Pulse compression for sample assessment

Abstract

A charged particle-optical device configured to assess a sample, the charged particle-optical device comprising: an emitter configured to emit a beam of charged particles along a beam path; a chopper configured to chop the beam into pulses; a compressor configured to compress the pulses into compressed pulses, wherein the charged particle-optical device is configured to direct the compressed pulses along the beam path onto a sample location; and a detector configured to detect signal charged particles from the sample location so as to assess a sample located at the sample location.

Description

PULSE COMPRESSION FOR SAMPLE ASSESSMENTCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23201716.0 which was filed on October 04, 2023 and which is incorporated herein in its entirety by reference.TECHNICAL FIELD

[0002] The embodiments provided herein disclose a charged particle-optical device, a method for assessing a sample, non-transitory computer readable media, an e-beam inspection device and a method for inspecting a sample.BACKGROUND

[0003] In manufacturing processes of integrated circuits (ICs), unfinished or finished circuit components are inspected to ensure that they are manufactured according to design and are free of defects. Inspection systems utilizing optical microscopes or charged particle (e.g., electron) beam microscopes, such as a scanning electron microscope (SEM) can be employed. As the physical sizes of IC components continue to shrink, and their structures continue to become more complex, accuracy and throughput in defect detection and inspection become more important. The overall image quality depends on a combination of high secondary-electron and backscattered-electron signal detection efficiencies, among others. Backscattered electrons have higher emission energy to escape from deeper layers of a sample, and therefore, their detection may be desirable for imaging of complex structures such as buried layers, nodes, high-aspect-ratio trenches or holes of 3D NAND devices. For applications such as overlay metrology, it may be desirable to obtain high quality imaging and efficient collection of surface information from secondary electrons and buried layer information from backscattered electrons, simultaneously, highlighting a need for using multiple electron detectors in a SEM. Although multiple electron detectors in various structural arrangements may be used to maximize collection and detection efficiencies of secondary and backscattered electrons individually, the combined detection efficiencies remain low, and therefore, the image quality achieved may be inadequate for high accuracy and high throughput defect inspection and metrology of two- dimensional and three-dimensional structures.SUMMARY

[0004] An embodiment of the present disclosure provides a charged particle-optical device configured to assess a sample, the charged particle-optical device comprising: an emitter configured to emit a beam of charged particles along a beam path; a compressor configured to compress the beam to form compressed pulses, wherein the charged particle-optical device is configured to direct the compressed pulses along the beam path onto a sample location; and a detector configured to detect tsignal charged particles from the sample location so as to assess a sample located at the sample location.

[0005] An embodiment of the present disclosure provides a method for assessing a sample, the method comprising: emitting a beam of charged particles along a beam path; compressing the beam to form compressed pulses; directing the compressed pulses along the beam path onto a sample; and detecting signal charged particles from the sample so as to assess the sample.

[0006] An embodiment of the present disclosure provides a non-transitory computer readable medium that stores instructions for a processor of a controller to control a charged particle-optical device to carry out a method for assessing a sample, the method comprising: emitting a beam of charged particles along a beam path; compressing the beam to form compressed pulses; directing the compressed pulses along the beam path onto a sample; and detecting signal charged particles from the sample so as to assess the sample.

[0007] An embodiment of the present disclosure provides an e-beam inspection device, comprising: an emitter that emits an e-beam along a beam path; a chopper for chopping the e-beam into pulses; a compressor for compressing the pulses; and a stretcher for reducing an energy spread introduced by the compressor.

[0008] An embodiment of the present disclosure provides a method for inspecting a sample with an e-beam inspection device, the method comprising: emitting an e-beam along a beam path; chopping the e-beam into pulses; compressing the pulses; and reducing an energy spread introduced by the compressing.

[0009] An embodiment of the present disclosure provides a non-transitory computer readable medium that stores instructions for a processor of a controller of an e-beam inspection device to control the e-beam inspection device to carry out a method for inspecting a sample, the method comprising: emitting an e-beam along a beam path; chopping the e-beam into pulses; compressing the pulses; and reducing an energy spread introduced by the compressing.

[0010] Other advantages of the embodiments of the present disclosure will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the present invention.BRIEF DESCRIPTION OF FIGURES

[0011] 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 in which:

[0012] Fig. 1 is a schematic diagram illustrating an exemplary electron beam inspection (EBI) system, consistent with embodiments of the present disclosure.

[0013] Fig. 2A, Fig. 2B, and Fig. 2C are schematic diagrams illustrating exemplary electron beam tools, consistent with embodiments of the present disclosure that may be a part of the exemplary electron beam inspection system of Fig. 1.

[0014] Fig. 3 is a schematic diagram illustrating part of an electron beam tool for pulse compression, consistent with embodiments of the present disclosure.

[0015] Fig. 4 is a graph showing the relationship between pulse duration and probed voltage.

[0016] Fig. 5 is a schematic diagram illustrating parts of an electron beam tool for pulse compression, consistent with embodiments of the present disclosure.

[0017] Fig. 6 shows phase space plots for a pulse compression scheme.

[0018] Fig. 7 shows phase space plots for another pulse compression scheme.

[0019] Fig. 8 shows phase space plots for another pulse compression scheme.

[0020] Fig. 9 is a process flow chart representing an exemplary method for assessment of a sample, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0021] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the disclosed embodiments as recited in the appended claims. For example, although some embodiments are described in the context of utilizing electron beams, the disclosure is not so limited. Other types of charged particle beams may be similarly applied.

[0022] One component of improving yield is monitoring the chip making process to ensure that it is producing a sufficient number of functional integrated circuits. One way to monitor the process is to inspect the chip circuit structures at various stages of their formation. Inspection can be carried out using an SEM. An SEM can be used to image these extremely small structures, in effect, taking a “picture” of the structures. The image can be used to determine if the structure was formed properly and also if it was formed in the proper location. If the structure is defective, then the process can be adjusted so the defect is less likely to recur. It may be desirable to have higher throughput for defect detection and inspection processes to meet the requirements of IC manufacturers.

[0023] During inspection the SEM produces signals for different locations on the sample (e.g. wafer). These signals form brightness values for the pixels that build up the “picture” of the structures. Some types of defects lead to weaker signals that result in pixels harder to distinguish from locations without defects. A pulsed electron beam with pulse compression can be used for the inspection. By compressing the pulses, the brightness of the pulses is increased. The increased tbrightness leads to stronger signals during inspection. The stronger signals lead to pixels for defects that are more easily distinguished from pixels for locations without defects and / or allow the “picture” to be built up more quickly.

[0024] Relative dimensions of components in drawings may be exaggerated for clarity. Within the following description of drawings, the same or like reference numbers refer to the same or like components or entities, and only the differences with respect to the individual embodiments are described. As used herein, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, if it is stated that a component may include A or B, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or A and B. As a second example, if it is stated that a component may include A, B, or C, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

[0025] Reference is now made to Fig. 1, which illustrates an exemplary EBI system 10 that may include a detector, consistent with embodiments of the present disclosure. EBI system 10 may be used for imaging. As shown in Fig. 1, EBI system 10 includes a main chamber Il a load / lock chamber 20, an electron beam tool 100 (also referred to herein as an electron beam apparatus, an electron-optical device or an e-beam inspection device), and an equipment front end module (EFEM) 30. Electron beam tool 100 is located within main chamber 11. EFEM 30 includes a first loading port 30a and a second loading port 30b. EFEM 30 may include additional loading port(s). First loading port 30a and second loading port 30b receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other material(s)) or samples to be inspected (wafers and samples may be collectively referred to as “samples” herein).

