Electron beam device and method for generating pulsed electron beam and use thereof

Abstract

An electron beam apparatus (100) for generating a pulsed electron beam (1) comprising a sequence of electronic pulses, the electron beam apparatus comprising: a radiation source arrangement (10) for generating a sequence of emitter excitation pulses (2), in particular laser pulses; an electron source arrangement (20) having a photoemission electron source (21) for radiation-induced emission of a source electron pulse (3) in response to irradiation of the emitter excitation pulse (2); a digital distribution device (30) for spatially separating the source electron pulse (3) into sub-pulses (4, 4A), where each sub-pulse (4, 4A) comprises an integer number of electrons (n), n = 1, 2, 3,...; and digital state selector means (40) for selecting, as the pulsed electron beam (1) to be generated, sub-pulses (4, 4A) comprising at least one set of predetermined electron digital states. Preferably, the photoemission electron source (21) is configured to generate countable low charge electron pulses, e.g., each pulse comprising 1, 2, 3 or 4 electrons. Furthermore, a method of generating a pulsed electron beam (1) and a method of using an electron beam apparatus are described.

Description

Technical Field

[0001] The present invention relates to an electron beam device and / or method for generating a pulsed electron beam, the pulsed electron beam comprising a sequence of electron pulses. The present invention can be applied, for example, in the field of electron microscopy or electron lithography material processing. Background Art

[0002] In this specification, the technical background and related technologies of the present invention are described with reference to the following prior arts:

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[0067] Quantum statistical control of fermionic and bosonic modes is at the heart of nonclassical light sources [1, 2] and strongly correlated functional materials ([3, 4]), and enables the realization of (noise-corrected) quantum computers ([5]). Developing these concepts in the context of free-electron quantum optics holds promise for quantum coherent manipulation [6, 7], nanoscale sensing [8] and imaging, with new opportunities emerging in fields ranging from structural biology and materials science to electron-beam lithography. In particular, recent theoretical and experimental work has explored quantum-enhanced electron microscopy using interferometry [9] or multipass schemes [10, 11], quantum light emission

[12] or tailored interactions with optical modes [6, 8, 13–15].

[0068] Although correlated multi-electron states are ubiquitous in experimental condensed matter physics [16, 17], their analogues in free-particle beams have not been observed, and proposed mechanisms include indirect coupling through long-lived optical modes [13, 18], common-path interference

[19] , or correlated photoemission processes [20, 21] and ionization processes [22, 23]. As a hallmark feature and necessary condition for entanglement, particle correlations in free-electron beams have been studied in both transverse [19, 24] and longitudinal

[25] phase space, taking into account contributions from exchange-mediated

[24] or Coulomb interactions

[26] . In particular, classical Coulomb repulsion leads to random growth of transverse and longitudinal emittance in electron beams (described by the Boersch [27, 28] and Loeffler

[29] effects, respectively), limiting the brightness of state-of-the-art electron sources

[30] . In highly charged electron pulses, mean-field induced space charge effects determine the achievable pulse duration, energy broadening, and focusability

[31] , posing significant experimental challenges for ultrafast electron microscopy and diffraction, particle accelerators, and free-electron lasers.

[0069] Coincidence spectroscopy is well established in atomic and molecular science [33,34] and has revealed complex collision mechanisms and correlation effects in solids [20,35], COLTRIMS, reaction microscopy and other correlation detection techniques.

[0070] Miniaturization of electron sources to generate coherent electron pulses on nanoscale tip-shaped emitters enables ultrafast dark-field

[36] or phase-contrast imaging

[37] , nanoscale diffraction detection[38,39], and photon-induced near-field electron microscopy[40–43], which requires in-depth analysis of random Coulomb effects and mean-field effects even for low-charge electron beams

[31] .

[0071] However, distinguishing between these two contributions requires event detection based on single-particle resolution, which has only recently been introduced in electron microscopy with applications in high-speed STEM [44,45] and EELS

[42] , detection of electron-correlated X-ray emission

[46] , and cathodoluminescence from quantum materials

[47] and integrated photonic resonators

[15] .

[0072] Purpose of the Invention

[0073] The object of the present invention is to provide an improved electron beam device and / or method for generating a pulsed electron beam, which is able to avoid the limitations of conventional techniques for generating electron beams. In particular, the pulsed electron beam will be generated with tailored pulse statistics, thus allowing in particular to expand new application ranges, such as electron microscopy applications with reduced noise and / or improved imaging quality, and / or improved applications for electron lithography and / or quantum computing, and / or new applications in the field of electron manipulation. Summary of the invention

[0074] These objects are solved by an electron beam device and / or a method of generating a pulsed electron beam, respectively comprising the features of the independent claims. Advantageous embodiments and applications of the invention are defined in the dependent claims.

[0075] According to the first general aspect of the present invention, the above-mentioned purpose is solved by an electron beam device for generating a pulsed electron beam, the pulsed electron beam comprising a sequence of electron pulses, the electron beam device comprising: a radiation source device arranged to generate a sequence of emitter excitation pulses, the emitter excitation pulses are especially laser pulses; an electron source device having a photoelectric emission electron source, arranged to radiation-induced emit source electron pulses in response to irradiation of the emitter excitation pulses; a number state dispersion device, arranged to spatially separate the source electron pulses into sub-pulses, wherein each sub-pulse comprises an integer number of electrons (n), n=1, 2, 3, ...; and a number state selector device, configured to select sub-pulses comprising at least one set of predetermined electron number states as the pulsed electron beam to be generated.

[0076] According to the second general aspect of the present invention, the above-mentioned object is solved by a method for generating a pulsed electron beam, the pulsed electron beam comprising a sequence of electron pulses, the method comprising the following steps: using a radiation source device to generate a sequence of emitter excitation pulses, the emitter excitation pulses are particularly laser pulses; using the emitter excitation pulses to irradiate the photoelectric emission electron source of the electron source device, thereby generating source electron pulses by radiation-induced emission; using a number state dispersion device to spatially separate the source electron pulses into sub-pulses, wherein each sub-pulse comprises an integer number of electrons (n), n=1, 2, 3, ...; using a number state selector device to select sub-pulses comprising at least one set of predetermined electron number states as the pulsed electron beam to be generated. Preferably, the method of the second general aspect of the present invention or its embodiments is implemented using an electron beam device according to the first general aspect of the present invention or its embodiments. All preferred embodiments disclosed herein in relation to the device are regarded as corresponding preferred embodiments of the method, and vice versa.

[0077] According to a third general aspect of the present invention, the above object is solved by a method using an electron beam device according to the first general aspect or an embodiment thereof as a beam source, the electron beam device being used as a beam source in at least one of the following: an electron microscope device, an electron lithography device, an electron pair (source) device, an electron prediction device, an electron counting device (application to determine the exact number of electrons), an information processing device, a communication device, and a quantum computing device. An electron microscope device, an electron lithography device, and / or a quantum computing device, provided with an electron beam device according to the first general aspect or an embodiment thereof, and / or configured to perform the method according to the second general aspect of the present invention or an embodiment thereof, is considered as an independent subject matter of the present invention.

