Data retention after heavy ion exposure of floating gate memories: Analysis and simulation

Floating gate (FG) memories are the most important of current nonvolatile memory technologies. We are investigating the long-term retention issues in advanced Flash memory technologies submitted to heavy ion irradiation. Long tails appear in threshold voltage distribution of cells hit by ions after they have been reprogrammed. This phenomenon is more pronounced in devices with smaller gate area. Results are explained by a new physics-based model of the leakage current flowing through the damaged oxides of FG memory cells. The model calculates the trap-assisted tunneling current through a statistically distributed set of defects by using electron coupling to oxide phonons. The model is used to fit experimental data and to discuss retention properties after heavy ions exposure of future devices, featuring thinner tunnel oxide.