When one decides to use electron microscopy to investigate a specimen, the end goal of the project must be evaluated to properly choose the right path to obtain that goal. Some applications where the scanning electron microscope would be the instrument of choice might be: studies involving the exterior morphology of the sample, the localization of large (20-30 nm) colloidal gold markers on the surface of the sample, the localization of boundaries between regions of differing atomic number composition, and the qualitative and quantitative identification of the elemental content of the specimen. Each of these applications requires that the instrument be operated properly so as to maximize the excitation and collection of the desired signal.

All electron microscopes are high-vacuum instruments. Vacuums are needed to prevent electrical discharge in the gun assembly (lightning), and to allow the electrons to travel within the instrument unimpeded. There are many scales to measure vacuum levels, some being: mm/Hg, Pascals, Torr, and atmospheres. One undisputed area of vacuums is cost. If higher vacuum levels are desired, better pumping systems are required. Better systems cost more money. Why would one want better vacuums?

When designing the microscope, we started with the vacuum. The electron microscope source to be used should be a factor in the design of the vacuum system. Poor vacuum levels shorten the life of the electron emission source. Saving money in designing the vacuum system might prove costly if filaments are consistantly in need of replacement. Also, any contaminants in the vacuum can be deposited upon the surface of the specimen as carbon. Cleaner vacuums will minimize this artifact.

Different electron emission sources require different vacuum levels. There are 2 classes of emission sources, thermionic emitters and field emitters. Thermionic emitters use an electrical current to heat up the filament which lowers the work function of the filament material. When the work function is lowered, electrons can be more readily drawn off of the filament with an electric field. The two most common materials used for filaments are tungsten and lanthanum hexaboride. Cold cathode field emission sources do not heat the filament material. The electrons are drawn from field emission gun by placin the filament at a huge electrical potential gradient, so large that the work function of the material is overcome, and electrons are drawn off of the filament. Field emission systems require extremely high, clean vacuums in which to operate.