EP4659278A1

EP4659278A1 - Leem based surface profile determination

Info

Publication number: EP4659278A1
Application number: EP24700318.9A
Authority: EP
Inventors: Jasper Frans Mathijs VAN RENS; Mathijs Wouter Henk GARMING; Faruk Krecinic; Willem Marie Julia Marcel COENE; Marleen KOOIMAN
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2023-01-31
Filing date: 2024-01-15
Publication date: 2025-12-10
Also published as: EP4411783A1; CN120615227A; KR20250138736A; WO2024160526A1; TW202445632A

Abstract

A system and method for surface profile determination comprising generating multiple interactions between the surface (360) and a set of particle beams (332, 334) and recombining the set of particle beams after interaction with the surface to determine a surface depth profile. The set of particle beams may comprise a particle beam split (340, 342) into a reference (334) and sample (332) beam, a particle beam passed through a Zernike phase plate or other phase splitter, or a set of particle beams with different focal lengths. The particle beams may be recombined holographically, by interference, mathematically (such as based on intensity, phase, amplitude, etc.), where the recombination method may vary based on the type of particle beams used.

Description

LEEM BASED SURFACE PROFILE DETERMINATION

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23154317.4 which was filed on January 31, 2023 and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] The present disclosure relates generally to a system and method for low-energy, electron microscopy (LEEM) depth characterization.

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) and low-energy electron microscopy (LEEM) may 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. Imaging of complex structures such as high-aspect-ratio trenches or holes of 3D NAND devices, thick resists which may have low contrast, etc. may require imaging of 3D features which have projections in the electron beam direction which are larger than a focal length of a traditional scanning electron microscope (SEM) and which may have structures (such as resist layers) which may experience deterioration from high-energy electrons. For applications such as overlay metrology, defect inspection (including resist inspection, etch inspection), etc., it may be desirable to obtain high-quality, non-destructive imaging with information about surface profiles from reflected electrons from LEEM.

[0004] In the context of semiconductor manufacture, wafers need to be monitored and defects identified, preferably at stages where they may be corrected. Various solutions for imaging have been proposed.

SUMMARY

[0005] According to an embodiment, there is provided a particle beam system. The particle beam system comprises a beam splitter configured to split a particle beam into a first particle beam and a second particle beam, the first particle beam and the second particle beam being coherent along at least a portion of their path lengths; a reference line configured to enable passing of the first particle beam from the beam splitter; a sample line configured to enable the second particle beam to reflect from a sample; a delay segment configured to alter a path length of the first particle beam traveled in the reference line and/or to alter a path length of the second particle beam traveled in the sample line; and a beam combiner configured to recombine a) the first particle beam after it passes through the reference line, and b) the second particle beam after it passes through the sample line, wherein the recombination of the first particle beam and the second particle beam causes interference between the first particle beam and the second particle beam.

[0006] In an embodiment, the sample line comprises a low-energy electron microscopy (LEEM) arrangement.

[0007] In an embodiment, the second particle beam is elastically scattered by the sample.

[0008] In an embodiment, the delay segment comprises a particle mirror.

[0009] In an embodiment, the particle mirror is an electrostatic and/or magnetic mirror.

[0010] In an embodiment, the first particle beam after it passes through the reference line comprises particles elastically scattered by the particle mirror.

[0011] In an embodiment, the sample line comprises an electric, magnetic, electromagnetic and/or electrostatic objective lens.

[0012] In an embodiment, the objective lens comprises an immersion objective lens.

[0013] In an embodiment, the sample line and/or the reference line comprises a magnetic prism.

[0014] In an embodiment, further comprising a sensor to detect the recombination, the sensor configured to detect intensity of the interference between the first particle beam and the second particle beam.

[0015] In an embodiment, the sensor is a pixelated particle detector.

[0016] In an embodiment, further comprising: a processor; and one or more non- transitory, machine -readable medium having instructions thereon, the instructions when executed by the processor being configured to: determine phase and/or amplitude of a waveform of the second particle beam after it has interacted with the sample based on the interference between the first particle beam and the second particle beam.

[0017] In an embodiment, the instructions further comprise instructions to: reconstruct a depth profile of the sample based on the determined phase and/or amplitude of the waveform of the second particle beam after it has interacted with the sample.

[0018] In an embodiment, the instructions further comprise instructions to: identify defects in a depth profile of the sample based on the determined phase and/or amplitude of the waveform of the second particle beam after it has interacted with the sample.

[0019] In an embodiment, the instructions further comprise instructions to: apply an aberration correction to the determined phase and/or amplitude of the waveform of the second particle beam after it has interacted with the sample.

[0020] In an embodiment, the instructions further comprise instructions to determine an aberration correction.

[0021] In an embodiment, instructions to determine an aberration correction comprise instructions to determine an aberration correction based on a recombination of the first particle beam and the second particle beam obtained when the second particle beam has not interacted with a sample and/or when the particle beam has interacted with a known sample.

[0022] In an embodiment, the known sample is one or more point-scatterers.

[0023] In an embodiment, the one or more point-scatterers comprise an array in at least one dimension of point-scatterers.

[0024] In an embodiment, the delay segment further configured to alter the path length of the first particle beam and/or to alter the path length of the second particle beam such that the first particle beam and the second particle beam interfere within a coherence length at their recombination.

[0025] In an embodiment, the particle beam is a coherent electron beam.

[0026] In an embodiment, the second particle beam is a low-energy electron beam when interacting with the sample.

[0027] In an embodiment, a spot size of the particle beam illuminates a field of view (FOV) with a dimension greater than 200 nm and wherein the particle beam is substantially non-scanning. [0028] In an embodiment, the beam splitter is a biprism and wherein the beam combiner is a biprism.

[0029] In an embodiment, the beam combiner comprises electromagnetic deflectors.

[0030] According to an embodiment, there is provided a delay line for a particle beam system, wherein the delay line is configured to alter a path length of a particle beam of the particle beam system.

[0031] In an embodiment, the delay line comprises a particle mirror and wherein the particle mirror is an electrostatic and/or magnetic mirror.

[0032] In an embodiment, wherein the delay line is operable to alter a phase of the particle beam to align with a second particle beam to generate a hologram.

[0033] According to an embodiment, there is provided a particle beam system comprising: a sample line configured to enable a particle beam to reflect from a sample; a phase modulator configured to output a first portion of the particle beam with a first phase and a second portion of the particle beam with a second phase; and a sensor being configured to detect a recombination of the first portion and the second portion of the particle beam.

[0034] In an embodiment, the recombination of the first portion and the second portion of the particle beam comprises interference of the first portion of the particle beam and the second portion of the particle beam.

[0035] In an embodiment, the phase modulator comprises a Zernike phase plate and wherein the first portion of the particle beam comprises a diffracted portion of the particle beam and wherein the second portion of the particle beam comprises an undiffracted portion of the particle beam.

[0036] In an embodiment, the particle beam is a coherent electron beam.

[0037] In an embodiment, the particle beam is a low-energy electron beam when interacting with the sample. [0038] In an embodiment, the particle beam comprises particles elastically scattered by the sample.

[0039] In an embodiment, the sensor is a pixelated particle detector.

[0040] According to an embodiment, a method of particle beam microscopy is provided. The method comprising: splitting, by a beam splitter, a particle beam into a first particle beam and a second particle beam, the first particle beam and the second particle beam being coherent along at least a portion of their path lengths; passing the first particle beam by a reference line; passing a second particle beam by a sample line, the sample line configured to enable reflection of the second particle beam from a sample, wherein the reference line and/or the sample line further comprises a delay segment, the delay segment configured to alter a path length of the first particle beam and/or to alter a path length of the second particle beam; and recombining, by a beam combiner, a) the first particle beam after it passes through the reference line and b) the second particle beam after it passes through the sample line, wherein the recombination of the first particle beam and the second particle beam causes interference between the first particle beam and the second particle beam.

[0041] In an embodiment, passing the first particle beam by the reference line further comprises adjusting the path length of the first particle beam to correspond to the path length of the second particle beam.

[0042] In an embodiment, passing the second particle beam by the sample line further comprises adjusting the path length of the second particle beam to correspond to the path length of the first particle beam.

[0043] In an embodiment, further comprising applying an aberration correction to the recombination.

[0044] In an embodiment, further comprising determining the aberration correction, wherein determining the aberration correction comprises: passing the first particle beam by the reference line, the delay segment configured to alter the path length of the first particle beam to correspond to the path length of the second particle beam and/or to alter the path length of the second particle beam to correspond to the path length of the first particle beam; transmitting a second particle beam along the sample line, the sample line configured to enable reflection of the second particle beam from a null sample and/or a known sample; recombining, by the beam combiner, a) the first particle beam after it passes through the reference line and b) the second particle beam after it has been reflected by the null sample and/or after it has been reflected by the known sample to generate aberration-correction information; and determining an aberration correction based on the aberration-correction information. [0045] In an embodiment, further comprising determining phase and/or amplitude of a waveform of the second particle beam after it has interacted with the sample based on the recombination.

[0046] In an embodiment, further comprising reconstructing a depth profile of the sample based on the recombination. [0047] In an embodiment, further identifying defects in a depth profile of the sample based on the recombination.

[0048] In an embodiment, the particle beam is a coherent electron beam.

[0049] In an embodiment, the second particle beam is a low-energy electron beam when interacting with the sample.

[0050] In an embodiment, further comprising detecting the recombination by a pixelated particle detector.

[0051] In an embodiment, a spot size of the second particle beam illuminates a field of view (FOV) on the sample with a dimension greater than 200 nm and wherein the second particle beam is substantially non-scanning.

[0052] According to an embodiment, a method of particle beam microscopy is provided, the method comprising: transmitting a particle beam along a sample line, the sample line configured to enable reflection of the particle beam by a sample; splitting, by a phase modulator, the particle beam after interaction with the sample into a first portion and a second portion, the first portion having a first phase and the second portion having a second phase; and detecting, by a sensor, a recombination of the first portion of the particle beam and the second portion of the particle beam, the recombination containing information about a surface profile of the sample.

[0053] In an embodiment, the recombination of the first portion and the second portion of the particle beam causes interference between the first portion and the second portion of the particle beam, the interference between the first portion and the second portion of the particle beam containing information about relative phases of the first portion of the particle beam and the second portion of the particle beam, where a difference between the relative phases contains information about the surface profile of the sample.

[0054] In an embodiment, the phase modulator comprises a Zernike phase plate and wherein the first portion of the particle beam comprises a diffracted portion of the particle beam and wherein the second portion of the particle beam comprises an undiffracted portion of the particle beam.

[0055] In an embodiment, further comprising determining phase and/or amplitude of a waveform of the particle beam after it has interacted with the sample based on the detected recombination and determining information about the surface profile of the based on the determined phase and/or amplitude.

[0056] In an embodiment, information about the surface profile of the sample comprises information about sample depth, or a sample material, or a combination thereof for the surface.

[0057] In an embodiment, further comprising reconstructing a depth profile of the sample based on the detected recombination.

[0058] In an embodiment, further comprising identifying defects in a depth profile of the sample based on the detected recombination.

[0059] In an embodiment, the particle beam is a coherent electron beam. [0060] In an embodiment, the particle beam is a low-energy electron beam when interacting with the sample.

[0061] In an embodiment, the recombination particle beam comprises a recombination of the first portion of the particle beam and particles elastically scattered by the sample from the second portion of the particle beam.

[0062] In an embodiment, the second portion of the particle beam further comprises particles inelastically scattered by the sample, further comprising: detecting, by the sensor or a second sensor, inelastically scattered particles of the second portion of the particle beam in addition to the recombination.

[0063] In an embodiment, the recombination further comprises a first auto-interference between the first portion of the particle beam and itself and/or a second auto interference between the second portion of the particle beam and itself, further comprising: detecting, by the sensor or a second sensor, the first auto-interference and/or the second auto-interference.

[0064] In an embodiment, the sensor comprises a pixelated particle detector.

[0065] According to an embodiment, a method of particle beam microscopy is provided. The method comprising: for a plurality of focus depths, transmitting a particle beam along a sample line, the sample line configured to enable reflection of the particle beam by a sample; and detecting, by a sensor, the particle beam after interaction with the sample; and determining, based on the detected particle beams for the plurality of focus depths, a reconstruction corresponding to the sample.

[0066] In an embodiment, the sensor comprises a pixelated particle detector and wherein detecting, by the pixelated particle detector, the particle beam comprises generating an image corresponding to intensity of the particle beam after interaction with the sample.

[0067] In an embodiment, determining a reconstruction comprises: generating a reconstructed image corresponding to phase and/or amplitude of the sample based on the images corresponding to the intensity of the particle beam after interaction with the sample for the plurality of focus depths.

[0068] In an embodiment, further comprising determining a depth profile of the sample based on the reconstruction.

[0069] In an embodiment, further comprising identifying defects in a depth profile of the sample based on the reconstruction.

[0070] In an embodiment, the particle beam is a coherent electron beam.

[0071] In an embodiment, the particle beam is a low-energy electron beam when interacting with the sample.

[0072] In an embodiment, the particle beam after interaction with the sample comprises particles elastically scattered by the sample.

[0073] In an embodiment, determining the reconstruction further comprises applying an aberration correction to the reconstruction. [0074] In an embodiment, the reconstruction is a reconstructed wavefunction of the particle beam.

[0075] In an embodiment, the reconstruction is a reconstructed wavefunction of the particle beam after interaction with the sample.

[0076] According to an embodiment, a system is provided, the system comprising: a low- energy, electron microscope (LEEM) apparatus; a processor; and one or more non-transitory, machine -readable medium having instructions thereon, the instructions when executed by the processor being configured to: obtain, for a plurality of focus depths, images of a sample from the LEEM; and determine, based on the obtained images for the plurality of focus depths a wavefunction corresponding to interaction of a particle beam of the LEEM with the sample, the wavefunction containing information about a surface of the sample.

