Growth mechanism of thin oxide films under low-energy oxygen-ion bombardment

Bombardment of silicon surfaces by low-energy oxygen ions has been investigated as a possible process for growing films ofSiOzat room temperature. Broad ion beams ofenergy 40-200 eV and variable oxygen content have been used to grow ultrathin oxides of extremely uniform thickness. The ion beam oxides are similar to thin thermal oxides in many respects-composition, chemical binding, optical, and electrical properties. The dependence of the thickness and quality of the oxide films onion dose, ion energy, and substrate temperature have been investigated. The obtained thickness is observed to vary only slightly with increasing substrate temperature up to 650·C which indicates nonthermal process kinetics. The ion-beam oxides reach a limiting thickness of40-60 Awhich is largely independent of ion dose and is also found to be insensitive to ion energy. The observed oxidation is explained on the basis of surface implantation and radiation-enhanced diffusion and reaction processes. Limited thicknesses are observed even when sputtering is negligible because of the decreasing effective penetration of the ions due to the swelling of the target which accompanies the conversion ofSi to SiOz' Thus the film grows until the oxide-semiconductor interface moves beyond the current-ion penetration depth after which oxidation effectively stops. This model is equally applicable to high-energy, high-dose oxygen-ion implantation for production of buried oxides in silicon-on-insulator technology where it is observed that oxide growth occurs predominantly at the upper interface.