Electron beam apparatus and method for generating pulsed electron beams, and their applications.

Abstract

An electron beam device 100 for generating a pulsed electron beam 1 including a sequence of electron pulses comprises an irradiation source device 10 for generating a sequence of emitter excitation pulses 2, in particular laser pulses; an electron source device 20 having a photoemission electron source 21 for irradiation-induced emission of source electron pulses 3 in response to irradiation with the emitter excitation pulses 2; a number-state disperser 30 for spatially separating the source electron pulses 3 into sub-pulses 4, 4A, each sub-pulse 4, 4A including an integer number n of electrons, where n = 1, 2, 3, ...; and a number-state selection device 40 for selecting sub-pulses 4, 4A including at least one set of predetermined electron number states as the generated pulsed electron beam 1. Preferably, the photoemission electron source 21 is configured to generate countable low-charge electron pulses, e.g., including 1, 2, 3, or 4 electrons per pulse. Methods for generating the pulsed electron beam 1 and for using the electron beam device are also described.

Description

[Technical Field]

[0001] The present invention relates to an electron beam apparatus and / or method for generating a pulsed electron beam including a sequence of electron pulses. Applications of the present invention can be found, for example, in the field of material processing by electron microscopy or electron lithography. [Background technology]

[0002] This disclosure refers to the following prior art illustrating the technical background and related technologies of the present invention. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] DNKlyshko,Phys.-Usp.39,573(1996) [Non-Patent Document 2] MIKolobov,Rev.Mod.Phys.71,1539(1999) [Non-Patent Document 3] G. Kotliar and D. Vollhardt, Physics Today 57,53(2004) [Non-Patent Document 4] E.Morosan,D.Natelson,AHNevidomskyy,andQ.Si,Adv.Mater.24,4896(2012) [Non-Patent Document 5] TDLadd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe, and JLO'Brien, Nature 464, 45 (2010) [Non-Patent Document 6] O.Kfir,V.Di Giulio,FJGde Abajo,and C.Ropers,Sci.Adv.7,eabf6380(2021) [Non-Patent Document 7] S.Asban and FJGarc▲i▼a de Abajo,npj Quantum Inf 7,42(2021)

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[0004] Controlling the quantum statistics of fermion and boson modes is central to non-classical light sources [Non-Patent Literature 1, 2] and strongly correlated functional materials [Non-Patent Literature 3, 4], enabling (noise-corrected) quantum computers [Non-Patent Literature 5]. Developing these concepts in the context of free-electron quantum optics promises coherent manipulation of quanta [Non-Patent Literature 6, 7] and nanoscale sensing [Non-Patent Literature 8] and imaging, as well as creating new opportunities from structural biology and materials science to electron beam lithography. In particular, recent theoretical and experimental studies have explored quantum-enhanced electron microscopy relying on interference [Non-Patent Literature 9] or multi-pass [Non-Patent Literature 10, 11] methods, emission of quantum light [Non-Patent Literature 12], or coordinated interactions with optical modes [Non-Patent Literature 6, 8, 13-15].

[0005] While correlated multi-electron states are ubiquitous in experimental condensed matter physics [Non-Patent Literature 16,17], their analogues in free particle beams have not yet been observed. However, proposed mechanisms include indirect coupling in long-lived optical modes [Non-Patent Literature 13,18], common-path interference [Non-Patent Literature 19], or correlated photoemission [Non-Patent Literature 20,21] and ionization [Non-Patent Literature 22,23] processes. As a characteristic and necessary condition of entanglement, particle correlation in free electron beams has been studied considering contributions from exchange-mediated [Non-Patent Literature 24] or Coulomb [Non-Patent Literature 26] interactions in transverse [Non-Patent Literature 19,24] and longitudinal [Non-Patent Literature 25] phase spaces. In particular, classical Coulomb repulsion results in stochastic transverse and longitudinal emittance increases of the electron beam, described by the Belsch [Non-Patent Literature 27,28] effect and the Loeffler [Non-Patent Literature 29] effect, respectively, limiting the brightness of the leading electron source [Non-Patent Literature 30]. In high-charge electron pulses, mean-field-induced space charge effects govern the achievable pulse duration, energy diffusion, and focusing [Non-Patent Literature 31], raising major experimental challenges in ultrafast electron microscopy and diffraction, particle accelerators, and free-electron lasers [Non-Patent Literature 32].

[0006] Coincidence spectroscopy is well-established in atomic and molecular sciences [Non-Patent Documents 33, 34], and has revealed complex collision mechanisms and correlation effects in solids [Non-Patent Documents 20, 35], COLTRIMS, reaction microscopy, and other correlation detection techniques.

[0007] To miniaturize electron sources to generate coherent electron pulses in nanoscale pointed emitters, enabling ultrafast dark-field imaging [Non-Patent Literature 36] or phase-contrast imaging [Non-Patent Literature 37], nanoscale diffraction probes [Non-Patent Literature 38,39], and photon-induced near-field electron microscopy [Non-Patent Literature 40-43], a detailed analysis of stochastic Coulomb and mean-field effects on low-charge electron fluxes [Non-Patent Literature 31] is necessary.

[0008] However, unraveling both contributions requires single-particle resolution event-based detection, a very recent addition to electron microscopy, which is applied to high-speed STEM [Non-Patent Literature 44,45] and EELS [Non-Patent Literature 42], and detects cathodoluminescence in quantum materials [Non-Patent Literature 47] and integrated photonic resonators [Non-Patent Literature 15]. [Overview of the project] [Problems that the invention aims to solve]

[0009] The object of the present invention is to provide an improved electron beam apparatus and / or method for generating a pulsed electron beam that can overcome the limitations of the prior art for generating electron beams. In particular, the pulsed electron beam should be generated using tuned pulse statistics, and thus enable a wide range of new applications, particularly in electron microscopy applications with reduced noise and / or improved imaging quality, and / or improved applications in electron lithography and / or quantum computing, and / or novel applications in electronic manipulation. [Means for solving the problem]

[0010] These objectives are addressed by electron beam apparatus and / or methods for generating pulsed electron beams, each comprising features of an independent claim. Advantageous embodiments and applications of the present invention are defined in the dependent claims.

[0011] According to a first general aspect of the present invention, the above object is solved by an electron beam apparatus configured to generate a pulsed electron beam comprising a sequence of electron pulses, the apparatus comprising: an irradiation source apparatus arranged to generate a sequence of emitter excitation pulses, in particular laser pulses; an electron source apparatus having a photoemission electron source arranged for irradiation-induced emission of source electron pulses in response to irradiation of emitter excitation pulses; a number-state dispersive apparatus arranged to spatially separate the source electron pulses into subpulses, each subpulse comprising an integer number of electrons (n), where n = 1, 2, 3, ...; and a number-state selection apparatus arranged to select subpulses comprising at least one set of predetermined number of electron states as the pulsed electron beam to be generated.

[0012] According to a second general aspect of the present invention, the above object is solved by a method for generating a pulsed electron beam comprising a sequence of electron pulses, comprising: generating a sequence of emitter-excitation pulses, particularly laser pulses, using an irradiation source device; irradiating a photoemission electron source of the electron source device with the emitter-excitation pulses so that a source electron pulse is generated by irradiation-induced emission; spatially separating the source electron pulse into subpulses using a number-state dispersion device, each subpulse comprising an integer number of electrons (n), where n = 1, 2, 3, ...; and selecting a subpulse comprising at least one set of predetermined number of electron states as the pulsed electron beam to be generated using a number-state selection device. Preferably, the method or embodiments of the second general aspect of the present invention are performed using an electron beam apparatus according to the first general aspect or embodiments thereof. All preferred embodiments disclosed herein in relation to the apparatus are deemed to be corresponding preferred embodiments of the method, and vice versa.

