Enhanced defect generation in gate oxides of P-channel MOS transistors in the presence of water
Hydrogenous species play a key role in radiation induced charge buildup in metal oxide semiconductor field effect transistors (MOSFETs). The effects of water on defect formation in MOSFETs before and after radiation exposure have been studied. Transistors built in Sandia National Laboratories' 4/3-ìm technology were exposed to water at 130 °C for times up to three weeks. The n-channel transistors did not show as much sensitivity to water as the p-channel transistors. Irradiation of the n-channel transistors exposed to moisture, followed by a long-term biased anneal, resulted in a small increase in interface-trap and oxide-trap charge densities in the gate oxides. Greatly enhanced post-irradiation defect generation was observed in the gate oxides of p-channel MOS transistors that were exposed to water. Low frequency (1/f) noise measurements also showed enhanced noise power spectral densities in the moisture-exposed p-channel transistors consistent with the enhanced post-irradiation increase in defect density. Phosphorus and boron dopant atoms are present in the field oxides of the n-channel and p-channel transistors because of source and drain implant steps. Boron accelerates water penetration and phosphorus suppresses water diffusion in SiO2. This can lead to enhanced water-induced defect formation in the gate oxides of p-channel transistors compared to n-channel transistors before and after irradiation. These results are significant for the performance of MOS technologies in non-hermetic environments where water can be present; in particular, the degradation of devices and circuits may be larger in these cases than expected from reliability and radiation tests that do not account for the additional degradation that can occur because of water and its reactions in SiO2.