COPPER CONTAMINATION MECHANISM OF SILICON SUBSTRATES FROM HF SOLUTIONS

The contamination of silicon wafers from dilute HF solutions containin g ultratrace levels of metallic ion impurities is a subject of constan t interest. The mechanism of copper electroless deposition from HF ont o monocrystalline silicon was investigated using a new electrochemical cell, which proved to be a very sensitive detector for in situ charac terization of silicon surfaces. Upon addition of copper trace amounts, the open-circuit potential was observed to shift rapidly towards more positive values at a rate nearly proportional to the copper concentra tion. All potential/time curves tend to reach a limiting value of the potential, while quantitative measurements of radioactive tracers reve aled that during a few tens of minutes, copper ions were continuously reduced on the silicon surface. Electrochemical potentials and voltamm etric measurements were interpreted in terms of the mixed potential th eory and led to the conclusion that copper nuclei act as a catalyst wh ich enhances the cathodic activity for protons reduction. The model wa s supported by AFM observations which demonstrated the initiation of c orrosion pits around the nuclei.