[0026] 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 is subject 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.

[0027] Fig. 2A illustrates a charged particle beam apparatus in which an inspection system may comprise a multi-beam inspection tool that uses multiple primary electron beamlets to simultaneously scan multiple locations on a sample.

[0028] As shown in Fig. 2A, an 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 an 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 a beam separator 222, deflection scanning unit 226, and objective lens 228. Electron detection device 244 may comprise detection sub-regions 246, 248, and 250.

[0029] 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 apparatus 100A. Secondary optical system 242 and electron detection device 244 may be aligned with a secondary optical axis 252 of electron beam apparatus 100A.

[0030] 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 being emitted 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.

[0031] 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. Pat. 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.

[0032] 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 a moveable condenser lens that may be configured so that the position of its first principle plane is movable. The movable condenser lens may be configured to be magnetic, which may result in off-axis beamlets 216 and 218 tlanding 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 movable condenser lens. In some embodiments, the moveable condenser lens may be a moveable anti-rotation condenser lens, which involves an anti-rotation lens with a movable first principal plane. A moveable condenser lens is further described in U.S. Publication No. 2017 / 0025241, which is incorporated by reference in its entirety.

[0033] Objective lens 228 may focus beamlets 214, 216, and 218 onto a wafer 230 (i.e. a sample) for inspection and may form a plurality of probe spots 270, 272, and 274 on the surface of wafer 230.

[0034] 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.

[0035] Deflection scanning unit 226 may deflect beamlets 214, 216, and 218 to scan probe spots 270, 272, and 274 over a surface area 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 surface area of wafer 230.

[0036] Although Fig. 2A shows an example of electron beam tool 100 as a multi-beam tool that uses a plurality of beamlets, embodiments of the present disclosure are not so limited. For example, electron beam tool 100 may also be a single-beam tool that uses only one primary electron beam to scan one location on a wafer at a time.

[0037] As shown in Fig. 2B, an electron beam tool 100B (also referred to herein as electron beam apparatus 100B) may be a single -beam inspection tool that is used in EBI system 10. Electron beam apparatus 100B includes an electron-optical device configured to project electrons towards a sample location (i.e. where the wafer is) and a wafer holder 136 supported by motorized stage 134 to hold a wafer 150 (i.e. a sample) to be inspected. Electron beam tool 100B includes an electron emitter, which may comprise a cathode 103, an anode 121, and a gun aperture 122. Electron beam tool 100B further includes a beam limit aperture 125, a condenser lens 126, a column aperture 135, an objectivelens 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 an 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 primary 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.

[0038] 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 connect with detector 144 of electron beam tool 100B 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 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.

[0039] 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.

[0040] 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 100B may comprise a first quadrupole lens 148 and a second quadrupole lens 158. In some embodiments, the quadrupole lenses are used for controlling the electron beam. For example, tfirst quadrupole lens 148 can be controlled to adjust the beam current and second quadrupole lens 158 can be controlled to adjust the beam spot size and beam shape.

[0041] Fig. 2B illustrates a charged particle beam apparatus in which an inspection system may use a single primary beam that may be 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. 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, 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.

[0042] Another example of a charged particle beam apparatus will now be discussed with reference to Fig. 2C. Electron beam tool 100C (also referred to herein as an electron beam apparatus 100C or an electron-optical device) may be an example of electron beam tool 100 and may be similar to electron beam tool 100A shown in Fig. 2A.

[0043] As shown in Fig. 2C, 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. For a dispersion plane 224 of beam separator 222, Fig. 2C shows dispersion of beamlet 214 with nominal energy V0 and an energy spread AV into beamlet portions 262 corresponding to energy V0, beamlet portion 264 corresponding to energy VO+AV / 2, and beamlet portion 266 corresponding to energy VO-AV / 2. The total force exerted by beam separator 222 on an electron of secondary electron beams 236, 238, and 240 can 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.

[0044] A semiconductor electron detector (sometimes called a “PIN detector”) may be used in apparatus 100 in EBI system 10. EBI system 10 may be a high-speed wafer imaging SEM including an image processor. An electron beam generated by EBI system 10 may irradiate the surface of a sample or may penetrate the sample. EBI system 10 may be used to image a sample surface or structures under the surface, such as for analyzing layer alignment. In some embodiments, EBI system 10 may detect and report process defects relating to manufacturing semiconductor wafers by, for example, comparing SEM images against device layout patterns, or SEM images of identical patterns at other locations on the wafer under inspection. A PIN detector may include a silicon PIN tdiode that may operate with negative bias. A PIN detector may be configured so that incoming electrons generate a relatively large and distinct detection signal. In some embodiments, a PIN detector may be configured so that an incoming electron may generate a number of electron-hole pairs while a photon may generate just one electron-hole pair. A PIN detector used for electron counting may have numerous differences as compared to a photodiode used for photon detection, as shall be discussed as follows.

[0045] The controller may comprise a storage that is a storage medium such as a hard disk, random access memory (RAM), other types of computer readable memory, and the like. The storage may be used for saving scanned raw image data as original images, and post-processed images. A non- transitory computer readable medium may be provided that stores instructions for a processor of controller 109 to carry out charged particle beam detection, sampling period determination, image processing, or other functions and methods consistent with the present disclosure. 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 CD-ROM, any other optical data storage medium, any physical medium with patterns of holes, a ROM, a PROM, and EPROM, a FLASH- EPROM or any other flash memory, NVRAM, a cache, a register, any other memory chip or cartridge, and networked versions of the same.

[0046] Block diagrams in the figures may illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer hardware / 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.

[0047] An electron beam tool 100 may scan a sample (e.g. a wafer). The electron beam tool 100 may scan one or more electron beams across the surface of the sample. A detector of the electron beam tool 100 is configured to detect signal electrons from the sample. By detecting the signal electrons, information about the sample can be obtained. The signal electrons may include backscattered electrons and / or secondary electrons.

[0048] During the scan the electron beam tool 100 may scan sequential locations of the sample. The sequential locations may be adjacent to each other at the surface of the sample. The sample may tbe scanned row by row. Each location may correspond to a pixel in an image that can be constructed from the data obtained by the electron beam tool 100 during the scan.

[0049] Fig. 3 is a schematic diagram illustrating part of an electron beam tool 100, consistent with embodiments of the present disclosure. As shown in Fig. 3, in an embodiment the electron beam tool 100 comprises an emitter 91. The emitter 91 is configured to emit a beam of electrons along a beam path. The beam path generally extends from the emitter 91 to the sample 208.

[0050] In an embodiment the emitter 91 is, or forms part of, the electron source 202 shown in Fig. 2A or Fig. 2C and described elsewhere. In an embodiment, the emitter 91 corresponds to the electron emitter of the electron beam tool 100B shown in Fig. 2B. In an embodiment, the emitter 91 comprises a cathode configured to emit electrons. In an embodiment the emitter 91 comprises an accelerator such as an anode configured to accelerate electrons from the cathode. In an embodiment the emitter 91 comprises a gun aperture. Alternatively the gun aperture may be omitted. In an embodiment the electron beam tool 100 comprises a pulse generator configured to generate pulses of electrons. The pulse generator may comprise the emitter 91. However, it is not essential for a pulse generator to be provided. In an alternative embodiment a continuous beam 41 is transmitted to the compressor 80, which is described in more detail below.

