QUANTUM MOLECULAR DYNAMICS STUDY ON THE OXIDATION OF
Kazuo Teraishi, S. Salai Cheettu Ammal, Aruba Yamada, Akira Endou, Momoji Kubo, and Akira Miyamoto, Tohoku University, Department of Materials Chemistry, Graduate School of Engineering, Sendai, Japan; Masahiro Kitajima, National Research Institute for Metals, Tsukuba, Ibaragi, Japan.
With a reduction of device dimensions in ULSIs, the establishment of atomically controlled and low-temperature processes for the formation of ultrathin gate oxide films is urgent in semiconductor manufacturers. Growth rate of thermally grown oxide films on Si surface can be estimated excellently by Deal-Grove model  at the thickness of ca.40 nm. However, today's advanced ULSI fabrication technology relies on the gate oxide film of sub-10 nm thickness, and various models/mechanisms have been proposed for the oxidation of silicon surface [2-4]. We have investigated here the oxidation process in detail considering various competing reaction pathways. The reactions have been followed by estimating the potential of mean force that is regarded as free energy along the reaction coordinate through semi-empirical quantum MD and umbrella sampling.
Si(111) surface has been modeled by the S26H28 cluster and the following three competing path ways have been considered; (i) oxidation of hydrogen terminated surface (ii) oxidation of hydrogen defect site (iii) oxidation of partially oxidized site. These reactions involve multiple steps and they have been analyzed through activation free energy. Oxidation of both, complete hydrogen terminated surface and hydrogen defect site are found to have activation free energy of 50 kcal/mol but they have different rate determining steps; in the former insertion of oxygen is the slow step while in the latter oxygen migration from the precursor to form a stable oxide is the rate determining step. The desorption of hydrogen from the Si surface requires 50 kcal/mol activation free energy. The activation free energy for the oxidation of partially oxidized site is 40 kcal/mol, hence the preoxidation by the activated oxygen species may accomplish the selective oxidation prior to the hydrogen desorption.