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] This 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 present invention relates to a system for performing surface analysis, comprising a pulsed electron source capable of forming a high spectral resolution electron beam, i.e., an electron beam having an energy dispersion of 10 meV or less, preferably 5 meV or less.

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

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

[0006] Surface vibrational states are generally important in both surface physics and surface chemistry, and are particularly useful for determining the functionality of materials and the reactivity of their surfaces. Analysis of surface vibrational states must be performed at low energies (below 20 eV) and with high spectral resolution to enable the determination of chemical properties, phonon characteristics, and plasmon properties. Therefore, it is possible to predict and / or determine the relationship between the nature of a material's interaction 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 state of a surface with nanoscale spatial resolution (typically from a few nanometers to tens of nanometers). Non-limiting examples of the importance of these analyses include semiconductor devices in which 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 in which the interaction between charge carriers and phonons within the system may be unfavorable, as this may result in non-radiative losses compared with the desired purely radiative losses.

[0008] Another application relates to the modification of local properties, particularly the functionalization of molecular layers by optical or electron irradiation. Analysis of chemical changes and surface functionality requires very high spectral resolution of surface vibrational states and spatial resolution of several nanometers. Furthermore, using low-energy electron beams in chemical lithography is one method that advances the ability to better control induced changes.

[0009] Known solutions for performing the analysis either have high spatial resolution or high spectral resolution, or allow for probing the bulk rather than the surface of the material. Among the known solutions, the following three may be mentioned.

[0010] STEM-EELS (an acronym for Scanning Transmission Electron Microscope-Electron Energy Loss Spectroscopy) enables 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 a cross-section of the sample, with the electrons passing through the sample surface to reach the bulk. Therefore, STEM-EELS provides information about the bulk rather than the surface properties. Furthermore, excellent alignment, mechanical stability, and electrical stability are required to correct chromatic aberration and achieve very low counting rates, resulting in very high construction costs (millions of euros, or even tens of millions of euros). Finally, sample preparation is complex, and STEM-EELS is a destructive method because it requires cutting a portion of the sample using a FIB (an acronym for Focused Ion Beam). In conclusion, STEM-EELS cannot be used to investigate surface properties, is destructive, and has significant drawbacks. STEM-EELS is not suitable for performing high spatial resolution analysis of surface vibration states with high spectral resolution.

[0011] LEEM (an acronym for Low-Energy Electron Microscopy) is a technique for performing surface analysis, typically used to acquire images of material surfaces, atomic-surface interactions, and / or thin films. An example of a low-energy electron microscope is shown in Figure 1. In LEEM, high-energy (approximately 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 magneto-electron beam splitter 3. The electrons, referred to as "high-speed," are deflected by the splitter towards the material sample 9, pass through the objective lens 4, and are decelerated to low energy (approximately 1-20 eV) near the surface of the sample 9, because 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 sample surface can be modified by adjusting the energy of the incident electrons. The aforementioned depth is determined by the difference between the work function of the sample (i.e., the minimum energy, measured in eV units, required to pull an electron from the Fermi level of an atom to a point infinitely outside the atom) and the work function of the electron gun, plus a value called an initiation voltage of several eV. The initiation voltage roughly defines the kinetic energy of the electron relative to the surface potential of the sample, and by adjusting the initiation voltage, the electron can be given 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 through the electron beam splitter 3 again, and are guided through the projection optical system 5 (a set of electrostatic or magnetic lenses) that generates a visible image on the electron detector 6. This makes it possible, in particular, to observe dynamic surface processing of materials in real time, such as phase transitions, adsorption, reactions, separation, thin layer growth, etching, stress release, sublimation, and magnetic microstructure.

[0012] Therefore, LEEM makes it possible to determine the local work function and the characteristics of the surface crystallinity and chemical properties with a spatial resolution of about 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 usually 250 meV, which is too high to enable the resolution of the energy loss spectrum.

