Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement, ion-optical arrangement
  • H01J37/06—Electron sources; Electron guns
  • H01J37/073—Electron guns using field emission, photo emission, or secondary emission electron sources
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement, ion-optical arrangement
  • H01J37/06—Electron sources; Electron guns
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/22—Optical or photographic arrangements associated with the tube
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/244—Detectors; Associated components or circuits therefor
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/26—Electron or ion microscopes; Electron or ion diffraction tubes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/26—Electron or ion microscopes; Electron or ion diffraction tubes
  • H01J37/29—Reflection microscopes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/05—Arrangements for energy or mass analysis
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/06—Sources
  • H01J2237/063—Electron sources
  • H01J2237/06325—Cold-cathode sources
  • H01J2237/06333—Photo emission
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/06—Sources
  • H01J2237/063—Electron sources
  • H01J2237/06325—Cold-cathode sources
  • H01J2237/06341—Field emission
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/06—Sources
  • H01J2237/083—Beam forming
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/244—Detection characterized by the detecting means
  • H01J2237/24475—Scattered electron detectors
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/26—Electron or ion microscopes
  • H01J2237/262—Non-scanning techniques
  • H01J2237/2623—Field-emission microscopes
  • H01J2237/2626—Pulsed source

Definitions

• the present invention lies in the field of surface analysis using electrons.
• the invention relates to a pulsed source of electrons for a surface analysis system.
• the invention relates to a surface analysis system comprising a pulsed source of electrons capable of forming an electron beam with high spectral resolution, that is to say of which the energy dispersion is less than or equal. at 10 meV, and preferably less than or equal to 5 meV.
• the invention also makes it possible to form a collimated electron beam (divergence less than a milliradian) at low energy (electron energy less than or equal to 20 eV) and therefore with a very low beam emittance (dispersion in position and in l phase space of electrons).
• the surface analysis system can be for example a high resolution electron energy loss microscope (designated by “HREELM” for “High Resolution Electron Energy Loss Microscope”), allowing in particular the microscopy of vibrational surface states. .
• HREELM high resolution electron energy loss microscope
• Vibrational surface states are of general importance in both chemistry and surface physics, especially in determining the functionality of a material and its surface reactivity.
• the analysis of vibrational surface states must be carried out at low energy (energy less than or equal to 20 eV) and with high spectral resolution in order to allow the characterization of chemical functions, phonons and plasmons.
• low energy energy less than or equal to 20 eV
• high spectral resolution in order to allow the characterization of chemical functions, phonons and plasmons.
• vibrational surface states with nanometric spatial resolution (typically from a few nanometers to a few tens of nanometers) for a wide range of applications where the measurement and / or modification of the spatial distribution and spectral vibrational states are sought information.
• nanometric spatial resolution typically from a few nanometers to a few tens of nanometers
• Another application relates to the modification of local properties, and in particular, the functionalization of molecular layers by optical or electronic irradiation.
• the analysis of chemical transformations and surface functionality require a very high spectral resolution of the vibrational surface states and a spatial resolution of a few nanometers.
• the use of low-energy electron beams for chemical lithography is also a way towards better control of induced transformations.
• Known analysis solutions have either high spatial resolution or high spectral resolution, or allow matter to be probed in its volume and not at the surface. Among the known solutions, mention may be made of the following three solutions.
• STEM-EELS Electron Energy Loss Spectroscopy
• STEM-EELS Imaging transmission electron microscopy
• STEM-EELS electron energy loss spectroscopy
• STEM-EELS imaging transmission electron microscopy
• Electron Energy Loss Spectroscopy allows to obtain a nanometric spatial resolution and a high spectral resolution. Imaging is performed by scanning an electron beam (at very high energy, that is to say between 30 keV and 300 keV or even up to 1 MeV) over the cross section of the sample, the electrons passing through the surface of the sample to reach its volume.
• STEM-EELS provides information in volume and not in surface.
• LEEM Low Energy Electron Microscopy
• a low-energy electron microscope An example of a low-energy electron microscope is shown in Figure 1.
