Silicon layers, damaged by heavy As + implantation in self-anneal conditions and subsequently amorphized by a second low current implant at room temperature, are studied by channeling Rutherford backscattering spectroscopy, and transmission and high resolution electron microscopy. Moderate temperature anneals are investigated systematically. For some anneal conditions, the amorphized layer leaves a thin continuous buried amorphous layer, thus giving an indication that a different structure from the one which reverts to the crystal by so]id phase epitaxy may exist. Special attention is given to the hypothesis that the buried amorphous layer is obtained by a lattice collapse due to a strong local excess of self-interstitials.