[0078] The term "pulse" as used herein generally refers to a waveform that provides a predetermined, non-continuous time structure of source electrons. The pulses can be provided as a periodic pulse sequence. Selecting sub-pulses that include at least one set of predetermined electronic number states includes: selecting a single set of sub-pulses with a single electronic number state (e.g., n=2), or selecting multiple sets of sub-pulses, each set of sub-pulses having another electronic number state, such as n>1, n=1, or the like. Therefore, in the case of selecting multiple sets of sub-pulses, different electronic number states can be combined.

[0079] The state dispersion device spatially separates the pulse into integer electron numbers n=0, 1, 2, 3, ..., and projects the electron beam onto the state selector device. The state selector device includes, for example, an aperture, a slit and / or a beam blocker for blocking / rejecting a subset of quantized state, n=0, 1, 2, 3, ...

[0080] The inventors have observed Coulomb-correlated electron pairs and triplets generated by femtosecond pulsed photoemission from a nanoscale tip emitter in an ultrafast transmission electron microscope. Event-based electron spectroscopy enables the unambiguous identification of specific number states by their characteristic few-electron-volt kinetic energy separation between particles. State-sorted beam caustics show that higher numbers of electrons induced by random Coulomb scattering increase the size of the virtual source and that there is a separation of energy states in the average spectrum. The inventors propose a scheme in which simple filtering of the beam, such as spectral filtering, spatial filtering, and / or spin filtering, enables state selection and customization of pulse statistics. In addition, the use of electrostatic emitter configurations allows the electron kinetic energy separation and the relative occurrence of desired number states to be varied for optimization for specific applications. This will allow advanced control schemes, for example, for the generation of sub-Poissonian electron beams or the implementation of pre-announced single electron sources, which may remove limitations of electron microscope aberration correction and shot noise in electron beam lithography.

[0081] According to a preferred embodiment of the invention, the photoemissive electron source is configured to generate countable low-charge electron pulses, preferably each pulse comprising 1, 2, 3 or 4 electrons.

[0082] 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 arranged just downstream of the electron source arrangement and upstream of the optional accelerator and focusing optics.

[0083] According to a preferred embodiment of the present invention, the photoemissive electron source is configured to generate multiple electron states having distinguishable characteristics other than the impulse charge.

[0084] According to a preferred embodiment of the invention, the number dispersive device comprises an energy dispersive device, such as a spectrometer device. The spectrometer device can be arranged to perform a spectral decomposition of the source electron pulses and to apply an energy filter to the source electron pulses. The spatial filtering device can be arranged in an energy selective plane of the spectrometer device. A second spectrometer device can be provided for merging the selected subset of electrons into a common beam.

[0085] According to a preferred embodiment of the present invention, the energy dispersion device may include a beam monochromator and / or a spectrometer device, the beam monochromator being in particular an omega type, alpha type, Wien filter type, double Wien type or electron mirror type beam monochromator; the spectrometer device is configured to separate the spectral components of the source electron pulses, the spectrometer device being in particular a spectrometer device including a magnetic prism, a spectrometer device including an electrostatic multipole electron optical device or other types of spectrometer devices.

[0086] 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 can be designed as an (electronic) momentum dispersion device, a position dispersion device, an electron orbit momentum dispersion device or an electron spin dispersion device.

[0087] According to a preferred embodiment of the invention, the number state selector device comprises a spatial modulator of the electron beam intensity, in particular the spatial modulator is configured to pass selected sub-pulses having a predetermined electron number state and block the remaining sub-pulses.

[0088] According to a preferred embodiment of the present invention, the number state selector device comprises at least one of a mechanical slit, a grating, a linear beam blocker, an aperture, a ring or a disk. The selector device is preferably made of a conductive material, or is made into an electrode, and the selector device is suitable for absorbing the electronic number state to be rejected, preferably completely absorbing the electronic number state to be rejected.

[0089] According to a preferred embodiment of the present invention, the electron beam device also includes at least one of a detector device and a beam forming electron optical device, the detector device is configured to determine the number of electrons in at least one of the electron number states of the pulsed electron beam, and the beam forming electron optical device is configured to irradiate a sample and / or a workpiece, wherein the sample is, for example, a sample to be studied, and the workpiece is, for example, a semiconductor workpiece.

[0090] According to a preferred embodiment of the present invention, the photoemission electron source comprises a tip-shaped photoemission electron source, preferably configured as linear photoemission, in particular at least one of a Schottky-type emitter, a cold-field-type emitter and a thermionic emitter.

[0091] The photoemissive electron source may comprise a tungsten tip having a (100) crystal face covered by a zirconium oxide thin film, or a composite tip made of a material having an electron work function equal to or lower than that of lanthanum hexaboride (LaB6) or cerium hexaboride (CeB6), or a metal tip with a crystal face tip having a smaller electron work function than the rest of the tip material, or a pure metal tip, in particular a pure metal tip made of W, Mo, Re, Ir, Ta, Hc, Pt or Ni, or a transition metal carbide tip, in particular a transition metal carbide tip made of HfC, ZrC, NbC, TaC, TiC or VC, or a carbon cone emitter or a single carbon nanotube tip.

[0092] The radiation source device may include a laser source, and / or the radiation source device is configured to generate emitter excitation radiation, wherein the wavelength of the emitter excitation radiation is selected according to application conditions, in particular according to the tip material of the emitter, for example, the wavelength is at least one of the following ranges: 1nm to 200nm, 200nm to 1500nm, 1500nm to 16μm.

[0093] According to a preferred embodiment of the method of the present invention, at least one of the following steps may be provided:

[0094] - adjusting the radiation source means and / or the photoemission electron source to produce multi-electron states that can be distinguished by a property other than the impulse charge (equal to the number of electrons),

[0095] - adjusting the number dispersion device according to distinguishable properties of the source electron pulses, in particular energy, angle / momentum, spatial focus,

[0096] - adjusting the number state selector means to reject or block specific number states in the beam, of particular interest is rejecting or blocking all sub-pulses with n>2 or rejecting or blocking all sub-pulses with n≠2, and

[0097] - adjusting the radiation source means, the photo-emissive electron source, the number state dispersion means and / or the number state selector means based on the output of the additional number state sensitive detector. BRIEF DESCRIPTION OF THE DRAWINGS

[0098] Further details and advantages of the invention are described below with reference to the accompanying drawings, which schematically show:

[0099] Figures 1 to 8 illustrates the features of a preferred embodiment of the invention; and

[0100] Figure 9 to Figure 1 3 shows further features of a preferred embodiment of the invention and experimental tests. DETAILED DESCRIPTION

[0101] Figures 1 to 8 The general principle of the present invention is schematically shown (particularly Figure 1 ) and features of preferred embodiments for implementing the invention in practice (particularly Figures 2 to 8 ). According to the experimental findings of the inventors, Figure 9 to Figure 1 3 describes further details of the actual implementation.

