Numerical study of data retention due to direct tunneling for nonvolatile memory cell

A long-term transient device simulation is performed using a nonvolatile memory cell. The reached transient time is over several hundred years. An accuracy of the present simulation is confirmed by obtaining quite good agreements between simulated and measured characteristics of programming and erasing operations and between simulated and measured leakage currents from the floating-gate (FG). It is found that the limitation due to direct tunneling for tunnel oxide thickness is about 3.3-3.4 nm for ten years data retention, whose limitation is not affected by fluctuation due to grain boundaries in the FG of poly-Si.