[0013] According to a third general aspect of the present invention, the above object is solved by using an electron beam apparatus according to the first general aspect or an embodiment thereof as a beam source in at least one of the following: an electron microscope apparatus, an electron lithography apparatus, an electron pair (source) apparatus, an electron herald apparatus, an electron counting apparatus (for measuring the exact number of electrons), an information processing apparatus, a communication apparatus, and a quantum computing apparatus. An electron microscope apparatus, an electron lithography apparatus, and / or a quantum computing apparatus comprising an electron beam apparatus according to the first general aspect or an embodiment thereof, and / or configured to perform a method according to the second general aspect or an embodiment thereof, is considered an independent subject of the present invention.

[0014] As used in this disclosure, the term “pulse” typically refers to a waveform that provides a predetermined discontinuous time structure of source electrons. The pulse may be provided as a periodic pulse sequence. The step of selecting a subpulse containing at least one set of predetermined electron number states includes selecting a single set of subpulses having a single electron number state, such as n=2, or selecting multiple sets of subpulses, each having a different electron number state, such as n>1 or n=1. Thus, when selecting multiple sets of subpulses, different electron number states may be combined.

[0015] A number-state dispersion device spatially separates pulses with integer electron numbers n=0, 1, 2, 3, ... and projects the electron beam onto a number-state selection device. The number-state selection device includes, for example, apertures, slits, and / or beam blocks for blocking / rejecting subsets of quantized number states n=0, 1, 2, 3, ....

[0016] The inventors observed Coulomb-correlated electron-pair and triplet states generated by femtosecond pulsed photoemission from a nanoscale tip emitter in an ultrafast transmission electron microscope. Event-based electron spectroscopy allows for clear identification of specific number states through characteristic interparticle minority electron volt kinetic energy separation. State-selected beam caustics exhibit an increased virtual source size for higher electron counts induced by stochastic Coulomb scattering, and energy state separation affects the averaged spectrum. The inventors propose a scheme in which simple beam filtering, such as spectral filtering, spatial filtering, and / or spin filtering, allows for adjustment of state selection and pulse statistics. Furthermore, by employing an electrostatic emitter configuration, the separation of electron kinetic energy and the relative generation of a desired number of states may be modified to optimize for specific applications. This enables advanced control schemes for, for example, the generation of sub-Poisson electron beams or the implementation of heralded single electron sources, thereby eliminating limitations in electron microscope aberration correction and shot noise in electron beam lithography.

[0017] According to a preferred embodiment of the present invention, the photoemission electron source is configured to generate countable low-charge electron pulses, preferably containing 1, 2, 3, or 4 electrons per pulse.

[0018] According to a preferred embodiment of the present invention, the photoemission electron source comprises a beam-limiting aperture configured to reduce high-charge electron pulses into countable low-charge electron pulses. The beam-limiting aperture may be located directly downstream of the electron source device and upstream of an optional accelerator and focusing optical system.

[0019] According to a preferred embodiment of the present invention, the photoemission electron source is configured to generate a multi-electron state having distinguishable properties in addition to a pulsed charge.

[0020] According to a preferred embodiment of the present invention, the number-state dispersion device comprises an energy dispersion device, such as a spectrometer. The spectrometer may be configured to spectrally decompose a source electron pulse and apply an energy filter to the source electron pulse. A spatial filter device may be located within the energy selection plane of the spectrometer. A second spectrometer may be provided to merge a selected subset of electrons into a common beam.

[0021] According to a preferred embodiment of the present invention, the energy dispersion device may include a beam monochromator, particularly an omega-type, alpha-type, Wien filter-type, double Wien-type, electron mirror-type, or other type of monochromator, and / or a spectrometer configured to separate the spectral components of a source electron pulse, particularly a spectrometer including a magnetic prism, a spectrometer including an electrostatic multipole electron optical system, or other types of spectrometers.

[0022] According to a preferred embodiment of the present invention, the number-state dispersion device comprises a spatial dispersion device, in particular, at least one of a rotationally symmetric electron lens and a cylindrical electron lens. The spatial dispersion device may also be called an (electron) momentum dispersion device, a position dispersion device, an electron orbital momentum dispersion device, or an electron spin dispersion device.

[0023] According to a preferred embodiment of the present invention, the number state selection device includes a spatial modulator of electron beam intensity, in particular a spatial modulator configured to allow selected subpulses having a predetermined number of electron states to pass through and block the remaining subpulses.

[0024] According to a preferred embodiment of the present invention, the number state selection device includes at least one of a mechanical slit, a grid, a linear beam block, a hole, a ring, or a disk. The selection device is preferably made of a conductive material or electrode adapted to absorb, preferably completely absorb, the rejected electron number states.

[0025] According to a preferred embodiment of the present invention, the electron beam apparatus further comprises at least one of a detector apparatus configured to measure the number of electrons in at least one of the electron number states of a pulsed electron beam, and a beamforming electron optical system configured to illuminate a sample such as a sample under investigation and / or a workpiece such as a semiconductor workpiece.

[0026] According to a preferred embodiment of the present invention, the photoemission electron source preferably includes a pointed photoemission electron source configured for linear photoemission, in particular at least one of a Schottky emitter, a low-temperature field emitter, and a thermionic emitter.

[0027] The photoelectron emission electron source may include a tungsten tip having (100) crystal facets covered with a zirconium oxide thin film, or a composite tip made from a material having an electron work function less than or equal to that of lanthanum hexaboride, LaB6, or cerium hexaboride, CeB6, or a metal tip having crystal tip facets with a lower electron work function compared to the remaining tip material, or a pure metal tip, in particular pure metal tips made of W, Mo, Re, Ir, Ta, Hc, Pt, or Ni, a transition metal carbide tip, in particular transition metal carbide tips made of HfC, ZrC, NbC, TaC, TiC, or VC, or a carbon cone emitter or a single carbon nanotube tip.

[0028] The irradiation source device may include a laser source and / or the irradiation source device may be configured to generate emitter-excited emission having wavelengths selected according to the application conditions, in particular the emitter tip material, for example, in at least one of the ranges of 1 nm to 200 nm, 200 nm to 1500 nm, and 1500 nm to 16 μm.

[0029] According to a preferred embodiment of the method of the present invention, A step of adjusting the irradiation source device and / or photoemission electron source in order to generate a multi-electron state that can be identified by characteristics other than pulse charge (=number of electrons), Steps to adjust the multi-state dispersive device according to the identifiable characteristics of the source electron pulse, particularly energy, angular / momentum, and spatial focus. The step of adjusting the number state selection device to reject or block a specific number of states from the beam, of particular interest, to reject or block all n>2 states, or to reject or block all subpulses n≠2 states, At least one of the steps of adjusting the irradiation source device, photoemission electron source, number state disperser and / or number state selector device based on the output of an additional number state sensing detector may be provided.

