A Cameca IMS-4f secondary ion mass spectrometry instrument has been applied to the study of the diffusion of ultralow energy B implants in crystalline silicon. Several analyses on sub-keV B implants have been performed by using low energy O2+ beams both before and after thermal annealing. The limits and the accuracy of the technique are discussed. It is shown that a 1.5 keV beam provides the depth resolution needed to accurately characterize, beyond the equilibrium depth (∼5 nm), a 500 eV B implant. This measurement protocol provides at the same time a significantly low detection limit (1×1015 at/cm3) and a very fast sputter rate (25 nm/min), necessary to characterize deep diffused profiles. Several artifacts are discussed, with emphasis to those affecting the tail region of the profiles. © 2000 American Vacuum Society.