[0102] Embodiments of the present invention are described with particular reference to preparing and / or analyzing a pulsed electron beam having a well-defined number or charge state. 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 accelerating voltage, and / or a laser source (such as controlling the power of the laser source) for exciting the electron source of the transmission electron microscope are not described, as this is per se known conventional electron microscopy techniques.

[0103] The implementation of the present invention is not limited to the application of electron microscopy, but can be used for other applications mentioned above. Figure 10 to Figure 1 The application conditions are provided by the actual test shown in Figure 4. Depending on the application, the embodiments of the invention may be modified, for example with regard to the design of the number state dispersion device and / or the number state selector device.

[0104] Figure 1 Schematically, an electron beam device 100 for generating a pulsed electron beam 1 comprising a sequence of electron pulses is shown. The electron beam device 100 comprises a radiation source arrangement 10, in particular a pulsed radiation source, such as a pulsed laser, which is arranged to generate a sequence of emitter excitation pulses 2, in particular a pulsed laser beam of laser pulses. The electron beam device 100 further comprises an electron source arrangement 20 having a photoemission electron source 21. Exemplary details of the photoemission electron source 21 are in particular Figures 3 to 9 Shown in.

[0105] The photoemission electron source 21 is arranged to receive the emitter excitation pulse 2, and to radiation-induced emit the source electron pulse 3 in response to the emitter excitation pulse 2 being irradiated onto the photoemission electron source 21. The source electron pulse 3 provides a pulsed electron beam containing a small number (countable) of electrons per pulse. In particular, each source electron pulse 3 includes a number n of electrons, such as 1, 2, 3 or more electrons. The pulse charge of each source electron pulse 3 is correspondingly determined by the number of electrons per pulse. In addition to the pulse charge / number of electrons, the multiple electron states (number states) of the source electron pulse 3 can also be distinguished by at least one physical property, such as the energy, angle / momentum, position, spin and / or spatial defocusing generated after the number state dispersion device, which are determined by the angle / transverse momentum of the source electron pulse.

[0106] The source electron pulses 3 are directed along the optical axis z (electron beam axis z) to the number state dispersion device 30 of the electron beam apparatus 100. The number state dispersion device 30 is arranged to spatially separate the sequence of source electron pulses 3 into sub-sequences of source electron pulses 4, wherein each sub-sequence comprises source electron pulses 4 having a common integer number of electrons (n), n=1, 2, 3, ... The number state dispersion device 30 provides dispersion along at least one distinguishable physical property. Accordingly, the number state is dispersed in one or more parts. Depending on the physical properties of the number state, each source electron pulse 3 is directed to one of the different pulse paths of the sub-pulse 4 (see Figures 3 to 9 ).

[0107] Furthermore, the electron beam device 100 comprises a number state selector device 40, which is arranged to receive the spatially separated sub-pulses 4 and is arranged to select one of the sub-pulses 4 comprising at least one set of predetermined electron number states as the pulsed electron beam 1 to be generated. Based on the at least one distinguishable physical property, pulses having a specific number state are transmitted as the pulsed electron beam 1 by the number state selector device 40, while pulses having a number state different from the specific number state are rejected, in particular blocked, or optionally shunted by the number state selector device 40, such as in particular Figures 3 to 9 As shown exemplarily in FIG.

[0108] The pulsed electron beam 1 is directed to a sample and / or an application site 50, which accommodates, for example, a sample to be investigated with the pulsed electron beam 1 or a workpiece to be processed with the pulsed electron beam 1. At the sample and / or application site 50, only certain number states are used, for example the number states n<2 or n=2. The sample and / or application site 50 comprises, for example, a sample stage of a microscopy apparatus, a spectroscopic apparatus or a lithography apparatus. For example, only a subsequence of source electron pulses 4 with 2 electrons per pulse is used to investigate the sample by electron microscopy or for irradiating the workpiece.

[0109] Optionally, a number state reporting channel 60 may be provided, including an electron pulse detector arrangement. Using the number state reporting channel 60, one channel of number state n can be detected, while the n-1 state is used at the sample and / or application site 50, for example to generate a predicted single electron source.

[0110] Figure 2 Shows more details of optional applications for specific data states, including Figure 1The digital state selector device 40, the sample and / or application site 50 and the digital state reporting channel 60 are shown. With the digital state selector device 40, the pulsed electron beam 1 of state m=n-1 is transmitted to the sample and / or application site 50, while the unselected source electron pulses 4A of state n are diverted to the digital state reporting channel 60. The unselected subsequence of the source electron pulses 4A includes, for example, single electron pulses detected by a detector device included in the digital state reporting channel 60.

[0111] The number state reporting channel 60 can be coupled to the sample and / or the application site 50 via an information channel 61. For example, an exact countable number of electrons can be delivered to the number state reporting channel 60 and the sample or to the number state reporting channel 60 and the application site 50. Thus, knowledge of the exact number of electrons in the electronic state transmitted to the sample and / or the application site 50 is obtained, which is relevant for example for microscopy and lithography applications with a well-defined electron dose.

[0112] exist Figures 3 to 9 In the example, for example, Figure 1 or Figure 2 More details of the electron beam apparatus 100 are shown. Figures 3 to 9 In an embodiment of the present invention, the radiation source device 10 includes a laser that generates a pulsed laser beam, such as a femtosecond laser that generates laser pulses with a duration of, for example, 160 fs and a repetition period of, for example, about 2 μs. The pulsed laser beam is focused into a sequence of emitter excitation pulses 2 by a focusing optical device (such as a lens 11), and is focused onto a photoelectric emission electron source 21 of an electron source device 20.

[0113] Figures 3 to 7 and Fig. 9 The photoelectric emission electron source 21 includes an emitter tip (see Fig. 9 ). In response to the focused pulsed laser beam irradiation of the emitter excitation pulse 2, a corresponding pulsed electron beam of the source electron pulse 3 is generated at the emitter tip. In order to guide the source electron pulse 3 along the optical axis z to the number state dispersion device 30, the electron source device 20 of each illustrated embodiment can optionally further include a suppressor anode 22, an extractor anode 23 (see also Fig. 9 ), beam limiting aperture 20A and accelerator and focusing optics 24 (see Figure 3 ). Preferably, the beam limiting aperture 20A is arranged between the extractor anode 23 and the accelerator and focusing optics 24.

[0114] according to Figure 3 and Figure 4In an embodiment, the number state dispersion device 30 is an energy dispersion device including an energy dispersion magnetic prism 31, which can be configured in the manner described in, for example, [58, 61, 62]. Preferably, a single energy dispersion magnetic prism 31 can be provided. By means of the energy dispersion magnetic prism 31, different deflections relative to the optical axis z are applied to source electron pulses with different charges / energies in the deflection field of the energy dispersion magnetic prism 31. As an example shown, the sub-pulse 4 with the number state n=2 is deflected at a larger output angle than the sub-pulse 4A with the number state n=1. The spatially separated deflections of the sub-pulses are provided to the number state selector device 40.

