Driving Force in Diffusion and Redistribution of Reducing Agents During Redox Reaction on the Surface of CoFeB Film

Diffusion and redistribution of constituent elements on the CoFeB surface after natural oxidation and annealing were investigated by means of X-ray photoelectron spectroscopy with argon beam sputtering. Boron and iron atoms were found to redistribute into the ~3 nm-thick oxidized surface by the natural oxidation even at room temperature. After annealing up to 400 °C in vacuum, boron atoms drastically diffused into the oxidized surface to reduce the oxidized iron/cobalt. The migration of boron atoms was observed within the depth of ~6 nm. In contrast, iron atoms diffused little. The diffusion and redistribution behaviors during the redox reactions, such as the natural oxidation and annealing, are speculated to be related to the values of the standard free energy of formation of oxides. In other words, the reduction nature of elements on the CoFeB surface is speculated to be the driving force in the diffusion and redistribution of iron and boron on the surface during the redox reaction.