A method and system for ultra-high momentum resolution electron energy loss spectroscopy

Abstract

This invention discloses an ultra-high momentum resolution electron energy-loss spectroscopy (EELS) measurement method and system. By adjusting the rear objective projection system and applying a controllable sample height offset, an ultra-long camera length is achieved, and the momentum resolution can be improved to the order of magnitude. It eliminates the need to reduce the convergence half-angle, balancing high beam current and high signal-to-noise ratio, and allows for continuous tuning of momentum resolution and acquisition window using multiple parameters. Standardization is performed using the entrance aperture plane image. q The axis quantitative calibration can solve for a complete set of parameters such as momentum scale and pixel sampling interval. This invention reuses existing electron microscope hardware, has convenient mode switching, and can obtain the energy-momentum dispersion relationship of various elementary excitations such as phonons, plasmons, excitons, and magnons. It is suitable for the local quantitative detection of nanoscale materials in the extremely low momentum range and low-dimensional materials.

Description

Technical Field

[0001] This invention belongs to the field of transmission electron microscopy and electron energy loss spectroscopy, specifically relating to a method and system for measuring ultra-high momentum resolution electron energy loss spectroscopy. Background Technology

[0002] Electron energy-loss spectroscopy (EELS) for transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) can detect elementary excitation information such as phonons, plasmons, excitons, and magnons within a sample; further analysis of the momentum transfer corresponding to inelastic scattered electrons can yield energy. E With momentum q The dispersion relation between them is an important characterization method for studying the propagation, coupling and interaction of elementary excitations at the nanoscale.

[0003] While existing TEM / STEM-EELS can achieve high energy resolution and nanoscale spatial positioning, momentum resolution performance is constrained by multiple factors, including the incident beam convergence angle, probe, aperture, detector, projection optical path, and system stability, when performing quantitative detection in the long-wavelength limit and extremely low momentum range. The industry commonly relies on narrowing the convergence half-angle or using an external independent low-energy angle-resolved spectrometer to improve momentum resolution. However, these methods cannot simultaneously achieve high beam intensity, precise spatial positioning, ultra-high momentum resolution, standardization, and high reproducibility. q Axis quantitative calibration.

[0004] Currently, mainstream transmission-based momentum-resolved EELS detection methods generally adopt a design approach of stepwise switching of the convergence half-angle, using projection lenses to adjust the diffraction spot and slit apertures to limit the momentum acquisition range, relying on electron beam scanning to acquire four-dimensional energy spectrum data, and optimizing the incident momentum accuracy by reducing the convergence half-angle. However, due to the limitations of the condenser lens hardware structure, the convergence half-angle cannot be reduced indefinitely, and momentum resolution has an inherent upper limit; switching between spatial and momentum testing modes requires repeated adjustments to optical path parameters, making the adjustment process cumbersome and difficult to control continuously and smoothly; in addition, the hardware matching relationship between the aperture, projection lens, and detector is limited, resulting in insufficient momentum axis sampling density, further compressing the limiting momentum resolution capability.

[0005] Furthermore, high-resolution electron energy-loss spectroscopy (HREELS) utilizes a monochromatic low-energy electron beam and an angle-resolved energy analyzer to simultaneously resolve electron energy and momentum information, primarily obtaining signals from the sample surface and near-surface layers. However, for measurements of transmission thin films, suspended structures, and localized heterogeneous interfaces, substrate, support film, and surface contamination can introduce interference. These instruments typically require large, flat, clean, and uniform sample surfaces, making it difficult to simultaneously perform nanoscale selected-area and localized transmission measurements.

[0006] In summary, existing momentum-resolved EELS testing systems either have low momentum resolution limits and lack standardized, high-precision momentum quantitative calibration procedures, or are only suitable for surface reflection measurement scenarios. They cannot simultaneously achieve ultra-high momentum resolution, nanoscale local transmission characterization, flexible and continuous optical path adjustment, and complete quantitative solution of momentum parameters, making it difficult to meet the current demand for high-precision analysis of the excitation dispersion relationship of low-dimensional nanomaterials and heterogeneous interfaces. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention aims to improve the momentum resolution of electron energy loss spectra by adjusting and applying a controllable sample height offset through a rear objective projection system based on a scanning transmission electron microscope. Simultaneously, quantitative calibration of the momentum axis, momentum resolution, and momentum acquisition window is performed in the same image coordinates on the entrance aperture plane of the electron energy loss spectrometer.

