Atomic number and crystallographic contrast images with the SEM: a review of backscattered electron techniques

Backscattered electrons (BSE) are incident electrons reflected back from a target specimen and imaged with the scanning electron microscope (SEM). Three distinct BSE signals exist: atomic number or Z-contrast, in which composition determines image contrast; orientation contrast, in which specimen crystal structure determines image contrast; and electron channelling patterns (ECP), which are unique for a particular crystal orientation. The origins of these three signals are described, with particular attention being given to the necessary SEM operational and specimen preparation requirements. Z-contrast images are relatively simple to obtain and also have a familiar appearance such that their usage should become commonplace. ECP in comparison require subsequent interpretation which depends on the crystal structure and the relationship between crystal and specimen coordinate systems. A general solution to ECP interpretation is therefore presented, involving the construction of reference 'ECP-maps' over the surface of a sphere. A brief summary of the applications and potential use of the three BSE signals in the geological sciences is also given.