Copper Diffusion Behavior in SiO2/Si Structure During 400°C Annealing

The behavior of copper atoms diffusing from the back surface to the front surface of a silicon wafer during 400°C annealing was examined using total reflection X-ray fluorescence spectrometry. The behavior strongly depended on the back-surface copper concentration and on the front-surface SiO2 thickness. The higher the back-surface copper concentration, the harder it was for the copper to diffuse into the silicon. The number of copper atoms that diffused from the back surface to the front surface was limited by the copper solubility in silicon at 400°C. When the front SiO2 surface was thinner than approximately 3 nm, the atoms easily diffused to the SiO2 surface, but they did not when it was thicker than approximately 3 nm. This difference in the copper diffusion behavior apparently results from whether the SiO2 surface is electrically active or not and indicates that the effect of back-surface copper contamination on the electrical characteristics of semiconductor devices is very small during a back-end process.