Understanding the various interactions between electrons and the specimen chamber gas is essential to using environmental and low-vacuum SEM’s to their full potential. The problem can be divided into two main topics: distortion of the primary beam on its way to the sample surface, and amplification of the various electron signals leaving the sample. These issues are important for all low-vacuum SEM’s, regardless of the system design or signal detection method. All instruments must contend with scattering of the primary beam in the gas, and the subsequent effects on resolution, background signals,.and x-ray generation. Similarly, while some instrument designs rely more heavily than others on signal amplification in the gas, all benefit from the concomitant charge neutralisation.
As the electron probe travels through the gas on its way to the specimen, elastic collisions occur with molecules in the gas. Fortunately, for gas pressures of a few torr, the mean free path is on the order of tens of millimetres, and the vast majority of electrons reach the specimen unscattered.