Using erase self-detrapped effect to eliminate the flash cell program/erase cycling Vth window close

The electrons and holes trapped in the tunneling oxide and interface states generated in the Si-SiO₂ interface during program/erase (P/E) operations are known to cause reliability problems which can deteriorate the cell performance and cause V_th window closure. This deterioration is caused by the accumulation of electrons trapped in the oxide near the drain side after each P/E cycle. Although the trapped charges exist in the oxide layer during the erase operation, we find that these charges exist temporarily because the erase field eventually detraps the charges. We call this effect the erase self-detrapped effect (ESDE). In this paper, the influence of the ESDE on the cell degradation characteristics after P/E cycles is first studied. We then propose two new erase schemes to improve the cell´s endurance characteristics: (1) adding a channel erase cycle after the negative-gate erase (NGE) operation; (2) adding a channel erase cycle after the source erase (SE) operation. These two erase schemes have been implemented and shown to significantly reduce the V_th window closure problem