Thermal nitridation of ultrathin SiO ₂ on Si by NH ₃

Thermal nitridation of a 30 Å thick SiO 2 film on Si(100) was carried out at 750–850°C with NH 3 pressures of 100–500 Pa using an ultraclean low‐pressure chemical vapour deposition system, and the depth profile of the nitrogen atom concentration in the SiO 2 film was measured with atomic order resolution by repetitions of XPS measurement and etching with dilute HF solution dipping. It is suggested that nitrogen species are adsorbed at the outermost SiO 2 surface by NH 3 reaction, diffuse into the SiO 2 film and then nitride the Si substrate surface at the SiO 2 /Si interface. With increasing nitridation time, the surface nitrogen concentration initially increased and saturated to 2–3(×10 22 ) cm −3 ; the saturation time was dependent on the NH 3 pressure. The interface nitrogen concentration increased with a delay period and saturated to ∼6.5 × 10 14 cm −2 ; the time to increase of interface concentration was also dependent on the NH 3 pressure. The diffusion characteristics of the nitrogen atoms in SiO 2 were expressed by Fick's law and the temperature dependence of the diffusion coefficient was well determined as D = 2.2 × 10 −8 exp(−2.2 eV/ kT ) (cm 2 s −1 ). Finally, it is clarified that most of the nitrogen atoms can be localized to within a 10 Å thick surface region by choosing the nitridation conditions.