[0051] As shown in Fig. 3, in an embodiment the electron beam tool 100 comprises a chopper 70. The chopper 70 may be part of the pulse generator. The chopper 70 is configured to chop the beam 41 into pulses 43 of electrons. In an embodiment, the emitter 91 is configured to emit a substantially continuous beam 41 of electrons. The continuous beam 41 is separated into separate pulses 43 of electrons. The pulses 43 are spatially separated from each other. The pulses 43 continue along the beam path towards the sample 208.

[0052] In an embodiment the chopper 70 is configured to chop the beam 41 into pulses 43 by deflecting away and / or blocking (i.e. discarding) of the electrons of the continuous beam 41. As electrons are blocked from the continuous beam 41, gaps are formed downbeam of the chopper 70. The gaps separate the electron pulses 43 from each other.

[0053] As shown in Fig. 3, in an embodiment, the electron beam tool 100 comprises a compressor 80. The compressor 80 is configured to compress the beam (e.g. the pulses 43) to form compressed pulses 44. The compressor 80 is configured to compress the pulses 43 by shortening the length of the pulses 43 in the longitudinal direction. The longitudinal direction refers to the direction of the beam path. In the orientation shown in Fig. 3, the longitudinal direction is horizontal. In contrast, in Fig. 2A, Fig. 2B and Fig. 2C, the longitudinal direction is shown vertically.

[0054] In an embodiment, the electron beam tool 100 is configured to direct the compressed pulses 44 along the beam path onto a sample location i.e. onto the sample 208. For example, as described elsewhere the electron beam tool 100 may comprise one or more electron-optical elements configured to manipulate the electrons along the beam path from where the electrons are emitted to where theelectrons reach the sample 208. For example, the electron beam tool 100 may comprise an objective lens configured to focus the compressed pulses 44 of electrons onto the sample 208.

[0055] Merely as an example, Fig. 3 schematically shows a first electron-optical element 92, a second electron-optical element 93 and a third electron-optical element 94 configured to manipulate electrons along the beam path. The electron-optical elements 92-94 may be configured to apply electromagnetic fields to the beam path. For example, one or more of the electron-optical elements 92-94 may comprise a solenoid configured to generate an electromagnetic field.

[0056] Additionally, or alternatively one or more of the electron-optical elements 92-94 may comprise one or more metal plates and / or one or more exciting coils.

[0057] As shown in Fig. 3, in an embodiment the electron-optical elements 92-94 may be interspersed among the emitter 91, the chopper 70 and the compressor 80. For example, a first electron-optical element 92 may be located downbeam of the emitter 91 and a beam of the chopper 70. A second electron-optical element 93 may be located downbeam of the chopper 70 and upbeam of the compressor 80. A third electron-optical element 94 may be located downbeam of the compressor 80. The electron-optical elements 92-94 need not necessarily be singular electron-optical elements. For example, the first two electron-optical elements 92, 93 may be between the emitter 91 and the chopper 70.

[0058] In an embodiment, the electron beam tool 100 comprises a detector, for example the electron detection device 244 shown in Fig. 2A or Fig. 2C or the detector 144 shown in Fig. 2B. The detector is configured to detect signal electrons from the sample location (e.g. from the sample 208) so as to assess the sample 208 located at the sample location. The signal electrons may comprise secondary electrons.

[0059] In an embodiment, the electron beam tool 100 is configured to scan the compressed pulses 44 over the sample 208 while monitoring the returned secondary electron signal. Structures at the surface of the sample 208 may charge up depending on the extent to which they are electrically connected to ground. Some structures may have a high resistance towards ground but are not completely disconnected from ground. Where the resistance to ground is below a threshold, the signal received from a location on the surface of the sample 208 may be indistinguishable from locations that are well connected to ground.

[0060] An embodiment of the invention is expected to lower the detection threshold for defects where structures are not completely disconnected from ground. By forming compressed pulses from the emitted beam of electrons, the peak current in the beam is increased. By increasing the peak current in the beam, the contrast in the detected signals from well-connected structures and structures with low resistance to ground may be increased. This improves the sensitivity of the assessment of the sample 208.

[0061] In an embodiment a plurality of measurements for each location on the surface of a sample 208 are made. The measurement results are combined so as to provide an improved measurement tsignal, for example by averaging the measurement results. An embodiment of the invention is expected to reduce the time required to detect weakly connected structures. By increasing the peak current of electrons scanned on to the sample 208, the signal contrast is improved. A smaller number of repeat measurements may be required in order to sufficiently detect defects. The speed of assessing a sample 208 may be improved.

[0062] Fig. 4 is a graph showing the relationship between the duration of pulses and the probed voltage. Fig. 4 shows four plots corresponding to four different pulses having different durations. The total number of electrons in each pulse is substantially the same. However, the pulse duration differs. The longer pulses have lower peak voltages and corresponding lower peak beam currents. By concentrating the electron beam current into pulses of shorter duration, higher voltages may be accumulated. Such higher voltages improve the sensitivity of detecting defects in the sample 208.

[0063] In an embodiment, the electron beam tool 100 is configured such that the compressed pulses 44 have a duration at the sample location of at most 10ns, optionally at most Ins, optionally at most 500ps, optionally at most 200ps, optionally at most lOOps, optionally at most 50ps, optionally at most 20ps, and optionally at most lOps.

[0064] In an embodiment, the compressor 80 is configured to control speeds of the electrons along the beam path such that the compressed pulses 44 reduce in length downbeam of the compressor 80. This can be seen in Fig. 3, where the compressed pulses 44 immediately downbeam of the compressor 80 are longer than the compressed pulses 44 further downbeam (i.e. further to the right in Fig. 3). In an embodiment, the compressor is configured to selectively accelerate and decelerate electrons. For example, a pulse 43 of electrons comprises a plurality of electrons. The pulse may be referred to as a bunch of electrons. In an embodiment, the compressor 80 is configured to decelerate (i.e. slow down) electrons in the front of the bunch (i.e. at the downbeam end of the pulse 43). Additionally, or alternatively, the compressor 80 is configured to accelerate (or speed up) the electrons at the rear of the pulse (i.e. at the upbeam end of the pulse 43). In an embodiment the compressor 80 is configured to cause the electrons of the pulse 43 to congregate downbeam of the compressor 80. The duration of each compressed pulse 44 may gradually shorten as the pulse is directed downbeam away from the compressor 80.

[0065] In an embodiment, the compressor 80 is configured to control speeds of the electrons longitudinal to the beam path (i.e. along the beam path) such that for each pulse, the electrons are longitudinally focussed to a longitudinal focus. The longitudinal focus may be referred to as a temporal focus or a compression focus. At locations closer to the longitudinal focus the peak current of the compressed pulse 44 is greater. At locations further away from the longitudinal focus, the peak current of the compressed pulse 44 is lower.

[0066] In an embodiment the compressor 80 is configured such that a distance between the longitudinal focus and the sample location is less than a distance between the compressor 80 and the longitudinal focus. The distances are measured along the beam path. By providing that the sample iscloser to the longitudinal focus, the pulse duration is shorter compared to at the compressor 80. The peak current in the beam is increased compared to if the compressor 80 were not used. This helps to increase the sensitivity of the assessment of the sample 208.

[0067] In an embodiment, the compressor 80 is configured such that the longitudinal focus is located at or near the sample location. The sample 208 may be placed in the longitudinal focus where the pulse duration is at its shortest. The peak current in the beam is at its maximum. The longitudinal focus being near the sample location means that the distance between the longitudinal focus and the sample location is at most 50%, optionally at most 20%, optionally at most 10%, optionally at most 5% and optionally at most 2% of the distance between the longitudinal focus and the compressor 80.

