Segregation and Removal of Metallic Impurity at Interface of Silicon and Fluorine Etchant

Abstract

Realization of an ultraclean Si wafer surface is essential for achieving the advanced process in the ultralarge scale integrated production such as low‐temperature process and high selectivity. An ultraclean wafer surface is defined as a surface completely free from particles, organic impurities, metallic impurities, native oxide, surface microroughness, and adsorbed impurities. Since metallic impurities, one of the above contaminants, cause fatal damage to device characteristics, metallic impurities on the wafer surface need to be suppressed at least below 1010 atom/cm2 which is the level of the detection limit of total reflection x‐ray fluorescence. The current dry processes such as ion implantation and reactive ion etching cause metallic contamination of 1012 to 1013 atom/cm2. In order to remove the metallic contamination, the wet cleaning process plays an increasingly important role. When organic impurities remain on the wafer surface, native oxide and metallic impurities on the wafer cannot be completely removed. In order to establish an ultraclean wafer surface, therefore, it is crucial to remove organic impurities first of all. The wet cleaning process is the only possible method at present to remove trace organic contaminants on the wafer surface. We have studied the segregation and removal of metallic impurities on the solid/liquid interface between chemicals and various Si wafer surfaces (p, n, p+, n+). We tested several chemicals employed in the process to remove oxide on the Si surface. Metals featuring high electronegativity (such as Cu) are directly adsorbed on the bare Si surface while taking electrons away from the Si surface. It has been found that these metals are hard to remove. We used Cu as being representative of metals to be directly adsorbed on the bare Si surface and studied its segregation and removal on the solid/liquid interface between Si wafer and chemicals to keep the Si surface bare such as DHF, , and BHF. It has been found that Cu ion in DHF adheres on every Si wafer surface that we used in our study (p, n, p+, n+), especially on the n+‐Si surface. The solution is found to be effective in removing metals featuring high electronegativity such as Cu from the p‐Si and n‐Si wafers. Even when the solution has Cu ions at the concentration of 1 ppm, this solution is found effective in cleaning the wafer. In the case of the n+‐Si and p+‐Si wafers, however, their surfaces get contaminated with Cu when Cu ion of 10 ppb remains in the solution. When BHF is used, Cu in BHF is more likely to contaminate the p‐Si, n‐Si, and p+‐Si wafers but is less likely to contaminate the n+‐Si wafer. It is also revealed that the surfactant added to BHF to improve its wettability onto the Si wafer is effective in preventing Cu precipitation onto the p‐Si, n‐Si, and p+‐Si wafers. This effect of the surfactant, however, is not observed on the n+‐Si wafer. It is found also that the surface microroughness on the n+‐Si wafer is increased when it is immersed in the solution for 10 min. The etch rate of and BHF on the n+‐Si wafer is found to be much higher than that on the other Si wafers. In order to suppress the metallic contamination on every type of Si surface below , the metallic concentration in ultrapure water and high‐purity DHF which is employed at the final stage of the cleaning process must be lowered below the part per trillion level. cleaning is effective in removing metallic impurities on the p and n surfaces which are required to feature extremely high cleanliness level, such as the wafer surface before gate oxidation. The solution, however, degrades surface roughness on the substrate with the n+ and p+surfaces. In order to remove metallic impurities on these surfaces, there is no choice at present but to use the cleaning and the cleaning.