CN114678244A - Ultrafast scanning electron microscope system and application method thereof - Google Patents

Abstract

The invention discloses an ultrafast scanning electron microscope system and an application method thereof. The system comprises: the system comprises a scanning electron microscope, a detection laser system which converts a first near-infrared femtosecond laser into an ultraviolet femtosecond laser focused in a directional manner and generates a detection photoelectron pulse through cathode excitation, a pumping laser system which converts a second near-infrared femtosecond laser into a femtosecond laser pulse with adjustable wavelength and focused in a directional manner and used for pumping, an optical delay line which adjusts the optical path difference of the detection photoelectron pulse and the femtosecond laser pulse used for pumping reaching the surface of a sample to be detected, and a secondary electron detection system which detects transient secondary electron signals.

Description

Ultrafast scanning electron microscope system and application method thereof

Technical Field

The invention relates to the technical field of an ultrafast scanning electron microscope, in particular to the technical field of the ultrafast scanning electron microscope based on a pumping detection principle.

Background

In order to further improve the performance of the photoelectric functional device in service, the dynamic processes of carriers in the low-dimensional photoelectric materials, which usually occur in the nanometer spatial scale and the picosecond to femtosecond time scale, need to be deeply researched and understood, and the dual requirements on the time and spatial resolution of the detection means are provided. Therefore, the development of a carrier dynamics detection technology with ultrahigh time and spatial resolution is urgently needed, so that the visual understanding of the carrier dynamics process of the low-dimensional and non-uniform novel photoelectric functional material at the micro-nano-scale structure, interface, defect and the like is facilitated.

In the prior art, the measurement of the dynamic processes of generation, separation, transmission, recombination and the like of charge carriers in photoelectric materials is mainly based on the transient spectrum technology of a pure optical pumping-detection principle, such as transient absorption spectrum, reflection spectrum, harmonic spectrum and the like. However, due to the limitation of the wavelength limit of laser diffraction, the light spot size of micron order can be mostly converged, so that the commonly measured average kinetic information is in a larger range of the material, and the kinetic information of carriers with local fine structures, such as nano-scale semiconductor heterojunction, defects, quantum wells, organic or inorganic perovskite grain interfaces, metal nano superstructure and the like, is difficult to obtain. In addition, since laser light in the visible light band range has a large penetration depth in the photoelectric material, it is often difficult to analyze the contribution of surface carrier dynamics thereof. Therefore, a more suitable detection method is urgently needed to research the dynamic process of the excited-state carriers on the surface of the material at a high space-time resolution scale.

Scanning electron microscope (scanning electron microscope, SEM) mainly uses a high-energy electron beam with fine focusing to scan the surface of a sample, and secondary electrons (the energy range is 0-50eV) excited when the incident high-energy electrons bombard the surface of the sample are collected by a secondary electron detector (ETD), so that a clear amplified image of the three-dimensional micro-morphology of the surface of the sample can be obtained on a nanometer space scale almost without destructiveness. In addition, secondary electron signals excited by the action of high-energy electron beams on the surface of a sample in a scanning electron microscope are generally generated within a depth range which is less than 10nm from the surface of the sample, and are very sensitive to the distribution of charge states on the surface of the sample, so that the surface state information can be effectively reflected in a high spatial resolution scale. However, conventional scanning secondary electron imaging can only give static information of a sample surface state, and cannot obtain kinetic information of material surface carriers in an ultrafast time scale.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to provide an ultrafast electron microscope system and an application method thereof, which can detect the ultrafast dynamic processes of generation, drifting, diffusion, compounding and the like of photo-generated carriers on the surface of a photoelectric functional material in real time under an ultrahigh space-time resolution scale.

The invention firstly provides the following technical scheme:

an ultrafast scanning electron microscope system, comprising: the system comprises a scanning electron microscope, a first near-infrared femtosecond laser is converted into an ultraviolet femtosecond laser of directional focusing, the ultraviolet femtosecond laser is used for exciting an electron gun cathode of the scanning electron microscope to generate a detection photoelectron pulse detection laser system, a second near-infrared femtosecond laser is converted into a pumping femtosecond laser pulse of directional focusing with adjustable wavelength, an optical delay line for adjusting the optical path difference of the detection photoelectron pulse and the pumping femtosecond laser pulse reaching the surface of a sample to be detected, and a secondary electron detection system for detecting transient secondary electron signals generated by scanning of the detection photoelectron pulse after the pumping femtosecond laser pulse is excited.

According to some preferred embodiments of the present invention, the ultrafast scanning electron microscope system comprises: the system comprises a femtosecond optical system and a scanning electron microscope system, wherein the femtosecond optical system comprises a femtosecond laser for generating near-infrared femtosecond laser, a beam splitter for carrying out beam splitting treatment on the near-infrared femtosecond laser, a frequency doubling system for carrying out frequency doubling treatment on different beam-split laser, a detection laser system for obtaining the photoelectric pulse for detection for exciting the cathode of an electron gun according to the beam-split laser, a pumping laser system for obtaining focused femtosecond laser pulse for pumping according to the other beam-split laser, and an optical delay line for adjusting the optical path difference of the femtosecond laser pulse for pumping and the photoelectric pulse for detection; the scanning electron microscope system comprises a scanning electron microscope, an electron gun, an electron optics and scanning deflection system for deflecting and/or focusing the detection photoelectron pulse obtained by exciting the electron gun, a sample bin, an electric control vacuum system for providing a vacuum environment for the sample bin, a secondary electron detector for detecting transient secondary electron signals generated by scanning the detection photoelectron pulse after the pumping is excited by the femtosecond laser pulse on the surface of the sample, a digital scanning generator and a synchronous data acquisition system.

According to some preferred embodiments of the present invention, the ultrafast scanning electron microscope system comprises: the laser instrument of transmission near-infrared femtosecond laser divides into mutually perpendicular's first beam splitting laser and second beam splitting laser and corresponds near-infrared beam splitter or the beam splitter that forms first space light path and second space light path, and wherein, first space light path includes: obtaining a second frequency multiplier device and a third frequency multiplier device of mixed infrared femtosecond laser, wherein the mixed infrared femtosecond laser is femtosecond laser which is obtained by frequency multiplication processing of first beam splitting laser and contains un-frequency-multiplied near infrared femtosecond laser and frequency-multiplied near infrared femtosecond laser with different wavelengths, a first dichroic mirror which is used for obtaining frequency-multiplied femtosecond laser with other wavelengths except for excitation laser and pumped femtosecond laser through the mixed infrared femtosecond laser, a first light collector which is used for collecting the frequency-multiplied femtosecond laser, a polarizing device which is used for polarizing the pumped femtosecond laser, a beam expander device which is used for expanding the laser after the polarization processing to obtain expanded ultraviolet femtosecond laser, an ultraviolet focusing lens which is used for focusing the expanded laser to obtain focused ultraviolet femtosecond laser, and the ultraviolet femtosecond laser enters a scanning electron microscope system through a first optical flange window, exciting a photocathode filament tip of an electron gun of a scanning electron microscope, and exciting an obtained photoelectron beam to sequentially pass through a drawing electrode, a first diaphragm, a second diaphragm, a first electromagnetic coil, a third diaphragm, a second electromagnetic coil, a fourth diaphragm, a deflection coil and a pole shoe from top to bottom to obtain a focused detection photoelectron pulse which is emitted to the surface of a sample to be detected on a sample platform in a sample bin; the second spatial light path includes: a first frequency doubling device for frequency doubling processing of the second beam splitting laser, a second dichroic mirror which is positioned on the light path of the first beam splitting laser and behind the first frequency doubling device and can obtain un-doubled redundant near-infrared femtosecond laser and pumped femtosecond pulse laser, a second light collector for collecting the redundant near-infrared femtosecond laser, a retro-reflector which is arranged in a one-dimensional electric optical delay line and adjusts the direction of the pumped femtosecond pulse laser back to enable the adjusted laser to be parallel and opposite to the transmission direction of the pumped femtosecond pulse laser, the adjusted laser is focused by a second focusing lens to obtain pumped femtosecond laser pulse, the pumped femtosecond laser pulse enters a sample bin through a second optical flange window and is emitted to the surface of a sample to be detected on a sample platform from the direction different from the detection optoelectronic pulse, and a secondary electronic detector is also arranged in the sample bin, the secondary electronic detector is electrically connected with the digital scanning generator and the synchronous data acquisition system.

