Study of Lanthanum Diffusion in HfO2-Based High-k Gate Stack

La2O3 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 SiO2/HfO2 interface. In this paper, we study the factors which influence the diffusion of La into HfO2-based gate stack during drive-in anneal. These factors include: (1) Initial La2O3 thickness, HfO2 thickness and drive-in anneal temperature; (2). Crystallization state of HfO2; (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 HfO2 and the TiN cap adjacent to La2O3 are also found to greatly affect the amount of diffused La, which ties closely to the Vt tuning ability.