30 quick questions and answers on basic knowledge of the electron microscope industry

Scanning electron microscope (SEM) is a high-resolution electron microscope used to observe the surface structure of objects. The following are 30 basic knowledge about SEM. Do you know them all?

1. What is SEM?

Answer: SEM is the abbreviation of Scanning Electron Microscope. Unlike traditional optical microscopes, SEM uses electron beams as the "illumination source." The electron beam is focused onto the sample, and the system scans the sample and captures the electron signals reflected or emitted by the sample. These signals are then converted into images that reflect the surface topography, composition or other properties of the sample. SEM is a key tool in many fields such as materials science, biology and industrial inspection.

2. What is the difference between SEM and conventional optical microscope?

Answer: The main difference between SEM and optical microscope is the "light source" they use. An optical microscope uses visible light to illuminate a sample, while an SEM uses an electron beam. Because the wavelength of electrons is much smaller than visible light, SEM can achieve resolutions that far exceed those of optical microscopes. In addition, SEM provides a three-dimensional image of the sample surface, attributes to the way electrons interact with the sample surface, while an optical microscope gives a perspective two-dimensional image.

3. What is the resolution of SEM?
Answer: The resolution of SEM is mainly affected by the diameter of the electron beam, the interaction volume between electrons and the sample, and the performance of the detector. Generally speaking, conventional SEM can achieve a resolution of 1 to 10 nanometers. But this is related to the equipment used, operating conditions and sample types. For example, some SEMs can achieve sub-nanometer resolution when using field emission guns and ultra-high vacuum conditions.

4. Why can SEM provide three-dimensional surface images?
Answer: The images generated by SEM are based on the interaction of the sample surface with the incident electron beam. These interactions occur not only on the surface but also within the sample, but the interactions are strongest near the surface. As a result, SEM images capture minute details and textures on the sample surface, producing a natural three-dimensional visual effect. Compared with optical microscopy, SEM images show the actual surface morphology of the sample rather than the morphology obtained by projection in two dimensions.

5. What are secondary electrons?
Answer: Secondary electrons refer to low-energy electrons that are excited from the sample when the incident electron beam of the SEM hits the surface of the sample. Their energies are usually less than 50 electron volts. When an electron beam strikes a sample, it transfers some of its energy to the electrons of sample, causing those electrons to gain enough energy to escape from the atoms. These escaped electrons are called secondary electrons. Since secondary electrons mainly originate from tiny areas on the sample surface, they provide critical information for producing high-resolution SEM images.

6. Do SEM samples require special treatment?
Answer: Sample preparation is crucial to obtaining high-quality SEM images. For non-conductive samples, in order to avoid charge accumulation under electron beam irradiation, it is usually necessary to coat the sample surface with a thin layer of conductive material, such as gold or carbon. This helps to derive any charge accumulated on the sample, thus avoiding image distortion. Additionally, since SEM operates under high vacuum, the sample must be able to withstand vacuum conditions. For biological samples, steps of fixation, dehydration, and drying are often required to maintain their form and adapt them to the vacuum environment.

7. What is the working principle of SEM?
Answer: The core principle of SEM is to use an electron gun to generate a high-energy electron beam. This electron beam is focused by a series of electromagnetic lenses and scans the sample surface in an orderly manner. When an electron beam interacts with a sample, the sample emits or reflects various electrons (such as secondary electrons and backscattered electrons). These electrons are captured by a detector and converted into electrical signals. These signals are then amplified and displayed on a monitor, forming an image that represents the surface properties of the sample.

8. What are backscattered electrons?
Answer: Backscattered electrons are electrons reflected back by the elastic collision between the incident electron beam and the core of the atoms inside the sample. Backscattered electrons have higher energy than secondary electrons because they are essentially incident electrons that bounced off the nuclei of sample atoms. The generation of backscattered electrons is related to the atomic number (or nuclear charge). This means that heavier elements (such as gold, lead, etc.) produce more backscattered electrons. Therefore, using a backscattered electron detector we can obtain information about the elemental composition of the sample.

