Low Vacuum SEM
Overview
Traditional SEMs are operated under high vacuum. With specialized detectors, certain SEMs may be operated in a low vacuum mode to image nonconductive or vacuum sensitive samples. In the case of the TFS Quanta 600 FEG ESEM, a water vapor environment is introduced into the sample chamber.
Theory
Surface Charge Neutralization
When imaging a non-conductive sample under high vacuum, electrons tend to build up near the surface of the specimen. When a water vapor or other low vacuum environment is introduced into the specimen chamber and exposed to an electron beam, the water vapor produces positive ions. The positively ionized particles in a low vacuum SEM are attracted to the negative charge accumulation on the specimen and can neutralize the surface charge.
Scattered Electron Amplification
In addition, the low vacuum environment contributes to signal amplification. As electrons are emitted from the sample surface, they scatter off the low vacuum particles (e.g. water vapor.) These additionally scattered electrons create a cascade effect of even more electrons, which increases the signal yield from an otherwise low yield non-conductive specimen. However, too high pressure can cause images to look washed out or streaky due to overamplification or delay between signal generation and collection.
Accelerating Voltage Selection
High accelerating voltages encourage both surface charge neutralization and amplification of scattered electrons, in addition to better penetration through the higher pressure environment and formation of a sharper electron probe. Too high of a voltage for the selected pressure may result in streaked images due to the electron cascade effect. Chamber pressure and accelerating voltage are typically increased together.