Background
Historically the visual examination of very small features of an object has been carried out using visible light microscopy. However, as the resolution of a light microscope is limited by the wave nature of light (i.e. diffraction effects), it was realised that the wave nature of the electron could be exploited. The scanning electron microscope or SEM was developed in the 1950s and became commercially available in the 1960s.

Cambridge SIIA SEM c.1975 RGU's First SEM

The SEM employs a finely focused beam of electrons, which is made to scan across the surface of a specimen housed in an evacuated chamber. Interaction of the beam with the specimen generates a range of signals at or near the specimen surface. One of these signals involves the inelastic scattering of outer electrons in the specimen. These low energy (<50eV) scattered electrons are termed secondary electrons. It is the strength of this secondary electron signal, which is used to provide a visual picture of the specimen surface.

The SEM column can be broken down into several component parts;

• An electron gun which provides a source of electrons.
• A pair of electromagnetic condenser lenses which produce a beam with a very small diameter.
• An objective lens to focus the beam on the specimen surface and scan coils to cause the beam to traverse across the specimen.
• The specimen chamber containing a manipulative stage and secondary electron detector.
• A vacuum system to remove the air from the electron optical column thus providing a free path for the beam electrons.

The electron optical column of an SEM

 
An image is produced in the following manner:
The beam strikes a point on the specimen surface generating a release of SE electrons. These SE electrons are picked up by the detector, which is composed of a light guide with a phosphorescent material at one end. When the SE electrons collide with the phosphor a burst of light is transmitted up the light guide to a photomultiplier tube. The light signal is then converted to an electrical signal, which is used to modulate the scanning beam of the computer monitor. The strength of the SE signal  is represented by a point of light on the computer monitor; the stronger the SE signal, the brighter the point of light.
Schematic of the secondary electron detection system

As the electron beam scans across the specimen a light intensity map of the SE signal is produced on the computer monitor.  Picture contrast occurs as secondary electrons emitted from high points on the sample are more easily detected than those emitted from any low points. Therefore the higher regions appear brighter than the lower regions. The fact that the picture contrast is related to the surface shape of the specimen means that the image tends to look 3D making the SEM a useful tool for the examination of material surfaces.

The magnification of the image is defined as:

Length of line scan on the display monitor


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Length of line scan on the specimen

As the system uses a very finely focused electron beam, the length of line scan may be as small as 10^-4mm or 0.1mm giving magnifications of up to 300 000x. Compared to light microscopes, the SEM offers far greater resolution (approx. 5nm), a huge dynamic range of magnification (10x - 300,000x) and greater depth of focus (3D effect).


LEO S430 digital SEM, RGU's present SEM

The University has two SEMs, a Cambridge S90b used for undergraduate teaching and a Leo S430 used for teaching, analysis and postgraduate work. Although housed in the School of Applied Sciences at the Saint Andrews Street site, the two SEMs are accessable to all schools in the university. The S430 is also fitted with backscattered electron and Energy Dispersive X-ray detectors. They are employed by both undergraduate students and researchers from virtually all areas of science and engineering. Anyone interested in making use of the scanning electron microscope facility should contact:
 

Iain Tough
The Robert Gordon University
School of Applied Sciences
Saint Andrews Street
Aberdeen

Telephone (01224) 262807
[Extension 2807 for internal calls]
e-mail i.tough@rgu.ac.uk

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