Effect of organic acids in dilute HF solutions on removal of metal contaminants on silicon wafer

With continued advances in semiconductor devices below 10 nm, the required specification for ultraclean wafer surfaces (i.e., achieving metal contamination < 1 x 10(9) atoms/cm(2) on the final substrate surface) becomes very challenging. During the cleaning process, Cu contamination occurs by drawing electrons from silicon atoms according to their reduction potential difference, whereas Al forms a hydroxide or oxide upon reaction with H2O. In this paper, the effect of chelating agents on the removal of Cu and Al metals from Si surfaces was investigated in DHF (Dilute hydrofluoric acid) solutions. In solutions with a higher concentration of chelating agent (8 mM), the Cu removal efficiency in DHF/oxalic acid was higher than that in DHF/citric acid. In this case, oxalic acid was more ionized than citric acid at lower pH, which led to the observed results. In lower concentration (6 mM), DHF/citric acid exhibited higher Cu removal efficiency. This occurs because citric acid has more carboxylate groups than oxalic acid. In contrast to Cu, which reacted only with the chelating agents, the Al removal efficiency was > 95% in all conditions because it reacted with both the chelating agents and HF.