9. Why are SEM images black and white?
Answer: SEM images are generated based on electronic signals collected from the sample. These electronic signals carry no color information themselves, so the resulting image is grayscale. Color here is a representation of brightness, where brightness is determined by the number of electrons on the sample. However, false color techniques are often used to color SEM images for specific scientific or visualization purposes.

10. Can SEM provide chemical composition information?
Answer: Yes, although SEM is mainly used to obtain morphological information, the chemical composition information of the sample can be obtained by combining it with an energy dispersive X-ray spectrometer (EDS or EDX). When an electron beam hits a sample, it excites the atoms in the sample to emit specific X-rays. The energy of these X-rays is related to the type of elements in the sample. By analyzing these X-rays, we can determine the elemental composition of the sample.

11. Will the interaction between the electron beam and the sample cause damage to the sample?
Answer: Yes, especially for certain sensitive or organic samples, the electron beam may cause radiation damage. This damage may manifest as localized heating, changes in chemical structure, or other forms of physical changes. In order to minimize this damage, you can adjust the parameters of the electron beam, use a low beam current, or reduce the irradiation time.

12. Can liquid samples be observed under SEM?
Answer: Traditional SEM needs to operate under high vacuum conditions, which means that liquid samples need to be dried or frozen first. However, with the development of technology, environmental scanning electron microscopes (ESEMs) have been developed that can operate at higher pressures, making it possible to observe liquid, wet or frozen samples.

13. Will the size of the sample affect SEM observation?
Answer: The size of the sample does affect SEM observation, especially when its size is close to or exceeds the size of the SEM sample chamber. Most SEMs have sample size limitations. In addition, the shape and height of the sample may also affect focusing and scanning of the electron beam. A sample that is too tall may result in inability to observe properly due to working distance issues.

14. How to interpret the contrast of SEM images?
Answer: The contrast of SEM images comes from the differences in electron generation and detection in different areas of the sample surface. Contrast can be affected by sample topology, composition, charge state, electron beam parameters, and other factors. For example, the interface of different materials will show obvious contrast in the SEM image due to the different electron generation and ejection characteristics of the materials.

15. Why do we need to coat non-conductive samples?
Answer: When a non-conductive sample is irradiated by an electron beam, electrons will accumulate on the surface of the sample because they cannot be efficiently exported. This can lead to charge accumulation, which can lead to image distortion, drift, or other problems. By coating the sample with a thin layer of conductive material such as gold, silver or carbon, this charge buildup can be effectively reduced or eliminated, resulting in clear, distortion-free images.

16. How does SEM achieve deep focus?
Answer: Because the electron beam used by SEM has a very small wavelength, it can produce a large focus range in depth. This allows the SEM to demonstrate great depth definition on a given image, showing even complex and rough sample surfaces clearly. In addition, the depth of focus can be further optimized by adjusting the working distance and the accelerating voltage of the electron beam.

17. Can SEM observe samples in real time?
Answer: Yes, SEM can observe samples in real time. As the electron beam scans the sample, a detector instantly collects the signal from the sample and converts it into an image. This real-time imaging capability is extremely valuable for observing dynamic processes or performing real-time manipulations such as nanomanipulation.

18. Does SEM operation require special training?
Answer: Correct operation of SEM does require special training. Although modern SEMs often feature user-friendly interfaces and automated features, to obtain optimal image quality and ensure long life of the equipment, operators need to have an understanding of SEM fundamentals, sample preparation techniques and underlying sample characteristics, as well as electron beam interactions. Have a deep understanding of the role. In addition, specific training and knowledge are required for the safe use High voltage and vacuum systems.

19. What does "working distance" mean in SEM?
Answer: In SEM, the working distance (WD) refers to the distance between the last lens and the sample surface. Working distance has an impact on image resolution, depth focus, and signal collection. Generally, shorter working distances provide higher resolution but may limit the viewing area of the sample, especially for larger or raised samples.