According to some preferred embodiments of the present invention, the retroreflective mirror includes a first mirror and a second mirror.

According to some preferred embodiments of the invention, the optical flange window is provided with leaded glass; and/or the laser is selected from a high repetition frequency high power femtosecond laser.

According to some preferred embodiments of the present invention, the near infrared beam splitter or beam splitter has a splitting ratio of 1: 1.

According to some preferred embodiments of the invention, the frequency doubling device is selected from a frequency doubler and/or a nonlinear optical parametric amplifier.

According to some preferred embodiments of the present invention, the beam expander is selected from a beam expander and/or a lens combination, and the diameter of the expanded light spot is 5-10 mm.

According to some preferred embodiments of the invention, the one-dimensional electro-optical delay line is selected from a one-dimensional electro-dynamic displacement stage.

According to some preferred embodiments of the present invention, the ultraviolet focusing lens and/or the second focusing lens are mounted on a three-dimensional electrically controlled displacement stage.

According to some preferred embodiments of the present invention, the photocathode is selected from thermal field emission schottky photocathodes.

According to some preferred embodiments of the present invention, the optical flange window and the sample stage are arranged such that: the femtosecond laser for pumping can be made to be incident to the surface of a sample at an included angle of 50-57 degrees with the vertical direction and form a light spot with the diameter ranging from 50-100 mu m.

According to some preferred embodiments of the present invention, the photosensitive surface of the front end of the scintillator of the secondary electron detector is coated with an aluminum film with a thickness of 150-300nm, and/or a narrow band filter that only allows the fluorescence band of the scintillator to pass is additionally arranged between the light guide tube and the photomultiplier of the secondary electron detector.

According to some preferred embodiments of the present invention, the one-dimensional electro-optical delay line has a first application programming interface, the scanning electron microscope has a second application programming interface, and the ultrafast scanning electron microscope system further includes a processing system for performing operation management on the one-dimensional electro-optical delay line and the scanning electron microscope according to the first application programming interface and the second application programming interface.

The invention further provides the use of any of the ultrafast scanning electron microscope systems described above for characterizing the microstructure of a material.

According to some preferred embodiments of the invention, the characterizing comprises obtaining a secondary electron image of the material.

According to some preferred embodiments of the invention, the characterization includes quantitative measurement of carrier dynamics of the opto-electronic functional material under the action of a femtosecond laser.

According to some preferred embodiments of the invention, the application comprises the following synchronized data acquisition process:

before formal testing, the ultrafast scanning electron microscope system is used for measuring a surface carrier dynamics curve of a standard sample, and the time when a pumping laser pulse and a detection electron pulse simultaneously reach an excitation region on the surface of the sample is used as a time zero point of the ultrafast scanning electron microscope system;

adjusting the moving position of the optical delay line to realize the adjustment of the optical path difference between the femtosecond laser pulse for pumping and the photoelectron pulse for detection;

setting the test parameters of the scanning electron microscope;

according to a set acquisition process, obtaining scanning images of the surface of the material to be detected at different moving positions of the optical delay line;

the test parameters comprise one or more of electron gun heating current, electromagnetic diaphragm aperture, electron beam accelerating voltage, image magnification and contrast brightness, electron beam dwell time of single pixel point in the image, single-line scanning times and superposition number of integral scanning of the image.

According to some preferred embodiments of the invention, the acquisition process is performed by one or more of the following means:

(1) sequential acquisition

Continuously collecting the same number of transient secondary electronic images at each collecting position along the same direction of the optical delay line, wherein the collecting conditions of each image are the same;

after the whole acquisition process is finished, the femtosecond laser pulse for pumping is closed, and the photoelectron images with the same quantity at the position of a single delay line or the transient secondary electron images with the same quantity at the position of the delay line far away from a time zero point in the negative time direction are acquired and taken as background signals;

(2) cyclic collection

Only a single transient secondary electronic image is collected at each delay line position by keeping the same collection condition, the single transient secondary electronic image is collected along a displacement direction until the delay line reaches the end point position, the single-pass collection is finished, the femtosecond laser pulse for pumping is immediately closed, and a photoelectron image is collected as a background signal;

starting reverse direction one-way acquisition by taking the terminal position of the delay line as a new acquisition starting point, wherein all acquisition positions on the delay line are fixed, only a single transient secondary electronic image is acquired when each acquisition position returns, and the single transient secondary electronic image is acquired until the initial position of the previous round is returned, and then the femtosecond laser pulse for pumping is closed again, and a photoelectron image is acquired as a new background signal;

taking the above one round-trip acquisition as one cycle, carrying out a plurality of cycles of reciprocating acquisition, and acquiring one additional photoelectron image for subsequent data processing after two opposite single-pass acquisition in each cycle are finished.

According to some preferred embodiments of the invention, the data acquisition comprises data point taking by one or more of:

(1) taking the delay time corresponding to each acquisition point on the optical delay line as a point taking position;

(2) and taking the actual delay line position corresponding to each acquisition point on the optical delay line as a point-taking position.

According to some preferred embodiments of the invention, the application comprises the following data processing procedures:

carrying out image superposition, subtraction, rotation, slicing and pixel value integration processing on the obtained secondary electronic images, and finally fitting to obtain corresponding carrier dynamics curves;

wherein the subtraction process includes: and deducting the transient secondary electronic image acquired after the time zero point from the transient secondary electronic image acquired before the time zero point is far away under the same condition.

The invention is based on the pumping detection principle, a Scanning Electron Microscope (SEM) and a femtosecond laser are combined to form an Ultrafast Scanning Electron Microscope (SUEM) system, the sensitivity characteristic of secondary Electron yield to the surface charge state of a sample when a Scanning Electron beam acts on the surface of the sample is utilized, the Ultrafast dynamic processes of generation, drifting, diffusion, compounding and the like of photo-generated carriers on the surface of a photoelectric functional material can be detected in real time under the ultrahigh space-time resolution scale, the Ultrafast time resolution of the femtosecond laser and the ultrahigh space resolution of the Scanning Electron Microscope are realized at the same time, and the dynamic information of the photo-generated carriers on the surface of the sample is obtained from the ultrahigh space-time scale.

