Direct aberration retrieval for charged particle apparatus

Abstract

A system and method for aberration detection in charged particle beam systems such as scanning electron microscopes. The method uses spatial information of the incidence locations of charged particle arrival events on a detector surface to determine characteristics of the incident beam on a sample. By irradiating a flat, open surface area on a sample, beam parameters such as beam tilt, 5 defocus, spherical aberration, chromatic aberration, coma, diffraction error, astigmatism, field curvature, distortion and others may have an identifiable relationship with the resulting spatial distribution that may be used to identify and correct beam aberrations.

Description

DIRECT ABERRATION RETRIEVAL FOR CHARGED PARTICLE APPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 514,543 which was filed on July 19, 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The description herein relates to charged particle beam systems, and more particularly, to systems and methods for determining beam aberrations in such charged particle beam systems.BACKGROUND

[0003] Detectors may be used for sensing physically observable phenomena. For example, charged particle beam tools, such as electron microscopes, may comprise detectors that receive charged particles projected from a sample and that output detection signals. Detection signals may be used to reconstruct images of sample structures under inspection and may be used, for example, in metrology processes or to reveal defects in the sample. Metrology relates to precision measurements of sample structures and other miniaturized features. For example, in a semiconductor wafer, metrology may include measurements of circuit pattern features such as critical dimension (width of the smallest device feature), critical dimension uniformity, linewidth, overlay, line edge roughness, line end shortening, floor tilt, sidewall angle, and other dimensional parameters. Detection of defects in a sample is also increasingly important in the manufacturing of semiconductor devices, which may include large numbers of densely packed, miniaturized integrated circuit (IC) components. Inspection systems may be provided for these and other purposes.

[0004] With continuing miniaturization of semiconductor devices, performance demands for inspection systems may continue to increase. To ensure optimal imaging performance, it is important to measure the parameters of a charged particle beam with a high degree of accuracy, consistency, and speed. For example, beam aberrations such as beam tilt, defocus, spherical aberration, chromatic aberration, coma, diffraction error, astigmatism, field curvature, distortion and others, must be determined in order to correct such aberrations and maintain high performance imaging.SUMMARY

[0005] Embodiments of the present disclosure provide systems and methods for determining beam aberrations in a charged particle beam system. Some embodiments of the present disclosure provide a charged particle beam method. The method may comprise: irradiating a surface with a charged particle beam using a charged particle beam system; obtaining spatial distribution information of detected charged particle arrivals on a charged particle detector; determining an aberration value ofthe charged particle beam based on the spatial distribution information; and performing an adjustment to the charged particle beam system based on the determined aberration value.

[0006] Some embodiments of the present disclosure provide a charged particle beam apparatus. The charged particle beam apparatus may comprise: a charged particle beam source configured to generate a beam of primary charged particles; a charged particle optical system configured to direct the beam of primary charged particles at a surface; and a controller comprising one or more processors and configured to cause the charged particle beam apparatus to perform operations comprising the method steps above.

[0007] Some embodiments of the present disclosure provide a non-transitory computer-readable medium storing a set of instructions. The set of instructions may be executable by at least one processor of a device to cause the device to perform operations comprising the method steps above.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The above and other aspects of the present disclosure will become more apparent from the description of exemplary embodiments, taken in conjunction with the accompanying drawings.

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

[0010] Figs. 2A-C are diagrams illustrating charged particle beam apparatus that may be examples of an electron beam tool, consistent with embodiments of the present disclosure.

[0011] Fig. 2D illustrates an electron arrival distribution on an exemplary detector in a charged particle beam apparatus, consistent with embodiments of the present disclosure.

[0012] Figs. 3A-B are diagrammatic representations of exemplary structures of a detector in a charged particle beam apparatus, consistent with embodiments of the present disclosure.

[0013] Figs. 4A-F illustrate example spatial distribution information of electron arrival locations on a pixelated detector surface, consistent with embodiments of the present disclosure.

[0014] Fig. 5 illustrates example spatial distribution information of electron arrival locations on a segmented detector surface, consistent with embodiments of the present disclosure .

[0015] Fig. 6A illustrates a flowchart of an example method of determining beam aberrations in a charged particle beam detection process, consistent with embodiments of the present disclosure.

[0016] Figs. 6B-C illustrate flowcharts of example methods of determining reference information for use in an aberration measurement in a charged particle beam detection process, consistent with embodiments of the present disclosure

[0017] Fig. 7 illustrates a flowchart of an example method of determining beam aberrations in a charged particle beam detection process, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0018] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations consistent with the invention. Instead, they are merely examples of apparatuses, systems, and methods consistent with aspects related to subject matter that may be recited in the appended claims. For example, although some embodiments are described in the context of utilizing charged-particle beams (e.g., electron beams), the disclosure is not so limited. Other types of charged particle beams (e.g., proton beams) may be similarly applied. Furthermore, other imaging systems may be used, such as optical imaging, photodetection, x-ray detection, or the like.

[0019] Electronic devices are constructed of circuits formed on a piece of silicon called a substrate. Many circuits may be formed together on the same piece of silicon and are called integrated circuits or ICs. With advancements in technology, the size of these circuits has decreased dramatically so that many more of them can fit on the substrate. For example, an IC chip in a smart phone can be as small as a fingernail and yet may include over 2 billion transistors, the size of each transistor being less than 1 / 1, 000th the width of a human hair.

[0020] Making these ICs with extremely small structures or components is a complex, timeconsuming, and expensive process, often involving hundreds of individual steps. Errors in even one step have the potential to result in defects in the finished IC, rendering it useless. Thus, one goal of the manufacturing process is to avoid such defects to maximize the number of functional ICs made in the process, that is, to improve the overall yield of the process.

[0021] One component of improving yield is monitoring the chip making process to ensure that it is producing a sufficient number of functional integrated circuits. One way to monitor the process is to inspect the chip circuit structures at various stages of their formation. Inspection can be carried out using a scanning charged-particle microscope, such as a scanning electron microscope (SEM). A SEM can be used to image these extremely small structures, in effect, taking a “picture” of the structures. The image can be used to determine if the structure was formed properly, and also if it was formed in the proper location. If the structure is defective, then the process can be adjusted so the defect is less likely to recur. To enhance throughput (e.g., the number of samples processed per hour), it is desirable to conduct inspection as quickly as possible.

[0022] The working principle of a SEM is similar to a camera. A camera takes a picture by receiving and recording intensity of light reflected or emitted from people or objects, pixel by pixel. A SEM takes a “picture” by receiving and recording energies or quantities of electrons reflected or emitted from the structures of the wafer. Before taking such a “picture,” an electron beam may be projected onto the structures, and when the electrons are reflected or emitted (“exiting”) from the structures (e.g., from the wafer surface, from the structures underneath the wafer surface, or both), a detector ofthe SEM may receive and record the energies or quantities of those electrons to generate an inspection image. To take such a “picture,” the electron beam may scan through the wafer (e.g., in a line-by-line or zig-zag manner), and the detector may receive exiting electrons coming from a region under electron-beam projection (referred to as a “beam spot”). The detector may receive and record exiting electrons from each beam spot one at a time and join the information recorded for all the beam spots to generate the inspection image. Some SEMs use a single electron beam (referred to as a “singlebeam SEM”) to take a single “picture” to generate the inspection image, while some SEMs use multiple electron beams (referred to as a “multi-beam SEM”) to take multiple “sub-pictures” of the wafer in parallel and, in some instances, stitch them together to generate the inspection image. By using multiple electron beams, the SEM may provide more electron beams onto the structures for obtaining these multiple “sub-pictures,” resulting in more electrons exiting from the structures. Accordingly, the detector may receive more exiting electrons simultaneously and generate inspection images of the structures of the wafer with higher efficiency and faster speed.

[0023] In some inspection processes, such as critical dimension (CD) metrology, a charged particle image may be used to measure widths of the smallest features printed on the sample to determine how well the printed pattern meets design specifications. For instance, an electron beam may be scanned across the surface topography of the pattern, such as a set of repeating lines and spaces, taking pixel- by-pixel intensity measurements as it goes. Each line may have a substantially flat, raised upper surface. The lower spaces between each line may also be relatively flat, while the transition between line and space may comprise a sharp vertical line edge. In the electron beam image, the pixel intensities at these edge transitions may contrast sharply with the flat regions around them, providing a means of measuring the widths of these features.

[0024] To ensure high-quality imaging, various characteristics of an electron beam must be measured and adjusted. For instance, the spread of angles at which electrons in the beam strike a sample, the different energies of electrons in the beam, the cross-sectional shape of the beam, the focal point(s) of the beam, etc. can all become distorted or offset from their desired values in complex ways. These offsets and distortions of the beam are commonly referred to as aberrations. Typical low-order aberrations may include, e.g., focus, beam tilt, distortion or astigmatism. In general, any non-ideality of, e.g., the intended position, angle or energy of the electron on the specimen may be considered to be caused by an aberration. Because even small aberration values can degrade the quality of an electron beam image, it is critical to measure them and make appropriate corrections.

[0025] One problem with existing methods for aberration measurement is that they can be very timeconsuming. For example, a measurement process may require taking a series of test images of a test pattern. The test pattern may be located on a test wafer or on a surface of the chuck supporting the wafer. The test pattern may have a number of different micro- or nano-scale test structures formed on it, and may be designed to be sensitive to different types of aberrations. The electron beam system may scan the test pattern at several different focus level settings, or at a variety of other settings of theelectron beam optics. For instance, the electron beam system may scan the test pattern at several different landing energies to gain information about, e.g., chromatic aberration. The resulting images may be inspected to determine the various aberration values that may be present in the electron beam system and perform adjustments. Because electron beam scanning is very time-consuming, valuable production time is lost to this monitoring process. For example, obtaining a single high-resolution defect inspection image of a full 300 mm wafer may take, e.g., 20 hours or more at 12.5 million pixels per second. Taking time to measure test regions through focus lowers throughput in this process.

[0026] For example, measuring focus may require taking a series of, e.g., ten images at different focus levels over each of a plurality of test locations. While each series of images may require time on the order of e.g., seconds, there may be a number of test locations over a wafer and the measurement may be performed frequently. Thus the total interruption in scanning time is harmful in the aggregate. Other aberration measurements may take longer. For example, to characterize stigmation in an electron beam system, a plurality of images may be taken at a plurality of focus and stigmation values. Although typically performed less frequently than focus measurements (e.g., on the order of hours) the measurements may take longer, such as, e.g., several minutes.

[0027] A particularly difficult problem in conventional aberration measurement involves more complex measurements, such as determining distortions and troubleshooting in the electron beam tools. Distortion monitoring may require, e.g., hundreds of images, and troubleshooting can cause a machine to be taken offline for days.

[0028] A further problem with existing methods for aberration measurement is that they are not performed under real-world conditions. For example, the act of relocating a scanning beam to a specific location for testing requires additional time, and introduces the risk that an aberration state at the test region may differ from that of the actual inspection locations. Further, as mentioned above, the test patterns may be scanned under a series of different focus settings rather than the settings that would be applied during an actual imaging inspection. Because the beam is measured under different optics settings than what it uses in operation, it is difficult to accurately predict what the true beam aberrations will be.

[0029] Embodiments of the present disclosure provide systems and methods for determining beam aberrations in a charged particle beam system, such as a scanning electron microscope. Rather than obtaining a series of test images at various focus settings and analyzing those images, the system may use information of about the spatial distribution of electrons that are emitted from the sample to the detector surface. It has been found that the sizes, shapes and other characteristics of these electron distributions have a direct relationship with the aberration values of the incoming electron beam. By way of example, an ideal electron beam might strike a wafer and produce a stream of emitted electrons that land on the detector with a symmetrical circular distribution. On the other hand, an aberrated beam might produce a stream of emitted electrons that yields a distorted or asymmetrical distribution. By mapping the distribution shape to the corresponding beam characteristics, aberrationvalues may be measured much faster, and even be performed in real time during a normal scanning operation. Furthermore, the beam may be measured under real-world beam settings, leading to greater accuracy and improved electron imaging. The time required for measuring distortions, troubleshooting, and other time-consuming tasks can be dramatically reduced.

[0030] In some conventional systems, it may not matter where on a detector surface an emitted electron lands. For any given image pixel, only the total intensity (determined by the total number / energy of electrons) from the detector is recorded. Therefore, not all detectors may be configured to perform the disclosed method because they cannot tell where on the surface an electron has landed. However, by using a detector surface that is capable of distinguishing the different electron landing locations, the spatial distribution of electrons on the detector can be determined.

[0031] Objects and advantages of the disclosure may be realized by the elements and combinations as set forth in the embodiments discussed herein. However, embodiments of the present disclosure are not necessarily required to achieve such exemplary objects or advantages, and some embodiments may not achieve any of the stated objects or advantages.

[0032] As used herein, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, if it is stated that a component includes A or B, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or A and B. As a second example, if it is stated that a component includes A, B, or C, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

[0033] Reference is now made to Fig- 1, which illustrates an exemplary electron beam inspection (EBI) system 10 that may be used for wafer inspection, consistent with embodiments of the present disclosure. As shown in Fig. 1, EBI system 10 includes a main chamber I l a load / lock chamber 20, an electron beam tool 100 (e.g., a scanning electron microscope (SEM)), and an equipment front end module (EFEM) 30. Electron beam tool 100 is located within main chamber 11 and may be used for imaging. EFEM 30 includes a first loading port 30a and a second loading port 30b. EFEM 30 may include additional loading ports. First loading port 30a and second loading port 30b receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other materials) or samples to be inspected (wafers and samples may be collectively referred to as “wafers” herein).

