Negative bias temperature instability of pMOSFETs with ultra-thin SiON gate dielectrics

The negative bias temperature instability (NBTI) of pMOSFETs with ultra-thin gate dielectrics was investigated from four points of view: basic mechanism of NBTI, dependence of NBTI on gate dielectric thickness, mechanism of NBTI enhancement caused by addition of nitrogen to the gate dielectrics, and possibility of applying SiON gate dielectrics with a high concentration of nitrogen. By investigating the behavior of FET characteristics after NBT stresses were stopped, it was clarified that a portion (60%, in our case) of hydrogen atoms released by the NBT stress remains in the gate dielectric in the case of a 1.85-nm-thick NO-oxynitride gate dielectric. The existence of the hydrogen was shown to lead to the generation of positive fixed charges in the gate dielectric. It was also found that NBTI depends little on gate dielectric thickness. Moreover, we revealed that the origin of NBTI enhancement by incorporating nitrogen into gate dielectrics is the property of attracting H₂O or OH. We speculate that this property is due to the existence of positive fixed charges induced by undesirable nitrogen. We evaluated NBTI immunity of SiN gate dielectrics with an oxygen-enriched interface (OI-SiN) in which high carrier mobility was obtained by reducing positive fixed charges. OI-SiN gate dielectrics with EOTs of 1.4 and 1.6 nm were found to have sufficient lifetime for practical use under 1 V operation.