First-principles Study of Interfacial Reaction Mechanism during Silicon
Thermal Oxidation Process

Hiroyuki Kageshima and Masashi Uematsu
Physical Science Laboratory

 Silicon thermal oxide films are essential elements in fabricating silicon-based devices as well as even in future silicon devices. We have been studying the unified understanding on the mechanism of the silicon thermal oxidation process so as to control the process precisely in the atomic scale.
 The oxidation process is generally thought to consist of two sequential processes; the oxygen diffusion process through the covering oxide films and the oxygen reaction process with the silicon substrate at the interface. However, we think the oxygen reaction process should be further classified in two processes; oxygen incorporating process with the substrate and the strain releasing process by the structural transformation. Our picture is supported by recent isotope experimental results, which show the back flow of some reduction species from the interface into the oxide [1].
 For the oxygen incorporation process, first principles studies have revealed that there are not so high reaction barriers as expected by the general understandings (Fig. 1) [2]. Thus, this process does not necessarily govern the interfacial reaction speed. This is because the bonds of the oxygen molecule and of the Si substrate do not completely break through the incorporation process.
 For the strain releasing process, first principles studies have revealed that the high-density oxide regions with oxygen vacancies (Fig. 2) formed at the interface play important roles [3]. Even though these regions accompany the oxygen vacancy, there is no broken dangling bond but only the covalent bonds, being consistent with experimental quite low densities of electric and magnetic defects. These regions can be regarded as regions with interstitial SiO molecules, being consistent with the isotope experiments mentioned above.
[1] S. Fukatsu et al., Appl. Phys. Lett. 83 (2003) 3897.
[2] T. Akiyama and H. Kageshima, Surf. Sci. 576 (2005) L65.
[3] H. Kageshima et al., Jpn. J. Appl. Phys. 43 (2004) 8223.
Fig. 1. Energy profile in the oxygen incorporation process into the substrate
Fig. 2. Structure of high-density oxide regions with oxygen vacancies

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