Eliminating charging in the SEM
Electron charging is the build up of negative charges on a specimen irradiated with an electron beam. Charging may occur in a SEM when there is poor electrical conductivity of the specimen. To eliminate electron charging most mineral samples will need to be coated with a thin film of a conductive material before being placed into the SEM chamber. If a sample has a high electrical conductivity then it may not need a conductive coat. Carbon is most commonly used as it provides a good electron transparent, conductive coating, but other materials can be used such as gold. A lot of carbon coaters use a rod of carbon as an evaporation source. Before use the carbon rods will need to be prepared using a carbon coater rod sharpener to create a sharp tip with the required angle and shape for the coating unit, if this is not done the carbon coat may not be even across the sample. The thickness of the carbon coat needs to be controlled accurately, as if the coat is too thin the sample will charge and if the coat is too thick it will reduce the strength of the Kikuchi patterns and decrease the number of indexed points. While reports vary as to optimal carbon coat thicknesses, such as 2.5-5nm for biominerals (Perez-Huerta and Cusack, 2009) and 8-97nm for Mg and Si coated standards (Limandri et al., 2010), 7-15nm for various geological materials (Lloyd and Hall, 1981), all of the literature agrees that the carbon coat should be as thin as possible.
After the conductive coat has been applied to the sample either carbon paint or carbon tape is used to make sure any charge build up can be dissipated via the sample holder. The choice of whether to use carbon paint or carbon tape comes down to the geometry of your sample and personal preference. If your sample is a thin section, then you have a slice of rock sitting on top of a glass slide with sharp edges, these edges will attract a build up of electrons therefore carbon paint is the better choice as you can paint over all of the edges and eliminate any possible charging locations. If your sample is completely flat (thin wafer or sample in resin), then either can be used, although carbon paint is more conductive than carbon tape.