Formation and Evolution of Conductive Filament in TaOx Resistive Random-Access Memory

Abstract

The increasing demand on memory from the next-generation technologies facilitated the pathfinding and development of emerging memories, among which Resistive Random-Access Memory (RRAM) is one of the most competitive options and least understood. Many attempts have been made to understand the resistive switching phenomena in metal-oxide RRAM, most of which suffered artifacts introduced from the testing or characterization processes. In my PhD work, I selected TiN/TaO2/TiN-based RRAM to understand the mechanism of electroformation, resistive switching, filament evolution, and endurance failure of such memory cells using electrical characterization and electron microscopy techniques.The findings of my thesis work indicate that the behavior of TaO2-based RRAM is a mixture of both Valence Change Model (VCM) and Thermochemical Model (TCM). During electroformation, the Ta moves laterally towards the hot spot and down the direction of the electric field, whereas the O moves up the electric field. The motion of both Ta and O results in the TaO0.4 filament core and Ta2O5 gap after forming, corresponding to the high resistance state (HRS). The localized heating during forming also induces temperature activated ionic interdiffusion of O and Ti across the interfaces. The resistive switching is induced by the electric field applied across the device leading to Ta-rich sub-filaments forming and breaking the connection between the filament core and the electrode. Repeated resistive switching provides a virtual annealing in the conductive filament, causing the material in proximity of the filament to phase separate into metallic Ta and Ta2O5. The phase separation continues if provided with longer electrical stressing and will result in SET failure due to the Ta particles isolated by the Ta2O5 matrix. The intrinsic cycle-to-cycle variability of metal-oxide RRAM causing stochastic sub-filament overgrowth can lead the device to RESET failure. Endurance can be potentially improved by reducing interdiffusion and controlling the temperature in the memory cell during programming operations.