[0068] An embodiment of the invention is expected to reduce the time taken to measure a defect of a given resistance. By compressing the pulses, the peak beam current is increased, which reduces measurement time. In an embodiment, the compressor 80 is configured such that the pulse duration for a given number of electrons is shorter at the sample location than in the continuous beam emitted by the emitter 91 by a factor of at least 2, optionally at least 5, optionally at least 10, optionally at least 20, optionally at least 50 and optionally at least 100.

[0069] There are gaps between the pulses that arrive at the sample location. The pulse repetition frequency is the frequency with which pulses arrive at the sample location. A lower pulse repetition frequency in general reduces the measurement time. Lower pulse repetition frequencies allow for more electrons to be combined into a pulse, and therefore result in a shorter measurement time. In an embodiment the electron beam tool 100 is configured such that the repetition frequency of pulses is at most 5 GHz, optionally at most 2 GHz, optionally at most 1 GHz and optionally at most 0.5 GHz.

[0070] When the compressed pulse 44 is nearer to the longitudinal focus, the duration of the compressed pulse 44 is shorter. The shorter duration is associated with a higher peak current of the beam. Near or even at the longitudinal focus, the electrons within the compressed pulse 44 may have different energies. There may be an energy spread amongst the electrons. The energy spread may have been increased by the compressor 80 because the compressor 80 accelerates and / or decelerates the electrons.

[0071] Fig. 5 is a schematic diagram illustrating an electron beam tool 100, consistent with embodiments of the present disclosure. Features of the electron beam tool 100 of Fig. 5 which are also shown in Fig. 3 are not described in detail below so as to avoid redundant description.

[0072] As shown in Fig. 5, in an embodiment the electron beam tool 100 comprises a spread reducer 50 (which may also be referred to as a stretcher or a decompressor). The spread reducer is configured to reduce an energy spread among electrons of each of the compressed pulses 44. In an embodiment, the spread reducer 50 is configured to transform the compressed pulses 44 into despreaded pulses 45. The despreaded pulses 45 have a lower energy spread among their electrons compared to the compressed pulses 44.

[0073] By providing the spread reducer 50, the energy spread within each pulse may be reduced. By reducing the energy spread within each pulse, resolution may be improved. The resolution may be improved by reducing chromatic aberrations. Chromatic aberrations are generally expected to be greater for pulses that have a greater energy spread. An embodiment of the invention is expected to improve measurement sensitivity and / or throughput without unduly reducing resolution.

[0074] However, it is not essential for a spread reducer 50 to be provided. The spread reducer 50 may be omitted. The compressor 80 may be configured to introduce an energy spread among the electrons of the pulses that is sufficiently low that the chromatic aberrations are kept sufficiently low and resolution is maintained sufficiently high. For example, the compressor 80 may introduce only a small speed difference between electrons at different ends of the pulses. When only a small speed difference is introduced by the compressor 80, there may be a longer distance between the compressor 80 and the longitudinal focus. The distance between the compressor and the longitudinal focus may be referred to as the drift space length L. In an embodiment, the compressor 80 is configured such that a distance between the compressor 80 and the longitudinal focus is less than a distance between the longitudinal focus and the sample location.

[0075] The drift space length L is directly proportional to the electron pulse duration before compression. The drift space length L is inversely proportional to the difference in speed introduced by the compressor 80. The drift space length L is directly proportional to the square of the speed of the electrons in a pulse.

[0076] The compressor 80 may be configured to introduce a difference in speed so as to result in a target value for the drift space length L. By increasing the difference in speed introduced by the compressor 80, the drift space length L may be reduced. By reducing the drift space length L, the electron beam tool 100 may be made more compact.

[0077] In an embodiment the compressor 80 is configured such that the drift space length L is at most 2m, optionally at most Im, optionally at most 0.5m, optionally at most 0.2m and optionally at most 0.1m. By reducing the drift space length L, the size of the electron beam tool 100 may be reduced.

[0078] In an embodiment, the spread reducer 50 is configured to control speeds of the electrons along the beam path such that the compressed pulses 44 remain substantially constant in length (i.e. duration and size) downbeam of the spread reducer 50. For example, the spread reducer 50 may be configured to accelerate electrons at one end of the pulse and decelerate electrons at the other end of the pulse. For example, the spread reducer 50 may be configured to accelerate electrons ahead in bunches and slow those in the rear. The spread reducer 50 is configured to remove or substantially reduce electron energy spread introduced by the compressor 80.

[0079] An embodiment of the invention is expected to reduce the size of the electron beam tool 100 without significantly increasing the risk of chromatic aberrations. The compressor 80 may be configured to provide for a short drift space L. This can help to reduce the size of the electron beam ttool 100. The greater energy spread introduced by the compressor 80 may be reduced by the spread reducer 50. The spread reducer 50 allows for the use of stronger compression to introduce a larger energy spread and achieve a shorter drift space length L. The spread reducer 50 reduces the energy spread once the pulse has been appropriately compressed. The addition of the spread reducer 50 allows for shortening of the drift space length L by using stronger compression, without unduly increasing chromatic aberrations.

[0080] In an embodiment, a distance between the spread reducer 50 and the longitudinal focus (onto which the compressor 80 is configured to longitudinally focus the electrons of each pulse) is less than a distance between the compressor 80 and the spread reducer 50. The distances are measured along the beam path. The distance between the compressor 80 and the spread reducer 50 allows the pulses to gradually compress (i.e. reduce in size and duration). The compressed pulses 44 may then have their energy spread reduced by the spread reducer 50. By reducing the distance between the spread reducer 50 and the longitudinal focus, the pulse duration may be lower.

[0081] In an embodiment, the spread reducer 50 is located near the longitudinal focus. By providing that the spread reducer 50 is located near the longitudinal focus, the pulse duration may be reduced. By reducing the pulse duration, the peak current of the beam may be increased, which may increase the sensitivity and / or throughput of the electron beam tool 100.

[0082] In an embodiment, the spread reducer 50 is distanced from the longitudinal focus. By distancing the spread reducer 50 from the longitudinal focus, there remains in each pulse a difference in electron energy that is dependent on the position of electrons within the pulse. This allows the spread reducer 50 to accelerate and decelerate electrons differently dependent on their position within the pulse. This helps the spread reducer 50 to control how the energy spread is reduced, without causing the compressed pulse 44 to undesirably spread out in space.

[0083] In an embodiment, the spread reducer 50 is located upbeam of the longitudinal focus. By locating the spread reducer 50 upbeam of the longitudinal focus, the pulse may continue to compress downbeam of the spread reducer 50 until it reaches the sample 208. However, it is not essential for the spread reducer to be located upbeam of the longitudinal focus. In an alternative embodiment, the spread reducer 50 is located downbeam of the longitudinal focus.

[0084] In an embodiment, the electron beam tool 100 comprises an objective lens such as the objective lens 228 or the objective lens assembly 132 described elsewhere. The objective lens is configured to focus the electrons onto the sample location.

[0085] In an embodiment, a distance between the spread reducer 50 and the objective lens 228 is less than a distance between the compressor 80 and the spread reducer 50. The distance between the compressor 80 and the spread reducer 50 allows for the electrons within a pulse to drift together, i.e. compress and reduce in duration. By reducing the distance between the spread reducer 50 and the objective lens 228, a greater distance for the drift space length L. This may help to reduce the extent to which the compressor 80 introduces an energy spread within the pulse.

[0086] In an embodiment, the spread reducer 50 is located near the objective lens 228. By locating the spread reducer 50 near the objective lens 228, the drift space length L may be substantially maximised for a given location of the compressor 80.

