Characteristics of thermal SiO2 films during nitridation

Abstract

Thin thermal SiO2 films were annealed in pure NH3 gas in a polycrystalline silicon (poly Si) resistance-heated reactor. The nitrogen concentration in the nitridized oxide films was found to be a function of annealing temperature, annealing time, NH3 partial pressure, initial SiO2 film thickness, and the degree of oxidant contamination in the nitridation ambient. After nitridation, an increase in hydrogen concentration with increasing annealing time was observed by nuclear reaction analysis and an increase in N–H bonds with increasing annealing time was detected by infrared analysis. A negative flatband shift was caused by the nitridation process. The amount of negative flatband shift was found to be a function of annealing time, annealing temperature, NH3 partial pressure, and initial SiO2 film thickness. Positive charges inside the nitridized films were the main cause of the negative flatband shift. The positive charge distribution was calculated for a thick SiO2 film (75 nm) annealed at 1000 °C in NH3 for 10 min. A model which included two mechanisms, the indiffusion of radicals and the formation of Si–N bonds, was proposed to explain the negative flatband shift behavior. The midgap surface-state density of SiO2/Si structures increased 3–5 times when annealed in NH3 gas. The conduction of the nitridized oxide films depended on the amount of nitrogen concentration incorporated into these films. Electron trapping was observed for nitridized oxide films during current-voltage ( I-V ) measurements. The breakdown behavior of the nitridized oxide films was found to depend on the quality of the initial SiO2 films, the amount and distribution of the generated positive charge, and the amount of nitrogen incorporated into these nitridized films.