In situ TEM study of electron-beam radiation induced boron diffusion and effects on phase and microstructure evolution in nanostructured CoFeB/SiO_2 thin film

摘要

Using in situ transmission electron microscopy (TEM), we studied boron diffusion and segregation in CoFeB/SiO_2 nanostructured thin film stacks. We also investigated how these phenomena affected the phase and microstructure of CoFeB thin films under electron beam irradiation at 300 kV. A unique phase transformation was observed in CoFeB thin films under high-dose electron irradiation, from a polycrystalline Co-3Fe to a unilateral amorphous phase of Co_3Fe and nanocrys-talline Fe_xCo_(23-x)B_6. The unilateral amorphization of the Co_3Fe film showed an electron-dose-rate sensitivity with a threshold dose rate. Detailed in situ TEM studies revealed that the unilateral amorphization of the Co_3Fe film arose from boron segregation at the bottom of the Co_3Fe thin film induced by radiation-enhanced diffusion of boron atoms that were displaced by electron knock-on effects. The radiation-induced nanocrystallization of Fe_xCo_(23-x)B_6 was also found to be dose-rate sensitive with a higher electron beam current leading to earlier nucleation and more rapid grain growth. The nanocrystallization of Fe_xCo_(23-x)B_6 occurred preferentially at the CoFeB/SiO_2 interface. Kinetic studies by in situ TEM revealed the surface crystallization and diffusion-controlled nucleation and grain growth mechanisms. The radiation-enhanced atomic diffusivity and high-concentration of radiation-induced point defects at the Co_3Fe/SiO_2 interface enhanced the local short-range ordering of Fe, Co, and B atoms, favoring nucleation and grain growth of Fe_xCo_(23-x)B_6 at the interface.