In this work we demonstrate that He co-implantation can be a powerful tool to control B diffusion in crystalline silicon (c-Si). In particular, the He induced damage leads to the formation of a distribution of nanovoids near the surface that locally suppress the amount of self-interstitials (Is) generated by further B implantation. Thus, B diffusion is reduced and the B implanted profile assumes a box-like shape. In particular, we analyze the microscopic mechanisms leading to the implanted B -nanovoids interaction, demonstrating that the Is injected from the B implanted region annihilate at nanovoids while eroding the nanovoid front layer. By means of a simulation of B diffusion, we will demonstrate that the consequent non-uniform interstitial supersaturation within the sample determines the peculiar B box-like shape, which is particularly appealing for device fabrication.