[0008] To achieve the above objectives, in a first aspect, the present invention provides a method for measuring ultra-high momentum resolution electron energy loss spectroscopy, comprising the following steps: S1. Sample positioning: Load the sample to be tested into a scanning transmission electron microscope and use high-angle annular dark field image and convergent beam electron diffraction image to determine the test area and the direction of the momentum to be measured. S2. Set the incident electron beam and spectroscopic conditions: Set the accelerating voltage, convergence half angle and beam current, selectively enable the monochromator according to the required energy accuracy to adjust the incident electron beam energy broadening, and set the energy acquisition channel of the electron energy loss spectrometer. S3. Adjust the objective lens projection system: Adjust the intermediate mirror and the projection mirror to refocus the divergent electron beam emitted from the sample on the plane of the electron energy loss spectrometer entrance aperture, and make the direction of the measured momentum consistent with the direction of the major axis of the electron energy loss spectrometer entrance aperture. S4. Apply sample height offset: Control the sample support mechanism to move along the optical axis of the electron microscope, so that the sample to be tested is offset by a sample height Δ relative to the positive focal plane of the electron probe along the optical axis of the electron microscope. f ; S5. Set the momentum selection stop: Insert a narrow strip momentum selection stop into the entrance plane of the electron energy loss spectrometer and confirm that the direction of the major axis is consistent with the direction of the momentum to be measured. S6. Acquiring Energy-Momentum Data: Recording electron energy loss using one detection dimension of an electron energy loss spectrometer. E Another detection dimension records momentum transfer. q Adjusting EELS astigmatism to obtain a two-dimensional intensity distribution I ( E , q Single-point acquisition is performed at a fixed electron probe position, or a four-dimensional dataset containing two-dimensional spatial coordinates, energy coordinates, and momentum coordinates is obtained by scanning the electron probe. S7. Calibrate the momentum axis and momentum resolution: While keeping the optical path parameters constant, induce a known displacement of the electron beam in the focal plane. Acquire images of the entrance aperture plane before and after the displacement in the electron energy loss spectrometer imaging mode. Measure the beam spot displacement, the full width at half maximum (FWHM) of the beam spot along the direction of the momentum to be measured, and the effective length of the momentum selection aperture along the direction of the momentum to be measured in the same image coordinates. Combined with the sample height shift and electron wavelength, calculate the actual momentum resolution, the momentum acquisition window, and the single-pixel momentum interval. S8. Data Correction and Analysis: Perform energy drift correction and momentum axis drift correction on the acquired spectrum, subtract dark current and background as needed, and normalize the intensity of each momentum position to obtain the energy-momentum dispersion relationship.

[0009] In step S2, the convergence half-angle can be 0.5 mrad to 30 mrad; the energy resolution, characterized by the full width at half maximum (FWHM) of the zero-loss peak, can be 5 meV to 300 meV. Preferably, the accelerating voltage is 30 kV to 60 kV, the convergence half-angle is 4 mrad to 15 mrad, and the energy resolution is 5 meV to 30 meV.

[0010] In step S4, the absolute value of the offset of the sample plane relative to the electron probe focal plane. The offset can range from 1 μm to 800 μm, preferably from 20 μm to 400 μm. This offset is generated by the mechanical displacement of the sample stage along the optical axis of the electron microscope controlled by the goniometer stage. If the width of the pole piece has a large limitation on the mechanical displacement, the relative height offset can also be generated by adjusting the objective lens excitation to change the convergence point of the electron probe.

[0011] In step S7, fixing the current optical path parameters means maintaining the excitation conditions of the intermediate mirror and the projection mirror, and the sample height offset Δ. f The accelerating voltage, convergence half-angle, and spectrometer entrance plane imaging settings remain unchanged. Except for inserting or removing the momentum selection stop according to the calibration procedure, no other optical path or spectrometer parameters are altered.

[0012] First, exit the momentum selection diaphragm, stop the electron beam at the center of the scanning field of view and turn off descan compensation. In spectrometer imaging mode, acquire a plane image of the first electron energy loss spectrometer entrance diaphragm excluding the momentum selection diaphragm. Extract the beam intensity distribution along the direction of the measured momentum, and denote the full width at half maximum (FWHM) of the beam spot as... .

[0013] Subsequently, a scanning coil causes the electron beam to be displaced by a known Δ relative to the scanning center in the sample's focal plane. x Under the same conditions, a second electron energy loss spectrometer entrance aperture plane image without a momentum selection aperture is acquired; the displacement of the beam spot in the second image relative to the first image is measured and recorded as... .

