Effects of water on the aging and radiation response of MOS devices

We report theoretical calculations and new controlled-humidity-and-temperature experiments to elucidate the role of water molecules in the aging of MOS devices. These assist in the interpretation of ionizing radiation-response experiments, namely the increase of interface trap density in aged devices as a function of irradiation and subsequent positive-bias annealing. First-principles calculations are used to demonstrate the existence of a low-energy configuration of a water molecule, bonded within the network. The complex has two silanol (Si-O-H) groups, from which H+ can be released when holes are available, as is the case under radiation. Migration of H+ to the Si-SiO2 interface then leads to depassivation of dangling bonds, i.e., an increase in interface trap density. Radiation-response measurements performed on samples exposed to elevated humidity and temperature conditions prior to irradiation exhibit a markedly greater increase in interface trap density than pre-baked control samples. We further report Monte Carlo simulations of proton transport in SiO2, showing that H+ builds up at the interface. The complex processes of depassivation and passivation by H+ are then modeled by differential rate equations. The final simulation results are in accord with the data.