Dynamic behavior of the buildup of fixed charge and interface states during hot-carrier injection in encapsulated MOSFET´s

The aging behavior of MOSFET´s encapsulated with various types of capping layers was studied. Aging consisted of room-temperature pulsed gate bias operation with a drain-to-source voltage sufficient to cause avalanche multiplication in the channel. It was verified by secondary ion mass spectroscopy (SIMS) profiling that plasma silicon nitride capping layers introduce 2-4 times more hydrogen at the Si-gate oxide interface than exists in uncapped devices. Capping materials that serve as hydrogen barriers contribute to device aging by trapping hydrogen that is liberated during hot-carrier emission into the gate oxide. The aging dynamics begin with buildup of negative fixed charge in the gate oxide near the drain, followed by the buildup of positive fixed charge and interface states. The generation of these interface states and the negative fixed charge was found to have a spatial and time dependence. Long anneals at temperatures above 350°C delay the onset of the aging process. A model that accounts for these observations is proposed.