[0087] In an embodiment the spread reducer 50 is located upbeam of the objective lens 228. By locating the spread reducer 50 upbeam of the objective lens 228, the energy spread of the pulse may be reduced before the pulse is focused by the objective lens 228. By reducing the energy spread before focusing, the possibility of undesirable chromatic aberrations may be reduced. By locating the spread reducer 50 upbeam of the objective lens 228, it is not necessary to allow sufficient space between the objective lens 228 and the sample location to provide for the spread reducer 50. An embodiment of the invention is expected to make the electron beam tool 100 more compact.

[0088] In an embodiment a distance between an emitter 91 and the compressor 80 is less than a distance between the compressor 80 and the sample location. This helps to allow for a greater drift space length L for a given location of the emitter 91.

[0089] In an embodiment, the compressor 80 is configured to introduce into each pulse an energy spread of at least 5 eV, optionally at least 10 eV, optionally at least 20 eV and optionally at least 50 eV. When the spread reducer 50 is provided, the compressor 80 may be configured to introduce into each pulse higher energy spreads. For example, in an embodiment, the compressor 80 is configured to introduce into each pulse an energy spread of at least 100 eV, optionally at least 200 eV, optionally at least 500 eV, optionally at least 1 keV, optionally at least 2 keV, optionally at least 5 keV, optionally at least 10 keV and optionally at least 20 keV. By increasing the introduced energy spread, the drift space length L may be reduced. By reducing the drift space length L, the size of the electron beam tool 100 may be reduced. In an embodiment, the compressor 80 is configured to introduce into each pulse an energy spread among the electrons of at most 20 keV, optionally at most 10 keV, optionally at most 5 keV, optionally at most 2 keV, optionally at most 1 keV, optionally at most 500 eV, optionally at most 200 eV, optionally at most 100 eV, at most 50 eV, and optionally at most 20 eV. By reducing the energy spread, the risk of chromatic aberration may be reduced.

[0090] In an embodiment, the chopper 70 is configured to substantially conserve emittance of the electrons transverse to the beam path. In an embodiment the compressor 80 is configured to substantially conserve emittance of the electrons transverse to the beam path. In an embodiment, the spread reducer 50 is configured to substantially conserve emittance of the electrons transverse to the beam path. By conserving emittance of the electrons, the focusability of the beam is maintained.

[0091] It is possible that one or more or the chopper 70, the compressor 80 and the spread reducer 50 may function as a lensing element on the beam. In an embodiment, the electron-optical elements 92-94 and / or the objective lens 228 may be configured to compensate for any lensing effect of any of the chopper 70, the compressor 80 and the spread reducer 50.

[0092] Fig. 6 shows phase space plots for a pulse compression scheme. The vertical dimension of the polygon shown on each plot relates to the size of the energy spread of electrons within each pulse.A greater vertical dimension corresponds to a greater energy spread. The horizontal dimension of the polygon is related to the pulse duration. A greater horizontal dimension corresponds to a greater pulse duration.

[0093] The left-hand plot represents the energy spread and pulse duration of the separated pulses 43 of electrons before the pulses 43 reach the compressor 80. The pulses 43 undergo a compression 61. As a result of the compression, the pulses have a greater energy spread introduced. As the compressed pulses 44 undergo a drift 62 through the drift space length L, the pulse duration gradually decreases. This is shown in the change from the middle plot to the right hand plot.

[0094] The pulse compression scheme shown in Fig. 6 does not require any spread reducer 50. However, the drift space length L may be relatively long.

[0095] Fig. 7 shows phase space plots for a different pulse compression scheme. The left-hand plot is the same as the left-hand plot in Fig. 6 because the properties of the separated pulses 43 are the same. The compression 61 in Fig. 7 is stronger than the compression 61 in Fig. 6. After the stronger compression 61, a greater energy spread introduced into the compressed pulses 44. This is shown by the second plot in Fig. 7 being more skewed than the second plot in Fig. 6.

[0096] The compressed pulses 44 drift 62 through the drift space length L. The drift space length L may be shorter than in the pulse compression scheme of Fig. 6. As a result of the drift 62, the pulse duration is reduced in length, as shown by the horizontal dimension of the third plot in Fig. 7 being smaller than in the first two plots.

[0097] The compressed pulses 44 having shortened in duration then undergo a decompression 63 (which may also be referred to as a spread reduction or a stretch). The decompression 63 causes the energy spread of the pulse to be reduced. This is indicated by the right-hand plot in Fig. 7 being skewed relative to the third plot such that a smaller vertical dimension is provided (corresponding to a lower energy spread).

[0098] The pulse compression scheme of Fig. 7 has a shorter drift space length L. This may allow the electron beam tool to be made more compactly. The spread reducer 50 is provided to perform the decompression 63. The decompression may be performed to remove energy when the pulse is close to the longitudinal focus. This helps to achieve the combination of a short drift space length L with a low energy spread.

[0099] Fig. 8 shows phase space plots form a different pulse compression scheme. As shown in Fig. 8, the strength of the compression 61 is the same as the strength of the compression 61 of Fig. 7. This is shown by the second plot having the same skew as in Fig. 7.

[0100] As shown in Fig. 8, after a short drift space length L through which the electrons of each pulse undergo drift 62, the pulse duration is reduced. This is shown by the narrow horizontal dimension in the third plot of Fig. 8. However, the pulse has a relatively high energy spread, which may be undesirable for increasing chromatic aberration and / or increasing the size of the spot on thesample 208. A high energy spread is introduced by the compression 61 and the high energy spread is retained throughout the beam path. This may negatively impact focusing of the electron beam.

[0101] Fig. 9 is a flow chart showing a method for assessing a sample 208, consistent with embodiments of the present disclosure. As shown in Fig. 9, in an embodiment the method comprises emitting 64 a beam of electrons along a beam path. The beam may be a continuous beam 41 emitted by the emitter 91.

[0102] As shown in Fig. 9, in an embodiment the method comprises chopping 65 the beam 41 into pulses 43. Gaps are provided between the pulses 43 so that the pulses 43 are separated from each other.

[0103] As shown in Fig. 9, in an embodiment the method comprises compressing 66 the pulses 43 into compressed pulses 44. The compressor 80 may perform the compression. The compressor 80 may be tuned so as to control the strength of the compression, i.e. the energy spread introduced into the pulses 43.

[0104] As shown in Fig. 9, in an embodiment the method comprises reducing 67 an energy spread of each compressed pulse 44. Alternatively, the step of reducing 67 the energy spread may be omitted.

[0105] As shown in Fig. 9, in an embodiment the method comprises focusing 68 the electrons onto a surface of a sample 208. The focusing may be performed by, for example, an objective lens 228.

[0106] As shown in Fig. 9, in an embodiment the method comprises detecting 69 signal electrons from the sample 208 so as to assess the sample 208.

[0107] In an embodiment a non-transitory computer readable medium stores instructions for a processor of a controller (e.g. the controller 109) to carry out a method as described above.

[0108] There are different possible implementations for the chopper 70. In an embodiment, the chopper 70 is configured to generate a time-dependent electromagnetic field to deflect the electrons transverse to the beam path. This is shown in Fig. 3 for example where the continuous beam 41 from the emitter becomes a deflected beam 42.

[0109] As shown in Fig. 3, for example, in an embodiment the chopper 70 comprises a blocker 73 (which may also be referred to as a screen). The blocker 73 is configured to block deflected electrons and transmit pulses 43 of electrons. By blocking deflected electrons, these deflected electrons are prevented from undesirably landing on the sample 208 at uncontrolled times. By blocking some of the electrons from the initial continuous beam 41, the overall current of the beam is reduced.However, the peak current is increased by compressing the pulses 43 into compressed pulses 44.

