In the present work.a Cz-Silicon wafer is implanted with helium ions to produce a buried porous layer,and then thermally annealed in a dry oxygen atmosphere to make oxygen transport into the cavities.The formation of the buried oxide layer in the case of inteal oxidation(ITOX)of the buried porous layer of cavities in the silicon sample is studied by positron beam annihilation(PBA).The cavities are formed by 15 keV He implantation at a fluence of 2×1016cm-2 and followed by thermal annealing at 673 K for 30 min in vacuum.The inteal oxidation is carried out at temperatures ranging from 1073 to 1473 K for 2 h in a dry oxygen atmosphere.The layered structures evolved in the silicon are detected by using the PBA and the thicknesses of their layers and nature are also investigated.It is found that rather high temperatures must be chosen to establish a sufficient flux of oxygen into the cavity layer.On the other hand high temperatures lead to coarsening the cavities and removing the cavity layer finally.