Are soft breakdown and hard breakdown of ultrathin gate oxides actually different failure mechanisms?

By means of a statistical analysis, the soft breakdown and hard breakdown of thin gate SiO2 films in MOS devices are shown to have a common physical origin. Being triggered by identical microscopic defects, these breakdown modes can be actually considered to be the same failure mechanism. In particular, it is shown that the soft breakdown conduction path is not precursor of the final hard breakdown event, which generally appears at a different spatial location. The huge differences between the soft and hard post-breakdown current-voltage (I-V) characteristics are attributed to differences in the breakdown spot area and to point contact energy funneling effects.