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FinFETs have been made successfully for mass manufacturing on bulk and silicon-on-insulator wafers. When choosing the bulk option, additional process steps are needed for substrate leakage suppression. Typically, heavy substrate doping for punchthrough stopping between the source and drain is used, but precise control of the doping profile to prevent its up-diffusion into the channel has been a challenging task, especially for continuously shrinking device dimension. In this paper, we propose a stack gate structure with doping-free substrate while punchthrough leakage can be suppressed. The proposed technique can be integrated in conventional gate-last high-k metal gate process. Both polysilicon and metal gates are shown to be feasible in the proposed stack gate based on 3-D TCAD simulation. In addition, the stack gate structure without substrate doping is immune to its random dopant fluctuations.