Invented some 50 years ago, the scanning electron microscope is applied widely in many scientific applications such as metallurgy, the semiconductor industry, medicine, geology, physics, biology, archaeology – virtually anywhere that people need to image something or to analyse it. Scanning Electron Microscopy is now a mature and and well-established technique. “SEM” can be applied to either the scanning electron microscope itself, or to the technique of scanning electron microscopy.
A normal scanning electron microscope ( SEM ) operates at a high vacuum. The basic principle is that a beam of electrons is generated by a suitable source, typically a tungsten filament or a field emission gun. The electron beam is accelerated through a high voltage (eg: 20 kV) and pass through a system of apertures and electromagnetic lenses to produce a thin beam of electrons that passes down the SEM column. Just above the specimen, the beam is deflected by scan coils so that it scans a rectangular shape over the surface of the specimen. This is similar to the spot scanning the screen in a cathode-ray tube “old-style” television tube.
Electrons are emitted from the specimen by the action of the scanning beam and collected by a suitably-positioned detector.
The SEM operator is watching the image on a screen. Imagine a spot on the screen scanning across the screen from left to right. At the end of the screen, it drops down a line and scans across again, the process being repeated down to the bottom of the screen.
The key to how the scanning electron microscope works is that the beam scanning the specimen surface is exactly synchronised with the spot on the screen that the operator is watching. When the SEM electron beam is at the top left corner of the rectangle as it scans the specimen surface, the spot on the operator’s screen is also at the top left. As the electron beam in the SEM moves to the top right corner, so does the spot on the operator’s screen. As the SEM beam moves down to scan the next line, so does the spot on the operator’s screen. And so on.
The electron detector controls the brightness of the spot on the operator’s screen. As the detector “sees” more electrons from a particular feature on the specimen, the screen brightness is increased. When there are fewer electrons, the spot on the screen gets darker. These days, the screen is generally a digital monitor, not a glass crt, but the principle is the same.
The magnification of the image is the ratio of the size of the screen to the size of the area scanned on the specimen. If the screen is 300 mm across and the scanned area on the specimen is 3 mm across, the magnification is x100. To go to a higher magnification, the operator scans a smaller area, the screen size remaining the same of course. If the scanned area is 0.3 mm across, the magnification is x 1000, and so on.
A typical scanning electron microscope produces images ranging in magnification form approximately x30 to x300,000. While a magnification of x300,000 may sound impressive, sometimes at very high magnifications the image is just magnified fuzz that shows little detail, although much depends on the SEM itself, the specimen and the operator. In most cementitious work, typical magnifications used are at the lower end of this range – perhaps x30 to x5000.
There are several different types of electron image. The two most common are the secondary electron image (sei) and the backscattered electron image (bei). Others include specimen current imaging and cathodoluminescence. Examples of secondary electron imaging and backscattered electron imaging in the context of cementitious materials are shown on the SEM imaging page.
Chapters 1 and 2 of “Scanning Electron Microscopy of Cement and Concrete” contain much more information on the scanning electron microscope itself, and how it can be used in studying cementitious materials.