Transmission micro-grid for realizing atomic-scale precision lossless electron microscopic characterization

Abstract

The invention discloses a transmission micro-grid for realizing atomic-scale precision lossless electron microscopy characterization, and belongs to the technical field of transmission electron microscopes. The transmission micro-grid is prepared from glassy carbon, and a glassy carbon microscopic substrate with high thermal stability is obtained by using a femtosecond laser processing technology and is used as a dual carrier for material growth and TEM characterization, so that the problems of sample morphology change and original information loss caused by a transfer process in the existing micro-grid prepared from a metal material are solved; the problem that a large number of pollutants are brought to a sample due to the fact that the Si-based material reacts with growth raw materials such as S and NaCl at the high temperature is also avoided, and seamless connection between material growth and microscopic characterization is achieved.

Description

Technical Field

[0001] The present invention relates to a transmission microgrid for realizing atomic-precision non-destructive electron microscopic characterization, belonging to the technical field of transmission electron microscopes. Background Art

[0002] A transmission electron microscope (TEM) is one of the important tools for studying the microstructure of materials. Its working principle is to project an accelerated and focused electron beam onto a very thin sample. The electrons collide with the atoms in the sample and change direction, generating a solid-angle scattering. The size of the scattering angle is related to the density and thickness of the sample. Therefore, different bright and dark images can be formed, and these images are magnified and focused and then displayed on the imaging device, thereby helping people observe the microstructure of the sample and achieving atomic-level resolution.

[0003] According to the above TEM working principle, when observing the microstructure of a sample using TEM, the sample needs to be placed on a microgrid. If the sample is prepared and then transferred to the microgrid, it will cause changes in the sample morphology and loss of original information. If the microgrid is directly used as a carrier to prepare the sample, the microgrid needs to not contaminate the sample under the sample preparation conditions (usually the sample preparation process is a gas-phase high-temperature growth process), be able to withstand various extreme preparation conditions, and maintain a relatively stable structure.

[0004] Traditional microgrids are usually made by covering a layer of porous organic film on a metal or Si-based material carrier such as a copper mesh or a nickel mesh, and then evaporating a layer of amorphous carbon film. Metal materials are not resistant to the gas-phase high-temperature growth process and are prone to chemical reactions with raw materials and atmospheres. Although Si-based materials have higher thermal stability and chemical stability, they will react with substances such as S and NaCl at high temperatures, bringing a large amount of contaminants to the sample. In addition, considering the charge accumulation generated during the imaging process of the transmission electron microscope due to the irradiation of the electron beam, it is also required that the microgrid structure has good conductivity. Other materials such as sapphire, ceramics, and quartz will generate charging effects due to lack of sufficient conductivity, affecting the imaging quality. Therefore, the prior art has not been able to provide a method for realizing seamless connection between material growth and microscopic characterization. Summary of the Invention

[0005] To solve the existing problems, the present invention provides a transmission microgrid for realizing atomic-precision non-destructive electron microscopic characterization. The microgrid is prepared from glassy carbon. By using femtosecond laser processing technology, a glassy carbon microscopic substrate with high thermal stability is obtained, which is used as a dual carrier for material growth and TEM characterization. Combined with a vacuum sample exchange chamber, the traditional destructive sample preparation process is avoided, and thus non-destructive electron microscopic characterization without transfer and exposure is realized.

[0006] The first object of the present invention is to provide a transmission microgrid for realizing atomically precise non-destructive electron microscopic characterization. The transmission microgrid is prepared from glassy carbon, and the glassy carbon is a glassy carbon sheet with a thickness of 100-200 microns; the glassy carbon sheet has a microgrid structure formed by an n×m rectangular array pattern processed by a femtosecond laser system.

[0007] Optionally, in the rectangular array pattern, each rectangle has a size of a×b, and the center of the array is aligned with the center of the transmission microgrid, where the value range of a is 200-300 microns and the value range of b is 100-200 microns.

