Charge trapping in ultrathin hafnium silicate/metal gate stacks

Charge trapping characteristics of MOCVD HfSi_xO_y (20% SiO₂) gate stack of n-MOSFETs during substrate injection have been investigated. Positive constant voltage stress (CVS) and constant current stress (CCS) were applied at the gate of TiN-HfSi_xO_y-SiO₂/p-Si n-MOSFETs having EOT of 2 nm. Significant electron trapping is observed from the positive shift of threshold voltage (ΔV_t) after stress. Curve fitting of the threshold voltage shift data confirms power law dependence for Hf-silicate gate stacks. Charge pumping measurements for both cases showed significant electron trapping at bulk Hf-silicate while interface trap generation was comparatively insignificant. A turn-around effect is noticed for ΔV_t as the stress current and voltage increases under CCS and CVS. Dependence of spatial distribution of charge trapping at shallow traps on stress level in the Hf-silicate film and redistribution of trapped charges during and after removal of stress is possibly responsible for the turn-around effect.