[0077] In an embodiment, the instructions further comprising instructions to identify defects in a depth profile of the sample based on the wavefunction.

[0078] In an embodiment, instruction to identify defects in a depth profile comprise instructions to: identify a plurality of regions in the sample corresponding to features of substantially similar planned depth; determine a phase and/or amplitude of the wavefunction corresponding the plurality of regions; and determine if any of the plurality of regions substantially vary in phase and/or amplitude from others of the plurality of regions.

[0079] In an embodiment, instructions to determine if any of the plurality of regions substantially vary in phase and/or amplitude from others of the plurality of regions comprise instructions to identify lithography defects.

[0080] In an embodiment, the instructions further comprising instructions to determine a depth profile of the sample based on the wavefunction.

[0081] In an embodiment, the sample comprises one of the following: a lithography sample, an after etch sample, an after development sample, a trenched sample, a sample comprising an array of contact holes, a sample comprising contact holes other than in an array, a sample comprising elongated slots, a sample comprising lines and spaces, or a combination thereof.

[0082] In an embodiment, the instructions further comprising instructions to determine, based on the wavefunction, a process defect rate.

[0083] According to an embodiment, an apparatus configured to perform the method of any other embodiment is provided.

[0084] According to another embodiment, one or more non-transitory, machine-readable medium is provided having instructions thereon, the instructions when executed by a processor being configured to perform the method of any other embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS [0085] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0086] Figure 1 is a schematic diagram illustrating an exemplary particle beam system, according to an embodiment.

[0087] Figure 2 is a schematic diagram of an exemplary particle beam tool, according to an embodiment.

[0088] Figure 3 is a schematic diagram of an exemplary particle beam tool configured for holography based on a sample line and a reference line, according to an embodiment.

[0089] Figure 4 is a schematic diagram of an exemplary particle beam tool configured for holography based on a phase modulator, according to an embodiment.

[0090] Figure 5 is a schematic diagram of an exemplary particle beam tool delay line, according to an embodiment.

[0091] Figure 6 is a flowchart that illustrates a method of particle beam holography, according to an embodiment.

[0092] Figure 7 is a schematic diagram of an exemplary recombination of a first particle beam and a second particle beam, according to an embodiment.

[0093] Figure 8 is a schematic diagram of an exemplary aberration correction, according to an embodiment.

[0094] Figure 9A is a graph of a simulated height for an exemplary surface after resist development, according to an embodiment.

[0095] Figure 9B is a simulated SEM image of the exemplary surface of Figure 9A, according to an embodiment.

[0096] Figure 10 is a schematic diagram of multiple object planes, obtained by varying a focal length of an objective lens, at which EEEM images may be acquired for a surface, according to an embodiment.

[0097] Figures 11 A-l 1G are simulated low-energy microscopy (EEEM) images at various object planes, obtained by varying a focal length of an objective lens, for an exemplary surface with contact holes of varying depth, according to an embodiment.

[0098] Figure 12 is a schematic diagram of reconstruction of a reflected wavefront based on multiple LEEM images at different object planes, obtained by varying a focal length of an objective lens, of Figures 11A-11G, according to an embodiment.

[0099] Figure 13 is a block diagram of an example computer system, according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[00100] Embodiments of the present disclosure are described in detail with reference to the drawings, which are provided as illustrative examples of the disclosure so as to enable those skilled in the art to practice the disclosure. Notably, the figures and examples below are not meant to limit the scope of the present disclosure to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present disclosure can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present disclosure will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the disclosure. Embodiments described as being implemented in software should not be limited thereto, but can include embodiments implemented in hardware, or combinations of software and hardware, and vice-versa, as will be apparent to those skilled in the art, unless otherwise specified herein. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the disclosure is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present disclosure encompasses present and future known equivalents to the known components referred to herein by way of illustration.

[00101] Although specific reference may be made in this text to the manufacture of ICs, it should be explicitly understood that the description herein has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” in this text should be considered as interchangeable with the more general terms “substrate” and “target portion”, respectively. A patterning device (for example, a lithography device) can comprise, or can form, one or more patterns, which may correspond to one or die. The patterns can be generated utilizing CAD (computer-aided design) programs, based on a pattern or design layout, this process often being referred to as EDA (electronic design automation).

[00102] Reference is now made to Figure 1, which illustrates an exemplary electron beam system 100 consistent with embodiments of the present disclosure. As shown in Figure 1, the electron beam system 100 includes a main chamber 110, a load-lock chamber 120, an electron beam tool 140, and an equipment front end module (EFEM) 130. Electron beam tool 140 is located within main chamber 110. The exemplary electron beam system 100 may be a single or multi-beam system. While the description and drawings are directed to an electron beam, it should be appreciated that the embodiments of the present disclosure are not limited to specific particles. Electrons may instead or additionally include other charged particles, such as ions, protons, etc. [00103] EFEM 130 includes a first loading port 130a and a second loading port 130b. EFEM 130 may include additional loading port(s). First loading port 130a and second loading port 130b may 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 are collectively referred to as “wafers” hereafter). One or more robot arms (not shown) in EFEM 130 transport the wafers to load-lock chamber 120.

[00104] Load-lock chamber 120 is connected to a load/lock vacuum pump system (not shown), which removes gas molecules in load-lock chamber 120 to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robot arms (not shown) transport the wafer from load-lock chamber 120 to main chamber 110. Main chamber 110 is connected to a main chamber vacuum pump system (not shown), which removes gas molecules in main chamber 110 to reach a second pressure below the first pressure. The second pressure may correspond to an ultra- high vacuum (UHV) pressure, such as on the order of 10'¹⁰ torr. After reaching the second pressure, the wafer is subject to inspection by electron beam tool 140. In some embodiments, the electron beam tool 140 may comprise a single -beam inspection tool. In some embodiments, the electron beam tool 140 may comprise a multi-beam inspection tool.

[00105] Controller 150 may be electronically connected to electron beam tool 140 and may be electronically connected to other components as well. Controller 150 may be a computer configured to execute various controls of electron beam system 100, including control of one or more lens, electron beam source, phase modulator, energy filter, etc. Controller 150 may also include processing circuitry configured to execute various signal and image processing functions. While controller 150 is shown in Figure 1 as being outside of the structure that includes main chamber 110, load-lock chamber 120, and EFEM 130, it is appreciated that controller 150 can be part of the structure.

[00106] FIG. 2 illustrates schematic diagram of an exemplary imaging system 200 according to embodiments of the present disclosure. Electron beam tool 140 of FIG. 2 may be configured for use in electron beam system 100. Electron beam tool 140 may be a single beam apparatus or a multi-beam apparatus. As shown in FIG. 2, electron beam tool 140 includes a motorized sample stage 201, and a wafer holder 202 supported by motorized sample stage 201 to hold a wafer 203 to be inspected.

Electron beam tool 140 further includes an electron beam source 220. The electron beam source 220 is depicted as having a cathode 218, an anode 216, a gun aperture 214, beam limiting apertures 205 and 212, a condenser lens 210, a deflector 204, and an objective aperture 208. The electron beam source 220 is also depicted as having an electron detector 206, which may be an electron detector used to detect secondary electrons (SEs), backscattered electrons (BSEs), etc. In some embodiments, the electron detector 206 may be located in the electron beam source 220 (e.g., as depicted), including embodiments in which the electron beam source 220 is configured to perform multiple types of electron microscopy. For example, the electron detector 206 may be an electron detector used in the electron beam tool 140 in an scanning electron microscopy (SEM) configuration, low energy electron diffraction (LEED) configuration, etc. in addition to or instead of a LEEM configuration, including at different times. The electron detector 206 may also be omitted. In some embodiments the electron detector 206 may be located in a different location in the imaging system 200, such as in the beam path after the electron beam has interacted with a sample, such as a wafer 203. The electron detector 206 may be located after an energy filter, which may select SEs and BSEs from a beam path for detection. Other electron detectors, such as electron detector 280, may have similar or different components to the electron detector 206. The electron beam source 220 may have more or fewer components, including multiple of the components (e.g., the cathode 218, the anode 216, the gun aperture 214, the beam limiting apertures 205 and 212, the condenser lens 210, the deflector 204, the objective aperture 208, and the electron detector 206) depicted. In embodiments where the electron beam tool 140 is configured to operate in at least some instances as an SEM or for LEED, the electron beam tool 140 may also include deflectors or scanning coils configured to scan an electron beam across the wafer 203. In some embodiments, the electron detector 206 may be omitted or may instead be a lens, collimator, or other beam conditioning element. The components may be integrated into one or more components, such that components depicted as separate elements may be instead integrated elements. For example, the cathode, the anode, and the gun aperture 214 may be part of an electron “gun” which may be an encapsulated electron source.

[00107] The electron detector 206 may be a single electron detector, multiple electron detectors, contain multiple electron detector elements, etc. In one embodiment, the electron detector 206 may be an electron detector for the electron beam tool 140, which is a multi-beam tool, a parallel beam tool, a massively parallel beam tool, etc. The electron detector 206 may have one or more apertures, such as to allow penetration of one or more electron beam (such as in a forward direction from the cathode 218 to the motorized sample stage 201). The electron detector 206 may have multiple apertures, such as apparatus which correspond to one or more of multiple electron beams. The electron beam source 220, in some embodiments, may include a modified swing objective retarding immersion lens (SORIL), which may include a pole piece, a control electrode, a deflector, and an exciting coil. Electron beam tool 140 may additionally include an Energy Dispersive X-ray Spectrometer (EDS) detector (not shown) to characterize the materials on wafer 203.

[00108] An electron beam 230 is emitted from cathode 218 by applying a voltage between anode 216 and cathode 218. Electron beam 230 passes through gun aperture 214 and beam limiting aperture 212, both of which may determine the size of electron beam entering condenser lens 210, which resides below beam limiting aperture 212. Condenser lens 210 focuses electron beam 230 before the beam enters objective aperture 208 to set the size of the electron beam before entering objective lens assembly, which may contain a deflector 204. The deflector 204 may deflect electron beam 230, such as to facilitate focusing of the beam on the wafer 203, beam scanning on the wafer 203, alignment of the electron beam 230, etc. Further, in some embodiments, anode 216 and cathode 218 may be configured to generate multiple electron beams 230. [00109] The electron beam source 220 emits the electron beam 230, which is a coherent electron beam. The coherent electron beam may be substantially coherent, such as coherent for part but not all of its path. The coherency may be spatial (e.g., lateral) coherency. In some embodiments, the coherency may include substantial temporal (e.g., longitudinal) coherency, where the temporal coherency of the electron beam may be limited by a non-zero energy range imparted by the electron beam source 220. The electron beam 230, as emitted from the electron beam source 220, may be a high-energy electron beam (e.g., where high energy is a relative measurement and may correspond to 15-20 keV). The electron beam 230, as emitted from the electron beam source 220 may be an electron beam which has been accelerated (e.g., to higher than emission energy) such as by one or more of the cathode 218, the anode 216, a voltage difference between the cathode 218 and the anode 216, the gun aperture 214, the beam limiting apertures 205 and 212, the condenser lens 210, the deflector 204, the objective aperture 208, and the electron detector 206. The electron beam 230, as emitted from the electron beam source 220, may have a relatively small spot size (e.g., on the order of an SEM spot size or other spot size useful for scanning applications or approximately 0.4 nm to 5 nm). The electron beam 230 may instead have a relatively larger spot size (e.g., a broad beam on the order of with a diameter of approximately 10 pm, including from a diameter of 200 nm or smaller to a diameter of 80 pm or larger) or a spot size otherwise configured for sample area illumination (e.g., of a field of view (FOV)). The electron beam 230 may be produced by a set of illumination optics 222, which may include the electron beam source 220. The illumination optics 222 may also include one or more auxiliary lens, such as lens 270A, for focusing, collimation, direction, etc.

[00110] From the illumination optics 222, the electron beam 230 may pass into a magnetic prism 272. The magnetic prism 272 may direct the electron beam 230 towards the wafer 203. The magnetic prism 272 may also receive electrons reflected by a sample (e.g., the wafer 203), including elastically scattered electrons, inelastically scattered electrons, etc. The magnetic prism 272 may be any appropriate type of electron beam separator, including a magnetic beam separator. The magnetic prism 272 may focus, shape, broaden, narrow, etc. the electron beam 230, including by selecting electrons of a particular energy or range of energies for direction (such as towards the wafer 203 or towards an electron detector 280). The magnetic prism 272 may deflect the electron beam towards a surface of the sample (e.g., the wafer 203). The magnetic prism 272 may deflect electrons reflected from the surface of the sample (e.g., the wafer 203) towards the electron detector 280.

[00111] The electron detector 280 may be any appropriate sensor, including a pixelated sensor, a pixelated image detector, a phosphorescent screen (such as for FEED), etc. The electron detector 280 may be used additionally to the electron detector 206 of the electron beam source 220 or may replace the electron detector 206 of the electron beam source 220. Additional electron detectors, not depicted, may be used to detect electrons in other parts of the electron beam tool 140, such as electron detectors which may be placed near the wafer 203 and which may be used to detect inelastically scattered electrons from a surface of the wafer 203, where such inelastically scattered electrons may have energies such that they are not transmitted by the magnetic prism 272 to the electron detector 280. The electron detector 280 may be substantially planar. The electron detector 280 may not have apertures and may be configured to function as a backstop for electrons. The electron detector 280 may time average or time integrate detected electrons. The electron detector 280 may detect a combination of multiple electron beams, including interference between multiple electron beams. The electron detector 280 may detect electrons diffracted by the sample (e.g., the wafer 203), including in a low-energy electron diffraction (LEED) configuration. The electron detector 280 may detect electrons reflected — e.g., elastically — by the sample (e.g., the wafer 203) which are not diffracted. The electron detector 280 may detect an intensity of the incident electron beam on the electron detector 280.