[0034] One or more robotic arms (not shown) in EFEM 30 may transport the wafers to load / lock chamber 20. Load / lock chamber 20 is connected to a load / lock vacuum pump system (not shown) which removes gas molecules in load / lock chamber 20 to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robotic arms (not shown) may transport the wafer from load / lock chamber 20 to main chamber 11. Main chamber 11 is connected to a main chamber vacuum pump system (not shown) which removes gas molecules in main chamber 11 to reach a second pressure below the first pressure. After reaching the second pressure, the wafer issubject to inspection by electron beam tool 100. Electron beam tool 100 may be a single-beam system or a multi-beam system. A controller 109 is electronically connected to electron beam tool 100, and may be electronically connected to other components as well. Controller 109 may be a computer configured to execute various controls of EBI system 10. While controller 109 is shown in Fig. 1 as being outside of the structure that includes main chamber 11, load / lock chamber 20, and EFEM 30, it is appreciated that controller 109 can be part of the structure.

[0035] In some embodiments, controller 109 may include one or more processors (not shown). A processor may be a generic or specific electronic device capable of manipulating or processing information. For example, the processor may include any combination of any number of a central processing unit (or “CPU”), a graphics processing unit (or “GPU”), an optical processor, a programmable logic controllers, a microcontroller, a microprocessor, a digital signal processor, an intellectual property (IP) core, a Programmable Logic Array (PLA), a Programmable Array Logic (PAL), a Generic Array Logic (GAL), a Complex Programmable Logic Device (CPLD), a Field- Programmable Gate Array (FPGA), a System On Chip (SoC), an Application-Specific Integrated Circuit (ASIC), and any type circuit capable of data processing. The processor may also be a virtual processor that includes one or more processors distributed across multiple machines or devices coupled via a network.

[0036] In some embodiments, controller 109 may further include one or more memories (not shown). A memory may be a generic or specific electronic device capable of storing codes and data accessible by the processor (e.g., via a bus). For example, the memory may include any combination of any number of a random-access memory (RAM), a read-only memory (ROM), an optical disc, a magnetic disk, a hard drive, a solid-state drive, a flash drive, a security digital (SD) card, a memory stick, a compact flash (CF) card, or any type of storage device. The codes and data may include an operating system (OS) and one or more application programs (or “apps”) for specific tasks. The memory may also be a virtual memory that includes one or more memories distributed across multiple machines or devices coupled via a network.

[0037] A charged particle beam microscope, such as that formed by or which may be included in EBI system 10, may be capable of resolution down to, e.g., the nanometer scale, and may serve as a practical tool for inspecting IC components on wafers. With an e-beam system, electrons of a primary electron beam may be focused at probe spots on a wafer under inspection. The interactions of the primary electrons with the wafer may result in secondary particle beams being formed. The secondary particle beams may comprise backscattered electrons, secondary electrons, or Auger electrons, etc. resulting from the interactions of the primary electrons with the wafer. Characteristics of the secondary particle beams (e.g., intensity) may vary based on the properties of the internal or external structures or materials of the wafer, and thus may indicate whether the wafer includes defects.

[0038] The intensity of the secondary particle beams may be determined using a detector. The secondary particle beams may form beam spots on a surface of the detector. The detector maygenerate electrical signals (e.g., a current, a charge, a voltage, etc.) that represent intensity of the detected secondary particle beams. The electrical signals may be measured with measurement circuitries which may include further components (e.g., analog-to-digital converters) to obtain a distribution of the detected electrons. The electron distribution data collected during a detection time window, in combination with corresponding scan path data of the primary electron beam incident on the wafer surface, may be used to reconstruct images of the wafer structures or materials under inspection. The reconstructed images may be used to reveal various features of the internal or external structures or materials of the wafer and may be used to reveal defects that may exist in the wafer. There are two ways for CD-SEM to perform image reconstruction based on the signal: based on amplitude integral of the signal in each scanning pixel; or based on signal pulse edge detection and discrimination. At each scanned pixel location, multiple electrical signal pulses collected by different detector segments may have different pulse shapes. These various pulse shapes contain information about the electron energy distribution collected by the detector. Electrons with higher energy can cause faster rising times in the electrical pulse shape. With multiple segments of the detector (4 channels), up to 4 pulse edge detection image channels can be used to perform electron energy analysis, yielding higher SEM resolution.

[0039] Fig. 2A illustrates a charged particle beam apparatus that may be an example of electron beam tool 100, consistent with embodiments of the present disclosure. Fig. 2A shows an apparatus that uses a plurality of beamlets formed from a primary electron beam to simultaneously scan multiple locations on a wafer.

[0040] As shown in Fig. 2A, electron beam tool 100A may comprise an electron source 202, a gun aperture 204, a condenser lens 206, a primary electron beam 210 emitted from electron source 202, a source conversion unit 212, a plurality of beamlets 214, 216, and 218 of primary electron beam 210, a primary projection optical system 220, a wafer stage (not shown in Fig. 2A), multiple secondary electron beams 236, 238, and 240, a secondary optical system 242, and electron detection device 244. Electron source 202 may generate primary particles, such as electrons of primary electron beam 210. A controller, image processing system, and the like may be coupled to electron detection device 244. Primary projection optical system 220 may comprise beam separator 222, deflection scanning unit 226, and objective lens 228. Electron detection device 244 may comprise detection sub-regions 246, 248, and 250.

[0041] Electron source 202, gun aperture 204, condenser lens 206, source conversion unit 212, beam separator 222, deflection scanning unit 226, and objective lens 228 may be aligned with a primary optical axis 260 of electron beam tool 100A. Secondary optical system 242 and electron detection device 244 may be aligned with a secondary optical axis 252 of electron beam tool 100 A.

[0042] Electron source 202 may comprise a cathode, an extractor or an anode, wherein primary electrons can be emitted from the cathode and extracted or accelerated to form a primary electron beam 210 with a crossover (virtual or real) 208. Primary electron beam 210 can be visualized as beingemitted from crossover 208. Gun aperture 204 may block off peripheral electrons of primary electron beam 210 to reduce size of probe spots 270, 272, and 274.

[0043] Source conversion unit 212 may comprise an array of image-forming elements (not shown in Fig. 2A) and an array of beam-limit apertures (not shown in Fig. 2A). An example of source conversion unit 212 may be found in U.S. Patent No 9,691,586; U.S. Publication No. 2017 / 0025243; and International Application No. PCT / EP2017 / 084429, all of which are incorporated by reference in their entireties. The array of image-forming elements may comprise an array of micro-deflectors or micro-lenses. The array of image-forming elements may form a plurality of parallel images (virtual or real) of crossover 208 with a plurality of beamlets 214, 216, and 218 of primary electron beam 210. The array of beam-limit apertures may limit the plurality of beamlets 214, 216, and 218.

[0044] Condenser lens 206 may focus primary electron beam 210. The electric currents of beamlets 214, 216, and 218 downstream of source conversion unit 212 may be varied by adjusting the focusing power of condenser lens 206 or by changing the radial sizes of the corresponding beam-limit apertures within the array of beam- limit apertures. Condenser lens 206 may be an adjustable condenser lens that may be configured so that the position of its first principle plane is movable. The adjustable condenser lens may be configured to be magnetic, which may result in off-axis beamlets 216 and 218 landing on the beamlet-limit apertures with rotation angles. The rotation angles change with the focusing power and the position of the first principal plane of the adjustable condenser lens. In some embodiments, the adjustable condenser lens may be an adjustable anti-rotation condenser lens, which involves an anti-rotation lens with a movable first principal plane. An example of an adjustable condenser lens is further described in U.S. Publication No. 2017 / 0025241, which is incorporated by reference in its entirety.

[0045] Objective lens 228 may focus beamlets 214, 216, and 218 onto a wafer 230 for inspection and may form a plurality of probe spots 270, 272, and 274 on the surface of wafer 230. Secondary electron beamlets 236, 238, and 240 may be formed that are emitted from wafer 230 and travel back toward beam separator 222.

[0046] Beam separator 222 may be a beam separator of Wien filter type generating an electrostatic dipole field and a magnetic dipole field. In some embodiments, if they are applied, the force exerted by electrostatic dipole field on an electron of beamlets 214, 216, and 218 may be equal in magnitude and opposite in direction to the force exerted on the electron by magnetic dipole field. Beamlets 214, 216, and 218 can therefore pass straight through beam separator 222 with zero deflection angle. However, the total dispersion of beamlets 214, 216, and 218 generated by beam separator 222 may also be non-zero. Beam separator 222 may separate secondary electron beams 236, 238, and 240 from beamlets 214, 216, and 218 and direct secondary electron beams 236, 238, and 240 towards secondary optical system 242.

[0047] Deflection scanning unit 226 may deflect beamlets 214, 216, and 218 to scan probe spots 270, 272, and 274 over an area on a surface of wafer 230. In response to incidence of beamlets 214, 216,and 218 at probe spots 270, 272, and 274, secondary electron beams 236, 238, and 240 may be emitted from wafer 230. Secondary electron beams 236, 238, and 240 may comprise electrons with a distribution of energies including secondary electrons and backscattered electrons. Secondary optical system 242 may focus secondary electron beams 236, 238, and 240 onto detection sub-regions 246, 248, and 250 of electron detection device 244. Detection sub-regions 246, 248, and 250 may be configured to detect corresponding secondary electron beams 236, 238, and 240 and generate corresponding signals used to reconstruct an image of the surface of wafer 230.

[0048] The generated signals may represent intensities of secondary electron beams 236, 238, and 240 and may be provided to an image processing system (e.g. such as image processing system 199 provided in Fig. 2B below) that is in communication with detection device 244, primary projection optical system 220, and motorized wafer stage. The movement speed of motorized wafer stage may be synchronized and coordinated with the beam deflections controlled by deflection scanning unit 226, such that the movement of the scan probe spots (e.g., scan probe spots 270, 272, and 274) may orderly cover regions of interests on the wafer 230. The parameters of such synchronization and coordination may be adjusted to adapt to different materials of wafer 230. For example, different materials of wafer 230 may have different resistance-capacitance characteristics that may cause different signal sensitivities to the movement of the scan probe spots.

[0049] The intensity of secondary electron beams 236, 238, and 240 may vary according to the external or internal structure of wafer 230, and thus may indicate whether wafer 230 includes defects. Moreover, as discussed above, beamlets 214, 216, and 218 may be projected onto different locations of the top surface of wafer 230, or different sides of local structures of wafer 230, to generate secondary electron beams 236, 238, and 240 that may have different intensities. Therefore, by mapping the intensity of secondary electron beams 236, 238, and 240 with the areas of wafer 230, the image processing system may reconstruct an image that reflects the characteristics of internal or external structures of wafer 230.

[0050] Detection sub-regions 246, 248, and 250 may include separate detector packages, separate sensing elements, or separate regions of an array detector. In some embodiments, each detection subregion may include a single sensing element.

[0051] While Fig. 2A shows detector 244 having several detection sub-regions aligned with secondary optical axis 252, it is appreciated that other multi-beam detector schemes may exist. For example, it is appreciated that a detector may correspond with each beamlet, such as a different detector for each of beamlets 214, 216, 218. It is appreciated that these different detectors may be positioned under the primary column corresponding to primary axis 260. For example, these different detectors could be positioned between primary projection optical system 220 and the wafer stage.

[0052] Another example of a charged particle beam apparatus will now be discussed with reference to Fig. 2B. An electron beam tool 200B (also referred to herein as charged particle apparatus 200B) may be an example of electron beam tool 100 and may be similar to electron beam tool 100A shownin Fig. 2A. However, different from electron beam tool 100 A, electron beam tool 200B may be a single-beam tool that uses only one primary electron beam to scan one location on the wafer at a time.

[0053] As shown in Fig. 2B, apparatus 200B includes a wafer holder 136 supported by motorized stage 134 to hold a wafer 150 to be inspected. Electron beam tool 200B includes an electron emitter, which may comprise a cathode 103, an anode 121, and a gun aperture 122. Electron beam tool 200B may further include a beam limit aperture 125, a condenser lens 126, a column aperture 135, an objective lens assembly 132, and a detector 144. Objective lens assembly 132, in some embodiments, may be a modified SORIL lens, which includes a pole piece 132a, a control electrode 132b, a deflector 132c, and an exciting coil 132d. In a detection or imaging process, an electron beam 161 emanating from the tip of cathode 103 may be accelerated by anode 121 voltage, pass through gun aperture 122, beam limit aperture 125, condenser lens 126, and be focused into a probe spot 170 by the modified SORIL lens and impinge onto the surface of wafer 150. Probe spot 170 may be scanned across the surface of wafer 150 by a deflector, such as deflector 132c or other deflectors in the SORIL lens. Secondary or scattered particles, such as secondary electrons or scattered primary electrons emanated from the wafer surface may be collected by detector 144 to determine intensity of the beam and so that an image of an area of interest on wafer 150 may be reconstructed.

[0054] In some embodiments, wafer holder 136 or stage 134 may comprise a measurement section 146. An upper surface of measurement section 146 may be substantially flush with an upper surface of wafer 150. Measurement section 146 may comprise fiducials or other test patterns for measuring, e.g., alignment, focus or beam aberrations. In some embodiments, as further discussed below, measurement section 146 may include high-flatness test surface configured to provide an ideal irradiation surface for detecting aberrations according to systems and methods of the present disclosure. For example, measurement section 146 may comprise a high-flatness portion having a layer of, e.g., a heavy metal such as silver, gold, titanium, cobalt, copper, ruthenium or another high atomic number (Z) material. Such a material may be configured to yield a large number of backscattered electrons, thus improving a signal to noise ratio of the measurement. Measurement section 146 may be coupled to a power supply configured to apply a desired electric potential to the test surface to control the emission properties of emitted electrons.

