Structural damage resulting from the implantation of 5 keV ions into FZ-Si has been investigated by positron annihilation spectroscopy (PAS) using a tuneable monoenergetic beam. Four samples, exposed to ion fluences from to , were studied. The PAS results demonstrate the applicability of the technique to the study of vacancy-type defects in small-scale device structures created by very low-energy ion implantation. Ion depth profiles determined by SIMS exhibited tails extending well beyond the limit predicted by the code TRIM, attributed to ion channelling. PAS, when extended by repeated measurements after precise etching of 40 and 140 nm of material via anodic oxidation, showed that the vacancy-type defect depth profiles also extended far beyond the limit predicted by TRIM. The ratio of defects to ions increases with depth, suggesting that the defect tails are not simply correlated to the implanted ions but that there may also be a contribution from post-implantation defect diffusion.