The ultrafast scanning electron microscope system provided by the invention is used as an ultrahigh space-time resolution carrier dynamics detection system, not only retains the original advantages of nano spatial resolution, large span of the space size of a research object (from nano to centimeter magnitude), capability of obtaining information of the surface and three-dimensional shape of a sample, capability of integrating various detectors and the like of a scanning electron microscope, but also combines the femtosecond time resolution owned by a femtosecond laser pumping-detection technology, and greatly breaks through the limitations of single obtained dynamics information, low spatial resolution and the like of carrier dynamics detection means such as the traditional ultrafast spectroscopy technology and the like on detection mode and performance. In addition, the system can be complementary with ultrafast substance structure dynamics research technologies such as ultrafast electron diffraction, four-dimensional ultrafast transmission electron microscope, ultrafast X-ray diffraction and the like in research objects and research fields, and can be widely applied to ultrafast carrier dynamics research of high space-time resolution on material surfaces or interfaces.

The system and the application method of the invention greatly break through the bottleneck of the traditional ultrafast spectrum detection technology on the spatial resolution, can visualize the influence of the surface morphology, defects, passivation treatment and the like of the optical active material on the whole photon-generated carrier compounding process, and have very important application value for the research on the aspects of photoelectric functional materials and devices, semiconductor photoelectric chips and the like.

Drawings

Fig. 1 is a schematic overall frame diagram of an ultrafast scanning electron microscope system according to an embodiment of the present invention.

Fig. 2 is a schematic view of an overall apparatus of an ultrafast scanning electron microscope system according to an embodiment of the present invention.

Fig. 3 is a schematic diagram of a detection laser system in the femtosecond optical system according to the embodiment of the invention.

Fig. 4 is a schematic diagram of a pump laser system according to an embodiment of the present invention.

Fig. 5 is a physical diagram of a light spot ablated on the surface of the conductive carbon adhesive by the irradiation of the pump femtosecond laser according to the embodiment of the invention.

Fig. 6 is a secondary electron image according to an embodiment of the present invention, wherein (a) is a conventional scanning secondary electron image (SEM image) of a copper grid according to an embodiment of the present invention, (b) is a secondary electron image (laser-on) of photoelectron detection without pump laser irradiation in an ultrafast mode, and (c) is a secondary electron image (laser-off) when neither a photocathode nor pump laser acts on a detection laser to excite a sample surface;

fig. 7 is an ultrafast scanning secondary electron image of the entire photogenerated carrier generation, diffusion and recombination kinetics of an n-doped GaAs (100) single crystal according to an embodiment of the present invention, wherein (a) - (d) are ultrafast scanning secondary electron images obtained in a negative delay time, (e) are darkest near an ultrafast scanning secondary electron image obtained near a zero point in time, and (e) - (i) are ultrafast scanning secondary electron images obtained in a positive delay time.

Fig. 8 is a carrier kinetic curve obtained from the process according to fig. 7.

Detailed Description

The present invention is described in detail below with reference to the following embodiments and the attached drawings, but it should be understood that the embodiments and the attached drawings are only used for the illustrative description of the present invention and do not limit the protection scope of the present invention in any way. All reasonable variations and combinations that fall within the spirit of the invention are intended to be within the scope of the invention.

Referring to fig. 1, according to an embodiment of the present invention, an ultrafast scanning electron microscope system includes:

the system comprises a femtosecond optical system and a scanning electron microscope system, wherein the femtosecond optical system comprises a femtosecond laser for generating near-infrared femtosecond laser, a beam splitter for carrying out beam splitting treatment on the near-infrared femtosecond laser, a frequency doubling system for carrying out frequency doubling treatment on different beam-split laser, a detection laser system for obtaining detection photoelectron pulses for exciting a cathode of an electron gun according to the beam-split laser, a pump laser system for obtaining focused pump femtosecond laser pulses according to the other beam-split laser, and an optical delay line for adjusting optical path difference of the pump femtosecond laser pulses and the detection photoelectron pulses; the scanning electron microscope system comprises a scanning electron microscope, an electron gun, an electron optics and scanning deflection system for deflecting and/or focusing detection photoelectron pulses obtained by exciting the electron gun, a sample bin, an electric control vacuum system for providing a vacuum environment for the sample bin, a secondary electron detection system for detecting transient secondary electron signals generated by scanning the sample surface with the detection photoelectron pulses after being excited by pump laser pulses, a digital scanning generator and a synchronous data acquisition system.

Preferably, the femtosecond optical system further comprises a laser beam expander according to the actual requirement on the size of the pump laser spot, and the frequency doubling system comprises a plurality of frequency doublers and/or nonlinear optical parametric amplifiers.

In accordance with the above system, and with reference to fig. 2-4, a particular ultrafast scanning electron microscope system comprises: the laser 1 of transmission near-infrared femtosecond laser 2 divides into mutually perpendicular's first beam splitting laser and second beam splitting laser and corresponds the near-infrared beam splitter or beam splitter 3 that forms first space free path and second space light path, wherein, includes in the first space light path: a second frequency multiplier 23 and a third frequency multiplier 24 for performing frequency doubling processing on the first beam-splitting laser to obtain a mixed infrared femtosecond laser, wherein the mixed infrared femtosecond laser comprises un-doubled near-infrared femtosecond laser and frequency-doubled near-infrared femtosecond laser with different wavelengths, the frequency-doubled femtosecond laser 27 with other wavelengths except the excited laser and a first dichroic mirror 26 for pumping the femtosecond laser 29 are obtained by the mixed infrared femtosecond laser 25, a first light collector (light garbage can) 28 for collecting the femtosecond laser 27 is used for collimating the pumping femtosecond laser 29 through two diaphragms 30 and 31, then the collimated pumping femtosecond laser 29 passes through a polarization device (such as a half-wave plate) 32 for polarization processing, the beam-expanded laser is subjected to beam expanding processing to obtain a beam expander 33 for expanded ultraviolet femtosecond laser 34, and the beam-expanded laser 34 is focused to obtain an ultraviolet focusing lens 36 for focused ultraviolet femtosecond laser 38, the ultraviolet focusing lens is arranged on a three-dimensional precise electric displacement table 36, so that the incident angle and direction of ultraviolet femtosecond laser 38 can be finely regulated, the ultraviolet femtosecond laser 38 enters a scanning electron microscope system through a first optical flange window (with leaded glass) 37, and excites a photocathode filament tip 39 of an electron gun 40 of the scanning electron microscope, a focused detection photoelectron pulse 51 is obtained after an obtained photoelectron beam 44 is excited to sequentially pass through a pull-out electrode 41, a first diaphragm 42, a second diaphragm 43, a first electromagnetic coil 45, a third diaphragm 46, a second electromagnetic coil 47, a fourth diaphragm 48, a deflection coil 49 and a pole shoe 50 from top to bottom, and the photoelectron pulse is emitted to the surface of a sample 52 to be measured on a sample table 53 in a sample bin; the second spatial light path comprises: a first frequency multiplier 4 for frequency multiplication processing of the second sub-beam laser, a second dichroic mirror 5 which is positioned on the light path of the first sub-beam laser and behind the first frequency multiplier 4 and can obtain un-multiplied frequency surplus near-infrared femtosecond laser 6 and pump femtosecond pulse laser 8, a second light collector (light garbage bin) 7 for collecting the surplus near-infrared femtosecond laser 6, a reflector 13 and a reflector 14 which adjust the direction of the pump femtosecond pulse laser 8 in a backward direction and are arranged in a one-dimensional electric optical delay line (such as a one-dimensional electric high-precision displacement table) 12, a first reflector (which can select a reflector with different types of antireflection films plated on the surface according to the actual pump laser wavelength, such as a surface silvered reflector of the pump light of a visible light wave band) for readjusting the transmission direction of the laser parallel and opposite to the transmission direction of the pump femtosecond pulse laser 8 obtained by the reflector 14, a surface-plated aluminum film reflecting mirror 17 and a second reflecting mirror 18 for pumping light in ultraviolet band, wherein the adjusted laser is focused by a focusing lens 19 (which can select optical lenses with different types of antireflection films plated on the surfaces according to the actual pumping laser wavelength) to obtain a pumping femtosecond laser 22, the focusing lens 19 is arranged on a three-dimensional precise electric displacement platform 20 so as to finely regulate and control the incident angle and direction of the pumping femtosecond laser 22, and the pumping femtosecond laser 22 can enter a sample bin through a second optical flange window 21 and is incident to the same region on the surface of a sample 52 to be detected on a sample platform 53 in a direction different from that of a focusing detection photoelectron pulse 51.

