Ion channeling and electrical characterization techniques have been used in order to study the effects of thermal annealing on phosphorus implanted silicon wafers. A low energy thermally activated process (0.15-0.28 eV) is clearly observed after annealing at low temperature (less than or equal to 500degreesC). This electrical activation mechanism is found to be well described by a local relaxation model involving point defect migration. It is shown that in order to achieve a complete electrical activation of the implanted impurities, an annealing must be performed at temperatures higher than 700 degreesC.