The specimen chamber is maintained at high vacuum that minimises scattering of the electron beam before reaching the specimen. This is important as scattering or attenuation of the electron beam will increase the probe size and reduce the resolution, especially in the SE mode. A high vacuum condition also optimises collection efficiency, especially of the secondary electrons.
The specimen holder is fixed to the specimen stage by the dovetail locating slide. The stage can be moved manually along the X, Y (in the specimen plane), and Z directions (at right angles to the specimen plane). The Z adjustment is also known as the specimen height. The specimen stage can also rotate continuously.
See Principles of operation - High vacuum mode and pump system for more on specimen chamber theory.
This will vary depending on the age and type of machine but as an example, it consists of an oil rotary (backing) pump and an oil diffusion pump. The backing pump is used for rough evacuation while the diffusion pump achieves higher vacuums. A fail safe circuit controls the vacuum sequence and maintains the appropriate vacuum (~10-6 torr) in the optical column and specimen chamber. (Note: The diffusion pump will not work until the backing pump vacuum is adequate. Attempting to operate the diffusion pump without a good backing vacuum will lead to 'backstreaming' and oil contamination of the specimen chamber.) Pump down times will be longer if a specimen is moist or degassing. In these circumstances it is advisable to dry the sample before introduction into the microscope.