The effects of aging on MOS irradiation and annealing response

We find that, after approximately 17 years of room-temperature storage, the irradiation and annealing responses of poly-Si-gate nMOS transistors can change significantly. For devices with 32 nm gate oxides that were stored in a nonhermetic environment, the magnitude of the threshold-voltage rebound during postirradiation annealing is much larger now than in previous tests on devices from the same wafer and packaging lot in 1988. These changes in threshold-voltage shifts during storage are primarily due to a more than 50% increase in interface-trap generation during irradiation and annealing. When these parts are baked before irradiation, the aging-related increase in threshold-voltage shift is reduced significantly. Water molecules absorbed in the device are likely candidates for causing aging-related degradation in a nonhermetic environment, so we investigated the properties of H₂O in amorphous SiO₂ using first-principles quantum-mechanical calculations based on density functional theory. We find a complex with energy that is 0.3 eV smaller than interstitial H₂O, with an activation energy of formation of <1.5 eV. These results may help to account for the enhanced interface-trap buildup. Devices from the same lot with 60 nm oxides stored in hermetically sealed packages showed a ∼25% increase in interface-trap generation as compared to 1988 results. The increase in threshold-voltage rebound in nonhermetically stored devices is larger than standard testing margins, e.g., in MIL-STD 883, Test Method 1019. These results reinforce the importance of allowing for changes in radiation response with aging and temperature when performing hardness assurance testing of MOS and linear bipolar devices, especially for low-dose-rate applications in which hermeticity cannot be guaranteed.