[0110] In an embodiment the chopper 70 is configured to transmit at least 10%, optionally at least 20%, optionally at least 50% of the continuous beam 41. For example, the chopper 70 may be configured to block 75% of the emitted electrons and to transmit 25% of the emitted electrons. By deflecting and blocking, the chopper 70 may separate the continuous beam 41 into separated pulses 43.

[0111] In an embodiment, the time-dependent electromagnetic field is a magnetic field 72 transverse to the beam path. For example, in an embodiment the chopper 70 comprises a chopper field generator configured to generate the time-dependent electromagnetic field (e.g. the magnetic field 72).

[0112] In an embodiment the time-dependent electromagnetic field is an oscillating electromagnetic field. Alternative waveforms may be used.

[0113] As shown in Fig. 3, for example, in an embodiment the chopper 70 comprises a chopper chamber 71. The chopper chamber 71 may be configured to define a cavity within the chamber. The chopper field generator may be configured to generate the time-dependent electromagnetic field in the chopper chamber 71. The chopper 70 may comprise a radio-frequency (RF) cavity.

[0114] It is not essential for the chopper 70 to comprise an RF cavity. It is desirable for the timedependent electromagnetic field that deflects the electrons to be changed rapidly. In an embodiment the chopper 70 comprises one or more (e.g. two) solenoids. The solenoids may be arranged to have a relatively low induction. Solenoids having a low induction may be configured to generate a quickly changing time-dependent electromagnetic field. Alternatively, a capacitive beam blanker may be used. The capacitive beam blanker may comprise two parallel plates at a voltage difference.However, the chopper 70 is not essential. It is possible to make a pulsed electron beam in a different way than a chopper. In an embodiment the electron beam tool 100 comprises an illuminator configured to illuminate the continuous beam 41 with radiation pulses. Electrons are released upon photo-excitation, resulting in electron pulses. In an embodiment the illuminator comprises a laser configured to illuminate the beam 41 with a modulated (pulsed) laser. It is not essential to separate the continuous beam 41 into separate pulses 43. In an alternative embodiment the continuous beam 41 is incident on the compressor 80.

[0115] In an embodiment the compressor 80 is configured to generate a time-dependent electromagnetic field to selectively accelerate and decelerate electrons. For example, the timedependent electromagnetic field may be generated to accelerate electrons at one end of a pulse 43 and to decelerate electrons at another end of the pulse 43. In an embodiment the time-dependent electromagnetic field is an oscillating electromagnetic field.

[0116] In an embodiment the time-dependent electromagnetic field of the compressor 80 is an electric field 82. The electric field 82 may be longitudinal to the beam path. In an embodiment the electric field comprises a component longitudinal to the beam path.

[0117] For example, in one implementation the compressor 80 comprises a compressor chamber 81, as shown in Fig. 3, for example. In an embodiment, the compressor 80 comprises a compressor field generator configured to generate the time-dependent electromagnetic field (e.g. the electric field 82) in the compressor chamber 81. The compressor chamber 81 may define a cavity within the compressor chamber 81.

[0118] In an embodiment, the compressor 80 is implemented as an RF cavity. Alternatively, the compressor 80 may be implemented as one or more electron-optical elements configured to produce a rapidly varying electromagnetic field. For example, a plurality of surfaces of metal plates may be used to produce a rapidly varying electric field. For example, two metal plates across the beam path may be provided with a potential difference between them. In an embodiment, the compressor 80 may comprise a photoconductive switch configured to rapidly change the electric field over time.

[0119] In an embodiment the spread reducer 50 is configured to generate a time-dependent electromagnetic field to selectively accelerate and decelerate electrons. For example, the timedependent electromagnetic field may be generated to accelerate electrons at one end of a compressed pulse 44 and to decelerate electrons at another end of the compressed pulse 44. In an embodiment the time-dependent electromagnetic field is an oscillating electromagnetic field.

[0120] In an embodiment the time-dependent electromagnetic field of the spread reducer 50 is an electric field. The electric field may be longitudinal to the beam path. In an embodiment the electric field comprises a component longitudinal to the beam path.

[0121] For example, in one implementation the spread reducer 50 comprises a reducer chamber 51, as shown in Fig. 5. In an embodiment, the spread reducer 50 comprises a reducer field generator configured to generate the time-dependent electromagnetic field (e.g. the electric field) in the reducer chamber 51. The reducer chamber 51 may define a cavity within the reducer chamber 51.

[0122] In an embodiment, the spread reducer 50 is implemented as an RF cavity. Alternatively, the spread reducer 50 may be implemented as one or more electron-optical elements configured to produce a rapidly varying electromagnetic field. For example, a plurality of surfaces of metal plates may be used to produce a rapidly varying electric field. For example, two metal plates across the beam path may be provided with a potential difference between them. In an embodiment, the spread reducer 50 may comprise a photoconductive switch configured to rapidly change the electric field over time.