[0030] Further details and advantages of the present invention are described below with reference to the accompanying drawings, which are schematically shown below. [Brief explanation of the drawing]

[0031] [Figure 1] This is an illustration of a preferred embodiment of the present invention. [Figure 2] This is an illustration of a preferred embodiment of the present invention. [Figure 3] This is an illustration of a preferred embodiment of the present invention. [Figure 4] This is an illustration of a preferred embodiment of the present invention. [Figure 5] This is an illustration of a preferred embodiment of the present invention. [Figure 6] This is an illustration of a preferred embodiment of the present invention. [Figure 7] This is an illustration of a preferred embodiment of the present invention. [Figure 8] This is an illustration of a preferred embodiment of the present invention. [Figure 9] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 10] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 11A] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 11B]Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 11C] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 11D] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 11E] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 11F] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 11G] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 11H] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 12] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 13A] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 13B] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 13C] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 13D] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 13E] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Figure 13F] Further features of preferred embodiments of the present invention and illustrations of experimental tests are shown. [Modes for carrying out the invention]

[0032] Figures 1 to 8 schematically illustrate the general principles of the present invention (particularly Figure 1) and the features of preferred embodiments for actually carrying out the present invention (particularly Figures 2 to 8). Further details of actual embodiments based on the inventors' experimental findings are described with reference to Figures 9 to 13.

[0033] Embodiments of the present invention will be described with particular reference to the preparation and / or analysis of pulsed electron beams having clearly defined number states or charge states. The electron beam apparatus may be implemented based on a transmission electron microscope as outlined below. Details and / or operating parameters of the transmission electron microscope, such as adjusting the acceleration voltage, and details and / or operating parameters of the laser source for exciting the electron source of the transmission electron microscope, such as controlling the output of the laser source, will not be described insofar as the conventional electron microscopy techniques themselves are known.

[0034] The implementation of the present invention is not limited to electron microscopy applications but can also be carried out in other applications as described above. Furthermore, the present invention is not limited to the applicable conditions provided for the actual tests shown in Figures 10 to 14. Depending on the application, embodiments of the present invention may be modified, for example, with respect to the design of a multi-state dispersion device and / or a multi-state selection device.

[0035] Figure 1 schematically shows an electron beam apparatus 100 for generating a pulsed electron beam 1 including a sequence of electron pulses. The electron beam apparatus 100 comprises an irradiation source apparatus 10, in particular a pulsed irradiation source such as a pulsed laser, which is arranged to generate a pulsed laser beam of a sequence of emitter excitation pulses 2, in particular laser pulses. The electron beam apparatus 100 further comprises an electron source apparatus 20 having a photoemission electron source 21. Exemplary details of the photoemission electron source 21 are shown in particular in Figures 3 to 9.

[0036] The photoemission electron source 21 is configured to receive an emitter excitation pulse 2 and to irradiate source electron pulses 3 in response to irradiation with the emitter excitation pulse 2 directed onto the photoemission electron source 21. The source electron pulses 3 provide a pulsed electron beam having a small number of (countable) electrons per pulse. In particular, each source electron pulse 3 contains a number n of electrons, for example, 1, 2, 3 or more. The pulse charge of each source electron pulse 3 is determined in correspondence with the number of electrons per pulse. In addition to pulse charge / number of electrons, the multi-electron states (number states) of the source electron pulses 3 are identifiable by at least one physical property such as electron energy, angle / momentum, position, spin, and / or spatial focus shift, which are determined by the angle / transverse momentum of the source electron pulses, for example, after a number-state dispersion device.

[0037] The source electron pulse 3 is directed along the optical axis z (electron beam axis z) to the number-state disperser 30 of the electron beam apparatus 100. The number-state disperser 30 is configured to spatially separate the sequence of source electron pulses 3 into separate subsequences of source electron pulses 4, each subsequence containing a source electron pulse 4 having a common integer number of electrons (n), i.e., n = 1, 2, 3, ... The number-state disperser 30 provides dispersion along at least one identifiable physical property. Thus, the number states are dispersed into one or more parts. Depending on the physical characteristics of the number states, each source electron pulse 3 is directed to one of the different pulse paths of the subpulse 4 (see Figures 3 to 9).

[0038] Furthermore, the electron beam apparatus 100 includes a number state selection device 40, which is configured to receive spatially separated subpulses 4 and to select one of the subpulses 4 containing at least one set of predetermined electron number states as the generated pulsed electron beam 1. Based on at least one identifiable physical characteristic, pulses having a specific number state are transmitted by the number state selection device 40 as the pulsed electron beam 1, while pulses having a number state other than the specific number state are rejected, in particular blocked, or arbitrarily split by the number state selection device 40, as illustrated particularly in Figures 3 to 9.

[0039] The pulsed electron beam 1 is directed towards the sample and / or application area 50 which contains the sample to be investigated with the pulsed electron beam 1 or the workpiece to be processed with the pulsed electron beam 1. Only a specific number of states, such as n<2 or n=2, are used in the sample and / or application area 50. The sample and / or application area 50 includes, for example, the sample stage of a microscope, spectrometer, or lithography apparatus. For example, only a subsequence of source electron pulses 4 having 2 electrons per pulse is used for investigating the sample with an electron microscope or irradiating the workpiece.

[0040] Optionally, a number-state reporting channel 60 including an electron pulse detector may be provided. Using the number-state reporting channel 60, one channel of n states may be detected, while state n-1 is used in the sample and / or application site 50 to realize, for example, a heralded single electron source.

[0041] Figure 2 shows further details of an optional application with a specific number of states herald, including a number of states selection device 40, a sample and / or application site 50, and a number of states reporting channel 60, as shown in Figure 1. The number of states selection device 40 delivers a pulsed electron beam 1 in state m=n-1 to the sample and / or application site 50, and a non-selected source electron pulse 4A in state n is split into the number of states reporting channel 60. The non-selected subsequence of the source electron pulse 4A includes, for example, a single electron pulse detected by a detector device, which is included in the number of states reporting channel 60.

[0042] The numerical state reporting channel 60 may be coupled to the sample and / or application site 50 via the information channel 61. For example, a precisely countable number of electrons may be introduced into the numerical state reporting channel 60 and the sample, or into the numerical state reporting channel 60 and the application site 50. This provides insight into the precise number of electrons in the electronic state transferred to the sample and / or application site 50, which is suitable, for example, for microscopy and lithography applications with clearly defined electron doses.

[0043] Further details of the electron beam apparatus 100 according to Figure 1 or Figure 2 are illustrated in Figures 3 to 9. In the embodiments of Figures 3 to 9, the irradiation source apparatus 10 includes a laser that generates a pulsed laser beam, for example, a femtosecond laser that generates laser pulses having a duration of 160 fs and a repetition period of approximately 2 μs. The pulsed laser beam is focused onto the photoelectron source 21 of the electron source apparatus 20 as a sequence of emitter-excited pulses 2 having a focusing optical system such as a lens 11.

[0044] The photoemission electron source 21 in Figures 3-7 and 9 includes an emitter tip (see Figure 9 in particular). In response to irradiation with a focused pulsed laser beam accompanied by an emitter excitation pulse 2, a corresponding pulsed electron beam of the source electron pulse 3 is generated at the emitter tip. To direct the source electron pulse 3 along the optical axis z towards the multi-state dispersive device 30, the electron source device 20 of each illustrated embodiment may optionally further include a suppressor anode 22, an extractor anode 23 (see also Figure 9), a beam limiting aperture 20A, and an accelerator and concentrator optical system 24 (see Figure 3). Preferably, the beam limiting aperture 20A is located between the extractor anode 23 and the accelerator and concentrator optical system 24.