[0008] The second object of the present invention is to provide a preparation method for a transmission microgrid for realizing atomically precise non-destructive electron microscopic characterization. The method includes:

[0009] Step 1, prepare a glassy carbon sheet with a thickness of 100-200 microns;

[0010] Step 2, set the parameters of the femtosecond laser system, including laser pulse parameters and cutting speed;

[0011] Step 3, optimize the laser pulse parameters and the cutting speed, cut the glassy carbon sheet into the required shape to obtain a lens substrate;

[0012] Step 4, process an n×m rectangular array pattern on the lens substrate to form a microgrid structure to obtain a lens microgrid.

[0013] Optionally, the laser pulse parameters include pulse width and repetition frequency; in the optimized laser pulse parameters, the pulse width is 150 fs and the repetition frequency is 1 kHz, and the optimized cutting speed is 30-50 μm / s.

[0014] Optionally, in the rectangular array pattern, each rectangle has a size of a×b, and the center of the array is aligned with the center of the lens substrate, where the value range of a is 200-300 microns and the value range of b is 100-200 microns.

[0015] Optionally, the method further includes using air flow purging to remove glassy carbon debris and residues generated during cutting and processing the rectangular array.

[0016] The third object of the present invention is to provide a transmission electron microscope. The transmission microgrid of the transmission electron microscope is prepared from glassy carbon and is prepared by the above method.

[0017] The fourth object of the present invention is to provide a transmission electron microscopic system. The transmission electron microscopic system includes the above transmission electron microscope and a vacuum sample exchange chamber. The vacuum sample exchange chamber is used to transfer the transmission microgrid and the sample thereon under the condition of isolating air.

[0018] The beneficial effects of the present invention are as follows:

[0019] The present invention provides a transmission microgrid capable of achieving atomically precise non-destructive electron microscopic characterization. Prepared from glassy carbon, it can not only meet the good electrical conductivity requirements for the substrate of a transmission electron microscope but also satisfy the high-temperature gas-phase conditions for sample preparation, and will not contaminate the sample. Further, the present invention also provides a method for preparing this transmission microgrid. By optimizing the femtosecond laser processing parameters, the problem of the swelling effect during the processing of glassy carbon using femtosecond laser technology is overcome, and a relatively flat transmission microgrid is prepared, which can achieve atomically precise non-destructive electron microscopic characterization. Description of the Drawings

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following-described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0021] Figure 1 are the front and back optical images of the transmission microgrid after femtosecond laser processing.

[0022] Figure 2 is a schematic diagram of the influence of femtosecond laser parameters on the thermal expansion effect caused by processing glassy carbon.

[0023] Figure 3 is the optical image of the glassy carbon transmission microgrid after the thermal expansion effect is alleviated.

[0024] Figure 4 is the SEM image of the glassy carbon transmission microgrid.

[0025] Figure 5 is a comparison diagram of SEM images with and without a carbon nanotube film at the same position.

[0026] Figure 6 is a comparison diagram of SEM images of the carbon nanotube film before and after high-temperature deposition of boron nitride at the same position.

[0027] Figure 7 is a comparison diagram of SEM images of the carbon nanotube film before and after high-temperature deposition of molybdenum sulfide at the same position.

[0028] Figure 8 is the transmission electron microscopic imaging diagram of the carbon nanotube-boron nitride nanotube-molybdenum sulfide nanotube ternary structure.

[0029] Figure 9 is a comparison diagram of transmission electron microscopic images of carbon nanotubes at the same position before and after chemical vapor deposition of boron nitride. Detailed implementation manners

[0030] To make the objectives, technical solutions and advantages of the present invention clearer, the following will further describe in detail the implementation manners of the present invention with reference to the accompanying drawings.

[0031] First, the material vitreous carbon used in the preparation of the microgrid of the present invention is introduced as follows: Vitreous carbon, also known as glassy carbon or vitreous carbon, is an amorphous carbon material with some unique characteristics and excellent properties:

[0032] ① It has relatively high hardness and mechanical strength and can resist scratching and abrasion;

[0033] ② It shows excellent thermal stability at non-oxygen high temperatures and can maintain its structure and properties in high-temperature environments;

[0034] ③ It has excellent chemical corrosion resistance and can resist the erosion of acids, alkalis and organic solvents;

[0035] ④ It has good electrical conductivity.

[0036] Secondly, the femtosecond laser processing technology adopted in the solution of the present invention is introduced as follows: Femtosecond laser processing is a technology that uses femtosecond laser pulses for material processing. It has extremely high time resolution and energy density, so it has important applications in the fields of micro-nano processing, precision manufacturing, etc.

