Role of oxygen vacancies in VFB/Vt stability of pFET metals on HfO2

We demonstrate experimentally that the flatband/threshold voltages (V_FB/V_t) of pFET metal gates are strongly dependent on the post-deposition annealing conditions of the gate stacks. By varying the temperature and the O₂ partial pressure during post-deposition N₂/O₂ and/or forming gas annealing (FGA) with Re, Ru and Pt, the gate stack V_FB can change by as much as 750 mV. However, using Re as an example, it is shown that conditions can be optimized and V_FB/V_t-tuning for pFETs can be achieved for aggressively scaled stacks. It is proposed that charge transfer from oxygen vacancies to the gate electrode, possible only for high workfunction metal gates, leads to the formation of a dipole layer near the gate which can shift V_FB/V_t. The results indicate that V_FB/V_t control remains a formidable processing challenge with high workfunction metals on HfO₂.