 155 may be separated by an optimal distance 155G, as shown in FIG. 5. Although it is desirable to minimize the distance 155G between the pre-limiting aperture plate 155P and the secondary beam limiting aperture array 155 to reduce the possibility of peripheral secondary electrons leaking out or irradiating other apertures of the secondary beam limiting aperture array 155, the distance 155G may be optimized so that the movement of the pre-limiting aperture plate 155P and the secondary beam limiting aperture array 155 is not restricted. In some embodiments, the distance between the pre-limiting aperture plate 155P and the secondary beam limiting aperture array 155 may be 5 mm. In some embodiments, the distance 155G may be determined based on mechanical design considerations, available space, manufacturability, and cost efficiency, among others. For example, using several techniques, it may be possible to reliably and reproducibly achieve a distance of 3 mm to 5 mm between the pre-limiting aperture plate 155P and the secondary beam limiting aperture array 155. In some embodiments, distance 155G may be greater than 5 mm (e.g., 10 mm) based on factors including, but not limited to, space availability, design constraints, cost efficiency, materials, and intended application.

[0070]

[0078] 6A-6D, which are schematic diagrams illustrating an example arrangement of apertures in the secondary beam-limiting aperture array 155 of the secondary imaging system 150 of the electron beam tool 500 shown in FIG. 5, consistent with an embodiment of the present disclosure. It is understood that the size of the apertures, the size of the secondary beam-limiting aperture array 155, the secondary electron beam size before the pre-limiting aperture plate 155P, and the secondary electron beam size incident on the secondary beam-limiting aperture array 155 after passing through the pre-limiting aperture plate 155P are for illustrative purposes only and are not to scale.

[0071]

[0079] As shown in Figures 6A-6D, cross section 102se represents the contour of a beam of secondary electrons generated by a primary beamlet (e.g., primary beamlets 102_1, 102_2, 102_3 in Figure 2) after interacting with the sample at probe spots 102_1S, 102_2S, 102_3S, respectively.

[0072]

[0080] FIG. 6A shows a secondary beam limiting aperture array 155A including three apertures 155_1, 155_2, 155_3 arranged in one direction such that the geometric centers of all the apertures are aligned along an axis (e.g., the x-axis). Although only three apertures are shown, any number of apertures can be used. In some embodiments, the number of apertures in such a configuration may be limited by the allowable size of the secondary beam limiting aperture array 155A based on the physical space available in the secondary imaging system 150. One of several advantages of using a pre-limiting aperture plate 155P is that it allows for a reduction in the separation distance between two adjacent apertures by blocking all peripheral secondary electrons from being irradiated into the apertures of the secondary beam limiting aperture array 155A that are not used for detection. In addition, the use of a pre-limiting aperture plate 155P can reduce crosstalk.

[0073]

[0081] The reduction in the separation distance between two adjacent apertures is shown in FIG. 6A. In this context, the separation distance between two apertures can be referred to as the linear distance between the geometric centers of the two adjacent apertures. In some embodiments, the separation distance between two apertures across the array can be uniform. For example, the separation distance between aperture 155_1 and aperture 155_2 can be similar to the separation distance between aperture 155_2 and aperture 155_3. In some embodiments, the separation distance can be non-uniform.

[0074]

[0082] In some embodiments, the secondary beam limiting aperture array 155A can be configured to move along a single axis (such as the x-axis) to adjust the size of the aperture through which the overlapping secondary electron beams can pass.

[0075]

[0083] As shown in FIG. 6A, the secondary beam-limiting aperture array 155A may include apertures of different sizes. In some embodiments, the apertures 155_1, 155_2, 155_3 of the secondary beam-limiting aperture array 155A may be uniform in size, shape, cross-section, and pitch. In some embodiments, the radius of the apertures of the pre-limiting aperture plate 155P may be slightly larger than the radius of the largest aperture of the secondary beam-limiting aperture array 155. The pre-limiting aperture plate 155P and the secondary beam-limiting aperture array 155 may be aligned such that the geometric centers of the apertures of the pre-limiting aperture plate 155P and the secondary beam-limiting aperture array 155 are aligned with the secondary optical axis 150_1.

