Electrochemical and radiochemical study of copper contamination mechanism from HF solutions onto silicon substrates

The mechanism of copper contamination of silicon wafers from dilute HF solutions containing ultratrace levels of metallic ion impurities, was investigated using a new electrochemical cell, which proved to act as a very efficient sensor for in situ characterization. Upon copper contamination, the open-circuit potential was observed to shift rapidly toward more positive values at a rate nearly proportional to the copper concentration. All potential/time curves tend to reach a plateau, while quantitative measurements using radioactive tracers revealed that during a few tens of minutes, copper ions were continuously reduced on the silicon surface. Results are interpreted in terms of the mixed-potential theory and lead to the conclusion that copper nuclei act as a catalyst which enhances the cathodic activity for proton reduction. The model was supported by atomic force microscopy observations which showed the initiation of corrosion pits around the nuclei.