[0045] According to embodiments of Figures 3 and 4, the number-state dispersion device 30 is an energy dispersion device including an energy-dispersive magnetic prism 31, which may be configured as described, for example, in [Non-Patent Documents 58, 61, 62]. Preferably, a single energy-dispersive magnetic prism 31 is provided. The energy-dispersive magnetic prism 31 imparts different deflections with respect to the optical axis z to source electron pulses with different charge / energy in the deflection field of the energy-dispersive magnetic prism 31. As in the illustrated example, the subpulse 4 with number state n=2 is deflected at a larger output angle compared to the subpulse 4A with number state n=1. The deflection due to the spatial separation of the subpulses is provided toward the number-state selection device 40.

[0046] The number-state selection device 40 includes a mechanical beamblock 41 that acts as a spatial modulator of the electron beam intensity of spatially separated subpulses. The mechanical beamblock 41 in Figure 3 is configured to allow only selected subpulses 4 of number state n=2 to pass through the sample and / or application site 50, while blocking subpulses 4A of number state n=1. For this purpose, the mechanical beamblock 41 may include a beamblock material that is positioned in the path of the subpulses 4A to be blocked and through which the selected subpulses 4 pass. Alternatively, the mechanical beamblock 41 may be described as a slit, for example, a circular slit through which the selected subpulses 4 pass. As a further alternative, in order to select only subpulses 4A of number state n=1 and block subpulses 4 of number state n>1, in particular n=2, the mechanical beamblock 41 may include a circular beamblock material with a central hole, as shown in Figure 4.

[0047] Figure 5 shows further details of an electron beam apparatus 100, for example, according to Figure 1 or Figure 2, where the multi-state dispersion apparatus 30 comprises an in-column omega monochromator filter 32 including a multi-state selection apparatus 40 provided by a slit 42. The in-column omega monochromator filter 32 may be configured as described, for example, in [Non-Patent Documents 58, 61]. The monochromator filter 32 comprises four magnetic prisms 32A to 32D that deflect the pulsed electron beam of the source electron pulse 3 into the shape of the Greek letter Ω and act as an energy filter.

[0048] Due to the deflection fields of the first pair of magnetic prisms 32A and 32B, and depending on the number of states of the received source electron pulse 3, the source electron pulse 3 is dispersed as subpulses 4, 4A on different beam paths toward the slit 42 after passing through the monochromator filter 32. The slit 42 can block the outer n>1, for example, n=2 state subpulses 4 and allow the central n=1 state subpulse 4A to pass through. Alternatively, the slit 42 may be replaced with a beam stop (not shown) at a slit position that allows only the outer n>1 state subpulses to pass through and blocks the central n=1 state subpulses.

[0049] Due to the deflection field effect of the second pair of magnetic prisms 32C and 32D, the selected subpulse passed through the slit 42 (or beam stop) is re-deflected in the initial direction of the source electron pulse 3 and guided as electron pulse 1 obtained toward the sample and / or application site 50. To adjust the electron kinetic energy to the requirements of the application / sample 50, the accelerator 33 is positioned behind the monochromator 32, but can optionally be positioned in front of 32 as well.

[0050] According to the embodiment of Figure 6, for example, the electron beam apparatus 100 according to Figure 1 or Figure 2 includes a multi-state dispersive apparatus 30 having an in-column alpha monochromator 34 including a slit 43. The slit 43 provides a mechanical beam block for the multi-state selection apparatus 40. The pulsed electron beam of the source electron pulse 3 generated at the emitter tip 21 is directed to the alpha monochromator 34 via a focusing optical system and accelerator 25.

[0051] The alpha-type monochromator 34 may be configured, for example, as described in [Non-Patent Documents 58, 61, 64]. The monochromator 34 includes an imaging optical system and three magnetic prisms 34A-34C that deflect the pulsed electron beam of the source electron pulse 3 into the shape of the Greek letter α and act as energy filters. Similar to the monochromator filter 32 in Figure 5, the source electron pulse 3 is dispersed as subpulses 4, 4A on different beam paths toward the energy-selective slit 43 after passing through the monochromator 34. The slit 43 can block the outer n>1, e.g., n=2 state subpulses 4 and allow the central n=1 state subpulse 4A to pass through. Alternatively, a central beam stop (not shown) at the slit position may be used to transmit only the outer n>1 state subpulses and block the central n=1 state subpulses. The selected subpulses transmitted by the slit 43 (or around the central beam stop) are imaged toward the sample and / or application area 50.

[0052] As a further alternative embodiment, the electron beam apparatus 100 in Figure 7 includes a Wien monochromator 35 as a number-state dispersion device 30. The Wien monochromator 35, which can be configured, for example, as described in [Non-Patent Literature 61, 64], is arranged in combination with an accelerator 36 located directly downstream of the extractor anode 23. As an alternative to the Wien monochromator 35 shown, a double Wien monochromator may be used. The Wien monochromator 35 functions as an energy-selective imaging system that guides pulses with different number states of electrons onto beam paths having different angles with respect to the optical axis z of the electron source apparatus 20. Thus, subpulses 4, 4A are spatially separated, and subpulses can be selected according to their number states. The schematic color scale diagram in Figure 7 encodes the kinetic electron energies dispersed by the Wien filter.

[0053] The multi-state selection device 40 includes a slit 44 positioned at a distance downstream of the Wien monochromator 35. The slit 44 acts as a spatial modulator of the electron beam intensity of spatially separated subpulses. Depending on the position of the slit 44 with respect to the optical axis z, the slit 43 can either block the n>1 state or transmit only the n=2 state.

[0054] Figure 8 shows the formation of the virtual source size and position at the apex of the pointed photoemission electron source 21 of the electron beam apparatus 100. In response to focused pulsed laser beam irradiation from the irradiation source apparatus 10 via lens 11, the emitter excitation pulse 2 generates a pulsed electron beam of source electron pulse 3 by virtual source size and virtual source shift at the emitter tip depending on the number state of the pulse. In particular, the virtual source size increases further as the number of excited electron pulses increases (see double arrow), and the virtual source shifts further along the optical axis z (see arrow) as the number of excited electron pulses increases.

[0055] The multi-state dispersion device 30 in the embodiment shown in Figure 8 includes a pair of electron lenses 37, 38 arranged with a common optical axis z perpendicular to the surface of the emitter-tip photoemission electron source 21. The electron lenses 37, 38 are spaced apart from each other in the axial direction, including a multi-state selection device 40. The multi-state selection device 40 includes a circular aperture 45 made of beam-blocking material having a hole centered on the optical axis z. Alternatively, the multi-state selection device 40 includes a circular beam blocker (not shown) centered on the optical axis z.

[0056] The emitter tip of the photoemission electron source 21 provides a virtual source size depending on the number of states of the excited source electron pulse 3. This characteristic creates an angular / spatial dispersion of the source electron pulse 3 visible in the beam caustics after the first electron lens 37, as schematically shown in Figure 8 (see also Figure 13). As the virtual source size increases, i.e., as the number of states increases, the diameter of the beam caustics after the first electron lens 37 also increases. Therefore, depending on the type of aperture or blocker and the position of the number-state selection device 40, certain number states, e.g., subpulse 4 with n=1, can pass through the number-state selection device 40, while other number states, e.g., subpulse 4A with n=2 and n=3, are blocked by the number-state selection device 40. The second electron lens 38 images the subpulse 4 (the generated pulsed electron beam) transmitted by the number-state selection device 40 onto the sample and / or application site 50.

