EFFECT OF CHEMICALS ON METAL CONTAMINATION ON SILICON-WAFERS

Chemicals from various suppliers were tested for the metal contamination levels left on silicon wafers after cleaning. During the investigations, hydrochloric acids and ammonium hydroxides from three vendors and hydrogen peroxides from four vendors, were evaluated. The wafers were cleaned in RCA standard solutions, after which particle counts and metal contamination were measured. Total reflection X-ray fluorescence analysis was used for metal contamination measurements. Hot ammonium hydroxide-hydrogen peroxide mixture (APM), an efficient particle remover, results in iron deposition on silicon. The amount of deposited iron is strongly dependent on the quality of the peroxide used in the mixture, and it can be reduced by using a shorter cleaning time. The deposition rate of iron on a wafer is probably diffusion limited for short times before the equilibrium is reached between the metal concentrations on the wafer and in the solution. The iron concentration after APM is not very sensitive to the age of the chemical mixture. There is, however, more zinc deposited in an aged chemical mixture. The same type, but weaker, dependence of iron concentration on chemical supplier observed after APM was also found after hot hydrochloric acid-hydrogen peroxide mixture (HPM). The iron concentrations were much lower. The HPM mixture however, causes a considerable increase in particle counts. These particles are, fortunately, loosely bound to the wafer surface and can therefore be easily removed. The copper concentration on silicon remained essentially constant, independent of chemical supplier, cleaning solution, or the time used for the cleaning. If APM is used as the last cleaning solution, the choice of the peroxide supplier is critical. However, if HPM is used, all the tested chemical grades give eseentially the same metal contamination levels.