[0055] There may also be provided an image processing system 199 that includes an image acquirer 120, a storage 130, and controller 109. Image acquirer 120 may comprise one or more processors. For example, image acquirer 120 may comprise a computer, server, mainframe host, terminals, personal computer, any kind of mobile computing devices, and the like, or a combination thereof. Image acquirer 120 may be communicatively coupled with detector 144 of electron beam tool 200B through a medium such as an electrical conductor, optical fiber cable, portable storage media, IR, Bluetooth, internet, wireless network, wireless radio, or a combination thereof. Image acquirer 120 may receive a signal from detector 144 and may construct an image. Image acquirer 120 may thus acquire images of wafer 150. Image acquirer 120 may also perform various post-processing functions, such as imageaveraging, generating contours, superimposing indicators on an acquired image, and the like. Image acquirer 120 may be configured to perform adjustments of brightness and contrast, etc. of acquired images. Storage 130 may be a storage medium such as a hard disk, random access memory (RAM), cloud storage, other types of computer readable memory, and the like. Storage 130 may be coupled with image acquirer 120 and may be used for saving scanned raw image data as original images, and post-processed images. Image acquirer 120 and storage 130 may be connected to controller 109. In some embodiments, image acquirer 120, storage 130, and controller 109 may be integrated together as one electronic control unit.

[0056] In some embodiments, image acquirer 120 may acquire one or more images of a sample based on an imaging signal received from detector 144. An imaging signal may correspond to a scanning operation for conducting charged particle imaging. An acquired image may be a single image comprising a plurality of imaging areas that may contain various features of wafer 150. The single image may be stored in storage 130. Imaging may be performed on the basis of imaging frames.

[0057] The condenser and illumination optics of the electron beam tool may comprise or be supplemented by electromagnetic quadrupole electron lenses. For example, as shown in Fig. 2B, electron beam tool 200B may comprise a first quadrupole lens 148 and a second quadrupole lens 158. In some embodiments, the quadrupole lenses may be used for controlling the electron beam. For example, first quadrupole lens 148 may be controlled to adjust the beam current and second quadrupole lens 158 may be controlled to adjust the beam spot size and beam shape.

[0058] Fig. 2B illustrates a charged particle beam apparatus that may use a single primary beam configured to generate secondary electrons by interacting with wafer 150. Detector 144 may be placed along optical axis 105, as in the embodiment shown in Fig. 2B. The primary electron beam may be configured to travel along optical axis 105. Accordingly, detector 144 may include a hole at its center so that the primary electron beam may pass through to reach wafer 150. Fig. 2B shows an example of detector 144 having an opening at its center. However, some embodiments may use a detector placed off-axis relative to the optical axis along which the primary electron beam travels. For example, as in the embodiment shown in Fig. 2A, discussed above, a beam separator 222 may be provided to direct secondary electron beams toward a detector placed off-axis. Beam separator 222 may be configured to divert secondary electron beams by an angle a toward an electron detection device 244, as shown in Fig. 2A.

[0059] In some embodiments of the disclosure, a PIN detector may be used as an in-lens detector in a retarding objective lens SEM column of EBI system 10. The PIN detector may be placed between a cathode for generating an electron beam and the objective lens. The electron beam emitted from the cathode may be potentialized at -BE keV (typically around -10 kV). Electrons of the electron beam may be immediately accelerated and travel through the column. The column may be at ground potential. Thus, electrons may travel with kinetic energy of BE keV while passing through the opening of detector 144. Electrons passing through the pole piece of the objective lens, such as polepiece 132a of objective lens assembly 132 of Fig. 2B, may be steeply decelerated down to landing energy LE keV as the wafer surface potential may be set at -(BE - LE) keV.

[0060] Fig. 2C illustrates an example of a charged particle beam apparatus 200C, consistent with embodiments of the present disclosure. Charged particle beam apparatus 200C may be, e.g., charged particle beam apparatus 200A of Fig. 2A or 200B of Fig. 2B. Emitted electrons 171, comprising e.g., secondary or backscattered electrons, are emitted from the wafer surface by the impingement of electrons of the primary electron beam 105. A retarding electric field, which may slow the primary electrons as they approach probe spot 170, may act as an acceleration electric field to accelerate the emitted electrons backwards toward a detector 144 surface. For example, as shown in Fig. 2C, due to interactions with wafer 150 at probe spot 170, emitted electrons 171 may be generated that travel back toward detector 144. Emitted electrons 171 from the wafer surface travelling along optical axis 105 may arrive at the surface of detector 144 with a distribution of positions that depends mainly upon the emission angles and energies of the emitted electrons, as well as a smaller influence from the precise emission positions within a probe spot. For example, secondary and backscattered electrons may be emitted from the area with an angular distribution that substantially follows Lambert’s cosine law. That is, the distribution of emission angles may be substantially proportional to the cosine of the angle of incidence of the incoming electron. This distribution is schematically illustrated by the length of arrows in the closeup view of probe spot 170.

[0061] Fig. 2D illustrates an example of an emitted electron arrival point distribution on a detector surface. Detector 144 may be 10 mm in diameter, or larger, for example. In some embodiments, a detector may be, e.g., about 4 to 10 mm in diameter. Electrons 171a may land at different points on the surface of detector 144 while, generally, most may be clustered around the central portion of detector 144 when there is no deflection field. The shape, size and position of a spatial distribution of the arrival positions on the detector surface may vary according to the emission angles and energies of the emitted electrons. These emission angles and energies may in turn depend on the incident angles and energies of incoming electrons from primary electron beam 105. For example, when a flat area is scanned at the center of a field of view at normal incidence by an ideal, unaberrated beam, the arrival positions of emitted electrons may form a substantially gaussian distribution on detector 144. For a beam exhibiting aberrations, the distribution may be different. Thus it may be possible under the right conditions to map the beam aberrations of primary electron beam 105 to the resulting spatial distribution of emitted electrons arriving at the surface of detector 144.

[0062] As discussed above, emitted electrons may comprise, e.g., secondary or backscattered electrons. In some embodiments, for example, a distribution may comprise between 60-85% secondary electrons and between 40-15% backscattered electrons. In some embodiments, it may be desirable to discriminate or filter out secondary and other electrons so that a detected distribution is composed primarily of backscattered electrons. Backscattered electrons result from the elastic scattering of incoming electrons by atomic nuclei in the irradiated sample. These backscatteredelectrons may be emitted from the sample with substantially the same energies as the incoming electrons from the primary electron beam. Because their energies can be related to the incoming energies, and their emission angles can be related to the electron incident angles using an approximately Lambertian distribution, the spatial distribution of backscattered electrons is particularly well-suited for obtaining information about the incoming beam. Therefore, in some embodiments the distribution may advantageously comprise up to, e.g., 70%, 80%, 90%, 95%, 99% or more backscattered electrons. For example, secondary electrons may be separated using, e.g., a Wien filter or by biasing the sample to suppress emission of secondary electrons. For example, a bias that decelerates the incoming electrons may reduce the degree of secondary electron generation relative to backscattering, while causing all emitted electrons to accelerate from the surface at higher energies. A bias that accelerates the electrons toward the surface, on the other hand, will attract the emitted electrons back to the surface. Therefore a bias may be applied with a value that retains many of the lower energy secondary electrons while allowing backscattered electrons to escape to the detector surface. Alternatively, the detection signals from secondary electron arrivals may be filtered out using, e.g., energy discrimination.

[0063] Detector 144 may be placed along optical axis 105. The primary electron beam may be configured to travel along optical axis 105. Accordingly, detector 144 may include a hole 145 at its center so that the primary electron beam may pass through to reach wafer 150. Figs. 2B-C show examples of a detector 144 having an opening at its center. However, some embodiments may use a detector placed off-axis relative to the optical axis along which the primary electron beam travels. For example, as in the example shown in Fig. 2A, a beam separator 222 may be provided to direct emitted electron beams toward a detector placed off-axis. Beam separator 222 may be configured to divert emitted electron beams by an angle a toward an electron detection device 244, as shown in Fig. 2A. Therefore, in some embodiments of the present disclosure, a detector may be provided that has no central opening.

[0064] Detectors 244 of Fig. 2A or 144 of Figs. 2B-D may include sensing elements such as diodes, or elements similar to diodes, that may convert incident energy into a measurable signal. For example, sensing elements in a detector may include a SPAD, APD, scintillator, or PIN diode. Throughout this disclosure, sensing elements may be represented as a diode, although sensing elements or other components may deviate from ideal circuit behavior of electrical elements such as diodes, resistors, capacitors, etc. In embodiments of the present disclosure, a detector in a charged particle beam system may comprise a pixelated array of multiple sensing elements. In some embodiments, the sensing elements may be configured for charged particle counting. Pixelated sensing elements of a detector that may be useful for charged particle counting are discussed in U.S. Publication No. 2019 / 0378682, which is incorporated by reference in its entirety.

[0065] For ease of explanation without causing ambiguity, electrons are used as examples in some of the descriptions herein. However, it should be noted that any charged particle may be used in anyembodiment of this disclosure, not limited to electrons. For instance, a source in a charged-particle beam tool can emit one or more charged particles, such as electrons, protons, ions, muons, or any other particle carrying electric charges. Furthermore, some embodiments of the present disclosure may use photons instead of charged particles, such as light in the visible, UV, DUV, EUV, x-ray, or any other wavelength range. Therefore, while detectors in the present disclosure may be disclosed with respect to electron detection, some embodiments of the present disclosure may be directed to detecting other charged particles or photons.

[0066] Figs. 3A-B illustrate exemplary structures of a pixelated electron detector, consistent with embodiments of the present disclosure. A detector such as detector 344a of Fig. 3A or 344b of Fig. 3B may be provided as detector 244 as shown in Fig. 2A or 144 as shown in Figs. 2B-D. In Fig. 3A, detector 344a includes a sensor layer 301 and a signal processing layer 302. Sensor layer 301 may include a sensor die made up of multiple sensing elements, including sensing elements 311, 312, 313, and 314. In some embodiments, the multiple sensing elements may be provided in an array of sensing elements, each of which having a uniform size, shape, and arrangement.

[0067] Signal processing layer 302 may include multiple signal processing circuits, including circuits 321, 322, 323, and 324. The circuits may include interconnections (e.g., wiring paths) configured to communicatively couple or decouple the sensing elements to or from each other. Each sensing element of sensor layer 301 may have a corresponding signal processing circuit in signal processing layer 302. Sensing elements and their corresponding circuits may be configured to operate independently. As shown in Fig. 3A, circuits 321, 322, 323, and 324 may be configured to communicatively couple to outputs of sensing elements 311, 312, 313, and 314, respectively, as shown by the four dashed lines between sensor layer 301 and signal processing layer 302.

[0068] In some embodiments, signal processing layer 302 may be configured as a single die with multiple circuits provided thereon. Sensor layer 301 and signal processing layer 302 may be in direct contact. In some embodiments, components and functionality of different layers may be combined or omitted. For example, signal processing layer 302 may be combined with sensor layer 301 into a single layer. Furthermore, a circuit for charged particle counting may be integrated at various points in a detector, for example in a separate read-out layer of a detector or on a separate chip. Further details of electron counting circuitry and alternative structures for sensor layer 301 and signal processing layer 302 may be found in International Publication WO 2022 / 008518, the entirety of which is incorporated herein by reference.

[0069] As shown in Fig. 3B, a detector 344b having an array of sensing elements 311 may be provided. Detector 344b may be, e.g., a pixelated electron detector. In some embodiments, the pixelated electron detector may be a pixelated electron counting detector. Detector 344b may include sensor layer 301 and signal processing layer 302 as seen in Fig. 3A. Detector circuitry may be configured to separately detect electron arrivals from each sensing element individually. Detector344b may include a plate 351, wherein the plurality of sensing elements 311 are formed thereon. Plate 351 may include opening 345 for allowing a primary electron beam to pass through plate 351.

[0070] When an individual sensing element 311 of a detector is made small compared to the spatial distribution of emitted electrons incident on the detector, the spatial distribution may be mapped on the detector surface with sufficient resolution to gather useful information about the primary beam. For example, an individual sensing element may be a square or other shape having dimensions of, e.g.5, 10, 25, 50, 100, 200, or 300 pm on a side. Sensing elements may be larger, e.g., on the order of millimeters. A typical detector may comprise, e.g., 100, 500, 1,000, 5,000, 10,000, 50,000 or 100,000 or more sensing elements 311. The sensing elements may be configured for individual detection, or they may be coupled into groups. For example, when the pixel size of individual sensing elements 311 is much smaller than the required resolution for mapping a spatial distribution on detector 344b, sensing elements may be grouped together so that their signals may be processed as if they were one element. This may reduce a signal processing load while still achieving the goals of the present disclosure.

[0071] Embodiments of the present disclosure utilize information gathered from individual sensing elements or groups of sensing elements to characterize the properties of a primary beam. In particular, spatial distribution information of electron arrivals on the detector may be used to determine aberration values of the primary beam. In some embodiments of the present disclosure, the spatial distribution information may comprise, e.g., a shape, size, centroid location, or intensity map of the distribution of electron arrival events.

