Role of Nitrogen on the Gate Length Dependence of NBTI

Gate length (L _g) dependence of NBTI-induced threshold voltage shift (|DeltaV _t|) is examined. A significant increase in |DeltaV _t| is observed for heavily nitrided gate p-MOSFET with L _g less than 100 nm. The |DeltaV _t| ldquorolluprdquo is reduced in lightly nitrided gate devices and is absent in the SiO₂ gate devices, indicating that it is linked to the presence of nitrogen in the gate dielectric. Electron-energy loss spectroscopy (EELS) of the gate dielectric reveals a stronger nitrogen K edge signal (~401 eV) at the source and drain extensions (SDEs) as compared to that in the middle of the channel. The EELS data imply that |DeltaV _t| roll-up in scaled nitrided gate p-MOSFETs is a consequence of locally higher nitrogen concentration near the SDEs. This finding has serious implication in view of the increasing level of nitridation employed in sub-100-nm technology.