This breakthrough result, say the collaborators, paves the way to realising 28-nm embedded applications. The results were presented at this year’s VLSI technology symposium (Kyoto, June 15-19 2015).
One of today’s most promising concepts for scaled memory is RRAM which is based on the electronic (current-or voltage-induced) switching of a resistor element material between two metals. The two states of the resistive memory cell are differentiated by the formation and erasure of a ‘filament’ of higher conductivity. Imec and Panasonic developed a method that overcomes filament instability in RRAM, one of the critical parameters that impacts the memory state during read operation in the memory.
The method was realised using a combination of process technologies such as low-damage etching, cell side oxidation, and an innovative encapsulated cell structure with an Ir/Ta 2O5/TaOx/TaN stacked film structure featuring a filament at the cell centre. [The switchable resistivity is in part the result of transitions between different oxides of tantalum, e.g. TaO and Ta 2O5.] With these methods, a 2-Mbit 40 nm TaOx-based RRAM cell with precise filament positioning and high thermal stability was achieved. The memory array showed excellent reliability of 100k cycles and 10 years’ retention at 85°C. Additionally, the filament control and thermal stability technologies offer the potential to realise 28 nm cell sizes.
Gosia Jurczak, director of imec’s research program on RRAM devices stated: “With these breakthrough results, we have proven the potential of this promising memory concept as embedded nonvolatile memory in 28nm technology node where conventional NOR Flash shows scaling limitations. This result is a confirmation of our leadership position in research and development on resistive memory.”
The image above shows a electron-microscopy cross-section of a 40-nm Ir(TE)/Ta2O5/TaOx/TaN (BE) RRAM.