Role of oxygen vacancies in V/sub FB//V/sub t/ stability of pFET metals on HfO/sub 2/

We demonstrate experimentally that the flatband/threshold voltages (V/sub FB//V/sub t/) of pFET metal gates are strongly dependent on the post-deposition annealing conditions of the gate stacks. By varying the temperature and the O/sub 2/ partial pressure during post-deposition N/sub 2//O/sub 2/ and/or forming gas annealing (FGA) with Re, Ru and Pt, the gate stack V/sub 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/sub FB//V/sub 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/sub FB//V/sub t/. The results indicate that V/sub FB//V/sub t/ control remains a formidable processing challenge with high workfunction metals on HfO/sub 2/.