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The negative threshold voltage (Vt) 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 Vt shift. Hole transport in nitride is characterized by monitoring gate induced drain leakage current and using a thermionic emission model. The hole emission induced Vt 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 Vt drift is proposed.
Date of Publication: Sept. 2004