[0013] HREELS (an acronym for High-Resolution Electron Energy-Loss Spectroscopy) is a technique for performing analysis that involves analyzing the energy and direction of electrons scattered by a surface when the surface is irradiated with a monochromatic electron beam, defined as a monochromatic electron beam with an energy of a few eV within a few meV. An example of HREELS is shown in Figure 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. The electrons are scattered by the surface of the sample 9 exposed to the beam. The scattering is mainly due to the interaction between the electron's Coulomb potential and the dipole field of the sample surface. By measuring the energy loss and their wave vector dispersion parallel to the surface, it becomes possible to probe most energy excitations, for example, as follows: -Surface phonons, - Vibrational modes of isolated atoms or molecules, - Surface and interface plasmons, - In particular, low-energy electron excitations resulting from states within the band gap.

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

[0015] However, the use of a monochromator implies deceleration of the electron beam and an accompanying increase in angular divergence, which results in a reduction of the transmission function and a substantial increase in aberrations. Thus, the electron beam diverges and impinges on the sample at 1 mm. 2 The size of the monochromatic beam implies a three-digit loss of intensity, which hinders microscopy, and the aberrations resulting from angular divergence also hinder imaging. Thus, in high-resolution energy-loss spectroscopy, spatial imaging with nanoscale resolution cannot be achieved.

[0016] Therefore, in the prior art, spatial imaging combining nanoscale resolution and high spectral resolution cannot be achieved.

[0017] U.S. Patent Application Publication No. 2008 / 173829 discloses an electron source intended particularly for use in low-energy electron microscopy. This electron source uses a pulsed laser to generate an interfering beam of waves in rubidium atoms, and these beams are then ionized via the electric field effect by a pulsed electric field synchronized with the laser. This source generates, on average, one electron per laser pulse and thus has a very low current, less than 1 picoampere (pA).

[0018] U.S. Patent No. 4,414,509 discloses a low-energy magnetometer having an electron source based on the field-effect ionization of rubidium atoms generated 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. This paper also mentions the possibility of using pulsed electric fields, but only for the purpose of improving the accuracy of energy measurements of long-lived Rydberg states. On this subject, the paper refers to "Field-Ionization Processes in Excited Atoms" by MGLittman, 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., Ultramicroscopy207(2019)112848, discloses several energy loss microscope configurations that can utilize a continuous electron source based on the field-effect ionization of Rydberg atoms.

[0021] The objective of this invention is to overcome the aforementioned drawbacks of the prior art.

[0022] More specifically, the object of the present invention is to provide a novel electron source that enables the generation of a monochromatic electron beam, i.e., a monochromatic electron beam with an energy dispersion of 10 meV or less, preferably 5 meV or less, without the use of a monochromator. Therefore, the object of the present invention is to control the energy dispersion of electrons in the formed electron beam.

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

[0024] [Patent Document 1] U.S. Patent Application Publication No. 2008 / 173829 [Patent Document 2] U.S. Patent Application 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 [Overview of the project] [Means for solving the problem]

[0026] A device that can overcome these shortcomings is a system for performing surface analysis of materials. - A pulsed monochromatic electron source that enables the formation of a monochromatic incident electron beam, - A means for transporting all or part of incident electrons to the surface of a material sample and for transporting all or part of the backscattered electrons to a detection means, wherein the transport means includes at least one electron optical system, preferably a plurality of electron optical systems. - A system including means for detecting all or part of backscattered electrons, A pulse monochromatic electron source is, - Source of atoms, - At least one continuous-wave laser beam configured to form a laser excitation zone capable of exciting at least some of the aforementioned atoms to the Rydberg state, - A system comprising means for generating pulsed electric fields on both sides of a laser excitation zone, wherein the pulsed electric fields are configured to ionize at least some of the excited atoms to form a monochromatic electron beam.

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

[0028] The term "monochromatic" electron beam (this type of beam is also called a "single-energy" electron beam) refers to an electron beam in which the energy dispersion of electrons is 10 meV or less, preferably 5 meV or less.

