Scanning Electron Microscopy Part I: Imaging

What is a scanning electron microscope?

Have you ever wondered why lotus leaves are water-repellent or what keeps your pantyhose from easily tearing? Using the scanning electron microscope (SEM), scientists/engineers are able to observe a variety of materials and life-forms to better understand the world around them. The SEM is an investigative tool that scans the surface of a specimen with a beam of electrons. An image of the specimen is formed by collecting the signals produced by the specimen/beam interaction. Because electrons are used for imaging rather than light, the SEM has a greater resolving power (minimum distance between two points that can still be seen separately) and depth of focus (distance that image remains sharp) compared to a traditional light microscope (Figure 1). For example Hitachi and FEI claim their S-5500 and Magellan™ 400, high-resolution SEMs, are capable of 0.4 nm at 30 kV and 0.8 nm at 15 kV, respectively. To put this into context 0.4 nm is about the diameter of a single potassium atom. It is important to remember that resolution capability is generally more significant than magnification power, blowing up a blurry image doesn’t result in any additional information gained.

Figure 1. The resolving power of the SEM is significantly better than the light microscope or human eyes.

How it works

The SEM forms images using either backscattered or secondary electrons collected as the electron beam scans across the surface of the specimen. The two types of images are often used together to gain a full understanding of the material. A comparison of these two types of images is presented in Figure 2 [3]. The image on the left is formed from secondary electrons, showing contrast from topographical differences. We can see the surface roughness of the alumina fibre balls from this image [3]. The image on the right is formed from backscattered electrons showing darker and lighter regions, which correspond to areas with lighter and heavier elements. This image reveals the location of compositional differences (i.e. precipitates containing heavier elements are observed in the brighter ring around the fibre bundles) [3].

Figure 2. SEM images formed from secondary electrons (left) and backscattered electrons (right) showing topography/surface roughness and elemental differences, respectively.

Backscattered electrons are the electron beam electrons, resulting from the collision between the electron beam and atoms deep within the specimen (Figure 3) [4]. Every element has a different nucleus size. More signals are produced from an element with a larger atomic nucleus (element appears brighter in the image). Secondary electrons originate from the specimen, resulting from the collision between the electron beam and near surface atoms in the specimen (Figure 4) [5]. The interaction causes the electron beam to repel the specimen electrons in the outer shell of the atom. The outer shell electrons are pushed out of the atom and collected to produce an image.

Figure 3. Interaction between electron beam and specimen producing backscattered electrons.

Figure 4. Interaction between electron beam and specimen producing secondary electrons.

Why is the SEM important?

A wide range of materials can be observed under the SEM, from biological tissue to fuel cells. The instrument’s versatility is why it can be found in a variety of educational institutions, industries (e.g. aeroplane manufactures, mining companies), and public sectors (e.g. hospitals, police departments). The average price of an SEM ranges upwards from ~US$100,000 due to the variety of SEM accessories and capabilities. In 2010, world market sales for SEMs were approximately US$132.56 million [6]. The versatility of the SEM and strong market sales are good indications that SEMs will continue to be a staple tool in research and industry. 

Images: Figure 3 REL Inc. Figure 4 Iowa State University. Figure 5 Iowa State University.

References

[1]  Hitachi High-Technologies Canada, Inc. Ultra-high Resolution Scanning Electron Microscope S-5500 [Online]. Available: http://www.hhtc.ca/microscopes/sem/s5500.htm

[2]  FEI Company. (2011). Magellan™ XHR Scanning Electron Microscope [Online]. Available: http://www.fei.com/products/scanning-electron-microscopes/magellan.aspx

[3]  REL Inc. (2008). Scanning Electron Microscopy. Available: http://www.relinc.net/AdvancedMaterials/Analysis/scanningElectronMicroscope.php

[4]  Iowa State University. Backscattered Electrons. Available: http://mse.iastate.edu/microscopy/backscat2.html

[5]  Iowa State University. Secondary Electrons and Detection. Available: http://mse.iastate.edu/microscopy/second2.html

[6]Electronics.ca Publications. (2011). The World Market for Scanning Electron Microscopes. Available: http://www.electronics.ca/publications/products/The-World-Market-for-Scanning-Electron-Microscopes.html

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