[0076]

[0084] FIG. 6B illustrates a secondary beam-limiting aperture array 155B including six apertures 155_1-155_6 arranged in a rectangular matrix along the x-axis and y-axis. In some embodiments, the secondary beam-limiting aperture array 155B may include apertures of different sizes. In some embodiments, the secondary beam-limiting aperture array 155B may include at least two apertures of similar sizes. Cross section 102se_155P represents the profile of a beam of secondary electrons (represented by 102se) generated by the primary beamlets 102_1, 102_2, 102_3 and passed through the pre-limiting aperture plate 155P. It is understood that cross sections 102se and 102se_155P represent the profile of the secondary electron beam in different planes along the secondary optical axis 150_1.

[0077]

[0085] In some embodiments, the secondary beam-limiting aperture array 155B can be configured to move along both the x-axis and the y-axis to adjust the size of the aperture through which the overlapping secondary electron beams can pass. Although Figures 6A and 6B show rectangular secondary beam-limiting aperture arrays (e.g., 155A and 155B), other shapes can also be used, including, but not limited to, circular, triangular, elliptical, etc. It is understood that the size and shape of the secondary beam-limiting aperture array can be determined based on available physical space, mechanical design considerations, cost efficiency, etc.

[0078]

[0086] Referring now to FIG. 6C, FIG. 6C illustrates a circular secondary beam-limiting aperture array 155C including seven apertures 155_1-155_7. In some embodiments, the secondary beam-limiting aperture array 155C may include a central aperture 155_1 and six off-center apertures 155_2-155_7 arranged radially around the central aperture 155_1. As shown in FIG. 6C, the off-center apertures may be positioned along an imaginary circle 155R such that the separation distance between the center of the central aperture 155_1 and the center of each of the off-center apertures 155_2-155_7 is uniform. In other words, the separation distance between the center of the central aperture 155_1 and the center of each of the off-center apertures 155_2-155_7 may be similar or substantially similar to the radius of 155R. In such an embodiment, the separation distance or the radius of the virtual circle 155R can be determined based on the radius of the cross section 102se_155P and the radius of the largest aperture (e.g., aperture 155_6 in FIG. 6C) of the secondary beam-limiting aperture array 155C. In some embodiments, the separation distance can be greater than the sum of the radius of the cross section 102se_155P and the radius of, for example, the largest aperture 155_6 of the secondary beam-limiting aperture array 155C.

[0079]

[0087] 6C, the secondary beam-limiting aperture array 155C may include apertures of different sizes. The central aperture 155_1 may be of a different size as compared to the off-center apertures 155_2-155_7. In some embodiments, each of the off-center apertures may be of a different size and may be randomly arranged along the perimeter of the imaginary circle 155R.

[0080]

[0088] In some embodiments, the secondary beam limiting aperture array 155C can be configured to move along both the x and y axes to adjust the size of the aperture through which the overlapping secondary electron beams can pass. One of several advantages of the circular arrangement of apertures in the secondary beam limiting aperture array 155C is that a variety of aperture sizes can be accessed with limited movement in the x and y axes.

[0081]

[0089] FIG. 6D illustrates a circular secondary beam-limiting aperture array 155D including seven apertures 155_1-155_7. In comparison to FIG. 6C, the size of two or more apertures in the secondary beam-limiting aperture array 155D may be similar or substantially similar. For example, the size of off-center apertures 155_3 and 155_7 are similar. Although FIG. 6D illustrates two off-center apertures 155_3 and 155_7 having similar sizes, it is understood that any two or more apertures may have similar sizes.

[0082]

[0090] In some embodiments, the apertures may become contaminated after extended use, for example due to particles, debris and gases generated by secondary electrons incident on the secondary beam limiting aperture array 155D. Contamination may change the effective size or shape of the aperture, which may affect the collection efficiency of the detection elements (e.g., 140_1, 140_2, 140_3 in FIG. 3) of the electron detection device 140, and thus impact the overall throughput and image resolution. The problem of contamination may be exaggerated for smaller apertures, such as 155_3, in part because a smaller aperture blocks more of the secondary electrons incident on the array than a larger aperture. In addition, a smaller aperture has a smaller circumference, and therefore may reduce the effective size of the aperture more for the same exposure time to secondary electrons than a larger aperture. One of several advantages of having two or more apertures of similar size may be reduced tool downtime due to maintenance. For example, if one of the apertures becomes contaminated, a second aperture of similar size can be used, thereby allowing uninterrupted tool usage and therefore increasing the overall throughput of wafer inspection.