• high-energy electrons (of the order of 15-20 keV) are emitted by an electron gun 1, then focused using of an electronic optics assembly 2 (electrostatic or magnetic lens system), and sent through a magnetic splitter 3 of electron beams.
• the so-called "fast" electrons are deflected by the separator in the direction of a sample 9 of material, pass through an objective lens 4 and decelerate at low energies (of the order of 1-20 eV) near the surface of the sample 9 due to the maintenance of the sample at a potential V 9 close to the potential Vi of the electron gun 1.
• These low energy electrons are sensitive to the surface.
• the sampling depth near the surface of said sample can be varied by adjusting the energy of the incident electrons. It is defined by the difference between the output work (i.e.
• the elastically backscattered low energy electrons return through the objective lens 4, accelerate again to the electron gun voltage and pass again through the electron beam splitter 3, to be guided through an optic of projection 5 (electrostatic or magnetic lens system) which generates an image made visible on an electron detector 6.
• This makes it possible in particular to observe in real time dynamic surface processes of the material such as: phase transitions, adsorption , reactions, segregation, the growth of thin layers, etching, stress relief, sublimation, magnetic microstructure ...
• the LEEM thus makes it possible to characterize the local output work, the chemistry and the surface crystallinity with a spatial resolution of the order of 15 nm.
• the spatial mapping of the vibrational states of the surface by the energy losses of the electrons is impossible with a LEEM because the energy dispersion of the electron source is typically 250 meV, which is very insufficient to resolve the loss spectra. of energy.
• High-resolution energy loss spectroscopy (referred to as "HREELS” for "High Resolution Energy Electron Loss Spectroscopy”) is an analytical technique that consists of analyzing in energy and direction the electrons scattered by a surface when it- ci is irradiated by a monochromatic electron beam whose energy of a few eV is defined to within a few meV.
• HREELS High-resolution energy loss spectroscopy
• Figure 2 An example of HREELS is shown in Figure 2.
• Sample 9 of material to be analyzed is exposed to a monochromatic electron beam from an electron gun 1 and a first monochromator 7a.
• the electrons are scattered from the surface of Sample 9 exposed to the beam. Scattering is mainly due to the interaction of the Coulomb potential of electrons with the dipole field of the sample surface.
• the measurement of energy losses and their dispersion in a wave vector parallel to the surface can be used to probe most low energy excitations, for example:
• the energy loss is measured by an electron detector 8 placed at the output of a second monochromator 7b and which makes it possible to perform a high spectral resolution spectroscopy of the energy losses of electrons, with a lower energy dispersion. or equal to 10 meV, or even less than or equal to 1 meV under the best conditions.
• the use of monochromators involves a deceleration of the electron beam and a concomitant increase in angular divergence, which reduces the transmission function and significantly increases aberrations.
• the electron beam is divergent and incident on 1 mm 2 of the sample.
• the size of the monochromatic beam implies a loss of intensity of three orders of magnitude, prohibitive for microscopy, and the aberrations resulting from the angular divergence are prohibitive for imaging.
• high-resolution energy loss spectroscopy does not make it possible to obtain spatial imagery at nanometric resolution.
• the invention aims to overcome the aforementioned drawbacks of the prior art.
• the invention aims to have a new source of electrons making it possible to produce a monochromatic electron beam, that is to say the energy dispersion of which is less than or equal to 10 meV, and preferably less than or equal to 5 meV, and without resorting to a monochromator.
• the invention thus aims to control the energy dispersion of electrons in the electron beam formed.
• the invention also aims to have a surface analysis system, such as a high resolution electron energy loss microscope ("HREELM”) integrating such an electron source, with low energy electrons. (less than or equal to 20 eV), making it possible to carry out an analysis of the vibrational surface states of a material, and more precisely to carry out spatial imaging with nanometric resolution and high spectral resolution (energy dispersion is less than or equal to 10 meV, and preferably less than or equal to 5 meV).
