Beyond optical microscopy  (Page 2/5)

How the SEM works

In the SEM, electromagnets are used to bend an electron beam which is then utilized to produce the image on a screen. The beam of electrons is produced at the top of the microscope by heating a metallic filament. The electron beam follows a vertical path through the column of the microscope. It makes its way through electromagnetic lenses which focus and direct the beam down towards the sample. Once it hits the sample, other electrons are ejected from the sample. Detectors collect the secondary or backscattered electrons, and convert them to a signal that is sent to a viewing screen similar to the one in an ordinary television, producing an image.

By using electromagnets an observer can have more control over how much magnification he/she obtains. The SEM has a large depth of field, which allows a large amount of the sample to be in focus at one time. The electron beam also provides greater clarity in the image produced. The SEM allows a greater depth of focus than the optical microscope. For this reason the SEM can produce an image that is a good representation of the three-dimensional sample.

The SEM also produces images of high resolution, which means that closely spaced features can be examined at a high magnification. Preparation of the samples is relatively easy since most SEMs only require the sample to be conductive. The combination of higher magnification, larger depth of focus, greater resolution, and ease of sample observation makes the SEM one of the most heavily used instruments in research areas today.

SEM Usage

The SEM is designed for direct studying of:

• Topography: study of the surfaces of solid objects
• Morphology: study of shape and size
• Brief history of each microscope
• Composition: analysis of elements and compounds
• Crystallographic information: how atoms are arranged in a sample

SEM has become one of the most widely utilized instruments for material characterization. Given the overwhelming importance and widespread use of the SEM, it has become a fundamental instrument in universities and colleges with materials-oriented programs. [2] Institutions of higher learning and research have been forced to take extremely precautious measures with their equipment as it is expensive and maintenance is also costly.

Rice University, for example, has created what is called the Rice Shared Equipment Authority (SEA) to organize schedules, conduct training sessions, collect usage fees and maintain the usage of its high tech microscopic equipment. The following chart indicates prices, location, and necessary training for three of the most popular instruments under SEA jurisdiction:

Advantages and Disadvantages

Among the advantages is the most obvious, better resolution and depth of field than light microscopes. The SEM also provides compositional information for small areas, is relatively easy to use (after training), and the coatings make it semi non-destructive to beam damage. Its disadvantages, however, are all related to the specimen being examined. There are occasions when vacuum compatibility does not allow clear visibility. Specimen preparation can also cause contamination by introducing unwanted artifacts. Lastly, specimen must also be conductive for maximum visibility.