[0083]

[0091] 7, which illustrates an example configuration 700 of a secondary imaging system 150 of a multi-beam apparatus consistent with embodiments of the present disclosure. It is understood that the secondary imaging system 150 may be part of a charged particle beam inspection system (e.g., the electron beam inspection system 100 of FIG. 1).

[0084]

[0092] As shown in Fig. 7, the secondary imaging system 150 may include a movable secondary beam limiting aperture array 155 configured to move in three directions along the x-axis, y-axis, and z-axis. In some embodiments, while the pre-limiting aperture plate 155P is fixed in its position, the secondary beam limiting aperture array 155 may be moved along the x-axis and y-axis to select a desired aperture size, and along the z-axis to coincide with the position of the secondary beam crossover plane. The x-axis and y-axis may be perpendicular to the secondary optical axis 150_1, and the z-axis may be parallel to the secondary optical axis 150_1.

[0085]

[0093] In some embodiments, the position of the secondary beam crossover plane along the secondary optical axis 150_1 can be determined based at least on factors including, but not limited to, the landing energy of the primary beamlets and the excitation of the objective lens 131. In some embodiments, the corresponding positions of the secondary beam crossover plane for a range of landing energies of the primary beamlets can be determined based on simulation and data modeling algorithms.

[0086]

[0094] Based on the landing energy of the primary beamlets, the secondary electron beams may overlap at different planes perpendicular to the secondary optical axis 150_1, thus forming a range of positions of the secondary beam crossover plane for a corresponding range of landing energies. In some embodiments, the position of the crossover plane can be determined based on a simulation of the landing energy, and thus the range of positions of the secondary beam crossover plane can be determined based on the range of landing energies used in the system. For example, a user or a system can theoretically determine the coordinates of the secondary beam crossover plane for a given value of the landing energy of the primary beamlets based on a simulation such as an algorithm.

[0087]

[0095] In some embodiments, when the position of the secondary beam limiting aperture array 155 is adjusted along the z-axis to align with the position of the secondary beam crossover plane, the position of the pre-limiting aperture plate 155P may be out of the range of the position of the secondary beam crossover plane. This may increase the size of the secondary electron beam (e.g., represented as 102se_155P in FIGS. 6A-6D) incident on the secondary beam limiting aperture array 155. The larger beam size incident on the secondary beam limiting aperture array 155 enhances the number of secondary electrons incident on the sensing element of the electron detection device, but may also increase the possibility of crosstalk.

[0088]

[0096] 8, which illustrates an example configuration 800 of a secondary imaging system 150 of a multi-beam apparatus consistent with embodiments of the present disclosure. It is understood that the secondary imaging system 150 may be part of a charged particle beam inspection system (e.g., the electron beam inspection system 100 of FIG. 1).

[0089]

[0097] 8, the secondary imaging system 150 may include a movable pre-limiting aperture plate 155P and a movable secondary beam limiting aperture array 155. The movable pre-limiting aperture plate 155P may be configured to move independently in three directions along the x-axis, the y-axis, and the z-axis. In some embodiments, the movable pre-limiting aperture plate 155P may be configured to move in conjunction with the movable secondary beam limiting aperture array 155 such that the distance 155G is maintained.

[0090]

[0098] In some embodiments, the movable pre-limiting aperture plate 155P and the secondary beam-limiting aperture array 155 can be moved together such that the position of the secondary beam-limiting aperture array 155 is aligned with the position of the secondary beam crossover plane. In such a scenario, based on the distance 155G and the range of crossover positions, the pre-limiting aperture plate 155P and the secondary beam-limiting aperture array 155 can both be positioned within the range of crossover positions. In some embodiments, it is desirable to have the pre-limiting aperture plate 155P and the secondary beam-limiting aperture array 155 within the range of crossover positions along the secondary optical axis 150_1 to effectively block peripheral electrons and reduce the possibility of crosstalk.

[0091]

[0099] 9, which illustrates a process flow diagram depicting an exemplary method 900 performed by a secondary imaging system (e.g., secondary imaging system 150 of FIG. 3) to form an image of a sample, consistent with an embodiment of the present disclosure. Method 900 may be performed, for example, by a controller 50 of an EBI system 100 as shown in FIG. 1. The controller 50 may be programmed to perform one or more steps of method 900. For example, the controller 50 may instruct a module of a charged particle beam device to activate a charged particle source to generate a charged particle beam, which may generate a secondary charged particle beam upon interaction with a sample.

