SEM ELECTRON CHANNELING PATTERNS AS A TECHNIQUE FOR THE CHARACTERIZATION OF ION-IMPLANTATION DAMAGE

Electron channelling patterns (ECPs) formed in back-scattered images in the scanning electron microscope (SEM) have been used occasionally to confirm surface amorphization during ion implantation. In order to place such observations on a more quantitative basis, the study reported here has explored the variation of ECP appearance with both specimen damage levels (and thus subsurface structures) and SEM accelerating voltage (i.e. sampled depth). Polished and annealed (0001) single crystal sapphire discs were implanted to various damage levels up to both subsurface and full surface amorphization. Damage levels were measured independently by Rutherford back-scattering (RBS). Selected-area ECPs were obtained in a Jeol-840 electron microscope operating over the range 5-40 kV in 5-kV steps. Progressive ECP degradation-in terms of high-order line disappearance-was observed with increasing dose, culminating in total pattern loss when full surface amorphization occurred. However, ECP information could still be obtained from the damaged near-surface material even when a subsurface amorphous layer was present, thus demonstrating the shallow retrieval depth of information from the ECP technique. Indeed, because the spatial distribution of damage from ion implantation is both calculable and measurable, these experiments have also allowed us, for the first time, to explore and demonstrate the shallow sample depths from which the majority of ECP contrast originates (< 150 nm in sapphire at an accelerating voltage of 35 kV), even when the beam penetration is considerable by comparison (approximately 5-mu-m). Furthermore, the way in which this sampled depth varies with SEM accelerating voltage is both demonstrated and shown to be a powerful diagnostic technique for studying the distribution of near-surface structural damage.