[0123] Exemplary embodiments of the present disclosure are set out in the following numbered clauses:1. A charged particle-optical device configured to assess a sample, the charged particle-optical device comprising: an emitter configured to emit a beam of charged particles along a beam path; a compressor configured to compress the beam to form compressed pulses, wherein the charged particle-optical device is configured to direct the compressed pulses along the beam path onto a sample location; and a detector configured to detect signal charged particles from the sample location so as to assess a sample located at the sample location.2. The charged particle-optical device of clause 1, comprising: a spread reducer configured to reduce an energy spread among charged particles of each of the compressed pulses.3. The charged particle-optical device of clause 2, wherein the spread reducer is configured to control speeds of the charged particles along the beam path such that the compressed pulses remain substantially constant in length downbeam of the spread reducer.4. The charged particle-optical device of clause 2 or 3, wherein the spread reducer is configured to generate a time-dependent electromagnetic field to selectively accelerate and decelerate charged particles.5. The charged particle-optical device of clause 4, wherein the time-dependent electromagnetic field is an electric field longitudinal to the beam path.6. The charged particle-optical device of clause 4 or 5, wherein the spread reducer comprises: a reducer chamber; and a reducer field generator configured to generate the time-dependent electromagnetic field in the reducer chamber.7. The charged particle-optical device of any of clauses 2-6, wherein the spread reducer is configured to substantially conserve emittance of the charged particles transverse to the beam path.8. The charged particle-optical device of any of clauses 2-7, wherein a distance between the spread reducer and a longitudinal focus onto which the compressor is configured to longitudinally focus the charged particles of each pulse is less than a distance between the compressor and the spread reducer.9. The charged particle-optical device of clause 8, wherein the spread reducer is located near the longitudinal focus.10. The charged particle-optical device of clause 8 or 9, wherein the spread reducer is located upbeam of the longitudinal focus.11. The charged particle-optical device of any of clauses 2- 10, comprising an objective lens configured to focus the charged particles onto the sample location.12. The charged particle-optical device of clause 11, wherein a distance between the spread reducer and the objective lens is less than a distance between the compressor and the spread reducer.13. The charged particle-optical device of clause 12, wherein the spread reducer is located near the objective lens.14. The charged particle-optical device of any of clauses 11-13, wherein the spread reducer is located upbeam of the objective lens.15. The charged particle-optical device of any preceding clause, wherein the compressor is configured to control speeds of the charged particles along the beam path such that the pulses reduce in length downbeam of the compressor.16. The charged particle-optical device of any preceding clause, wherein the compressor is configured to generate a time-dependent electromagnetic field to selectively accelerate and decelerate charged particles.17. The charged particle-optical device of clause 16, wherein the time-dependent electromagnetic field is an electric field longitudinal to the beam path.18. The charged particle-optical device of clause 16 or 17, wherein the compressor comprises: a compressor chamber; and a compressor field generator configured to generate the time-dependent electromagnetic field in the compressor chamber.19. The charged particle-optical device of any preceding clause, wherein the compressor is configured to control speeds of the charged particles longitudinal to the beam path such that for each pulse, the charged particles are longitudinally focused to a longitudinal focus.20. The charged particle-optical device of clause 19, wherein the compressor is configured such that a distance between the compressor and the longitudinal focus is less than a distance between the longitudinal focus and the sample location.21. The charged particle-optical device of clause 19, wherein the compressor is configured such that a distance between the longitudinal focus and the sample location is less than a distance between the compressor and the longitudinal focus.22. The charged particle-optical device of clause 21, wherein the compressor is configured such that the longitudinal focus is located at or near the sample location.23. The charged particle-optical device of any preceding clause, wherein the compressor is configured to substantially conserve emittance of the charged particles transverse to the beam path.24. The charged particle-optical device of any preceding clause, wherein a distance between the emitter and the compressor is less than a distance between the compressor and the sample location.25. The charged particle-optical device of any preceding clause, comprising: a pulse generator comprising the emitter configured to generate pulses of charged particles, wherein the compressor is configured to compress the pulses to form the compressed pulses.26. The charged particle-optical device of clause 25, wherein the pulse generator comprises a chopper configured to chop the emitted beam into the pulses to be compressed.27. The charged particle-optical device of clause 26, wherein the chopper is configured to generate a time-dependent electromagnetic field to deflect the charged particles transverse to the beam path.28. The charged particle-optical device of clause 27, wherein the time-dependent electromagnetic field is a magnetic field transverse to the beam path.29. The charged particle-optical device of clause 27 or 28, wherein the chopper comprises: a chopper chamber; anda chopper field generator configured to generate the time-dependent electromagnetic field in the chopper chamber.30. The charged particle-optical device of any of clauses 27-29, wherein the chopper comprises: a blocker configured to block deflected charged particles and transmit pulses of charged particles.31. The charged particle-optical device of any of clauses 26-30, wherein the chopper is configured to substantially conserve emittance of the charged particles transverse to the beam path.32. A method for assessing a sample, the method comprising: emitting a beam of charged particles along a beam path; compressing the beam to form compressed pulses; directing the compressed pulses along the beam path onto a sample; and detecting signal charged particles from the sample so as to assess the sample.33. The method of clause 32, comprising: reducing an energy spread among charged particles of each of the compressed pulses.34. The method of clause 33, wherein the reducing comprises controlling speeds of the charged particles along the beam path such that the compressed pulses remain substantially constant in length downbeam of where the reducing is performed.35. The method of clause 33 or 34, wherein the reducing comprises generating a time-dependent electromagnetic field to selectively accelerate and decelerate charged particles.36. The method of clause 35, wherein the time-dependent electromagnetic field is an electric field longitudinal to the beam path.37. The method of any of clauses 33-36, wherein the reducing comprises substantially conserving emittance of the charged particles transverse to the beam path.38. The method of any of clauses 33-37, wherein a distance between the reducing and a longitudinal focus onto which the compressing longitudinally focuses the charged particles of each pulse is less than a distance between the where compressing is performed and where the reducing is performed.39. The method of clause 38, wherein the reducing is performed near the longitudinal focus.40. The method of clause 38 or 39, wherein the reducing is performed upbeam of the longitudinal focus.41. The method of any of clauses 33-40, comprising focusing the charged particles onto the sample.42. The method of clause 41, wherein a distance between where the reducing is performed and where the focusing is performed is less than a distance between where the compressing is performed and where the reducing is performed.43. The method of clause 42, wherein the reducing is performed near where the focusing is performed. t44. The method of any of clauses 41-43, wherein the reducing is performed upbeam of where the focusing is performed.45. The method of any of clauses 32-44, wherein the compressing comprises controlling speeds of the charged particles along the beam path such that the pulses reduce in length downbeam of where the compressing is performed.46. The method of any of clauses 32-45, wherein the compressing comprises generating a timedependent electromagnetic field to selectively accelerate and decelerate charged particles.47. The method of clause 46, wherein the time-dependent electromagnetic field is an electric field longitudinal to the beam path.48. The method of any of clauses 32-47, wherein the compressing comprises controlling speeds of the charged particles longitudinal to the beam path such that for each pulse, the charged particles are longitudinally focused to a longitudinal focus.49. The method of clause 48, wherein a distance between where the compressing is performed and the longitudinal focus is less than a distance between the longitudinal focus and the sample.50. The method of clause 49, wherein a distance between the longitudinal focus and the sample is less than a distance between where the compressing is performed and the longitudinal focus.51. The method of clause 50, wherein the longitudinal focus is located at or near the sample.52. The method of any of clauses 32-51, wherein the compressing comprises substantially conserving emittance of the charged particles transverse to the beam path.53. The method of any of clauses 32-52, wherein a distance between where the charged particles are emitted and where the compressing is performed is less than a distance between where the compressing is performed and the sample.54. The method of any of clauses 32-53, comprising: generating pulses of charged particles, wherein the compressing compresses the pulses to form the compressed pulses.55. The method of clause 54, comprising chopping the emitted beam into the pulses to be compressed.56. The method of clause 55, wherein the chopping comprises generating a time-dependent electromagnetic field to deflect the charged particles transverse to the beam path.57. The method of clause 56, wherein the time-dependent electromagnetic field is a magnetic field transverse to the beam path.58. The method of clause 56 or 57, comprising blocking deflected charged particles and transmit pulses of charged particles.59. The method of any of clauses 55-58, wherein the chopping substantially conserves emittance of the charged particles transverse to the beam path.60. A non-transitory computer readable medium that stores instructions for a processor of a controller to control a charged particle-optical device to carry out a method for assessing a sample, the tmethod comprising: emitting a beam of charged particles along a beam path; compressing the beam to form compressed pulses; directing the compressed pulses along the beam path onto a sample; and detecting signal charged particles from the sample so as to assess the sample.61. The non-transitory computer readable medium of clause 60, the method comprising: reducing an energy spread among charged particles of each of the compressed pulses.62. The non-transitory computer readable medium of clause 61, wherein the reducing comprises controlling speeds of the charged particles along the beam path such that the compressed pulses remain substantially constant in length downbeam of where the reducing is performed.63. The non-transitory computer readable medium of clause 61 or 62, wherein the reducing comprises generating a time-dependent electromagnetic field to selectively accelerate and decelerate charged particles.64. The non-transitory computer readable medium of clause 63, wherein the time-dependent electromagnetic field is an electric field longitudinal to the beam path.65. The non-transitory computer readable medium of any of clauses 61-64, wherein the reducing comprises substantially conserving emittance of the charged particles transverse to the beam path.66. The non-transitory computer readable medium of any of clauses 61-65, wherein a distance between the reducing and a longitudinal focus onto which the compressing longitudinally focuses the charged particles of each pulse is less than a distance between the where compressing is performed and where the reducing is performed.67. The non-transitory computer readable medium of clause 66, wherein the reducing is performed near the longitudinal focus.68. The non-transitory computer readable medium of clause 66 or 67, wherein the reducing is performed upbeam of the longitudinal focus.69. The non-transitory computer readable medium of any of clauses 61-68, comprising focusing the charged particles onto the sample.70. The non-transitory computer readable medium of clause 69, wherein a distance between where the reducing is performed and where the focusing is performed is less than a distance between where the compressing is performed and where the reducing is performed.71. The non-transitory computer readable medium of clause 70, wherein the reducing is performed near where the focusing is performed.72. The non-transitory computer readable medium of any of clauses 69-71, wherein the reducing is performed upbeam of where the focusing is performed.73. The non-transitory computer readable medium of any of clauses 60-72, wherein the compressing comprises controlling speeds of the charged particles along the beam path such that the pulses reduce in length downbeam of where the compressing is performed.74. The non-transitory computer readable medium of any of clauses 60-73, wherein the compressing comprises generating a time-dependent electromagnetic field to selectively accelerate and decelerate charged particles.75. The non-transitory computer readable medium of clause 74, wherein the time-dependent electromagnetic field is an electric field longitudinal to the beam path.76. The non-transitory computer readable medium of any of clauses 60-75, wherein the compressing comprises controlling speeds of the charged particles longitudinal to the beam path such that for each pulse, the charged particles are longitudinally focused to a longitudinal focus.77. The non-transitory computer readable medium of clause 76, wherein a distance between where the compressing is performed and the longitudinal focus is less than a distance between the longitudinal focus and the sample.78. The non-transitory computer readable medium of clause 77, wherein a distance between the longitudinal focus and the sample is less than a distance between where the compressing is performed and the longitudinal focus.79. The non-transitory computer readable medium of clause 78, wherein the longitudinal focus is located at or near the sample.80. The non-transitory computer readable medium of any of clauses 60-79, wherein the compressing comprises substantially conserving emittance of the charged particles transverse to the beam path.81. The non-transitory computer readable medium of any of clauses 60-80, wherein a distance between where the charged particles are emitted and where the compressing is performed is less than a distance between where the compressing is performed and the sample.82. The non-transitory computer readable medium of any of clauses 60-81, the method comprising: generating pulses of charged particles, wherein the compressing compresses the pulses to form the compressed pulses.83. The non-transitory computer readable medium of clause 82, comprising chopping the emitted beam into the pulses to be compressed.84. The non-transitory computer readable medium of clause 83, wherein the chopping comprises generating a time-dependent electromagnetic field to deflect the charged particles transverse to the beam path.85. The non-transitory computer readable medium of clause 84, wherein the time-dependent electromagnetic field is a magnetic field transverse to the beam path.86. The non-transitory computer readable medium of clause 84 or 85, comprising blocking deflected charged particles and transmit pulses of charged particles.87. The non-transitory computer readable medium of any of clauses 83-86, wherein the chopping substantially conserves emittance of the charged particles transverse to the beam path.88. An e-beam inspection device, comprising: an emitter that emits an e-beam along a beam path; a chopper for chopping the e-beam into pulses; a compressor for compressing the pulses; and a stretcher for reducing an energy spread introduced by the compressor.89. The e-beam inspection device of clause 88, wherein the e-beam inspection device is configured to direct the pulses along the beam path onto a sample location, the e-beam inspection device comprising: a detector for detecting signal electrons from the sample location so as to inspect a sample located at the sample location.90. A method for inspecting a sample with an e-beam inspection device, the method comprising: emitting an e-beam along a beam path; chopping the e-beam into pulses; compressing the pulses; and reducing an energy spread introduced by the compressing.91. The method of clause 90, comprising: directing the pulses along the beam path onto a sample; and detecting signal electrons from the sample so as to inspect the sample.92. A non-transitory computer readable medium that stores instructions for a processor of a controller of an e-beam inspection device to control the e-beam inspection device to carry out a method for inspecting a sample, the method comprising: emitting an e-beam along a beam path; chopping the e-beam into pulses; compressing the pulses; and reducing an energy spread introduced by the compressing.93. The non-transitory computer readable medium of clause 92, the method comprising: directing the pulses along the beam path onto a sample; and detecting signal electrons from the sample so as to inspect the sample.