[0029] To obtain a monochromatic beam, electrons must be generated at substantially the same voltage. Here, the source according to the present invention enables almost instantaneous ionization in a zone where the voltage hardly changes spatially. Therefore, since electrons are generated in the same electric field environment, the resulting electron beam is monochromatic.

[0030] The term "Rydberg state" refers to an excited atomic state in which ionization can occur, and this state contains one or more electrons with a very high principal quantum number n, usually 20 or more, preferably 50 or more.

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

[0032] According to the present invention, a monochromatic electron source is a pulsed source, which is formed by rapidly changing an electric field (pulsed electric field) around an electric field intensity required to ionize atoms pre-excited to a Rydberg state by a laser beam (the change is typically 1 nanosecond or shorter). The laser excitation is configured to excite atoms to a Rydberg state with quantum number n, where the quantum number n is sufficiently high to allow for a low and rapid change in the electric field intensity that needs to be applied.

[0033] To achieve an energy dispersion of less than 10 meV for the electron beam, on average, 10 per second 8 ~10 9It is necessary to supply electrons between 10 and 100 picoamperes (pA), which corresponds to an average current between 10 and 100 picoamperes (pA). Above this, i.e., the number of electrons per second increases (e.g., 1 nanoampere (nA), which corresponds to 10 per second). 10 If the number of electrons exceeds a certain number (equivalent to more than 100), the spatial charge effect reduces the spectral resolution, making it difficult or even impossible to obtain a monochromatic electron beam. Furthermore, if the number of electrons falls below this level, it becomes insufficient to achieve imaging within a reasonable time.

[0034] 10 per second 8 ~10 9 At these scales of individual electrons, it is preferable to perform electron analysis using time-of-flight detection devices, as the known detection limit for most hybrid pixel detectors of this type is 1 nanosecond (ns), and the detection limit can 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] It should be recalled that time-of-flight (ToF) detection devices measure the energy of an electron over the time it takes for the electron to travel from its departure (from the electron source) to its arrival (at the detector). As with the detector described above, and as with the detectors mentioned in the embodiments for carrying out the invention, such a detection device comprises a flight tube, to which a potential difference may be applied, allowing electrons to travel through the flight tube to the detector, which "counts" the electrons and measures the time of arrival of the electrons.

[0036] When using a continuous electron source, even if the electron beam from the source is monochromatic, after the electrons collide with the sample and are backscattered, they will no longer remain monochromatic. In order to analyze the energy distribution of the backscattered electrons, it is necessary to sequentially scan the electron energy in a continuous flow. Therefore, for example, it is necessary to select the electron energy range (spectral band), such as by placing a monochromator in the path of the backscattered electrons. By selecting this narrow electron spectral band, most of the electrons generated by the source will be lost, and this loss can reach 100 times. Therefore, when using a 100 pA electron source, only 7 10 electrons (backscattered and selected by the monochromator) per second can be obtained.

[0037] However, in order to enable nanoscale spatial imaging, the inventors have identified that more than 10 9 electrons are required to form an image. Specifically, for a 500×500 pixel image, the inventors estimated that at least 100 electrons per pixel are required to lose energy (inelastically scatter) during the interaction with the surface in order to achieve a sufficiently resolved loss spectrum. Assuming a magnification 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, the source needs to generate approximately 2.5×10 9 (500×500×10×10×100) electrons.

[0038] Therefore, when using a continuous electron source, in order to achieve imaging with high spatial resolution, it is necessary to wait for at least 250 seconds, which is not practical. In addition, during this image acquisition time, noise may distort the results displayed in the image.

[0039] Conversely, if it is desirable to increase the number of electrons to account for this loss at 100x magnification and to shorten the imaging time, the average current of the electron source must be increased. However, beyond 100 pA, the spectral resolution decreases due to the space charge effect.

[0040] Therefore, the present invention makes it possible to eliminate the need for one or more monochromators, thereby limiting electron loss and also within a reasonable time (this is from a few seconds, i.e., 10 pA per second in the case of a 10 pA supply source). 8 This enables imaging with high spectral and spatial resolution (which can take several tens of seconds in the case of individual electrons).

