Sputter coating in scanning electron microscopy is a process of covering a specimen with a very thin layer of
conducting material, typically a metal, such as agold/palladium (Au/Pd) alloy. Conductive coating is needed to prevent charging of a specimen with an electron beam in conventional SEM mode (high vacuum, high voltage). While metal coatings could be useful also for increasing signal to noise ratio (heavy metals are good secondary electron emitters), they are of inferior quality when X-ray spectroscopy employed. Therefore, when X-ray spectroscopy needed, the preferred coating is a carbon coating.