This study presents the use of atomistic process simulations to optimize p+/n ultra-shallow junctions fabrication process. We first bring to the fore that a high injection of interstitials close to the boron profile decreases the sensibility of boron diffusion to thermal budget. Preamorphization of the substrate is thus necessary to decrease boron diffusion by thermal budget reduction, the latter being obtained by the use of the thermal conduction tool (Levitor) instead of the classical lamp-type rapid thermal annealing. At the same time we show that the use of Levitor does not enhance boron activation, the substrate being preamorphize or not. So Levitor anneal can improve sheet resistance/junction depth trade-off only with preamorphization implant. Experiments are performed that confirm the predictions of our simulations. Further discussions explain activation path of boron during temperature cycle, as a function of amorphous depth, and for both lamp-type and Levitor anneal.