[00112] The electron beam 230, such as when it contacts the sample (e.g., the wafer 203), may be any electron beam appropriate for low energy electron microscopy (LEEM), such as an electron beam which produces a relatively broad and parallel 1-100 eV electron beam. Low energy may be a relative descriptor, such that low energy of an electron corresponds to an electron energy that is substantially non-ionizing when interacting with a surface. Low energy may include energies lower than an electron emission energy, such as from the cathode 218. Broad may likewise be a relative descriptor relating to spot size (when interacting with a sample) or beam dispersion angle, implying that an entire FOV may be illuminated by the electron beam at one time without substantial scanning. A broad beam may have a diameter of approximately 10 pm, including from a diameter of 200 nm or smaller to a diameter of 80 pm or larger. Parallel may be a descriptor applied to a relatively coherent particle beam. A broad and parallel beam may be a beam which has both a larger spot size and a small dispersion angle.

[00113] The electron beam 230 at the wafer 203 may be a low energy electron beam, such as an electron beam of approximately electron emission energy. The electron beam 230 may be decelerated from a higher energy state by one or more lens. The electron beam 230 at the wafer 203 may have a relatively large spot size (e.g., as previously described), such as may illuminate a significant portion of a sample, of a field of view (FOV), etc. The area of illumination may be larger than the image resolution — including significantly larger. The spot size or illumination on the sample may include one or more portions which may be blocked, such as by an intervening aperture, including in cases of selected area mode, diffraction mode, etc. The electron beam 230 may be a relatively narrow (e.g., as used for traditional SEM) and coherent ~15 keV electron beam in some portions of its path, which may be decelerated and focused into a relatively broad and parallel (e.g., coherent) 1-100 eV electron beam by one or more electric, magnetic, or electromagnetic objective lens, which may include one or more objective lens, such as lens 250. The sample (e.g., the wafer 203) may be held at substantially the same electric potential as the electron beam source 220, such that the electrons reaching the sample are (e.g., relatively) low-energy (e.g., 0-100 eV) electrons which may experience elastic scattering by the sample. In some embodiments, the electrons reaching the sample may have very low energy, including equal to zero, substantially equal to zero, etc. In some embodiments, the electrons may not contact the surface directly, such as if they are reflected (e.g., in a mirror-mode) before the sample, but may still experience deflection, reflection, energy change, momentum change, etc. due to deformations of the surface or deformations of the electric field close to the surface, where the deformations of the electric field may correspond to surface deformations. The electron beam 230 may be decelerated, broadened, accelerated, narrowed, etc., such as by one or more lens 270A-270D, along a path of the electron beam 230. The electron beam 230 may be focused, shaped, deflected, redirected, etc., by one or more element along the paths of the respective electron beams, such as the lenses 270A-270C, magnetic prisms 272, etc. The electrons of the electron beam 230 may elastically scatter at one or more sample (e.g., the wafer 203) or particle mirror, where the particle mirror may be an electrostatic mirror, a magnetic mirror, an electromagnetic mirror, etc. which causes electrons (or other particles) to reflect while maintaining coherence.

[00114] The magnetic prism 272 may contain one or more apertures, where an aperture may be configured to select a contrast for the electron beam (e.g., dark field imaging or bright field imaging). The aperture may be configured to be moveable, e.g., in and out of the electron beam, to select a portion of the beam, etc. The aperture may be configured to select a through beam or one or more diffraction beam received from the sample (e.g., the wafer 203). The one or more apertures may be alternatively or additionally located in other locations along the path of the electron beam 230.

[00115] The lenses 270B and 270C may be transfer lenses. Additional transfer lenses or fewer transfer lenses may be used. Transfer lenses may be used to alter the path of the electron beam 230 without changing the image contained therein. The lens 250 may be an electric, magnetic, electromagnetic or electrostatic lens. In some embodiments, the lens 250 may be an immersion lens. In some embodiments, the lens 250 may be an electric -field-free objective lens. The lenses 270C and 270D may be parts of a set of condenser optics 224, which may include more or fewer lenses. The lens 270D may be one or more projector lens, such as a lens configured to magnify the electron beam 230 for projection on the electron detector 280. The projector lens may conserve the phase of the wavefront (or applying the same phase change across the wavefront) of the electron beam, while spreading the beam size such as to a size visible to the human eye. The projector lens may spread the beam size while conserving phase to a size such that the pixels of the electron detector 280 may detect intensity variations corresponding to recombination of one or more electron beam. The electron detector 280 or associated processing (such as may occur in imaging system 200) may also enlarge, such as to a size visible to the human eye, any detected image. The condenser optics 224 may be or include projection optics, including one or more projector lens.

[00116] In some embodiments, the imaging system 200 may include one or more correction optics, which may correct aberrations of in a particle beam before or after beam splitting, diffraction, interaction with the sample, etc. The correction optics may be an in-line electron energy filter, such as described in Tromp, R.M., Hannon, J.B., Ellis, A.W., et al. A new aberration-corrected, energy- filtered LEEM/PEEM instrument. I. Principles and design. Ultramicroscopy 110 (2010) 852-861. htps://www.sciencedirect.com/science/article/abs/pii/S030439911000Q835, the contents of which are hereby incorporated by reference in their entirety. While the correction optics are described in reference to the imaging system 200, it should be understood that they may be used with any imaging system and/or method described herein. In some embodiments, aberrations (such as beam aberrations, lens aberrations, mirror aberrations, etc.) may be corrected using correction optics and/or digital image correction techniques, such as described hereafter.

[00117] Imaging system 200 may be used for inspecting a wafer 203 on motorized sample stage 201, and comprises an electron beam tool 140, as discussed above. Imaging system 200 may also comprise an image processing system 251 that includes an image acquirer 260, storage 270, and controller 150 (which may be the controller 150 of Figure 1). Image acquirer 260 may comprise one or more processors. For example, image acquirer 260 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 260 may connect with an electron detector 206 or electron detector 280 of electron beam tool 140 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 260 may receive a signal from electron detector 206 or electron detector 280 and may construct an image. Image acquirer 260 may thus acquire images of wafer 203. Image acquirer 260 may also perform various post-processing functions, such as generating contours, superimposing indicators on an acquired image, determining a surface profile, determine a difference in surface depth or material, determining a reconstruction of multiple de-focused images, determining an aberration correction, applying an aberration correction, including by applying an aberration correction in post-processing (for example, by applying a digital aberration correction to a digital image), determining a reconstruction of an image from images at different focus conditions, etc.. Image acquirer 260 may be configured to perform adjustments of brightness, contrast, skew, size, etc. of acquired images. Storage 270 may be a storage medium such as a hard disk, cloud storage, random access memory (RAM), other types of computer readable memory, and the like. Storage 270 may be coupled with image acquirer 260 and may be used for saving raw image data as original images, and post-processed images. Image acquirer 260 and storage 270 may be connected to controller 150. In some embodiments, image acquirer 260, storage 270, and controller 150 may be integrated together as one control unit.

[00118] In some embodiments, image acquirer 260 may acquire one or more images of a sample based on an imaging signal received from electron detector 206 or electron detector 280. An imaging signal may correspond to an imaging operation for conducting electron imaging. An acquired image may be a single image comprising a plurality of imaging areas or comprising an imaging area of which some regions are illuminated while other regions may not be illuminated. The single image may be stored in storage 270. The single image may be an original image that may be divided into a plurality of regions. Each of the regions may comprise one imaging area containing a feature of wafer 203.

[00119] Figure 3 is a schematic diagram of an exemplary particle beam tool configured for holography based on a sample line and a reference line. Figure 3 is a plan view of an exemplary particle beam apparatus comprising an electron beam source 220, which may be any appropriate electron beam source, such as that of the electron beam tool 140 of Figure 2. The electron beam source 220 is depicted as having a cathode 218, an anode 216, a gun aperture 214, beam limiting apertures 205 and 212, a condenser lens 210, a deflector 204, an objective aperture 208, and an electron detector 206. The electron beam source 220 may have more or fewer components, including multiple of the components (e.g., the cathode 218, the anode 216, the gun aperture 214, the beam limiting apertures 205 and 212, the condenser lens 210, the deflector 204, the objective aperture 208, and the electron detector 206) depicted. The components may be integrated into one or more components, such that components depicted as separate elements may be instead integrated elements. The components may be any appropriate components, such as those described with reference to Figure 2.

[00120] The electron beam source 220 emits an electron beam 330, which is a coherent electron beam. The electron beam 330 may be any appropriate electron beam, such as described in reference to the electron beam 230 of Figure 2. The electron beam 330 is split by a beam splitter 340 into a sample electron beam 332 and a reference electron beam 334. The sample electron beam 332 and the reference electron beam 334 are coherent and coherent with one another. The sample electron beam 332 and the reference electron beam 334 may have the same characteristics as the electron beam 330. The electron beam 330, the sample electron beam 332, and the reference electron beam 334 may be any electron beams appropriate for low energy electron microscopy (EEEM), such as an electron beam which produces a broad and parallel 1-100 eV electron beam, as previously described. The electron beam 330, the sample electron beam 332, and the reference electron beam 334 may be narrow (e.g., as used for traditional SEM) and coherent ~15 keV electron beams, which may be decelerated into a broad and parallel 1-100 eV electron beams by one or more objective lens, such as objective lenses 350 and 352, as previously described. The electron beam 330, the sample electron beam 332, and the reference electron beam 334 may be decelerated or broadened, or accelerated or narrowed, such as by one or more lens 370A-370F, along path of the respective electron beams. The electron beam 330, the sample electron beam 332, and the reference electron beam 334 may be focused, shaped, etc., by one or more element along the paths of the respective electron beams, such as the lenses 370A-370F, magnetic prisms 372A and 372B, etc. The electrons of the electron beam 330, the sample electron beam 332, and the reference electron beam 334 may elastically scatter at one or more sample 360 or particle mirror 362, where the particle mirror may be an electrostatic mirror, a magnetic mirror, an electromagnetic mirror, etc. which causes electrons (or other particles) to reflect while maintaining coherence. The electron beam 330, the sample electron beam 332, and the reference electron beam 334 may interfere with each other, such as by recombination. The sample electron beam 332, and the reference electron beam 334 may be recombined such as by a beam combiner 344.

[00121] The sample electron beam 332 passes through a sample line 336, which includes the lenses 370A-370C, the magnetic prism 372A, the objective lens 350, and the sample 360. The sample line 336 may include additional or fewer elements. The reference electron beam 334 passes through a reference line 338, which includes the lenses 370D-370F, the objective lens 352, and the particle mirror 362. The reference line 338 may include additional or fewer elements, including elements such that it mimics or mirrors the sample line 336. The reference line 338 may include different components than the sample line 336 while maintaining coherence of the reference electron beam 334 with the sample electron beam 332.

[00122] The beam splitter 340 may be a biprism. The beam splitter 340 may be supplemented by an additional beam splitter 342, which may be a biprism. The beam combiner 344 may also be a biprism. The beam combiner 344 may be electromagnetic deflectors. The beam splitter 340, the additional beam splitter 342, and the beam combiner 344 may additionally or instead be elements which split, recombine, or redirect a portion of the electron beam 330, the sample electron beam 332, and the reference electron beam 334 while maintaining coherence. The magnetic prisms 37A and 37B may be other elements which alter a direction of the sample electron beam 332 of the reference electron beam 334 without reducing coherence. The sample 360 and the particle mirror 362 may additionally (or alternatively) be placed at a different position in the sample line 336 and the reference line 338, respectively, such that more or fewer magnetic prisms are placed within the sample line 336 and the reference line 338.

[00123] The sample electron beam 332 and the reference electron beam 334 may have the same coherence characteristics, and be coherent, at least along a part of their path lengths, with one another. The sample electron beam 332 may illuminate the sample 360, which may be supported by a sample holder (not show). The sample electron beam 332 is coherent over the extent of the sample (or the portion of the sample within the sample electron beam 332 path. The sample electron beam 332 has a transverse coherence length given by Equation 1, below: where L_± is the transverse coherence length, 2 is the electron wavelength, ag is a measure of angular spread, a_x is a measure of transverse spread (which may be 5 pm), and e_{N x} is a measure of electric field stability (which may be 3 pm rad for a Schottky FEG at 1 nA). The transverse coherence may then be calculated as approximately 644 nm, which gives a measurement spot size larger than 20 nm pitches, and which may therefore be useful for measurement of multiple 3D devices, trenches, etc. The number of fringes (such as interference fringes caused by recombination of the sample electron beam 332 and the reference electron beam 334) resolvable in the transverse direction may be given by Equation 2, below as:

L_± 2n = (2)

Which, given the values above, may be n~ 1651 . The sample electron beam 332 has a longitudinal coherence length given by Equation 3, below:

Which for AE=0.25cV for a 1-100 eV LEEM beam, may be given by the values in Chart 1, below:

The number of fringes in the recombination (e.g., of the sample electron beam 332 and the reference electron beam 334) in the longitudinal direction may be given by Equation 4, below

Which is analogous to Equation 2, above, for the transverse direction, and where it is assumed that the L|l is greater than the diameter of the objective lens d_obj. If the depth of interrogation (e.g., a resist thickness) is approximately 10 nm, then an electron energy E_o > 10 eV will allow imaging of the sample within the coherence length.

[00124] The path length difference between the sample electron beam 332 as it traverses the sample line 336 and the reference electron beam 334 as it traverses the reference line 338 is smaller than the longitudinal coherence length, such that coherence between the sample electron beam 332 and the reference electron beam 334 is maintained. The path length difference may be altered by a delay segment. The delay segment may lie in the sample line 336 or the reference line 338. The delay segment may include an element to adjust a position of the sample 360 or the particle mirror 362. The delay segment may include an element that measures the path length of the sample electron beam 332 or the reference electron beam 334 or a difference between the sample electron beam 332 and the reference electron beam 334. The sample 360 may be a wafer (or portion of a wafer), such as fabricated during IC construction, which may have one or more layer of unknown or variable thickness. The delay segment may adjust the path length of the sample line 336 or the reference line 338 such that the path length of the sample line 336, which includes interaction of the sample electron beam 332 with the sample 360, is equivalent to the path length of the reference line 338. The delay segment may adjust the path length of the sample line 336 in the direction 364 (or in a direction perpendicular to the direction 364). The delay segment may adjust the path length of the reference line 338 in the direction 366 (or in a direction perpendicular to the direction 366). The delay segment may allow adjustment of the path length of the sample line 336 or the reference line 338 for each sample, during measurement, iteratively, etc. The delay segment may allow adjustment of the path length of the sample line 336 or the reference line 338 based on a recombination of the sample electron beam 332 and the reference electron beam 334, such as based on the interference pattern of the recombination.

