Mechanism of low temperature nitridation of silicon oxide layers by nitrogen plasma generated by low energy electron impact

Abstract

Thermal silicon oxide layers formed on the Si substrate can be nitrided at low temperatures ranging between 25 and 700 °C by nitrogen plasma generated by low energy electron impact. The nitrogen concentration is high near the oxide surface, and the nitrogen atomic concentration ratio [N/(Si+O+N)] at the surface ranges between ∼10% and ∼25%, depending on the nitridation conditions. For nitridation above 450 °C, only N≡Si3 (i.e., a nitrogen atom bound to three Si atoms) is observed in the nitrided oxide films by x-ray photoelectron spectroscopy measurements, while both N+=Si2 (i.e., an N+ ion bound to two Si atoms) and N≡Si3 are present with nitridation below 400 °C. When a negative bias voltage is applied to the Si with respect to the grid used for the generation of nitrogen plasma, the nitrogen concentration in the film increases, indicating that nitrogen cations play a dominant role in the nitridation. It is concluded that, first, N+ ions react with SiO2 to form N+=Si2, after which inward movement of N+ ions occurs with the assistance of an electrical field induced in the nitrided oxide layers by nitrogen ions at the surface. Ultimately, N+=Si2 is transformed to N≡Si3.