[0092]

[0100] In step 910, multiple secondary electron beams (e.g., 102_1se, 102_2se, 102_3se in FIG. 3) can be generated from the sample (e.g., Sample 1 in FIG. 2) when the primary beamlets (e.g., 102_1, 102_2, 102_3 in FIG. 2) interact with the probe spots (e.g., 102_1S, 102_2S, 102_3S in FIG. 2) of the sample. In some embodiments, the number of secondary electron beams generated can be equal to the number of primary beamlets incident on the sample. The three secondary electron beams 102_1se, 102_2se, 102_3se can be redirected by a beam separator such as a Wien filter (e.g., beam separator 160 in FIG. 2) to enter the secondary imaging system 150 along its secondary optical axis 150_1.

[0093]

[0101] Step 910 may further include directing the secondary electron beams 102_1se-102_3se to overlap at a common area of ​​intersection along the secondary optical axis, so that a secondary beam crossover (e.g., a relatively sharp secondary beam crossover, etc.) may be formed. The plane in which the common area of ​​intersection or secondary beam crossover is located is referred to as the intersection plane or secondary beam crossover plane. A beam-limiting aperture array (e.g., secondary beam-limiting aperture array 155 in FIG. 3) may be positioned at or near the secondary beam crossover plane. In some embodiments, a pre-limiting aperture plate (e.g., pre-limiting aperture plate 155P in FIG. 5) may be positioned in front of the secondary beam-limiting aperture array.

[0094]

[0102] In step 920, a pre-limiting aperture plate can be used to block peripheral secondary electrons of the secondary electron beam prior to illumination of the secondary beam-limiting aperture array. The pre-limiting aperture plate can include a plate having apertures configured to block peripheral electrons while allowing passage of on-axis electrons of the secondary electron beam. In some embodiments, the pre-limiting aperture plate can be aligned with the secondary beam-limiting aperture array and the secondary optical axis such that a majority of peripheral electrons of the off-axis secondary electron beam are blocked.

[0095]

[0103] In some embodiments, the pre-limiting aperture plate and the secondary beam-limiting aperture array can be separated by an optimal distance (e.g., distance 155G as shown in FIG. 5). Although it is desirable to minimize the distance between the pre-limiting aperture plate and the secondary beam-limiting aperture array to reduce the possibility of peripheral secondary electrons leaking or projecting into other unintended apertures of the secondary beam-limiting aperture array, the distance 155G can be optimized so that the movement of the pre-limiting aperture plate and the secondary beam-limiting aperture array is not restricted. In some embodiments, the distance between the pre-limiting aperture plate and the secondary beam-limiting aperture array can be 5 mm. In some embodiments, the distance can be determined based on mechanical design considerations, available space, manufacturability, and cost efficiency, among others. For example, using several techniques, it may be possible to reliably and reproducibly achieve a distance of 3 mm to 5 mm between the pre-limiting aperture plate 155P and the secondary beam-limiting aperture array 155. In some embodiments, the distance may be greater than 5 mm (e.g., 10 mm) based on factors including, but not limited to, space availability, design constraints, cost efficiency, materials, and intended use.

[0096]

[0104] In step 930, the secondary electron beam can be further reduced using a beam-limiting aperture array. The secondary beam-limiting aperture array can be positioned at or within a range of motion of the position of the secondary beam crossover plane. The secondary beam-limiting aperture array can be moved along the secondary beam crossover plane so that a desired aperture or aperture size can be used to block surrounding secondary electrons that are directed towards the charged particle detection element (e.g., electron detection device 140 of FIG. 3). The position of the secondary beam crossover plane can depend, among other things, on the landing energy of the primary beamlets or the excitation of the objective lens (e.g., objective lens 131 of FIG. 2). The secondary beam-limiting aperture array can be positioned at an optimal position within the range of motion of the secondary beam crossover plane.