In the above system:

the laser 1 is preferably a high power high repetition frequency femtosecond laser, such as in some embodiments, the present invention employs a high power fiber femtosecond laser with a center wavelength of 1030nm, a repetition frequency range of 200kHz to 25.2MHz, an average power of up to 20W, a laser pulse width of over 300fs, and a single pulse energy of up to 10 μ J.

The emergent near infrared femtosecond laser 2 is divided into two beams according to a certain beam splitting ratio through a beam splitter or a beam splitter 3, and in some specific embodiments, the beam splitting ratio is generally set to be 1: 1.

after beam splitting, a beam of femtosecond laser sequentially passes through a frequency doubling system to obtain ultraviolet femtosecond laser for exciting an electron gun to generate photoelectrons. In order to obtain a high-quality pulsed photoelectron beam, photoelectron excitation wavelengths and energies of different wavelengths can be used according to the difference of ultrafast photocathode materials, for example, in some embodiments, a femtosecond laser is quadrupled to obtain an ultraviolet femtosecond laser with a central wavelength of 258 nm.

In some embodiments, the split beam of femtosecond laser may be subjected to secondary frequency doubling and fourth or third frequency doubling to obtain the uv femtosecond laser, or the uv femtosecond laser may be obtained through a nonlinear optical parametric amplifier.

The generated ultraviolet femtosecond laser can realize the expansion of laser facula after entering the beam expanding system, and is convenient for being converged into smaller facula to act on the tip of the photocathode through a lens. In some embodiments, the diameter of the resulting expanded beam spot, i.e., the beam spot, is about 5-10 mm. In some embodiments, the beam expanding system may use a beam expander or a combination of lenses built on its own.

The expanded ultraviolet femtosecond laser is preferably incident to an ultraviolet focusing lens arranged on a high-precision three-dimensional electric control displacement platform for focusing, and the position of the ultraviolet lens is accurately adjusted by the electric control displacement platform, so that the size and the position of a light spot of the femtosecond laser incident to the tip of the photocathode are adjusted. In some embodiments, the ultraviolet focusing lens has a focal length of 30 cm.

The focused ultraviolet femtosecond laser is used for exciting the cathode to generate photoelectron pulses, and in some embodiments, the excitation mode can adopt two modes that the laser is excited from the side surface of the cathode or from the bottom to the upper front surface of the cathode. In some embodiments, the photocathode is selected from thermal field emission schottky photocathodes, which typically emit ultraviolet femtosecond laser single pulse energy in the range of 3-6 nJ.

And the other beam of split infrared laser passes through a frequency doubling or Nonlinear Optical Parametric Amplifier (NOPA) and then is used as pump laser to excite the surface of the sample to generate carriers or a structural dynamic process. In some embodiments, the wavelength of the pump laser passing through the NOPA is in the range of 258-900nm and is continuously adjustable, and the pulse width is in the range of 30-300 fs. In some embodiments, the present invention selects 517nm as the excitation center wavelength. In some embodiments, different pump laser wavelengths may be selected for different materials of interest.

The frequency-doubled pump laser preferably passes through a high-precision one-dimensional electrically controlled translation stage (i.e., a one-dimensional optical time delay line) to precisely adjust the optical path difference between the pump laser pulse and the probe electronic pulse to the position to be measured of the sample, which in some embodiments is a high-precision one-dimensional electrically controlled translation stage, such as a Thorlabs ODL600(/ M) type displacement stage, including but not limited to such a displacement stage.

After the optical path difference is adjusted, the pump laser after being expanded by the beam expanding system is preferably incident to the center of a focusing lens arranged on a high-precision three-dimensional electric control displacement platform so as to accurately adjust the position and the size of the pump femtosecond laser acting on the surface of the sample.

The pump laser after passing through the focusing lens passes through a flange window with leaded glass arranged on a sample bin of the scanning electron microscope, and enters the surface of the sample at a certain angle, for example, the included angle range of 50-57 degrees with the vertical direction is generally, so as to excite the ultra-fast carrier dynamics, and the incident light generally forms an elliptical light spot with the diameter of 50-100 mu m.

The scanning electron microscope system comprises a scanning electron microscope, an ultrafast electron gun, an electron optics and scanning deflection system, a sample bin, an electric control vacuum system, a digital scanning generator, a synchronous data acquisition system and a secondary electron detector signal detection system.

The femtosecond laser excites the ultrafast electron gun to generate pulse electron beams, the pulse electron beams are accelerated and then reach the surface of a sample to be detected in the sample chamber to be scanned after being focused and deflected by an electron optical and scanning deflection system, transient secondary electron signals are generated on the surface of the sample excited by the pumping laser, and then the transient secondary electron signals are detected by a secondary electron detector.

The time difference between the pump laser and the detection electronic pulse reaching the sample surface can be realized by controlling the optical path of the femtosecond laser by the one-dimensional precise displacement table.

The data acquisition and storage process of the ultrafast scanning secondary electron imaging is automatically completed by triggering and controlling the precise optical displacement table, the pulsed electron beam scanning system and the secondary electron detector according to the set logic through a data acquisition control program. In some embodiments, the scanning electron microscope used in the present invention is FEI Quanta S SEM, and the practical scheme is not limited to this type. In some embodiments, the present invention employs an electron gun based on schottky thermal field emission, and the practical solution is not limited to this type.

