Long-term annealing study of midgap interface-trap charge neutrality

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Abstract
Non-radiation-hardened metal-oxide-semiconductor (MOS) devices were irradiated to doses from 30 to 90 krad(SiO2) and baked at 100 °C under positive bias for 2.75 yr. Threshold-voltage shifts due to radiation-induced oxide- and interface-trap charge were estimated via the subthreshold current-voltage technique of McWhorter and Winokur [Appl. Phys. Lett. 48, 133 (1986)], which depends on the assumption that radiation-induced interface traps are neutral at midgap surface potential. The oxide-trap charge inferred via this technique asymptotically approached zero ∼9 months after irradiation and remained constant for the next 2 yr, despite large changes in the subthreshold current-voltage characteristics caused by changes in interface-trap density (which increased during the first 4–9 months of 100 °C annealing, then decreased during the remaining 2–2.5 yr). This strongly reinforces the often controversial idea that radiation-induced interface traps are charge-neutral at midgap. Finally, a new figure of merit is developed to assess the self-consistency of interface-trap charge measurements on irradiated MOS devices.