QUASI-BREAKDOWN IN ULTRATHIN GATE DIELECTRICS

During constant current stressing (CCS) of ultrathin gate oxide, the g ate bias (Vg) variation with time exhibits three distinct regimes. Dur ing the first regime, Vg decreases slightly indicative of a positive t rapping. After that, the gate bias is seen to strongly fluctuates. Thi s second regime, characteristic of the quasi breakdown (QB), is follow ed 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 o f the physical parameters used to fit the experimental data is questio nable. In fact, a potential barrier as high as 6 eV is required. An al ternative phenomenological model is proposed. The experimental data ar e interpreted assuming that after the QB occurrence conducting paths b etween the electrodes are generated. Further stressing the structure l eads to the breakdown which is interpreted as a merging of some of the se narrow conducting paths.