Further, in the scanning electron microscope system:

under the optimal femtosecond laser irradiation condition, in order to obtain the optimal ultrafast secondary electron imaging quality, preferably, the pulsed electron beam has better spatial coherence, the number of electrons in a single electron pulse and the pulse width meet the response requirements, i.e., a high-quality femtosecond ultrafast pulsed electron beam is obtained, specifically, a single-crystal tungsten needle cathode, a ZrOx/W (100) cathode (the work function of the cathode material is 2.8eV, the diameter of a front emitting surface is 0.2-0.5 μm, the cathode material has lower work function and better electron emission direction and electron coherence) and the like are selected as the photocathode material of a suitable ultrafast electron source, usually a metal or a semiconductor, such as a field emission electron gun, and the photocathode of the thermal emission electron gun is selected from a tungsten cathode and yttrium iridium oxide (Y)₂O₃-Ir) cathodes and borides of alkaline earth or rare earth metals (e.g. LaB)₆、CeB₆、YB₆Etc.) a cathode material, etc.; further selecting specific photocathode shape (such as hairpin shape, needle tip shape, etc. obtained by electron bombardment method, electrochemical corrosion method, etc.), and photocathode surface treatment mode (coating a small zirconium oxide (ZrO) at the upper end of the needle tip of the metal tungsten filament)₂) Coating a layer of yttrium iridium oxide (Y) on the surface of the metal iridium₂O₃Ir)) and emission techniques (introducing femtosecond laser through fiber or focusing the femtosecond laser to the tip of the photocathode filament using a long focusing lens, therebyBased on the einstein photoelectric effect to excite and generate ultrafast photoelectrons), and the like, and the used photocathode material has the excellent characteristics of high quantum efficiency, long operation life, fast time response, high damage threshold, concentrated energy distribution of emitted electrons (small electron dispersion) and the like.

In a preferred embodiment, the invention adopts a Schottky thermal field emission electron gun with high coherence, the photocathode of the Schottky thermal field emission electron gun is a ZrO/W (100) photocathode, the work function of the material is 2.8eV, the diameter of the front-end emission surface is 0.2-0.5 μm, and the Schottky thermal field emission electron gun has lower work function and better electron emission direction and electron coherence.

Compared with the common field emission, the filament is heated to about 1800K through the filament heating current (generally between 2.0 and 2.5A), the electric field of the tip of the filament is changed by adjusting the extraction voltage and the suppression voltage of an electron gun to generate field emission continuous thermal electrons, the invention preferably realizes the ultra-fast photoelectron emission, namely, parameters such as the heating current, the extraction voltage, the suppression voltage and the like of the filament are firstly set to ensure that the filament does not just emit the field emission continuous thermal electrons, and then the ultraviolet femtosecond pulse laser is focused on the tip of the filament of the photocathode, so that the tip is excited to emit the ultra-fast pulsed photoelectron based on the Einstein photoelectric effect, and dozens of micrometers of ultraviolet laser focusing light spots are required to be accurately irradiated on the tip of the filament of the photocathode with the diameter of hundreds of nanometers to ensure that the filament efficiently and stably emits the photoelectron pulses.

In a preferred embodiment, the invention is characterized in that a 1-inch leaded glass optical flange window is arranged at the position of a Schottky field emission electron gun, which is opposite to a cathode Filament, the position of an ultraviolet lens is controlled by a high-stability three-dimensional precise electric control displacement platform arranged near the optical flange window, ultraviolet femtosecond laser is precisely focused at the tip of the Filament from the side surface, and a high-quality ultrafast pulse electron beam is obtained by adjusting the heating current (Ifil) of the Schottky field emission electron gun Filament, the extraction voltage (Vext), the suppression voltage (Vsup) and the first-order convergence deflection voltage (C1 lens) and the series parameters of centering, focusing and astigmatism of the system, the wavelength, the pulse energy and the pulse width of the ultraviolet femtosecond laser and the like, and is used for detecting the ultrafast dynamic process of a sample surface carrier.

Compared with the standard secondary electron detector (ETD) in the traditional scanning electron microscope, the ETD mainly utilizes the scintillator and the photomultiplier tube with a certain emission wavelength range (usually 350-450nm) to perform electro-optic-photoelectric conversion on the received secondary electron signal to realize the detection of the secondary electron signal, the invention performs ultrafast scanning secondary electron imaging based on the pumping-detection principle, needs to use pumping laser with specific wavelength to excite the surface of a sample, because the pumping laser is easily influenced by the reflection/scattering of the surface of the sample and the multiple reflection/scattering on the wall of the sample cavity to generate stray light, part of the stray light enters the secondary electron detector to excite the front end scintillator thereof or directly penetrates through the scintillator to reach the photomultiplier tube at the rear end to generate stronger background signals, so that the noise is larger than the secondary electron signal generated by pulsed light, affecting the quality and signal-to-noise ratio of ultrafast scan secondary electron imaging. In this regard, the present invention further provides a preferred secondary electron detector.

The secondary electron detector is coated with an aluminum film with a certain thickness on the photosensitive surface of the front end of the scintillator by electron beam evaporation or thermal evaporation to block stray pumping laser, for example, in a further embodiment, the aluminum film has a thickness of 150-300 nm.

In the preferred embodiment, because the atomic mass of aluminum is relatively small, secondary electrons can easily penetrate through the aluminum film with the thickness to excite the scintillator behind the aluminum film, and the detection of secondary electron signals cannot be influenced.

And/or a narrow-band filter with a specific transmission waveband is additionally arranged between the light guide pipe and the photomultiplier of the secondary electron detector so as to block possible noise signals. The transmission band is sufficient to transmit only the fluorescence of the scintillator and block the possible leaking pump laser.

In order to enhance the signal-to-noise ratio of the acquired image as much as possible, highlight the light spot signal, set proper acquisition conditions and acquisition modes according to actual conditions such as the characteristics of the sample and the influence of the system by the external environment in the image acquisition process, and accurately and efficiently acquire the photocarrier dynamics information of the material surface through specific image acquisition and data processing programs based on the conditions of high operation repetition degree and large image data amount in the acquisition, the invention preferably adopts the following data acquisition method:

(1) confirming a time zero position of an acquisition

Before formal test, semiconductor material with extremely high response speed, such as highly-doped gallium arsenide and the like, is selected as a standard sample, and the time zero point of an ultrafast scanning electron microscope system is determined by testing the surface ultrafast carrier kinetic curve.

(2) Automated data acquisition

The mutual triggering between the optical delay line and the scanning electron microscope is automatically controlled by calling an interface (API) provided by the optical delay line and the scanning electron microscope, the delay time change of the one-dimensional delay line is controlled in real time, and a large amount of repeated data acquisition work of an ultrafast scanning electron microscope system is carried out, referring to the attached drawing 6, the method can further comprise the following steps:

the movement of a one-dimensional high-precision electric control translation stage (namely the one-dimensional delay line) is controlled by calling a program interface API 1 of the translation stage so as to adjust the optical path difference between a pump laser pulse and a detection photoelectron pulse;

the method comprises the steps that API 2 is called to realize electron beam scanning image electron microscope parameter setting, such as electron gun heating current, electromagnetic diaphragm aperture, electron beam accelerating voltage, image magnification and contrast brightness, electron beam dwell time of a single pixel point in an image, single-line scanning times, number of superposed images of image integral scanning and the like, of a scanning electron microscope by calling a program interface of the scanning electron microscope;

before starting to collect, naming the secondary electronic scanning image to be obtained according to a certain rule, using the naming serial number as a collection cyclic variable to call a displacement table to a specified delay line position so as to enable the program to automatically enter the image scanning after starting, and automatically collecting according to a delay line position list input into the program in advance, wherein the collection can determine different delay line collection positions in the list according to requirements;

the program controls a displacement platform interface API 1 to command the displacement platform interface to move to a corresponding initial acquisition position in the list according to the sequence number of a first acquired image, then the computer receives a trigger signal for ending the current movement of the displacement platform, triggers a program to control an electron microscope interface API 2 to acquire according to preset electron microscope scanning parameters and store the scanning image in the computer, then the API 2 notifies the program to continue to operate, and the program is commanded to interface API 1 to control the displacement platform to enter the next delay line position to start a new acquisition until the whole delay line acquisition list is finished and the acquisition program is automatically ended.

