Scanning Electron Microscope (SEM) Knowledge

What is a Scanning Electron Microscope (SEM)?

The Scanning Electron Microscope (SEM), invented around 1965, utilizes signals such as secondary electrons, backscattered electrons, and characteristic X-rays to observe and analyze the morphology and features of sample surfaces. It is a method for observing microstructures that lies between transmission electron microscopy and optical microscopy.

SEM can be equipped with accessories such as Energy Dispersive X-ray Spectroscopy (EDS), Wavelength Dispersive X-ray Spectroscopy (WDS), and Electron Backscatter Diffraction (EBSD), enabling simultaneous analysis of microstructure, texture, orientation differences, and micro-area composition. It can also be equipped with devices such as heating and tensile testing apparatus inside the sample chamber, allowing for in-situ and dynamic analysis of samples.

Today, SEM finds wide applications in fields such as materials science, physics, chemistry, biology, archaeology, geology, and the microelectronics industry.

Figure 1: Schematic Diagram of SEM Operation

The Basic Principles of Scanning Electron Microscopy (SEM)

A scanning electron microscope (SEM) operates by emitting an electron beam from an electron gun. When high-energy electrons from this beam strike the surface of the sample, they excite regions which then emit secondary electrons, backscattered electrons, absorbed electrons, Auger electrons, cathodoluminescence, and characteristic X-rays. By receiving, amplifying, and displaying these signals as images, the features of the sample surface can be observed, allowing for analysis of its morphology, structure, composition, and other characteristics. SEM primarily utilizes signals such as secondary electrons, backscattered electrons, and characteristic X-rays to analyze the features of the sample surface.

Figure 2:Diagram of Electron Emission

Secondary electrons refer to the outer-shell electrons of sample atoms that are excited by incident electrons. Secondary electrons have low energy and can only escape the surface to a depth of a few nanometers. Therefore, they are highly sensitive to the surface state of the sample and are primarily used for observing the morphology of the sample surface in scanning electron microscopy.

Figure 3:Schematic Diagram of Secondary Electron Detection

Backscattered electrons are high-energy electrons that emerge from the sample surface after being scattered (both elastically and inelastically) by incident electrons within the sample. Their energy is similar to that of the incident electrons. The yield of backscattered electrons increases with the atomic number of the sample elements. Therefore, the intensity of backscattered electron signals is related to the chemical composition of the sample and can display atomic number contrast, making it useful for qualitative analysis of sample composition.

Figure 4:Schematic Diagram of Backscattered Electron Detection

Difference between Secondary Electron Image and Backscattered Electron Image:

A secondary electron image is formed by imaging the outer-shell electrons of the sample that are ejected by incident electrons. These electrons have low energy and can only characterize the surface of the sample, providing relatively high resolution.

Backscattered electrons, on the other hand, are formed by imaging electrons that are scattered by the sample after interaction with incident electrons. They have high energy, similar to that of the incident electrons, and can reflect deeper information within the sample. However, their resolution is relatively lower compared to secondary electrons.

Figure 5:Schematic Diagram of Secondary Electron Scanning Imaging

Scanning Electron Microscope (SEM) Equipment

A scanning electron microscope (SEM) mainly consists of electron optical system, signal collection and processing system, signal display and recording system, vacuum system, computer control system, etc.

The electron optical system comprises components such as an electron gun, electromagnetic lenses, scanning coils, and the specimen chamber. The high-energy electron beam emitted from the electron gun is focused by two stages of electromagnetic lenses to converge into a spot several nanometers in size. The electron beam is then deflected by the scanning coils and synchronously scanned over the specimen surface and screen, exciting various signals from the specimen surface.

The secondary electrons and backscattered electrons excited by the interaction between the electron beam and the sample surface in the specimen chamber first hit the scintillator in the secondary electron detector and backscattered electron detector, generating light. The light signal is then converted into an electrical signal by a photomultiplier tube, further amplified by a preamplifier to produce a sufficiently powerful output signal, and ultimately displayed as an amplified image on a cathode ray tube (CRT).

