Surface analysis system with pulsed electron source

Abstract

The present invention relates to a system for performing surface analysis of materials, the system comprising: a pulsed electron source (10) forming a monochromatic beam (100) of incident electrons; - means (20) for transporting the incident electrons (100) to the surface of the material sample (55) so as to form backscattered electrons (110) and for transporting the backscattered electrons (110) to detection means, said transport means including at least one electron optical system; - means (30) for detecting backscattered electrons (110), The aforementioned pulsed electron source is - a source of atoms (16); a continuous wave laser beam (15) configured to form a laser excitation zone (15a) capable of exciting atoms (16) up to the Rydberg state; - a pulsed electric field (F) on either side of the laser excitation zone, configured to ionize at least the excited atoms and form a monochromatic electron beam (100).

Description

[Technical field]

[0001] The present invention relates to the field of performing surface analysis using electrons.

[0002] The present invention relates to a pulsed electron source for a system for performing surface analysis.

[0003] More precisely, the invention relates to a system for performing surface analysis equipped with a pulsed electron source capable of generating an electron beam with high spectral resolution, i.e. an electron beam with an energy dispersion of less than 10 meV, preferably less than 5 meV.

[0004] The present invention further enables the formation of a collimated beam (divergence less than 1 milliradian) of low energy electrons (electron energy below 20 eV), and therefore a beam with very small beam emittance (very small dispersion in electron position and phase space).

[0005] The system for performing the surface analysis may for example be a high-resolution electron energy loss microscope (HREELM), which in particular allows the microscopic observation of the vibrational state of the surface. [Background technology]

[0006] The vibrational state of surfaces is generally important both in surface physics and surface chemistry, and in particular allows to determine the functionality of materials and the reactivity of their surfaces. The analysis of the vibrational state of surfaces must be carried out at low energies (energy below 20 eV) and with high spectral resolution in order to be able to determine the chemical functionality, phonon and plasmon properties. It is therefore possible to predict and / or determine the relationship between the manner of interaction of the material with its environment and its electronic properties.

[0007] Furthermore, in a wide range of applications where the spatial and spectral distribution of vibrational states is to be measured and / or modified, it is necessary to be able to analyze the vibrational states of surfaces with nanoscale spatial resolution (typically from a few nanometers to tens of nanometers). As non-limiting examples of the importance of these analyses, reference may be made to systems or devices such as semiconductor devices, where these analyses can be used to optimize the mobility of charge carriers as a function of local stress, particularly in semiconductor devices, and photovoltaic systems, where the interaction of charge carriers with phonons can be unfavorable, since this can result in non-radiative losses as opposed to losses in a purely radiative manner that is sought.

[0008] Another application concerns the modification of local properties, in particular the functionalization of molecular layers by optical or electron irradiation. The analysis of chemical changes and surface functionality requires a very high spectral resolution of the vibrational state of the surface and a spatial resolution of a few nanometers. Furthermore, the use of low-energy electron beams for chemical lithography is one way of progressing to better control the changes induced.

[0009] Known solutions for performing the analysis either have a high spatial resolution or a high spectral resolution or make it possible to probe the bulk of a material rather than its surface. Among the known solutions, mention may be made of the following three solutions:

[0010] STEM-EELS (acronym for Scanning Transmission Electron Microscope-Electron Energy Loss Spectroscopy) allows to obtain nanoscale spatial resolution and high spectral resolution. Imaging is performed by scanning a beam of electrons (at very high energies, i.e. between 30 keV and 300 keV, or even up to 1 MeV) over the cross section of the sample, which passes through the surface of the sample and reaches the bulk. STEM-EELS therefore provides information about the bulk and not about the surface properties. Furthermore, to achieve the correction of chromatic aberration and a very low count rate, excellent alignment, good mechanical and electrical stability are required, which leads to very high construction costs (several millions or even tens of millions of euros). Finally, the sample preparation is complex and STEM-EELS is a destructive method, since it requires cutting a part of the sample by FIB (acronym for Focused Ion Beam). In conclusion, STEM-EELS cannot be used to investigate surface properties, it is destructive and has major negative factors. STEM-EELS is not suitable for performing high spatial resolution analysis at high spectral resolution of surface vibrational states.

[0011] LEEM (acronym for Low-Energy Electron Microscopy) is a technique for performing surface analysis, usually used to obtain images of surfaces, atom-surface interactions, and / or thin films of materials. An example of a low-energy electron microscope is shown in FIG. 1. In LEEM, high-energy (around 15-20 keV) electrons are emitted by an electron gun 1, then focused using a set of electron optics 2 (a set of electrostatic or magnetic lenses) and pass through a magnetic electron beam splitter 3. The electrons, which are said to be "fast", are deflected by the splitter towards a sample 9 of material, pass through an objective lens 4, and slow down to low energy (around 1-20 eV) near the surface of the sample 9, since the sample is kept at a potential V9 close to the potential V1 of the electron gun 1. These low-energy electrons can be used to investigate surface properties. The sampling depth from the surface of the aforementioned sample can be altered by adjusting the energy of the incident electrons. Said depth is determined by the difference between the work function of the sample (i.e. the minimum energy, measured in eV, required to pull an electron from the Fermi level of the atom to a point at an infinite distance outside the atom) and the work function of the electron gun, plus a value called the starting voltage, which is on the order of a few eV, which roughly determines the kinetic energy of the electrons with respect to the surface potential of the sample and can be adjusted to give the electrons the desired kinetic energy. The elastically backscattered low energy electrons return through the objective lens 4, are accelerated again to the electron gun voltage, pass again through the electron beam splitter 3 and are directed through the projection optics 5 (a set of electrostatic or magnetic lenses) which generate a visible image on the electron detector 6. This makes it possible, among other things, to observe in real time dynamic surface processing of materials, such as phase transitions, adsorption, reactions, separation, growth of thin layers, etching, stress release, sublimation, magnetic microstructures, etc.