[0124] A non-transitory computer readable medium may be provided that stores instructions for a processor of a controller (e.g., the controller 109 of Fig. 1) to carry out image inspection, image acquisition, activating charged-particle source, adjusting electrical excitation of stigmators, adjusting landing energy of electrons, adjusting objective lens excitation, adjusting secondary electron detector position and orientation, stage motion control, beam separator excitation, applying scan deflection tvoltages to beam deflectors, receiving and processing data associated with signal information from electron detectors, configuring an electrostatic element, detecting signal electrons, adjusting the control electrode potential, adjusting the voltages applied to the electron source, extractor electrode, and the sample, etc. 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 optical data 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.

[0125] It will be appreciated that the embodiments of the present disclosure are not limited to the exact construction that has been described above and illustrated in the accompanying drawings, and that various modifications and changes may be made without departing from the scope thereof. The present disclosure has been described in connection with various embodiments, other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

[0126] The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A charged particle-optical device configured to assess a sample, the charged particle-optical device comprising: an emitter configured to emit a beam of charged particles along a beam path; a compressor configured to compress the beam to form compressed pulses, wherein the charged particle-optical device is configured to direct the compressed pulses along the beam path onto a sample location; and a detector configured to detect signal charged particles from the sample location so as to assess a sample located at the sample location.

2. The charged particle-optical device of claim 1, comprising: a spread reducer configured to reduce an energy spread among charged particles of each of the compressed pulses.

3. The charged particle-optical device of claim 2, wherein the spread reducer is configured to generate a time-dependent electromagnetic field to selectively accelerate and decelerate charged particles.

4. The charged particle-optical device of claim 3, wherein the spread reducer comprises: a reducer chamber; and a reducer field generator configured to generate the time-dependent electromagnetic field in the reducer chamber.

5. The charged particle-optical device of claim 2, wherein a distance between the spread reducer and a longitudinal focus onto which the compressor is configured to longitudinally focus the charged particles of each pulse is less than a distance between the compressor and the spread reducer.

6. The charged particle-optical device of claim 1, wherein the compressor is configured to control speeds of the charged particles along the beam path such that the pulses reduce in length downbeam of the compressor.

7. The charged particle-optical device of claim 1, wherein the compressor is configured to control speeds of the charged particles longitudinal to the beam path such that for each pulse, the charged particles are longitudinally focused to a longitudinal focus.

8. The charged particle-optical device of claim 7, wherein the compressor is configured such that a distance between the compressor and the longitudinal focus is less than a distance between the longitudinal focus and the sample location.

9. The charged particle-optical device of claim 8, wherein the compressor is configured such that a distance between the longitudinal focus and the sample location is less than a distance between the compressor and the longitudinal focus.

10. The charged particle-optical device of claim 9, wherein the compressor is configured such that the longitudinal focus is located at or near the sample location.

11. The charged particle-optical device of claim 1, wherein the compressor is configured to substantially conserve emittance of the charged particles transverse to the beam path.

12. The charged particle-optical device of claim 11, comprising: a pulse generator comprising the emitter configured to generate pulses of charged particles, wherein the compressor is configured to compress the pulses to form the compressed pulses.

13. The charged particle-optical device of claim 12, wherein the pulse generator comprises a chopper configured to chop the emitted beam into the pulses to be compressed.

14. The charged particle-optical device of claim 13, wherein the chopper comprises: a blocker configured to block deflected charged particles and transmit pulses of charged particles.

15. A non-transitory computer readable medium that stores instructions for a processor of a controller of an e-beam inspection device to control the e-beam inspection device to carry out a method for inspecting a sample, the method comprising: emitting an e-beam along a beam path; chopping the e-beam into pulses; compressing the pulses; and reducing an energy spread introduced by the compressing.

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