Hf-based dielectric stack is replacing SiON as gate dielectric even though our understanding of it is incomplete. It has been reported that a thermal exposure above 450 degrees C can lead to positive charging in both unoptimized SiO2 layer and Hf-based dielectric stack. At present, there is little information on how this process-induced positive charging (PIPC) occurs in the Hf-based stack and how to suppress it. The objective of the current work is to improve our understanding by addressing three key issues. First, the activation of PIPC precursors after device fabrication is investigated and it will be shown that the loss of certain species from the gate edge through lateral diffusion is responsible for it. Second, the passivation of the precursor is studied and the relevant species are explored. It is found that both water- and chlorine-related species play a role. Finally, the reactivation of the passivated precursor is examined and the results show that it is not thermally accelerated.