X-ray signals are collected by the spectrometer (or spectroscope) accessory equipped in the SEM specimen chamber. These signals are then displayed on a monitor as X-ray spectra (or spectrograms) by a lithium-drifted silicon detector, preamplifier, main amplifier, and pulse processor, used for qualitative and quantitative analysis of elements.

Figure 6

Applications of Scanning Electron Microscope (SEM):

1. Observation of Nanomaterials: SEM has high resolution and can observe particles or microcrystal sizes (0.1-100 nm) of composite materials.

2. Analysis of Material Fracture Surfaces: SEM has great depth of field, rich three-dimensional images, and can present the essence and fracture mechanism of material fracture morphology. It is suitable for analyzing material fracture causes, accident reasons, and process rationality.

3. Observation of Large Samples: It can directly observe samples with diameters of 100 mm, heights of 50 mm, or even larger sizes, without any restrictions on the shape of the sample. It can also observe rough surfaces, which eliminates the trouble of preparing samples and allows for the observation of the different contrasts of the sample's own material composition (backscattered electron image).

4. Observation of Thick Samples: It provides high resolution and the most realistic morphology when observing thick samples.

5. Detailed Observation of Various Areas of the Sample: The sample has a large movable range in the specimen chamber and can move in six degrees of freedom in three-dimensional space (i.e., three-dimensional translation, three-dimensional rotation), making it convenient to observe various areas of irregular samples.

6. Observation of Samples at Low Magnifications with Large Field of View: Large field of view and low magnification observation of samples are necessary in some fields such as forensic investigation and archaeology.

7. Continuous Observation from High to Low Magnifications: The magnification range of SEM is wide (continuously adjustable from 5 to 200,000 times), and once focused, it can continuously observe from high to low magnifications or from low to high magnifications without refocusing, which is particularly convenient for analysis.

8. Observation of Biological Samples: The minimal damage and contamination caused by electron irradiation to the sample surface is particularly important for observing some biological samples.

9. Dynamic Observation: If accessories such as heating, cooling, bending, stretching, and ion etching are installed in the specimen chamber, dynamic changes such as phase transitions and fractures can be observed, i.e., in-situ analysis.

10. Obtaining Comprehensive Material Information from Sample Surface Morphology: SEM combined with spectroscopy can determine the segregation of various elements in metals and alloys, observe and identify phases such as intermetallic compounds, carbides, nitrides, and niobides; observe and identify inclusions or second phases at grain boundaries in steel structures; analyze failure modes of components (such as distortion failure, fracture failure, wear failure, and corrosion failure) and identify precipitates and corrosion products on the surface of failed parts. For polished metal samples, SEM combined with EBSD can further analyze crystal structures, textures, and orientation differences.

Advantages and Disadvantages of Scanning Electron Microscope (SEM):

Advantages:

1. Offers high magnification, typically adjustable continuously between 20 and 200,000 times.
2. Provides significant depth of field, large field of view, and three-dimensional imaging, allowing direct observation of fine structures on various uneven sample surfaces.
3. Sample preparation is simple, sample sizes are relatively large (usually the specimen chamber can accommodate samples of several tens of millimeters), and various forms of samples can be tested (fractures, bulk materials, powders, etc.).
4. Can be equipped with accessories such as Energy Dispersive X-ray Spectroscopy (EDS), Wavelength Dispersive X-ray Spectroscopy (WDS), and Electron Backscatter Diffraction (EBSD), enabling simultaneous analysis of microstructure, texture, orientation differences, and micro-area composition.

Disadvantages:

1. Lower resolution compared to Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM), cannot observe molecular and atomic structures.
2. Samples need to be observed in a vacuum environment, limiting the types of samples.
3. Only able to observe surface morphology of samples; structures below the surface cannot be detected.
4. Lacks height-directional information.
5. Cannot observe liquid samples.