Performance and reliability of Ultra-Thin HfO2-based RRAM (UTO-RRAM)

We report on the performance and reliability of the Hf/HfO₂ RRAM cell with Ultra-Thin Oxide (UTO-RRAM). We show that cells with an oxide thickness of 3 nm have basic performance (including speed, switching voltages, and the on/off window) similar to that of the cells with reference oxide (5-10 nm thickness), while their operation requires a forming step at a voltage of only about 1.5 V for a 40 nm size. This performance can be further optimized by tuning the cap layer thickness. We also demonstrate endurance of at least 10⁸ cy and observe failure modes similar to the reference cells. Endurance optimization needs to take into account, next to the stack structure and pulse characteristics, the target on/off states. UTO-RRAM retention is strongly temperature-activated, with a median cell extrapolating at 125°C/10 yr. Furthermore, we analyze in detail the on-state loss and show how emergence of tail bits relates to the strength (initial level) of the state.