[0072] Figs. 4A-F illustrate example spatial distributions 475 of charged particle arrivals on a charged particle detector 444, consistent with embodiments of the present disclosure. Detector 444 may correspond to, e.g., detector 144 of Figs. 2A-2D, or detectors 344a or 344b of Figs 3A-3B. Detector 444 may be used in a charged particle apparatus, such as those described in Figs. 1-2C. The arrival events may be, e.g., emitted electron arrivals, and the charged particle detector may be an electron detector such as a pixelated electron detector. The distribution may correspond to a backscattered electron distribution.

[0073] Fig. 4A depicts an example spatial distribution 475 of emitted electrons on detector 444 under ideal conditions, consistent with embodiments of the present disclosure. For example, spatial distribution 475 may be a distribution of backscattered electrons produced by an electron beam that is free of aberrations. The electron beam may be irradiated onto a flat featureless portion of a test surface at a center of a field of view the electron beam apparatus in which it is used. Distribution 475 may be a substantially gaussian distribution that is substantially centered on the detector surface, where a substantially continuous intensity gradient of electron arrival signals received by detector 444 is schematically illustrated as high, medium, and low intensity signals respectively.

[0074] As illustrated in Fig. 4A, even an ideal electron beam may not yield a perfect spatial distribution 475, due to random variations in emission angle, etc. However, the distribution willgenerally converge toward the expected ideal shape with increasing electron collection. In general, the number of imaging frames collected to form spatial distribution 475 may be varied as a function of competing considerations, such the desired resolution of spatial distribution 475 and the desired amount of measurement time spent. For example, while a high resolution image of a pattern feature on a wafer may require the integration of, e.g., between 10 and 100 imaging frames or more, the spatial distribution 475 may be measured in some embodiments using as little as one imaging frame. In some embodiments, spatial distribution 475 may comprise more than one imaging frame, such as 5, 10, 15 or 20 frames or more. For example, spatial distribution 475 may comprise as many frames as are used to form an inspection image during an inspection process, or it may comprise even more. As further discussed below, in some embodiments the spatial distribution may be continuously or repeatedly collected while iterative feedback adjustments are performed to fine-tune the electron beam system in real time.

[0075] In some embodiments, the irradiated test surface may be a region of a sample under inspection, such as a semiconductor wafer surface. Because emitted electrons can be obstructed by neighboring pattern features, a test surface may be chosen that is located far from any pattern features or other structures. For example, if a test surface portion is located adjacent to an elevated line feature, electrons emitted from the test surface toward, e.g., a sidewall of the line feature may be absorbed by it, while electrons emitted away from the line feature may easily reach the detector surface. This would produce an asymmetry in the spatial distribution on detector 444 that is not representative of beam characteristics. Therefore, in some embodiments, an empty flat region such as a scribe lane, large flat structure, or a peripheral region of a wafer may be selected as the test surface. For example, the test surface may comprise a featureless portion of the sample that is at least as large as a field of view of the electron beam. In some embodiments, the test surface may comprise a comparatively large, flat pattern structure, such as a metrology target in a scribe lane or elsewhere. In some embodiments, in-die test surfaces may be selected within die regions of a wafer based on a determination that the region is sufficiently flat and empty, or that the region includes a sufficiently large, flat in-die structure. For example, this determination may be based on, e.g., a GDS file or other pattern data. Alternatively, the determination may be based on previous scans of the same area, or previous scans of the corresponding area in a similar die region. In some embodiments, the test surface may be a printed flat region on the wafer surface. For example, a test surface may comprise, e.g., a square patch having a deposited layer of high-Z material configured to yield a large number of backscattered electrons.

[0076] In some embodiments, the test surface may comprise an area on a support structure of the sample under inspection, such as measurement section 146 as discussed with respect to Fig. 2B above. Providing a test surface on a support structure may ensure a wide-area, high-flatness surface to eliminate some unwanted factors that may affect a spatial distribution on detector 444, such as wafer warp, elevation uncertainty, neighboring structures or suboptimal surface materials.

[0077] In some embodiments, the test surface may be a plane located just above a sample surface. For example, in some embodiments, a charged particle apparatus may be operated in “mirror mode.” In mirror mode, the surface of a sample may have an applied electric potential that is tuned precisely to prevent incoming charged particles from actually impacting the surface, instead being reflected back just before they reach it. In this way, the incoming electrons are reflected in a manner reminiscent of light being reflected from a mirror. An advantage of this mode is that it may eliminate the distribution noise inherent in the Lambertian distribution of emitted particles.

[0078] However, the divergence of such reflected beams may be very close to that of the original incoming beam, and therefore it may be difficult to achieve a distribution on the detector surface, with much of the beam passing back through the opening 445 from which the original incoming beam was passed. Nevertheless, such an arrangement may be beneficial for measuring, e.g., off-axis information such as the presence of distortions.

[0079] Fig. 4B depicts an example signal intensity map 476 on detector 444, consistent with embodiments of the present disclosure. Signal intensity map 476 may correspond to the spatial distribution 475 of Fig. 4A, and may represent spatial distribution information of spatial distribution 475. The intensity of electron arrival signals received by each pixel 411 on detector 444 is schematically illustrated as high, medium, and low intensity signals respectively. For example, a signal intensity at each pixel 411 may vary according to the number or energies of electrons arriving at that pixel. Signal intensity map 476 may represent a total intensity map of electron arrival events received at detector 444, or it may comprise, e.g., an energy-discriminated signal representing only backscattered electrons.

[0080] In some embodiments, signal intensity map 476 may be analyzed to determine geometric parameters of spatial distribution 475. Fig. 4C depicts an example boundary 477 in signal intensity map 476 on detector 444, consistent with embodiments of the present disclosure. Because an intensity gradient may taper off very gradually in a radial direction, and a small number of electron arrival signals may be dispersed over a large area of detector 444 due to false positives or random shot noise, there may be no precise geometric boundary of spatial distribution 475. Therefore, a boundary 477 may be determined for the purpose of defining, e.g., a shape, size, centroid location, etc., of spatial distribution 475. Further, boundary 477 may be used to simplify or optimize the analysis of intensity map 476 by considering only those pixels 411 within boundary 477. For example, a boundary may be drawn around a substantially continuous cluster of pixels having a signal intensity above a predetermined threshold. Alternatively, boundary 477 may be drawn around a predetermined percentage of total detected energy. For example, boundary 477 may be chosen to enclose, e.g., 60%, 80%, 90%, 95%, 99% or more of the energy detected at detector 444. In some embodiments, a boundary may be defined using a clustering algorithm or a contour-fitting algorithm. In some embodiments, no boundary 477 may be used, and instead a full signal intensity map 476 may be relied upon to determine beam properties.

[0081] Spatial distribution information, such as signal intensity map 476 or boundary 477, may be analyzed to determine the presence of aberrations, by, e.g., comparing the spatial distribution information to one or more reference distributions. For example, the spatial distribution of electron arrivals on detector 444 may be related to a spatial distribution of the primary beam in a pupil plane of the charged particle beam system. Thus, in some embodiments, the reference distribution may relate to an expected pupil plane distribution of the electron beam system, or to a model aberration of a pupil plane distribution of the electron beam system.

[0082] The expected pupil plane distribution may further account for other system parameters such as, e.g., lens settings downstream of a pupil plane, landing energy, field of view location and other parameters of the charged particle beam system. The expected distribution may further account for other effects as well, such as, e.g., conditions of the irradiated sample such as wafer topography and surface materials. For example, a number of fixed parameters associated with a charged particle beam system may affect a location and distribution of electron arrivals on a detector surface, such as, e.g., placement and settings of electrodes, landing energy and detector height. Deflection electrodes, focusing electrodes, or Wien filters may affect the location or shape of an electron distribution. Voltages on deflector electrodes can affect the divergence of the electrons as they move from a sample towards the detector. Focusing electrodes in a separate detector branch (such as 242 of Fig. 2A) may also affect location or shape. Wien filters may also be used to offset incident electrons from a detector center, e.g. to minimize loss of electron through the primary beam hole. Further, a geometric spread of electron arrivals may increase with increasing landing energy as well.Additionally, an increase in detector height may result in a greater spread of electrons for a given divergence value. In some embodiments, comparing a spatial distribution of electron arrivals to an expected pupil plane distribution may account for these factors.

[0083] In some embodiments, the one or more reference distributions may correspond to a set of model aberrations. In such a case, comparing the spatial distribution to the one or more reference distributions may comprise fitting the spatial distribution to a model aberration, or to a weighted combination of model aberrations. In some embodiments, the model aberrations may comprise a set of orthogonal or orthonormal functions such as, e.g., a set of functions that are orthogonal on a disk. For example, in some embodiments the model aberrations may comprise, e.g., a set of intensity distributions represented by Zernike polynomials.

[0084] Model aberrations may include one or more low-order aberrations, such as defocus, astigmatism, beam tilt, chromatic aberration, and beam shift. Model aberrations may further include higher-order aberrations such as, e.g., coma, pincushion or barrel distortion, spherical aberration, field curvature, and others. In general, model aberrations may represent any typical aberration or combination of aberrations such as, e.g., one or more first, second or third order aberrations, etc. Model aberrations may comprise both off- and on-axis aberrations. For example, a test surface may beirradiated at an off-axis location in the field of view of an electron beam system to yield an off-axis signal intensity map 476, which may be compared to off-axis model aberrations.

[0085] In some embodiments, the one or more reference distributions may correspond to a model or a lookup table of experimental or simulated reference distributions. The reference distributions may comprise, e.g., reference signal intensity maps or reference boundaries corresponding to signal intensity maps 476 and boundaries 477 as discussed above. For example, in some embodiments an electron beam system may irradiate a test surface under a series of different exposure settings. The settings may include, e.,g., focus settings, other lens settings, beam energy settings, field of view settings, etc. In some embodiments, the plurality of different exposure settings may be configured to produce a plurality of induced aberrations or combinations of induced aberrations. The obtained reference distributions may then be used for comparison in a later aberration measurement process to determine aberrations within the electron beam system.

[0086] In some embodiments, a model may be constructed to relate the spatial distribution in a detector plane to its corresponding pupil distribution. This may be done by directly measuring the pupil plane under a plurality of different exposure settings. For example, an electron beam system may be configured with a pupil detector located at or near the pupil plane, and the system may irradiate the pupil detector under the plurality of different exposure settings to yield a plurality of pupil distributions. The pupil detector may be configured to, e.g., face the charged particle source and directly receive charged particles at the pupil surface to directly measure the pupil distribution. Then with the pupil detector removed, the electron beam system may irradiate a test surface under the same plurality of different exposure settings to obtain a relationship between each spatial distribution and its corresponding pupil distribution. The obtained relationship may then be used for comparison in a later aberration measurement process to determine aberrations within the electron beam system.

[0087] Fig. 4D depicts an example model aberration 452, consistent with embodiments of the present disclosure. Model aberration 452 may comprise reference distribution 478, and may correspond to an ideal, unaberrated beam. When comparing, e.g., the idealized spatial distribution information of Figs. 4A-C with a set of model aberrations or other reference distributions, the idealized spatial distribution information of Figs. 4A-C may become closely identified with the unaberrated model 452. Therefore, it may be determined that for the electron beam used to produce spatial distribution 475 of Fig. 4A, aberration values are within acceptable limits.

[0088] Fig. 4E depicts a further example spatial distribution 475 of emitted electrons on detector 444, consistent with embodiments of the present disclosure. As seen in Fig. 4F, spatial distribution 475 remains centered and substantially symmetrical similar to the ideal version of Fig. 4A. However, the distribution is larger and more diffuse, with a smaller high intensity region. This distribution may result from a focusing error, in which a probe spot on the test surface is larger and more diffuse due to improper focus. Thus, the defocused spatial distribution of Fig. 4E may become identified with areference distribution corresponding to the focus error, and appropriate focus corrections may be determined and applied.

[0089] Fig. 4F depicts another example spatial distribution 475 of emitted electrons on detector 444, consistent with embodiments of the present disclosure. Here, the spatial distribution 475 may exhibit further aberrations, resulting in a lateral shift and skewed shape on the surface of detector 444. Spatial distribution information, such as an intensity map 476 or boundary 477 as discussed above, may be compared to a set of reference distributions. For example, the shift and skewed spatial distribution of Fig. 4F may become identified with a weighted combination of reference distributions corresponding to, e.g., astigmatism and beam tilt. Thus, appropriate aberration corrections may be determined and applied.

[0090] While the above embodiments are discussed with respect to high-resolution pixelated detectors, embodiments of the present disclosure are not limited to this. For example, in some embodiments, limited information may be obtained from low-resolution segmented detectors. Fig. 5 schematically illustrates a plurality of spatial distributions of electron arrival events on a plurality of segmented detectors, consistent with embodiments of the present disclosure. Here, three different classes of segmented detectors are depicted under two different representative distributions 575. The representative distributions 575 may correspond to, e.g., the symmetric and asymmetric spatial distributions of Figs. 4E and 4F to illustrate the capabilities and limitations of each segmented detector. Two-segment detector 580, for example, may be configured to discern low-resolution information about the asymmetry of a spatial distribution, but may not be capable of determining symmetrical errors such as defocus. A similar issue may occur with four-quadrant segmented detector 581, where a small amount of asymmetry information may be obtained in two dimensions, but symmetrical errors such as defocus may still not be identifiable. On the other hand, a segmented ring detector 582 may have the opposite problem. For example, in some embodiments, defocus may be roughly measured by comparing relative signal intensities from a plurality of concentric annular segments. However, beam shifts, deformations and other asymmetric errors may not be identifiable. Therefore, while some embodiments may utilize spatial distribution information to obtain beam information using simpler detector systems such as the segmented detectors 580-582, of Fig. 5, beam aberrations can be much more accurately determined using, e.g., pixelated electron detectors according to Figs. 4A-F.

