Eliminating charging in the SEM

Electron charging is the buildup 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 conductive film before being placed into the high-vacuum SEM chamber. Depending upon the SEM it is also possible to work under low vacuum and in this case no coating would be required. Also if a sample has a high electrical conductivity then it may not need a conductive coat. When coating is required, carbon is most commonly used as it provides a good electron transparent (low atomic number), conductive coating, but other materials can be used such as gold. Many carbon coaters use a rod of carbon as an evaporation source. Before use, the carbon rods will need to be sharpened to create a 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 a thin coating will cause the sample to charge and a thick coating will reduce the strength of the diffraction patterns and decrease the fraction of successful indexing. While reports vary as to optimal carbon coat thicknesses, such as 2.5-5nm for biominerals (Perez-Huerta and Cusack, 2009) and 8nm for Mg coated standards (Limandri et al., 2010), all of the literature agrees that the carbon coat should be as thin as possible and 3-5nm is generally accepted for EBSD (Bestmann et al., 2011; Trimby and Prior, 1999).

After the conductive coat has been applied to the sample either conductive paint (carbon or silver) 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 only be very thinly coated. In this situation conductive paint is generally the better choice as all edges can be painted to eliminate any possible charging. 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 and silver paint is more conductive than carbon paint.