[0097]

[0105] In some embodiments, the order of steps 920 and 930 may be interchangeable. For example, the pre-limiting aperture plate may be disposed upstream or downstream of the secondary beam-limiting aperture array, so that the secondary electron beams 102_1se-102_3se may be incident on the pre-limiting aperture plate before or after being projected onto the secondary beam-limiting aperture array, respectively. In some embodiments, a pre-limiting aperture plate (e.g., the primary pre-limiting aperture plate 155P_1 (not shown)) may be disposed upstream of the secondary beam-limiting aperture array, and another pre-limiting aperture plate (e.g., the pre-limiting aperture plate 155P_2 (not shown)) may be disposed downstream of the secondary beam-limiting aperture array 155. In such a configuration, one pre-limiting aperture plate (e.g., 155P_1) can be configured to block a majority of peripheral secondary electrons from being projected into unintended apertures of the secondary beam limiting aperture array 155, and another pre-limiting aperture plate (e.g., 155P_2) can be configured to block stray peripheral secondary electrons that could not be blocked by the first pre-limiting aperture plate, thus reducing the possibility of crosstalk occurring. It is understood that other combinations of numbers of pre-limiting aperture plates and their arrangements can be used as desired.

[0098]

[0106] In step 940, the reduced secondary electron beam may be projected towards detection elements of an electron detection device (eg, 140_1, 140_2, 140_3 in FIG. 3) to generate an image of the interrogated area of ​​the sample.

[0099]

