Growth mechanism of thin silicon oxide films on Si(100) studied by medium-energy ion scattering

The growth of ultrathin oxide films by the thermal oxidation of Si(100) at 1020--1170 K and in 10^-1--10^-3 Torr O₂ pressure has been studied by high-resolution medium-energy ion-scattering spectroscopy (MEIS). To develop a fundamental understanding of very thin oxide film growth, we utilize sequential isotopic exposures (¹⁸{O}₂$ followed by $¹⁶O₂$). MEIS readily distinguishes $¹⁸O$ from $¹⁶O$ and the depth distribution for both species can be determined quantitatively with high accuracy. Our results show that the traditional phenomenological models for silicon oxidation cannot be applied to the initial oxidation. For very thin oxide films (15--25 {}), we find overlapping isotope depth profiles in the film. For thicker films (>40 {}), we find that several key aspects of the Deal-Grove model (oxygen diffusion to the Si-SiO₂ interface and oxide formation at and/or near that interface) are consistent with our results. We also observe ¹⁸O loss from the surface after reoxidation in ¹⁶O₂. The complex oxidation behavior during the initial oxidation is likely to be a combination of interfacial, near-interfacial, and surface reactions.