[0012] LEEM therefore allows the determination of the local work function and the characteristics of the crystallinity and chemistry of surfaces with a spatial resolution of the order of 15 nm. However, spatial mapping of surface vibrational states via electron energy loss is not possible with LEEM because the energy dispersion of the electron source is typically 250 meV, which is far too high to allow resolution of energy loss spectra.

[0013] HREELS (acronym for High-Resolution Electron Energy-Loss Spectroscopy) is a technique for performing analyses consisting in analyzing the energy and direction of electrons scattered by a surface when it is illuminated by a monochromatic electron beam, defined as a monochromatic beam with an energy of a few eV to within a few meV. An example of HREELS is shown in FIG. 2. A sample 9 of the material to be analyzed is exposed to a monochromatic electron beam generated by an electron gun 1 and a first monochromator 7a. Electrons are scattered by the surface of the sample 9 exposed to the beam. The scattering is mainly due to the interaction of the Coulomb potential of the electrons with the dipole field of the sample's surface. Measurement of the energy losses and their wave vector dispersion parallel to the surface makes it possible to probe most energetic excitations, for example: -surface phonons, - vibrational modes of isolated atoms or molecules, - surface and interface plasmons, -In particular, low-energy electronic excitations due to states within the band gap.

[0014] The energy loss is measured by an electron detector 8, which is positioned at the exit of the second monochromator 7b and which is capable of performing high spectral resolution spectroscopic measurements of the electron energy loss with an energy dispersion of less than 10 meV or even under best conditions less than 1 meV.

[0015] However, the use of a monochromator implies a deceleration of the electron beam and an associated increase in angular divergence, which reduces the transmission function and increases aberrations substantially. The electron beam therefore diverges and is measured within 1 mm of the sample. 2 The size of the monochromatic beam implies a three-order of magnitude loss of intensity, which precludes microscopy, and aberrations arising from the angular divergence preclude imaging. Thus, spatial imaging with nanoscale resolution cannot be achieved with high-resolution energy-loss spectroscopy.

[0016] Thus, prior art techniques do not allow spatial imaging that combines nanoscale resolution with high spectral resolution.

[0017] US 2008 / 173829 discloses an electron source, particularly intended for use in low energy electron microscopy, which uses a pulsed laser to generate coherent wave packets in Rydberg atoms, which are then ionized via field effect by a pulsed electric field synchronized with the laser. The source generates one electron on average per laser pulse, and is therefore very low current, less than 1 picoamp (pA).

[0018] US Pat. No. 4,414,509 discloses a low energy magnetometer with an electron source based on field effect ionization of Rydberg atoms produced by a pulsed laser.

[0019] The paper "Forced field ionization of Rydberg states for the production of monochromatic beams" by E. Moufarej et al., Phys.Rev.A, 95, 043409 (2017) studies the field-effect ionization of Rydberg atoms in a continuous electric field. The paper also mentions the possibility of using pulsed electric fields, but only for the purpose of improving the accuracy of the energy measurement of long-lived Rydberg states. On this subject, the paper refers to "Field-Ionization Processes in Excited Atoms" by M G Littman, Phys.Rev.Lett., vol. 41, n.2 (1978).

[0020] The paper "Design for a high resolution electron energy loss microscope" by M. Mankos et al., Ultramicroscopy 207 (2019) 112848, discloses several energy loss microscope configurations that can use a continuous electron source based on field effect ionization of Rydberg atoms.

[0021] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to overcome the above-mentioned drawbacks of the

[0022] More specifically, it is an object of the present invention to provide a new electron source that allows to generate a monochromatic electron beam, i.e. a monochromatic electron beam with an energy spread of 10 meV or less, preferably 5 meV or less, without the use of a monochromator. It is therefore an object of the present invention to control the energy spread of the electrons in the formed electron beam.

[0023] It is also an object of the present invention to provide a system for performing surface analysis, such as a high-resolution electron energy loss microscope ("HREELM") incorporating such an electron source, the electrons of which are of low energy (20 eV or less) and which makes it possible to perform an analysis of the surface vibrational state of a material and, more precisely, to perform spatial imaging with nanoscale resolution and with high spectral resolution (with an energy dispersion of 10 meV or less, preferably 5 meV or less). [Prior art documents] [Patent documents]

[0024] [Patent Document 1] US Patent Application Publication No. 2008 / 173829 [Patent Document 2] U.S. Patent Application Serial No. 4,414,509 [Non-patent literature]