[0115] The digital state selector device 40 comprises a mechanical beam stop 41 which acts as a spatial modulator of the electron beam intensity of the spatially separated sub-pulses. Figure 3 The mechanical beam stop 41 is configured to pass only selected sub-pulses 4 with a number state n=2 to the sample and / or application site 50, while sub-pulses 4A with a number state n=1 are blocked. To this end, the mechanical beam stop 41 may include a beam blocking material that is located in the path of the sub-pulses 4A to be blocked and that transmits the selected sub-pulses 4. Alternatively, the mechanical beam stop 41 may be described as a slit, such as a circular slit that allows the selected sub-pulses 4 to pass. As a further alternative, in order to select only sub-pulses 4A with a number state n=1 and block sub-pulses 4 with a number state n>1 (particularly n=2), the mechanical beam stop 41 may include a circular beam blocking material that includes a central hole, such as Figure 4 As shown in .

[0116] Figure 5 For example, according to Figure 1 or Figure 2 Further details of the electron beam apparatus 100, wherein the number state dispersion device 30 includes an in-column Omega type monochromator filter 32, which includes a number state selector device 40 provided by a slit 42. The in-column Omega type monochromator filter 32 can be configured as described in [58,61]. The monochromator filter 32 includes four magnetic prisms 32A to 32D, which 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.

[0117] By the action of the deflection field of the first pair of magnetic prisms 32A and 32B, and according to the number state of the received source electron pulse 3, the source electron pulse 3 is diffused to different beam paths toward the slit 42 through the monochromator filter 32 as sub-pulses 4, 4A. The slit 42 is capable of blocking the peripheral sub-pulses 4 of the n>1 (e.g., n=2) number state, and allowing the sub-pulses 4A of the central n=1 number state to pass. Alternatively, the slit 42 can be replaced by a beam blocker (not shown), which is located at the slit position, transmits only the peripheral sub-pulses of the n>1 number state and blocks the central sub-pulses of the n=1 number state.

[0118] The selected sub-pulse passing through the slit 42 (or beam stopper) is deflected again to the original direction of the source electron pulse 3 by the deflection field of the second pair of magnetic prisms 32C and 32D, and is guided to the sample and / or application site 50 as the electron pulse 1 to be obtained. In order to adjust the electron kinetic energy to meet the requirements of the application / sample 50, the accelerator 33 is placed after the monochromator 32, but can also be placed before the monochromator 32 as an alternative.

[0119] according to Figure 6 Embodiments, for example according to Figure 1 or Figure 2 The electron beam device 100 comprises a number dispersion device 30 having an in-column Alpha-type monochromator 34 including a slit 43. The slit 43 provides a mechanical beam stop for the number selector device 40. The pulsed electron beam of the source electron pulse 3 generated at the emitter tip 21 is guided to the Alpha-type monochromator 34 via focusing optics and an accelerator 25.

[0120] The alpha type monochromator 34 can be configured as described in, for example, [58, 61, 64]. The monochromator 34 includes imaging optics and three magnetic prisms 34A to 34C, which deflect the pulsed electron beam of the source electron pulse 3 into the shape of the Greek letter α and act as an energy filter. Figure 5 Like the monochromator filter 32 of the embodiment of the present invention, the source electron pulse 3 is diffused by the monochromator 34 onto different beam paths toward the energy selection slit 43 as sub-pulses 4, 4A. The slit 43 is capable of blocking the peripheral sub-pulses 4 of the n>1 (e.g., n=2) number state and passing the central sub-pulse 4A of the n=1 number state. Alternatively, a central beam blocker (not shown) located at the slit position can be used to transmit only the peripheral sub-pulses of the n>1 number state and block the central sub-pulse of the n=1 number state. The selected sub-pulses transmitted by the slit 43 (or around the central beam blocker) image the sample and / or application site 50.

[0121] As another alternative embodiment, Figure 7 The electron beam device 100 comprises a Wien-type monochromator 35 as a number state dispersion device 30. The Wien-type monochromator 35 can be configured as described, for example, in [61, 64], and is arranged in combination with an accelerator 36 directly downstream of the extractor anode 23. As an alternative to the Wien-type monochromator 35 shown, a double Wien-type monochromator can be used. The Wien-type monochromator 35 acts as an energy selective imaging system, which guides pulses with different electron number states onto beam paths at different angles relative to the optical axis z of the electron source device 20. Accordingly, the sub-pulses 4, 4A are spatially separated, allowing the sub-pulses to be selected according to their number states. Figure 7The schematic color scale diagram shown in encodes the kinetic electron energy dispersed by the Wien filter.

[0122] The state selector device 40 comprises a slit 44 arranged at a distance downstream from the Wien-type monochromator 35. The slit 44 acts as a spatial modulator of the electron beam intensity of the spatially separated sub-pulses. Depending on the position of the slit 44 relative to the optical axis z, the slit 44 can block the n>1 state, or only transmit the n=2 state.

[0123] Figure 8 The process of forming the virtual source size and position at the tip of the tip-shaped photoelectric emission electron source 21 of the electron beam device 100 is shown. In response to the irradiation of the focused pulsed laser beam from the radiation source device 10 via the lens 11, the emitter excitation pulse 2 generates a pulsed electron beam of the source electron pulse 3, which has a virtual source size on the emitter tip and a virtual source offset depending on the pulse number state. In particular, the virtual source size increases additionally with the increase of the excited electron pulse number state (see double arrows), and the virtual source is additionally offset along the optical axis z (see arrows) with the increase of the excited electron pulse number state.

[0124] Figure 8 The digital state dispersion device 30 of the embodiment includes a pair of electron lenses 37, 38, which are arranged to have a common optical axis z perpendicular to the surface of the emitter tip-shaped photoelectric emission electron source 21. A digital state selector device 40 is arranged in the axial distance between the electron lenses 37, 38. The digital state selector device 40 includes a circular aperture 45, which is made of a beam blocking material and whose hole is centered on the optical axis z. Alternatively, the digital state selector device 40 includes a circular beam blocker (not shown) centered on the optical axis z.

[0125] The emitter tip of the photoemission electron source 21 provides a virtual source size according to the number state of the excited source electron pulses 3. This characteristic causes the source electron pulses 3 to produce angular dispersion / spatial dispersion after passing through the first electron lens 37, which is visible in the beam caustic, as shown in FIG. Figure 8 13 ). As the size of the virtual source increases, i.e. as the number of number states increases, the beam caustic diameter after passing through the first electron lens 37 also increases. Thus, depending on the type of aperture or blocker, and depending on the position of the number state selector device 40, sub-pulses 4 with a specific number state, such as n=1, are able to pass through the number state selector device 40, while sub-pulses 4A with other number states, such as n=2 and n=3, are blocked by the number state selector device 40. Using the second electron lens 38, the sub-pulses 4 transmitted by the number state selector device 40 (the pulsed electron beam to be generated) are imaged to the sample and / or the application site 50.

[0126] for Figures 5 to 8 In each of these, it should be noted that the average kinetic energy of the electron pulse can be obtained using an electrostatic electron accelerator (see Figure 5 33 or Figure 7 36) in, the electrostatic electron accelerator can be placed anywhere in the beam path between the emitter unit and the application / sample, but is preferably placed before the number state dispersion device, or after the number state selector device.