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

[0042] Furthermore, the present invention enables both microscopic and spectroscopic analysis to be performed using a single system for surface analysis, thereby avoiding the transfer of the material to be analyzed between various systems, and thus avoiding the loss of measurement reproducibility and adverse effects on measurement and analysis times. Therefore, by providing a single system capable of performing both microscopic and spectroscopic analysis, measurement and analysis times are shortened while improving the reliability of the analysis.

[0043] Other features and advantages of the present invention will become apparent from the following non-limiting description, given as an example with reference to the accompanying figures. [Brief explanation of the drawing]

[0044] [Figure 1] This shows a low-energy electron microscope using conventional technology. [Figure 2] This shows a high-resolution energy-loss spectrometer using conventional technology. [Figure 3] This shows a pulse monochromatic electron source according to the present invention. [Figure 4] This shows an electron energy loss microscope incorporating the pulsed monochromatic electron supply source according to the present invention. [Modes for carrying out the invention]

[0045] Figures 1 and 2 have already been explained in the chapter on "Background Technology," so they will not be explained again here.

[0046] Figure 3 shows a pulse monochromatic electron supply source according to the present invention.

[0047] A pulsed monochromatic electron source (with an energy dispersion of 10 meV or less, preferably 5 meV or less) is 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, so as to form a monochromatic electron beam.

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

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

number

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

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

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

[0053] An ionization electric field F is applied to both sides of the excitation zone 15a formed by at least one laser beam 15. The atomic beam 16 is directed towards the aforementioned excitation zone, and as a result, at least some of the atoms are excited to the Rydberg state without being ionized. The selected Rydberg state is close to the ionization limit. It is the application of the pulsed ionization electric field F to both sides of the excitation zone that enables the ionization of the atoms and forms a monochromatic, pulsed electron beam 100.

[0054] The amplitude of the ionization electric field is preferably between 5 and 50 V / cm. Such an amplitude allows for the formation and extraction of electrons without causing chromatic aberration.

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

[0056] The pulse repetition rate is preferably 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 enables a spectral resolution of 1 meV over an energy range of 1 eV.

[0057] The illustrated pulsed electron source also includes a gun lens 13 composed of multiple electrodes (here, three electrodes 13a, 13b, and 13c), for example, a focusing electrode 13b which may operate at a voltage approximately 1kV lower than the electron beam voltage when the electron beam voltage is approximately 3kV, and two ambient electrodes 13a and 13c which are together at ground potential. In addition to focusing and collimating the electron beam, the gun lens also allows the pulsed electron source to be coupled with a desired analysis system.

[0058] The laser beam is obtained 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 less expensive than pulsed lasers and, on average, can excite a larger number of atoms in a resonant manner.

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

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

[0062] For example, the laser outputs of the first beam, second beam, and third beam (in this case, the beam of the final laser stage) are approximately 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. This may 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 may be diode lasers.

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

[0064] The Rydberg state defined by the principal quantum number n is preferably located between 50 and 100.

[0065] The laser beam is preferably configured to excite atoms to identical Rydberg states characterized by a single quantum number n (particularly through the selection of the laser beam's emission wavelength). This allows for better atomic excitation efficiency, and one or more wavelengths of one or more laser beams can be precisely tuned to one or more target atomic transitions. This also makes it possible to maximize the monochromaticity of the electron beam obtained by the ionization of Rydberg atoms.

[0066] The atomic beam can be obtained, for example, using an effusion cell 161.

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

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

[0069] The flux of an atom is 10 11 at / s / mm 2 ~10 13 at / s / mm 2 It is preferable that it be included between them.

[0070] For example, 10 12 at / s / mm 2 A flux of atoms equivalent to can sometimes be obtained using cesium.

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

[0072] The electron supply source is approximately 10 -6 The electrons are preferably confined within a chamber (not shown) pumped to less than millibars. The chamber is preferably equipped with an active or passive magnetic shield, such as a mu-metal shield, to avoid or at least limit the effects that the magnetic field may otherwise have on the electron paths.

