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.
Highlights
Oxide resistive random access memories (OxRRAMs) are based on the reversible voltage-controlled resistance switching [1] from a high resistive state (HRS) to a low resistive state (LRS)
It is known that the nature of the top and bottom electrodes has a great impact on the OxRRAM electrical performance [7,8]
XANES is very sensitive both to the local atomic arrangement and electronic structure of the system and a simple fingerprint analysis can provide a valuable insight into the changes of the local and electronic environment, as we show in the following
Summary
Oxide resistive random access memories (OxRRAMs) are based on the reversible voltage-controlled resistance switching [1] from a high resistive state (HRS) to a low resistive state (LRS). Hafnia associated with a titanium [9,10,11] active top electrode constitutes one of the best metal-oxide-metal (MIM) capacitors for resistive switching memories The latter have demonstrated an excellent electrical behavior with low voltage set and reset, high writing speed, and a long retention of information [12,13]. A great deal of basic research on HfO2-based RRAM switching mechanism was focused on understanding the electric-field-induced physical chemistry occurring during voltage cycling. The annealing was performed to mimic the thermal budget of BEOL The latter has a fundamental impact on the device electrical properties because it determines the number of oxygen atoms gettered by the Ti electrodes from the HfO2 layer, prior to the forming operation. This study experimentally determines the migration mechanism of O atoms from HfO2 to the Ti electrode due to the thermal budget
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