[0014] Then, insert the momentum selection stop and measure the effective length of the momentum selection stop along the direction of the momentum to be measured under the same inlet plane imaging settings. Switch to energy dispersion mode and maintain the same momentum selection aperture state. Determine the number of effective detector pixels occupied by the zero-loss peak along the momentum direction in the electron energy loss spectrum. N . and All measurements were taken in the same entrance aperture plane image coordinates and using the same image length unit.

[0015] The sample experiences a height offset Δ relative to the focal plane. f Under the given conditions, the electron beam displacement Δ within the focal plane is known. x The corresponding angle change is approximately: ;

[0016] The full width at half maximum (FWHM) of the beam spot in the entrance aperture plane and beam spot displacement The angular resolution determined by electron beam broadening is obtained as follows: ;

[0017] Using electronic wave vector Momentum conversion is performed, where λ Given the relativistic electron wavelength determined by the accelerating voltage, the momentum resolution resulting from electron beam broadening is: ;

[0018] The momentum acquisition window corresponding to the momentum selection stop is: ;

[0019] The momentum interval per pixel is: ;

[0020] Based on the above relationships, the momentum axis is uniformly converted to... Or other selected reciprocal length units.

[0021] Step S7 may also include a calibration and verification process: applying known displacements Δ of the same magnitude but opposite direction along the direction of the momentum to be measured. x The average of the two calibration results was taken; the offset Δ at different sample heights was considered. f The calibration was repeated for each step, and a calibration lookup table corresponding to the optical path parameters was established. The momentum scale was cross-checked using known lattice diffraction patterns or pixelated detector diffraction patterns. The sample spectra and corresponding calibration data used the same intermediate mirror and projection mirror conditions, sample height, accelerating voltage, convergence half angle, and spectrometer projection settings. The momentum selection aperture shape was consistent with the corresponding calibration stage, and the same relativistic electron wavelength was used for conversion.

[0022] In a second aspect, the present invention provides an ultra-high momentum resolution electron energy loss spectroscopy measurement system for performing the above-described measurement method, comprising an electron beam generation and control unit, a sample carrying and height control unit, a rear objective projection unit, a momentum selection unit, an energy analysis and detection unit, and a control and data processing unit.

[0023] The electron beam generation and control unit includes an electron gun, an accelerating voltage control module, a monochromator, a condenser system, and scanning and descanning coils, which are used to output an incident electron beam with a set convergence half-angle, energy broadening and beam current intensity, and to make the electron beam produce a known displacement required for calibration.

[0024] The sample support and height control unit includes a sample rod, a sample stage, and a height adjustment mechanism that moves along the optical axis of the electron microscope to generate a controllable sample height offset Δ. f .

[0025] The rear objective projection unit includes an intermediate mirror and a projection mirror, which are used to refocus the electron beam emitted from the sample under test on the plane of the entrance aperture of the electron energy loss spectrometer.

[0026] The momentum selection unit includes a narrow strip momentum selection aperture set on the entrance plane of the electron energy loss spectrometer, which is used to select the direction of the momentum to be measured and the corresponding momentum acquisition range.

[0027] The energy analysis and detection unit includes an electron energy loss spectrometer and a two-dimensional electron detector, used to acquire images of the entrance aperture plane and perform energy dispersion and two-dimensional detection on electrons passing through the momentum-selective aperture, outputting the intensity distribution I( E , q ).

[0028] The control and data processing unit is connected to each of the above units and is used to control optical path adjustment, sample height offset, calibration image acquisition, and energy-momentum data acquisition; its calibration calculation module extracts data from the entrance aperture plane image. and And according to Δ x Δ f , λ and N Calculate momentum resolution, momentum window, and single-pixel momentum interval.

[0029] Compared with other existing technologies, the present invention has the following advantages: 1. Significantly improved momentum resolution upper limit and better testing conditions: By adjusting and controlling the sample height offset through the rear objective projection system, the traditional resolution bottleneck relying on reducing the convergence half-angle is overcome, and the momentum resolution can be improved to [missing information]. The signal strength is high; at the same time, it can be tested under normal convergence half-angle conditions, taking into account both high beam current and high signal-to-noise ratio, and the momentum resolution, acquisition window and signal flux can be continuously and flexibly adjusted by multiple parameters.

