We implanted 3keV B ions into a crystalline Si film, grown by molecular-beam epitaxy and masked by SiO2 stripes with opening widths ranging from 3.2μm down to 0.38μm. Thermal anneals were performed at 800°C for several times. By quantitative high-resolution scanning capacitance microscopy, we demonstrated that the electrical reactivation of inactive B after postimplant annealing is obtained at faster rates as the window width decreases. Total electrical activation is gained first in the narrowest window, with times shorter by nearly a factor of 4 compared to the widest one. In addition, since inactive B seems to be caused by B clustering induced by implantation, our results put in evidence a strong effect of implantation confinement also on B clusters dissolution mechanism. These results have a strong impact on the modern silicon-based device engineering.