In the scanning electron microscope (SEM) a beam of electrons is focused and scanned across the specimen. The signal from the detected scattered and emitted electrons is used to form a magnified image with dramatically better resolution and depth of view than an optical microscope could give. The SEM has become an indispensable tool in virtually all fields of research, development, manufacturing and analysis. 

The diverse needs of so many applications have resulted in many different models. In general we can group the scanning electron microscopes into the following categories. 

Field emission SEMs
These instruments are superior in giving very high resolution especially at low accelerating voltages.

Conventional SEMs
General purpose instruments available in different sizes and complexities. These are the most common instruments found in most material research laboratories.

Low vacuum SEMs
In addition to the capabilities of the conventional SEMs, these microscopes can also be used to study samples with poor or no electrical conductivity without any additional conductive coating.

Wafer inspection SEMs
Automated instruments used by the semiconductor industry to investigate defects on the surface of wafers and to measure the critical dimensions of structures.