[0030] 2. Possesses a complete standardized quantitative calibration system: By obtaining the full width at half maximum (FWHM), beam response shift, and effective length of the momentum selection aperture in the same image coordinates on the entrance aperture plane of the electron energy loss spectrometer, and combining the optical path and electron wavelength parameters, the momentum scale, momentum window, pixel momentum sampling interval, and system momentum resolution are accurately calculated, achieving... q The axis parameters are reproducible and can be fully quantitatively analyzed.

[0031] 3. The platform is highly versatile and applicable to a wide range of testing scenarios: The entire process can be completed using only the electron microscope's native sample stage, intermediate mirror, projector mirror, momentum selective aperture, and energy dispersive spectrometer. It can freely switch between conventional characterization and ultra-high momentum resolution modes. For platforms that are already equipped with a usable momentum selective aperture and EELS spectrometer, there is no need to change the main structure of the electron microscope. It can perform fine tests on the dispersion relations of various elementary excitations such as phonons, plasmons, and excitons, and is especially suitable for transmission quantitative characterization in the long-wavelength ultra-low momentum range and nano-selective regions. Attached Figure Description

[0032] Figure 1 The diagram illustrates the EELS optical path configuration and sample height offset principle. (A) In a conventional small convergence angle configuration, the electron probe is focused on the sample, and the diffraction pattern is projected onto the slit aperture. The momentum resolution is limited by the incident beam convergence half-angle. (B) After the projection mirror is readjusted, the diverging beam or diffraction-related beam spot is refocused on the EELS entrance aperture plane, forming a small beam spot suitable for slit selection. (C) Introducing a larger sample height offset. Subsequently, small-angle scattering is converted into a larger lateral displacement at the aperture plane, while essentially maintaining the beam spot diameter; the angular resolution can be calculated according to... Estimate, spatial resolution trade-offs can be based on Estimate, of which d The size of the electron beam spot. For converging half-angle; Figure 2 A comparison of momentum resolution between conventional small convergence angle schemes and the scheme of this invention. (A) Momentum resolution varies with convergence half-angle. Or sample height offset The relationship between the changes is as follows: The gray curve represents the conventional convergence angle limitation case, and the orange curve and data points represent the theoretical estimation and experimental calibration results of the configuration of this invention; increasing the convergence half-angle... The momentum resolution can be continuously adjusted to (B) Effective corner spread function and corresponding real space scan image examples under different configurations. Compared with the conventional small convergence angle condition, the effective angle broadening is significantly compressed after introducing sample height offset, which can enter the microradian level, while reflecting the trade-off between momentum resolution and spatial resolution / field of view.

[0033] Figure 3 Schematic diagrams of the experiment involving projector lens refocusing and slit aperture selection. (A) Diffraction / beam pattern obtained at a smaller convergence half-angle, used to show the incident beam angle range. (B) Diffraction / beam pattern at a larger convergence half-angle, showing the increased incident momentum range. (C) After projector lens refocusing, the beam spot is refocused near the momentum selection aperture plane; the inset shows a magnified view of the refocused beam spot. (D) The refocused beam spot is placed within the slit aperture, and a momentum direction is selected by the slit for momentum-resolved EELS acquisition; the inset shows a magnified view of the slit region. Detailed Implementation

[0034] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the following embodiments are given for illustrative purposes only and are not intended to limit the scope of the present invention. Those skilled in the art can make various modifications and substitutions to the present invention without departing from its spirit and essence.

[0035] This invention provides a method for measuring ultra-high momentum resolution electron energy loss spectroscopy. This embodiment uses a scanning transmission electron microscope equipped with a monochromator and an electron energy loss spectrometer as an example. The electron microscope should be able to adjust the excitation of the intermediate mirror and the projection mirror; the sample support mechanism should be able to adjust the sample height along the optical axis of the electron microscope; and the electron energy loss spectrometer should be able to image at the entrance aperture plane and retain a momentum coordinate in energy-dispersion mode. The specific implementation method is as follows: S1. Sample Positioning: Load the sample to be tested onto the sample holder, adjust the sample orientation and select the area to be tested in scanning transmission electron microscopy mode. A high-angle annular dark-field image is used to determine the measurement position, with the imaging convergence half-angle set to 23 mrad and the detector collection angle range set to 80 mrad–210 mrad. A converging beam electron diffraction pattern is used to determine the sample orientation. To facilitate subsequent alignment of the momentum direction to be measured with the long axis of the momentum selection aperture, the correlation information between the sample area characteristics (such as sample edges) and the diffraction momentum direction can be observed as an auxiliary judgment criterion.

