New Method to Analyze the Shift of Floating Gate Charge and Generated Tunnel Oxide Trapped Charge Profile in NAND Flash Memory by Program/Erase Endurance
A new test system was devised and used to separate the amount of floating gate (FG) charge (QFG) from the oxide trapped charge (QOX) generated by program-and-erase (P/E) cycles. We also extracted the pure Vmid shift caused by the generation of QOX, which is separated from the part of Vt shift coming from QFG deviation. The identification of QFG and Vmid shift makes it possible to analyze the detailed the oxide trapped charge profile. The QFG shift generated by P/E cycles displays asymmetry between the programmed and erased states: the absolute value of QFG exhibits a maximum at ~100 cycles in the programmed states, while QFG monotonically decreases in the erased one. Considering that the Fowler-Nordheim tunneling current is sensitive to the oxide trap near the cathode, itself the source of electron tunneling current, our results indicate that the hole trap is dominant near to Si, whereas the electron trap is dominant near FG.