• HREELM high resolution electron energy loss microscope
• One device to overcome these drawbacks is a system for analyzing the surface of a material comprising:
• At least one continuous laser beam configured to form a laser excitation zone capable of exciting at least part of said atoms to Rydberg states
• - a means of generating a pulsed electric field on either side of the excitation zone laser, said pulsed electric field being configured to ionize at least part of the excited atoms and form a monochromatic electron beam.
• the electron beam is of low emittance.
• emittance refers to the electron beam and refers to the positional and spatial dispersion of the phases of electrons.
• monochromatic electron beam also called “monocinetic” is meant an electron beam whose energy dispersion of the electrons which compose it is less than or equal to 10 meV, and preferably less than or equal to 5 meV. .
• the source according to the invention allows almost instantaneous ionization and in an area where the voltage varies little spatially, the electrons produced are therefore in the same electric field environment, so the electron beam formed is monochromatic.
• Reniberg state is meant the excited state of an atom from which ionization can occur, having one or more electrons whose principal quantum number n is very high, typically greater than or equal to 20, of preferably greater than or equal to 50.
• pulsed electric field is meant an electric field the amplitude of which varies rapidly and transiently (duration of the pulse) between a base value and a higher or lower value, followed by a rapid return to the base value, and and so on.
• the delay between two pulses is called the repetition rate
• the pulse can be rectangular in shape, in this case the height of the rectangle is the amplitude, the width is the duration of the pulse and the distance between two rectangles is the rate of repetition.
• the pulse can have other shapes, such as a Gaussian or other shapes known to those skilled in the art.
• the monochromatic electron source is a pulsed source, and it is formed by rapid variations (typically less than or equal to one nanosecond) of an electric field (pulsed electric field) around the value of the field d ionization of atoms, the atoms being previously excited by laser beam to Rydberg states.
• the laser excitation is adapted to excite atoms to Rydberg states with a quantum number n high enough so that the value of the electric field to be applied is low and its rapid variations are achievable.
• a time-of-flight detection device whose known detection limit is 1 nanosecond (ns) for most Hybrid pixel detectors, or even 100-200 picoseconds (ps), for certain pixel hybrid detectors or for detectors with an anode microchannel array ("multianode microchannel array detector” in English). English) or even lower for other delay line detectors (“Delay Line Detector” or “DLD” in English).
• a time-of-flight (“Time of Flight” or “ToF”) detection device measures the energy of an electron as a function of the travel time (referred to as “time of flight”) between the departure of the electron (at the level of the electron source) and the arrival (at the level of the detector).
• time of flight a time-of-flight
• Such a detection device comprises a flight tube, to which a potential difference can be applied, through which the electrons reach the detector which "counts" the electrons and measures their arrival times, as do the detectors described above. above and those cited in the detailed description.
• the inventors have identified that more than 10 9 electrons are necessary to form an image. Indeed, for an image of 500 x 500 pixels, the inventors estimated that at least 100 electrons per pixel, losing energy (inelastic scattering) during the interaction with the surface, are necessary to have a spectrum of sufficiently resolved loss. Added to the factor of 1 to 10 between the inelastically scattered electrons and the elastically scattered electrons and a ratio of 1 to 10 between the reflected or backscattered electrons and the incident electrons, this imposes a number of electrons from the source of about 2 , 5.10 9 (500 x 500 x 10 x 10 x 100).
• the invention makes it possible to dispense with the monochromator (s), and thus to limit the losses of electrons, and makes it possible to obtain imaging at high spectral and spatial resolutions, in a reasonable time (which may be a few seconds, a few tens of seconds if we take a source at 10pA or 10 8 electrons per second).
• the system according to the invention makes it possible to obtain imaging at high spectral and spatial resolutions.
• FIG.l shows a low-energy electron microscope of the state of the art.
• FIG. 2 shows a state of the art high resolution energy loss spectroscope.
• FIG. 3 shows a pulsed monochromatic electron source according to the invention.
• FIG. 4 shows an electron energy loss microscope incorporating a pulsed monochromatic electron source according to the invention.