[0036] S2. Setting the Incident Electron Beam and Spectroscopic Conditions: Based on sample stability, the required energy range, and signal intensity, set the accelerating voltage, convergence half-angle, and beam current. Selectively activate the monochromator according to the required energy accuracy to adjust the incident electron beam energy broadening, and set the spectrometer's energy acquisition channel. The convergence half-angle is 0.5 mrad to 30 mrad, and the energy resolution, characterized by the full width at half maximum (FWHM) of the zero-loss peak, is 5 meV to 300 meV. Under one preferred condition, the accelerating voltage is 30 kV to 60 kV, the convergence half-angle is 4 mrad to 15 mrad, and the energy resolution is 5 meV to 30 meV. More specifically, a larger convergence angle results in a smaller electron beam spot, ultimately achieving higher momentum resolution, but subsequent refocusing adjustments are more difficult and can be determined based on instrument usage and testing requirements. Without the monochromator, the electron beam energy broadening is approximately 200 meV to 300 meV, suitable for acquiring signals from targets at tens of eV and higher energies. With the monochromator enabled, the required energy resolution and signal-to-noise ratio can be balanced according to the target. For signals from a few eV to tens of eV, the monochromator slit can be appropriately opened to improve the signal-to-noise ratio, and the energy resolution can be controlled to 20 meV to 30 meV. For signals from tens of meV to a few eV, the monochromator slit can be reduced, and the energy resolution can be controlled to 5 meV to 20 meV.

[0037] S3. Adjusting the rear objective projection system: Adjust the excitation of the intermediate mirror and the projector mirror to refocus the diverging electron beam emitted from the sample onto the entrance aperture plane of the electron energy loss spectrometer. Through beam spot translation, rotation, scaling, and astigmatism correction, ensure the refocused beam spot stably falls within the usable area of ​​the entrance aperture, and align the direction of the measured momentum with the long axis of the narrow strip momentum selection aperture (the long axis of the pre-configured aperture should be as parallel as possible to the non-energy dispersive direction of the two-dimensional detector). If the diffraction projection distortion is large and the momentum direction is difficult to distinguish, it can be confirmed by referring to the characteristics of the sample area under appropriate defocus using the correspondence in S1.

[0038] S4. Apply sample height offset: Move the sample stage along the optical axis of the electron microscope to introduce a shift in the sample relative to the original focal plane. The absolute value of the sample height offset. The range is 1μm to 800μm, preferably 20μm to 400μm. Increasing... While this can improve momentum resolution, it requires a larger, more uniform sample area. Furthermore, with the number of pixels in the momentum direction of the detector remaining constant, the measured momentum window size decreases accordingly. Therefore, the selection should consider the size of the area to be measured, mechanical stability, and the characteristic momentum range of the signal to be measured. Additionally, the width of the electron microscope's pole piece physically limits the amount of momentum that can be obtained simply by moving the sample stage. At this point, the relative offset can be adjusted by changing the electron beam convergence point by adjusting the objective lens excitation. After adjustment, step S3 needs to be repeated.

[0039] S5. Set the momentum selection aperture: Place the slit aperture or equivalent momentum selection structure in the EELS inlet plane. The slit length can be 1 mm to 3 mm, and the width can be 0.02 mm to 0.15 mm.

[0040] S6. Acquire Energy-Momentum Spectroscopy: Enter EELS spectrometer acquisition mode, expand the energy loss in the first direction of the detector, retain the momentum coordinates corresponding to the slit major axis in the second direction, select an appropriate energy dispersion channel, adjust EELS astigmatism, and obtain I( E , q Spectrum. Two-dimensional data can be acquired at a fixed probe position. E - q Spectra can also be obtained by scanning an electron beam. x - y - E - q Dataset.

[0041] S7. Calibrate the momentum axis and momentum resolution: Perform a complete calibration once for each set of sample height offsets and rear objective projection conditions. At the start of calibration, exit the momentum selection aperture, stop the electron beam at the center of the scanning field of view and turn off descan compensation, acquire the first entrance aperture plane image, and measure the full width at half maximum (FWHM) of the refocused beam spot along the direction of the momentum to be measured. .

[0042] Keeping the excitation settings for the intermediate mirror and projector mirror, sample height offset, accelerating voltage, convergence half-angle, and spectrometer entrance plane imaging constant, the electron beam is displaced by a known Δ on the sample's focal plane using a scanning coil. x The second entrance aperture plane image is acquired, and the displacement of the beam spot relative to the first image is measured. To reduce the effects of drift and nonlinearity of the scanning coil, equal displacements can be applied in both the positive and negative directions and the results averaged, or multiple known displacements can be used for linear fitting.

