Investigation of copper removal efficiency on reclaimed wafers with HF-based solutions

摘要

Eliminating Cu contaminants is essential to preserve material integrity in silicon device fabrication. Cu must indeed be efficiently removed from wafer back-sides and front-sides to avoid any cross-contamination. It is all the more important when wafers are repetitively processed in a reclaim industry. In this paper, we investigate copper removal efficiency with different HF-based wet-cleans. We show that copper deposits on the Si surface thanks to an electrochemical reaction between Si and Cu. As this mechanism is influenced by the pH and the redox-potential of the solution, we have explored the effects of reagents such as HCl and oxidant additives such as H_2O_2 on copper removal. We have concentrated on the efficiency of these cleanings and on the front-side Si surface morphology after treatment. We find that for our applications, additives in the HF solution of the SC2/HF/SC2 clean do not improve the surface morphology. On the contrary, they underline a heterogeneous surface topographic aspect. We suspect that aggregates form, resulting from Cu gettering by impurities in the silicon lattice following an out-diffusion of copper. However, the morphology can be significantly improved by adding an oxidizing agent such as HF-HNO_3 in HF-based solutions. A smooth surface is obtained with a good copper removal efficiency. We thus conclude that Cu etching is strongly dependant on the nature and on the amount of oxidants in the HF solutions as well as the oxido-reduction potential of the solution.