[00125] The sample electron beam 332 and the reference electron beam 334 are recombined by the beam combiner 344. The recombination of the sample electron beam 332 and the reference electron beam 334 is detected by an electron detector 380. The electron detector 380 may be any appropriate sensor, including a pixelated sensor, a pixelated image detector, a phosphorescent screen, etc. The electron detector 380 may be used additionally to the electron detector 206 of the electron beam source 220 or may replace the electron detector 206 of the electron beam source 220. The electron detector 380 may be substantially planar. The electron detector 380 may detect, as the recombination, interference of the sample electron beam 332 and the reference electron beam 334. The interference may include constructive and destructive interference, analogous to the interference detected during LEEM. The recombination may be reconstructed to determine amplitude, phase, or wavefront (e.g., waveform, waveform shape, etc.) characteristics of the sample electron beam 332 as reflected (or otherwise interacted) by the sample 360. The recombination may contain information about the surface materials (such as carried by back scattered electrons (BSEs)), surface profile, and other characteristics of the sample 360. The recombination or a reconstruction of the electron beam 332 as reflected by the sample 360 may be used to detect defects in the sample, such as incomplete etching at the end of 3D trenches or holes. The recombination or the reconstruction of the sample electron beam 332 as reflected by the sample 360 may be used to determine critical dimension (CD) metrology, as it may be free from edge blooming, have no substantial interaction volume, reduce charging artifacts, reduce grid distortion, etc. The recombination or the reconstruction of the sample electron beam 332 as reflected by the sample 360 may be used to determine local 3D metrology, profilometry (e.g., floor tilt, side wall angle, roughness), etc. The recombination may be corrected using computational aberration corrections in order to improve resolution. The aberration correction may be applied to a complex- valued wavefront that results from digital post-processing of a hologram, such as a hologram realized by the recombination. The recombination or the reconstruction of the sample electron beam 332 as reflected by the sample 360 may have a field of view (FOV) of up to 6 pm and a resolution of up to 1 nm, which may be determined by the transverse and longitudinal coherence lengths.

[00126] Figure 4 is a schematic diagram of an exemplary particle beam tool configured for holography based on a phase modulator. Figure 4 is a plan view of an exemplary particle beam apparatus, which will be described with reference to the electron beam source 220 of Figure 2, though it should be understood that the electron beam source 220 may be any appropriate electron beam source. The electron beam source 220 has a cathode 218, an anode 216, a gun aperture 214, beam limiting apertures 205 and 212, a condenser lens 210, a deflector 204, an objective aperture 208, and an electron detector 206, as previously described.

[00127] The electron beam source 220 emits an electron beam 430, which is a coherent electron beam, such as previously described for the electron beam 230 of Figure 2 and the electron beam 330 of Figure 3. The electron beam 430 may be any electron beam appropriate for low energy electron microscopy (LEEM), such as an electron beam which produces a broad and parallel 1-100 eV electron beam, as previously described. The electron beam 430 may be a narrow (e.g., as used for traditional SEM) and coherent ~ 15 keV electron beam, which may be decelerated into a broad and parallel 1-100 eV electron beams by one or more objective lens 450, as previously described. The electron beam 430 may be decelerated or broadened, or accelerated or narrowed, such as by one or more lens 430A- 470D. The electron beam 430 may be focused, shaped, etc., by one or more element along the paths of the electron beam 430, such as the lenses 470A-479D, the objective lens 450, magnetic prism 472, etc. The electrons of the electron beam 430 may elastically scatter at sample 460. The electron beam 430 may be modulated, such as by phase modulator 490. The phase modulator 490 may be a Zernike phase plate. The phase modulator 490 may diffract a portion of the electron beam 430, while leaving another portion of the electron beam 430 undiffracted. The phase modulator 490 may be a reflective phase modulator, for example a charge -particle mirror plate, which may be programmable. A reflective phase modulator, which may have a fixed shape, an adjustable shape (e.g., a programmable shape), etc., may receive the electron beam 430 and, by reflection, influence a shape and/or phase of the beam, as described in U.S. Patent Application 2023/0048580 Al, which is hereby incorporated by reference in its entirety. The phase modulator 490 is depicted as a transmissive phase modulator in Figure 4, but may instead be a reflective phase modulator which may alter the beam path, in which case an electron detector (e.g., electron detector 480) may be located in the path of the reflection. The phase modulator 490 modulate the electron beam 430 such a first portion of the electron beam 430 has a first phase and a second portion of the electron beam 430 has a second phase (e.g., after the electron beam 430 passes through the phase modulator 490). The phase modulator may be placed in any back focal plane (e.g., Fourier planes) of the electron beam 430, such as after it has interacted with the sample 460. The first portion of the electron beam 430 and the second portion of the electron beam 430 may recombine (e.g., interfere with each other, such as by constructive or destructive interference) and be detected by an electron detector 480. The first portion of the electron beam 430 and the second portion of the electron beam 430 may have the same coherence characteristics but different phases.

[00128] The recombination of the first portion of the electron beam 430 and the second portion of the electron beam 430 is detected by the electron detector 480, which may be used additionally to the electron detector 206 of the electron beam source 220 or may replace the electron detector 206 of the electron beam source 220. The electron detector 480 may be substantially planar. The electron detector 480 may detect, as the recombination, interference of the sample electron beam 332 and the reference electron beam 334. The interference may include constructive and destructive interference, analogous to the interference detected during phase contrast transmission electron microscopy (TEM). The recombination may be reconstructed to determine amplitude, phase, or wavefront characteristics of the electron beam 430 as reflected (or otherwise interacted) by the sample 460. The recombination may contain information about the surface materials (such as carried by electron wave intensity, diffracted electrons, secondary electrons (SEs) and back scattered electrons (BSEs) which may or may not constructively and destructively interfere with other electrons), surface profile, and other characteristics of the sample 460. The recombination or a reconstruction of the electron beam 430 as reflected by the sample 460 may be used to detect defects in the sample, such as incomplete etching at the end of 3D trenches or holes. The recombination or the reconstruction of the electron beam 430 as reflected by the sample 460 may be used to determine critical dimension (CD) metrology, as it may be free from edge blooming, have no substantial interaction volume, reduce charging artifacts, reduce grid distortion, etc. The recombination or the reconstruction of the electron beam 430 as reflected by the sample 460 may be used to determine local 3D metrology, profilometry (e.g., floor tilt, side wall angle, roughness), etc. The recombination may be corrected using computational aberration corrections in order to improve resolution.

[00129] Figure 5 is a schematic diagram of an exemplary particle beam tool delay line. Figure 5 is a plan view of an exemplary delay line 500 containing an exemplary delay segment 502, which may be placed in a particle beam tool within an electron beam. The delay segment 502 may be used in the sample line 336 or the reference line 338 of Figure 3, with a path of the electron beam 430 of Figure 4, with a path of the electron beam 230 of Figure 2, etc. The delay segment 502 may be used to alter a path length within an electron beam (or another particle beam) within any appropriate particle beam tool. Herein, “path length” for an electron beam (or particle beam) may include a geometric path length (e.g., a length in meters, mm, nm, etc.), a number of wavelengths (e.g., a number of full or partial wavelength cycles which may occur at different frequencies in different mediums or at different electron energies), a phase (e.g., a point in the wavelength cycle), etc. Two electron beams may have an identical path length even if they travel different physical lengths, where the speed, number of wavelength cycles, wavelength, frequency, etc. of the electron beams may vary during their transit of the path length. In some embodiments, electron beams may have an identical (or substantially identical) path length if they have identical (to within a wavelength) phase and frequency when they recombine. For example, a first electron beam may travel n wavelength, while a second electron beam may travel n+1 wavelengths. At recombination, these electron beams may still have substantially equal path lengths, with phase and wavelength such that the recombination effects constructive and destructive interference. The path length of the electron beam may be taken to be analogous to an optical path length, in which a phase shift or wavelength change may occur due to electric fields, lenses, etc., but in which the phase shift of wavelength change may be balanced by a corresponding reverse change to maintain coherence between particle beams.

[00130] The delay segment 502 may alter a path length of an electron beam within the delay segment 502 itself. The delay segment 502 may alter a phase, such as by altering an electron energy of the electron beam, within the delay segment 502. The delay segment 502 may alter a wavelength, such as by deceleration or acceleration of the particle beam, within the delay segment 502. The delay segment 502 may alter a path length of an electron beam external to the delay segment 502, such as by altering an angle of incident of an electron beam and thereby changing a path length of the electron beam inside or outside of extents of the delay segment 502. The path length may be altered by making it longer (in geometric or wavelength number terms), such as longer by a small amount (e.g., on the order of an electron wavelength, 1 nm, etc.) or longer by a large amount (e.g., on the order of a sample thickness, 10 nm, 1 mm, up to on the order of a longitudinal coherence length), or by making it shorter, such as by a small amount or by a large amount. The path length may be altered by increasing a path length in one part and decreasing a path length in another part, such as by tilting of a reflector or lens. The path length may be altered by steps consistent with alignment, such as by adjustment of lens parameters, angles of reflection, positions in one or more directions, etc. The path length may be altered by altering an angle of incidence, such that a path angle is altered. The path length may be altered by changing an angle of reflection or transmission medium, which may alter a transit time of the path length with or without altering a path length distance. The path length may be altered by changing a potential, such as an electrostatic potential distribution, inside one or more element of the delay segment 502. The path length may be altered in any appropriate manner where the path length is changed (or modified) without destroying coherence.

[00131] The delay line 500 receives an electron beam 530 from an input direction 510 and provides the electron beam to an output direction 512. The delay line 500 may also or instead receive the electron beam 530 (or an additional electron beam) from the output direction 512 and provide the electron beam 530 (or an additional electron beam) to the input direction 510. The electron beam 530 may be a coherent electron beam during at least some portions of the operation of the delay line 500. The electron beam 530 may be an incoherent electron beam during some portions of the operation of the delay line 500, such as during tuning, set up, during operations other than holography, etc.

[00132] The electron beam 530 may be any electron beam appropriate for low energy electron microscopy (LEEM), such as an electron beam which produces a broad and parallel 1-100 eV electron beam, as previously described. The electron beam 530 may be a narrow (e.g., as used for traditional SEM) and coherent ~ 15 keV electron beam, which may be decelerated into a broad and parallel 1-100 eV electron beams by objective lens 550, as previously described. The electron beam 530 may be decelerated or broadened, or accelerated or narrowed, such as by one or more lens 530A-530C. The electron beam 530 may be focused, shaped, etc., by one or more element along the paths of the electron beam 530, such as the lenses 570A-570C, the objective lens 550, magnetic prism 572, etc. The electrons of the electron beam 530 may elastically scatter at scatterer 560, which may be a sample, a particle mirror, a point scatterer, an array of point scatterers in one or more dimension, etc. The point scatterer may be any appropriate point scatter, which may be of a size smaller than the resolution of the LEEM system, such that to the LEEM system the point scatter appears as a “point” scatterer. The scatterer 560 may be adjusted, such as by altering the position along the direction 566, to change a path length (or time) of the delay line 500. The scatterer 560 may decelerate or accelerate the electron beam, which may change the wavelength of electrons of the beam. The scatterer 560 may decelerate and reverse (e.g., accelerate back to an initial energy in an opposite direction) the electrons of the electron beam. The electron beam 530 may be output to a phase modulator, such as via the output direction 512, as previously described in reference to Figure 4. The electron beam 530 may be recombined (or otherwise interfered with another electron beam), such as via the output direction 512, as previously described in reference to Figure 3. The electron beam 530 may comprise multiple electron beams, such as having different phases, different energies, operating in different directions, etc. The delay line 500 may output the electron beam 530, such as in the output direction 512, to one or more electron detector. The delay line 500 may output the electron beam 530 to additional path segments, such as to interact with an additional scatterer (e.g., a sample, particle mirror, point scatterer, etc.).

[00133] The delay segment 502 may be made up of the objective lens 550 and the scatterer 560. The delay segment 502 may be placed in any appropriate location in the delay line 500 or another line, such as the sample line 336 or the reference line 338 of Figure 3, a path of the electron beam 430 of Figure 4, a path of the electron beam 230 of Figure 2, etc. The scatterer 560 may be a sample, such as the sample 360 of the Figure 3, a particle mirror, a point scatterer, etc. The delay segment 502 may contain the objective lens 550 or may receive input from any appropriate lens or electron beam source. For example, the delay segment 502 may be comprised of the scatterer 560 at a distance from another lens in the delay line 500. The delay segment 502 may receive an input electron beam 532, which may be the electron beam 530. The input electron beam 532 may transit the objective lens 550 to become an input electron beam 532 which may transit a distance 580 between the objective lens 550 and the scatterer 560. The input electron beam 532 or any portion thereof may then be reflected, scattered, or otherwise returned from the scatterer 560 as the output electron beam 536. The output electron beam 536 may transit the distance 580 to the objective lens 550, which may emit a corresponding output electron beam 538.