[0025] [Non-Patent Document 1] E. Moufarej et al. “Forced field ionization of Rydberg states for the production of monochromatic beams”, Phys.Rev.A, 95, 043409 (2017) [Non-Patent Document 2] MGLittman “Field-Ionization Processes in Excited Atoms”, Phys.Rev.Lett.Vol.41, n.2 (1978) [Non-Patent Document 3] M.Mankos et al. “Design for a high resolution electron energy loss microscope”, Ultramicroscopy207(2019)112848 Summary of the Invention [Means for solving the problem]

[0026] A device that can overcome these drawbacks is a system for carrying out surface analysis of materials, a pulsed monochromatic electron source enabling the formation of a monochromatic incident electron beam; - means for transporting all or a part of the incident electrons to a surface of the sample of material so as to form backscattered electrons, and for transporting all or a part of the backscattered electrons to detection means, said transport means comprising at least one electron optics, preferably a plurality of electron optics; - means for detecting all or a portion of the backscattered electrons, The pulsed monochromatic electron source is a source of atoms; at least one continuous wave laser beam configured to form a laser excitation zone capable of exciting at least some of said atoms to a Rydberg state; - means for generating a pulsed electric field on either side of a laser excitation zone, said pulsed electric field being configured to ionize at least some of the excited atoms and form a monochromatic electron beam.

[0027] The electron beam preferably has low emittance. The term "emittance" refers to a beam of electrons and indicates the dispersion of the electrons in position and phase space.

[0028] By "monochromatic" electron beam (this type of beam is also called a "monoenergetic" electron beam) is meant a beam of electrons in which the energy spread of the electrons is 10 meV or less, preferably 5 meV or less.

[0029] To obtain a monochromatic beam, the electrons must be generated at substantially the same voltage. Now, the source according to the invention allows almost instantaneous ionization in a zone where the voltage varies little spatially. The electrons are thus generated in the same electric field environment, so that the electron beam formed is monochromatic.

[0030] By "Rydberg state" is meant an excited atomic state in which ionization can occur, which state contains one or more electrons whose principal quantum number n is very high, typically 20 or more, preferably 50 or more.

[0031] By pulsed electric field is meant an electric field whose amplitude changes rapidly and transiently (during the duration of the pulse) from a base value to a higher or lower value, then rapidly returns to the base value, etc. The delay between two pulses is called the repetition rate. The pulses may be rectangular in shape, where the height of the rectangle is the amplitude, the width is the duration of the pulse, and the distance between the two rectangles corresponds to the repetition rate. The pulses may have other shapes, such as Gaussian or other shapes known to those skilled in the art.

[0032] According to the invention, the monochromatic electron source is a pulsed source, which is created by rapidly varying (the variation is typically for 1 nanosecond or less) the electric field (pulsed electric field) around the electric field strength required to ionize atoms previously excited into the Rydberg state by a laser beam. The laser excitation is configured to excite the atoms into a Rydberg state with quantum number n, which is high enough that the electric field strength that needs to be applied is low and the rapid variation thereof can be achieved.

[0033] To achieve an energy spread of less than 10 meV for the electron beam, an average of 10 8 ~10 9It is necessary to provide between 10 and 100 electrons per second, which corresponds to an average current of between 10 and 100 picoamperes (pA). Above this, the number of electrons per second increases (for example, 1 nanoampere (nA), which corresponds to an average current of 10 10 Above 1000 uV (corresponding to 1000 uV or more electrons), space charge effects degrade the spectral resolution and make it difficult or even impossible to obtain a monochromatic electron beam. Furthermore, below this, the number of electrons becomes insufficient to achieve imaging in a reasonable time.

[0034] 10 per second 8 ~10 9 At these scales of electrons, it is preferable to perform the analysis of the electrons using a time-of-flight detection device, since the known detection limit of most hybrid pixel detectors of this type is 1 nanosecond (ns), and the detection limit can even reach 100-200 picoseconds (ps) for certain hybrid pixel detectors or multi-anode microchannel array detectors, and even smaller for other DLDs (acronym for Delay Line Detector).

[0035] Recall that a time-of-flight (ToF) detection device measures the energy of an electron via the time of flight between its departure (from the electron source) and its arrival (at the detector). Like the detectors described above and those referred to in the detailed description, such a detection device comprises a flight tube across which a potential difference may be applied, through which the electrons reach the detector, which "counts" the electrons and measures their arrival time.

[0036] With a continuous electron source, even if the electron beam from the source is monochromatic, it does not remain monochromatic after the electrons impinge on the sample and are backscattered. To analyze the energy distribution of the backscattered electrons, it is necessary to scan the energy of the electrons sequentially in a continuous flow. It is therefore necessary to select an energy range (spectral band) of electrons, for example by placing a monochromator in the path of the backscattered electrons. This narrow selection of the electron spectral band results in the loss of most of the electrons generated by the source, which can be as much as 100 times. Thus, with a 100 pA electron source, the electrons can be scattered at a rate of 100 times per second. 7 Only electrons (backscattered and selected by a monochromator) are obtained.

[0037] However, to enable nanoscale spatial imaging, the inventors 9 We have determined that many more electrons are needed to form an image. Specifically, for a 500 x 500 pixel image, we estimate that at least 100 electrons per pixel are needed to lose energy (inelastically scattered) during interaction with the surface to achieve a sufficiently resolved loss spectrum. Allowing for a 1 to 10 factor between inelastically and elastically scattered electrons, and a 1 to 10 ratio between reflected or backscattered electrons and incident electrons, the source would be approximately 2.5 x 10 9 It is necessary to produce (500 x 500 x 10 x 10 x 100) electrons.

