Latent interface-trap buildup and its implications for hardness assurance (MOS transistors)

Long-term anneals at temperatures from 25 degrees C to 135 degrees C were performed on irradiated MOS transistors. Following the normal saturation of interface-trap density (within 10/sup 2/ to 10/sup 5/ s after irradiation), large increases in the number of interface traps were observed for both commercial and radiation-hardened transistors at very long times after irradiation (>10/sup 6/ s at 25 degrees ). This latent buildup of interface traps can be significant, up to a factor of four times larger than the normal saturation value. The latent buildup is thermally activated with an activation energy of 0.47+or-0.08 eV. As a natural consequence of the delay between the normal and the latent buildup, there is a window in time in which little or no interface-trap buildup occurs. Two possible mechanisms for the latent buildup are explored: (1) the direct conversion of oxide traps into interface traps or border traps and (2) the diffusion of molecular hydrogen into the gate oxide from adjacent structures. The latent buildup of interface traps can degrade the performance of ICs in space systems and may cause IC failure at long times. Recommendations are provided for characterizing latent interface-trap buildup.>