[0043] Subsequently, a momentum selection aperture is inserted, and the effective length of the aperture along the direction of the measured momentum is measured in the same entrance plane image coordinates. Switch to energy dispersion mode and measure the number of effective detector pixels occupied by the zero-loss peak along the momentum direction in the electron energy loss spectrum. N .Will and Substituting the aforementioned calculation relationship, we obtain... δq , and Different sample height offsets or rear objective lens projection conditions can be calibrated and stored separately to form a calibration lookup table.

[0044] S8. Data Correction and Analysis: Energy drift and momentum axis drift corrections are performed on repeatedly acquired spectra, with zero-loss peak cross-correlation results used as alignment benchmarks. Subsequently, dark current correction, detector gain correction, and background subtraction are performed as needed. Based on the characteristics of the signal under test, deconvolution or peak fitting can be further performed. For ease of observation, the intensity of each momentum position can be normalized in the target energy range, and finally, the dispersion relationship of elementary excitation energy as a function of momentum is obtained.

[0045] After the measurement is completed, the sample height, projection lens conditions and aperture status can be restored to the conditions of conventional STEM imaging, ordinary EELS or conventional momentum-resolved EELS, realizing multi-mode switching within the same platform.

[0046] The geometric principle behind this invention for improving momentum resolution is as follows: An electron probe is formed under conventional converging beam conditions to obtain sufficient beam current. The intermediate mirror and projection mirror are adjusted to refocus the electron beam emitted from the sample at the spectrometer entrance aperture plane. After the sample deviates from the focal plane along the optical axis, the minute scattering angle is converted into a lateral displacement at the entrance aperture plane. Ideal probe size. d It can be approximated as: ; in, α To converge half-angle, λ Let be the relativistic electron wavelength determined based on the accelerating voltage. Under the geometric approximation, the ideal angular resolution, ideal momentum resolution, and real space broadening can be estimated as follows: ; ; ;

[0047] Therefore, increasing It can compress the effective angle and broaden the illumination range in real space. The actual momentum resolution is also affected by factors such as electron beam astigmatism, optical path stability, mechanical drift, sample homogeneity, multiple scattering, and momentum sampling rate. The measured calibration results obtained from the entrance aperture plane image in step S7 provide a judgment standard.

[0048] In one specific embodiment, the focus is on the elementary excitation signal near 6 eV. The accelerating voltage is set to 30 kV, the convergence half-angle is set to 8 mrad, and the electron beam current at the sample after monochromatization is approximately 0.3 pA, corresponding to an initial electron beam current of approximately 3.8 pA before monochromatization. The energy resolution, characterized by the full width at half maximum (FWHM) of the zero-loss peak, is approximately 20 meV. The intermediate mirror and projection mirror are adjusted to refocus the electron beam on the entrance aperture plane of the electron energy loss spectrometer. The sample is shifted 400 μm along the optical axis, and a narrow strip momentum selection aperture of 2 mm × 0.125 mm is used. The spectrometer's energy sampling interval is set to 10 meV / channel, with 1028 acquisition channels in the energy direction, corresponding to a total recorded energy span of approximately 10.28 eV. The effective acquisition pixels in the momentum direction are... N The value is 423, thus obtaining a two-dimensional I-shaped structure with 1028 channels in the energy direction and 423 effective pixels in the momentum direction. E , q (Figure). The single-frame exposure time was set to 10 s, and 400 frames of spectra were continuously acquired for subsequent drift correction and overlay.

[0049] At an accelerating voltage of 30 kV, the relativistic electron wavelength λ Approximately 6.98 pm, electron wave vector 2π / λ Approximately For a convergence half-angle of 8 mrad, according to d ≈ 0.61 λ / sin α Estimate the ideal electron probe size d Approximately 0.53 nm; when the absolute value of the sample height shift... Ideal angular resolution at 400 μm Approximately 1.3 μrad, corresponding to an ideal momentum resolution of approximately The actual momentum resolution is calibrated according to step S7, and the full width at half maximum (FWHM) of the refocused beam spot is measured along the direction of the momentum to be measured. The value is 0.62 au, which causes the electron beam to produce Δ at the positive focal plane of the sample. x Given a displacement of 100 nm, the beam spot displacement was measured. After inserting a narrow strip momentum selection aperture, the effective length of the aperture along the direction of the momentum to be measured is measured in the same image coordinates. To obtain the actual angular resolution Approximately 2.1 μrad, momentum resolution Approximately The total momentum window is obtained. Approximately Single pixel momentum interval Approximately / pixel.