[00134] A position of the scatterer 560 may be adjustable. The position of the scatterer may be adjustable by use of piezoelectric or other actuators. The position of the scatterer may be adjusted along an axis of an electron beam, such as along the axis of the input electron beam 534 or the output electron beam 536. The input electron beam 534 and the output electron beam 536 may be coaxial. The adjustment of the position of the scatterer 560 may increase or decrease the distance 580. The increase or decrease of the distance 580 may result in a corresponding increase or decrease of the path length of an electron beam which makes up the input electron beam 534 and the output electron beam 536. The adjustment of the position of the scatterer 560 may occur in another direction, such as from the position depicted for the scatterer 560 to the position depicted for a scatterer 562, such as along a vector 568. The adjustment of the position of the scatterer 560 may be included in one or more alignment step, such as of the electron beam.

[00135] A strength or other parameter of the scatterer 560 may be adjustable. For example, electric fields, voltages, etc. which make up a particle mirror may be adjusted. The parameters of the scatterer 560 may be adjusted to change a path length within the scatterer 560 — such as to change a transit path length within the scatterer 560. The parameters of the scatterer 560 may be adjusted to change an electron speed, wavelength, phase, angle of reflectance, etc. within the scatterer 560, which may change an effective path length of transit path within the scatterer 560. Any appropriate adjustment to the position or other parameters (e.g., various voltages, electrode separation distances, angle of orientation, etc.) may be made to the scatterer 560 to change a path length of the electron beam.

[00136] A position or other parameters of the objective lens 550 may be adjustable. The objective lens 550 may be adjusted in response to adjustments of the scatterer 560. For example, the objective lens 550 may be adjusted in focus, voltage, position, etc. to correspond to an adjusted objective lens 552 when the scatterer 560 is adjusted to the position of the scatterer 562. The objective lens 550 may be adjusted in order to compensate for the adjustment of the scatterer 560 while maintaining focus of the electron beam 530. The objective lens 550 may be adjusted independently of the adjustment of the scatterer 560 in order to increase or decrease a path length of the electron beam (for example, the distance 580). The objective lens 550 may be adjusted in position by the same or different actuators that adjust a position of the scatterer 560. The objective lens 550 may operate at a fixed distance to another object in the delay line 500, such as the magnetic prism 572, or may move in position in concert with the scatterer 560, including at a fractional distance where the objective lens 550 moves a distance of x/r for each distance x moved by the scatterer 560. [00137] Figure 6 is a flowchart that illustrates a method of particle beam holography. Each of these operations is described in detail below. The operations of method 600 presented below are intended to be illustrative. In some embodiments, method 600 may be accomplished with one or more additional operations not described, and/or without one or more of the operations discussed.

Additionally, the order in which the operations of method 600 are illustrated in Figure 6 and described below is not intended to be limiting. In some embodiments, one or more portions of method 600 may be implemented (e.g., by simulation, modeling, etc.) in one or more processing devices (e.g., one or more processors). The one or more processing devices may include one or more devices executing some or all of the operations of method 600 in response to instructions stored electronically on an electronic storage medium. The one or more processing devices may include one or more devices configured through hardware, firmware, and/or software to be specifically designed for execution of one or more of the operations of method 600, for example.

[00138] At an operation 610, a particle beam is split into a first particle beam and a second particle beam. The particle beam may be an electron beam. The first particle beam and the second particle beam are coherent along at least a portion of their path lengths. The first particle beam and the second particle beam may be split by a beam splitter, which may be a biprism. The first particle beam and the second particle beam may instead be phase modulated, such that the first particle beam corresponds to a first portion of the particle beam with a first phase and the second particle beam corresponds to a second portion the particle beam with a second phase different from the first phase. The phase modulator may preserve a phase, such that the first phase may be the phase of the particle beam and the second phase may be different than the phase of the particle beam or vice versa. The phase modulator may be a Zernike phase plate.

[00139] At an operation 620, the first particle beam is transmitted along a reference line. The reference line may include one or more particle transmission elements, such as lenses, prisms, mirrors, segment lines, etc.

[00140] At an operation 630, the second particle beam is transmitted along a sample line. The sample line may include one or more particle transmission elements, such as lenses, prisms, mirrors, delay segment, etc. The sample line includes interaction of the second particle beam with a sample. [00141] The transmission of the first particle beam along the reference line and the transmission of the second particle beam along the sample line may occur substantially simultaneously, such as in parallel. The transmission of the first particle beam along the reference line and the transmission of the second particle beam along the sample line may start and stop at different times, while remaining substantially simultaneous. For example, the transmission along the sample line may begin before the transmission along the reference line, such as if the reference line includes a delay segment which may be adjusted or if the sample line is used for acquisition of an image prior to acquisition of holographic information. Transmission along the reference line or the transmission line does not require that either particle beam transit the entirety of the reference line or the transmission line, such as during alignment.

[00142] The first particle beam or the second particle beam includes a delay segment configured to alter a path length of the first particle beam of the second particle beam. The delay segment may include a particle mirror or a sample (e.g., sample holder) which may be positionally adjusted, such as on the order of the electron wavelength, the sample thickness, a coherence length, etc. to increase or decrease a path length of at least one of the sample line or the reference line. Transmitting of either the first particle beam or the second particle beam may include adjusting a path length of either particle beam to correspond to a path length of the other particle beam.

[00143] In one embodiment, the particle beam may be transmitted along the sample line, such that the particle beam interacts with the sample. After the particle beam interacts with the sample, the operation 610 may be performed, where the particle beam is split (e.g., modulated) to create the first particle beam, being a first portion of the particle beam with a first phase, and the second particle beam, being a second portion of the particle beam with a second phase.

[00144] At an operation 640, the first particle beam and the second particle beam are recombined. The first particle beam and the second particle beam may cause interference between the first particle beam and the second particle beam. The recombination may be detected by a particle detector. The recombination may include application of an aberration correction to the recombination. The recombination may include determination of an aberration correction to be applied to the recombination. The recombination may be used to determine a phase or amplitude of the first particle beam after it has interacted with the sample. The recombination may be used to determine a depth profile of the sample. The recombination may be used to determine a 3D image of the sample.

[00145] In one embodiment, recombining the first particle beam and the second particle beam may be recombing a first portion of the particle beam with a first phase and a second portion of the particle beam with a second phase.

[00146] As described above, method 600 (and/or the other methods and systems described herein) is configured for particle beam holography.

[00147] Figure 7 is a schematic diagram of an exemplary recombination of a first particle beam and a second particle beam. Figure 7 is a cross-sectional view of an electron wavefront 710 incident on a surface 716 with and a corresponding phase contrast image 760. The phase contrast image 760 may be a hologram image of the surface 716, and may be used to reconstruct information about the surface 716, the electron wavefront 710 as it reflects of the surface 716, etc.

[00148] The electron wavefront 710 is a wavefront corresponding to a low energy (where low energy is relative and herein corresponds to energies at which electrons backscatter elastically (e.g., coherently)) electron beam. The electron wavefront 710 is broad, with a large (relatively large, e.g., larger than on the scale of the features of the surface 716), diffuse peak such that the electron wavefront 710 is roughly parallel to a longitudinal axis of the surface 716. The electron wavefront 710 approaches the surface along a direction d 702, which is roughly parallel to depth in the surface 716 and such that multiple peaks of the electron beam creating the electron wavefront 710 are coherent, parallel, etc. This direction is selected for ease of description, but it should be understood that the electron wavefront 710 may instead impinge on the surface 716 with a perpendicular angle of arrival, either intentionally (for example, such as parallel to a sidewall angle) or unintentionally (such as due to sample defects, drift, etc.). The electron wavefront 710 has an electron wavelength X 712, which may be on the order of 1 nm.

[00149] The surface 716 has a surface profile represented in cross section by the line containing segments 718, 726, 736, and 746, which correspond to raised plateaus of the sample, segments 720, 730, and 740, which correspond to trenches of the sample, and segments 722, 724, 732, 734, 742, and 744, which correspond to sidewalls of the sample. The surface profile has a height h 714 of approximately 10 nm, where the height h 714 is the difference between the depth of the plateaus of segments 718, 726, 736, and 746 and the trenches of segments 720, 730, and 740. The surface profile is provided as an example, where other surface profiles may be interrogated.

[00150] When the electron wavefront 710 reaches the surface 716, it is elastically (e.g., coherently) scattered by the surface (as shown in scattering event 750). The electron wavefront 710 incident on the surface 716 is scattered which creates a reflected electron wavefront 752 which radiates outwards from the sample. The reflected electron wavefront 752 contains information about the change in depth of the surface 716. For example, the electron wavefront 710 incident on the plateau of the segment 726 and the electron wavefront 710 incident on the trench of the segment 720 travel different distances (e.g., on both the journey to the sample and the journey away from the sample), corresponding to electron wavefronts 754. The reflected electron wavefront 752 may also contain information about the material of the sample, such as encoded in the intensity of BSE electrons.

[00151] The reflected electron wavefront 752 may then be recombined with a reference electron wavefront which has not reflected off the sample, and which is coherent with the electron wavefront 710. The recombination between the reflected electron wavefront 752 and the reference electron wavefront may generate a recombination which may be measured as an interference pattern at an electron detector, such as by the corresponding phase contrast image 760.

[00152] The phase contrast image 760 corresponds to the intensity of the interference pattern caused by the recombination of the reflected electron wavefront 752 and the reference electron wavefront. The difference in intensity of phase contrast image 760 is shown as binary (e.g., black or white) pattern for ease of depiction, but — depending on resolution — is a sinusoidal pattern (e.g., in intensity) displaying grayscale values as well. The phase contrast image 760, based on the relative phase contrast of surfaces (for example the trench of the segment 720 with contrast 766 and the trench of the segment 730 with contrast 774) contains information about whether surfaces are in-plane or offset along the direction d 702. The phase contrast image 760, based on the number of fringes, also contains information about the rate of depth change (or slope) or surfaces. For example, the sidewall of the segment 732 is sloped less steeply than the sidewall of the segment 734 but is approximately as deep, and correspondingly a larger wavelength is displayed in the interference fringe 772 that in that of interference fringe 776 but both the interference fringe 772 and the interference fringe 776 have the same number of maximums and minimums in intensity. In another example, the sidewall of the segment 742 and the sidewall of the segment 744 have substantially the same width but different depths, where the sidewall of the segment 742 has a steeper slope than the sidewall of the segment 744. Correspondingly, interference fringe 780 has an additional number of interference fringes (e.g., 1 as depicted corresponding to the difference in depth between the sidewall of the segment 742 and the sidewall of the segment 744 which is approximately equal to the electron wavelength X 712 as depicted) when compared to interference fringe 784. The interference fringe 780 and the interference fringe 784 occupy roughly the same lateral extent in the phase contrast image 760, corresponding to the substantially similar widths of the sidewall of the segment 742 and the sidewall of the segment 744, which leads the interference fringe 780 to have a smaller wavelength than the interference fringe 784. The phase contrast image 760, based on the presence or absence of fringes, contains information about the relative flatness of features. For example, the trench of segment 740 is angled relative to the direction d 702, which appears as a corresponding gradient in the contrast 782.

[00153] The phase contrast image 760 or other signals corresponding to the recombination of the reflected electron wavefront 752 (e.g., the sample electron beam) and the reference electron wavefront (e.g., the reference electron beam) may be used, based on these and other principles, to reconstruct information about the surface 716.

[00154] In one embodiment, the reflected electron wavefront 752 may be phase modulated instead of recombined with a reference electron wavefront, such as through the use of a Zernike phase plate. The reflected electron wavefront 752 may be used to produce a first portion of an electron beam having a first phase and a second portion of the electron beam having a second phase. The first phase or the second phase may be a phase shift from the phase of the electron beam before modulation. Either the first phase or the second phase may be the phase of the electron beam before modulation (e.g., a negligible phase shift). The first portion of the electron beam (e.g., of the reflected electron wavefront 752) and the second portion of the electron beam (e.g., of the reflected electron wavefront 752) may then be recombined with each other through interference. The recombination may generate a phase contrast image, such as the phase contrast image 760.

[00155] An image of the recombination, which is a digital hologram, may be used to acquire information about both the phase and amplitude of the reflected electron wavefront 752. For example, an intensity of the image may be given by Equation 5, below:

I = |E₀ P + \E_r\² + 2 |E₀1 l^rl cos(<p₀ - fcx) (5)

Where E_o is the electron wavefront corresponding to the reflection from the sample, E_r is the reference electron wavefront, |E₀1 is the amplitude of the reflection from the sample, <p₀ is the phase of the reflection from the sample, x is the displacement, and k is the wavenumber. The recombination may be processed, such as by a fast Fourier transform (FFT) or other appropriate transform, to generate information about both the phase and amplitude of the reflection from the sample (e.g., lEo l l^rl exp(/ o))- [00156] Figure 8 is a schematic diagram of an exemplary aberration correction. Figure 8 is a view of an example digital hologram 810 as well as aberration corrections 820 and 830. If the lenses of the example particle beam are magnetic lenses, spherical or chromatic aberrations can be corrected using aberration corrections determined by holographic reconstruction. The aberration correction may be applied to a complex-valued wavefront that results from digital post-processing of a hologram, such as a hologram realized by the recombination. An example digital hologram 810 may be acquired for a sample which is a null sample. A null sample may be a known sample, such as a particle reflector, point scatterer, absence of a sample, etc. The digital hologram 810 may be subjected to a transform 812, such as an FFT, and averaged over multiple data acquisitions to determine an average of the aberration corrections 820 and 830 in amplitude and phase. The aberration corrections 820 and 830 can then be transformed back into the intensity space and applied to data acquired for samples in order to correct for lens aberrations.

[00157] The hologram (e.g., the recombination of the first particle beam and the second particle beam or of the first portion of the particle beam with a first phase and the second portion of the particle beam with the second phase) may be related to the object wave (e.g., the reflected electron wavefront 752 of Figure 7, the reflection from the sample E_o, etc.) by the relationship given by Equation 6, below: hologram = I = |R + 0\² = |R|² + 101² + R*0 + RO* (6)

Where R represents the reference wave (e.g., the reference electron wavefront as described in Figure 7, the reference electron wave E_r, etc.) and O represents the object wave. The reconstruction of the object wave may be determined by a multiplication of the reference wave and the hologram, such as by using Equation 7, below: reconstruction = RI = R\R\² + R\0\² + |R \²0 + RRO* (7)

Where the object wave can be determined from the |R | ²0 term. Aberration correction information, which may be a direct measurement of a complex-valued point-spread function (which may include information about all aberrations in the image process), may be obtained from a hologram obtained for a known sample, point scatterer, etc.

