Ultrafast scanning electron microscope imaging method and device based on low-voltage deceleration mode

Abstract

The invention discloses an ultrafast scanning electron microscope imaging method and device based on a low-voltage deceleration mode, and the method combines a pumping-detection ultrafast scanning electron microscope technology and a low-voltage deceleration imaging technology, and comprises the steps: setting a high acceleration electric field from a photocathode electron gun to a pole shoe outlet, and setting a reverse deceleration electric field from a pole shoe opening to a sample; transient imaging with high time resolution, high spatial resolution, low damage, low charge and high signal-to-noise ratio is realized at the same time. Compared with the prior art, the technical difficulty that spatial resolution, time resolution and signal-to-noise ratio of a traditional ultrafast scanning electron microscope are limited under low voltage is solved at the same time, and the problems of electron damage, surface charge and the like caused by high electron acceleration voltage to a sample are solved at the same time.

Description

Technical Field

[0001] The present invention belongs to the field of high spatio-temporal resolution imaging and materials science applications, and in particular relates to an ultrafast scanning electron microscope imaging method and apparatus, especially adding a deceleration mode, and simultaneously obtaining high time and space resolution capabilities at a high acceleration voltage, low damage and low charging advantages at a low landing voltage, and a high imaging signal-to-noise ratio. Background Art

[0002] Ultrafast electron microscopes [Science 328, 187 (2010), Micron 43, 1108 (2012)] add a time resolution in the range of nanoseconds to hundreds of femtoseconds on the basis of traditional electron microscopes; as one of them, ultrafast scanning electron microscopes [PNAS 107, 14993 (2010), J. Phys. Chem. Lett. 6, 3884 (2015)] combine a conventional scanning electron microscope with a femtosecond ultrafast laser, focusing on the ultrafast structure and morphology dynamics of the surface and interface. It gives full play to the original multiple imaging modes of the scanning electron microscope (secondary electron imaging, cathodoluminescence imaging, backscattered imaging, energy-dispersive X-ray imaging, etc.) and endows it with the new feature of sub-picosecond dynamic imaging. Among them, the secondary electron detection method it adopts, the basic principle is that the instantaneous concentration of local electrons or holes is positively correlated with the instantaneous efficiency of secondary electrons [JACS 133, 7708 (2011), PNAS 111, 2094 (2014)], which is particularly suitable for carrier dynamics research. In order to overcome the time and space broadening of electron pulses caused by the space charge effect and obtain higher time and space resolution, currently known ultrafast scanning electron microscopes generally need to operate at a high acceleration voltage (such as 30 kV).

[0003] However, the interaction between the high-energy electron beam corresponding to high voltage and the sample also brings many drawbacks, specifically including: (1) significant electron damage: A prerequisite for photoexcitation-electron detection experiments is that the influence of the detection pulse on the sample should be negligible compared to that of the excitation pulse (i.e., significantly smaller than that of the excitation pulse). However, the high-energy electron beam causes severe damage to the sample, posing a huge challenge to ensuring this prerequisite; (2) charging phenomenon: After the high-energy electron beam interacts with a sample with poor conductivity, it is easy to generate excessive electrons or holes on the sample surface, correspondingly forming an unstable electric field on the sample surface, resulting in bright or dark streaks in the scanning electron microscope image, that is, the charging phenomenon. Severe static charging will affect the analysis and understanding of transient imaging results and also limit the applicable range of the ultrafast scanning electron microscope (only applicable to samples with good conductivity); (3) deep detection depth: The penetration depth of the detection pulse being less than or comparable to that of the excitation pulse is the best condition for obtaining information on the excitation region. However, the penetration depth of the high-energy electron beam is often greater than that of the detection pulse, thus reducing the signal-to-noise ratio of the longitudinal transient signal and increasing the lateral scale of interaction with the sample, reducing the spatial resolution.

