Effect of Extremely Thin Nitrogenous Surface Films on Phosphorus-impurity Profiles in Silicon

In open-tube diffusion furnaces, nitrogen is often used as a carrier gas for impurities. Nitrogen flowing over silicon wafers at the diffusion temperature creates extremely thin surface films, which act as strong diffusion barriers. Similar barriers are formed on silicon wafers even in capsule-diffusion systems because of the residual atmosphere. Phosphorus-impurity profiles are found to be kinked due to these diffusion barriers. The position and dependence of the kinks on various diffusion parameters such as time, temperature and impurity concentration in the carrier gas are discussed. The kinks are eliminated if the formation of the thin-film barriers is avoided through use of argon or other noble gases. A mathematical model is presented to explain the kinked behavior of the profiles.