[0127] In the following, reference will be made to Figure 9 to Figure 1 3 describes further practical embodiments and experimental tests of the present invention.

[0128] Through actual tests by the inventors, strong Coulomb correlations were shown in the two-electron and three-electron states generated at the laser-driven Schottky field emitter, as shown below. Using event-based electron energy spectrum and imaging, the kinetic energy distribution of the electron collection emitted by a single laser pulse was recorded, and the events were classified according to the number of free electrons. It was found that events containing, for example, two and three electrons presented characteristic two-lobe spectra and three-lobe spectra, respectively. The present invention allows for quantitative characterization of inter-particle correlations in both energy and transverse momentum, and observes that random few-body interactions dominate the mean field (space charge) effect. The two-particle energy correlation function shows a clear peak at an energy difference of about 1.7 eV, indicating that the effective joint emission region of the electron pair state is much smaller than the physical and virtual source sizes. These findings reveal the fundamental correlation of multi-electron emission and achieve statistical control of the electron beam, providing the possibility for on-demand correlated few-particle imaging and energy spectrum analysis.

[0129] Fig. 9 The creation of Coulomb-correlated few-electron states in an electron beam device 100 according to an embodiment of the invention is shown. The electron beam device 100 is based on a transmission electron microscope, whose microscope column 101 has an optical axis z, such as an ultrafast transmission electron microscope (UTEM) as described in

[48] .

[0130] The microscope electron source located at the upper end of the microscope tube 101 constitutes an electron source device 20, which has a pointed photoelectric emission electron source 21, such as Fig. 9 The tip-shaped photoemission electron source 21 includes a Schottky field emitter, such as a tungsten (W) / ZrOx nanotip with a W(100) crystal plane 21A covered with a ZrOx layer. The curvature radius of the photoemission electron source 21 is, for example, r=490 nm, which is used to extract a voltage such as U ext =2kV, bias voltage U bias = -0.3 kV. The photoemissive electron source 21 may be cooled during operation. Cooling may be provided just below the continuous Schottky emission threshold.

[0131] The photoemission electron source 21 is arranged to perform focused illumination and generate pulsed photoemission, in particular linear photoemission, using the emitter excitation pulse 2. The emitter excitation pulse 2 (e.g. 160 fs pulse duration, 515 nm center wavelength) with a repetition rate of, for example, 600 kHz is generated by a femtosecond laser source 10 and is focused onto the tip-shaped photoemission electron source 21 through a lens 11.

[0132] In addition, the electron source device 20 is provided with a suppressor anode 22 and an extractor anode 23 for extracting electrons emitted from the laser-assisted Schottky field emitter (nano-tip) with a repetition period of T rep Ultrashort source electron pulses 3 are directed and accelerated onto the optical axis z. Few-electron states are prepared by pulsed photoemission, i.e., after passing through the extractor anode 23 and optionally another beam limiting aperture 23A, source electron pulses 3 with a small electron pulse charge are created.

[0133] Each source electron pulse 3 represents n electron events, such as Fig. 9 As shown in the upper right part of . Each emitter excitation pulse 2 will result in, 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) or even n exceeding 3. However, according to experimental tests by the inventors, the probability of generating pulses with n>4 is low. The source electron pulse 3 can have a low pulse charge, that is, the source electron pulse 3 includes less than one electron per pulse on average in the sample plane.

[0134] A digital state dispersion device 30 is arranged at the lower side of the microscope barrel 101. The digital state dispersion device 30 includes an imaging energy filter (such as Figure 3 / 4), which deflects the source electron pulse 3 at a deflection angle relative to the optical axis z. The deflection angle depends on the number state of the source electron pulse 3, so that the source electron pulse 3 with different number states is spatially separated into sub-pulses 4 of the electron pulse sequence. This spatial separation is schematically shown in the form of a diagram of the total source electron pulse 3, where the total source electron pulse 3 includes any number state and further separated sub-pulses 4, for example, with number states n=1, n=2 and n=3.

[0135] Subsequently, the sub-pulse 4 having one of the digital states is post-selected using the digital state selector device 40, which transmits the selected sub-pulse as the pulsed electron beam 1 to be obtained. The digital state selector device 40 can be, for example, as described above. Figures 1 to 8 One of the described methods is provided.

[0136] Downstream of the number state selector device 40, a time-resolved event-based electron detector camera 105, such as a Timepix3 ASIC (EM CheeTah T3, Amsterdam Scientific Instruments), can be arranged for event-based electron energy spectrum analysis, thereby enabling selective beam analysis of the number states, in particular for the tests described herein. The time resolution of the electron detector camera 105 allows distinguishing between consecutive incident electron pulses, thereby unambiguously measuring the number of electrons n transmitted by each laser pulse. In practical applications of the present invention, the detector camera 105 is preferably replaced by the sample and / or application site 50 (e.g., a sample stage), or alternatively, the detector camera 105 can be combined with the sample and / or application site 50.

[0137] Fig. 9 A typical configuration of a microscope is shown, including objective lenses 102, 103 and a sample stage 104 in a sample plane, for illustration purposes only. Electrons pass through the sample plane of the microscope. In various embodiments, objective lenses 102, 103 and sample stage 104 are not used to implement the techniques of the present invention and may be omitted. However, in Figure 8 In the embodiment, the objective lenses 102 and 103 can provide the electron lens pair 37, 38, the sample stage 104 can be used to provide the digital state selector device 40, and the units 30, 40 at the downstream end of the lens barrel 101 can be omitted.

[0138] Fig.10 A shows the use of Fig. 9 The number of n-electron states per pulse detected by the electron detector camera 105 is plotted as a function of the laser power of the emitter excitation pulse 2. To make the measurement, the detector camera 105 may be operated as follows. The detector camera 105 may generate a stream of data packets containing the locations of the detector pixels of the electron activations, their arrival times (ToA) (digitized in 1.56 ns time bins), and the energies (time-to-threshold, ToT) associated with the incident electron events. At a beam voltage of, for example, 200 kV, each individual electron activation has a variable size (N pixels,avg ≈8 pixels), shape and energy (ToT avg≈280 a.u.). Single electron events are localized on the raw data stream after ToT correction using, for example, a clustering code from the Maastricht University Nanoscience (M4I) department (Hierarchical Density Spatial Clustering in Python3 (HDBSCAN)). The algorithm reconstructs the time and position of each electron incident on the detector based on the activated pixel clusters (hit points). Thereby, individual electrons are distinguished according to their ToA, and three to nine adjacent pixels activated within a time window of 100 ns and with a total pixel energy ToT ranging from 200 a.u. to 400 a.u. are grouped into the same cluster (see

[60] ). The photoelectrons are clustered according to the femtosecond laser pulse that produced them. The temporal resolution of the detector (e.g. 1.56 ns) is much faster than the temporal pulse separation given by the laser repetition rate (≈1 μs), but much slower than the temporal separation of related electrons at the detector (≈1 ps). Therefore, at Δt n =The electrons reaching the detector within 50ns are assigned to a number of electronic states n=1, 2, 3... determined by the number of electrons in one laser pulse.