[0038] Therefore, using a continuous electron source would require waiting at least 250 seconds to achieve high spatial resolution imaging, which is impractical; additionally, during this image acquisition time, noise may distort the results displayed in the image.

[0039] Conversely, if it is desired to increase the number of electrons to account for this loss of magnification of 100 and to reduce imaging time, the average current of the electron source must be increased. However, above 100 pA, space charge effects degrade the spectral resolution.

[0040] The invention thus makes it possible to dispense with one or more monochromators, thus limiting the loss of electrons, and to reduce the number of electrons lost within a reasonable time (which may range from a few seconds to 10 pA sources, i.e. 10 per second). 8 This allows imaging with high spectral and spatial resolution to be achieved over a period of time (which can be tens of seconds in the case of 100 electrons).

[0041] The system according to the invention makes it possible to achieve imaging with high spectral and spatial resolution.

[0042] Furthermore, the present invention allows both microscopic and spectroscopic analysis to be performed using a single system for performing surface analysis, thereby avoiding the need to transfer the material to be analyzed between various systems, thus compromising the reproducibility of the measurements and adversely affecting measurement and analysis times. Thus, by providing a single system capable of performing both microscopic and spectroscopic analysis, measurement and analysis times are advantageously reduced while improving the reliability of the analysis.

[0043] Other characteristics and advantages of the invention will become apparent from the following non-limiting description, given by way of example with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0044] [Figure 1] 1 shows a low energy electron microscope according to the prior art. [Diagram 2] 1 shows a high-resolution energy loss spectrometer according to the prior art. [Diagram 3] 1 shows a pulsed monochromatic electron source according to the present invention. [Figure 4] 1 shows an electron energy loss microscope incorporating a pulsed monochromatic electron source according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] 1 and 2 have already been described in the section on "Background Art" and will not be described again here.

[0046] FIG. 3 shows a pulsed monochromatic electron source according to the present invention.

[0047] Pulsed monochromatic electron sources (energy spread ≦10 meV, preferably ≦5 meV) are based on laser excitation of a beam of atoms into a Rydberg state and ionization of at least some of the excited atoms by a pulsed electric field to form a monochromatic electron beam.

[0048] The illustrated pulsed electron source 10 has a voltage V 11 a first electrode 11 to which a voltage V 12 The first electrode includes a second electrode 12 to which a voltage is applied. An electric field F, referred to as the "ionizing field," is formed between the first and second electrodes. The voltage applied to one or both of the first and second electrodes is pulsed, resulting in a pulsed ionizing field.

[0049] If the distance between the two electrodes is D, the ionization field is given by

number

[0050] A pulsed voltage may be applied to the first electrode 11, which then conventionally forms the "ionization electrode", and a non-pulsed voltage may be applied to the second electrode 12, which then conventionally forms the "extraction electrode".

[0051] Alternatively, a pulsed voltage may be applied to the second electrode 12, which then forms the ionization electrode, and a non-pulsed voltage may be applied to the first electrode 11, which then forms the extraction electrode.

[0052] Alternatively, a pulsed voltage may be applied to the first and second electrodes.

[0053] An ionization electric field F is applied on both sides of an excitation zone 15a formed by at least one laser beam 15. A beam 16 of atoms is directed into said excitation zone, so that at least some of the atoms are excited to a Rydberg state without being ionized. The selected Rydberg state is close to the ionization limit. It is the application of a pulsed ionization electric field F on both sides of the excitation zone that allows the ionization of the atoms and the formation of a monochromatic pulsed electron beam 100.

[0054] The amplitude of the ionizing electric field is preferably comprised between 5 and 50 V / cm, which makes it possible to form and extract electrons without inducing chromatic aberration.

[0055] The duration of the pulse is preferably comprised between 100 ps and 1 ns.

[0056] The pulse repetition rate is preferably comprised between 1 MHz (1 μs) and 10 MHz (0.1 μs). For example, a ratio of 1000 between a 1 μs rate and a 1 ns electron pulse allows a spectral resolution of 1 meV over a 1 eV energy range.

[0057] The illustrated pulsed electron source also comprises a gun lens 13, which is made up of a number of electrodes (here three electrodes 13a, 13b, and 13c), for example a focusing electrode 13b, which may be operated at a voltage of about 1 kV less than the voltage of the electron beam, which is about 3 kV, and two surrounding electrodes 13a and 13c, both at ground potential. In addition to allowing the electron beam to be focused and collimated, the gun lens also allows the pulsed electron source to be coupled to a desired analysis system.

[0058] The laser beam may be derived from a continuous wave laser, such as a laser diode or a continuous wave Ti:Sa laser.

[0059] In particular, continuous wave lasers are generally cheaper than pulsed lasers and, on average, are capable of exciting a greater number of atoms in a resonant manner.

[0060] The laser beam may further be formed by several laser stages.

[0061] The laser output of the final laser stage is preferably 100 mW or more at a wavelength comprised between 300 nm and 2000 nm.

[0062] As an example, the laser powers of the first, second and third beams (in this case the beam of the last laser stage) are about 10 μW at 852 nm, 100 μW at 1470 nm and 1 W at 780-830 nm, respectively. The third laser is called a Rydberg pumped laser. It can be an amplified diode laser or a Ti laser. Its continuity and wavelength may be monitored using a high-precision optical lambdameter. The first and second lasers can be diode lasers.

