Lateral migration of trapped holes in a nitride storage flash memory cell and its qualification methodology

The negative threshold voltage (V_t) shift of a nitride storage flash memory cell in the erase state will result in an increase in leakage current. By utilizing a charge pumping method, we found that trapped hole lateral migration is responsible for this V_t shift. Hole transport in nitride is characterized by monitoring gate induced drain leakage current and using a thermionic emission model. The hole emission induced V_t shift shows a linear correlation with bake time in a semi-logarithm plot and its slope depends on the bake temperature. Based on the result, an accelerated qualification method for the negative V_t drift is proposed.