Cobalt and iron segregation and nitride formation from nitrogen plasma treatment of CoFeB surfaces

Cobalt-iron-boron (CoFeB) thin films are the industry standard for ferromagnetic layers in magnetic tunnel junction devices and are closely related to the relevant surfaces of CoFe-based catalysts. Identifying and understanding the composition of their surfaces under relevant processing conditions is therefore critical. Here we report fundamental studies on the interaction of nitrogen plasma with CoFeB surfaces using infrared spectroscopy, x-ray photoemission spectroscopy, and low energy ion scattering. We find that, upon exposure to nitrogen plasma, clean CoFeB surfaces spontaneously reorganize to form an overlayer comprised of Fe₂N₃ and BN, with the Co atoms moved well below the surface through a chemically driven process. Subsequent annealing to 400 °C removes nitrogen, resulting in a Fe-rich termination of the surface region.