• Figure 3 shows a pulsed monochromatic electron source according to the invention.
• the pulsed monochromatic electron source (energy dispersion less than or equal to 10 meV, and preferably less than or equal to 5 meV) is based on the laser excitation of a beam of atoms towards Rydberg states and the ionization of at least part of the atoms excited by a pulsed electric field so as to form a monochromatic electron beam.
• the illustrated pulsed electron source 10 comprises a first electrode 11 to which a voltage Vu is applied, and a second electrode 12 to which a voltage Vi 2 is applied. Between the first electrode and the second electrode is formed an electric field F designated by “ionization electric field”. On one and / or the other of the first and second electrodes, the applied voltage is pulsed so that the ionization electric field is pulsed.
• a pulsed voltage can be applied to the first electrode 11, then forming by convention “the ionization electrode” and a non-pulsed voltage to the first electrode 12 then forming by convention "the extraction electrode”.
• a pulsed voltage can be applied to the second electrode 12, then forming the ionization electrode, and a non-pulsed voltage to the first electrode 11 then forming the extraction electrode. Still alternatively, a pulsed voltage can be applied to the first and to the second electrode.
• the ionization electric field F is applied on either side of an excitation zone 15a formed by at least one laser beam 15.
• a beam of atoms 16 is directed towards said excitation zone, so that 'at least some of the atoms are excited to Rydberg states, without being ionized.
• the selected Rydberg states are close to the ionization limit. It is the application of the pulsed electric ionization field F on either side of the excitation zone that ionizes the atoms and forms a beam of 100 monochromatic and pulsed electrons.
• the amplitude of the ionization electric field is preferably between 5 and 50 V / cm. Such amplitude values allow electrons to be formed and extracted without inducing chromatic aberration.
• the duration of a pulse is between 100 ps and 1 ns.
• the pulse repetition rate is between 1 MHz (1 ps) and 10 MHz (0.1 ps).
• the 1000 ratio between a rate of 1 ps and an electron pulse of 1 ns also allows a spectral resolution of 1 meV over an energy range of 1 eV.
• the illustrated pulsed source of electrons also comprises a gun lens 13 composed of several electrodes (here three electrodes 13a, 13b and 13c), for example a focusing electrode 13b which can operate at approximately less than 1 kV with respect to the voltage of the electron beam, when it is at about 3 kV and two surrounding electrodes 13a and 13c at earth potential.
• a gun lens also allows the pulsed source of electrons to be coupled to the desired analysis system.
• the laser beam is obtained by a continuous laser, for example a laser diode or a continuous Ti: Sa laser.
• a continuous laser is generally less expensive than a pulsed laser and can excite, on average, and in a resonant manner, a greater number of atoms.
• the laser beam can also be formed by a staging of several lasers.
• the laser power of the last laser stage is greater than or equal to 100 mW for wavelengths between 300 nm and 2000 nm.
• the laser powers of the first, second and third beams are respectively of the order of 10pW for 852 nm, 100pW for 1470 nm and 1W for 780-830 nm.
• the third laser is called the Rydberg excitation laser. It can be an amplified diode laser or a continuous Ti: Sa laser and its wavelength can be monitored using a high precision optical lambdameter.
• the first and second lasers can be diode lasers.
• the average diameters of the laser beams are on the order of ten micrometers, or less, to ensure homogeneity of the laser excitation field.
• the Rydberg states defined here by their principal quantum number n, are preferably between 50 and 100.
• the laser beams are adapted (in particular, by the choice of their emission wavelength) to excite the atoms towards one and the same Rydberg state, characterized by a unique quantum number n.
• n a unique quantum number
• the atom bundle is for example obtained using an effusion cell 161.
• a collimator or an atom focuser (not shown).
• an effusion cell it can be an element such as an alkali metals dispenser.
• the flow of atoms is between 10 11 at / s / mm 2 and 10 13 at / s / mm 2 .
• an atom flux is equal to 10 12 at / s / mm 2 for cesium.
