Impact of high tunneling electric fields on erasing instabilities in NOR flash memories

Experimental data and analysis show that overerase effects in NOR Flash memories increase with the electric field used during erasing. We found that the electric field is an accelerating factor for cell degradation during cycling. Tunnel oxide degradation reaches a critical level above which the cell starts showing erased threshold voltage instabilities possibly leading to single bit failure. Experimental data show that cell degradation during erasing has to be ascribed to hole injection rather than to electron injection and that both hole trapping and detrapping increase with the electric field. The total stress time required to reach the critical degradation level has been found to follow a 1/E_ox exponential dependence which is similar to the oxide breakdown phenomena thus establishing a physical link between the two phenomena. Anode Hole Injection has been suggested as hole generation and injection mechanism occurring during erasing and it is shown to be consistent with the experimental data.