[0037] For the micrographic substrate processing of vitreous carbon, the femtosecond laser is an effective means for processing transparent and high-hardness materials, but it also faces several difficulties:

[0038] ① The pulse width is very short. In the case of high power, local heat may still cause material structure changes and uneven oxidation;

[0039] ② Extremely high focusing accuracy is required to achieve the processing accuracy of vitreous carbon with a micron thickness. This requires the optical system to have high-quality focusing ability and stability;

[0040] ③ Different materials have different responses to femtosecond lasers. Vitreous carbon usually reacts with oxygen in the air at high temperatures to form carbon dioxide. However, if the laser parameters are not set properly, the carbon oxidation reaction process will be insufficient, and amorphous carbon will accumulate on the surface of vitreous carbon, resulting in an obvious swelling effect, and further causing the material surface to be uneven. As a carrier for microscopic characterization of samples, the lens microgrid has relatively high requirements for surface flatness. Therefore, if the femtosecond laser processing technology is used for processing, the laser parameters need to be optimized for vitreous carbon materials.

[0041] These difficulties make it necessary to comprehensively consider various factors in the practical application of femtosecond laser processing technology and to carry out precise regulation and optimization. The present invention solves these problems one by one, and the specific solutions are introduced as follows:

[0042] Example 1

[0043] This example provides a transmission microgrid for realizing atomic-precision non-destructive electron microscopic characterization and a preparation method thereof. The transmission microgrid is prepared from glassy carbon. By using femtosecond laser processing technology, a glassy carbon microscopic substrate with high thermal stability is obtained and used as a dual carrier for sample growth and TEM characterization. The preparation method includes:

[0044] Step 1: Prepare a glassy carbon sheet with a thickness of 100-200 microns;

[0045] Step 2: Set the parameters of the femtosecond laser system, including laser pulse parameters and cutting speed;

[0046] The laser pulse parameters include pulse width and repetition frequency.

[0047] Step 3: Optimize the laser pulse parameters and the cutting speed, cut the glassy carbon sheet into the required shape to obtain a lens substrate;

[0048] In the optimized laser pulse parameters, the pulse width is 150 fs and the repetition frequency is 1 kHz. The optimized cutting speed is 30-50 μm / s.

[0049] Step 4: Process an n×m rectangular array pattern on the lens substrate to form a microgrid structure, and obtain a lens microgrid.

[0050] In the rectangular array pattern, the size of each rectangle is a×b, and the center of the array is aligned with the center of the lens substrate, where a ranges from 200 to 300 microns and b ranges from 100 to 200 microns.

[0051] The following takes the preparation of a circular substrate with a diameter of 3 mm as an example for detailed introduction:

[0052] I. Preparation work

[0053] 1.1 Equipment preparation: Femtosecond laser (pulse width <200 fs, wavelength 800 nm), high-precision laser focusing system (such as microscope objective, numerical aperture NA>0.8), precision positioning platform, microscope system, computer control system and software.

[0054] 1.2 Material preparation: 200-micron-thick glassy carbon sheet, support substrate.

[0055] The performance parameters of the glassy carbon used in this example include: apparent density of 1.51 g / cm3 , the resistivity is 42 μΩm. The flexural strength is 147 Mpa. The coefficient of thermal expansion is 2.1x10^-6 / °C. The thermal conductivity is 5.8 W / mK. The impurity content is lower than a certain ppm value. For example, the impurity content of aluminum (Al) is less than 0.08 ppm, and the impurity content of calcium (Ca) is less than 0.04 ppm, and so on.

[0056] II. Femtosecond Laser System Parameter Settings

[0057] 2.1. Laser Pulse Parameters: The pulse width is 200 fs, and the repetition frequency is 100 kHz.

[0058] 2.2. Focusing System Adjustment: Use a microscope objective with a high numerical aperture and adjust the focal length to focus the laser on the surface of the glassy carbon.

[0059] 2.3. Positioning Platform Calibration: Use the microscope system to calibrate the positioning platform to ensure the motion accuracy.