[0004] Patent application CN201210385701.2 discloses an ultrafast lensless coherent electron diffraction imaging method and device. By combining an electron pulse precisely synchronized with a process excitation source (such as a femtosecond laser pulse) and lensless coherent diffraction imaging technology, analyzing the intensity distribution of the diffracted coherent electron pulse, and inversely calculating to determine the electron scattering phase, three-dimensional transient atomic-scale structure and morphology reconstruction are achieved, solving the technical dilemma that traditional electron microscopy imaging methods do not have high time resolution or the current ultrafast electron imaging has limited time and space resolution. However, the lensless coherent diffraction imaging technology it adopts and the ultrafast scanning electron microscope are two different technical routes. It is applicable to detecting crystal structures, and due to the large scattering cross-section of electrons, there are requirements for the thickness of the sample (<200 nm), and it is not applicable to thicker samples. In addition, this technology can only provide diffraction images in reciprocal space, and then obtain the dynamic changes of the crystal structure through a data processing process, and cannot provide real-space imaging of the sample. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the above-mentioned existing technologies and provide an ultrafast scanning electron microscope imaging method and device based on a low-voltage deceleration mode, which can simultaneously achieve high-time-resolution, high-space-resolution, low-damage, low-charging, and high-signal-to-noise-ratio transient scanning imaging without changing the structure of the original electron microscope optical system.

[0006] The object of the present invention can be achieved by the following technical solutions: An ultrafast scanning electron microscope imaging method based on a low-voltage deceleration mode, which combines pump-probe ultrafast scanning electron microscope technology and low-voltage deceleration imaging technology. A high-acceleration electric field is set between the photocathode electron gun and the exit of the pole piece, and at the same time, a reverse deceleration electric field is set between the pole piece opening and the sample to simultaneously achieve high time resolution, high spatial resolution, low damage, low charging, and high signal-to-noise ratio transient imaging.

[0007] Further, under the action of the high-acceleration electric field, electrons can be accelerated to 5 - 30 keV or above.

[0008] Further, the distance from the pole piece opening to the sample is the working distance, and the working distance can be as low as 0.5 - 4 mm.

[0009] Further, under the action of the reverse deceleration electric field, electrons can be decelerated to 1 - 5 keV or below.

[0010] Further, the high-acceleration electric field enables photoelectrons to quickly pass through the lens barrel, reducing the time and spatial broadening of electron pulses caused by space charge effects;

[0011] The reverse deceleration electric field significantly reduces the landing voltage of the electron beam, and the expansion range of the low-energy electron beam in the sample is significantly reduced compared to the high-energy electron beam;

[0012] At the sample, the deceleration mode is adopted. Without changing the structure of the original electron microscope optical system, the brightness and sufficient signal-to-noise ratio under high acceleration voltage are maintained, as well as high time resolution and high spatial resolution. At the same time, the interaction range between the electron beam and the sample is reduced, improving the spatial resolution of microanalysis. The leading edge of the electron beam pulse decelerates first relative to the trailing edge, compressing the electron pulse to a certain extent and improving the time resolution.

[0013] Further, at the sample, the deceleration mode is adopted, and the electron beam interacting with the sample is a low-energy electron beam, reducing the damage of the detected electrons to the sample and ensuring the effectiveness of the basic premise of the pump-electron detection technology;

[0014] The deceleration mode effectively alleviates the charging of the sample surface, ensuring low-charging scanning imaging, and greatly expanding the sample application range of the ultrafast scanning electron microscope without the need for conductive coating; The deceleration mode accelerates secondary electrons or backscattered electrons signals under the action of the deceleration voltage, and the energy of these signal electrons becomes higher when detected by the detector, thereby improving the collection efficiency of secondary electrons or backscattered electrons and increasing the signal-to-noise ratio.

[0015] The present application also provides an ultrafast scanning electron microscope imaging device based on a low-voltage deceleration mode for implementing the above method. The device includes a process excitation source, a data processing system, and a high-vacuum sample chamber. A pulsed electron system, a pulsed electron control system, a deceleration system, a sample stage, and a detection system are provided in the high-vacuum sample chamber;

[0016] The process excitation source generates process excitation pulses and inputs them into the high-vacuum sample chamber to excite a sample located on the sample stage; the pulsed electron system generates electron pulses that are precisely synchronized with the process excitation pulses. The electron pulses are accelerated, shaped, and focused by the pulsed electron control system into a high-brightness and high-energy pulsed electron beam that exits from the pole piece. After being decelerated by the deceleration system, the pulsed electron beam irradiates the sample area excited by the process excitation pulses on the sample stage. After being scattered by the sample, secondary electron or backscattered electron signals are formed and received by the detection system. The signals are input into the data processing system, and the spatial distribution of the signals is obtained by scanning the electron beam through the pulsed electron control system.