[0107] The embodiments are further described using the following clauses. 1. A first pre-limiting aperture plate including a first aperture configured to block peripheral charged particles of a plurality of secondary charged particle beams from a sample; a beam-limiting aperture array including a second aperture configured to reduce a plurality of secondary charged particle beams; 1. An electro-optical system comprising: 2. The system of claim 1, further comprising a charged particle detector including a plurality of detection elements, each detection element of the plurality of detection elements being associated with a corresponding attenuation beam of the plurality of secondary charged particle beams. 3. A system as described in clause 1 or 2, wherein the distance between the first pre-limiting aperture plate and the beam-limiting aperture array is 5 mm or less. 4. A system described in any one of clauses 1 to 3, wherein a first pre-limiting aperture plate is positioned upstream of the beam-limiting aperture array. 5. A system described in any one of clauses 2 to 4, wherein a first pre-limiting aperture plate is positioned downstream of the beam-limiting aperture array. 6. The system of clause 1 or 2, further comprising a second pre-limiting aperture plate. 7. A system as described in clause 6, wherein a first pre-limiting aperture plate is positioned upstream of the beam-limiting aperture array and a second pre-limiting aperture plate is positioned downstream of the beam-limiting aperture array. 8. A system described in any one of clauses 1 to 7, wherein the multiple secondary charged particle beams include at least one of secondary electrons or backscattered electrons generated from the sample in response to interactions between the multiple primary charged particle beams and the sample. 9. A system as described in any one of clauses 1 to 8, wherein the beam-limiting aperture array comprises a plurality of apertures of different sizes. 10. The system of claim 9, wherein at least two of the plurality of apertures have a similar size. 11. The system of clause 9 or 10, wherein the plurality of apertures are arranged in a rectangular, circular or spiral pattern. 12. A system as described in any one of clauses 1 to 11, wherein multiple secondary charged particle beams overlap to produce a crossover area on a crossover plane perpendicular to a secondary optical axis of the electro-optical system. 13. A system as described in clause 12, wherein a beam-limiting aperture array is positioned at or within a range of positions of the crossover plane and perpendicular to the secondary optical axis. 14. The system of claim 12 or 13, wherein the range of positions of the crossover plane is determined based on the landing energies of the multiple primary charged particle beams on the sample. 15. A system as described in any one of clauses 12 to 14, wherein the second aperture is centered in the crossover area. 16. A system according to any one of clauses 12 to 15, wherein the centres of the first and second apertures are aligned with the secondary optical axis. 17. A system as described in any one of clauses 12 to 16, wherein the beam-limiting aperture array is movable to align each aperture of the plurality of apertures with the crossover area. 18. A system as described in any one of clauses 14 to 17, wherein the beam-limiting aperture array is movable along the secondary optical axis based on a range of positions of the crossover surface. 19. A system as described in any one of clauses 12 to 18, wherein the plane of the first pre-limiting aperture plate is outside the range of positions of the crossover surface. 20. A system as described in clause 19, wherein the planes of the beam-limiting aperture array and the first pre-limiting aperture plate are within the range of the position of the crossover plane. 21. An electro-optical system for projecting a plurality of secondary charged particle beams from the sample to a charged particle detector. A multi-charged particle beam device, comprising: a first pre-limiting aperture plate including a first aperture configured to block peripheral charged particles of the plurality of secondary charged particle beams; a beam-limiting aperture array including a second aperture configured to reduce a plurality of secondary charged particle beams; Including, A multiple charged particle beam device, wherein the charged particle detector includes a plurality of detection elements, each detection element of the plurality of detection elements being associated with a corresponding attenuation beam of the plurality of secondary charged particle beams. 22. An apparatus as described in clause 21, wherein the distance between the first pre-limiting aperture plate and the beam-limiting aperture array is 5 mm or less. 23. An apparatus according to clause 21 or 22, wherein a first pre-limiting aperture plate is positioned upstream of the beam-limiting aperture array. 24. An apparatus as described in any one of clauses 21 to 23, wherein a first pre-limiting aperture plate is positioned downstream of the beam-limiting aperture array. 25. The apparatus of claim 21, further comprising a second pre-limiting aperture plate. 26. An apparatus as described in clause 25, wherein a first pre-limiting aperture plate is positioned upstream of the beam-limiting aperture array and a second pre-limiting aperture plate is positioned downstream of the beam-limiting aperture array. 27. An apparatus described in any one of clauses 21 to 26, wherein the multiple secondary charged particle beams include at least one of secondary electrons or backscattered electrons generated from the sample in response to interactions between the multiple primary charged particle beams and the sample. 28. An apparatus according to any one of clauses 21 to 27, wherein the beam-limiting aperture array comprises a plurality of apertures of different sizes. 29. The apparatus of claim 28, wherein at least two of the plurality of apertures have a similar size. 30. The apparatus of clause 28 or 29, wherein the plurality of apertures are arranged in a rectangular, circular, or spiral pattern. 31. An apparatus as described in any one of clauses 21 to 30, wherein multiple secondary charged particle beams overlap to produce a crossover area on a crossover plane perpendicular to a secondary optical axis of the electro-optical system. 32. Apparatus according to clause 31, wherein a beam-limiting aperture array is arranged at or within a range of positions of the crossover plane and perpendicular to the secondary optical axis. 33. An apparatus as described in clause 31 or 32, wherein the range of positions of the crossover plane is determined based on the landing energies of the multiple primary charged particle beams on the sample. 34. The apparatus of any one of clauses 31-33, wherein the second aperture is centered in the crossover area. 35. An apparatus as described in any one of clauses 31 to 34, wherein the centers of the first and second apertures are aligned with the secondary optical axis. 36. An apparatus as described in any one of clauses 31 to 35, wherein the beam-limiting aperture array is movable to align each aperture of the plurality of apertures with the crossover area. 37. An apparatus according to any one of clauses 33 to 36, wherein the beam-limiting aperture array is movable along the secondary optical axis based on a range of positions of the crossover surface. 38. An apparatus as described in any one of clauses 31 to 37, wherein the plane of the first pre-limiting aperture plate is outside the range of positions of the crossover surface. 39. An apparatus as described in clause 38, wherein the planes of the beam-limiting aperture array and the first pre-limiting aperture plate are within the position of the crossover plane. 40. A method performed by a secondary imaging system to form an image of a sample, comprising: generating a plurality of secondary charged particle beams from the sample; blocking peripheral charged particles of the plurality of secondary charged particle beams using a pre-limiting aperture plate; reducing the plurality of secondary charged particle beams using apertures of a beam-limiting aperture array; projecting the plurality of reduced secondary charged particle beams onto corresponding detection elements of a charged particle detector; A method comprising:

[0100]

[0108] A non-transitory computer readable medium can be provided that stores instructions for a processor of a controller (e.g., controller 50 of FIG. 1 ) to perform image inspection, image acquisition, collection lens adjustment, activation of a charged particle source, beam deflection, positioning of a beam limiting aperture array (e.g., secondary beam limiting aperture array 155), positioning of a pre-limiting aperture plate (e.g., pre-limiting aperture plate 155P), etc. Common forms of non-transitory media include, for example, a floppy disk, a flexible disk, a hard disk, a solid state drive, a magnetic tape or any other magnetic data storage medium, a compact disk read only memory (CD-ROM), any other optical data storage medium, any physical medium having a pattern of holes, a random access memory (RAM), a programmable read only memory (PROM), an erasable programmable read only memory (EPROM), a flash EPROM or any other flash memory, a non-volatile random access memory (NVRAM), a cache, a register, any other memory chip or cartridge, and network-connected versions thereof.