During the collection, the scanning process can be controlled in real time by using a program, real-time monitoring can be further carried out on a user operation interface, problem prompts in the collection process can be obtained, the current errors can be conveniently screened and corrected, and therefore operations such as scanning collection can be suspended, stopped or restarted at any time according to actual conditions.

In some embodiments, the present invention employs the following two image acquisition modes:

(1) sequential acquisition

And continuously collecting the same number of transient secondary electronic images at each collecting position along the same direction of the whole delay line (namely starting unidirectional collection from a negative time point at one end of the delay line far away from a time zero point, and passing the time zero point to the end point of the delay line), wherein the collecting conditions of each image are the same.

After the whole acquisition process is finished, the pump laser is turned off, and the same number of photoelectron images as that at a single delay position or the same number of transient secondary electron images as that at a delay line position (negative delay time) far away from a time zero point are acquired as background signals.

(2) Cyclic collection

Only a single transient image is collected at each delay line position by keeping the same collection condition, the transient images are collected along a displacement direction until the delay line reaches the end point position, the single-pass collection is finished, and then the pumping laser is turned off to collect a photoelectron image as a background signal;

starting one-way acquisition in the opposite direction by taking the terminal position of the delay line as a new acquisition starting point, wherein all acquisition positions on the delay line are fixed, only one transient secondary electronic image is acquired when one acquisition position is returned until the initial position of the previous round is returned, and then, the pumping laser is turned off again to acquire one photoelectron image as a new background signal;

and taking the above one round-trip acquisition as a cycle, carrying out a plurality of cycles of round-trip acquisition according to the actual situation of the acquired signals, and respectively acquiring an additional photoelectron image for subsequent data processing after the two opposite single-pass acquisition in each cycle is finished.

In some embodiments, the invention uses the following two data point-taking modes:

(1) the Delay Time (Delay Time List) corresponding to each acquisition point on the Delay line is used for pointing.

(2) The actual delay line Position (Stage Position List, the length range of the delay line in the present invention is 0-600mm) of each acquisition point on the delay line is used for fixing the point.

In some embodiments, the present invention employs the following image data processing method:

and carrying out image superposition, subtraction, rotation, slicing and integration on the obtained mass of ultrafast secondary electronic image data, and finally fitting to obtain an ultrafast carrier dynamics curve corresponding to the image.

In the image subtraction processing, preferably, an image acquired before the pump laser excitation (i.e., before the pump laser is far away from the time zero point and before the dynamic process is considered to not start at the moment) under the same condition is subtracted from a transient secondary electron image acquired after the pump laser excitation (i.e., after the time zero point), so as to obtain an ultrafast secondary electron differential signal in which noise signals are basically eliminated, and finally, the change between the brightness and the contrast of a light spot displayed by the image can directly reflect the change of the relative concentration of a photon-generated carrier on the surface of the sample, thereby being beneficial to analyzing the dynamic information of the ultrafast carrier on the surface of the material.

Example 1

In the embodiment of the invention, a femtosecond laser is adopted to be excited from the side, namely, an optical window is arranged on the side of a lens cone which is just opposite to an electronic gun of a scanning electron microscope, and ultraviolet laser is introduced into the tip of an internal photocathode filament; in addition, the laser polarization direction is made parallel to the photocathode tip (i.e., vertical direction) by rotating a half-wave plate placed in the path of the ultraviolet laser light to obtain the optimum photoelectron excitation efficiency. And then, the generated pulsed light electrons are accelerated under the acceleration voltage of 1-30kV and pass through a condenser lens, an stigmator, a scanning coil, an objective lens and other elements in an electronic light path of a scanning electron microscope, and the focused pulsed light electrons are used as a detection pulsed electron beam to scan on the surface of a sample to excite transient secondary electrons. Transient secondary electrons are collected in real time by a secondary electron detector (ETD) in the sample bin and corresponding ultrafast scanning secondary electron images are presented.

Example 2

The system of the invention is used for testing gallium arsenide (GaAs) or silicon (Si) semiconductor materials, and the set image acquisition parameters comprise two parts of ultrafast laser parameter setting and scanning electron microscope parameter setting, as follows:

(1) ultrafast laser parameter setting

The repetition frequency of the adopted femtosecond laser is 8MHz, the repetition frequency is continuously adjustable in the range of 200KHz-8 MHz, wherein the incident power of the pumping femtosecond laser is about 3mW, and the incident power of the detection laser is about 50 mW.

(2) Electron microscope parameter setting

Setting the collection parameters of a scanning electron microscope to cumulatively collect about 10-25 scanning images at each delay line position, enabling a photoelectron pulse to be scanned back and forth twice for each line of pixel points in each transient photoelectron scanning image of a region to be detected, setting the Dwell Time (Dwell Time) of each pixel point to be 100ns, enabling each transient secondary electron image to be superposed to be obtained by superposing (integrating) 128 transient photoelectron scanning images, wherein the Resolution (Resolution) of each photoelectron scanning image is 1536 1024pixels, namely 10-128 to 25-128 transient photoelectron scanning images can be obtained at the same delay line position finally, averaging and denoising the images to obtain an ultrafast secondary electron signal, and adjusting the Contrast (Contrast) and the Brightness (Brightness) of the images through the electron microscope, wherein the setting values are respectively 80% -90% and 35% -45%, the ultrafast secondary electron spot signal is more clearly visible.

Example 3

Placing conductive carbon adhesive on a sample stage of the ultrafast scanning electron microscope system, adjusting the conductive carbon adhesive to a height which is 10mm from the working distance under the pole shoe, focusing pump femtosecond laser with 60-70mW power on the surface of the carbon adhesive under the pole shoe, and obtaining a spot ablation trace at a femtosecond laser action site through a scanning secondary electron image of a 30kV field emission electron beam, wherein the spot ablation trace in the image is an ellipse with a short axis of about 50 μm as shown in figure 5, and the range and the shape are the range and the shape of an ultrafast dynamic region which can be excited when the pump light path system focuses the femtosecond pump laser on the surface of a sample under the embodiment.

Example 4

Referring to fig. 6, the imaging quality of the ultrafast scanning electron microscope system of the present invention was tested by a copper grid,

the process comprises the following steps: under a common imaging mode, a clear continuous secondary electron image is obtained by focusing a copper grid mesh under the conditions that a continuous electron beam (the accelerating voltage (HV) is 10kV), the focusing Working Distance (WD) is 10mm, the diameter of a diaphragm is 100 mu m, the Spot size is 3.5 correspondingly selected, and the magnification is 1200 times. And then, switching to an ultrafast mode, keeping the working condition basically unchanged, and detecting photoelectron images of the same area by using the femtosecond pulse photoelectron beams. And finally, in an ultrafast mode, turning off the ultraviolet femtosecond detection laser to obtain a scanning secondary electron image as a reference when the laser pulse excites the photocathode tip without thermal field emission influence.