[0057] For each of Figures 5 to 8, it should be noted that the average kinetic energy of the electron pulse can be adjusted using an electrostatic electron accelerator (see, for example, 33 in Figure 5 or 36 in Figure 7), which may be placed at any point in the beam path between the emitter unit and the application / sample, preferably before the multi-state dispersion or after the multi-state selection device.

[0058] Further practical embodiments and experimental tests of the present invention will be described below with reference to Figures 9 to 13.

[0059] Actual tests by the inventors demonstrate strong Coulomb correlations in two- and three-electron states generated in a laser-driven Schottky field emitter, as shown below. Using event-based electron spectroscopy and imaging, the kinetic energy distribution of electron ensembles emitted by a single laser pulse is recorded, and events are classified by the number of free electrons. For example, characteristic double and triple lobe spectra are found for events containing two and three electrons, respectively. The invention enables the quantitative characterization of interparticle correlations in both energy and transverse momentum, and observes that stochastic small-many-body interactions prevail over mean-field (space charge) effects. The two-particle energy correlation function reveals a prominent peak at an energy difference of approximately 1.7 eV, indicating an effective junction emission region for electron-pair states much smaller than the physical and virtual source sizes. This finding sheds light on fundamental correlations in multi-electron emission and enables statistical control of electron beams for on-demand correlated small-particle imaging and spectroscopy.

[0060] Figure 9 shows the generation of Coulomb-correlated minority electron states in an electron beam apparatus 100 according to an embodiment of the present invention. The electron beam apparatus 100 is based on a transmission electron microscope equipped with a microscope column 101 having an optical axis z, for example, an ultrafast transmission electron microscope (UTEM) described in [Non-Patent Literature 48].

[0061] The microscope electron source at the upper end of the microscope column 101 is provided with a pointed photoemission electron source 21 in the electron source device 20, which is shown in an enlarged view in the upper right section of Figure 9. The pointed photoemission electron source 21 includes a Schottky field emitter, for example, a tungsten (W) / ZrOx nanochip having a W(100) facet 21A covered with a ZrOx layer. The photoemission electron source 21 has a radius of curvature of, for example, r=490nm, and for example, U ext =2kV draw voltage and U bias It operates with a bias voltage of -0.3kV. The photoemission electron source 21 may be cooled during operation. Cooling may be performed to just below the continuous Schottky emission threshold.

[0062] The photoemission electron source 21 is positioned for focused irradiation by the emitter excitation pulse 2 and for generating pulsed photoemission, particularly linear photoemission. The emitter excitation pulse 2 (e.g., pulse duration of 160 fs, center wavelength of 515 nm) is generated using the femtosecond laser source 10 at a repetition rate of, for example, 600 kHz and focused onto the pointed photoemission electron source 21 by the lens 11.

[0063] Furthermore, the electron source device 20 receives a repetition period T from a laser-assisted Schottky field emitter (nanochip). rep A suppressor anode 22 and an extractor anode 23 are provided to direct and accelerate the ultrashort source electron pulse 3 emitted along the optical axis z. The minority electron state is prepared by pulsed photoemission; that is, the source electron pulse 3 is generated by a pulse charge of a minority of electrons, and then passes through the extractor anode 23 and optionally another beam limiting aperture 23A.

[0064] Each source electron pulse 3 represents an n-electron event, as shown in the upper right section of Figure 9. Each emitter excitation pulse 2 results in the generation of n greater than 3, for example, n=0 (no electron pulse), n=1 (1 electron per pulse), n=2 (2 electrons per pulse), n=3 (3 electrons per pulse). However, according to our experimental tests, pulses with n>4 have a lower probability. The source electron pulse 3 may have a low pulse charge, i.e., the source electron pulse 3 contains less than 1 electron per pulse on average in the sample plane.

[0065] A multi-state disperser 30 is positioned at the lower end of the microscope column 101. The multi-state disperser 30 includes an imaging energy filter (shown in Figures 3 / 4) that deflects the source electron pulse 3 at a deflection angle with respect to the optical axis z. The deflection angle depends on the number of states in the source electron pulse 3, and as a result, source electron pulses 3 with different numbers of states are spatially separated as subpulses 4 of the electron pulse sequence. This spatial separation is schematically illustrated by the diagram of the total source electron pulse 3 containing any number of states, e.g., further separated subpulses 4 of states n=1, n=2, and n=3.

[0066] Next, a selection is provided for the subpulse 4 having one of several states, using a multi-state selection device 40 that transmits the subpulse selected as the acquired pulsed electron beam 1. The multi-state selection device 40 may be configured, for example, as described in one of the above Figures 1 to 8.

[0067] Downstream of the number-state selection device 40, a time-resolved event-based electron detector camera 105, such as the Timepix3 ASIC (EMCheeTah T3, Amsterdam Scientific Instruments BV), may be positioned for event-based electron spectroscopy, particularly enabling number-state selective beam analysis for the tests described herein. The time resolution of the electron detector camera 105 allows for the distinction of consecutive incident electron pulses and provides a clear measure of the number of transmitted electrons n per laser pulse. In practical applications of the present invention, the detector camera 105 may preferably be replaced by a sample and / or application site 50, such as a sample stage, or the detector camera 105 may be combined with the sample and / or application site 50.

[0068] A microscope, in a typical configuration including objective lenses 102, 103 and a sample stage 104 in the sample plane, is shown in Figure 9 for illustrative purposes only. Electrons pass through the sample plane of the microscope. In some embodiments, the objective lenses 102, 103 and the sample stage 104 are not used to carry out the art of the present invention and may be omitted. However, in the embodiment of Figure 8, the objective lenses 102 and 103 may provide a pair of electron lenses 37 and 38, the sample stage 104 may be used to provide a number-state selection device 40, and the units 30 and 40 at the downstream end of the column 101 may be omitted.

[0069] Figure 10A shows the number of n-electron states per pulse detected by the electron detector camera 105 in Figure 9, depending on the laser output of the emitter excitation pulse 2. For measurement, the detector camera 105 may operate as follows: The detector camera 105 may generate a stream of data packages including the position of the electron-activated detector pixels, their arrival times (ToA) digitized in a 1.56 ns time bin, and the energy associated with the incident electron event (time over threshold, ToT). For example, at a beam voltage of 200 kV, for each individual electron, a variable size (N) may be used. pixel,avg (approximately 8 pixels), shape and energy (ToT avgClusters of pixels having an energy of approximately 280 a.u. are activated. Localization of single-electron events in the ToT-corrected raw data stream is achieved, for example, using the event clustering code of the Nanoscopy Division M4I at Maastricht University [Non-Patent Literature 59], based on Hierarchical Density-Based Spatial Clustering (HDBSCAN) in Python3. The algorithm reconstructs the timing and position of individual electrons incident on the detector from the clusters (hits) of activated pixels. Thereafter, individual electrons are distinguished by their ToA and assigned to the same cluster between three adjacent pixels and nine adjacent pixels activated within a 100 ns time window, and the total pixel energy ToT in the range of 200 a.u. to 400 a.u. (see [Non-Patent Literature 60]). Photoelectrons are clustered according to the femtosecond laser pulse that generated them. The time resolution of the detector (e.g., 1.56 ns) is much faster than the time pulse separation given by the laser repetition rate (≒1 μs), but much slower than the time resolution of correlated electrons in the detector (≒1 ps). This results in Δt n Electrons that reach the detector within 50 ns are assigned to one of several classes of electronic states n=1, 2, 3, ..., determined by the number of electrons in a single laser pulse.