[0060] III. Processing a Circular Substrate with a Diameter of 3 mm

[0061] 3.1. Design and import a circular pattern with a diameter of 3 mm, ensuring that the center of the pattern is aligned with the center of the glassy carbon sheet.

[0062] 3.2. Use the laser to cut along the designed path at a cutting speed of 50 μm / s.

[0063] IV. Processing a 3×3 Rectangular Array

[0064] 4.1. Design and import a 3×3 rectangular array pattern, with each rectangle having side lengths of 200 μm×300 μm, and the center of the array is aligned with the center of the circular substrate.

[0065] 4.2. The cutting speed is 50 μm / s, and adjust the laser focus to accurately focus on the surface of the glassy carbon.

[0066] 4.3. Use the laser to process each rectangle in the rectangular array one by one.

[0067] V. Post-Processing

[0068] Clean the processing surface and use an air flow to blow away debris and residues. The optical microscope characterization of the transmission microgrid after processing is as Figure 1 shown. It can be seen that the surface of the transmission microgrid processed under the above parameters is uneven, and the cutting edge shape is irregular, seriously affecting the focusing and imaging during the subsequent microscopic characterization process.

[0069] VI. Parameter Optimization

[0070] Continuously fine-tune the horizontal height of the glassy carbon substrate to make the laser focus as much as possible on the middle height of the glassy carbon. At the same time, continuously fine-tune the pulse width, repetition frequency, and cutting speed to reduce the thermal expansion effect of the glassy carbon( Figure 2 ), and finally, the optimized pulse width is 150 fs, the repetition frequency is 1 kHz, and the cutting speed is 30 μm / s. Figure 2 The left figure in shows the cutting surface when cutting glassy carbon without optimizing the laser parameters. It can be seen that the expansion effect of the glassy carbon is relatively obvious, and the cutting surface has obvious protrusions. Figure 2 The right figure in shows the cutting surface when cutting glassy carbon after optimizing the laser parameters. It can be seen that although there is still a certain expansion effect, the cutting surface no longer has obvious protrusions.

[0071] The optical microscope characterization of the optimized transmission microgrid is as Figure 3 shown. By comparing Figure 1 it can be seen that after optimizing the parameters, the surface of the processed transmission microgrid is flat, and the shape of the cutting edge is regular, which can ensure that the samples are basically in the same focal plane during the subsequent microscopic characterization process.

[0072] VII. Inspection and Testing

[0073] 7.1. Use a Scanning Electron Microscope (SEM) to characterize the morphology of the prepared transmission microgrid, and confirm whether its diameter size and the position of the rectangular array meet the design requirements. As Figure 4 shown, it can be seen that its diameter size is 3 mm, which meets the requirements, and the rectangular array on it is located at the center of the transmission microgrid.

[0074] 7.2. Use high-quality thin-film carbon nanotubes as a standard sample to evaluate the resolution and imaging stability of the above-prepared transmission microgrid. As Figure 5 , the left figure is the SEM image in the state without a sample, and the right figure is the SEM image with a thin-film carbon nanotube. It can be seen that the carbon nanotube film can be placed very flat on the transmission microgrid, and according to the imaging quality, it can also be inferred that the transmission microgrid has good conductivity.

[0075] 7.3. Use the above-processed transmission microgrid as a carrier, and deposit boron nitride on the thin-film carbon nanotubes by chemical vapor deposition at a high temperature of 1000 °C. Then, evaluate the high-temperature stability of the above-prepared transmission microgrid according to the SEM imaging quality. As Figure 6For the comparison of thin-film carbon nanotubes at the same position before and after depositing boron nitride by chemical vapor deposition, it can be seen that the prepared transmission microgrid itself has excellent high-temperature stability and can still maintain a stable structure after extremely high temperatures above 1000 degrees Celsius, and the SEM imaging quality is still very good. Since the high-temperature stability of the transmission microgrid structure is one of the prerequisites for achieving atomic-scale structure comparison at the same position, if the transmission microgrid structure does not have good stability at high temperatures, that is, it deforms at high temperatures, then the same position before and after high-temperature treatment cannot be found during microscopic characterization, and thus the effect of the boron nitride deposition process on the sample cannot be studied. And currently, Figure 6 In the two figures, it can be clearly determined that they are thin-film carbon nanotubes at the same position.

