The role of the spacer oxide in determining worst-case hot-carrier stress conditions for NMOS LDD devices

In this paper the underlying mechanisms that produce the crossover in worst-case hot-carrier stress condition observed at room temperature in some deep submicron lightly-doped-drain (LDD) NMOS devices and at cryogenic temperatures for devices with longer channel lengths are investigated. Experiments were performed that demonstrate the generality of the cross-over. The role of stress temperature, measurement temperature and stress condition were experimentally addressed. The temperature dependence of the mobility was measured, and an analysis is presented that shows that mobility changes alone do not explain the observed changes in the transconductance. A model is proposed that allows for changes in the source-drain resistance with stress time. It is suggested that the origin of the time-dependent increasing source-drain resistance was the injection of charge, either in the form of fixed charge or as interface states, into the spacer oxide above the LDD region. This model is used to explain the qualitative dependence of the worst-case stress condition on channel length and temperature. Finally, it is suggested that the methodology used to design the LDD structure be modified to account for these new observations.