• Suitable atoms are, for example, those of: cesium, lithium, sodium, potassium, rubidium, magnesium, calcium, strontium, barium, chromium, erbium, silver, ytterbium, mercury, helium, neon, argon, krypton, xenon.
• the electron source is preferably confined in a vacuum chamber (not shown) to less than about 10 6 millibars.
• the chamber preferably comprises magnetic shielding, active or passive, for example in mu-metal, in order to eliminate or at least limit the disturbances that a magnetic field can generate on the trajectories of the electrons.
• the pulsed electron source according to the invention can find several applications beyond its integration as a monochromatic source for the HREELM. Besides the application in an electron energy loss microscope, as described later, the pulsed source of electrons according to the invention can be used for other surface analysis systems, which require have a very monocinetic low-energy electron beam (energy dispersion of less than 10 meV or even less) or for structures for functionalization of thin molecular layers by electrons with a precise energy to optimize the chemical reaction to the origin of functionalization.
• the monocinetic source can also be used for electron-atom or electron-molecule collision or diffraction studies.
• the analysis of the electrons is preferably carried out by a time-of-flight (ToF) detection system, the known detection limit of which is of 1 nanosecond (ns) for hybrid pixel detectors, and can go down to 100-200 picoseconds (ps) for detectors with an array of microchannel anodes (“multianode microchannel array detector ”) or more advanced pixel hybrid detectors.
• TOF time-of-flight
• the lateral resolution is to be understood with reference to the surface of the sample which includes structures to be analyzed including the smallest have sizes of the order of 15 nanometers or less, the size of the sample being larger, a few millimeters in diameter or in width.
• G pulse of the electric ionization field F must allow the ionization of atoms in 1 ns, i.e. an ionization rate G, i.e. a number of electrons emitted per second of 10 9 s 1 .
• G pulse of the electric ionization field F must allow the ionization of atoms in 1 ns, i.e. an ionization rate G, i.e. a number of electrons emitted per second of 10 9 s 1 .
• G pulse of the electric ionization field F must allow the ionization of atoms in 1 ns, i.e. an ionization rate G, i.e. a number of electrons emitted per second of 10 9 s 1 .
• This modification in 1 nanosecond of the ionization conditions of atoms requires modifying their energy E.
• h is the reduced Planck constant and is equal to approximately 1.054.10 34 Js
• n is the principal quantum number.
• F the value of F given by the above formula is in atomic unit.
• k is an integer included in - ⁇ n-1) and (n-1).
• This formula also uses atomic units (which are 5.14.10 11 V / m for the field, 2.4.10 17 s for time and 4.36.10 18 J for energy). In other words, the electric field must undergo a variation of at least dF for the Rydberg atom to be "destabilized".
• the typical size of the dz laser excitation zone is on the order of 1 Opm.
• Figure 4 shows a high resolution HREELM electron energy loss microscope according to the invention.
• the energy loss microscope shown includes:
• a pulsed source 10 as illustrated in FIG. 3 and making it possible to form a beam 100 of incident monochromatic and pulsed electrons;
• means 20 for conveying all or part of the incident electrons towards the surface of a sample of material 55, so as to form backscattered electrons 110, and all or part of the backscattered electrons towards the detection means 30, said conveying means comprising a plurality of electronic optics;
• the means 30 for detecting all or part of the backscattered electrons 110 is the means 30 for detecting all or part of the backscattered electrons 110.
• the illustrated routing means include a plurality of electronic optics, which are typically electrostatic or magnetic lenses.
• the term lens therefore denotes an electrostatic or magnetic lens.
• the conveying means 20 comprise an electron beam splitter 25, said splitter being able to deflect the incident electrons by 90 ° towards the surface of the sample 55, to separate backscattered electrons 110 from the incident electrons 100 and to deflect said electrons backscattered towards the detection means 30.
• the separator 25 comprises for example a magnetic prism which deflects the electron beam by 90 °.
• a set of electronic optics designated by "illumination optics", transports the incident electrons between the source 10 and the separator 25 and between the separator 25 and the surface of the sample 55, and the electrons backscattered between the surface of sample 55 and separator 25.