[00158] Aberration correction can account for imperfections in the lenses or reference wave. By choosing the object wave to be equal to the reference wave (for example, by replacing a sample with a point scatterer, particle mirror, etc.), the reference wave itself may be reconstructed. For example, the hologram of the reference wave with itself is governed by Equation 8, below

I = |R + R|² = 4|R |² (6)

Which may be used to determine a difference between the measured intensity I and the reference wave R and therefore determine a mathematical correction to the measured intensity I to account for aberration in one or more lens.

[00159] In some embodiments, random stochastic failures may occur in lithography processes, including in extreme ultraviolet (EUV) lithography processes. Failures may include non-printing (e.g., non-development, incomplete development, failure in in-fill, etc.) of features. Failures may include printing (e.g., development, fabrication, etc.) of extraneous features. In order to identify these failures, various metrology techniques may be used to measure surface profiles, materials, patterning, etc. During a lithography process, a chip may be inspected after lithography — in after development inspection (ADI) — which may measure the patterning and presence of photoresist on the chip. The chip may also or instead be inspected after etch — in after etch inspection (AEI). In some embodiments, chips may be inspected at additional or alternative points in the lithography process. In some embodiments, measurement at ADI may be preferred to measurement at AEI, because ADI may provide more direct measurement of the lithography process, chips at ADI which contain failures may be more easily corrected (e.g., re -processed through lithography) than chips at AEI which contain failures, etc. In some cases, it has been suggested that some failures (e.g., missing feature defects) may be caused by remaining resist, including resist scum (e.g., portions of a resist layer which may be incompletely removed during development, including portions which may re-deposit in different locations during development), resist footings, etc., which may remain in contact holes or other lithographic features (such as along edges of exposed photoresist) and may cause decrease in width of etched features, up to and including missing features at AEI.

[00160] In some embodiments, inspection by LEEM (including LEEM holography) may be used to detect defects that are less likely to be detected by SEM or other metrology tools, including photoresist scum or other incomplete development in lithography features at ADI. Photoresist which remains in lithography features after development may not be substantially detectable using SEM inspection, because of the non-crystalline nature of the photoresist scum, because of the thinness of any remaining layer, because of intensity of SEs and BSEs emitted from the photoresist, etc. In order to improve the fabrication process in general and AEI in particular, it may be beneficial to measure scum and other defects which may cause missing lithography features, such as to better characterize, control, etc. the lithography process, to allow for wafer re-processing in a certain threshold of defects are detected, to improve metrology, etc.

[00161] Figure 9A is a graph of a simulated height for an exemplary surface after resist development. Figure 9A displays in grayscale an example height (e.g., along a z-axis parallel to the direction of fabrication) for contact holes arrayed (e.g., in the x-y longitudinal plane of the chip), where light colors represent higher elevation (e.g., height) and dark color represent low elevation (e.g., depth). The contact holes appear as roughly circular dark marks arrayed in a three-by-three grid. Some of the contact holes of Figure 9A, as simulated, are depicted as containing photoresist scum defect or other defect causing defect holes to be less deep, as may be detected in some embodiments. For each of the contact holes, a contour as obtained from SEM metrology is outlined by solid line 900 (as will be further discussed in reference to Figure 9B). The solid line 900 outlines a contact hole CD, as identified by image processing from SEM images. For eight of the contact holes (e.g., all the depicted contact holes other than the centrally located contact hole of Figure 9A), a contour of the simulated height, which corresponds to the location at which the resist (after development) is equal to half the as-deposited resist thickness, is outlined by dashed line 910 (e.g., a full width at half max (FWHM) for the distribution of the contact hole depths). FWHM is used as an approximation of the CD of each contact hole, but any other appropriate CD measurement scheme may be used, such as a depth threshold other than half of the resist thickness (for example, 90% of resist thickness). As seen with respect to the centrally located contact hole, in some cases photoresist scum (or another lithography defect) may occupy up to half (or less or more) of the depth of the contact hole development area, such that there is no contact hole (e.g., feature) topology which extends to a depth of half (or less or more) of the photoresist thickness. In such a case, a pattern feature (e.g., contact hole) may not correctly print during lithography (e.g., may not be etched, may not be filed, etc.), which may cause missing contact holes or other features. Likewise, in the simulate height, it is shown that the SEM contour outlined by solid line 900 may have a substantially different shape, including larger size, different CD, etc., than the actual cross-sectional area of the simulated contact hole represented by the dashed line 910.

[00162] Figure 9B is a simulated SEM image of the exemplary surface of Figure 9A. Figure 9B displays in grayscale an example SEM image, simulated for the exemplary surface of Figure 9A, including contact holes and photoresist scum. In Figure 9A, the contact holes are rendered in darker pixels than the surrounding areas. Similar to Figure 9A, the solid line 900 outlines contact hole contours measured from the SEM image (the solid line 900 outlines the same contact hole contours in Figures 9A and 9B, where the contours from Figure 9B — e.g., based on the simulated SEM image — are added to the simulated height of Figure 9A). The contact hole contours may be determined by any appropriate method, such as template matching, intensity contrast, etc. Similar to Figure 9A, the dashed line 910 outlines contours of the contact holes at half of the resist thickness (e.g., based on the simulated height of Figure 9A). Again, the centrally located contact hole shows no contour at half of the resist thickness, corresponding to a photoresist scum or other lithography defect. However, even though the centrally located contact hole is less deep that the other contact holes (e.g., as depicted in the grayscale depth of Figure 9A), the SEM image shows the substantially the same pixel value (e.g., change in pixel value from base height of the surrounding areas to contact hole depth) for the centrally located contact hole and its neighbors. In fact, the contact hole below the centrally located contact hole shows less contrast in the SEM image than the centrally located contact hole, even though the neighboring contact hole is correctly developed and the centrally located contact hole has a height defect. In such a manner, SEM metrology may incompletely characterize surface profiles, such as of features containing photoresist scum, during fabrication. In some applications, error or failure analysis may be based on SEM contrast, contour extraction, determined CD, etc., all of which may incorrectly detect feature (e.g., contact hole) depth, completeness, successful printing, etc., particularly for features which are narrow when compared to their depth (e.g., where Az»CD).

[00163] In some cases, whether a feature is patterned (and filled) correctly may be determined by voltage-contrast SEM imaging at AEI. However, this measurement may require that a fill step be performed before measurement (e.g., after an etched feature is filled with a metal and biased), which may eliminate chances to re-pattern the wafer while still in a photoresist (e.g., lithography) step and which may conflate lithography, etch, and fill defects (such as by detecting failure on any of multiple steps) and thus not correspond specifically to a lithography failure. In some embodiments described herein, LEEM may be used to improve measurement of lithography outcomes and may thereby improve a lithography patterning process or knowledge of a lithography patterning process.

[00164] As described previously, LEEM may be used to determine a surface profile, including my reconstruction of a waveform through interference, phase shifting, etc. In some embodiments, instead of or in addition to LEEM holography in which reference and sample waveforms (including phase shifted waveforms) are interfered, a set of LEEM images at different object planes, obtained by varying the focal length of the objective lens, may be used to reconstruct a wavefront reflected from the sample, which may provide information about a sample depth profile, surface material, defect prevalence, etc.

[00165] Figure 10 is a schematic diagram of multiple object planes, obtained by varying a focal length of an objective lens, at which LEEM images may be acquired for a surface. Figure 10 depicts an example sample 1060, with a surface profile 1062 (represented by a dashed line). The surface profile 1062 may be the actual surface profile — e.g., a surface profile which may experience defects, such as photoresist scum, incomplete photoresist development, overdevelopment, photoresist footing, etc. The surface profile 1062 may be different from an ideal surface profile 1062 (represented by a solid line), which may be the intended surface profile, a defect free surface profile, etc. In some embodiments, the surface profile 1062 may be reconstructed, such as from a reconstructed waveform, a recombined waveform, etc. In some embodiments, a difference between the ideal surface profile 1064 and the surface profile 1062 may be reconstructed (e.g., as an offset map, edge placement error, etc.), such as from the reconstructed waveform, a recombined waveform, etc. The example sample 1060 may be comprised of substantially one material, such as to within an electron penetration depth. For example, the sample 1060 at the surface profile 1062 may be substantially comprised of photoresist. In some embodiments, the example sample may be comprised of multiple materials. For example, the sample 1060, though depicted as a single shade, may be made up of different materials at different depths, such as photoresist at higher elevations (e.g., at the top of the plateaus depicted) and silicon at lower elevations (e.g., at the bottom of one or more of the valleys depicted). In some embodiments, the materials that make up the sample 1060 may have different scattering coefficients, electron scattering intensities, etc. In some embodiments, the number, energy, intensity, etc. of electrons scattered, such as elastically, from areas of the sample 1060 may provide information about the material found (such as on the surface) in those areas of the sample 1060.

[00166] Figure 10 depicts the sample 1060 in a portion of an electron microscope set up for LEEM. Any appropriate LEEM microscopy apparatus may be used, such as the imaging system 200 of Figure 2, of the imaging systems of any one of Figures 3-5 such as in place of holography or in addition to holography. The sample 1060 may be irradiated by particles, such as from a low-energy electron beam (such as previously described), in a coherent wavefront. The sample 1060 may be imaged at different object planes by changing the focal length of objective lens 1050 or any other appropriate focusing device. The wavefront may enter the objective lens 1050 along the direction 1051, reflect from the sample, re-enter the objective lens 1050 antiparallel to the direction 1051, and be directed to an electron detector or another device suitable for characterizing the reflected wavefront. The objective lens 1050 may be configured to vary the object plane at any one of multiple depths, such as object plane 1052, object plane 1054, object plane 1056, object plane 1058, etc. The objective lens 1050 may be configured to vary the object plane that is imaged onto the detector (e.g., imaged by the electrons reflected by the sample) by varying the focal length (e.g., vary the focus), where each object plane may have an associated depth of focus. The objective lens 1050 is depicted as having a scale on the order of the features of the sample 1060 for ease of description only. The objective lens 1050, and the spot size of the electron beam on the surface of the sample 1060, may be larger than the individual features of the sample 1060, including much larger. For example, although ~1 contact hole is depicted as within the electron beam at the object plane 1054 (and thus within the FOV), the FOV may be many times larger than a single contact hole, such as 10 times larger, 100 times larger, 1000 times larger, etc. In some embodiments, the depth of focus for an object plane may be configured to be relatively small (e.g., smaller than a depth of a contact hole or other feature) such that imaging at different object planes images significantly different regions in depth.

[00167] In some embodiments, images acquired by LEEM at different depths may image significantly different portions of the sample 1060. For example, a LEEM image taken of the object plane 1052 may image a top portion of the sample (e.g., of the contact holes of the sample), such as the area represented by ellipse 1000A, ellipse 1000B, and ellipse 1000C and the intervening plateaus. A LEEM image taken at the object plane 1054 may image a deeper portion of the sample (e.g., of the contact holes of the sample), such as the area represented by the ellipse 1010A and 1010C. At the depth corresponding to object plane 1054, a photoresist scum defect is depicted in the central contact hole for part of the contact hole width. Thus, for the object plane 1054, a LEEM image may indicate that the central contact hole is neither completely open nor completely closed. A LEEM image at the object plane 1056 may image a depth of the sample 1060 where the right and left contact holes are open and the center contact hole is filled. A LEEM image at the object plane 1058 may image a depth deeper than any of the contact holes. The object planes at various focal lengths (e.g., the object planes

1052, 1054, 1056, and 1058) are provided for illustrative purposes only, and more or fewer object planes may be imaged. The depths at which images (e.g., LEEM images) are obtained may be equally spaced, such as based on a defocus range, defocus increment, focal length range, etc. The depths at which images are obtained may be unequally spaced.

[00168] In some embodiments, one or more LEEM image of a sample (e.g., the sample 1060) may be obtained for each of a plurality of focal depths, defocus increments, etc. The multiple LEEM images from the different focal depths may be combined, such as in order to improve image resolution in lateral (e.g., in the plane of fabrication) and/or horizontal (e.g., in depth) of the sample. For each of the focal depths, a wavefront reflected from the sample may be measured, such as by detection of an image at an electron detector. For example, by assuming a linear approximation for image intensity, the wavefront may be approximated as shown in Equation 9, below:

I_L(r) ~ 1 + 2Re ip( )] (9) where I_L(r~) is the linear image intensity as a function of position r, Re[ )(r) ] is the real (e.g., not imaginary) portion of the wavefunction, and ip(r) is the wavefunction in the image plane where '/(^r) = 1 + ’X^r) and I '(^r)l « 1-

[00169] In some embodiments, by summing (or performing another mathematical operation, such as a weighted sum, frequency-based sum (e.g., in Fourier space), etc.), a the wavefunction in the object plane (e.g., as reflected from the sample) may be reconstructed. For example, an object-plane wavefunction may be reconstructed from a set of N varying focus images using Equation 10, below: where ip₀ is the wavefunction in the object plane, g is the spatial frequency (where g = 1/r), I_{L n}(jf) is the Fourier transform of the intensity of the n-th image of the multiple focal depths, and f_n(g) are complex-value weighting coefficients. The complex- value weighted coefficients f_n(g) may be given by Equation 11, below, as: where t_n(g) is the complex-valued transfer function which describes the propagation of the wavefront from the n-th image plane at z_n (e.g., depth z_n) to the plane of zero-defocus. Any appropriate methods and equations for generating composite LEEM images from multiple LEEM images take at various focal depths may be used, such as those discussed in “Focal-series reconstruction in low-energy electron microscopy” by Thomas Duden, Andrea Thust, Christian Kumpf, and F. Stephan Tautz in Microscopy and Microanalysis in June 2014 20(3):pages 968-73. (doi: 10.1017/S1431927614000403), which is hereby incorporated by reference in its entirety. [00170] Figures 11 A-l 1G are simulated low-energy microscopy (LEEM) images at various object planes, obtained by varying a focal length of an objective lens, for an exemplary surface with contact holes of varying depth. In each of the Figures 11A-11G, image intensity is depicted in grayscale. Figure 11 A corresponds to a defocus value of -6, Figure 1 IB corresponds to a defocus value of -4, Figure 11C corresponds to a defocus value of -2, Figure 11D corresponds to a defocus value of 0 (e.g., the plane of zero defocus), Figure HE corresponds to a defocus value of 2, Figure 1 IF corresponds to a defocus value of 4, and Figure 11G corresponds to a defocus value of 6. As can be seen in the image series depicted in Figures 11 A-l 1G, out-of-focus images may contain more (e.g., additional) information about variations in depth or reflected wavefront that in-focus images.

