Statistical analysis of soft and hard breakdown in 1.9-4.8-nm-thick gate oxides

We have systematically analyzed the time-to-hard breakdown (t_BD) and time-to-soft breakdown (t_SBD) distributions for 1.9-4.8-nm-thick oxides. The t_BD and t_SBD distributions well fit to the Weibull distribution function. Also, the Weibull slope β of the t_SBD distributions coincides with that of the t_BD distributions over the oxide thickness range 1.9-4.8 nm, and β linearly decreases with decrease of oxide thickness. It is also shown that decrease in β of both t_SBD and t_BD distributions is well correlated to measured Si-O-Si bond angle reduction induced by compressive stress in the oxide network near the SiO₂-Si(100) interface. These results suggest that soft breakdown as well as hard breakdown in ultrathin gate oxides are triggered by a common physical mechanism such as defect generation from strained-Si-O-Si bonds under strong electric field.