• the illumination optic comprises two arms: the source arm 21, which transports the incident electrons between the source 10 and the separator 25; and the objective arm 22 which transports the incident electrons between the separator 25 and the surface of the sample 55, and the electrons backscattered between the surface of the sample 55 and the separator 25 (to send them to the detection means 30) .
• the optics of the source arm 21 illustrated comprises a series of four lenses, arranged one after the other in the following order, indicated according to the direction of the incident electrons (that is to say from the source to the separator ):
• a field lens makes it possible to recreate the image of the electron source (also referred to as "source” in this description).
• a condenser lens helps maintain source collimation.
• This optics of the source arm makes it possible to illuminate the surface (which may be designated by “plane” in the remainder of the present description) of the sample 55 with a collimated and uniform electron beam and to obtain an optical zoom of. 10 ⁇ , that is to say for a field of view (“FoV” for “Field of View”) of the sample which varies between 10 and 100 ⁇ m.
• Uniformity of the beam at the plane of the sample is obtained by mapping the uniform source profile at the intersection on said plane.
• Beam collimation is achieved by mapping the Gaussian point of the source image to the back focal plane (diffraction) of the objective lens, which results in a small divergence angle at the sample.
• a single condenser lens may suffice, while two condenser lenses are required for a 10 ⁇ m field of view.
• the optics of the objective arm 22 illustrated comprises a series of four lenses, arranged one after the other in the following order, indicated according to the direction of the incident electrons (that is to say from the separator towards the sample) :
• the transfer lens 221 makes it possible to transfer the source (incident electrons 100) from the separator 25 to the plane of the sample, and to return the backscattered electron beam 110 to the separator 25.
• the diffraction lens 222 also makes it possible to transfer the source (incident electrons 100) from the separator 25 to the plane of the sample, and to return the backscattered electron beam 110 to the separator 25, but can delay or accelerate the electrons. .
• the objective lens 224 and the surface of the sample form an immersion lens.
• the immersion lens is configured to slow down the electrons and illuminate the sample with a low energy electron beam (0-100 eV): for this, a high negative voltage is applied to the sample to decelerate the incident electrons then accelerate backscattered electrons. It makes it possible to produce an intermediate image of the sample.
• the diffraction lens 222 is not excited.
• the third field lens 223 maps the diffraction pattern at the center of the transfer lens 221, which is also the plane of the slit of the prism 25; and the transfer lens maps the center of the third field lens achromatically to the prism.
• the diffraction lens 222 is excited with the other lenses in order to switch between the sample image and the diffraction pattern in the principal planes of the prism 25.
• the third field lens 223 maps the diffraction plane. at the center of the diffractive lens, which is then mapped by transfer lens 221 on the achromatic plane of the prism; and the diffractive lens maps the image of the sample in the third field lens to the center of the transfer lens, which coincides with the plane of the prism slit.
• the incident electron beam is collimated by the illumination optics, and decelerated to the desired landing energy at the surface of the material 55 where it is diffused.
• the electrons backscattered by the sample are accelerated in the opposite direction and transported to the beam splitter 25.
• the beam splitter deflects the incoming flow of electrons again by 90 °, this time to the detection means 30, with a energy resolution in meV for detection.
• the real image of the sample is transferred to the achromatic plane of a dispersive beam splitter, while the diffraction pattern is transferred to the slit plane of the splitter, which has an energy dispersion generally of a few pm / eV, which is not suitable for the invention.
• the HREELM microscope of the invention as illustrated is capable of switching between the two modes: in the image mode, the real image of the sample is placed in the achromatic plane of the beam splitter, and in the diffraction mode, the diffraction pattern of the sample is also placed in the achromatic plane of the beam splitter.
• the source arm of the illumination optics comprises two additional lenses (field lenses) to respect the energy dispersion of 10 meV at most, and the objective arm of Illumination optics require three additional lenses to maintain flexibility of illumination optics (between the two modes).
