Sodium Contamination in SiO2 Films Induced by Plasma Ashing

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Abstract
A sodium contamination phenomenon in a SiO2 film caused by O2 plasma ashing of a resist film was studied using the triangular voltage sweep method and neutron activation analysis. The contaminant sodium atoms have been proven to come from the resist film. The contamination occurs in two stages: (i) Before ashing end point, the sodium atoms dissolve only in the SiO2 surface layer by 1012 cm-2 surface area density or less. (ii) After the endpoint, the sodium amount in the SiO2 film increases markedly and eventually saturates at 1013–1014 cm-2. The sodium atoms in the SiO2 film migrate toward the SiO2/Si interface during over-ashing in O2 plasma. Ashing both in O2-CF4 plasma and in a metal shield cylinder are effective in reducing the sodium transportation rate and in preventing contamination.