The redistribution phenomena (such as pile-up, push-back) of arsenic impurities in silicon during thermal oxidation are dependent upon the oxidation rate. the diffusivities of arsenic in silicon and SiO2, and the segregation rate of arsenic impurities at the interface between the oxide and silicon. The diffusivity of arsenic in SiO2 is known to be negligible compared with the diffusivity of arsenic in silicon and the oxidation rate of silicon. The diffusivity of arsenic in silicon is also dependent on the arsenic concentration. The pile-up at the Si-SiO2 interface as a result of the concentration dependence of arsenic. has net reported so far. For silicon samples implanted with low fluences (1 x 10(15) or 3 x 10(15) cm(-2)) of arsenic at 100 keV, a pile-up of arsenic was observed during thermal oxidation at 1050 degrees C, using Rutherford backscattering spectroscopy. For silicon samples implanted with fluences greater than 3 x 10(16) cm(-2), push-back phenomena were observed. These phenomena can be explained only by the diffusivity of arsenic, dependent upon the concentration of arsenic in the silicon.