[00171] Figure 12 is a schematic diagram of reconstruction of a reflected wavefront based on multiple LEEM images at different object planes, obtained by varying a focal length of an objective lens, of Figures 11A-11G. In Figure 12, grayscale color differences are used to represent differences in wavefunctions, with lighter colors representing larger differences in graphs 1210 and 1220 and darker colors representing larger magnitude changes (e.g., larger in a negative direction) in graph 1230. The graph 1210 represents, on an intensity scale, a phase of an input wavefront, corresponding to the same surface depicted in the images of Figures 11 A-l 1G. The input wavefront is the simulated wavefront which would be reflected from the surface used to produce Figures 11A-11G. A dark background field represents a surface (e.g., a set of plateaus or mesas as depicted in Figure 10 in one- dimension and which may be a photoresist surface), in which contact holes have been patterned. Various contact holes of different depths are represented by lighter, circular portions of the graph 1210, where the difference in contact hole depth corresponds to difference in grayscale values. The graph 1220 represents, on an intensity scale, a reconstructed phase of the input wavefront, corresponding to a reconstruction obtained, as previously described, from the LEEM images of Figures 11 A-l 1G. As similar to the graph 1210, a dark background field represents the while the various contact holes of different depths are represented by lighter, circular portions of the graph 1220. Contact holes of the graph 1210 have similar depth (e.g., pixel intensity) values in the graph 1220. The graph 1220 is obtained from the LEEM images of Figures 11 A-l 1G using the multiple focal depth process, as previously described.

[00172] The graph 1230 represented a delta between the graph 1210 and the graph 1220, which shows that the wavefront reconstructed based on the LEEM images of Figure 11A-11G corresponds well to the actual input wavefront of the graph 1210. In the graph 1230, the filed color of medium gray represents substantially zero difference in the input wavefront and the reconstructed wavefront, while the darker shades of the various contact holes represent a negative difference between the input wavefront and the reconstructed wavefront. The variation in the difference between the input wavefront and the reconstructed wavefront (as depicted for the contact holes in the graph 1230) appears to increase with increasing hole depth variation, which, in some embodiments, may be corrected by a fitting factor.

[00173] In some embodiments, a reconstructed phase (such as depicted in the graph 1220) may be used to determine a surface profile, a surface profile defect rate, etc., such as previously described for wavefronts determined, such as by recombination, in holography examples provided above. [00174] In some embodiments, LEEM microscopy may provide benefits over other metrology techniques, such as SEM, for imaging in semiconductor fabrication. For example, LEEM, which may use relatively low energy electrons (e.g., with energies less than 100 eV, in some embodiments), may allow for imaging of photoresist without generating damage to the photoresist, where photoresist damage (such as burn in) may occur in some SEM applications. In some embodiments, LEEM microscopy may provide information about both phase and amplitude of reflected wavefronts, which may be used to reach resolution of up to 1 nm in both lateral and depth directions. In some embodiments, the depth resolution of LEEM images may be limited only by the de Broglie wavelength of the electron, which may be controlled by the electron energy (e.g., electron landing energy). For example electrons at 1 eV may have a de Broglie wavelength of 1.2 nm, electrons at 10 eV may have a de Broglie wavelength of 0.39 nm, and electrons at 100 eV may have a de Broglie wavelength of 0.12 nm. In some embodiments, LEEM may have a FOV of approximately 6 pm (of a greater or smaller FOV dimension in one or more direction), which may allow for measuring of defect rates for many contact holes in one image, and which may be acceptable for high throughput wafer fabrication, calibration, process control, etc.

[00175] Figure 13 is a diagram of an example computer system CS that may be used for one or more of the operations described herein. Computer system CS includes a bus BS or other communication mechanism for communicating information, and a processor PRO (or multiple processors) coupled with bus BS for processing information. Computer system CS also includes a main memory MM, such as a random-access memory (RAM) or other dynamic storage device, coupled to bus BS for storing information and instructions to be executed by processor PRO. Main memory MM also may be used for storing temporary variables or other intermediate information during execution of instructions by processor PRO. Computer system CS further includes a read only memory (ROM) ROM or other static storage device coupled to bus BS for storing static information and instructions for processor PRO. A storage device SD, such as a magnetic disk or optical disk, is provided and coupled to bus BS for storing information and instructions.

[00176] Computer system CS may be coupled via bus BS to a display DS, such as a cathode ray tube (CRT) or flat panel or touch panel display for displaying information to a computer user. An input device ID, including alphanumeric and other keys, is coupled to bus BS for communicating information and command selections to processor PRO. Another type of user input device is cursor control CC, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor PRO and for controlling cursor movement on display DS. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. A touch panel (screen) display may also be used as an input device.

[00177] In some embodiments, portions of one or more methods described herein may be performed by computer system CS in response to processor PRO executing one or more sequences of one or more instructions contained in main memory MM. Such instructions may be read into main memory MM from another computer-readable medium, such as storage device SD. Execution of the sequences of instructions included in main memory MM causes processor PRO to perform the process steps (operations) described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory MM. In some embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software. [00178] The term “computer-readable medium” and/or “machine readable medium” as used herein refers to any medium that participates in providing instructions to processor PRO for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device SD. Volatile media include dynamic memory, such as main memory MM. Transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise bus BS. Transmission media can also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Computer- readable media can be non-transitory, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH- EPROM, any other memory chip or cartridge. Non-transitory computer readable media can have instructions recorded thereon. The instructions, when executed by a computer, can implement any of the operations described herein. Transitory computer-readable media can include a carrier wave or other propagating electromagnetic signal, for example.

[00179] Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor PRO for execution. For example, the instructions may initially be borne on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system CS can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to bus BS can receive the data carried in the infrared signal and place the data on bus BS. Bus BS carries the data to main memory MM, from which processor PRO retrieves and executes the instructions. The instructions received by main memory MM may optionally be stored on storage device SD either before or after execution by processor PRO.

[00180] Computer system CS may also include a communication interface CI coupled to bus BS. Communication interface CI provides a two-way data communication coupling to a network link NDL that is connected to a local network LAN. For example, communication interface CI may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface CI may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface CI sends and receives electrical, electromagnetic, or optical signals that carry digital data streams representing various types of information.

[00181] Network link NDL typically provides data communication through one or more networks to other data devices. For example, network link NDL may provide a connection through local network LAN to a host computer HC. This can include data communication services provided through the worldwide packet data communication network, now commonly referred to as the “Internet” INT. Local network LAN (Internet) may use electrical, electromagnetic, or optical signals that carry digital data streams. The signals through the various networks and the signals on network data link NDL and through communication interface CI, which carry the digital data to and from computer system CS, are exemplary forms of carrier waves transporting the information.

[00182] Computer system CS can send messages and receive data, including program code, through the network(s), network data link NDL, and communication interface CL In the Internet example, host computer HC might transmit a requested code for an application program through Internet INT, network data link NDL, local network LAN, and communication interface CL One such downloaded application may provide all or part of a method described herein, for example. The received code may be executed by processor PRO as it is received, and/or stored in storage device SD, or other non-volatile storage for later execution. In this manner, computer system CS may obtain application code in the form of a carrier wave.

[00183] Embodiments of the disclosure are defined in the following numbered clauses: Clause 1: A particle beam system comprising: a beam splitter configured to split a particle beam into a first particle beam and a second particle beam, the first particle beam and the second particle beam being coherent or at least partially coherent along at least a portion of their path lengths; a reference line configured to enable passing of the first particle beam from the beam splitter; a sample line configured to enable the second particle beam to reflect from and/or interact with a sample; a delay segment configured to alter a path length of the first particle beam traveled in the reference line and/or to alter a path length of the second particle beam traveled in the sample line; and a beam combiner configured to recombine a) the first particle beam after it passes through the reference line, and b) the second particle beam after it passes through the sample line, wherein the recombination of the first particle beam and the second particle beam causes interference between the first particle beam and the second particle beam.

Clause 2: The particle beam system of clause 1, wherein the sample line comprises a low-energy electron microscopy (LEEM) arrangement.

Clause 3: The particle beam system of any of the previous clauses, wherein the second particle beam is elastically scattered by the sample.

Clause 4: The particle beam system of any of the previous clauses, wherein the delay segment comprises a particle mirror.

Clause 5 : The particle beam system of clause 4, wherein the particle mirror is an electrostatic and/or magnetic mirror. Clause 6: The particle beam of clause 4 or 5, wherein the first particle beam after it passes through the reference line comprises particles elastically scattered by the particle mirror.

Clause 7 : The particle beam system of any of the previous clauses, wherein the sample line comprises an electric, magnetic, electromagnetic and/or electrostatic objective lens.

Clause 8: The particle beam system of clause 7, wherein the objective lens comprises an immersion objective lens.

Clause 9: The particle beam system of any of the previous clauses, wherein the sample line and/or the reference line comprises a magnetic prism.

Clause 10: The particle beam system of any of the previous clauses, further comprising a sensor to detect the recombination, the sensor configured to detect intensity of the recombination of the first particle beam and the second particle beam.

Clause 11 : The particle beam system of clause 10, wherein the sensor is a pixelated particle detector.

Clause 12: The particle beam system of any of the previous clauses, further comprising: a processor; and one or more non-transitory, machine-readable medium having instructions thereon, the instructions when executed by the processor being configured to: determine phase and/or amplitude of a waveform of the second particle beam after it has interacted with the sample based on the interference between the first particle beam and the second particle beam.

Clause 13: The particle beam system of clause 12, wherein the instructions further comprise instructions to: reconstruct a depth profile of the sample based on the determined phase and/or amplitude of the waveform of the second particle beam after it has interacted with the sample.

Clause 14: The particle beam system of clause 12 or 13, wherein the instructions further comprise instructions to: identify defects in a depth profile of the sample based on the determined phase and/or amplitude of the waveform of the second particle beam after it has interacted with the sample.

Clause 15: The particle beam system of any of the clauses 12 to 14, wherein the instructions further comprise instructions to: apply an aberration correction to the determined phase and/or amplitude of the waveform of the second particle beam after it has interacted with the sample.

Clause 16: The particle beam system of clause 15, wherein the instructions to apply the aberration correction comprise instructions to apply the aberration correction via post-processing.

Clause 17: The particle beam system of clause 15 or 16, further comprising one or more phase modulators, the one or more phase modulators in a beam path of at least one of the first particle beam, the second particle beam, and a combination thereof, wherein the instructions further comprise instructions to: determine a shape of the phase modulator to apply the aberration correction; and apply the aberration correction via the phase modulator of the determined shape.

Clause 18: The particle beam system of any of the clauses 15 to 17, wherein the instructions further comprise instructions to determine an aberration correction.

Clause 19: The particle beam system of clause 18, wherein instructions to determine an aberration correction comprise instructions to determine an aberration correction based on a recombination of the first particle beam and the second particle beam obtained when the second particle beam has not interacted with a sample and/or when the particle beam has interacted with a known sample.

Clause 20: The particle beam system of clause 19, wherein the known sample is one or more point- scatterers.

Clause 21: The particle beam system of clause 20, wherein the one or more point-scatterers comprise an array in at least one dimension of point-scatterers.

Clause 22: The particle beam system of any of the previous clauses, the delay segment further configured to alter the path length of the first particle beam and/or to alter the path length of the second particle beam such that the first particle beam and the second particle beam interfere within a coherence length at their recombination.

Clause 23: The particle beam system of any of the previous clauses, wherein the particle beam is a coherent electron beam or a partially coherent electron beam.

Clause 24: The particle beam system of clause 23, wherein the second particle beam is a low-energy electron beam when reflecting from or interacting with the sample.

Clause 25 : The particle beam system of any of the previous clauses, wherein a spot size of the particle beam illuminates a field of view (FOV) with a dimension greater than 200 nm and wherein the particle beam is substantially non-scanning.

Clause 26: The particle beam system of any of the previous clauses, wherein the beam splitter is a biprism and wherein the beam combiner is a biprism.

Clause 27 : The particle beam system of any of the previous clauses, wherein the beam combiner comprises electromagnetic deflectors.

Clause 28: A delay line for a particle beam system, wherein the delay line is configured to alter a path length of a particle beam of the particle beam system.

Clause 29: The delay line of clause 28, wherein the delay line comprises a particle mirror and wherein the particle mirror is an electrostatic and/or magnetic mirror.

Clause 30: The delay line of clause 28 or 29, wherein the delay line is operable to alter a phase of the particle beam to align with a second particle beam to generate a hologram.

Clause 31: A particle beam system comprising: a sample line configured to enable a particle beam to reflect from a sample; a phase modulator configured to output a first portion of the particle beam with a first phase and a second portion of the particle beam with a second phase; and a sensor being configured to detect a recombination of the first portion and the second portion of the particle beam.