[0101]

[0109] It will be understood that the embodiments of the present disclosure are not limited to the exact construction as described above and shown in the accompanying drawings, and that various modifications and changes can be made without departing from the scope thereof. The present disclosure has been described in conjunction with various embodiments, and other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as merely exemplary, with a true scope and spirit of the invention being indicated by the following claims.

[0102]

[0110] The above description is intended to be illustrative rather than limiting. Thus, it will be apparent to one skilled in the art that modifications can be made as described without departing from the scope of the claims set out below.

Claims

1. a beam-limiting aperture array comprising circular apertures configured to reduce a plurality of secondary charged particle beams from a sample, the apertures include a central aperture and a plurality of off-center apertures radially arranged about the central aperture; A beam-limiting aperture array, wherein a size of one of the plurality of off-center apertures is different from a size of another one of the plurality of off-center apertures.

2. 2. The beam-limiting aperture array of claim 1, wherein the off-center apertures are positioned along an imaginary circle such that a separation distance between a center of the central aperture and a center of each of the off-center apertures is uniform.

3. The beam-limiting aperture array of claim 2 , wherein the separation distance is similar to a radius of the imaginary circle.

4. 3. The beam-limiting aperture array of claim 2, wherein the separation distance is determined based on a radius of a cross section of the secondary charged particle beam incident on the beam-limiting aperture array and a radius of a maximum aperture of the beam-limiting aperture array.

5. 5. The beam-limiting aperture array of claim 4, wherein the separation distance is greater than the sum of the radius of the cross section of the secondary charged particle beam incident on the beam-limiting aperture array and the radius of the largest aperture of the beam-limiting aperture array.

6. 3. The beam-limiting aperture array of claim 2, wherein the radius of the imaginary circle is determined based on a radius of a cross section of the secondary charged particle beam incident on the beam-limiting aperture array and a radius of a maximum aperture of the beam-limiting aperture array.

7. 7. A beam-limiting aperture array according to claim 1, wherein the central aperture is of a different size than the off-centre apertures.

8. 8. A beam-limiting aperture array according to claim 1, wherein each of the off-centre apertures is of different size.

9. 9. The beam-limiting aperture array of claim 1 , configured to move along both the x-axis and the y-axis to adjust the size of the aperture through which the overlapping secondary charged particle beams can pass.

10. 10. The beam-limiting aperture array of claim 9, wherein the x-axis and the y-axis are perpendicular to a secondary optical axis of an electro-optical system in which the beam-limiting aperture array is provided.

11. In a case where the plurality of secondary charged particle beams overlap each other to generate a crossover area on a crossover plane perpendicular to the secondary optical axis, 11. The beam-limiting aperture array of claim 10, wherein the beam-limiting aperture array is positioned at or within a range of positions of the crossover plane and perpendicular to the secondary optical axis.

12. 12. The beam-limiting aperture array of claim 11 configured to move along a z-axis parallel to the secondary optical axis to coincide with the position of the crossover surface.

13. 10. The beam-limiting aperture array of claim 1, wherein two or more of said apertures are similar in size.

14. a charged particle source configured to emit primary charged particles to form a primary charged particle beam; a lens configured to focus a plurality of beamlets of the primary charged particle beam onto a sample; a detector configured to detect a secondary charged particle beam emitted from the sample in response to the plurality of beamlets interacting with the sample; a beam-limiting aperture array having a circular aperture configured to reduce the secondary charged particle beam; the apertures include a central aperture and a plurality of off-center apertures radially arranged about the central aperture; a size of one of the plurality of off-center apertures is different from a size of another of the plurality of off-center apertures; Electro-optical system.

15. A non-transitory computer-readable medium storing instructions for performing a method, comprising: The method comprises: sending a signal causing a charged particle source to emit primary charged particles to form a primary charged particle beam; sending a signal causing a lens to focus a plurality of beamlets of the primary charged particle beam onto a sample; receiving data from a detector that detects a secondary charged particle beam emitted from the sample in response to the beamlets interacting with the sample; sending a signal to position a beam-limiting aperture array having a circular aperture for reducing the secondary charged particle beam; the apertures include a central aperture and a plurality of off-center apertures radially arranged about the central aperture; A non-transitory computer readable medium, wherein a size of one of the plurality of off-center apertures is different from a size of another one of the plurality of off-center apertures.

Images