[0139] As combined Fig.10 As shown in the power calibration curves of the one-electron state, the two-electron state, and the three-electron state in A, the ratio of single-electron pulses to the total number of emission events is linearly related to the photoexcitation laser power, which is consistent with the near-threshold laser-assisted Schottky photoemission process used [48,49]. Fig.10 B shows in a schematic manner how single-photon laser-assisted near-threshold Schottky emission produces single-electron states.

[0140] Similarly, the ratio of two and three electrons increases with the power of n. Considering the relative distribution of n electron events at a given laser power, weak sub-Poissonian statistics are determined. Specifically, define P n is the probability of detecting n electrons in a pulse, the Poisson process predicts the probability distribution to be r n = 1. The measured ratios for n ≥ 2 are slightly lower, corresponding to bunching ratios of r2 = 0:85 and r3 = 0:57, respectively. This confirms the moderate antibunching of the minority-electron states, as recently observed from different field emitters for n = 2 [19, 24, 25].

[0141] Fig.10 C to Fig.10 F shows the study of the kinetic energy of electronic states classified by number state, showing the event average spectrum ( Fig.10 C) can be separated into contributions of number resolution (n = 1, 2, 3, Fig.10 D to Fig.10 F). The two-electron and three-electron spectra show a unique shape with n peaks, indicating that the electrons involved have discrete energy separations.

[0142] Energy spectrum distribution of single electron events ( Fig.10 D) also dominates the total spectrum (average of all events), which consists of a single peak centered at the accelerating voltage E0 = 200 keV. In sharp contrast, the energy spectra of two-electron events and three-electron events show a clear double-lobe and triple-lobe structure, respectively, with an average energy of E0.

[0143] In addition to the average of similar events, e.g. Fig.10 C to Fig.10 As shown in FIG. 11 , the inventors' measurement scheme also allows the energy spectrum features to be linked to two-particle and three-particle correlations within each electron pulse, as shown in FIG. 11 . FIG. 11 shows the generation of electron pair states at a tip-shaped photoemission electron source 21 (see Fig. 9 ), where FIG. 11A shows an energy histogram of overlapping electron pairs, revealing a strong correlation of the relative kinetic energies, visible in the spectral correlation function (inset, integrated along the diagonal), and FIG. 11B shows a normalized one-sided pair correlation function (n=2) at different laser powers. FIG. 11C shows the power variation of the peak position of the n=2 correlation function versus the spectral width (FWHM) of the n=1 state (the energy spectrum is shown in 11D as a function of laser power). FIG. 11E shows the normalized n=2 spectrum as a function of laser power, and FIG. 11F shows the pair correlation function of the photoemission of two delayed laser pulses. In the temporal overlap, a strong correlation gap is observed, which disappears after a pulse delay of approximately 200 fs (see the cross section in the inset). FIG. 11G shows the extraction voltage U ext Figure 11H shows the lateral (r tra ) and longitudinal ((r lon ) size (Inset: Comparison of the correlation volume and the virtual source size).

[0144] For the dual laser pulse generation described with reference to FIG. 11F and FIG. 11H , a Michelson interferometer can be used to split the incident laser pulse into two separate pulses. One of the interferometer arms has a variable optical path length, which is realized by a retro-reflector mounted on a delay stage, with a bidirectional repeatability of (value). The optical path difference can be adjusted to the delay time difference between the two pulses, for example, up to 10 ps.

[0145] In more detail, FIG. 11A shows the electron energy E associated with two electrons A and B assigned to the same electron pulse. A and E B The Coulomb repulsion leads to the energy difference E A -E B With a clear gap and widening of the total energy E A +E B, thus partially reducing the observed gap in the total spectral density. This strong correlation proves that the observed splitting of the n = 2 spectrum into a two-lobe structure is the result of two-electron interactions caused by Coulomb repulsion.

[0146] Similar to a conventional (non-laser triggered) Schottky source, only a small fraction of the electrons generated at the emitter surface are transmitted into the microscope column 101 (see Fig. 9 ). Thus, the mean field (space charge) as well as random interactions with random nearby electrons that did not enter the beam can be considered and distinguished from the correlations observed in the electron pair states. The laser power-dependent measurements allow these different contributions to be evaluated. The corresponding n = 1 and n = 2 (Fig. 11D, Fig. 11E) spectral distributions show a broadening trend with increasing laser power (i.e., average photocurrent) (see Fig. 11C, circle C1). This is closely related to previous non-event selective measurements [28, 31, 50], which were usually attributed to random Coulomb interactions and mean field effects.

[0147] In contrast, the set of two-electron correlation functions shown in Figure 11B is remarkably independent of laser power, showing a clear gap, about 1 eV wide, a peak at about 1.8 eV, and a tail extending toward high-energy separations exceeding 4 eV. Increasing the photocurrent causes only modest changes in the depth of the gap and the shape of the high-energy tail. In particular, the main correlation peak (Figure 11C, circle C2) tends to a fixed value of 1.7 eV as the laser power approaches zero and the average current approaches zero. This suggests that the observed correlations are only weakly modified by the multi-Coulomb interactions of the electrons blocked by the aperture and are dominated solely by two-electron correlations.

[0148] To investigate the time range in which such strong Coulomb correlations hinder the observation of independent single electrons, measurements were performed at a constant integrated laser power using a pair of laser pulses with variable delay (see FIG11F ). Two different ranges were determined: temporally overlapping laser pulses reproduce the described n=2 correlation function. In contrast, a temporal separation of more than 200 fs is consistent with a significantly reduced energy difference, indicating the presence of two independent, uncorrelated electron emission events.

[0149] Although the correlation is weakly dependent on the emission current, the inventors found that the extraction field applied to the tip has a more significant effect. A reduction in the extraction voltage produces a large change in the slope of the observed gap and high-energy tail (see the semi-log plot in Figure 11G). Physically, changes in the extraction voltage affect the height of the Schottky barrier and the acceptance angle of the beam. The spectral shape of these correlation functions can be directly simulated by a simple model that assumes that the electron pairs are prepared with Gaussian distributions of interparticle distances and that the initial interparticle Coulomb energy is amplified when accelerated in an electrostatic field, resulting in a larger kinetic energy difference. By using separate standard deviations r for the longitudinal distribution (perpendicular to the surface) and the transverse distribution (parallel to the surface), the spectral shape of the correlation function can be directly simulated by a simple model that assumes that the electron pairs are prepared with Gaussian distributions of interparticle distances and that the initial interparticle Coulomb energy is amplified when accelerated in an electrostatic field, resulting in a larger kinetic energy difference. lon and r tra , this simple model successfully describes the main features of the measured correlation function (see the solid line in Figure 11G).

[0150] The physical source size of the Schottky field emitter and the back-projected virtual source size should be above 20 nm

[51] . However, since the correlation function almost vanishes when the energy difference is zero, there are almost no additional events containing two uncorrelated (or weakly correlated) electrons. This is remarkable because the fraction of Coulomb-correlated electron pairs reaches 85% of the expected fraction from the Poisson number distribution and the single electron fraction. The 15% missing two-electron events (antibunching) can be attributed to local Coulomb blockade [52,53], Pauli blockade

[24] or lateral interparticle deflection and spatial filtering.