[0063] The average diameter of the laser beam is about 10 micrometers or less to ensure uniformity of the laser excitation field.

[0064] Here, the Rydberg state defined by the principal quantum number n is preferably included in the range of 50-100.

[0065] The laser beams are preferably configured (in particular through the selection of the emission wavelength of the laser beam) to excite the atoms into the same Rydberg state characterized by a single quantum number n. This allows to obtain a better atomic excitation efficiency, and the wavelength(s) of the laser beam(s) can be precisely tuned to the target atomic transition(s). This also allows to maximize the monochromaticity of the electron beam resulting from the ionization of the Rydberg atoms.

[0066] The beam of atoms is obtained, for example, using an effusion cell 161 .

[0067] An atomic collimator or focuser (not shown) may be disposed between the effusion cell 161 and the first electrode 11 .

[0068] Instead of an effusion cell, elements such as an alkali metal dispenser may be used.

[0069] The flux of atoms is 10 11 at / s / mm 2 ~10 13 at / s / mm 2 It is preferable that the ratio is between 1 and 2.

[0070] For example, 10 12 at / s / mm 2 An atomic flux equal to may be obtained with cesium.

[0071] Suitable atoms are, for example, cesium, lithium, sodium, potassium, rubidium, magnesium, calcium, strontium, barium, chromium, erbium, silver, ytterbium, mercury, helium, neon, argon, krypton, xenon atoms.

[0072] The electron source is about 10 -6 It is preferably confined in a chamber (not shown) pumped to less than millibar, which preferably includes an active or passive magnetic shield, e.g. a mu-metal shield, to avoid or at least limit the effects that a magnetic field might otherwise have on the path of the electrons.

[0073] The pulsed electron source according to the invention may have several applications other than as a monochromatic source in HREELM. In addition to applications in electron energy loss microscopy, as described in more detail below, the pulsed electron source according to the invention may be used in other systems for performing surface analysis that require a highly monochromatic (energy dispersion of 10 meV or less) low energy electron beam, or even in tools for functionalizing thin molecular layers using electrons with precise energies to optimize the chemical reactions responsible for the functionalization. Monochromatic or monoenergetic sources may also be used in electron-atom or electron-molecule collision or diffraction studies.

[0074] Examples of characteristics of pulsed monochromatic electron sources In the above, to achieve a spectral resolution of 10 meV and a lateral spatial resolution better than 15 nm, it is necessary to perform an average of 10 8 ~10 9 It is necessary to provide electrons per second, which corresponds to an average current of between 10 and 100 picoamperes (pA). 8 ~10 9It has been shown that at these scales of electrons, the analysis of electrons is preferably performed by a Time of Flight (ToF) type detection system, since the known detection limit of this type of hybrid pixel detector is 1 nanosecond (ns), and can even reach 100-200 picoseconds (ps) for multianode microchannel array detectors or more advanced hybrid pixel detectors. Examples of such detectors are given below. The lateral resolution should be understood as being relative to the surface of the sample, which contains the structures to be analyzed, the smallest of which have a size of the order of 15 nanometers or less, and the size of the sample is larger (the sample is several millimeters in diameter or width).

[0075] Considering the detection limit of 1 ns, we estimated that the ionizing field should have a pulse duration of 1 ns. In other words, a pulse of the ionizing field F should be able to ionize an atom in 1 ns, i.e., 10 9 s -1 To achieve this, an ionization rate Γ, i.e. the number of electrons released per second, must be achieved. Thus, changing the ionization state of an atom in one nanosecond requires a change in the energy E of the atom. This change in energy dE of the atoms in the beam is due to the finite lifetime of the Rydberg state and is given by:

number

number

[0076] Therefore, the ionization field F must vary quickly (1 ns) by dF around the value of the field that ionizes the atoms toward the Rydberg state.

number

[0077] The following relationship, called the "first-order Stark effect", between the energy change dE of the Rydberg state n and the change in the ionization field dF is also used:

number

[0078] Furthermore, a typical size dz of the laser excitation zone is on the order of 10 μm.

[0079] However, the energy dispersion Δe of the electrons (i.e., up to the desired spectral resolution of 10 meV) is given by: Δe=F×dz=10meV (5)

[0080] The maximum ionization field F is then estimated from this equation as follows:

number

[0081] The minimum value of the principal quantum number n of the Rydberg state can be estimated from relation (2).

number

[0082] The change in the ionizing field dF can be estimated from relation (3) if k is assumed to be at most: k=n-1 (8)

[0083] Therefore, we obtain the following:

number

[0084] Therefore, 1 ns is sufficient to modify the electric field F by a value dF equal to 0.3 V / m.

[0085] Thus, the voltage V will be increased by only 0.3 mV if D is equal to 1 mm, or 1.2 mV if D is equal to 4 millimeters. 11 1ns is sufficient to change

[0086] The calculations presented above are non-limiting and simplified. A person skilled in the art will be able to adapt the calculations if other effects such as level crossing appear or if basic parameters such as the desired spectral resolution, the pulse duration or the size of the laser excitation zone are changed. In particular, different formulas may be used to obtain more accurate results.

[0087] FIG. 4 shows a high-resolution electron energy loss microscope (HREELM) according to the present invention.