Comparing the successively obtained scanning secondary electron image in the continuous electron beam mode, as shown in figure 6(a), with the obtained transient scanning secondary electron image in the ultrafast pulsed light electron beam mode, as shown in figure 6(b), it can be seen that the image qualities of resolution, signal-to-noise ratio and the like of the two images are basically at the same level, which fully shows that the imaging quality of the ultrafast scanning electron microscope system of the present invention is very excellent, and the generated pulsed photoelectrons can be completely used as the dynamic process of detecting the photo-generated carriers excited in the electron beam detection material. In addition, under the same test conditions, if no pulse laser acts on the photocathode, the obtained scanning secondary electron image is completely black, as shown in fig. 6(c), namely, no continuous electrons emitted by a thermal field act on the surface of the sample in an ultrafast mode, namely, a transient secondary electron image generated by only ultrafast pulse light electrons acting on the surface of the sample is obtained when only the femtosecond pulse laser excites the cathode tip, which provides a favorable guarantee for researching the ultrafast carrier dynamics and the ultrafast lattice dynamics process of the surface of the photoelectric semiconductor by using the system of the invention.

Example 5

Referring to fig. 7-8, the ultrafast scanning electron microscope system of the present invention is used to characterize n-doped GaAs (100) single crystal, wherein the center wavelength of the pump femtosecond laser is 517nm, the repetition frequency is 8MHz, the surface of the n-doped GaAs (100) single crystal is irradiated with 3mW of incident power, the spot diameter is about 50 μm, the accelerating voltage for pulsed light electrons is 30kV, the focusing working distance is about 10mm, a secondary electron detector is used to collect transient secondary electrons generated by ultrafast electronic pulse synchronous scanning excitation region, and the dynamic evolution process of the excitation region at different times is detected by finely adjusting the relative delay time between the pump optical path and the detection electronic optical path through a one-dimensional mechanical displacement stage.

In fig. 7, the image shows dark spots at negative delay times as shown in fig. 7(a) - (d), the image is darkest near the time zero point as shown in fig. 7(e), the image gradually returns to an equilibrium state at positive delay times as shown in fig. 7(e) - (f), and the intensity of the black spot in the image gradually weakens and the spot diameter becomes larger as the delay time increases as shown in fig. 7(g) - (i). The series of images can visually display the dynamic change process of diffusion and recombination of photo-generated unbalanced carriers of a sample surface in a pumping femtosecond laser action region along with time, specifically, pumping femtosecond laser acts on the sample surface to excite valence band electrons into a conduction band, and then detecting the energy gain of conduction band electrons caused by collision of photoelectrons and excited state electrons in the conduction band, so that the yield of secondary electrons is increased. When the laser irradiation area generates photon-generated carriers, the local photon-generated carriers have very high concentration and form a concentration gradient with the non-irradiation area, so that the local photon-generated carriers are diffused to the periphery, and along with the recombination of electron-hole pairs, namely, conduction band electrons are transited back to energy states with lower energy, such as valence band or defect energy level in band gap along with the delay time, and the excess energy is released in the form of photons or phonons and finally gradually recovered to a dynamic equilibrium state. Analyzing the corresponding physical process that the initial electron pulse causes the nonequilibrium response of the surface of the gallium arsenide material in the negative delay time, and then the energy of the internal secondary electrons is lost through processes of inelastic electron-electron scattering, electron-phonon scattering, electron-impurity scattering and the like under the action of the laser pulse, so that the yield of the secondary electrons is reduced to form a black spot with dark contrast relative to the background; near the time zero point and in the positive delay time, electrons in a gallium arsenide valence band or a donor energy level are excited into a conduction band by pumping laser, pulse detection photoelectrons are scattered as initial electrons and photon-generated carriers in the conduction band to generate secondary electrons capable of escaping from the surface of a material, but the secondary electrons are more difficult to escape from the surface due to the large effective scattering cross section of the conduction band electrons, so that a black spot with a stronger signal appears (the deeper the spot color represents the stronger the signal).

The ultrafast scanning secondary electron image is processed to obtain its time-varying kinetic curve, as shown in fig. 8, whose ordinate is the dark contrast of the spot normalized by the minimum value and whose abscissa is the delay time between the pump pulse laser and the probe pulse electrons. By fitting the dynamic curve, different service lives of the gallium arsenide surface photon-generated carriers can be obtained, and different types of radiation recombination and non-radiation recombination processes are correspondingly generated.

The above examples are merely preferred embodiments of the present invention, and the scope of the present invention is not limited to the above examples. All technical schemes belonging to the idea of the invention belong to the protection scope of the invention. It should be noted that modifications and embellishments within the scope of the invention may be made by those skilled in the art without departing from the principle of the invention, and such modifications and embellishments should also be considered as within the scope of the invention.

Claims (10)

1. An ultrafast scanning electron microscope system, comprising: the scanning electron microscope is used for converting the first near infrared femtosecond laser into directionally focused ultraviolet femtosecond laser, and exciting an electron gun cathode of the scanning electron microscope through the ultraviolet femtosecond laser to generate a detection laser system for detecting photoelectron pulses; the system comprises a pump laser system for converting second near-infrared femtosecond laser into a femtosecond laser pulse with adjustable wavelength and directional focusing for pumping, an optical delay line for adjusting the optical path difference between the photoelectric pulse for detection and the femtosecond laser pulse for pumping reaching the surface of a sample to be detected, and a secondary electron detection system for detecting transient secondary electron signals generated by scanning of the photoelectric pulse for detection after being excited by the femtosecond laser pulse for pumping.

2. The ultrafast scanning electron microscope system of claim 1, comprising: the system comprises a femtosecond optical system and a scanning electron microscope system, wherein the femtosecond optical system comprises a femtosecond laser for generating near-infrared femtosecond laser, a beam splitter for carrying out beam splitting treatment on the near-infrared femtosecond laser, a frequency doubling system for carrying out frequency doubling treatment on different beam-split laser, a detection laser system for obtaining the detection photoelectron pulse for exciting the cathode of an electron gun according to the split laser, a pumping laser system for obtaining the focused pumping femtosecond laser pulse according to the other beam-split laser, and an optical delay line for adjusting the optical path difference of the pumping femtosecond laser pulse and the detection photoelectron pulse; the scanning electron microscope system comprises a scanning electron microscope, an electron gun, an electron optics and scanning deflection system for deflecting and/or focusing the detection photoelectron pulse obtained by exciting the electron gun, a sample bin, an electric control vacuum system for providing a vacuum environment for the sample bin, a secondary electron detector for detecting transient secondary electron signals generated by scanning the detection photoelectron pulse after the pumping is excited by the femtosecond laser pulse on the surface of the sample, a digital scanning generator and a synchronous data acquisition system.