[0070] As shown in the output scaling of the one-electron, two-electron, and three-electron states in Figure 10A, the ratio of single-electron pulses to the total number of emission events scales linearly with the photo-excitation laser power, consistent with the process used for near-threshold laser-assisted Schottky photoemission [Non-Patent Literature 48,49]. Figure 10B illustrates, in an exemplary manner, how one-photon laser-assisted near-threshold Schottky emission generates a one-electron state.

[0071] Similarly, the ratio of 2-electron to 3-electron events increases with powers of n. Considering the relative distribution of n-electron events at a given laser power, a small sub-Poisson statistic can be identified. Specifically, the probability of detecting n electrons in a pulse is given by P. n Therefore, the Poisson process is r n =1

number

[0072] Figures 10C to 10F show the kinetic energies of the numerically selected electronic states, illustrating how the event-averaged spectrum (Figure 10C) can be separated into numerical state decomposition contributions (n=1, 2, 3, Figures 10D to 10F). The 2-electron and 3-electron spectra exhibit distinct shapes with n peaks, demonstrating the discrete energy separation of the electrons involved.

[0073] The spectral distribution of a one-electron event (Figure 10D), which occupies the entire spectrum (averaged across all events), consists of a single peak centered at the accelerating voltage E0 = 200 keV. In stark contrast, the spectra of two-electron and three-electron events exhibit a prominent double-lobe and triple-lobe structure, respectively, with the average energy at E0.

[0074] As shown in FIGS. 10C to 10F, beyond the average over similar events, the inventors' measurement method further enables, as shown in FIG. 11, associating spectral characteristics with two-particle and three-particle correlations within individual electron pulses. FIG. 11 shows the generation of electron pair states in the pointed photoelectron emission electron source 21 (see FIG. 9). FIG. 11A shows an energy histogram of coincident electron pairs that reveals strong correlations in relative kinetic energy, as seen in the spectral correlation function (insert, integrated along the diagonal line). FIG. 11B shows the normalized one-sided pair correlation function (n = 2) for various laser outputs. FIG. 11C shows the output scaling of the peak position of the n = 2 correlation function with respect to the spectral width (FWHM) of the n = 1 state (the spectra of various laser outputs are shown in FIG. 11D). FIG. 11E shows the normalized n = 2 spectra for various laser outputs, and FIG. 11F shows the pair correlation function for photoelectron emission by two delayed laser pulses. In temporal overlap, a strong correlation gap is observed, which disappears at a pulse delay of about 20 fs (see the cross-section of the insert). FIG. 11G shows the extraction voltage U ext dependent spectral correlation function (solid line: fitted spectral distribution), and FIG. 11H shows the horizontal (r tra ) and vertical (r lon ) dimensions of the correlation volume extracted from FIG. 11G (insert: explanatory diagram of the comparison between the correlation quantity and the virtual source size).

[0075] Regarding the generation of the two laser pulses described with reference to FIGS. 11F and 11H, a Michelson interferometer that splits the incident laser pulse into two separate pulses may be used. One of the interference arms has a variable optical path length implemented by a retroreflector attached to a delay stage with (value) bidirectional reproducibility. The optical path difference can be adjusted to the delay time difference between the two pulses up to, for example, 10 ps.

[0076] More specifically, FIG. 11A shows the electron energies E A and E BThe pair density distribution is shown. Coulomb repulsion is due to the energy difference E A -E B This creates a significant gap, and the total energy E partially reduces the observed gap in the overall spectral density. A +E B This leads to a broader spectrum. This strong correlation proves that the observed splitting of the n=2 spectrum into a double lobe structure is a result of two-electron interactions arising from Coulomb repulsion.

[0077] Similar to conventional (non-laser-triggered) Schottky sources, only a portion of the electrons generated on the emitter surface are transferred to the microscope column 101 (see Figure 9). Therefore, the mean field (space charge) and stochastic interactions with random neighboring electrons not incident on the beam can be considered and distinguished from the correlations observed in electron-pair states. Laser power-dependent measurements allow for the evaluation of these different contributions. The corresponding spectral distributions for n=1 and n=2 (Figures 11D and 11E) show broadening with increasing laser power (see circle C1 in Figure 11C), i.e., with the mean photocurrent. This corresponds well to previous non-event-selective measurements [Non-Patent Literature 28, 31, 50], typically attributed to stochastic Coulomb interactions and mean-field effects.

[0078] In contrast, the set of two-electron correlation functions shown in Figure 11B is remarkably independent of laser power, exhibiting a prominent gap of approximately 1 eV width, a peak at approximately 1.8 eV, and an extended tail toward high-energy separation above 4 eV. Increasing the photocurrent only introduces moderate variation in the gap depth and the shape of the high-energy tail. In particular, the position of the main correlation peak (circle C2 in Figure 11C) approaches a fixed value of 1.7 eV toward the disappearing laser power and therefore the average current. This demonstrates that the observed correlation is dominated by two-electron correlation alone, with only slight variations due to multiple Coulomb interactions with electrons blocked by the aperture.

[0079] To investigate the temporal range over which such strong Coulomb correlations interfere with the observation of independent single electrons, measurements were performed at a constant integral laser output using a pair of laser pulses with variable delay (see Figure 11F). Two distinct ranges were identified. Temporally overlapping laser pulses reproduce the n=2 correlation function described earlier. In contrast, temporal separations above 200 fs are consistent with a significantly reduced energy difference and indicate two individual, uncorrelated electron emission events.

[0080] Although the correlation has only a slight dependence on the emission current, the inventors found that the extraction electric field applied to the tip has a more significant effect. A decrease in the extraction voltage substantially alters the observed gap and the gradient of the high-energy tail (see the semi-logarithmic plot in Figure 11G). Physically, changes in the extraction voltage affect the height of the Schottky barrier and the beam's receiving angle. The spectral shapes of these correlation functions can be simply modeled by an ensemble of electron doublets prepared using Gaussian-distributed interparticle distances, and it can be assumed that the initial interparticle Coulomb energy is amplified by acceleration in the electrostatic field, resulting in a larger kinetic energy difference. Separate standard deviations r for the longitudinal (perpendicular to the surface) and transverse (parallel to the surface) distributions, respectively. lon and r tra Using this simple model, we can successfully describe the main features of the measured correlation function (see the solid line in Figure 11G).

[0081] The physical source size and back-projected virtual source size of a Schottky field emitter should be in the range of greater than 20 nm [Non-Patent Literature 51]. However, since the correlation function almost vanishes at zero energy difference, there are few additional events involving two uncorrelated (or weakly correlated) electrons. This is noteworthy because the proportion of doublet electrons with Coulomb correlation reaches 85% of what would be expected from the Poisson number distribution and the proportion of single electrons. The remaining 15% without two-electron events (anti-bunching) may be due to local Coulomb blockade [Non-Patent Literature 52, 53], Pauli blocking [Non-Patent Literature 24], or lateral inter-particle deflection and spatial filtering.

