It is crucial to make Si wafer surfaces ultraclcan 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 ~~ atom/cm ~. The current dry' processes, such as reactive ion etching or ion implantation, cause metallic contamination as high as 10~-10 ~: atom/cm-. 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 electroncgativity 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 10~2-10 ~: atom/cm 2. Therefore, to decrease the metallic contamination level on the Si surface to levels less than 1 • i0 ~~ atom/cm 2, 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-H~O2 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-H2Q cleaning, tt 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.As the ultralargc scale integrated (ULSI) device featuring increasingly finer patterns is further integrated, it is becoming more critical to keep the Si wafer surface ultraclean in order to improve the device reliability and performance. The establishment of the advanced process technology requires the perfect control of the Si-g...

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 10 l~ atom/cm 2 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/cm 2. 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, DHF-H~O2, 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 DHF-H202 solution is found to be effective in removing metals featuring high electronegativity such as Cu from the p-St and n-St wafers. Even when the DHF-H202 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 DHF-H203 solution. When BHF is used, Cu in BHF is more likely to contaminate the p-St, n-St, 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-St, n-St, 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 § wafer is increased when it is immersed in the DHF-H202 soluti...

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The minimization of particle contamination during wet processing of Si wafers, such as from particles in solutions with surfactants, was studied. It was demonstrated that particle deposition on wafer surfaces in solutions is determined by an attractive or a repulsive electrostatic force through the zeta potentials of particles and substrates. In practice, wafer surfaces consist of materials such as St, poly-Si, SiO2, Si3N4, Al-alloy, and photoresist, and all these materials exhibit different zeta potentials in wet chemical solutions. In solutions with a pH < 5, the Si surface exhibits a negative zeta potential while SiO2 and Si3N4 surfaces have positive zeta potentials. This means that particle deposition will always occur either on Si surfaces or on SiO2 and Si3N4 surfaces in acidic solutions that contain particles. Therefore, in order to suppress particle deposition on the wafer surfaces, it is essential, that the wafer surface and the particles must have the same polarity of the zeta potential. This can be achieved by adding an anionic or cationic surfactant, where the wafer surfaces and the particles exhibit the same polarity of the zeta potential.