[0017] Further, the sample stage is placed on a five-dimensional adjustment frame in the high-vacuum sample chamber.

[0018] Further, the pulsed electron control system is composed of an electron accelerator, a shaping aperture, multi-stage focusing lenses, and deflection lenses.

[0019] Further, the deceleration system is grounded at the pole piece and has a negative voltage at the sample, so as to ensure that the electron landing voltage is less than the acceleration voltage, and the reduction amplitude is adjusted according to the requirements of different samples.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The ultrafast scanning electron microscope imaging method and device based on the low-voltage deceleration mode of the present invention, compared with directly using a low acceleration voltage, this method adopts a deceleration mode to reduce the influence of electron lens aberration, thereby ensuring a small electron beam spot diameter on the sample surface; this method suppresses the space charge effect through a high acceleration voltage, obtains a short electron pulse width, a high time resolution and a high spatial resolution corresponding to a small focusing spot, and maintains the electron brightness and sufficient signal-to-noise ratio under a high acceleration voltage.

[0022] 2. The ultrafast scanning electron microscope imaging method and device based on the low-voltage deceleration mode of the present invention, compared with not using the deceleration mode, this method uses the interaction between a low-energy electron beam and a sample, significantly reducing sample damage, reducing the charging effect, and improving the signal-to-noise ratio; the expansion range of the low-energy electron beam in the sample is significantly reduced compared with that of the high-energy electron beam, further improving the spatial resolution of microscopic analysis; decelerating in front of the sample, the leading edge of the electron beam pulse decelerates first relative to the trailing edge, compressing the electron pulse to a certain extent and further improving the time resolution.

[0023] 3. The imaging method and device of the ultrafast scanning electron microscope based on the low-voltage deceleration mode according to the present invention, compared with the case without using the deceleration mode, adopt the interaction between low-energy electron beams and samples, broaden the applicable range of the ultrafast scanning electron microscope from conductive materials to general materials including poor conductors, and there is no need to conduct conductive spraying on the samples. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a structural schematic diagram of the ultrafast scanning electron microscope imaging device based on the low-voltage deceleration mode of the present invention;

[0025] wherein 01 - process excitation source, 02 - pulsed electron system, 03 - pulsed electron control system, 04 - deceleration system, 05 - sample stage, 06 - detection system, 07 - data processing system, and 08 - high-vacuum sample chamber.

[0026] Figure 2 is a schematic principle diagram of the ultrafast scanning electron microscope imaging device based on the low-voltage deceleration mode of the present invention, that is, a schematic diagram of the deceleration system;

[0027] Figure 3 is a gold sample photographed in the deceleration mode (decelerated from 1 kV to 500 V);

[0028] Figure 4 is a gold sample photographed in the non-deceleration mode (1 kV);

[0029] Figure 5 is Figure 3 and Figure 4 is a comparison diagram showing them together in the same field of view. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] To facilitate the understanding of the present invention, the present invention will be described more comprehensively below with reference to related embodiments. The following gives the preferred embodiments of the present invention. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the description of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0032] The experimental methods used in the following embodiments are all conventional methods unless otherwise specified. The materials, reagents, etc. used in the following embodiments can be obtained from commercial sources unless otherwise specified.

[0033] The object of the present invention is to overcome the deficiencies of the above high-acceleration voltage technology, and to provide an ultrafast scanning electron microscope imaging method and device based on a low-voltage deceleration mode; without changing the structure of the original electron microscope optical system, high-time-resolution, high-spatial-resolution, low-damage, low-charging, and high-signal-to-noise ratio transient scanning imaging can be achieved simultaneously. Specifically, through the pump-probe technology, photoelectrons are accelerated into high-energy electron beams at high voltages inside the lens barrel, and a deceleration mode is adopted at the sample to obtain low-energy electron beams to interact with the sample.