[0082] In other words, the statistical frequency of two-electron events is quite close to what would be expected from uncorrelated emission events from expanded nanoscale sources.

[0083] To explain the simultaneous enhancement of electron-pair emission, the inventors consider several mechanisms previously conceived in atomic and molecular contexts, as well as two-electron photoemission. Enhanced “discontinuous” double ionization has been observed in atoms exposed to a strong laser field, first in helium [Non-Patent Literature 54], and subsequently in many other elements. Various mechanisms, including cotuning, shake-up processes, and field-driven electron rescattering, have been used to explain this observation, but in most cases, the latter mechanism appears to be the cause [Non-Patent Literature 33, 34]. At the moderate local intensity in the inventors' experiments, the ponderomotive potential is well below 1 meV, so recollision can be ruled out as the dominant factor. In the linear region, one-photon two-electron emission is common in the Auger effect, with the core hole resulting from photoemission being filled during the simultaneous emission of the second electron. Coulomb interaction is the dominant factor in this process [Non-Patent Literature 55].

[0084] Figure 12 shows the characterization of the spatial beam properties of a small number of electron states. Figure 12A is a schematic diagram of spatial filtering due to the effect of Coulomb interaction. For integer increasing pulse charge, the virtual source increases in size (see double arrow) and is shifted along the electron beam axis z (see vertical arrow). Figure 12B shows the caustics of the n-selected electron beam, recorded by varying the last focusing lens 38 of the electron beam apparatus 100. The inset of Figure 12B shows images of the beam profile for n=2 in underfocus (left), focus (center), and overfocus (right). Figure 12C is an image of the beam profile in underfocus with a correlation angle φ between electron pairs relative to the beam center. The long angular leg in the underfocus state allows for accurate measurement of angular correlation, and Figure 12D shows that a strong anisotropic angular correlation is observed for n=2 for an isotropic distribution (the dataset used, shown in Figure 12B by black circles around the data points) to depict random events.

[0085] More specifically, Figure 12 shows that the minority electronic states observed here exhibit characteristic spatial properties, alongside the spectral distribution and correlation, as will be explained below. Specifically, Figure 12B shows n-dependent beam caustics, resulting in discrete differences in both minimum spot size and focal position, leading to separated subpulses. Variations in laser power result in changes in caustics (higher power increases the spot size somewhat), but these are far smaller than the differences between event classes. Under given conditions, focus can be limited by the spherical aberration of the objective lens and the virtual source size, resulting in typical spot profiles for positive and negative focus shifts (inset in Figure 12B). Clearly, caustics for n≧1 are a result of a larger effective source, and the beam waist is shifted toward a positive focus shift.

[0086] Both observations can be understood from the mutual lateral deflections shown in Figure 12A. Specifically, the lateral deflection is expected to spread the minority electron orbitals laterally so that the virtual source size increases and moves forward, as previously predicted in simulations [Non-Patent Literature 52, 57] [Non-Patent Literature 31].

[0087] A more detailed analysis of the spatial properties of minority-electron states is obtained by analyzing the correlation of transverse momentum. For this purpose, the inventors measured positional correlation for a sufficiently large negative focus shift (Figure 12C). The spatial correlation is quantified via the angle φ between the two electrons and the beam center. Figure 12D shows the angular correlation density of the two-electron state compared with the random correlation obtained from the corresponding single-electron state at the same spot size (15 nm). In the electron-pair state, a strong anisotropic correlation is obtained, peaking around an angle of 180 degrees, corresponding to electron events localized on both sides of the shifted beam and therefore having nearly opposite transverse momentums.

[0088] These observations indicate that averaging across multiple states significantly impacts beam properties, including uncorrectable stochastic aberrations. Therefore, controlling the numerical statistics of photoemission beams can directly benefit microscopy applications using such sources. More generally, stochastic Coulomb interactions are a fundamental challenge in electron microscopy, where the brightness of an electron source is limited by altering the lateral (Leffler) and longitudinal (Belsch) momentum distribution of the beam. Hereinafter, moderate antibunching, as observed in previous studies [Non-Patent Literature 25], means that the overall photocurrent exhibits a slight sub-Poisson noise characteristic, a highly desired feature in focusing scenarios (e.g., achieved by Coulomb cutoff). In the context of electron microscopy, this feature can be directly applied to reducing shot noise in imaging, spectroscopy, and lithography. However, perhaps even greater possibilities arise from the strong Coulomb correlation identified for the electron doublet (n=2) state. The fact that both electrons in this state are energetically sufficiently separated from each other and from the central energy allows for energy selection of each number state. This facilitates a powerful approach to controlling the statistics of one- and two-electron events through energy selection.

[0089] Figure 13 illustrates the statistical control of one-electron and two-electron states using spatial and spectral filtering. Figure 13A shows the spatial filtering method using a circular aperture, and Figure 13B shows the transmittance of spatial filtering. Figure 13C (two-electron state suppression) demonstrates that an energy slit significantly reduces the transmission of n=2 electronic states generated by the laser pulse compared to n=1 electronic states, and the spectra of n=1 and n=2 electronic states are shown. The electron energy in the dark shadow region is truncated by the energy slit. On the other hand, Figure 13D (two-electron state enhancement) demonstrates that an energy beam stop significantly reduces the transmission of n=1 electronic states compared to n=2 electronic states, and the spectra of n=1 and n=2 electronic states are shown. The electron energy in the dark shadow region is truncated by the energy beam stop.

[0090] Figure 13E shows the transmission T for various energy slit widths with up to 8x enhancement of the n=1 electronic state (for the settings shown in Figure 13C). n Figure 13F shows the transmission T1 / T2 plots for various energy stop widths with more than 20-fold enhancement of the n=2 electronic state (for the settings shown in Figure 13D). n The plot of the transmittance ratio T2 / T1 is also shown.

[0091] More specifically, certain number states can be selectively preferred using pre-sample energy filters commonly used in state-of-the-art electron microscopes [Non-Patent Literature 58]. Specifically, such an energy slit with a pinhole diameter d can strongly preferentially transmit n=1 states over n=2 states by truncating the transmitted energy spectrum and thus adjusting its width (see Figure 13C). Specifically, in experimentally measured one-electron and two-electron spectra (Figure 13E), the transmission probability of n=1 is eight times higher than that of n=2 with a small slit width, significantly enhancing the sub-Poisson characteristics of the electron number distribution and promoting electron currents with reduced shot noise. Conversely, a central beam stop at the energy can suppress a large proportion of one-electron states, increasing the transmission of pair states to n=1 states by up to 20 times (see Figures 13D and 13F). This approach enables new forms of microscopy and spectroscopy using correlated electrons for various novel two-point or two-shot measurement schemes in correlated materials and free-electron quantum optics.

