We show that high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM) can be applicable to quantitative examination of Hf chemical distribution at a HfO₂/SiO₂ interface in a ultra-thin gate dielectric of an advanced MOS-FET. We examined how the annealing at 1000^oC changes the chemical composition distribution at the interface and analyzed the observed change in the chemical composition in terms of Hf diffusion in SiO₂. The estimated diffusion coefficient of Hf in SiO₂ was 2.5×10^-18 cm²/s and the diffusion length was 1.6×10^-2 nm for annealing at 1000^oC for 1 s. These indicate that high-temperature annealing used to fabricate Si devices, such as conventional dopant activation process, hardly changes the chemical composition distribution at the interface.