[0073] The pulsed electron source according to the present invention may have several applications beyond a monochromatic source in HREELM. In addition to its use in electron energy loss microscopy, as will be described in more detail below, the pulsed electron source according to the present invention may be used in other systems for performing surface analysis that require a very 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 chemical reactions responsible for functionalization. Monochromatic or single-energy sources may also be used in studies of electron-atom or electron-molecular collisions or diffraction.

[0074] Examples of characteristics of pulsed monochromatic electron sources As described above, in order to achieve a spectral resolution of 10 meV and a lateral spatial resolution higher than 15 nm, an average of 10 per second is required. 8 ~10 9 It is necessary to supply a number of electrons, which corresponds to an average current between 10 and 100 picoamperes (pA), and 10 per second. 8 ~10 9At these scales of individual electrons, electron analysis is preferably performed by time-of-flight (ToF) detection systems, because the known detection limit of this type of hybrid pixel detector is 1 nanosecond (ns), while multi-anode microchannel array detectors or more advanced hybrid pixel detectors can reach even 100-200 picoseconds (ps). Examples of such detectors are shown below. The lateral resolution should be understood as being relative to the surface of the sample, where the sample contains the structure to be analyzed, the smallest of which is about 15 nanometers or less in size, and the sample is larger than that (the sample is several millimeters in diameter or width).

[0075] Considering the detection limit of 1 ns, the inventors estimated that the ionization electric field should have a pulse duration of 1 ns. In other words, the pulse of the ionization electric field F needs to be able to ionize an atom in 1 ns, i.e., 10 9 s -1 This ionization rate Γ, that is, the number of electrons emitted per second, must be achieved. In order to change the ionization state of an atom in 1 nanosecond, a change in the atom's energy E is necessary. This change in the beam's atomic energy dE is due to the finite lifetime of the Rydberg state and is given by the following equation.

number

number

[0076] Therefore, the ionization electric field F is the electric field that ionizes the atom, and it must change rapidly (1 ns) around the value of the electric field directed towards the Rydberg state by dF.

number

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

number

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

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

[0080] Next, the maximum ionization electric field F can be estimated from this equation as follows:

number

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

number

[0082] The change in the ionization electric field dF can be estimated from relation (3), assuming that k is at its maximum as follows. k=n-1 (8)

[0083] Therefore, the following is obtained:

number

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

[0085] Therefore, if D is equal to 1 mm, the value is only 0.3 mV, or if D is equal to 4 millimeters, the value is 1.2 mV, and the voltage V 11 1 ns is sufficient to make the change.

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

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

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

[0089] The illustrated transport means includes a plurality of electron optical systems, which are typically electrostatic or magnetic lenses. Therefore, in this specification, the term "lens" refers to an electrostatic or magnetic lens.

[0090] The transport means 20 includes an electron beam splitter 25, which can deflect the incident electrons by 90° toward the surface of the sample 55, separate the backscattered electrons 110 from the incident electrons 100, and deflect the backscattered electrons toward the detection means 30.

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

[0092] The electron optical system, called the "illumination optical system," transports incident electrons between the supply source 10 and the splitter 25, and between the splitter 25 and the surface of the sample 55, and also transports backscattered electrons between the surface of the sample 55 and the splitter 25.

[0093] The illumination optical system 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 (aiming to send the backscattered electrons to the detection means 30).

[0094] The optical system of the illustrated source arm 21 includes a series of four lenses, which are arranged in the following order in the direction of the incident electrons (i.e., from the source to the splitter): -First field lens 211, -First condensing lens 212, -Second field lens 213, -Second condensing lens 214.

[0095] The field lens allows for the reproduction of an image of the electron source (also referred to herein as the “source”). The focusing lens allows for the maintenance of collimation of the source.