[0034] As Figure 1 shown, the ultrafast scanning electron microscope imaging device based on the low-voltage deceleration mode includes: a process excitation source 01, a pulsed electron system 02, a pulsed electron control system 03, a deceleration system 04, a sample stage 05, a detection system 06, a data processing system 07, and a high-vacuum sample chamber 08. Each component is a conventional commercially available product in the art. For example:

[0035] The process excitation source 01 is a conventional commercially available pump that can generate process excitation pulses;

[0036] The pulsed electron system 02 is a commercially available product that can generate electron pulses that are precisely synchronized with the process excitation pump pulses generated by the process excitation source 01;

[0037] The pulsed electron control system 03 is composed of an electron accelerator, a shaping aperture, a multi-stage focusing lens, and a deflection lens.

[0038] The sample stage 05 is placed on a five-axis adjustment frame inside the high-vacuum sample chamber 08 for fixing the sample.

[0039] The pulsed electron system 02 generates electron pulses that are precisely synchronized with the process excitation pump pulses generated by the process excitation source 01. These pulsed electrons are accelerated, shaped, and focused by the pulsed electron control system 03 into a high-brightness high-energy pulsed electron beam that exits from the pole piece. After passing through the deceleration system 04, it becomes a low-energy pulsed electron beam and irradiates the sample area excited by the process excitation pulse source 01 on the sample stage 05 inside the high-vacuum sample chamber 08. After being scattered by the above sample, secondary electron or backscattered electron signals are formed and received by the detection system 06. The signals are input into the data processing system 07, and the spatial distribution of the transient signals is obtained by scanning the electron beam through the pulsed electron control system 03.

[0040] The principle of the technical solution of the present invention, as Figure 2Shown as follows: a high acceleration electric field from the photocathode electron gun to the exit of the pole piece (electrons can be accelerated to 5 - 30 keV or above) and a reverse deceleration electric field from the pole piece opening to the sample (i.e., the working distance, which can be as low as 0.5 - 4 mm) (electrons can be decelerated to 1 - 5 keV or below); the high acceleration electric field ensures that the photoelectrons quickly pass through the lens barrel, reducing the time and space broadening of the electron pulse caused by the space charge effect; the reverse deceleration electric field significantly reduces the landing voltage of the electron beam. Figure 2 The connection relationship between the sample and the deceleration voltage in the middle: the pole piece is grounded, and the deceleration voltage is loaded on the sample stage. When the electrons are emitted from the pole piece and are decelerated under the action of the reverse electric field, the voltage reaching the sample is the emission voltage minus the deceleration voltage, which is the landing voltage. Figure 2 The relationship between the middle positions 1 and 2: when at position 1, it means that the deceleration voltage is loaded, and when at position 2, it means that there is no deceleration voltage.

[0041] The working process, the specific steps include:

[0042] Step 1: Set the suppression voltage and extraction voltage of the photocathode of the electron gun in the pulsed electron system 02, the acceleration voltage of the pulsed electron control system 03, and the landing voltage of the sample in the deceleration system 04 respectively.

[0043] Step 2: Through femtosecond laser beam splitting, one beam can be converted into the process excitation source 01, and the other beam is used to excite the photocathode of the electron gun in the pulsed electron system 02 after frequency conversion to form an ultrafast electron beam; use this electron beam for scanning imaging.

[0044] Step 3: Adjust the time delay between the process excitation source 01 and the pulsed electron system 02 to achieve time-resolved scanning imaging.

[0045] Step 4: According to the set acceleration voltage, landing voltage, the distance from the photocathode to the pole piece, and the distance from the pole piece to the sample, actively correct the time zero point (i.e., the time point when the pump pulse and the probe electron pulse arrive simultaneously).

[0046] The following is a detailed description through specific examples.

[0047] Example 1

[0048] Use an ultrafast scanning electron microscope based on the low-voltage deceleration mode to characterize the carrier migration and recombination kinetics of nano-TiO2 (a widely used photocatalyst with a band gap of about 3.3 eV) under ultraviolet light illumination.

[0049] The working process is as follows.

