Impacts of gate electrode materials on threshold voltage (Vth) instability in nMOS HfO2 gate stacks under DC and AC stressing

Ultrathin nMOSFET hafnium oxide (HfO₂) gate stacks with TiN metal gate and poly-Si gate electrodes are compared to study the impact of the gate electrode on long term threshold instability reliability for both dc and ac stress conditions. The poly-Si/high-κ interface exhibits more traps due to interfacial reaction than the TiN/high-κ interface, resulting in significantly worse dc V_th instability. However, the V_th instability difference between these two stacks decreases and eventually diminishes as ac stress frequency increases, which suggests the top interface plays a minor role in charge trapping at high operating frequency. In addition, ac stress induced interface states (Nit) can be effectively recovered, resulting in negligible G_m degradation.