Effect of nitridation on Fowler–Nordheim tunneling coefficients in SiO2 MOS capacitors with WSi2-polysilicon gate

Abstract

In this work we have studied the influence of N2O nitridation of thin EEPROM SiO2 tunnel oxides on Fowler–Nordheim tunnel injection potential barrier heights in WSi2-polysilicon gate MOS structures. In particular we have paid attention to the effect of the nitridation temperature by means of a comparison with standard (dry-wet-dry) oxides annealed in N2 at the same temperature. The temperatures considered were 900 °C, 950 °C and 1000 °C. It has been shown that the Si/SiO2 barrier is systematically smaller than the poly-Si/SiO2 one (~0.05 eV) in the whole set of samples and that the nitridation in N2O systematically decreases both the Si/SiO2 and the poly-Si/SiO2 barrier heights all the more as the nitridation temperature increases. The effects of neglecting the contribution of Si/SiO2 interface states and of the gate depletion phenomenon in the evaluation of the oxide electric field and, as a consequence, in the evaluation of the tunnel injection potential barrier heights have also been considered. It has been shown that neglecting the polysilicon gate depletion phenomenon leads to a strongly over-evaluated value of the Si/SiO2 barrier height while neglecting the interface states gives an error of approximately 1%. Moreover, we have shown that N2O nitridation increases the oxide dielectric constant all the more as the nitridation temperature increases while fluorine incorporation into the oxide induced by the WSi2 polycide deposition process decreases the same constant.