[0151] In other words, the statistical occurrence frequency of the double electron events is fairly close to the expected occurrence frequency of uncorrelated emission events from an extended nanosource.

[0152] To explain the synchronous enhancement of electron pair emission, the inventors note some mechanisms previously cited in atomic and molecular backgrounds and in di-electron photoemission. Enhanced “non-sequential” double ionization has been observed in atoms exposed to strong laser fields, initially in helium

[54] and subsequently in many other elements. Various mechanisms, including co-tunneling, oscillatory processes, and rescattering of field-driven electrons, have been proposed to describe the observations, and in most cases the latter mechanism appears to be the dominant cause [33,34]. In the inventors’ experiments, the local intensities are quite modest and the ponderomotive potential is significantly below 1 meV, so recollisions can be ruled out as the dominant factor. In the linear regime, single-photon di-electron emission is common in the Auger effect, where the core hole created by the photoemission is filled during the concurrent emission of a second electron. The Coulomb interaction is the dominant factor in this process

[55] .

[0153] Fig.12 Characterization of the spatial beam properties of the few-electron states is shown. Fig.12A is a schematic diagram of spatial filtering using the Coulomb interaction effect. As the pulse charge increases in integer multiples, the virtual source size increases (see double arrows) and shifts along the electron beam axis z (see vertical arrows). Fig.12 B shows the n-ordered electron beam caustics recorded by changing the last condenser lens 38 of the electron beam device 100 . Fig.12 The inset of B shows the under-focus (left), focused (middle) and over-focus (right) beam profile images when n=2. Fig.12 C is the beam profile image under defocus, with the correlation angle between electron pairs relative to the beam center being φ. The long angle legs under defocus conditions allow accurate measurement of the angular correlation, Fig.12 D shows the isotropic distribution used for random event drawing (the data set used is in Fig.12 Compared with the results in Figure 5 (shown in Figure 5B, with black circles around the data points), a strong anisotropy angle dependence is observed when n = 2.

[0154] More detailed, Fig.12 It is shown that, in addition to their spectral distribution and correlations, the few-electron states observed here also exhibit characteristic spatial properties, as discussed below. Specifically, Fig.12 B shows the n-dependent beam caustics, which exhibit discrete differences in both the minimum spot size and the focus position, resulting in separated sub-pulses. The caustics vary with laser power (higher powers result in some increase in spot size), but are far less pronounced than the differences between the different event classes. Under given conditions, the focusing capability may be limited by the spherical aberration of the objective lens and the size of the virtual source, resulting in typical spot profiles with positive and negative defocus ( Fig.12 (Inset in B). Clearly, the n ≥ 1 caustics are caused by the increase in the effective source, and the beam waist shifts toward the positive defocusing direction.

[0155] Both of these observations can be seen from Fig.12 The lateral deflection is expected to spread the trajectory of the minority electrons laterally

[31] , causing the virtual source to grow in size and move forward, as previously predicted in simulations [52,57].

[0156] A more detailed analysis of the spatial properties of the minority-electron state can be obtained by analyzing the correlation of the transverse momentum. To this end, the inventors measured the positional correlation ( Fig.12 C) Spatial correlation through the angle between the two electrons and the beam center To quantify. Fig.12D shows the comparison of the angular correlation density of the two-electron state with the random correlations derived from the corresponding single-electron state at the same spot size (15 nm). In the electron pair state, we obtain a strongly anisotropic correlation peaking around an angle of 180 degrees, corresponding to electron events localized on opposite sides of the defocused beam and therefore with nearly opposite transverse momentum.

[0157] These observations suggest that averaging over number states can have severe consequences for the beam properties, including uncorrectable random aberrations. Control over the population statistics of photoemission beams could therefore directly benefit microscopy applications using such sources. More generally, random Coulomb interactions are a fundamental problem in electron microscopy, limiting the brightness of electron sources by altering the transverse (Loeffler) and longitudinal (Boersch) momentum distributions of the beam. The modest antibunching observed in this paper and in previous work

[25] suggests that the total photocurrent exhibits weak sub-Poissonian noise behavior, a highly sought-after property in condensed matter settings (e.g., via Coulomb blockade). In the context of electron microscopy, this feature has direct applications in imaging, spectroscopy, and shot noise reduction in lithography. However, perhaps even greater potential comes from the strong Coulomb correlations identified for the electron pair state (n = 2). The two electrons in this state are energetically separated from each other and from the central energy, which allows for energy selection of the corresponding number state. This provides a powerful approach to controlling the statistics of single- and di-electron events via energy selection.

[0158] FIG. 13 shows the statistical control of single and double electron states using spatial filtering and spectral filtering, wherein FIG. 13A shows a spatial filtering scheme using a circular aperture, and FIG. 13B shows the transmittance of the spatial filtering. FIG. 13C (double electron state suppression) shows that the energy slit significantly reduces the transmission of the n=2 electron state produced by the laser pulse relative to the n=1 electron state, wherein the spectra of the n=1 and n=2 electron states are shown. The electron energy in the dark shaded area is cut off by the energy slit. On the other hand, FIG. 13D (double electron state enhancement) shows that the energy beam blocker significantly reduces the transmission of the n=1 electron state produced by the laser pulse relative to the n=2 electron state, wherein the spectra of the n=1 and n=2 electron states are shown. The electron energy in the dark shaded area is cut off by the energy beam blocker.

[0159] FIG. 13E shows the transmission T of different energy slit widths when the n=1 electronic state is enhanced up to 8 times. n and transmittance ratio T1 / T2 (for the setup shown in FIG. 13C ), FIG. 13F shows the transmission T1 / T2 for different energy blocker widths when the n=2 electronic state is enhanced by more than 20 times. n and a graph of the transmittance T2 / T1 (for the setup shown in FIG13D ).

[0160] In more detail, the pre-sample energy filter used in state-of-the-art electron microscopes

[58] can be used to selectively favor specific number states. Specifically, such an energy slit with a pinhole diameter of d cuts off the transmission energy spectrum and can therefore be tuned to strongly favor the transmission probability of the n=1 state over that of the n=2 state (see FIG13C ). Specifically, for the experimentally measured single-electron and double-electron spectra ( FIG13E ), at small slit widths, the transmission probability of n=1 exceeds that of n=2 by a factor of 8, greatly enhancing the sub-Poissonian nature of the electron number distribution and promoting shot-noise-reduced electron currents. In contrast, a central beam energy blocker can suppress a large fraction of single-electron states, resulting in a transmission enhancement of up to 20 times for the pair state relative to the n=1 state (see FIG13D , FIG13F ). This approach enables new microscopes and spectrometers with correlated electrons for a variety of new two-point or two-time measurement schemes in correlated materials and free-electron quantum optics.

