Mechanism Investigation of Thermally Activated Lanthanum Diffusion in Dipole-Last Gate-Stacks

Abstract

In this study, La-dipole-inserted HfO2/SiO2 MOS capacitors were fabricated via a dipole-last process, with postcapping annealing (PCA) applied at varied temperatures to activate La dipoles. The thermal processing condition critically determines device performance by regulating La diffusion and oxygen redistribution in the gate stack. Scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX) confirms the gradual diffusion of La dipoles upon PCA temperature elevation, and X-ray photoelectron spectroscopy (XPS) verifies the formation of lanthanum silicate after thermal treatment. PCA temperature significantly modulates the flat-band voltage ( ${V}_{\textit {fb}}\text {)}$ shift: residual La dipoles at the HfO2/TiN interface induce a positive ${V}_{\textit {fb}}$ shift of ~70 mV at $750~^{\circ }$ C, while a conventional negative ${V}_{\textit {fb}}$ shift occurs at $880~^{\circ }$ C. Further heating to $1000~^{\circ }$ C drives thorough La diffusion toward the HfO2/SiO2 interface, forming substantial lanthanum silicate and yielding a large negative ${V}_{\textit {fb}}$ shift of ~350 mV. The contradictory ${V}_{\textit {fb}}$ polarities at low and high PCA temperatures stem from temperature-dependent La distribution, which alters the valence-band offset (VBO). With increasing PCA temperature, the interface trap density ( ${D}_{\textit {it}}\text {)}$ declines continuously, reaching a maximum reduction of 87% at $1000~^{\circ }$ C due to effective La dipole incorporation. The introduced La dipoles also suppress gate leakage up to one order of magnitude via oxygen vacancy compensation at $750~^{\circ }$ C. This work reveals that precise thermal budget control is essential for synergistically optimizing V modulation, leakage performance, and interfacial quality in dipole-last-processed gate stacks.