Introduction to Scanning Electron Microscope (SEM)

A Scanning Electron Microscope (SEM) is an advanced microscopy tool that overcomes the resolution limits of optical microscopes by using electrons instead of light. SEM have been commercialised for about 40 years; since then, the SEM has been developed for various applications. SEM is used in fields that deal with observing the features of specimens in micro- and nano-sized particles. Fig. 1 shows the SEM instrument by Hitachi.

Fig. 1 Scanning Electron Microscope (SEM) instrument by Hitachi [1]

Definition of Scanning Electron Microscope (SEM)

SEM is an advanced electron microscope that is used to observe specimens by irradiating the fine beam of high-energy electrons on the specimens. The variety of signals that are emitted from the surface of the specimen reveals information about the specimen. Information such as external morphology, chemical composition, and crystalline structure of the specimens can be obtained from this technology. The data that is collected from the surface of the specimen is generated as a 2-dimensional image, which describes the spatial variations in the surface properties of the specimen. The schematic representation of SEM is shown in Fig. 2.

Fig-2 Schematic diagram of Scanning Electron Microscope (SEM) [2]

Working Principle of Scanning Electron Microscope (SEM)

SEM works by directing a fine beam of high-energy electrons onto the specimen surface, generating various signals that reveal its properties. The electron-specimen interaction produces secondary electrons, backscattered electrons, diffracted electrons, photons, light, and heat. Among these, secondary and backscattered electrons are mainly used to form images — the former showing surface morphology and topography, and the latter highlighting compositional contrast in multiphase samples. The schematic of this process is shown in Fig-3.

Fig-3 Working principle of Scanning Electron Microscope (SEM) [3]

6 Components of Scanning Electron Microscope (SEM)

SEM is a very complex structure with a variety of components operating in it to analyze the data of the specimen surface. The essential components in the SEM constitute an electron gun, condenser and objective lens, specimen stage, secondary electron detector, image display, recording, and vacuum system.

1. Electron gun: The electron gun is used to produce the electron beam that mostly uses thermionic emission from the cathode source (tungsten filament). The filament is heated to a very high temperature (2800K) and the emitted thermoelectric is focused through a metal plate which acts like an anode. This is done in order to focus the current of the electron beam at the desired point.
2. Condenser and objective lens: These lenses are used to enable the adjustment of the diameter of the electron beam. A condenser lens helps in strengthening the electron beam and adjusting the diameter of the electron beam when it passes through this lens. An objective lens is used to focus the electron beam onto the specimen surface and it determines the final diameter of the electron beam.
3. Specimen stage: This acts as the supporting base of the specimen which stably supports the specimen by moving smoothly in vertical, horizontal, and rotational ways.
4. Secondary electron detector: This is used to detect the secondary electrons emitted from the specimens and it is placed above the objective lens. Magnetic fields are utilized in detecting secondary electrons.
5. Image display and recording: The output signals obtained from the secondary electron detector are amplified and sent to the display unit. Initially, cathode ray tubes were used for the display units however now the liquid crystal display is being used. Recording of these images is obtained in digital format.
6. Vacuum system: The electron optical system and the specimen chamber should be in vacuum condition and hence the components are evacuated by diffusion pumps. In the case of an oil-free environment, then, turbo molecular pumps are used.

Fig. 4 Components of Scanning Electron Microscope (SEM) [4]

Advantages and Disadvantages of Scanning Electron Microscope (SEM)

The image processing and data analysis of the topographical characteristics of any material specimen is very easy in the case of SEM. Using different techniques such as the backscattered electron beam technique, and diffracted backscattered electron techniques then various information such as crystal structure orientation and chemical compositions of the specimens can be obtained. SEM is one of the most used techniques in material surface analysis.

The disadvantages of the SEM include the sizing of the specimen, which should be fitted inside the specimen chamber. The specimen compositions also play an important role as the specimens which oxidize at low pressure or the specimens that are wet, such as organic materials, cannot be determined in this technique. Also, the materials should be electrically conductive or conductive coatings in order to analyze the data.

Applications of Scanning Electron Microscope (SEM)

The application of SEM is in various fields, it is used to determine high-resolution images of the material’s shapes and determine the chemical compositions by acquiring the elemental maps. SEM is used in identifying the phases based on qualitative chemical analysis or by crystalline structure. Backscattered electron images can be used to determine the discrimination of phases in multiphase specimens. Whereas diffracted backscattered electron detectors are used to determine the micro fabric and crystallographic orientation in specimens.

Do check Transmission electron Microscope

 

Reference

[1] https://analyticalscience.wiley.com/do/10.1002/imaging.6389

[2] https://www.britannica.com/technology/scanning-electron-microscope

[3]https://www.thermofisher.com/blog/materials/what-is-sem-scanning-electron-microscopy-explained/

[4] https://www.jove.com/v/5656/scanning-electron-microscopy-sem