[0091] Figs. 6A-C illustrate flowcharts of example methods 600, 650, 660 of determining aberrations in a charged particle beam using a charged particle detector, consistent with embodiments of the present disclosure. For example, method 600 of Fig. 6A may comprise a method of measuring aberrations, while methods 650 and 660 of Figs. 6B-C may comprise further steps of obtaining reference information for use in, e.g., method 600. The charged particle detector may be part of a charged particle beam apparatus. For example, the charged particle beam detector may be a pixelated electron detector in a SEM used for scanning inspection of a sample, such as a semiconductor wafer.The detector may be, e.g., detector 244 of Fig. 2A, detector 144 of Figs. 2B-D, detector 344a of Fig. 3A, detector 344b of Fig. 3B, detector 444 of Figs. 4A-F, or detectors 580-582 of Fig. 5.

[0092] Methods 600, 650, and 660 may be carried out by a processor and memory of a controller configured to control the charged particle beam apparatus to perform the method. Methods 650 and 660 may be carried out prior to an aberration measurement. With respect to actual aberration measurement of method 600, the method may be carried out periodically or on-demand during, e.g., high-volume production. For example, the method may be performed at regular time intervals such as, e.g., every 30 minutes, or after a predetermined exposure time or exposure dose has been irradiated by the beam during an inspection process. Further, the method may be performed with every wafer or after a predetermined number of wafers have been inspected, or at every die region, or after a predetermined number of die regions have been inspected. Further, the method may be performed on demand such as when, e.g., it is determined from an image analysis that aberrations have exceeded a predetermined threshold.

[0093] Finally, method 600 may be performed under any of these intervals “on the fly” during a charged particle beam inspection process. For example, a detector may be configured to selectively retain spatial distribution information of one or more imaging frames from a predetermined test surface. Thus, while a majority of imaging frames may not retain spatial distribution information (as the processing load of such captures may be prohibitively burdensome), a detector may be configured to retain such information when the beam scans over a location that is selected as a test surface. This may enable more frequent aberration monitoring without significantly harming throughput of charged particle beam inspection or placing any significant additional burden on the data processing load.

[0094] In Fig. 6A, at step 610, the controller may control a charged particle beam apparatus to irradiate a test surface with a charged particle beam. The test surface may comprise, e.g., a substantially flat, featureless area of a sample under inspection. Alternatively, the test surface may comprise, e.g., a flat measurement section located on a sample support stage.

[0095] At step 620, the controller may obtain spatial distribution information of detected charged particle arrivals on a surface of the charged particle detector. For example, the spatial distribution information may comprise the arrival locations of a plurality of emitted electrons on the detector surface. The spatial distribution information may comprise primarily backscattered electrons, or it may comprise a combination of, e.g., secondary and backscattered electrons. In some embodiments, the spatial distribution information may comprise a signal intensity map corresponding to signal intensity values at each detection pixel on the detector. In some embodiments, the spatial distribution information may comprise a boundary that is determined based on the signal intensity map. The boundary may be used to define geometric properties of the spatial distribution on the detector, such as a size, shape, or location.

[0096] At step 630, the controller may use the spatial distribution information to determine an aberration of the charged particle beam. For example, the aberration may be determined by comparingthe spatial distribution information to one or more reference distributions. The reference distributions may be based on, e.g., reference pupil plane distributions of the charged particle beam system. For example, comparing the spatial distribution to the one or more reference distributions may comprise fitting the spatial distribution to a model aberration, or to a weighted combination of model aberrations. The model aberrations may comprise, e.g., a set of intensity distributions represented by Zernike polynomials or a similar set of representative model aberrations. Model aberrations may include one or more of: defocus, astigmatism, beam tilt, beam shift, coma, pincushion or barrel distortion, spherical aberration, chromatic aberration, diffraction error, field curvature, and others. In some embodiments, the spatial distribution information may be decomposed into a weighted combination of model aberrations.

[0097] At step 640, the controller may determine an appropriate adjustment to the charged particle beam system based on the determined aberrations. In some embodiments, the controller may further perform said adjustments, in some cases in conjunction with an operator of the charged particle beam apparatus.

[0098] Using the method 600, beam aberration measurements may be performed faster and more accurately than with prior methods. The measurements may be better integrated into an inspection operation to reduce downtime. Further, the measurements may be performed using the actual focus values and other lens settings that are employed during an actual inspection, instead of performing the inspection “through focus” at a series of different focus values. This allows for observation of the beam properties under real -world conditions, yielding a better understanding of the aberrations that may be present.

[0099] Fig. 6B illustrates a flowchart of an example method 650 for obtaining reference information to be used in an aberration measurement method of a charged particle apparatus, consistent with embodiments of the present disclosure. Method 650 may be used to determine reference information for use in, e.g., method 600 of Fig. 6A. For example, the reference information may be applied at step 630 of determining aberrations based on spatial distribution information. The charged particle beam apparatus used in method 650 may be the same charged particle beam apparatus used in method 600, or it may a different charged particle beam apparatus. For example, the particular detector used to obtain reference information may be called a reference detector, but it should be understood that the reference detector may be the same as, or different from, the detector used in method 600.

[0100] At step 651, a controller may control a charged particle beam apparatus to irradiate a test surface with a charged particle beam under a plurality of different exposure conditions. The plurality of different exposure conditions may comprise a range of exposure settings, such as focus settings, lens settings, beam energy settings, field of view settings, etc. The plurality of different exposure conditions may be designed to capture an operating range of such settings, or a range that is greater than an intended operating range of such settings. The plurality of different exposure conditions maycomprise a set of induced aberrations configured to mimic the aberrations expected to be encountered during an aberration measurement.

[0101] At step 652, the controller may obtain spatial distribution information on a reference detector, in a manner similar to step 620 of method 600, for each of the plurality of different exposure settings. At step 653, the spatial distribution information may be evaluated based on the known values of the different exposure settings to construct a model describing the spatial distribution information as a function of different exposure settings. In some embodiments, the model may comprise a mathematical representation of spatial distributions as a function of the beam parameters induced under the plurality of different exposure settings. In some embodiments, the model may comprise a lookup table relating measured spatial distribution information to the beam parameters induced under the plurality of different exposure settings.

[0102] At step 654, the controller may use the model during an aberration measurement process to relate measured spatial distribution information to one or more aberration values based on the experimentally-derived relationships obtained at step 653. For example, step 630 of method 600 may include step 654 of method 650.

[0103] Fig. 6C illustrates a further flowchart of an example method 660 for obtaining reference information to be used in an aberration measurement method of a charged particle apparatus, consistent with embodiments of the present disclosure. Method 660 may be used to determine reference information for use in, e.g., method 600 of Fig. 6A. For example, the reference information may be applied at step 630 of determining aberrations based on spatial distribution information.

[0104] At step 661, the controller may control a charged particle beam apparatus to irradiate a pupil detector with a charged particle beam under a plurality of different exposure conditions. The plurality of different exposure conditions may correspond to the plurality of different exposure settings discussed above with respect to method 650.

[0105] At step 662, the controller may obtain first spatial distribution information on the pupil detector, in a manner similar to step 620 of method 600, or step 652 of method 650, for each of the plurality of different exposure settings. However, unlike the prior methods, the pupil detector may be located at or near the pupil plane of the charged particle beam apparatus to directly measure a pupil distribution of the charged particle beam. This distribution may be obtained without any downstream influences such as further lenses and electrodes, and without the effects of a Lambertian emission distribution from a test surface.

[0106] At step 663, with the pupil detector removed, the controller may control a charged particle beam apparatus to irradiate a test surface, in a manner similar to step 651 of method 650, for each of the plurality of different exposure settings used to irradiate the pupil detector at step 661. At step 664, the controller may obtain second spatial distribution information from a charged particle reference detector in a manner similar to step 652 of method 650. Thus the controller may obtain corresponding sets of first and second spatial distribution information for each of a plurality of different exposuresettings, where the first spatial distribution information is taken at the pupil plane, and spatial distribution information is taken at the charged particle beam inspection / reference detector.

[0107] At step 665, the controller may compare the first spatial distribution information and the second spatial distribution information to construct a model describing the relationship between a pupil distribution and the resulting spatial distribution on a charged particle detector during an inspection process. In some embodiments, the model may comprise a mathematical representation of spatial distributions as a function of the pupil distributions induced under the plurality of different exposure settings. In some embodiments, the model may comprise a lookup table relating measured spatial distribution information of the inspection detector to the pupil distributions induced under the plurality of different exposure settings.

[0108] At step 666, the controller may use the model during an aberration measurement process to relate measured spatial distribution information to one or more aberration values based on the experimentally-derived relationships obtained at step 665. For example, step 630 of method 600 may may include step 666 of method 660.

[0109] Fig. 7 illustrates a flowchart of an example method 700 of correcting aberrations in real time in a charged particle beam using a charged particle detector, consistent with embodiments of the present disclosure. Method 700 may comprise a special case of method 600 of Fig. 6A as an iterative feedback correction. For example, steps 710-730 may correspond to steps 610-630 of method 600. Therefore, a detailed description is omitted here.

[0110] At step 740, the controller may determine whether an aberration is below a predetermined threshold value. If it is, the method ends at step 760. However, if the aberration is not below a predetermined threshold, the method may proceed to step 750 where the controller may perform an incremental adjustment to reduce the aberration. After the correction is performed, or while the correction is still being performed, the controller may again proceed through steps 710-740 to evaluate the modified spatial distribution information and determine if the aberration has been brought below the desired threshold. This iterative loop of monitoring and performing incremental adjustments may continue until all aberration values are below the threshold.

[0111] Method 700 may advantageously be performed in real time during an inspection process, such as between wafers, between die areas, etc., to accurately determine whether all aberrations are at desired levels. Such an advantage is not feasible using conventional through-focus measurements.

[0112] A non-transitory computer-readable medium may be provided that stores instructions for a processor of a controller (e.g., controller 109 in Fig. 1) for determining aberrations using embodiments of the present disclosure. For example, the instructions stored in the non-transitory computer-readable medium may be executed by the circuitry of the controller for performing methods 600, 650, 660 or 700 in part or in entirety. Common forms of non-transitory media include, for example, a floppy disk, a flexible disk, hard disk, solid-state drive, magnetic tape, or any other magnetic data storage medium, a Compact Disc Read-Only Memory (CD-ROM), any other opticaldata storage medium, any physical medium with patterns of holes, a Random Access Memory (RAM), a Programmable Read-Only Memory (PROM), and Erasable Programmable Read-Only Memory (EPROM), a FLASH-EPROM or any other flash memory, Non-Volatile Random Access Memory (NVRAM), a cache, a register, any other memory chip or cartridge, and networked versions of the same.

