It is crucial to make Si wafer surfaces ultraclean in order to realize low-temperature processing and high-selectivity in ultralarge scale integrated production. The ultraclean wafer surface must be perfectly free from particles, organic materials, metallic impurities, native oxides, surface microroughness, and adsorbed molecular impurities. Metallic contamination, the major type of contaminants to be overcome, has a fatal effect on device characteristics and must be suppressed to at least below 10(10) atom/cm2. The current dry processes, such as reactive ion etching or ion implantation, cause metallic contamination as high as 10(12)-10(13) atom/cm2. The wet process becomes increasingly important to remove these metallic impurities introduced during dry processing. Employing a new evaluation method, the metallic impurity segregation at the interface between the Si wafer and the liquid in the wet cleaning process was studied. It has been found that metals, such as Cu having higher electronegativity than Si, are directly adsorbed on the Si surface by taking an electron from the Si. On the other hand, metals, such as Fe and K having lower electronegativity than Si, are not adsorbed on the Si surface. In the normal wet cleaning process when a native oxide is formed on the Si surface, metals such as Fe and K that are oxidized more easily than Si, are preferentially included into the native oxide. When the metals are in ultrapure water or chemicals with a concentration of 1 ppb, they are included into the Si surface, and native oxide with evaluation was 10(12)-10(13) atom/cm2. Therefore, to decrease the metallic contamination level on the Si surface to levels less than 1 x 10(10) atom/cm2, the metallic impurities must be suppressed to at least below the 1 ppt level in ultrapure water and high-purity HF, which are employed in the final step of the cleaning process- To prevent the metallic contamination on the wafer surface, it was found that it is important to maintain an inert atmosphere, such as N2 or Ar, to suppress native oxide growth and to reduce metallic impurities in the ultrapure water rinse. Moreover, it has been found that the diluted HF-H2O2 cleaning is effective in removing metals such as Cu, having high electronegativity, from the Si surface at room temperature and that it does not induce surface microroughness. This means the diluted HF cleaning, which has been employed in the final step of the conventional wet cleaning process to remove the native oxide, needs to be replaced with the diluted HF-H2O2 cleaning. It was also found that surfactants added to improve the wettability of chemicals on the Si surface were also able to prevent metallic impurity precipitation on the wafer surface.
