Investigation of the polarity asymmetry on the electrical characteristics of thin polyoxides grown on N+ polysilicon

Abstract

The polarity asymmetry on the electrical characteristics of the oxides grown on n(+) polysilicon (polyoxides) was investigated in terms of the oxidation process, the doping level of the lower polysilicon layer, the oxidation temperature and the oxide thickness, It was found that the thin polyoxide prepared by using a low-temperature wafer loading and N-2 preannealing process, has a smoother polyoxide/polysilicon interface and exhibits a lower oxide tunneling current, a higher dielectric breakdown field when the top electrode is positively biased, a lower electron trapping rate and a larger charge-to-breakdown than does the normal polyoxide. The polarity asymmetry is also strongly dependent on the doping level of the lower polysilicon layer, the oxidation temperature and the oxide thickness. It was found that only the thinner polyoxides (less than or equal to 240 Angstrom) grown on the heavily-doped polysilicon film (30 Ohm/sq) by using the higher-temperature oxidation process (greater than or equal to 950 degrees C) conduct a less oxide tunneling current when the top electrode is positively biased.