The role of border traps in MOS high-temperature postirradiation annealing response

A very-long-term study of the response of nonradiation-hardened MOS transistors to elevated-temperature, postirradiation biased anneals has been performed. The midgap-voltage shift of these devices returns to ~0 V during a 2.75-year, +6 V 100°C anneal, supporting the idea that in these devices interface traps and border traps (near-interfacial oxide traps which can exchange charge with the underlying Si) are charge-neutral at midgap. Subsequent switched-bias annealing reveals that a significant fraction of the radiation-induced trapped holes have not been removed from the device, but are compensated by electrons in border traps. These border traps can lead to large, reversible changes in midgap-voltage shifts and/or subthreshold stretchout during switched-bias anneals. Midgap-voltage and subthreshold-stretchout reversibility remains significant in these devices even after annealing at temperatures up to 350°C. Similar reversibility in postirradiation response is observed for hardened transistors and capacitors. These results suggest that border traps may lead to increased reliability problems in some irradiated devices as compared to their unirradiated counterparts