[0113] The embodiments may further be described using the following clauses:1. A non-transitory computer-readable medium storing a set of instructions that are executable by at least one processor of a device to cause the device to perform operations comprising: irradiating a surface with a charged particle beam using a charged particle beam system; obtaining spatial distribution information of detected charged particle arrivals on a charged particle detector; determining an aberration value of the charged particle beam based on the spatial distribution information; and performing an adjustment to the charged particle beam system based on the determined aberration value.2. The non-transitory computer-readable medium of clause 1, wherein the spatial distribution information comprises an intensity map of charged particle arrivals on the detector.3. The non-transitory computer-readable medium of clause 2, wherein: the spatial distribution information comprises a boundary of a distribution of charged particle arrivals on the detector; and the boundary is defined based on the intensity map.4. The non-transitory computer-readable medium of clause 3, wherein the boundary is defined based on an intensity threshold.5. The non-transitory computer-readable medium of clause 3, wherein the boundary is defined to include a threshold percentage of energy detected on the detector.6. The non-transitory computer-readable medium of clause 5, wherein the threshold percentage is greater than 90% of the energy detected on the detector, and less than 100% of the energy detected on the detector.7. The non-transitory computer-readable medium of clause 3, wherein the boundary is determined based on one of a clustering algorithm or a contour-fitting algorithm.8. The non-transitory computer-readable medium of clause 1, wherein the spatial distribution information comprises one of a shape, size or centroid location of a distribution of charged particle arrivals on the detector.9. The non-transitory computer-readable medium of clause 1, wherein determining the aberration value of the charged particle beam based on the spatial distribution information comprises comparing the spatial distribution information to a plurality of reference distributions.10. The non-transitory computer-readable medium of clause 9, wherein the plurality of reference distributions comprises a set of model aberrations.11. The non-transitory computer-readable medium of clause 10, wherein the model aberrations are based on a plurality of measurements on a reference charged particle detector under a plurality of different exposure settings.12. The non-transitory computer-readable medium of clause 10, wherein the charged particle detector is the reference charged particle detector.13. The non-transitory computer-readable medium of clause 10, wherein the model aberrations are based on a plurality of measurements on a pupil charged particle detector under a plurality of different exposure settings.14. The non-transitory computer-readable medium of clause 10, wherein the model aberrations are based on a set of orthogonal functions on a disk.15. The non-transitory computer-readable medium of clause 1, wherein the charged particle detector comprises a pixelated charged particle detector having an array of sensing elements.16. The non-transitory computer-readable medium of clause 15, wherein the pixelated charged particle detector comprises at least 100 sensing elements configured to detect independent signal intensity values.17. The non-transitory computer-readable medium of clause 1, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least four segments; and the spatial distribution information comprises an asymmetry parameter.18. The non-transitory computer-readable medium of clause 1, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least two annular segments; and the spatial distribution information comprises focus parameter.19. The non-transitory computer-readable medium of clause 1, wherein the aberration value comprises one of a first, second, or third order aberration.20. The non-transitory computer-readable medium of clause 1, wherein the aberration value comprises one of defocus, astigmatism, beam tilt, beam shift, coma, pincushion distortion, barrel distortion, spherical aberration, chromatic aberration, or field curvature.21. The non-transitory computer-readable medium of clause 1, wherein the surface comprises a surface of a sample under inspection.22. The non-transitory computer-readable medium of clause 21, wherein the surface of the sample under inspection comprises one of a peripheral unpatterned region, a scribe lane, a metrology mark, or a featureless in-die region.23. The non-transitory computer-readable medium of clause 1, wherein the surface comprises a surface of a sample stage configured to support a sample under inspection.24. The non-transitory computer-readable medium of clause 1, wherein the surface comprises a layer of heavy metal having an atomic number of at least 47.25. The non-transitory computer-readable medium of clause 1, wherein the surface comprises one of silver, gold, titanium, cobalt, copper, or ruthenium.26. The non-transitory computer-readable medium of clause 1, wherein the detected charged particle arrivals comprise at least 70% backscattered electron arrivals.27. The non-transitory computer-readable medium of clause 1, wherein the operations further comprise: after performing the adjustment, determining that the aberration value is not below a predetermined threshold; further irradiating the surface with the charged particle beam using the charged particle beam system; obtaining further spatial distribution information of detected charged particle arrivals on the charged particle detector; determining a further aberration value of the charged particle beam based on the further spatial distribution information; and performing a further adjustment to the charged particle beam system based on the further aberration value.28. A charged particle beam apparatus, comprising: a charged particle beam source configured to generate a beam of primary charged particles; a charged particle optical system configured to direct the beam of primary charged particles at a surface; a controller comprising one or more processors and configured to cause the charged particle beam apparatus to perform operations comprising: obtaining spatial distribution information of detected charged particle arrivals on a charged particle detector; determining an aberration value of the charged particle beam based on the spatial distribution information, and performing an adjustment to the charged particle beam system based on the determined aberration value.29. The charged particle beam apparatus of clause 28, wherein the spatial distribution information comprises an intensity map of charged particle arrivals on the detector.30. The charged particle beam apparatus of clause 29, wherein: the spatial distribution information comprises a boundary of a distribution of charged particle arrivals on the detector; and the boundary is defined based on the intensity map.31. The charged particle beam apparatus of clause 30, wherein the boundary is defined based on an intensity threshold.32. The charged particle beam apparatus of clause 30, wherein the boundary is defined to include a threshold percentage of energy detected on the detector.33. The charged particle beam apparatus of clause 32, wherein the threshold percentage is greater than 90% of the energy detected on the detector, and less than 100% of the energy detected on the detector.34. The charged particle beam apparatus of clause 30, wherein the boundary is determined based on one of a clustering algorithm or a contour-fitting algorithm.35. The charged particle beam apparatus of clause 28, wherein the spatial distribution information comprises one of a shape, size or centroid location of a distribution of charged particle arrivals on the detector.36. The charged particle beam apparatus of clause 28, wherein determining the aberration value of the charged particle beam based on the spatial distribution information comprises comparing the spatial distribution information to a plurality of reference distributions.37. The charged particle beam apparatus of clause 36, wherein the plurality of reference distributions comprises a set of model aberrations.38. The charged particle beam apparatus of clause 37, wherein the model aberrations are based on a plurality of measurements on a reference charged particle detector under a plurality of different exposure settings.39. The charged particle beam apparatus of clause 37, wherein the charged particle detector is the reference charged particle detector.40. The charged particle beam apparatus of clause 37, wherein the model aberrations are based on a plurality of measurements on a pupil charged particle detector under a plurality of different exposure settings.41. The charged particle beam apparatus of clause 37, wherein the model aberrations are based on a set of orthogonal functions on a disk.42. The charged particle beam apparatus of clause 28, wherein the charged particle detector comprises a pixelated charged particle detector having an array of sensing elements.43. The charged particle beam apparatus of clause 15, wherein the pixelated charged particle detector comprises at least 100 sensing elements configured to detect independent signal intensity values.44. The charged particle beam apparatus of clause 28, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least four segments; and the spatial distribution information comprises an asymmetry parameter.45. The charged particle beam apparatus of clause 28, wherein:the charged particle detector comprises a segmented charged particle detector comprising at least two annular segments; and the spatial distribution information comprises focus parameter.46. The charged particle beam apparatus of clause 28, wherein the aberration value comprises one of a first, second, or third order aberration.47. The charged particle beam apparatus of clause 28, wherein the aberration value comprises one of defocus, astigmatism, beam tilt, beam shift, coma, pincushion distortion, barrel distortion, spherical aberration, chromatic aberration, or field curvature.48. The charged particle beam apparatus of clause 28, wherein the surface comprises a surface of a sample under inspection.49. The charged particle beam apparatus of clause 48, wherein the surface of the sample under inspection comprises one of a peripheral unpatterned region, a scribe lane, a metrology mark, or a featureless in-die region.50. The charged particle beam apparatus of clause 28, wherein the surface comprises a surface of a sample stage configured to support a sample under inspection.51. The charged particle beam apparatus of clause 28, wherein the surface comprises a layer of heavy metal having an atomic number of at least 47.52. The charged particle beam apparatus of clause 28, wherein surface comprises one of silver, gold, titanium, cobalt, copper, or ruthenium.53. The charged particle beam apparatus of clause 28, wherein the detected charged particle arrivals comprise at least 70% backscattered electron arrivals.54. The charged particle beam apparatus of clause 28, wherein the operations further comprise: after performing the adjustment, determining that the aberration value is not below a predetermined threshold; further irradiating the surface with the charged particle beam using the charged particle beam system; obtaining further spatial distribution information of detected charged particle arrivals on the charged particle detector; determining a further aberration value of the charged particle beam based on the further spatial distribution information; and performing a further adjustment to the charged particle beam system based on the further aberration value.55. A charged particle beam method comprising: irradiating a surface with a charged particle beam using a charged particle beam system; obtaining spatial distribution information of detected charged particle arrivals on a charged particle detector;determining an aberration value of the charged particle beam based on the spatial distribution information; and performing an adjustment to the charged particle beam system based on the determined aberration value.56. The charged particle beam method of clause 55, wherein the spatial distribution information comprises an intensity map of charged particle arrivals on the detector.57. The charged particle beam method of clause 56, wherein: the spatial distribution information comprises a boundary of a distribution of charged particle arrivals on the detector; and the boundary is defined based on the intensity map.58. The charged particle beam method of clause 57, wherein the boundary is defined based on an intensity threshold.59. The charged particle beam method of clause 57, wherein the boundary is defined to include a threshold percentage of energy detected on the detector.60. The charged particle beam method of clause 59, wherein the threshold percentage is greater than 90% of the energy detected on the detector, and less than 100% of the energy detected on the detector.61. The charged particle beam method of clause 57, wherein the boundary is determined based on one of a clustering algorithm or a contour-fitting algorithm.62. The charged particle beam method of clause 55, wherein the spatial distribution information comprises one of a shape, size or centroid location of a distribution of charged particle arrivals on the detector.63. The charged particle beam method of clause 55, wherein determining the aberration value of the charged particle beam based on the spatial distribution information comprises comparing the spatial distribution information to a plurality of reference distributions.64. The charged particle beam method of clause 63, wherein the plurality of reference distributions comprises a set of model aberrations.65. The charged particle beam method of clause 64, wherein the model aberrations are based on a plurality of measurements on a reference charged particle detector under a plurality of different exposure settings.66. The charged particle beam method of clause 64, wherein the charged particle detector is the reference charged particle detector.67. The charged particle beam method of clause 64, wherein the model aberrations are based on a plurality of measurements on a pupil charged particle detector under a plurality of different exposure settings.68. The charged particle beam method of clause 64, wherein the model aberrations are based on a set of orthogonal functions on a disk.69. The charged particle beam method of clause 55, wherein the charged particle detector comprises a pixelated charged particle detector having an array of sensing elements.70. The charged particle beam method of clause 69, wherein the pixelated charged particle detector comprises at least 100 sensing elements configured to detect independent signal intensity values.71. The charged particle beam method of clause 55, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least four segments; and the spatial distribution information comprises an asymmetry parameter.72. The charged particle beam method of clause 55, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least two annular segments; and the spatial distribution information comprises focus parameter.73. The charged particle beam method of clause 55, wherein the aberration value comprises one of a first, second, or third order aberration.74. The charged particle beam method of clause 55, wherein the aberration value comprises one of defocus, astigmatism, beam tilt, beam shift, coma, pincushion distortion, barrel distortion, spherical aberration, chromatic aberration, or field curvature.75. The charged particle beam method of clause 55, wherein the surface comprises a surface of a sample under inspection.76. The charged particle beam method of clause 75, wherein the surface of the sample under inspection comprises one of a peripheral unpatterned region, a scribe lane, a metrology mark, or a featureless in-die region.77. The charged particle beam method of clause 55, wherein the surface comprises a surface of a sample stage configured to support a sample under inspection.78. The charged particle beam method of clause 55, wherein the surface comprises a layer of heavy metal having an atomic number of at least 47.79. The charged particle beam method of clause 55, wherein the surface comprises one of silver, gold, titanium, cobalt, copper, or ruthenium.80. The charged particle beam method of clause 55, wherein the detected charged particle arrivals comprise at least 70% backscattered electron arrivals.81. The charged particle beam method of clause 55, further comprising: after performing the adjustment, determining that the aberration value is not below a predetermined threshold; further irradiating the surface with the charged particle beam using the charged particle beam system;obtaining further spatial distribution information of detected charged particle arrivals on the charged particle detector; determining a further aberration value of the charged particle beam based on the further spatial distribution information; and performing a further adjustment to the charged particle beam system based on the further aberration value.82. A non-transitory computer-readable medium storing a set of instructions that are executable by at least one processor of a device to cause the device to perform operations comprising: irradiating a surface with a charged particle beam using a charged particle beam system to cause charged particles to be emitted from the surface; detecting the charged particles on a charged particle detector of the charged particle beam system to produce a charged particle detection signal; comparing the charged particle detection signal to a reference charged particle detection signal; determining an aberration value of the charged particle beam based on the comparison; performing an adjustment to the charged particle beam system based on the determined aberration value; and inspecting a sample using the detector.83. The non-transitory computer-readable medium of clause 82, wherein the detection signal comprises spatial distribution information of detected charged particle arrivals on the charged particle detector.84. The non-transitory computer-readable medium of clause 83, wherein the spatial distribution information comprises an intensity map of charged particle arrivals on the detector.85. The non-transitory computer-readable medium of clause 84, wherein: the spatial distribution information comprises a boundary of a distribution of charged particle arrivals on the detector; and the boundary is defined based on the intensity map.86. The non-transitory computer-readable medium of clause 85, wherein the boundary is defined based on an intensity threshold.87. The non-transitory computer-readable medium of clause 85, wherein the boundary is defined to include a threshold percentage of energy detected on the detector.88. The non-transitory computer-readable medium of clause 87, wherein the threshold percentage is greater than 90% of the energy detected on the detector, and less than 100% of the energy detected on the detector.89. The non-transitory computer-readable medium of clause 85, wherein the boundary is determined based on one of a clustering algorithm or a contour-fitting algorithm.90. The non-transitory computer-readable medium of clause 83, wherein the spatial distribution information comprises one of a shape, size or centroid location of a distribution of charged particle arrivals on the detector.91. The non-transitory computer-readable medium of clause 83, wherein determining the aberration value of the charged particle beam based on the comparison comprises comparing the spatial distribution information to a plurality of reference distributions.92. The non-transitory computer-readable medium of clause 91, wherein the plurality of reference distributions comprises a set of model aberrations.93. The non-transitory computer-readable medium of clause 92, wherein the model aberrations are based on a plurality of measurements on a reference charged particle detector under a plurality of different exposure settings.94. The non-transitory computer-readable medium of clause 92, wherein the charged particle detector is the reference charged particle detector.95. The non-transitory computer-readable medium of clause 92, wherein the model aberrations are based on a plurality of measurements on a pupil charged particle detector under a plurality of different exposure settings.96. The non-transitory computer-readable medium of clause 92, wherein the model aberrations are based on a set of orthogonal functions on a disk.97. The non-transitory computer-readable medium of clause 82, wherein the charged particle detector comprises a pixelated charged particle detector having an array of sensing elements.98. The non-transitory computer-readable medium of clause 97, wherein the pixelated charged particle detector comprises at least 100 sensing elements configured to detect independent signal intensity values.99. The non-transitory computer-readable medium of clause 82, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least four segments; and the charged particle detection signal comprises an asymmetry parameter among the at least four segments.100. The non-transitory computer-readable medium of clause 82, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least two annular segments; and the charged particle detection signal comprises focus parameter.101. The non-transitory computer-readable medium of clause 82, wherein the aberration value comprises one of a first, second, or third order aberration.102. The non-transitory computer-readable medium of clause 82, wherein the aberration value comprises one of defocus, astigmatism, beam tilt, beam shift, coma, pincushion distortion, barrel distortion, spherical aberration, chromatic aberration, or field curvature.103. The non-transitory computer-readable medium of clause 82, wherein the surface comprises a surface of a sample under inspection.104. The non-transitory computer-readable medium of clause 103, wherein the surface of the sample under inspection comprises one of a peripheral unpatterned region, a scribe lane, a metrology mark, or a featureless in-die region.105. The non-transitory computer-readable medium of clause 82, wherein the surface comprises a surface of a sample stage configured to support a sample under inspection.106. The non-transitory computer-readable medium of clause 82, wherein the surface comprises a layer of heavy metal having an atomic number of at least 47.107. The non-transitory computer-readable medium of clause 82, wherein the surface comprises one of silver, gold, titanium, cobalt, copper, or ruthenium.108. The non-transitory computer-readable medium of clause 82, wherein the detected charged particle arrivals comprise at least 70% backscattered electron arrivals.109. The non-transitory computer-readable medium of clause 82, wherein the operations further comprise: after performing the adjustment, determining that the aberration value is not below a predetermined threshold; further irradiating the surface with the charged particle beam using the charged particle beam system; detecting further charged particles on the charged particle detector to produce a further charged particle detection signal; determining a further aberration value of the charged particle beam based on the further charged particle detection signal; and performing a further adjustment to the charged particle beam system based on the further aberration value.110. A charged particle beam apparatus, comprising: a charged particle beam source configured to generate a beam of primary charged particles; a charged particle optical system configured to direct the beam of primary charged particles at a surface; a controller comprising one or more processors and configured to cause the charged particle beam apparatus to perform operations comprising: irradiating the surface with the charged particle beam to cause charged particles to be emitted from the surface; detecting the charged particles on a charged particle detector of the charged particle beam system to produce a charged particle detection signal; comparing the charged particle detection signal to a reference charged particle detection signal;determining an aberration value of the charged particle beam based on the comparison; performing an adjustment to the charged particle beam system based on the determined aberration value; and inspecting a sample using the detector.111. The charged particle beam apparatus of clause 110, wherein the detection signal comprises spatial distribution information of detected charged particle arrivals on the charged particle detector.112. The charged particle beam apparatus of clause 111, wherein the spatial distribution information comprises an intensity map of charged particle arrivals on the detector.113. The charged particle beam apparatus of clause 112, wherein: the spatial distribution information comprises a boundary of a distribution of charged particle arrivals on the detector; and the boundary is defined based on the intensity map.114. The charged particle beam apparatus of clause 113, wherein the boundary is defined based on an intensity threshold.115. The charged particle beam apparatus of clause 113, wherein the boundary is defined to include a threshold percentage of energy detected on the detector.116. The charged particle beam apparatus of clause 115, wherein the threshold percentage is greater than 90% of the energy detected on the detector, and less than 100% of the energy detected on the detector.117. The charged particle beam apparatus of clause 113, wherein the boundary is determined based on one of a clustering algorithm or a contour-fitting algorithm.118. The charged particle beam apparatus of clause 111, wherein the spatial distribution information comprises one of a shape, size or centroid location of a distribution of charged particle arrivals on the detector.119. The charged particle beam apparatus of clause 111, wherein determining the aberration value of the charged particle beam based on the comparison comprises comparing the spatial distribution information to a plurality of reference distributions.120. The charged particle beam apparatus of clause 119, wherein the plurality of reference distributions comprises a set of model aberrations.121. The charged particle beam apparatus of clause 120, wherein the model aberrations are based on a plurality of measurements on a reference charged particle detector under a plurality of different exposure settings.122. The charged particle beam apparatus of clause 120, wherein the charged particle detector is the reference charged particle detector.123. The charged particle beam apparatus of clause 120, wherein the model aberrations are based on a plurality of measurements on a pupil charged particle detector under a plurality of different exposure settings.124. The charged particle beam apparatus of clause 120, wherein the model aberrations are based on a set of orthogonal functions on a disk.125. The charged particle beam apparatus of clause 110, wherein the charged particle detector comprises a pixelated charged particle detector having an array of sensing