Detrimental impact of hydrogen on negative bias temperature instabilities in HfO2-based pMOSFETs

The impact of hydrogen on negative bias temperature instabilities (NBTI) in atomic layer deposited (ALD) HfO₂-based pMOSFETs is reported for the first time. After forming gas anneal (FGA) at high temperature (580°C), the saturated threshold voltage (V_th) shift of the devices is about 100 mV at 125°C and V_G = -1.5 V. The V_th instability is reduced to about 50 mV for devices annealed in forming gas at 520°C. Detailed analysis of the experimental results indicates that the defects responsible for NBTI are hydrogen-induced overcoordinated oxygen centers, induced by the transport and trapping of H⁺ in the gate stack. The V_th shift can be further reduced to less than 5 mV after subjecting the transistors to a higher thermal budget during the dopant activation anneal, which allows to release the strain at the Si/dielectric interface as well as to drive hydrogen out of the high-k gate stack. This finding is very important with respect to the thermal budget requirements for scaled CMOS processes.