Scanning capacitance microscopy (SCM) has been applied to monitor the two-dimensional (2D) diffusion of Si self- interstitials (I). A sub-micron laterally confined source has been generated by Si self-implantation through a sub-micron oxide mask. The structure was grown by molecular beam epitaxy on (0 0 1) Si, with three spikes of B at different depths used as markers for the interstitial concentration. The measured 2D SCM maps have been accurately quantified to 2D carrier concentration profiles, yielding quantitative information on the B diffusion induced by the I flux. The I supersaturation inside the wafer was monitored by the broadening and the consequent peak concentration lowering of the boron spikes. We show that the I depth- penetration strongly depends on the original source lateral size. Moreover, lateral diffusion of I has been observed, being independent of the source size.