[0088] The illustrated energy loss microscope includes: a pulsed source 10 capable of producing a monochromatic pulsed beam 100 of incident electrons, as shown in FIG. 3; means 20 for transporting all or a part of the incident electrons to a surface of the sample 55 of material so as to form backscattered electrons 110 and for transporting all or a part of the backscattered electrons to a detection means 30, said transport means including a plurality of electron optics; - Means 30 for detecting all or part of the backscattered electrons 110.

[0089] The illustrated transport means includes a number of electron optics, which are typically electrostatic or magnetic lenses, and therefore in this specification the term lens refers to either an electrostatic lens or a magnetic lens.

[0090] The transport means 20 includes an electron beam splitter 25, which is capable of deflecting the incident electrons by 90° towards the surface of the sample 55 and separating the backscattered electrons 110 from the incident electrons 100 and deflecting said backscattered electrons towards the detection means 30.

[0091] Splitter 25 includes, for example, a magnetic prism that deflects the electron beam by 90°.

[0092] A set of electron optics, called the “illumination optics”, transports incident electrons between the source 10 and the splitter 25, and between the splitter 25 and the surface of the sample 55, and transports backscattered electrons between the surface of the sample 55 and the splitter 25.

[0093] The illumination optics comprises two arms, a source arm 21 and an objective arm 22. The source arm 21 carries incident electrons between the source 10 and the splitter 25, and the objective arm 22 carries incident electrons between the splitter 25 and the surface of the sample 55, and carries backscattered electrons between the surface of the sample 55 and the splitter 25 (with the aim of delivering the backscattered electrons to the detection means 30).

[0094] The optical system of the illustrated source arm 21 comprises a series of four lenses, which are arranged one after the other in the direction of the incident electrons (ie from the source towards the splitter) in the following order: a first field lens 211, a first collecting lens 212, a second field lens 213, A second focusing lens 214.

[0095] The field lens makes it possible to reproduce the image of the electron source (herein also called "source") The collection lens makes it possible to maintain collimation of the source.

[0096] This optical system of the source arm allows illuminating with a collimated uniform electron beam the surface of the sample 55 (which may be denoted in the remainder of this specification by the term "plane"), obtaining an optical zoom of 10x, i.e. a FoV (acronym for Field of View) of the sample that varies between 10 and 100 μm. The uniformity of the beam at the plane of the sample is obtained by mapping the profile of the uniform source onto the aforementioned plane at its intersection. The collimation of the beam is obtained by mapping the Gaussian points of the source image onto the back focal plane of the objective lens (diffraction), resulting in a small angular divergence at the sample.

[0097] For a 100 μm field of view, a single focusing lens may be sufficient, but for a 10 μm field of view, two focusing lenses are required.

[0098] These conditions must be met regardless of the operating mode. There are two operating modes: in imaging mode, the surface of the sample is imaged onto the analyzer, and in diffraction mode, it is the diffraction pattern of the surface of the sample in the back focal plane of the objective that is imaged onto the analyzer. Whatever plane is chosen, the prism Achromatic It must first be imaged onto a plane that is substantially in the center of the prism, otherwise the dispersion imparted to the beam by the prism will introduce an energy-dependent shift in the detected image, pushing the beam outward from the center of the analyzer.

[0099] The optical system of the illustrated objective arm 22 comprises a series of four lenses, which are arranged one after the other in the direction of the incident electrons (ie from the splitter towards the sample) in the following order: -Transmission lens 221, - diffractive lenses 222, a third field lens 223, -224 objective lenses.

[0100] A transfer lens 221 carries the source (incident electrons 100 ) from the splitter 25 to the plane of the sample and allows the beam of backscattered electrons 110 to be returned to the splitter 25 .

[0101] The diffractive lens 222 also transports the source (incident electrons 100) from the splitter 25 to the plane of the sample and allows the beam of backscattered electrons 110 to be returned to the splitter 25, but may slow down or accelerate the electrons.

[0102] The objective lens 224 and the surface of the sample form an immersion lens. The immersion lens is configured to decelerate the electrons and illuminate the sample with a beam of low energy electrons (0-100 eV). To do this, a very negative voltage is applied to the sample, which decelerates the incident electrons and then accelerates the backscattered electrons. This makes it possible to generate an intermediate image of the sample.

[0103] In the imaging mode, the diffractive lens 222 is not excited. The third field lens 223 maps the diffraction pattern to the center of the transfer lens 221, which is also the slit face of the prism 25. The transfer lens aligns the center of the third field lens with the slit face of the prism. Achromatic Mapping to a face.

[0104] In the diffraction mode, the diffractive lens 222 is excited along with the other lenses to switch between an image of the sample at the principal plane of the prism 25 and a diffraction pattern. A third field lens 223 maps the diffraction plane to the center of the diffractive lens, which is then transferred by the transfer lens 221 to the principal plane of the prism. Achromatic The diffractive lens maps the image of the sample in the third field lens onto the center of the transfer lens, which coincides with the plane of the slit in the prism.

[0105] Thus, the beam of incident electrons is collimated by the illumination optics, decelerated to a desired arrival energy at the surface of material 55, and scattered at the surface of material 55.

[0106] Electrons backscattered by the sample are accelerated in the opposite direction and conveyed to the beam splitter 25, which again deflects the incoming electron flux by 90°, this time towards detection means 30, with meV energy resolution, for detection.