Clause 32: The particle beam system of clause 31, wherein the recombination of the first portion and the second portion of the particle beam comprises interference of the first portion of the particle beam and the second portion of the particle beam.

Clause 33: The particle beam system of clause 31 or 32, wherein the phase modulator comprises a Zernike phase plate and wherein the first portion of the particle beam comprises a diffracted portion of the particle beam and wherein the second portion of the particle beam comprises an undiffracted portion of the particle beam.

Clause 34: The particle beam system of any of the clauses 31 to 33, wherein the particle beam is a coherent electron beam or a partially coherent electron beam.

Clause 35 : The particle beam system of clause 34, wherein the particle beam is a low-energy electron beam when reflecting from or interacting with the sample.

Clause 36: The particle beam system of any of the clauses 31 to 35, wherein the particle beam comprises particles elastically scattered by the sample.

Clause 37 : The particle beam system of any of the clauses 31 to 36, wherein the sensor is a pixelated particle detector.

Clause 38: A method of particle beam microscopy comprising: splitting, by a beam splitter, a particle beam into a first particle beam and a second particle beam, the first particle beam and the second particle beam being coherent along at least a portion of their path lengths; passing the first particle beam by a reference line; passing a second particle beam by a sample line, the sample line configured to enable reflection of the second particle beam from a sample or interaction of the second particle beam with the sample, wherein the reference line and/or the sample line further comprises a delay segment, the delay segment configured to alter a path length of the first particle beam and/or to alter a path length of the second particle beam; and recombining, by a beam combiner, a) the first particle beam after it passes through the reference line and b) the second particle beam after it passes through the sample line, wherein the recombination of the first particle beam and the second particle beam causes interference between the first particle beam and the second particle beam.

Clause 39: The method of clause 38, wherein passing the first particle beam by the reference line further comprises adjusting the path length of the first particle beam to correspond to the path length of the second particle beam. Clause 40: The method of clause 38 or 39, wherein passing the second particle beam by the sample line further comprises adjusting the path length of the second particle beam to correspond to the path length of the first particle beam.

Clause 41: The method of any of the clauses 38 to 40, further comprising applying an aberration correction to the recombination.

Clause 42: The method of clause 41, further comprising determining the aberration correction, wherein determining the aberration correction comprises: passing the first particle beam by the reference line, the delay segment configured to alter the path length of the first particle beam to correspond to the path length of the second particle beam and/or to alter the path length of the second particle beam to correspond to the path length of the first particle beam; transmitting a second particle beam along the sample line, the sample line configured to enable reflection of the second particle beam from a null sample and/or a known sample; recombining, by the beam combiner, a) the first particle beam after it passes through the reference line and b) the second particle beam after it has been reflected by the null sample and/or after it has been reflected by the known sample to generate aberrationcorrection information; and determining an aberration correction based on the aberration-correction information.

Clause 43: The method of any of the clauses 38 to 42, further comprising determining phase and/or amplitude of a waveform of the second particle beam after it has interacted with or reflected from the sample based on the recombination.

Clause 44: The method of any of the clauses 38 to 43, further comprising reconstructing a depth profile of the sample based on the recombination.

Clause 45: The method of any of the clauses 38 to 44, further comprising identifying defects in a depth profile of the sample based on the recombination.

Clause 46: The method of any of the clauses 38 to 45, wherein the particle beam is a coherent electron beam or a partially coherent electron beam.

Clause 47 : The method of clause 46, wherein the second particle beam is a low-energy electron beam when interacting with the sample.

Clause 48: The method of any of the clauses 38 to 47, further comprising detecting the recombination by a pixelated particle detector.

Clause 49: The method of any of the clauses 38 to 48, wherein a spot size of the second particle beam illuminates a field of view (FOV) on the sample with a dimension greater than 200 nm and wherein the second particle beam is substantially non-scanning.

Clause 50: A method of particle beam microscopy comprising: transmitting a particle beam along a sample line, the sample line configured to enable reflection of the particle beam by a sample or interaction of the particle bean with the sample; splitting, by a phase modulator, the particle beam after interaction with the sample into a first portion and a second portion, the first portion having a first phase and the second portion having a second phase; and detecting, by a sensor, a recombination of the first portion of the particle beam and the second portion of the particle beam, the recombination containing information about a surface profile of the sample.

Clause 51 : The method of clause 50, wherein the recombination of the first portion and the second portion of the particle beam causes interference between the first portion and the second portion of the particle beam, the interference between the first portion and the second portion of the particle beam containing information about relative phases of the first portion of the particle beam and the second portion of the particle beam, where a difference between the relative phases contains information about the surface profile of the sample.

Clause 52: The method of clause 50 or 51, wherein the phase modulator comprises a Zernike phase plate and wherein the first portion of the particle beam comprises a diffracted portion of the particle beam and wherein the second portion of the particle beam comprises an undiffracted portion of the particle beam.

Clause 53: The method of any of the clauses 50 to 52, further comprising determining phase and/or amplitude of a waveform of the particle beam after it has interacted with the sample based on the detected recombination and determining information about the surface profile of the based on the determined phase and/or amplitude.

Clause 54: The method of any of the clauses 50 to 53, wherein information about the surface profile of the sample comprises information about sample depth, or a sample material, or a combination thereof for the surface.

Clause 55: The method of any of the clauses 50 to 54, further comprising reconstructing a depth profile of the sample based on the detected recombination.

Clause 56: The method of any of the clauses 50 to 55, further identifying defects in a depth profile of the sample based on the detected recombination.

Clause 57: The method of any of the clauses 50 to 56, wherein the particle beam is a coherent electron beam or a partially coherent electron beam.

Clause 58: The method of clause 57, wherein the particle beam is a low-energy electron beam when interacting with the sample.

Clause 59: The method of any of the clauses 50 to 58, wherein the recombination comprises a recombination of the first portion of the particle beam and particles elastically scattered by the sample from the second portion of the particle beam. Clause 60: The method of clause 59, wherein the second portion of the particle beam further comprises particles inelastically scattered by the sample, further comprising: detecting, by the sensor or a second sensor, inelastically scattered particles of the second portion of the particle beam in addition to the recombination.

Clause 61: The method of clause 59, wherein the recombination further comprises a first autointerference between the first portion of the particle beam and itself and/or a second auto interference between the second portion of the particle beam and itself, further comprising: detecting, by the sensor or a second sensor, the first auto-interference and/or the second autointerference.

Clause 62: The method of any of the clauses 50 to 61, wherein the sensor comprises a pixelated particle detector.

Clause 63: A method of particle beam microscopy comprising: for a plurality of focus depths, transmitting a particle beam along a sample line, the sample line configured to enable reflection of the particle beam by a sample or interaction of the particle beam with the sample; and detecting, by a sensor, the particle beam after reflection or interaction with the sample; and determining, based on the detected particle beams for the plurality of focus depths, a reconstruction corresponding to the sample.

Clause 64: The method of clause 63, wherein the sensor comprises a pixelated particle detector and wherein detecting, by the pixelated particle detector, the particle beam comprises generating an image corresponding to intensity of the particle beam after interaction with the sample.

Clause 65: The method of clause 64, wherein determining a reconstruction comprises: generating a reconstructed image corresponding to phase and/or amplitude of the sample based on the images corresponding to the intensity of the particle beam after interaction with the sample for the plurality of focus depths.

Clause 66: The method of any of the clauses 63 to 65, further comprising determining a depth profile of the sample based on the reconstruction.

Clause 67 : The method of any of the clauses 63 to 66, further comprising identifying defects in a depth profile of the sample based on the reconstruction.

Clause 68: The method of any of the clauses 63 to 67, wherein the particle beam is a coherent electron beam or a partially coherent electron beam.

Clause 69: The method of clause 68, wherein the particle beam is a low-energy electron beam when interacting with the sample.

Clause 70: The method of any of the clauses 63 to 69, wherein the particle beam after interaction with the sample comprises particles elastically scattered by the sample. Clause 71: The method of any of the clauses 63 to 70, wherein determining the reconstruction further comprises applying an aberration correction to the reconstruction.

Clause 72: The method of any of the clauses 63 to 71, wherein the reconstruction is a reconstructed wavefunction of the particle beam.

Clause 73: The method of clause 72, wherein the reconstruction is a reconstructed wavefunction of the particle beam after interaction with the sample.

Clause 74: A system comprising: a low-energy, electron microscope (LEEM) apparatus; a processor; and one or more non-transitory, machine-readable medium having instructions thereon, the instructions when executed by the processor being configured to: obtain, for a plurality of focus depths, images of a sample from the LEEM; and determine, based on the obtained images for the plurality of focus depths a wavefunction corresponding to interaction of a particle beam of the LEEM with the sample, the wavefunction containing information about a surface of the sample.

Clause 75: The system of clause 74, the instructions further comprising instructions to identify defects in a depth profile of the sample based on the wavefunction.

Clause 76: The system of clause 75, wherein instruction to identify defects in a depth profile comprise instructions to: identify a plurality of regions in the sample corresponding to features of substantially similar planned depth; determine a phase and/or amplitude of the wavefunction corresponding the plurality of regions; and determine if any of the plurality of regions substantially vary in phase and/or amplitude from others of the plurality of regions.

Clause 77: The system of clause 76, wherein instructions to determine if any of the plurality of regions substantially vary in phase and/or amplitude from others of the plurality of regions comprise instructions to identify lithography defects.

Clause 78: The system of any of the clauses 74 to 77, the instructions further comprising instructions to determine a depth profile of the sample based on the wavefunction.

Clause 79: The system of any of the clauses 74 to 78, wherein the sample comprises one of the following: a lithography sample, an after etch sample, an after development sample, a trenched sample, a sample comprising an array of contact holes, a sample comprising contact holes other than in an array, a sample comprising elongated slots, a sample comprising lines and spaces, or a combination thereof. Clause 80: The system of any of the clauses 74 to 79, the instructions further comprising instructions to determine, based on the wavefunction, a process defect rate.

Clause 81 : One or more non-transitory, machine-readable medium configured to perform the method of any one of clauses 38 to 73. [00184] While the concepts disclosed herein may be used for manufacturing with a substrate such as a silicon wafer, it shall be understood that the disclosed concepts may be used with any type of manufacturing system (e.g., those used for manufacturing on substrates other than silicon wafers). [00185] In addition, the combination and sub-combinations of disclosed elements may comprise separate embodiments. For example, one or more of the operations described above may be included in separate embodiments, or they may be included together in the same embodiment.

[00186] 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

1. A particle beam system comprising: a beam splitter configured to split a particle beam into a first particle beam and a second particle beam, the first particle beam and the second particle beam being coherent along at least a portion of their path lengths; a reference line configured to enable passing of the first particle beam from the beam splitter; a sample line configured to enable the second particle beam to reflect from a sample; a delay segment configured to alter a path length of the first particle beam traveled in the reference line and/or to alter a path length of the second particle beam traveled in the sample line; and a beam combiner configured to recombine a) the first particle beam after it passes through the reference line, and b) the second particle beam after it passes through the sample line, wherein the recombination of the first particle beam and the second particle beam causes interference between the first particle beam and the second particle beam.

2. The particle beam system of claim 1, wherein the sample line comprises a low-energy electron microscopy (LEEM) arrangement.

3. The particle beam system of claim 1, wherein the second particle beam is elastically scattered by the sample.

4. The particle beam system of claim 1, wherein the delay segment comprises a particle mirror.

5. The particle beam system of claim 4, wherein the particle mirror is an electrostatic and/or magnetic mirror.

6. The particle beam system of claim 1, further comprising a sensor to detect the recombination, the sensor configured to detect intensity of the recombination of the first particle beam and the second particle beam.

7. The particle beam system of claim 6, wherein the sensor is a pixelated particle detector.

8. The particle beam system of claim 1, further comprising: a processor; and one or more non-transitory, machine-readable medium having instructions thereon, the instructions when executed by the processor being configured to: determine phase and/or amplitude of a waveform of the second particle beam after it has interacted with the sample based on the interference between the first particle beam and the second particle beam.

9. The particle beam system of claim 8, wherein the instructions further comprise instructions to: reconstruct a depth profile of the sample based on the determined phase and/or amplitude of the waveform of the second particle beam after it has interacted with the sample.

10. The particle beam system of claim 8, wherein the instructions further comprise instructions to: identify defects in a depth profile of the sample based on the determined phase and/or amplitude of the waveform of the second particle beam after it has interacted with the sample.

11. The particle beam system of claim 8, wherein the instructions further comprise instructions to: apply an aberration correction to the determined phase and/or amplitude of the waveform of the second particle beam after it has interacted with the sample.

12. The particle beam system of claim 11, further comprising one or more phase modulators, the one or more phase modulators in a beam path of at least one of the first particle beam, the second particle beam, and a combination thereof, wherein the instructions further comprise instructions to: determine a shape of the phase modulator to apply the aberration correction; and apply the aberration correction via the phase modulator of the determined shape.

13. The particle beam system of claim 1, the delay segment further configured to alter the path length of the first particle beam and/or to alter the path length of the second particle beam such that the first particle beam and the second particle beam interfere within a coherence length at their recombination.

14. The particle beam system of claim 1, wherein the particle beam is a coherent electron beam or a partially coherent electron beam.

15. A method of particle beam microscopy comprising: splitting, by a beam splitter, a particle beam into a first particle beam and a second particle beam, the first particle beam and the second particle beam being coherent along at least a portion of their path lengths; passing the first particle beam by a reference line; passing a second particle beam by a sample line, the sample line configured to enable reflection of the second particle beam from a sample or interaction of the second particle beam with the sample, wherein the reference line and/or the sample line further comprises a delay segment, the delay segment configured to alter a path length of the first particle beam and/or to alter a path length of the second particle beam; and recombining, by a beam combiner, a) the first particle beam after it passes through the reference line and b) the second particle beam after it passes through the sample line, wherein the recombination of the first particle beam and the second particle beam causes interference between the first particle beam and the second particle beam.