• the electronic optics are thus adapted to be able to place the real image or the diffraction pattern of the source in the achromatic plane of the separator and thus allows the microscope to operate in imaging mode or in diffraction mode, without adding aberrations such as c This is the case in a LEEM of the state of the art.
• the voltage of the pulsed source and that of the electronic optics are for example between 3 kV and 10 kV with a stability of 10 6 so as not to compromise the spectral resolution of 10 meV.
• a voltage lower than 3 kV is generally avoided because it can generate excessive aberrations.
• a voltage greater than 10 kV is possible but would require power supplies with a stability of 10 7 , which are much more expensive.
• the backscattered electrons are sent to the detection means 30 in order to be analyzed and to form an image of the surface of the sample.
• At least one imaging optic 23 transports the backscattered electrons 110 to the detection means 30.
• the imaging optics 23 comprises one or more electrostatic or magnetic lenses, so as to generate an image made visible on the detection means 30. .
• the detection means 30 are advantageously synchronized with the frequency of the pulsed electron source 10, to make it possible to record the entire spectrum of the energy losses of the electrons during each electron pulse.
• the detection means consist of an electron time-of-flight detector.
• the detection means comprise, for example, an electron time-of-flight tube 31 through which the electrons reach a detector 32 which "counts" the electrons.
• the detector is for example a multi-pixel detector, which makes it possible to record an image of at least 500 x 500 pixels.
• the imaging device is integrated into the detector.
• This detector device avoids the inherent losses of a bandwidth analyzer which scans the detected kinetic energy to record a spectrum.
• the spectral resolution is defined by the potential difference on the tube and its length, allowing all electrons to be counted in a desired reading time, optimizing the detection efficiency compared to the primary electrons generated by the source.
• the desired reading time is typically between 100 ns (for example for a detector with 100 ps of resolution) and lps for example for a detector with 1 ns of resolution).
• the Timepix4 can be used, which is a pixel-based hybrid detector. It 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! 5-bits ofTOT energy resolution X”. It makes it possible to reach 195 picoseconds for 512 x 448 pixels (a pixel measuring 55 x 55 ⁇ m 2 ), ie a sensitive area of 6.94 cm 2 . Alternatively, one can use the Timepix3 which is also described in the aforementioned publication but which makes it possible to achieve only 1.56 nanoseconds for 256 ⁇ 256 pixels, ie a sensitive area of 1.98 cm 2 . The sampling part is designated by the reference 50.
• the sample of material 55 to be analyzed is placed on a support 52.
• the support 52 is made of molybdenum or any other material which makes it possible to limit the degassing of said support under vacuum.
• the energy loss microscope can include a precision five axis goniometer (x, y, z, 2 tilts) with optional azimuthal rotation (around the normal to the sample surface). This goniometer receives the sample holder system 51 and allows it to be oriented.
• the energy loss microscope may include a means for heating the sample, preferably adapted to heat the sample to 800 ° C (not shown).
• the energy loss microscope may include means for cooling the sample, for example by circulating cryogenic fluid, to the temperature of liquid nitrogen (not shown).
• the energy loss microscope according to the invention is capable of mapping the vibrational losses with an energy resolution of 10 meV, and preferably 5 meV, and a nanometric spatial resolution, typically from 15 to 20 nm, by probing the sample with electrons with an incident energy of 0 to 20 eV.
• the invention (electron source and / or surface analysis system) finds applications in particular for:
• thermoelectric composite materials and heat confinement by adjusting the spectrum of integrable phonons at microscopic scales

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• Analysing Materials By The Use Of Radiation (AREA)
• Electron Sources, Ion Sources (AREA)

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• 2020
  • 2020-05-05 FR FR2004444A patent/FR3110026B1/fr active Active
• 2021
  • 2021-04-28 EP EP21720785.1A patent/EP4147263A1/fr active Pending
  • 2021-04-28 WO PCT/EP2021/061068 patent/WO2021224079A1/fr unknown
  • 2021-04-28 US US17/921,587 patent/US20230170176A1/en active Pending
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