The structure of the Si(001)-$SiO_2$ interface formed by ion beam oxidation and thermal oxidation was studied with high resolution transmission electron microscopy(HRTEM) and medium energy ion scattering(MEIS) spectroscopy.
In spite of much research for years of decades, the understanding on the structure of the interface was not sufficient to back up ultra-thin oxide technology below 3 nm thickness. Issues still controversial are thickness, composition and structural changes at the interface transition layer. Especially, a use of separation by implanted oxygen(SIMOX) SOI for metal-oxide-semiconductor (MOS) transistor needs the understanding on beam bombardment effects on the Si-$SiO_2$ interface.
In this research, first, the Si(001)-$SiO_2$ interface of a SIMOX SOI formed by 100 keV $O^+$ ion implantation and 1300℃ 4 hours annealing was observed with HRTEM. The interface was very rough and it has many kinds of defect, oxide precipitate, stacking fault and coesite $SiO_2$ phase etc, before annealing. But, the diffusion of implanted oxygen and the growth of the $SiO_2$ precipitate followed by the oxygen diffusion made the continuous buried $SiO_2$ layer. And, the interface of this buried $SiO_2$ became sharp enough comparable to the roughness of the 3 keV ion beam oxide interface and thermal oxide interface.
And the second, a 3 keV ion beam oxide as a model system of ion beam oxidation and thermal oxides grown at 780-900℃ were observed comparatively with MEIS spectroscopy. In the case of the 3 keV ion beam oxide, it can be observed that the heavily disordered Si layer in the transition layer annealed out very fast at high temperature, during 15 seconds at 1200℃.
The interesting observation with MEIS was a blocking dip shift in the interface transition layer which reflect the displacement of the silicon atoms at the interface transition layer. From this observation, the fact that the crystalline silicon atoms at the interface are displaced upward the surface normal direction was conformed. This displacement was observed both in ion beam oxide and thermal oxide. The strain of the displacement were ~2.8% and ~0.96% for 3 keV ion beam oxide and thermal oxides with respect. This strained Si lattice at the interface transition layer was not mapped directly to the electronic states of the interface in this research. However it can be thought that this displacement affects the electronic properties of the Si(001)-$SiO_2$ interface.
