Study of Lanthanum Diffusion in HfO_2-bascd High-k Gate Stack

摘要

La_2O_3 has been selected as a threshold voltage (Vt) tuning material for advanced CMOS node because of its uniqueness of reducing effective work function towards nFET band edge by creating dipoles at SiO_2/HfO_2 interface. In this paper, we study the factors which influence the diffusion of La into HfO_2-based gate stack during drive-in anneal. These factors include: (1) Initial La_2O_3 thickness, HfO_2 thickness and drive-in anneal temperature; (2). Crystallization state of HfO_2; (3) Capping material during drive-in anneal. While La diffusion generally follows thermally activated process described by Fick's law and Arrhenius equation, the crystallization of HfO_2 and the TiN cap adjacent to La_2O_3 are also found to greatly affect the amount of diffused La, which ties closely to the Vt tuning ability.