Degradation of ultra-thin oxides

An analytical approach to the study of trap kinetics in ultra-thin oxides is presented here. In ultra-thin oxides electrons injected from the cathode are ballistic (or quasi-ballistic), which implies that their maximum energy is at the anode interface and is proportional to the oxide voltage drop. Conversely to thicker films, in ultra-thin oxides trap creation is observed also when the maximum electron energy is lower than the threshold energy for silicon bond-breaking. We propose that it can occur if two (or more) ballistic electrons release their energy to the network within a cooperation volume. The cooperation volume depends on the initial energy of electrons (and therefore on voltage), and is related to the electron relaxation time. In fact, the longer possible time delay between the two electrons for successful cooperation in bond-breaking increases with the energy carried by each one. Solution of the rate equation predicts a quadratic dependence of the density of new traps on the stress current, and a linear dependence on the stress time. Experiments validate the time and current dependence of defect generation rate.