[0050] During data processing, the normalized cross-correlation method was first used to analyze 400 consecutively acquired two-dimensional I( E , q The spectra are registered to correct for electron beam energy drift and momentum axis drift; then the registered spectra are superimposed to improve the signal-to-noise ratio. After completing dark current correction, detector gain correction, and background subtraction, the intensity of each momentum position is normalized one by one using the integrated intensity of the energy spectrum corresponding to each momentum position within a selected energy window as the normalization coefficient. The two-dimensional spectrum used for display can be Gaussian smoothed; for quantitative analysis, peak functions can be fitted to the corrected and normalized energy loss peaks to obtain the relationship between peak position, peak width, and intensity as a function of momentum.

[0051] This invention uses specific experimental data to illustrate the principles and implementation methods of the invention. The descriptions of the embodiments above are only for the purpose of helping to understand the method and core ideas of the invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the invention.

Claims

1. A method for measuring ultra-high momentum resolution electron energy loss spectroscopy, characterized in that, Includes the following steps: S1. Sample positioning: Load the sample to be tested into a scanning transmission electron microscope and use high-angle annular dark field image and convergent beam electron diffraction image to determine the test area and the direction of the momentum to be measured. S2. Set the incident electron beam and spectroscopic acquisition conditions: Set the accelerating voltage, convergence half angle and beam current, selectively enable the monochromator according to the required energy accuracy to adjust the incident electron beam energy broadening, and set the energy acquisition channel of the electron energy loss spectrometer. S3. Adjust the objective lens projection system: Adjust the intermediate mirror and the projection mirror to refocus the divergent electron beam emitted from the sample on the plane of the electron energy loss spectrometer entrance aperture, and make the direction of the measured momentum consistent with the direction of the long axis of the momentum selection aperture. S4. Apply sample height offset: Control the sample support mechanism to move along the optical axis of the electron microscope, so that the sample to be tested is offset by a height Δ relative to the positive focal plane of the electron probe along the optical axis of the electron microscope. f ; S5. Set the momentum selection stop: Insert a narrow strip momentum selection stop into the entrance plane of the electron energy loss spectrometer and confirm that its major axis is consistent with the direction of the momentum to be measured. S6. Acquire energy-momentum spectrum: The electron energy loss spectrometer records electron energy loss in one dimension of the detector and retains momentum transfer coordinates in another dimension. Astigmatism is adjusted to obtain a two-dimensional intensity distribution I. E , q ); A fixed electron probe is used for single-point acquisition, or a scanning electron probe is used to obtain a four-dimensional dataset containing two-dimensional real space coordinates, energy coordinates, and momentum coordinates; S7. Calibrate the momentum axis and momentum resolution: While keeping the optical path parameters constant, induce a known displacement of the electron beam in the focal plane. Acquire images of the entrance aperture plane before and after the displacement in the electron energy loss spectrometer imaging mode. Measure the beam spot displacement, the full width at half maximum (FWHM) of the beam spot along the direction of the momentum to be measured, and the effective length of the momentum selection aperture along the direction of the momentum to be measured in the same image coordinates. Combined with the sample height shift and electron wavelength, calculate the actual momentum resolution, the momentum acquisition window, and the single-pixel momentum interval. S8. Data Correction and Analysis: Energy drift and momentum axis drift alignment corrections are performed on multi-frame acquired spectra, and dark current correction, background subtraction, momentum intensity normalization and peak fitting are performed as needed to obtain the elementary excitation energy-momentum dispersion relationship.

2. The method for measuring ultra-high momentum resolution electron energy loss spectroscopy according to claim 1, characterized in that, In step S2, the convergence half-angle is 0.5 mrad to 30 mrad, and the energy resolution characterized by the full width at half maximum (FWHM) of the zero-loss peak is 5 meV to 300 meV.

3. The method for measuring ultra-high momentum resolution electron energy loss spectroscopy according to claim 2, characterized in that, In step S2, the accelerating voltage is 30 kV to 60 kV; the convergence half-angle is 4 mrad to 15 mrad; and the energy resolution characterized by the full width at half maximum (FWHM) of the zero-loss peak is 5 meV to 30 meV.