[0107] In conventional LEEM, the real image of the sample is reflected by the dispersive beam splitter. Achromatic The HREELM as illustrated in the present invention can be switched between two modes. In the image mode, a real image of the sample is projected onto the slit plane of the beam splitter, whereas the slit plane of the splitter projects a diffraction pattern, which typically has an energy dispersion of a few μm / eV and is not suitable for the present invention. Achromatic In diffraction mode, the diffraction pattern of the sample is also reflected by the beam splitter. Achromatic In a LEEM, two lenses are typically sufficient for the source arm of the illumination optics, and one lens is required for the objective arm. In an HREELM as illustrated in this invention, the source arm of the illumination optics includes two additional lenses (field lenses) to meet the requirement of a maximum energy dispersion of 10 meV, and the objective arm of the illumination optics requires three additional lenses to maintain the flexibility of the illumination optics (between the two modes).

[0108] Thus, the electron optics transfers the real image or diffraction pattern of the source to the splitter. Achromatic The LEEM can be placed on any plane, thus allowing the microscope to operate in imaging or diffraction mode without introducing additional aberrations, as is the case with prior art LEEMs.

[0109] The voltage of the pulse supply source and the voltage of the electron optical system are, for example, between 3 kV and 10 kV so as not to impair the spectral resolution of 10 meV, and-6 It is preferable to have a voltage of less than 3 kV, which is generally avoided since it can cause too much aberration. Voltages greater than 10 kV are possible, but are not recommended. -7 This requires a power supply with a high stability and such a power supply is much more expensive.

[0110] The aforementioned paper by M. Mankos et al., “Design for a high resolution electron energy loss microscope”, Ultramicroscopy 207 (2019) 112848, describes in detail an electron optical system suitable for implementing the present invention.

[0111] The backscattered electrons are analyzed and transmitted to a detection means 30 to enable imaging of the surface of the sample.

[0112] At least one imaging optics 23 conveys the backscattered electrons 110 to the detection means 30. The imaging optics 23 include one or more electrostatic or magnetic lenses to generate a visible image on the detection means 30.

[0113] The detection means 30 is advantageously synchronized with the frequency of the pulsed electron source 10 so as to be able to record the full spectrum of electron energy loss during each pulse of electrons.

[0114] In the illustrated example, the detection means consists of an electron time-of-flight detector. They therefore comprise, for example, an electron time-of-flight tube 31, via which the electrons reach a detector 32 which "counts" the electrons. The detector is, for example, a multi-pixel detector, which is able to record an image of at least 500x500 pixels. The imaging device is therefore integrated into the detector.

[0115] This detector device makes it possible to avoid the losses inherent in bandwidth analyzers that scan the detected kinetic energy and record a spectrum.

[0116] The spectral resolution is defined by the potential difference across the tube and the length of the tube, allowing all electrons to be counted within the desired readout time, optimizing the detection efficiency for the primary electrons generated by the source. The desired readout time is typically between 100 ns (e.g. for a detector with a resolution of 100 ps) and 1 μs (e.g. for a detector with a resolution of 1 ns).

[0117] As an example of a detector, a hybrid pixel detector, Timepix4, may be used, which is described in particular in the publication "The design of the Timepix4 chip: a 230 kpixel and 4-side buttable chip with 200 ps on-pixel time bin resolution and 15-bits of TOT energy resolution X". It has a resolution of 512 x 448 pixels (one pixel has dimensions of 55 x 55 μm 2 ), or 6.94 cm 2 Alternatively, the Timepix3, also described in the aforementioned publication, allows a resolution of 256 x 256 pixels, i.e. 1.98 cm 2 Although a maximum of 1.56 nanoseconds can be achieved for a sensing area of ​​1.56 nanoseconds, this may be used.

[0118] The sampling portion is indicated by the reference numeral 50 .

[0119] A sample 55 of the material to be analyzed is placed on the holder 52 .

[0120] The holder 52 is preferably made of molybdenum or any other material capable of limiting the outgassing of said holder under vacuum.

[0121] The energy loss microscope may include a precision goniometer that has five axes (x, y, z, and two tilts) and may optionally be azimuthally rotatable (about the normal to the surface of the sample), which may receive and orient the sample holding system 51.

[0122] The energy loss microscope may include means for heating the sample (not shown), which is preferably capable of heating the sample to temperatures as high as 800°C.

[0123] The energy loss microscope may include means (not shown) for cooling the sample to the temperature of liquid nitrogen, for example by a flow of cryogenic liquid.

[0124] The various embodiments described can be combined with each other.

[0125] Furthermore, the invention is not limited to the embodiments described above, but extends to any embodiment falling within the scope of the claims.

[0126] The energy loss microscope according to the present invention can map vibrational losses by probing a sample with electrons having incident energies between 0 and 20 eV, with an energy resolution of 10 meV, preferably 5 meV, and with nanoscale spatial resolution, typically between 15 and 20 nm.