[0096] The optical system of the source arm illuminates the surface of sample 55 (which may be referred to as the “plane” in the remainder of this specification) with a collimated, uniform electron beam, enabling a 10× optical zoom, i.e., obtaining a sample FoV (Field of View) that varies between 10 and 100 μm. The uniformity of the beam in the plane of the sample is obtained by mapping the profile of a uniform source onto the aforementioned plane at its intersection. Beam collimation is obtained by mapping the Gaussian point of the source image onto the back focal plane (diffraction) of the objective lens, resulting in a small angular divergence in the sample.

[0097] For a field of view of 100 μm, a single focusing lens may suffice, but for a field of view of 10 μm, 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, what is imaged onto the analyzer is the diffraction pattern of the sample surface at the back focal plane of the objective lens. Regardless of which plane is selected, it must first be imaged onto the achromatic surface of the prism, which is substantially at the center of the prism. Otherwise, the dispersion imparted to the beam by the prism will introduce an energy-dependent shift into the detected image, pushing the beam outward from the center of the analyzer.

[0099] The optical system of the illustrated objective arm 22 includes a series of four lenses, which are arranged in the following order in the direction of the incident electrons (i.e., from the splitter toward the sample): -Transmission lens 221, -Diffraction lens 222, -Third field lens 223, - Objective lens 224.

[0100] The transmission lens 221 allows the supply source (incident electrons 100) to be carried from the splitter 25 to the plane of the sample and the beam of backscattered electrons 110 to be returned to the splitter 25.

[0101] The diffraction lens 222 also allows the source (incident electrons 100) to be carried from the splitter 25 to the plane of the sample and the beam of backscattered electrons 110 to be returned to the splitter 25, but it may delay 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 electrons and irradiate the sample with a beam of low-energy electrons (0-100 eV). To do this, a very negative voltage is applied to the sample to decelerate the incident electrons, and then accelerate the backscattered electrons. This makes it possible to generate an intermediate image of the sample.

[0103] In image mode, the diffraction lens 222 is not excited. The third field lens 223 maps the diffraction pattern to the center of the transmission lens 221, which is also the slit surface of the prism 25, and the transmission lens maps the center of the third field lens to the achromatic surface of the prism.

[0104] In diffraction mode, the diffraction lens 222 is excited along with the other lenses so as to switch between the image of the sample and the diffraction pattern in the main plane of the prism 25. The third field lens 223 maps the diffraction plane to the center of the diffraction lens, which is then mapped onto the achromatic surface of the prism by the transmission lens 221. The diffraction lens maps the image of the sample in the third field lens onto the center of the transmission lens, which coincides with the plane of the prism slit.

[0105] Therefore, the incident electron beam is collimated by the illumination optics, slowed down 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 carried to the beam splitter 25. The beam splitter then deflects the flux of the arriving electrons again by 90°, this time towards the detection means 30, with an energy resolution of meV, for detection.

[0107] In conventional LEEMs, the real image of the sample is carried to the achromatic surface of a dispersive beam splitter, while the diffraction pattern is carried to the slit surface of the splitter, which generally has an energy dispersion of several μm / eV and is not suitable for the present invention. The HREELM illustrated in the present invention can switch between two modes. In imaging mode, the real image of the sample is placed within the achromatic surface of the beam splitter, and in diffraction mode, the diffraction pattern of the sample is also placed within the achromatic surface of the beam splitter. In LEEMs, two lenses are generally sufficient for the source arm of the illumination optics, and one lens is required for the objective arm. In the HREELM illustrated in the present 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] Therefore, the electron optical system can position the real image or diffraction pattern of the source on the achromatic surface of the splitter, thus allowing the microscope to operate in imaging or diffraction mode without adding aberrations as in the case of conventional LEEMs.

[0109] The voltage of the pulse source and the voltage of the electron optics are, for example, between 3kV and 10kV so as not to impair the spectral resolution of 10meV. -6Stability is preferable. Voltages below 3kV are generally avoided because they can cause excessively large aberrations. Voltages greater than 10kV are possible, but 10 -7 A power supply unit with high stability is required, and such a power supply unit is far more expensive.

