The behavior of Cu, diffusing from the rear surface to the front surface of a wafer during annealing (400C), was studied by using total reflection X-ray fluorescence spectrometry. It was found that the behavior depended strongly upon the rear-surface Cu concentration and upon the front-surface SiO2 thickness. The higher the rear-surface Cu concentration, the more difficult it was for the Cu to diffuse into the Si. The number of Cu atoms that diffused from the rear surface to the front surface was limited by the solubility of Cu in Si at 400C. When the front SiO2 surface was thinner than about 3nm, the atoms easily diffused to the SiO2 surface. They did not do so when it was thicker than 3nm. This difference in the Cu diffusion behavior appeared to depend upon whether the SiO2 surface was electrically active or not.

Copper Diffusion Behavior in SiO2/Si Structure During 400C Annealing. K.Hozawa, J.Yugami: Japanese Journal of Applied Physics – 1, 2004, 43[1], 1-8