High vacuum mode and pump system
This is the normal mode of operation for the SEM. A high vacuum 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 resolution, especially in the SE mode. The high vacuum condition also optimises collection efficiancy, especially of the secondary electrons.
Because backscattered electrons and characteristic X-rays are generally of higher energy than secondary electrons, their detection is not critically dependant on a high vacuum being maintained in the specimen chamber. Therefore the detectors can be used (but only semi-qualitavely) in the so-called 'low vacuum' operational mode. In this mode, a small amount of air is leaked into the chamber, where it ionises and reduces surface charging of insulating materials.
Note: In low vacuum operational mode it is not the low energies of SEs that limits their use; the main reason is due to arcing in the Everhart-Thornley detector as a result of the kV scale bias on the scintillator
Choose an option to change the pressure mode of the sample chamber.