20. Can the samples under SEM be moved?
Answer: The sample chamber of the SEM is usually equipped with a mechanical stage that allows the sample to be moved in the X, Y and Z axes. This allows the operator to select and adjust areas of interest while adjusting the height of the sample to ensure it is in optimal focus. For some complex applications, it may also be necessary to rotate or tilt the sample to obtain different viewing angles.

21. Why are some SEMs equipped with metal probes?
Answer: Metal probes are often used in nanomanipulation or nanoelectrolithography technology. When combined with an SEM, these probes can perform precise manipulations such as moving nanoparticles, creating nanostructures or even measuring electrical properties at the nanoscale, under real-time monitoring by an electron microscope.

22. Can SEM measure the electrical properties of samples?
Answer: Although traditional SEM is mainly used for morphology observation, SEM can be used to measure the electrical properties of samples by combining with other technologies, such as conductivity probes. This method is commonly used in semiconductor research to study the electrical behavior of materials or devices at the nanoscale.

23. What is the function of the "vacuum" often heard in SEM?
Answer: In SEM, the vacuum environment is crucial for the generation and transmission of the electron beam to the sample surface. In the presence of air or other gases, electrons interact with gas molecules, causing scattering and energy loss, which degrades image quality. The vacuum environment ensures electron beam stability and high-resolution imaging.

24. Can SEM be used to observe biological samples?
Answer: Yes. But because biological samples are mainly composed of moisture and organic matter, they may dry out, shrink or undergo structural changes in the high vacuum environment of traditional SEM. To reduce these problems, samples often require special processing such as fixation, dehydration, and coating. ESEM (environmental scanning electron microscopy) allows observation under higher pressure and humidity conditions, which is very convenient for observing hydrated or partially hydrated biological samples.

25. How is the magnification in SEM images controlled?
Answer: In SEM, the magnification is controlled by changing the area of the sample surface that the electron beam scans. When the electron beam scans a smaller area, the resulting image has a higher magnification. Most SEMs allow the user to easily adjust the magnification, from relatively low magnification (such as 10x or 100x) to very high magnification (such as 100,000x or higher).

26. How is the resolution of SEM determined?
Answer: Resolution describes the ability of a microscope to distinguish two close objects as separate entities. In SEM, resolution is determined by several factors, including the diameter of the electron beam, the nature of the sample, and the type of detector used. In theory, a smaller electron beam diameter could provide higher resolution. In practice, however, optimal resolution may not always be achieved due to sample drift, electron beam stability, and other factors.

27. Why do we need to place the sample in a specific position before SEM observation?
Answer: The sample is positioned to ensure that the electron beam can effectively interact with it, and also so that the detector can collect the signal scattered or emitted by the sample from the correct angle. Correct sample positioning also ensures that it does not interfere with the path of the electron beam or collide with other parts of the microscope.

28. How are the electrons used in SEM generated?
Answer: In SEM, electrons are generated by an electron gun, usually using a tungsten or field emission source. When these sources are heated or a strong electric field is applied, they release electrons. The electrons are then accelerated and focused through a series of electromagnetic lenses into a thin beam, which is then directed onto the sample.

29. How to control the energy of electron beam in SEM?
Answer: The energy of the electron beam is controlled by adjusting its accelerating voltage. The higher the accelerating voltage, the higher the energy of the electrons. Different applications and samples may require different electron beam energies. For example, to obtain high-resolution images of surfaces, a low-energy electron beam may be used, whereas for analysis of deep layers or scattered signals, higher energies may be required.

30. Why do some samples need to be coated before observation?
Answer: Non-conductive samples may accumulate charge when irradiated by electron beams. These accumulated charges can affect the quality of the electron beam and fault images. To eliminate this problem, the surface of the sample can be coated with a conductive material such as gold, silver or carbon. This coating provides a conductive path to the sample, allowing charges to leave the sample and ensuring clear, stable image acquisition.