[0110] The aforementioned paper by M. Mankos et al., "Design for a high resolution electron energy loss microscope," Ultramicroscopy207(2019)112848, describes in detail an electron optical system suitable for carrying out the present invention.

[0111] The backscattered electrons are sent to the detection means 30 for analysis and to enable imaging of the sample surface.

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

[0113] The detection means 30 is also advantageously synchronized with the frequency of the pulsed electron source 10 so that it can record the entire spectrum of electron energy loss during the duration of each electron pulse.

[0114] In the illustrated example, the detection means consists of an electron time-of-flight detector. Thus, the detection means includes, for example, an electron time-of-flight tube 31 through which electrons reach a detector 32 that "counts" the electrons. The detector is, for example, a multi-pixel detector capable of recording an image of at least 500 × 500 pixels. Therefore, an imaging device is incorporated 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 the spectrum.

[0116] Spectral resolution is defined by the potential difference across the tube and its length, enabling the counting of all electrons within a desired readout time and optimizing detection efficiency for 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, the Timepix4 hybrid pixel detector may be used. The Timepix4 is specifically described 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 512 × 448 pixels (1 pixel dimensions: 55 × 55 μm). 2 ), that is, 6.94 cm 2 This allows for a detection range of up to 195 picoseconds. Alternatively, the Timepix3, also mentioned in the aforementioned publication, has a resolution of 256 x 256 pixels, or 1.98 cm². 2 Although it can only reach a maximum of 1.56 nanoseconds in the sensing range, this can be used.

[0118] The sampling portion is indicated by 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 that can restrict gas release from the aforementioned holder under vacuum.

[0121] The energy loss microscope may include a precision goniometer having five axes (x, y, z, and two tilts) and which may be optionally rotatable in the azimuthal direction (around the normal to the surface of the sample). This goniometer may receive and orient a sample holding system 51.

[0122] An energy loss microscope may include means (not shown) for heating the sample, preferably capable of heating the sample to a high temperature of 800°C.

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

[0124] The various embodiments described can be combined with one another.

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

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

[0127] The present invention (electron source and / or system for performing surface analysis) is particularly applicable to the following: - In particular, analysis of surface vibration states can play a crucial role in the energy loss of charge carriers in high-efficiency photovoltaic systems. - Measurement of charge carrier mobility in semiconductor devices, particularly in transistor channels. - Imaging of the emergence of superconductivity on the surface of a domain or microdevice due to abrupt changes in the phonon spectrum. - Imaging of long-range electron-phonon interactions in two-dimensional electronic systems at the surface or oxide interface of transition metals. - Optimization of thermoelectric composite materials and thermal containment through tuning of integrable phonon spectra at the microscale. - Mapping of vibrational states that determine the chemical reactivity of the surface. - Functionalization of molecular layers by electron irradiation, and determination of the chemical properties 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) according to any one of claims 1 to 10, making it possible to form a monochromatic incident electron beam (100); - means (20) for transporting all or part of said incident electrons (100) to the surface of a sample (55) of material so as to form backscattered electrons (110) and for transporting all or part of said backscattered electrons (110) to 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) 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 the Rydberg state; - means for generating a pulsed electric field (F) on either side of said laser excitation zone (15a), said pulsed electric field being adapted to ionize at least some of said excited atoms and form a monochromatic electron beam (100).

2. 2. The system for performing 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. 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 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 contain 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 materials according to claim 1, wherein the system is formed by applying a voltage 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 surface analysis of materials according to claim 1, wherein the pulsed electron source of the system for performing surface analysis of materials comprises an effusion cell (161) capable of generating the source (16) of atoms.

10. 10. The system for performing surface analysis of materials 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 the same Rydberg state.

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 electron time-of-flight tube, a delay line, an imaging device.

13. about 10 －９ 13. The analysis 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 claim 1, wherein the transport means (20) comprises a plurality of electron optics (21, 22) and one electron beam splitter (25), the electron optics 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) positioned 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) with a second focusing lens (214); an objective arm (22) positioned between said splitter (25) and said sample (55) of material, 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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