4. The method for measuring ultra-high momentum resolution electron energy loss spectroscopy according to claim 1, characterized in that, In step S4, the sample height offset Δ f The absolute value ranges from 1 μm to 800 μm.

5. The ultra-high momentum resolution electron energy loss spectrum measurement method according to claim 4, characterized in that, The sample height offset Δ f The absolute value ranges from 20 μm to 400 μm.

6. The method for measuring ultra-high momentum resolution electron energy loss spectroscopy according to claim 1, characterized in that, During step S6, when acquiring the energy-momentum spectrum to be measured, and step S7, when acquiring the calibration image, maintain the lens excitation of the intermediate mirror and the projection mirror, and the sample height offset Δ. f With the accelerating voltage, convergence half-angle, and projection settings unchanged, and the spectrometer imaging unchanged during calibration, the electron beam is stopped at the center of the scanning field of view, the descan coil is turned off, and in the electron energy loss spectrometer imaging mode, without the momentum selection aperture inserted, the first image of the entrance aperture plane is acquired, and the full width at half maximum (FWHM) of the beam spot along the direction of the momentum to be measured is measured. The electron beam is displaced by a known Δ in the focal plane by a scanning coil. x Under the same conditions, a second image of the entrance aperture plane is acquired, and the displacement of the beam spot relative to the first image is measured. Then, a momentum selection aperture is inserted, and its effective length along the direction of the momentum to be measured is measured. ;in, and Using the same length unit in the same image coordinate system; with the momentum selection aperture inserted, the electron energy loss spectrometer is switched to energy dispersion mode to measure the number of effective detector pixels occupied by the zero-loss peak signal in the momentum transfer direction. N .

7. The method for measuring ultra-high momentum resolution electron energy loss spectroscopy according to claim 6, characterized in that, Step S7 is calculated according to the following relationships: ; ; ; ; ; Where Δ θ To be related to the known displacement Δ x The corresponding angle change, δθ This is the actual angular resolution. λ The relativistic electron wavelength determined based on the accelerating voltage. δq The momentum resolution determined for beam broadening. To determine the size of the momentum window, The momentum interval is for a single pixel.

8. The method for measuring ultra-high momentum resolution electron energy loss spectroscopy according to claim 6, characterized in that, Step S7 also includes a calibration and verification process: Apply a known beam spot displacement Δ of equal magnitude to two opposite displacement directions, positive and negative. x Two sets of calibration data were collected and the average value was taken as the benchmark calibration parameter; Changing the height offset Δ of different samples f Then repeat the complete calibration process, store the calibration data corresponding to each set of optical path parameters, and generate a calibration lookup table. The momentum scale obtained from the calibration was cross-checked using standard sample lattice diffraction spots and pixelated detector diffraction patterns. The sample spectra and calibration data were collected using the exact same projection lens conditions, sample height, accelerating voltage, convergence half angle, spectrometer projection settings, and aperture shape, and the same electronic wavelength was used for conversion.

9. A system for measuring ultra-high momentum resolution electron energy loss spectroscopy, characterized in that, The method for performing the measurement method according to any one of claims 1 to 8 includes an electron beam generation and control unit, a sample carrying and height control unit, a rear objective projection unit, a momentum selection unit, an energy analysis and detection unit, and a control and data processing unit. The electron beam generation and control unit is used to output an incident electron beam with a set convergence half angle, energy broadening and beam intensity, and to make the electron beam generate a known displacement required for calibration. The sample bearing and height control unit is used to generate a controllable sample height offset Δ. f ; The rear objective projection unit is used to refocus the electron beam emitted from the sample under test on the plane of the entrance aperture of the electron energy loss spectrometer. The momentum selection unit includes a narrow strip momentum selection aperture disposed on the inlet plane of the electron energy loss spectrometer; The energy analysis and detection unit is used to acquire the plane image of the entrance aperture and perform energy dispersion and two-dimensional detection on the electrons passing through the momentum selection aperture, outputting the intensity distribution I( E , q ); The control and data processing unit is used to control the optical path adjustment, sample height offset, calibration image acquisition and energy-momentum spectrum acquisition, and to complete momentum axis calibration and data processing.

10. The ultra-high momentum resolution electron energy loss spectroscopy measurement system according to claim 9, characterized in that, The control and data processing unit includes a calibration calculation module, which is used to extract data from the entrance aperture plane images before and after displacement. and And according to Δ x Δ f , λ Effective number of pixels acquired in the momentum direction N Calculate momentum resolution, momentum window, and single-pixel momentum interval.

Images