[0127] The invention (electron source and / or system for performing surface analysis) is particularly applicable to: - Analysis of surface vibrational states, which can play a crucial role in the energy losses of charge carriers in highly efficient photovoltaic systems, - measuring the mobility of charge carriers in semiconductor devices, in particular in transistor channels; - imaging the emergence of superconductivity on the surface of a domain or microdevice by abrupt changes in the phonon spectrum; - Imaging long-range electron-phonon interactions in two-dimensional electronic systems at transition metal surfaces or oxide interfaces, - Optimization of thermoelectric composites and thermal containment via tuning of integrable phonon spectra at the fine scale; - Mapping vibrational states that determine the chemical reactivity of surfaces, Functionalization of molecular layers by electron irradiation and chemical characterization of the effects of irradiation, etc.

Claims

1. 1. A system for performing surface analysis of a material, comprising: a pulsed monochromatic electron source (10) making it possible to form an incident monochromatic electron beam (100); - means (20) for transporting all or a part of said incident electrons (100) to a surface of a sample (55) of material so as to form backscattered electrons (110) and for transporting all or a part of said backscattered electrons (110) to a detection means, said transport means comprising at least one electron optics system, preferably a plurality of electron optics systems; - means (30) for detecting all or part of said backscattered electrons (110), The pulsed monochromatic electron source (10) comprises: A source of atoms (16), at least one continuous wave laser beam (15) configured to form a laser excitation zone (15a) capable of exciting at least some of said atoms to a Rydberg state; - means for generating a pulsed electric field (F) on either side of the laser excitation zone (15a), the pulsed electric field being configured to ionize at least some of the excited atoms and form a monochromatic electron beam (100).

2. 2. The system for performing a surface analysis of materials according to claim 1, wherein the duration of one pulse of said pulsed electric field is comprised between 100 picoseconds and 1 nanosecond.

3. A system for performing a surface analysis of materials according to claim 1 or claim 2, wherein the delay between two pulses is comprised between 0.1 microseconds and 1 microsecond.

4. A system for performing a surface analysis of materials according to any one of claims 1 to 3, wherein the amplitude of the pulsed electric field is comprised between 5 and 50 V / cm.

5. The pulsed electron source of the system for performing surface analysis of the material is about 10 －６ 5. A system for performing surface analysis of materials according to any one of claims 1 to 4, comprising a vacuum chamber pumped to less than millibar, said vacuum chamber configured to accommodate at least said laser excitation zone, said atoms if present in said excitation zone, said pulsed electric field, and said electrons if formed.

6. The pulsed electron source of the system for performing surface analysis of the material comprises at least one ionization electrode (11) arranged upstream or downstream of the laser excitation zone (15a), and the pulsed electric field (F) is applied to the at least one ionization electrode by a pulsed voltage (V １１ 6. The system for performing surface analysis of a material according to claim 1 , wherein the system is formed by applying a voltage of 0.5 V to the surface of the material.

7. 7. The system for performing surface analysis of materials according to claim 1, wherein the pulsed electron source of the system for performing surface analysis of materials comprises at least one extraction electrode, the extraction electrode being arranged upstream or downstream of the laser excitation zone (15a) and configured to generate an electric field for extracting the formed monochromatic electrons.

8. 8. A system for performing a surface analysis of a material according to any one of claims 1 to 7, wherein the pulsed electron source of the system for performing a surface analysis of a material comprises means (13) for collimating and / or focusing the beam of electrons.

9. 9. The system for performing a surface analysis of a material according to any one of claims 1 to 8, wherein the pulsed electron source of the system for performing a surface analysis of a material comprises an effusion cell (161) capable of generating the source (16) of atoms.

10. 10. The system for performing a surface analysis of a material according to any one of claims 1 to 9, wherein the atoms of the source (16) of atoms of the pulsed electron source are selected from the following elements: cesium, lithium, sodium, potassium, rubidium, magnesium, calcium, strontium, barium, chromium, erbium, silver, ytterbium, mercury, helium, neon, argon, krypton, xenon.

11. 11. The system for performing surface analysis of materials according to any one of claims 1 to 10, wherein the one or more continuous wave laser beams (15) are configured to excite at least some of the atoms to identical Rydberg states.

12. 12. The system for performing surface analysis according to any one of claims 1 to 11, wherein the detection means (30) comprises one or more elements selected from a time-of-flight detector, a multi-anode microchannel array detector, an electronic time-of-flight tube, a delay line, an imaging device.

13. about 10 －９ 13. The analytical system of claim 1, further comprising a second vacuum chamber at a pressure below millibar, said second vacuum chamber configured to accommodate at least said pulse source, said transport means and said detection means.

14. 14. The system for performing surface analysis according to any one of claims 1 to 13, wherein the transport means (20) comprises a plurality of electron optical systems (21, 22) and one electron beam splitter (25), the electron optical systems being configured to switch from an image mode, in which a real image of the sample is placed on an achromatic surface of the beam splitter, to a diffraction mode, in which a diffraction pattern of the sample is also placed on the achromatic surface of the beam splitter.

15. The conveying means (20) an electron beam splitter (25), a source arm (21) arranged between said source (10) and said splitter (25), A first field lens (211), A first focusing lens (212); A second field lens (213), and a source arm (21) having a second collecting lens (214); an objective arm (22) arranged between said splitter (25) and said sample (55) of material, A transfer lens (221), Diffractive lens (222), A third field lens (223), and 15. The analysis system of claim 14, further comprising: an objective arm (22) comprising an objective lens (224).

16. A system for performing surface analysis according to any one of claims 11 to 15, which is an electron energy loss microscope.

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