elements.126. The charged particle beam apparatus of clause 125, wherein the pixelated charged particle detector comprises at least 100 sensing elements configured to detect independent signal intensity values.127. The charged particle beam apparatus of clause 110, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least four segments; and the charged particle detection signal comprises an asymmetry parameter among the at least four segments.128. The charged particle beam apparatus of clause 110, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least two annular segments; and the charged particle detection signal comprises focus parameter.129. The charged particle beam apparatus of clause 110, wherein the aberration value comprises one of a first, second, or third order aberration.130. The charged particle beam apparatus of clause 110, wherein the aberration value comprises one of defocus, astigmatism, beam tilt, beam shift, coma, pincushion distortion, barrel distortion, spherical aberration, chromatic aberration, or field curvature.131. The charged particle beam apparatus of clause 110, wherein the surface comprises a surface of a sample under inspection.132. The charged particle beam apparatus of clause 131, wherein the surface of the sample under inspection comprises one of a peripheral unpatterned region, a scribe lane, a metrology mark, or a featureless in-die region.133. The charged particle beam apparatus of clause 110, wherein the surface comprises a surface of a sample stage configured to support a sample under inspection.134. The charged particle beam apparatus of clause 110, wherein the surface comprises a layer of heavy metal having an atomic number of at least 47.135. The charged particle beam apparatus of clause 110, wherein the surface comprises one of silver, gold, titanium, cobalt, copper, or ruthenium.136. The charged particle beam apparatus of clause 110, wherein the detected charged particle arrivals comprise at least 70% backscattered electron arrivals.137. The charged particle beam apparatus of clause 110, wherein the operations further comprise: after performing the adjustment, determining that the aberration value is not below a predetermined threshold; further irradiating the surface with the charged particle beam using the charged particle beam system; detecting further charged particles on the charged particle detector to produce a further charged particle detection signal; determining a further aberration value of the charged particle beam based on the further charged particle detection signal; and performing a further adjustment to the charged particle beam system based on the further aberration value.138. A charged particle beam method, comprising: irradiating a surface with a charged particle beam using a charged particle beam system to cause charged particles to be emitted from the surface; detecting the charged particles on a charged particle detector of the charged particle beam system to produce a charged particle detection signal; comparing the charged particle detection signal to a reference charged particle detection signal; determining an aberration value of the charged particle beam based on the comparison; performing an adjustment to the charged particle beam system based on the determined aberration value; and inspecting a sample using the detector.139. The charged particle beam method of clause 138, wherein the detection signal comprises spatial distribution information of detected charged particle arrivals on the charged particle detector.140. The charged particle beam method of clause 139, wherein the spatial distribution information comprises an intensity map of charged particle arrivals on the detector.141. The charged particle beam method of clause 140, wherein: the spatial distribution information comprises a boundary of a distribution of charged particle arrivals on the detector; and the boundary is defined based on the intensity map.142. The charged particle beam method of clause 141, wherein the boundary is defined based on an intensity threshold.143. The charged particle beam method of clause 141, wherein the boundary is defined to include a threshold percentage of energy detected on the detector.144. The charged particle beam method of clause 143, wherein the threshold percentage is greater than 90% of the energy detected on the detector, and less than 100% of the energy detected on the detector.145. The charged particle beam method of clause 141, wherein the boundary is determined based on one of a clustering algorithm or a contour-fitting algorithm.146. The charged particle beam method of clause 139, wherein the spatial distribution information comprises one of a shape, size or centroid location of a distribution of charged particle arrivals on the detector.147. The charged particle beam method of clause 139, wherein determining the aberration value of the charged particle beam based on the comparison comprises comparing the spatial distribution information to a plurality of reference distributions.148. The charged particle beam method of clause 147, wherein the plurality of reference distributions comprises a set of model aberrations.149. The charged particle beam method of clause 148, wherein the model aberrations are based on a plurality of measurements on a reference charged particle detector under a plurality of different exposure settings.150. The charged particle beam method of clause 148, wherein the charged particle detector is the reference charged particle detector.151. The charged particle beam method of clause 148, wherein the model aberrations are based on a plurality of measurements on a pupil charged particle detector under a plurality of different exposure settings.152. The charged particle beam method of clause 148, wherein the model aberrations are based on a set of orthogonal functions on a disk.153. The charged particle beam method of clause 138, wherein the charged particle detector comprises a pixelated charged particle detector having an array of sensing elements.154. The charged particle beam method of clause 153, wherein the pixelated charged particle detector comprises at least 100 sensing elements configured to detect independent signal intensity values.155. The charged particle beam method of clause 138, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least four segments; and the charged particle detection signal comprises an asymmetry parameter among the at least four segments.156. The charged particle beam method of clause 138, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least two annular segments; and the charged particle detection signal comprises focus parameter.157. The charged particle beam method of clause 138, wherein the aberration value comprises one of a first, second, or third order aberration.158. The charged particle beam method of clause 138, wherein the aberration value comprises one of defocus, astigmatism, beam tilt, beam shift, coma, pincushion distortion, barrel distortion, spherical aberration, chromatic aberration, or field curvature.159. The charged particle beam method of clause 138, wherein the surface comprises a surface of a sample under inspection.160. The charged particle beam method of clause 159, wherein the surface of the sample under inspection comprises one of a peripheral unpatterned region, a scribe lane, a metrology mark, or a featureless in-die region.161. The charged particle beam method of clause 138, wherein the surface comprises a surface of a sample stage configured to support a sample under inspection.162. The charged particle beam method of clause 138, wherein the surface comprises a layer of heavy metal having an atomic number of at least 47.163. The charged particle beam method of clause 138, wherein the surface comprises one of silver, gold, titanium, cobalt, copper, or ruthenium.164. The charged particle beam method of clause 138, wherein the detected charged particle arrivals comprise at least 70% backscattered electron arrivals.165. The charged particle beam method of clause 138, further comprising: after performing the adjustment, determining that the aberration value is not below a predetermined threshold; further irradiating the surface with the charged particle beam using the charged particle beam system; detecting further charged particles on the charged particle detector to produce a further charged particle detection signal; determining a further aberration value of the charged particle beam based on the further charged particle detection signal; and performing a further adjustment to the charged particle beam system based on the further aberration value.166. A non-transitory computer-readable medium storing a set of instructions that are executable by at least one processor of a device to cause the device to perform operations comprising: irradiating a surface with a charged particle beam using a charged particle beam system to cause charged particles to be emitted from the surface; obtaining spatial distribution information of the charged particle arrivals on a charged particle detector of the charged particle beam system; determining a value of a parameter of a pupil plane distribution of the charged particle beam based on the spatial distribution information;performing an adjustment to the charged particle beam system based on the determined value of the parameter; and inspecting a sample using the detector.167. The non- transitory computer-readable medium of clause 166, wherein the spatial distribution information comprises an intensity map of charged particle arrivals on the detector.168. The non-transitory computer-readable medium of clause 167, wherein: the spatial distribution information comprises a boundary of a distribution of charged particle arrivals on the detector; and the boundary is defined based on the intensity map.169. The non-transitory computer-readable medium of clause 168, wherein the boundary is defined based on an intensity threshold.170. The non-transitory computer-readable medium of clause 168, wherein the boundary is defined to include a threshold percentage of energy detected on the detector.171. The non-transitory computer-readable medium of clause 170, wherein the threshold percentage is greater than 90% of the energy detected on the detector, and less than 100% of the energy detected on the detector.172. The non-transitory computer-readable medium of clause 168, wherein the boundary is determined based on one of a clustering algorithm or a contour-fitting algorithm.173. The non-transitory computer-readable medium of clause 166, wherein the spatial distribution information comprises one of a shape, size or centroid location of a distribution of charged particle arrivals on the detector.174. The non-transitory computer-readable medium of clause 166, wherein determining the value of the parameter of the pupil plane distribution based on the spatial distribution information comprises comparing the spatial distribution information to a plurality of reference distributions.175. The non-transitory computer-readable medium of clause 174, wherein the plurality of reference distributions comprises a set of model aberrations.176. The non-transitory computer-readable medium of clause 175, wherein the model aberrations are based on a plurality of measurements on a reference charged particle detector under a plurality of different exposure settings.177. The non-transitory computer-readable medium of clause 175, wherein the charged particle detector is the reference charged particle detector.178. The non-transitory computer-readable medium of clause 175, wherein the model aberrations are based on a plurality of measurements on a pupil charged particle detector under a plurality of different exposure settings.179. The non-transitory computer-readable medium of clause 175, wherein the model aberrations are based on a set of orthogonal functions on a disk.180. The non- transitory computer-readable medium of clause 166, wherein the charged particle detector comprises a pixelated charged particle detector having an array of sensing elements.181. The non- transitory computer-readable medium of clause 180, wherein the pixelated charged particle detector comprises at least 100 sensing elements configured to detect independent signal intensity values.182. The non-transitory computer-readable medium of clause 166, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least four segments; and determining the value of the parameter of the pupil plane distribution comprises determining an asymmetry parameter among the at least four segments.183. The non-transitory computer-readable medium of clause 166, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least two annular segments; and determining the value of the parameter of the pupil plane distribution comprises determining a focus parameter.184. The non-transitory computer-readable medium of clause 166, wherein the parameter of the pupil plane distribution of the charged particle beam comprises one of a first, second, or third order aberration.185. The non-transitory computer-readable medium of clause 166, wherein the parameter of the pupil plane distribution of the charged particle beam comprises one of defocus, astigmatism, beam tilt, beam shift, coma, pincushion distortion, barrel distortion, spherical aberration, chromatic aberration, or field curvature.186. The non-transitory computer-readable medium of clause 166, wherein the surface comprises a surface of a sample under inspection.187. The non-transitory computer-readable medium of clause 186, wherein the surface of the sample under inspection comprises one of a peripheral unpatterned region, a scribe lane, a metrology mark, or a featureless in-die region.188. The non-transitory computer-readable medium of clause 166, wherein the surface comprises a surface of a sample stage configured to support a sample under inspection.189. The non-transitory computer-readable medium of clause 166, wherein the surface comprises a layer of heavy metal having an atomic number of at least 47.190. The non-transitory computer-readable medium of clause 166, wherein the surface comprises one of silver, gold, titanium, cobalt, copper, or ruthenium.191. The non-transitory computer-readable medium of clause 166, wherein the detected charged particle arrivals comprise at least 70% backscattered electron arrivals.192. The non-transitory computer-readable medium of clause 166, wherein the operations further comprise:after performing the adjustment, determining that the parameter of the pupil plane distribution is not below a predetermined threshold; further irradiating the surface with the charged particle beam using the charged particle beam system; obtaining further spatial distribution information of detected charged particle arrivals on the charged particle detector; determining a further value of the parameter of the pupil plane distribution based on the further spatial distribution information; and performing a further adjustment to the charged particle beam system based on the further value.193. A charged particle beam apparatus, comprising: a charged particle beam source configured to generate a beam of primary charged particles; a charged particle optical system configured to direct the beam of primary charged particles at a surface; a controller comprising one or more processors and configured to cause the charged particle beam apparatus to perform operations comprising: irradiating the surface with a charged particle beam using a charged particle beam system to cause charged particles to be emitted from the surface; obtaining spatial distribution information of the charged particle arrivals on a charged particle detector of the charged particle beam system; determining a parameter of a pupil plane distribution of the charged particle beam based on the spatial distribution information; performing an adjustment to the charged particle beam system based on the determined parameter; and inspecting a sample using the detector.194. The charged particle beam apparatus of clause 193, wherein the spatial distribution information comprises an intensity map of charged particle arrivals on the detector.195. The charged particle beam apparatus of clause 194, wherein: the spatial distribution information comprises a boundary of a distribution of charged particle arrivals on the detector; and the boundary is defined based on the intensity map.196. The charged particle beam apparatus of clause 195, wherein the boundary is defined based on an intensity threshold.197. The charged particle beam apparatus of clause 195, wherein the boundary is defined to include a threshold percentage of energy detected on the detector.198. The charged particle beam apparatus of clause 197, wherein the threshold percentage is greater than 90% of the energy detected on the detector, and less than 100% of the energy detected on the detector.199. The charged particle beam apparatus of clause 195, wherein the boundary is determined based on one of a clustering algorithm or a contour-fitting algorithm.200. The charged particle beam apparatus of clause 193, wherein the spatial distribution information comprises one of a shape, size or centroid location of a distribution of charged particle arrivals on the detector.201. The charged particle beam apparatus of clause 193, wherein determining the value of the parameter of the pupil plane distribution based on the spatial distribution information comprises comparing the spatial distribution information to a plurality of reference distributions.202. The charged particle beam apparatus of clause 201, wherein the plurality of reference distributions comprises a set of model aberrations.203. The charged particle beam apparatus of clause 202, wherein the model aberrations are based on a plurality of measurements on a reference charged particle detector under a plurality of different exposure settings.204. The charged particle beam apparatus of clause 202, wherein the charged particle detector is the reference charged particle detector.205. The charged particle beam apparatus of clause 202, wherein the model aberrations are based on a plurality of measurements on a pupil charged particle detector under a plurality of different exposure settings.206. The charged particle beam apparatus of clause 202, wherein the model aberrations are based on a set of orthogonal functions on a disk.207. The charged particle beam apparatus of clause 193, wherein the charged particle detector comprises a pixelated charged particle detector having an array of sensing elements.208. The charged particle beam apparatus of clause 207, wherein the pixelated charged particle detector comprises at least 100 sensing elements configured to detect independent signal intensity values.209. The charged particle beam apparatus of clause 193, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least four segments; and determining the value of the parameter of the pupil plane distribution comprises determining an asymmetry parameter among the at least four segments.210. The charged particle beam apparatus of clause 193, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least two annular segments; anddetermining the value of the parameter of the pupil plane distribution comprises determining a focus parameter.211. The charged particle beam apparatus of clause 193, wherein the parameter of the pupil plane distribution of the charged particle beam comprises one of a first, second, or third order aberration.212. The charged particle beam apparatus of clause 193, wherein the parameter of the pupil plane distribution of the charged particle beam comprises one of defocus, astigmatism, beam tilt, beam shift, coma, pincushion distortion, barrel distortion, spherical aberration, chromatic aberration, or field curvature.213. The charged particle beam apparatus of clause 193, wherein the surface comprises a surface of a sample under inspection.214. The charged particle beam apparatus of clause 213, wherein the surface of the sample under inspection comprises one of a peripheral unpatterned region, a scribe lane, a metrology mark, or a featureless in-die region.215. The charged particle beam apparatus of clause 193, wherein the surface comprises a surface of a sample stage configured to support a sample under inspection.216. The charged particle beam apparatus of clause 193, wherein the surface comprises a layer of heavy metal having an atomic number of at least 47.217. The charged particle beam apparatus of clause 193, wherein the surface comprises one of silver, gold, titanium, cobalt, copper, or ruthenium.218. The charged particle beam apparatus of clause 193, wherein the detected charged particle arrivals comprise at least 70% backscattered electron arrivals.219. The charged particle beam apparatus of clause 193, wherein the operations further comprise: after performing the adjustment, determining that the parameter of the pupil plane distribution is not below a predetermined threshold; further irradiating the surface with the charged particle beam using the charged particle beam system; obtaining further spatial distribution information of detected charged particle arrivals on the charged particle detector; determining a further value of the parameter of the pupil plane distribution based on the further spatial distribution information; and performing a further adjustment to the charged particle beam system based on the further value.220. A charged particle beam method, comprising: irradiating a surface with a charged particle beam using a charged particle beam system to cause charged particles to be emitted from the surface;obtaining spatial distribution information of the charged particle arrivals on a charged particle detector of the charged particle beam system; determining a parameter of a pupil plane distribution of the charged particle beam based on the spatial distribution information; performing an adjustment to the charged particle beam system based on the determined parameter; and inspecting a sample using the detector.221. The charged particle beam method of clause 220, wherein the spatial distribution information comprises an intensity map of charged particle arrivals on the detector.222. The charged particle beam method of clause 221, wherein: the spatial distribution information comprises a boundary of a distribution of charged particle arrivals on the detector; and the boundary is defined based on the intensity map.223. The charged particle beam method of clause 222, wherein the boundary is defined based on an intensity threshold.224. The charged particle beam method of clause 222, wherein the boundary is defined to include a threshold percentage of energy detected on the detector.225. The charged particle beam method of clause 224, wherein the threshold percentage is greater than 90% of the energy detected on the detector, and less than 100% of the energy detected on the detector.226. The charged particle beam method of clause 222, wherein the boundary is determined based on one of a clustering algorithm or a contour-fitting algorithm.227. The charged particle beam method of clause 220, wherein the spatial distribution information comprises one of a shape, size or centroid location of a distribution of charged particle arrivals on the detector.228. The charged particle beam method of clause 220, wherein determining the value of the parameter of the pupil plane distribution based on the spatial distribution information comprises comparing the spatial distribution information to a plurality of reference distributions.229. The charged particle beam method of clause 228, wherein the plurality of reference distributions comprises a set of model aberrations.230. The charged particle beam method of clause 229, wherein the model aberrations are based on a plurality of measurements on a reference charged particle detector under a plurality of different exposure settings.231. The charged particle beam method of clause 229, wherein the charged particle detector is the reference charged particle detector.232. The charged particle beam method of clause 229, wherein the model aberrations are based on a plurality of measurements on a pupil charged particle detector under a plurality of different exposure settings.233. The charged particle beam method of clause 229, wherein the model aberrations are based on a set of orthogonal functions on a disk.234. The charged particle beam method of clause 220, wherein the charged particle detector comprises a pixelated charged particle detector having an array of sensing elements.235. The charged particle beam method of clause 234, wherein the pixelated charged particle detector comprises at least 100 sensing elements configured to detect independent signal intensity values.236. The charged particle beam method of clause 220, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least four segments; and determining the value of the parameter of the pupil plane distribution comprises determining an asymmetry parameter among the at least four segments.237. The charged particle beam method of clause 220, wherein: the charged particle detector comprises a segmented charged particle detector comprising at least two annular segments; and determining the value of the parameter of the pupil plane distribution comprises determining a focus parameter.238. The charged particle beam method of clause 220, wherein the parameter of the pupil plane distribution of the charged particle beam comprises one of a first, second, or third order aberration.239. The charged particle beam method of clause 220, wherein the parameter of the pupil plane distribution of the charged particle beam comprises one of defocus, astigmatism, beam tilt, beam shift, coma, pincushion distortion, barrel distortion, spherical aberration, chromatic aberration, or field curvature.240. The charged particle beam method of clause 220, wherein the surface comprises a surface of a sample under inspection.241. The charged particle beam method of clause 240, wherein the surface of the sample under inspection comprises one of a peripheral unpatterned region, a scribe lane, a metrology mark, or a featureless in-die region.242. The charged particle beam method of clause 220, wherein the surface comprises a surface of a sample stage configured to support a sample under inspection.243. The charged particle beam method of clause 220, wherein the surface comprises a layer of heavy metal having an atomic number of at least 47.244. The charged particle beam method of clause 220, wherein the surface comprises one of silver, gold, titanium, cobalt, copper, or ruthenium.245. The charged particle beam method of clause 220, wherein the detected charged particle arrivals comprise at least 70% backscattered electron arrivals.246. The charged particle beam method of clause 220, further comprising: after performing the adjustment, determining that the parameter of the pupil plane distribution is not below a predetermined threshold; further irradiating the surface with the charged particle beam using the charged particle beam system; obtaining further spatial distribution information of detected charged particle arrivals on the charged particle detector; determining a further value of the parameter of the pupil plane distribution based on the further spatial distribution information; and performing a further adjustment to the charged particle beam system based on the further value.

