Anomalous charge loss from Floating-Gate Memory Cells due to heavy ions irradiation

We are presenting new data on the charge loss in large floating gate (FG) memory arrays subjected to heavy ion irradiation. Existing models for charge loss from charged FG and generation-recombination after a heavy ion strike are insufficient to justify (or in contrast with) our experimental results. In particular, the charge loss is by far larger than predicted by existing models, it depends on the number of generated holes, not on those surviving recombination, and it is larger for FGs with larger threshold voltage before irradiation. We show that these data can be explained as the effect of two different mechanisms. The first one is a semi-permanent multi trapassisted tunneling (TAT), which closely resembles anomalous stress induced leakage current (SILC) in electrically stressed devices. The second mechanism is a transient phenomenon responsible for the largest part of the lost FG charge. Detailed physical modeling of this mechanism is still not available, owing to the limited knowledge of the physical background under these phenomena, but three possible models are explored and discussed.