It is crucial to make Si wafer surfaces ultraclean in order to realize such advanced processes as the low-temperature process and the high-selectivity in the ULSI production. The ultra clean wafer surface must be perfectly free from particles, organic materials, metallic impurities, native oxide, surface microroughness, and adsorbed molecule impurities. Since the metallic contamination on the wafer surface, which is one of the major contaminants to be overcome in order to come up with the ultra clean wafer surface, has the fatal effect on the device characteristics, the metallic impurities in the wafer surface must be suppressed at least below 1010 atoms/cm2. Meanwhile the current dry processes such as reactive ion etching or ion implantation, suffer the metallic contamination of 10121013 atoms/cm2. The wet process becomes increasingly important to remove the metallic impurities introduced in the dry process. Employing a new evaluation method, the metallic impurity segregations at the inrerface between the Si and liquid employed in the wet cleaning process of the Si surface such as ultrapure water and various clemicals were studied. This article clearly indicate that it is important to suppress the metallic impurities, such as Cu, which can exchange electrons with Si to be segregated, at least below the 10 ppt level in ultrapure water and liquid chemical such as HF, H2O2, which are employed in the final step of the wet cleaning. When the ultrapure water rinsing is performed in the ambience containing oxygen, the native oxide grows accompanying an inclusion of metals featuring lower electron negativity than Si. It is revealed that, in order to provent the metallic impurity precipitation, it is require not only to remove metallic impurities from ultrapure water but also to keep the cleaning ambience without oxygen, such as the nitrogen ambience, so as to suppress the native oxide formation.