[0050] Step 1: Spread the TiO2 nano-powder evenly on the copper conductive tape, fix the conductive tape on the aluminum T-stage, and fix the T-stage on the sample stage 05; close the high-vacuum sample chamber 08 and evacuate to 2x10-4 Pa or below.

[0051] Step 2: Set the suppression voltage, extraction voltage, and filament current of the photocathode of the electron gun in the pulsed electron system 02, the acceleration voltage of the pulsed electron control system 03, and the landing voltage of the sample in the deceleration system 04.

[0052] Step 3: As shown in the appendix Figure 1 Adjust the process excitation source 01, such as a femtosecond fiber laser system with a central wavelength of 1030 nm, a pulse width of 350 fs, and a repetition frequency of 40 MHz. Convert the main frequency output into 515 nm through a frequency doubling crystal, and then convert it into a 257.5 nm femtosecond laser pulse through another frequency doubling crystal. Then divide it into two beams through a beam splitter. One beam is used as the pump light to irradiate the sample stage 05, and the other beam is used as the probe beam to irradiate the photocathode of the pulsed electron system 02. Temporarily block the two beams with a light stop.

[0053] Step 4: Remove the light stop of the probe beam, and use the automatic optical path positioning system to adjust the probe optical path to ensure that the probe beam is focused on the thermal field emission filament of the scanning electron microscope and generates photoelectron pulses.

[0054] Step 5: Turn on the deceleration system 04 and adjust the pulsed electron control system 03 to ensure that the detector in the lens barrel of the detection system 06 can collect the secondary electron signal generated by the photoelectron pulse at a high magnification (usually 500,000 times). Set the dwell time to 1 μs, and a clear secondary electron image can be observed on the data processing system 07. In the deceleration mode, adjust the landing voltage (usually 1 kV). In the case of no obvious charging, ensure that the crystal planes and crystal edges can be distinguished on the TiO2 particles.

[0055] Step 6: Adjust the optical path delay to ensure that the optical path of the pump light is 1 cm or more shorter than the equivalent optical path of the detected electron pulse.

[0056] Step 7: Return to a low magnification (usually 5000 times), remove the light stop of the pump light, and use the automatic optical path positioning system to adjust the pump optical path to ensure that the transient secondary electron signal is located at the center of the field of view of the scanning electron microscope; gradually increase the magnification and select a suitable area to ensure that the transient secondary electron signal is always located at the center of the field of view of the scanning electron microscope.

[0057] Step 8: Use program control to adjust the optical path delay (covering -0.5 to 5.0 ns), obtain secondary electron images at different delays in the data processing system 07, and realize the migration behavior of carriers between different crystal planes of TiO2 nanoparticles under 257.5 nm light excitation in the low-voltage deceleration mode. Obtain the migration and recombination kinetic information of carriers by analyzing the change of secondary electron intensity with the delay time.

[0058] The experiment compared the secondary electron images of the gold sample taken in the deceleration mode and the non-deceleration mode.Figure 3 A gold sample taken in the deceleration mode (from 1 kV to 500 V). Figure 4 A gold sample taken in the non-deceleration mode (1 kV). By comparison, it can be seen that after the deceleration mode is turned on, the edges of the gold particle images are clearer, and the resolution is improved from 1.6 nm to 1.1 nm. Figure 5 Put Figure 3 and Figure 4 together and shown in the same field of view, it can be seen that the deceleration mode reduces the damage to the sample.

[0059] The above description of the embodiments is to enable those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. An ultrafast scanning electron microscope imaging method based on a low-voltage deceleration mode, characterized in that, This method combines pump-probe ultrafast scanning electron microscopy technology and low-voltage deceleration imaging technology. A high-acceleration electric field is set between the photocathode electron gun and the exit of the pole piece, and at the same time, a reverse deceleration electric field is set between the pole piece opening and the sample to achieve transient imaging with high time resolution, high spatial resolution, low damage, low charging, and high signal-to-noise ratio simultaneously.

2. The ultrafast scanning electron microscope imaging method based on the low-voltage deceleration mode according to claim 1, characterized in that Under the action of the high-acceleration electric field, electrons can be accelerated to 5 - 30 keV or above.