[0076] 7.4. Using the processed transmission microgrid obtained above as a carrier, molybdenum sulfide is deposited by chemical vapor deposition, and then the chemical stability of the prepared transmission microgrid is evaluated according to the SEM imaging quality, such as Figure 7 For the comparison of carbon nanotubes at the same position before and after depositing molybdenum sulfide by chemical vapor deposition, it can be seen that the prepared transmission microgrid itself has excellent chemical stability and can still maintain a stable structure in an environment of sulfur vapor, and the SEM imaging quality is still very good. Since the high chemical stability of the transmission microgrid structure is also one of the prerequisites for achieving atomic-scale structure comparison at the same position, if the transmission microgrid structure does not have good chemical stability, then the same position before and after deposition cannot be found during microscopic characterization either. And currently, Figure 7 In the left and right figures, it can be clearly determined that they are thin-film carbon nanotubes at the same position.

[0077] 7.5. Under the high-vacuum environment of a transmission electron microscope, the processed transmission microgrid obtained above is heated to 300 °C, and the mechanical stability and imaging stability of the transmission microgrid are observed. Figure 8 For the TEM imaging of the ternary structure of carbon nanotube-boron nitride nanotube-molybdenum sulfide nanotube, it can be seen that the imaging quality is very good, the edge substrate of the nanotubes is obvious, the layers are distinct, and even a clear lattice structure can be seen. Therefore, during the TEM characterization process, it also shows that the prepared transmission microgrid has good structural stability and conductivity. Figure 9 For the comparison of thin-film carbon nanotubes at the same position before and after depositing boron nitride by chemical vapor deposition, it can also show that the prepared transmission microgrid has good high-temperature stability.

[0078] Some steps in the embodiments of the present invention can be implemented by software, and the corresponding software program can be stored in a readable storage medium, such as a CD or a hard disk, etc.

[0079] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A transmission micrograting for achieving atomic-level precision non-destructive electron microscopic characterization, characterized in that: The transmission micro-grating is made of glassy carbon, which is a glassy carbon sheet with a thickness of 100 to 200 microns; the glassy carbon sheet has a micro-grating structure formed by an n×m rectangular array pattern processed by a femtosecond laser system.

2. The transmission micrograting for realizing atomic-level precision non-destructive electron microscopic characterization according to claim 1, characterized in that: In the rectangular array pattern, the size of each rectangle is a×b, and the center of the array is aligned with the center of the transmission micro-grating, wherein a is in the range of 200 to 300 microns, and b is in the range of 100 to 200 microns.

3. A method for preparing a transmission micrograting for achieving atomic-level precision non-destructive electron microscopy characterization, characterized in that: The method comprises: Step 1, preparing a glassy carbon sheet with a thickness of 100 to 200 microns; Step 2, setting the parameters of the femtosecond laser system, including laser pulse parameters and cutting speed; Step 3, optimizing the laser pulse parameters and cutting speed, cutting the glassy carbon sheet into a desired shape, and obtaining a lens substrate; Step 4: Processing an n×m rectangular array pattern on the lens substrate to form a micro-grating structure to obtain a lens micro-grating.

4. The method according to claim 3, characterized in that The laser pulse parameters include pulse width and repetition frequency; the pulse width of the optimized laser pulse parameters is 150fs, the repetition frequency is 1kHz, and the optimized cutting speed is 30-50μm / s.

5. The method according to claim 4, characterized in that In the rectangular array pattern, the size of each rectangle is a×b, and the center of the array is aligned with the center of the lens substrate, wherein a is in the range of 200 to 300 microns, and b is in the range of 100 to 200 microns.

6. The method according to claim 5, characterized in that The method also includes using a gas flow purge to remove glassy carbon debris and residues generated during cutting and machining of the rectangular array.

7. A transmission electron microscope, characterized in that: The transmission micro-grating of the transmission electron microscope is made of glassy carbon and is prepared by the method described in any one of claims 3-6.

8. A transmission electron microscope system, characterized in that: The transmission electron microscope system comprises the transmission electron microscope as claimed in claim 7 and a vacuum sample exchange chamber, wherein the vacuum sample exchange chamber is used to transfer the transmission micrograting and the samples thereon under air-isolated conditions.

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