3. The ultrafast scanning electron microscope system of claim 2, comprising: the device comprises a laser for emitting near infrared femtosecond laser, a near infrared beam splitter or a beam splitter for dividing the near infrared femtosecond laser into a first beam splitter and a second beam splitter which are vertical to each other and correspondingly forming a first space light path and a second space light path; wherein the first spatial light path comprises: obtaining a second frequency multiplier device and a third frequency multiplier device of mixed infrared femtosecond laser, wherein the mixed infrared femtosecond laser is femtosecond laser which is obtained by frequency multiplication processing of first beam splitting laser and contains un-frequency-multiplied near infrared femtosecond laser and frequency-multiplied near infrared femtosecond laser with different wavelengths, a first dichroic mirror which is used for obtaining frequency-multiplied femtosecond laser with other wavelengths except for excitation laser and pumped femtosecond laser through the mixed infrared femtosecond laser, a first light collector which is used for collecting the frequency-multiplied femtosecond laser, a polarizing device which is used for polarizing the pumped femtosecond laser, a beam expander device which is used for expanding the laser after the polarization processing to obtain expanded ultraviolet femtosecond laser, an ultraviolet focusing lens which is used for focusing the expanded laser to obtain focused ultraviolet femtosecond laser, and the ultraviolet femtosecond laser enters a scanning electron microscope system through a first optical flange window, exciting a photocathode filament tip of an electron gun of a scanning electron microscope, exciting a obtained photoelectron beam to sequentially pass through a pull-out electrode, a first diaphragm, a second diaphragm, a first electromagnetic coil, a third diaphragm, a second electromagnetic coil, a fourth diaphragm, a deflection coil and a pole shoe from top to bottom to obtain a focused detection photoelectron pulse, and irradiating the focused detection photoelectron pulse to the surface of a sample to be detected on a sample platform in a sample bin; the second spatial light path includes: a first frequency doubling device for frequency doubling processing of the second beam splitting laser, a second dichroic mirror which is positioned on the light path of the first beam splitting laser and behind the first frequency doubling device and can obtain un-doubled redundant near-infrared femtosecond laser and pumping femtosecond pulse laser, a second light collector for collecting the redundant near-infrared femtosecond laser, a retro-reflector which is arranged in a one-dimensional electric optical delay line and adjusts the direction of the pumping femtosecond pulse laser back to enable the adjusted laser to be parallel and opposite to the transmission direction of the pumping femtosecond pulse laser, the adjusted laser is focused by a second focusing lens to obtain pumping femtosecond laser pulse, the pumping femtosecond laser pulse enters a sample bin through a second optical flange window and irradiates the surface of a sample to be detected on a sample platform from a direction different from the detection optoelectronic pulse, and a secondary electronic detector is also arranged in the sample bin and is electrically connected with the digital scanning generator and the synchronous data acquisition system.

4. The system of claim 3, wherein the optical flange window is leaded glass; and/or, the laser is selected from a high repetition frequency high power femtosecond laser; and/or the beam splitting ratio of the near-infrared beam splitter or the beam splitter is 1: 1; and/or, the frequency doubling device is selected from a frequency doubler and/or a nonlinear optical parametric amplifier; and/or the beam expander is selected from a beam expander and/or a lens combination, and the diameter of a light spot obtained after beam expansion is 5-10 mm; and/or the one-dimensional electric optical delay line is selected from a one-dimensional electric displacement table; and/or the ultraviolet focusing lens and/or the second focusing lens are/is arranged on the three-dimensional electric control displacement table; and/or the photocathode is selected from a thermal field emission Schottky photocathode; and/or the optical flange window and the sample stage are arranged to meet the following requirements: the femtosecond laser for pumping can be made to be incident to the surface of a sample at an included angle of 50-57 degrees with the vertical direction and form a light spot of 50-100 mu m; and/or the secondary electron detector is plated with an aluminum film with the thickness of 150-300nm on the photosensitive surface at the front end of the scintillator and/or a narrow band filter which only allows the fluorescence band of the scintillator to pass is additionally arranged between the light guide pipe and the photomultiplier of the secondary electron detector; and/or the one-dimensional electric optical delay line is provided with a first application programming interface, the scanning electron microscope is provided with a second application programming interface, and the ultrafast scanning electron microscope system further comprises a processing system for operating and managing the one-dimensional electric optical delay line and the scanning electron microscope according to the first application programming interface and the second application programming interface.

5. Use of the ultrafast scanning electron microscope system of any one of claims 1 to 4 for characterizing a microstructure of a material.

6. The use according to claim 5, wherein the characterizing comprises obtaining a secondary electron image of the material; and/or, the characterization comprises quantitative measurement of carrier kinetic processes of the photoelectric functional material under the action of femtosecond laser.

7. Use according to claim 5, characterized in that it comprises the following synchronized data acquisition process:

before formal testing, the ultrafast scanning electron microscope system is used for measuring a surface carrier kinetic curve of a standard sample, and the time when the femtosecond laser pulse for pumping and the electronic pulse for detection simultaneously reach an excitation region on the surface of the sample is used as a time zero point of the ultrafast scanning electron microscope system;

adjusting the moving position of the optical delay line to realize the adjustment of the optical path difference between the femtosecond laser pulse for pumping and the photoelectron pulse for detection;

setting the test parameters of the scanning electron microscope;

according to a set acquisition process, obtaining scanning images of the surface of the material to be detected at different moving positions of the optical delay line;

the test parameters comprise one or more of electron gun heating current, electromagnetic diaphragm aperture, electron beam accelerating voltage, image magnification and contrast brightness, electron beam dwell time of single pixel point in the image, single-line scanning times and superposition number of integral scanning of the image.

8. Use according to claim 7, wherein the acquisition process is performed by one or more of the following means:

(1) sequential acquisition

Continuously collecting the same number of transient secondary electronic images at each collecting position along the same direction of the optical delay line, wherein the collecting conditions of each image are the same;

after the whole acquisition process is finished, the femtosecond laser pulse for pumping is closed, and the same number of photoelectron images as the number of photoelectron images at the position of a single delay line or the same number of transient secondary electron images as background signals at the position of the delay line far away from the time zero point are acquired;

(2) cyclic collection

Only a single transient secondary electronic image is collected at each delay line position by keeping the same collection condition, the single transient secondary electronic image is collected along a displacement direction until the delay line reaches the end point position, the single-pass collection is finished, the femtosecond laser pulse for pumping is immediately closed, and a photoelectron image is collected as a background signal;

starting reverse direction one-way acquisition by taking the terminal position of the delay line as a new acquisition starting point, wherein all acquisition positions on the delay line are fixed, only a single transient secondary electronic image is acquired when each acquisition position returns, and the single transient secondary electronic image is acquired until the initial position of the previous round is returned, and then the femtosecond laser pulse for pumping is closed again, and a photoelectron image is acquired as a new background signal;

taking the above one round-trip acquisition as one cycle, carrying out a plurality of cycles of reciprocating acquisition, and acquiring one additional photoelectron image for subsequent data processing after two opposite single-pass acquisition in each cycle are finished.

9. The use of claim 7, wherein the data acquisition comprises data point taking by one or more of:

(1) taking the delay time corresponding to each acquisition point on the optical delay line as a point taking position;

(2) and taking the actual delay line position corresponding to each acquisition point on the optical delay line as a point-taking position.

10. Use according to any of claims 7-9, characterized in that it comprises the following data processing procedures:

carrying out image superposition, subtraction, rotation, slicing and integration treatment on the obtained secondary electronic images, and finally fitting to obtain corresponding carrier dynamics curves;

wherein the subtraction process includes: and deducting the transient secondary electronic image acquired after the time zero point from the transient secondary electronic image acquired before the time zero point is far away under the same condition.

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