[0114] Some embodiments of the present disclosure have been described with respect to electron beam systems, such as SEM, having an electron detector for detecting electron arrivals. However, the present disclosure is not limited to this. It should be understood that the above disclosed embodiments may be applicable to other systems, such as other non-SEM electron beam systems or non-electron based charged particle beam systems. Further, it should be understood that other charged particles, or other classes of electrons are contemplated within the scope of the present disclosure.

[0115] Block diagrams in the figures may illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer hardware or software products according to various exemplary embodiments of the present disclosure. In this regard, each block in a schematic diagram may represent certain arithmetical or logical operation processing that may be implemented using hardware such as an electronic circuit. Blocks may also represent a module, segment, or portion of code that comprises one or more executable instructions for implementing the specified logical functions. It should be understood that in some alternative implementations, functions indicated in a block may occur out of the order noted in the figures. For example, two blocks shown in succession may be executed or implemented substantially concurrently, or two blocks may sometimes be executed in reverse order, depending upon the functionality involved. Some blocks may also be omitted. It should also be understood that each block of the block diagrams, and combination of the blocks, may be implemented by special purpose hardware-based systems that perform the specified functions or acts, or by combinations of special purpose hardware and computer instructions.

[0116] It will be appreciated that the embodiments of the present disclosure are not limited to the exact construction that has been described above and illustrated in the accompanying drawings, andthat various modifications and changes can be made without departing from the scope thereof. For example, a charged particle inspection system may be but one example of a charged particle beam system consistent with embodiments of the present disclosure.

Claims

CLAIMS1. A non-transitory computer-readable medium storing a set of instructions that are executable by at least one processor of a device to cause the device to perform operations comprising: irradiating a surface with a charged particle beam using a charged particle beam system; obtaining spatial distribution information of detected charged particle arrivals on a charged particle detector; determining an aberration value of the charged particle beam based on the spatial distribution information; and performing an adjustment to the charged particle beam system based on the determined aberration value.

2. The non-transitory computer-readable medium of claim 1, wherein the spatial distribution information comprises an intensity map of charged particle arrivals on the detector.

3. The non-transitory computer-readable medium of claim 2, wherein: the spatial distribution information comprises a boundary of a distribution of charged particle arrivals on the detector; and the boundary is defined based on the intensity map.

4. The non-transitory computer-readable medium of claim 3, wherein the boundary is defined based on an intensity threshold.

5. The non-transitory computer-readable medium of claim 3, wherein the boundary is defined to include a threshold percentage of energy detected on the detector.

6. The non-transitory computer-readable medium of claim 5, wherein the threshold percentage is greater than 90% of the energy detected on the detector, and less than 100% of the energy detected on the detector.

7. The non-transitory computer-readable medium of claim 3, wherein the boundary is determined based on one of a clustering algorithm or a contour-fitting algorithm.

8. The non-transitory computer-readable medium of claim 1, wherein the spatial distribution information comprises one of a shape, size or centroid location of a distribution of charged particle arrivals on the detector.

9. The non-transitory computer-readable medium of claim 1, wherein determining the aberration value of the charged particle beam based on the spatial distribution information comprises comparing the spatial distribution information to a plurality of reference distributions.

10. The non-transitory computer-readable medium of claim 9, wherein the plurality of reference distributions comprises a set of model aberrations.

11. The non-transitory computer-readable medium of claim 10, wherein the model aberrations are based on a plurality of measurements on a reference charged particle detector under a plurality of different exposure settings.

12. The non-transitory computer-readable medium of claim 10, wherein the charged particle detector is the reference charged particle detector.

13. The non-transitory computer-readable medium of claim 10, wherein the model aberrations are based on a plurality of measurements on a pupil charged particle detector under a plurality of different exposure settings.

14. The non-transitory computer-readable medium of claim 10, wherein the model aberrations are based on a set of orthogonal functions on a disk.

15. The non-transitory computer-readable medium of claim 1, wherein the charged particle detector comprises a pixelated charged particle detector having an array of sensing elements.

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