XAFS atomistic insight of the oxygen gettering in Ti/HfO2 based OxRRAM

Abstract

Hafnia-based resistive memories technology has come to maturation and acceded to the market of nonvolatile memories. Nevertheless, the physical mechanisms involved in resistive switching are not yet fully understood and the numerous ab initio simulations studies have few many atomic-scale experimental counterparts. In this study we investigate the oxygen migration mechanism from an amorphous ${\mathrm{HfO}}_{2}$ layer to the Ti cap layer at a local scale before and after a thermal treatment. X-ray absorption spectroscopy at the Ti K edge and Hf ${\mathrm{L}}_{\text{III}}$ edge has been performed on samples as-deposited and annealed in Ar at $400{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ to mimic the back-end-of-line thermal budget (BEOL) of CMOS technology. The short-range Ti and Hf environments have been determined, showing that annealing promotes the migration of O from ${\mathrm{HfO}}_{2}$ to Ti, the amount of which is quantified. This provokes an expansion and an increase of atomic disorder in the Ti lattice. The nature of the oxygen gettering mechanism by the Ti metal is understood by comparing samples with increasing Ti-capping thickness. We show that the Ti getter effect has to be activated by thermal treatment and that the O diffusion takes place in a region of a few nanometers close to the $\mathrm{Ti}/{\mathrm{HfO}}_{2}$ interface. Therefore, the thermal budget history and the Ti cap-layer thickness determine the oxygen vacancy content in the ${\mathrm{HfO}}_{2}$ layer, which in turn controls the electrical properties, especially the forming operation.