3. The ultrafast scanning electron microscope imaging method based on the low-voltage deceleration mode according to claim 1, wherein The distance from the pole piece opening to the sample is the working distance, and the working distance can be as low as 0.5 - 4 mm.

4. The ultrafast scanning electron microscope imaging method based on a low-voltage deceleration mode according to claim 1, characterized in that Under the action of the reverse deceleration electric field, electrons can be decelerated to 1 - 5 keV or below.

5. The ultrafast scanning electron microscope imaging method based on the low-voltage deceleration mode according to claim 1, wherein, The high-acceleration electric field enables the photoelectrons to quickly pass through the lens barrel, reducing the time and spatial broadening of the electron pulse caused by the space charge effect. The reverse deceleration electric field significantly reduces the landing voltage of the electron beam, and the expansion range of the low-energy electron beam in the sample is significantly reduced compared to the high-energy electron beam. At the sample, the deceleration mode is adopted. Without changing the structure of the original electron microscope optical system, it maintains the brightness and sufficient signal-to-noise ratio at a high acceleration voltage, as well as high time resolution and high spatial resolution. At the same time, it reduces the interaction range between the electron beam and the sample, improves the spatial resolution of microscopic analysis, and the leading edge of the electron beam pulse decelerates first relative to the trailing edge, compressing the electron pulse to a certain extent and improving the time resolution.

6. The ultrafast scanning electron microscope imaging method based on the low-voltage deceleration mode according to claim 1, wherein At the sample, the deceleration mode is adopted, and the electron beam interacting with the sample is a low-energy electron beam, reducing the damage of the detected electrons to the sample and ensuring the effectiveness of the basic premise of the pump-electron detection technology. The deceleration mode effectively alleviates the charging of the sample surface, ensuring scanning imaging under low charging, and greatly expanding the sample application range of the ultrafast scanning electron microscope without the need for conductive coating. The deceleration mode accelerates the secondary electrons or backscattered electrons signals under the action of the deceleration voltage, and the energy of these signal electrons becomes higher when they are detected by the detector, thereby improving the collection efficiency of secondary electrons or backscattered electrons and increasing the signal-to-noise ratio.

7. An ultrafast scanning electron microscope imaging device based on a low-voltage deceleration mode for implementing the method according to any one of claims 1-6, characterized in that, This device includes a process excitation source (01), a data processing system (07), and a high-vacuum sample chamber (08). A pulsed electron system (02), a pulsed electron control system (03), a deceleration system (04), a sample stage (05), and a detection system (06) are provided in the high-vacuum sample chamber (08). The process excitation source (01) generates process excitation pulses, which are input into the high-vacuum sample chamber (08) to excite the sample located on the sample stage (05). The pulsed electron system (02) generates electron pulses that are precisely synchronized with the process excitation pulses. These electron pulses are accelerated, shaped, and focused by the pulsed electron control system (03) into a high-brightness high-energy pulsed electron beam that exits from the pole piece. After being decelerated by the deceleration system (04), it irradiates the sample area excited by the process excitation pulse on the sample stage (05). After being scattered by the sample, secondary electrons or backscattered electrons signals are formed and received by the detection system (06). The signals are input into the data processing system (07), and the spatial distribution of the signals is obtained by scanning the electron beam through the pulsed electron control system (03).

8. The ultrafast scanning electron microscope imaging device based on the low-voltage deceleration mode according to claim 7, characterized in that, The sample stage (05) described above is placed on a five-axis adjustment stage inside the high-vacuum sample chamber (08).

9. The ultrafast scanning electron microscope imaging device based on the low-voltage deceleration mode according to claim 7, wherein The pulsed electron control system (03) described above consists of an electron accelerator, a shaping aperture, a multi-stage focusing lens, and a deflection lens.

10. The ultrafast scanning electron microscope imaging device based on the low-voltage deceleration mode according to claim 7, characterized in that, The deceleration system (04) is grounded at the pole shoe and has a negative voltage at the sample, so as to ensure that the electron landing voltage is less than the acceleration voltage, and the reduction amplitude is adjusted according to the requirements of different samples.

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