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

Abstract

A new test system was devised and used to separate the amount of floating gate (FG) charge ( Q FG ) from the oxide trapped charge ( Q OX ) generated by program-anderase (P/E) cycles. We also extracted the pure V mid shift caused by the generation of Q OX , which is separated from the part of V t shift coming from Q FG deviation. The identification of Q FG and V mid shift makes it possible to analyze the detailed the oxide trapped charge profile. The Q FG shift generated by P/E cycles displays asymmetry between the programmed and erased states: the absolute value of Q FG exhibits a maximum at ∼100 cycles in the programmed states, while Q FG 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.

Technique for Profiling the Cycling-Induced Oxide Trapped Charge in NAND Flash Memories

“…Thus, we must consider all possible tunneling paths across the defective oxide [35,36], as schematically shown in Figure 2b, which increases the computational time and complexity of the method. The second category of methods is based on the direct extraction of Q T and Q FG using a special test device [37,38]. The cross-sectional view and equivalent circuit of the test structure are shown in Figure 3.…”

“…During P/E operations, FN tunneling occurs only through the oxide of the LV cell, thus degrading the oxide of this cell. The ES resulting from Q T is expressed as [38]:…”

“…Schematic cross-section view (a) and equivalent circuit (b) of the test device. Adapted from[37,38].…”

Reliability of NAND Flash Memories: Planar Cells and Emerging Issues in 3D Devices

“…Finally, the effect of non-uniform tunneling current flow and oxide charge distribution over the area of scaled NAND devices have been investigated in [74,75], pointing out their effect on the endurance curve and on the assessment of the endurance degradation with scaling. A recent technique for separating the endurance degradation components, including subthreshold slope and transconductance, which also influence the V T values, has been reported in [76].…”

Investigation of endurance degradation for 3-D charge trap NAND flash memory with bandgap-engineered tunneling oxide

“…FN current through the tunneling oxide generates interface traps (Nit) and increases the charges trapped in the tunneling oxide [6,7,8,9], resulting in the degradation of cell characteristics such as the mid-gap voltage (Vmg) shift, the subthreshold swing (SS) increase, and the transconductance (gm) decrease. Although several studies on the endurance degradation by P/E cycling stress have been conducted in 2-D FG NAND flash memory [10,11,12,13,14,15], they cannot be directly applied to 3-D CT NAND flash memory due to differences in cell structure and P/E operation mechanism. 3-D CT NAND flash memory has adopted bandgap-engineered tunneling oxide (BE-TOX) to enhance erase performance [16,17] and has a poly-silicon channel, and uses a charge trap nitride (CTN) as a storage node.…”