[0161] In summary, the present invention demonstrates a new method to exert unprecedented control over the statistical properties in pulsed charged particle beams, generating femtosecond pulses containing well-defined integer electron charges Q=ne. The number of electrons in each state n can not only be tracked directly by event-based detection, but also manifests itself significantly in the spectral and angular distribution of the beam. This will enable the particle statistics of the electron beam to be altered by simply blocking a portion of the time-averaged intensity in the energy dispersion plane, in the transverse momentum, or at the focused beam position, effectively filtering out specific number states. Rejecting beam states with n>1 enables the efficient generation of sub-Poissonian beams, which has direct implications for electron imaging and lithography with reduced shot noise. Furthermore, the selection of specific number states allows the borrowing of concepts from quantum optics. For example, n=2 constitutes a highly nonclassical state, which can be used to implement high-fidelity electron prediction of single electrons, thereby achieving shot noise-free (or reduced shot noise) electron imaging and lithography with a precisely countable number of electrons, thus breaking through the fundamental limitations previously considered. Furthermore, in the absence of additional entanglement-destroying reporter channels (such as potential residual electron-holes in photoemitters, or coupling to external heat baths), it can generally be assumed that the fundamental scattering processes involved in creating multi-electron states induce entanglement between multiple electrons. Future research will have to address the quantum coherence of such multi-electron states, promising new quantum technologies using free electrons, potentially enabling interaction-free measurement and ghost imaging, quantum teleportation and information processing, and ultimately the realization of entangled free-electron qubits for fermionic quantum computing.

[0162] The features of the invention disclosed in the above description, drawings and claims, individually and in combination or sub-combination, are of great significance for the implementation of the invention in its various embodiments. The invention is not limited to the preferred embodiments described above. On the contrary, there may be a number of variants and derivatives, which also use the concept of the invention and therefore fall within the scope of protection. Furthermore, the invention also claims the subject matter and features of the dependent claims independent of the features and claims to which they refer.

Claims

1. An electron beam device (100) for generating a pulsed electron beam (1), the pulsed electron beam (1) comprising a sequence of electron pulses, the electron beam device (100) comprising: - a radiation source device (10) arranged to generate a sequence of emitter excitation pulses (2), in particular laser pulses, - an electron source device (20) having a photoemissive electron source (21) arranged to radiation-inductively emit source electron pulses (3) in response to irradiation with emitter excitation pulses (2), - a numerical dispersion device (30) arranged to spatially separate the source electron pulse (3) into sub-pulses (4, 4A), wherein each sub-pulse (4, 4A) comprises an integer number of electrons (n), n=1, 2, 3, ..., and - a number state selector device (40) arranged to select sub-pulses (4, 4A) comprising at least one set of predetermined electron number states as the pulsed electron beam (1) to be generated.

2. The electron beam apparatus according to claim 1, wherein - the photoemissive electron source (21) is configured to generate countable low-charge electron pulses, preferably each pulse comprising 1, 2, 3 or 4 electrons.

3. Electron beam apparatus according to any one of the preceding claims, wherein - the photoemissive electron source (21) comprises a beam limiting aperture (20A) configured to reduce high charge electron pulses into countable low charge electron pulses.

4. Electron beam device according to one of the preceding claims, - the photoemissive electron source (21) is configured to generate multiple electron states having distinguishable characteristics other than impulse charges.

5. Electron beam device according to one of the preceding claims, wherein the number dispersing device (30) comprises an energy dispersing device.

6. The electron microscope apparatus according to claim 5, wherein the energy dispersive device comprises: - a beam monochromator, in particular of the omega type, alpha type, Wien filter type, double Wien type or electron mirror type.

7. The electron microscope apparatus according to claim 5, wherein the energy dispersive device comprises: - Spectrometer devices, in particular spectrometer devices comprising magnetic prisms or electrostatic multipole electron optics.

8. The electron beam device according to one of the preceding claims, wherein the number dispersion device (30) comprises a momentum dispersion device or a position dispersion device or an electron orbit momentum dispersion device, in particular at least one of a rotationally symmetric electron lens and a cylindrical electron lens.

9. Electron beam device according to one of the preceding claims, wherein the number dispersion device (30) comprises an electron spin dispersion device.

10. Electron beam apparatus according to any one of the preceding claims, wherein The number state selector device (40) comprises a spatial modulator of the electron beam intensity, in particular the spatial modulator is configured to pass selected sub-pulses (4, 4A) having a predetermined electron number state and block the remaining sub-pulses (4, 4A).

11. Electron beam apparatus according to any one of the preceding claims, wherein - the digital state selector device (40) comprises at least one of a mechanical slit, a grating, a linear beam blocker, an aperture, a ring or a disk.

12. The electron beam apparatus according to any one of the preceding claims, further comprising: - a detector arrangement configured to determine the number of electrons in at least one of the electron number states of the pulsed electron beam (1).

13. The electron beam apparatus according to any one of the preceding claims, further comprising: - beam-forming electron optics configured to illuminate a sample, such as a sample to be investigated, and / or a workpiece, such as a semiconductor workpiece.

14. Electron beam apparatus according to any one of the preceding claims, wherein - The photoelectric emission electron source (21) comprises a tip-shaped photoelectric emission electron source.

15. An electron microscope device provided with an electron beam device according to one of the preceding claims.

16. A method for generating a pulsed electron beam (1), the pulsed electron beam (1) comprising a sequence of electron pulses, the method comprising the following steps: - using a radiation source device (10) to generate a sequence of emitter excitation pulses (2), in particular laser pulses, - irradiating a photoemissive electron source (21) of an electron source device (20) with an emitter excitation pulse (2), thereby generating a source electron pulse (3) by radiation-induced emission, - using a number dispersion device (30) to spatially separate the source electron pulse (3) into sub-pulses (4, 4A), wherein each sub-pulse (4, 4A) comprises an integer number of electrons (n), n=1, 2, 3, ..., and - using a state selector device (40) to select sub-pulses (4, 4A) comprising at least one set of predetermined electron state as the pulsed electron beam (1) to be generated.

17. The method according to claim 16, comprising the steps of: - adjusting the radiation source arrangement (10) and / or the photoemission electron source (21) to produce multiple electron states that can be distinguished by properties other than the impulse charge.

18. The method according to any one of claims 16 to 17, comprising the steps of: - The numerical dispersion device (30) is adjusted according to distinguishable properties of the source electron pulses (3), in particular energy, angle / momentum, spatial focus.

19. The method according to any one of claims 16 to 18, comprising the steps of: - adjusting the number state selector means to reject or block specific number states in the beam, with particular interest in rejecting or blocking all sub-pulses with n>2 or rejecting or blocking all sub-pulses with n≠2 (4, 4A).

20. The method according to any one of claims 16 to 19, comprising the steps of: - adjusting the radiation source means (10), the photo-emissive electron source (21), the number state dispersion means (30) and / or the number state selector means (40) based on the output of the additional number state sensitive detector.

21. A method of using the electron beam device according to one of claims 1 to 15 as a beam source, wherein the electron beam device is used as a beam source in at least one of the following: electron microscope equipment, electron lithography equipment, electron pair (source) equipment, electron prediction equipment, electron counting equipment, information processing equipment, communication equipment and quantum computing equipment.

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