Quasi-breakdown in ultrathin gate dielectrics

During constant current stressing (CCS) of ultrathin gate oxide, the gate bias (Vg) variation with time exhibits three distinct regimes. During the first regime, Vg decreases slightly indicative of a positive trapping. After that, the gate bias is seen to strongly fluctuates. This second regime, characteristic of the quasi breakdown (QB), is followed by a sharp drop in Vg which corresponds to the breakdown. After the QB occurrence, a huge leakage current is observed. In a first attempt, this leakage current is modelled by a superposition of F-N tunneling and direct tunneling. Although a good agreement is found, the value of the physical parameters used to fit the experimental data is questionable. In fact, a potential barrier as high as 6 eV is required. An alternative phenomenological model is proposed. The experimental data are interpreted assuming that after the QB occurrence conducting paths between the electrodes are generated. Further stressing the structure leads to the breakdown which is interpreted as a merging of some of these narrow conducting paths.