[0092] In summary, the present invention demonstrates a novel method for generating femtosecond pulses containing clearly defined integer electron charges Q=ne, exhibiting unprecedented control over statistics in pulsed charged particle beams. The number of electrons n in each state can not only be directly tracked by event-based detection but also significantly influences the beam spectrum and angular distribution. This makes it possible to modify the particle statistics of the electron beam by simply blocking a portion of the time-averaged intensity at the energy dispersion plane, transverse momentum, or focusing beam position, thus efficiently filtering out a specific number of states. By rejecting beam states n>1, it is possible to efficiently generate sub-Poisson beams that directly impact electron imaging and lithography with reduced shot noise. Furthermore, by selecting a specific number of states, it becomes possible to borrow concepts from quantum optics. For example, n=2 constitutes a very classical state and can be used to implement a high-fidelity, electron-heralded single-electron, enabling shot-noise-free (or reduced-shot-noise) electron imaging and lithography with an accurately countable number of electrons, eliminating previously considered fundamental limitations. Furthermore, the fundamental scattering processes associated with the generation of multi-electron states can typically be assumed to induce multi-electron entanglement in the absence of additional entanglement-breaking reporting channels, such as electron-holes potentially remaining within the photoemission device, or coupling with an external heat bath. Future research must address the quantum coherence of such multi-electron states, promising promising novel quantum techniques using free electrons, potentially enabling interaction-free measurements and ghost imaging, quantum teleportation and information processing, and ultimately entangled free-electron qubits for fermion quantum computing.

[0093] The features of the present invention disclosed in the above specification, drawings, and claims may be important individually, in combination, or in partial combination for the realization of the invention in its various embodiments. The present invention is not limited to the preferred embodiments described above. Rather, multiple modifications and derivatives are possible using the concepts of the present invention and thus falling within the scope of protection. Furthermore, the present invention also seeks protection of the subject matter and features of dependent claims, separate from the features and claims referenced herein.

Claims

1. An electron beam apparatus (100) configured to generate a pulsed electron beam (1) including a sequence of electron pulses, An irradiation source device (10) is arranged to generate a sequence of emitter excitation pulses (2), An electron source device (20) having a photoemission electron source (21) arranged for irradiation-induced emission of a source electron pulse (3) in response to irradiation of the emitter excitation pulse (2), A number-state dispersive device (30) is arranged to spatially separate the source electron pulse (3) into subpulses (4, 4A), where each subpulse (4, 4A) contains an integer number of electrons (n), where n = 1, 2, 3, ..., A number state selection device (40) is configured to select a subpulse (4, 4A) containing at least one set of predetermined electron number states as the pulsed electron beam (1) to be generated. An electron beam apparatus equipped with the following features.

2. The electron beam apparatus according to claim 1, wherein the emitter excitation pulse (2) is a laser pulse.

3. The photoemission electron source (21) is configured to generate countable low-charge electron pulses. The electron beam apparatus according to claim 1.

4. The electron beam apparatus according to claim 3, wherein the low-charge electron pulse comprises 1, 2, 3, or 4 electrons per pulse.

5. The photoelectron emission electron source (21) includes a beam limiting aperture (20A) configured to reduce high-charge electron pulses into countable low-charge electron pulses. The electron beam apparatus according to claim 1.

6. The photoelectron emission electron source (21) is configured to generate a multi-electron state having distinguishable characteristics in addition to a pulsed charge. The electron beam apparatus according to claim 1.

7. The electron beam apparatus according to claim 1, wherein the aforementioned number-state dispersion device (30) includes an energy dispersion device.

8. The electron beam apparatus according to claim 7, wherein the energy dispersion device includes a beam monochromator.

9. The electron beam apparatus according to claim 8, wherein the beam monochromator is an omega type, alpha type, Wien filter type, double Wien type, or electron mirror type beam monochromator.

10. The electron beam apparatus according to claim 7, wherein the energy dispersion apparatus includes a spectrometer.

11. The spectrometer device is a spectrometer device that includes a magnetic prism or an electrostatic multipole electron optical system. The electron beam apparatus according to claim 10.

12. The electron beam apparatus according to claim 1, wherein the number state dispersing device (30) includes a momentum dispersing device, a position dispersing device, or an electron orbit momentum dispersing device.

13. The momentum dispersing device, the position dispersing device, or the electron orbit momentum dispersing device is at least one of a rotationally symmetric electron lens and a cylindrical electron lens. The electron beam apparatus according to claim 12.

14. The electron beam apparatus according to claim 1, wherein the number-state dispersion apparatus (30) includes an electron spin dispersion apparatus.

15. The aforementioned multi-state selection device (40) includes a spatial modulator of electron beam intensity, The electron beam apparatus according to claim 1.

16. The spatial modulator is configured to allow selected subpulses (4, 4A) having a predetermined electron number state to pass through and to block the remaining subpulses (4, 4A), The electron beam apparatus according to claim 15.

17. The aforementioned state selection device (40) includes at least one of a mechanical slit, a grid, a linear beam block, a hole, a ring, or a disk. The electron beam apparatus according to claim 1.

18. A detector device configured to measure the number of electrons in at least one of the electron number states of the pulsed electron beam (1) The electron beam apparatus according to claim 1, further comprising:

19. A beamforming electron optical system configured to illuminate a sample and / or workpiece. The electron beam apparatus according to claim 1, further comprising:

20. The electron beam apparatus according to claim 19, wherein the sample is the sample to be investigated.

21. The electron beam apparatus according to claim 19, wherein the workpiece is a semiconductor workpiece.

22. The photoemission electron source (21) includes a pointed photoemission electron source. The electron beam apparatus according to claim 1.

23. An electron microscope apparatus comprising the electron beam apparatus described in any one of claims 1 to 22.

24. A method for generating a pulsed electron beam (1) including a sequence of electron pulses, The irradiation source device (10) generates a sequence of emitter excitation pulses (2), The steps include irradiating the photoelectron emission electron source (21) of the electron source device (20) with the emitter excitation pulse (2) so that a source electron pulse (3) is generated by irradiation-induced emission, A step of spatially separating the source electron pulse (3) into subpulses (4, 4A) using a number-state dispersion device (30), wherein each subpulse (4, 4A) contains an integer number of electrons (n), where n = 1, 2, 3, ... The steps include: selecting a subpulse (4, 4A) containing at least one set of predetermined electron states as the pulsed electron beam (1) to be generated by a number state selection device (40); Methods that include...

25. The method according to claim 24, wherein the emitter excitation pulse (2) is a laser pulse.

26. The step of adjusting the irradiation source device (10) and / or the photoelectron emission electron source (21) in order to generate a multi-electron state that can be identified by characteristics other than pulsed charge. The method according to claim 24, including the method described in claim 24.

27. The step of adjusting the number-state dispersive device (30) according to the identifiable characteristics of the source electron pulse (3). The method according to claim 24 or 26, including the method described in claim 24 or 26.

28. The method according to claim 27, wherein the characteristics are energy, angle / momentum, and spatial focus.

29. The step of adjusting the number state selection device in order to reject or block subpulses of a specific number of states from the beam. The method according to claim 24 or 26, including the method described in claim 24 or 26.

30. The subpulses of the specific number of states are all subpulses (4, 4A) of all n > 2 or n ≠ 2. The method according to claim 29.

31. A step of adjusting the irradiation source device (10), the photoelectron emission electron source (21), the number state dispersion device (30), and / or the number state selection device (40) based on the output of an additional number state sensing detector. The method according to claim 24, including the method described in claim 24.

32. A method of using an electron beam apparatus according to any one of claims 1 to 22 as a beam source in at least one of an electron microscope apparatus, an electron lithography apparatus, an electron pair (source) apparatus, an electron herald apparatus, an electron counting apparatus, an information processing apparatus, a communication apparatus, and a quantum computing apparatus.

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