(Invited) Material and Device Design for Practical Application of Highly-Reliable and High-Density ReRAM
Authors: Yasuhara Ryutaro 
Published: February 2020

Electric-pulse-induced resistance change phenomena in PrCaMnO3 were reported in 2000 [1]. From this finding, Resistive Random Access Memory (ReRAM) using this phenomena attracted both academic and industrial researchers as a promising candidate of next-generation nonvolatile memory. In 2013, we started the world’s first mass-production of ReRAM-embedded micro-controller unit. Among issues to be solved for its practical application, the most fundamental problem was that the mechanism of resistive switching (RS) remained unclear, causing the lack of guide for the design of materials and devices for ReRAMs. Therefore, we investigated the mechanism of RS and considered the appropriate materials and device structures for ReRAM from the viewpoint of device reliability. Based on the clarified RS mechanism, we successfully developed highly-reliable Ta oxide ReRAM.
ReRAM cells with top electrode/Ta oxide/bottom electrode was fabricated. Ta oxide was deposited by reactive sputtering. Device structures, RS characteristics and reliability of ReRAMs were evaluated by electrical measurements and physical analyses.
In order to elucidate the mechanism of RS, chemical states in ReRAM cells for initial resistance state, high-resistance state (HRS) and Low-resistance state (LRS) were directly observed by hard x-ray photoemission spectroscopy. The ratio of Ta 4d photoemission peaks for Ta sub-oxide (TaOx) to Ta2O5 increases with the decrease in the resistance of ReRAM cells (from Initial to HRS to LRS). This indicates that the mechanism of RS is redox reactions in Ta oxide layer. Based on the RS model, the guidelines for the material selection of oxide and electrode were presented. Oxide material should be bi-stable: The Gibbs free energy for the reaction between oxides with different valence states such as TaO2/Ta2O5should be small. Electrode material should not be easy to oxidize: Standard electrode potential of the electrode metal should be sufficiently higher than that of material constituting the RS oxide [2].
We next investigated the detailed structure of Ta oxide ReRAM and found that the RS locally occurred in small conductive area (Conductive Filament: CF) consisting of oxygen-deficient Ta oxide [3]. Therefore, we evaluated the retention characteristics of Ta oxide ReRAMs focusing on the CF. Experimental results indicate that the retention degradation of LRS/HRS is caused by the oxygen diffusion into/out of CF. From detailed analyses, we clarified that the control of CF characteristics including filament size and the density of oxygen vacancy was the key for the improvement of the reliability [4]. Utilizing these insights, we established the reliability model of Ta oxide ReRAM and successfully started the mass-production of ReRAM.
The delivery of higher-density ReRAM is essential for further spread of ReRAM in the market. The following two the issues are inevitable for the realization of higher-density ReRAM; (1) scaling down of ReRAM cells and (2) low-current operation, which may degrade the reliability. Therefore, we addressed these challenges and successfully demonstrated the good array-level reliability with scaled-down cells and low-current operation by controlling the CF from the standpoint of fabrication process and device structure, respectively [5,6].

References

[1] S. Q. Liu, N. J. Wu, and A. Ignatiev, “Electric-pulse-induced reversible resistance change effect in magnetoresistive films,” Appl. Phys. Lett. 76, 2749-2751 (2000).
[2] Z. Wei et al., “Highly Reliable TaOx ReRAM and Direct Evidence of Redox Reaction Mechanism”, IEDM Tech. Dig., 293-296 (2008).
[3] Z. Wei et al., “Demonstration of High-density ReRAM Ensuring 10-year Retention at 85oC Based on a Newly Developed Reliability Model,” IEDM Tech. Dig., 721-724 (2011).
[4] S. Muraoka et al., “Comprehensive Understanding of Conductive Filament Characteristics and Retention Properties for Highly Reliable ReRAM,” VLSI. Tech. Dig., T62-63 (2013).
[5] Y. Hayakawa et al., “Highly reliable TaOxReRAM with centralized filament for 28-nm embedded application,” VLSI. Tech. Dig., T14-15 (2015)
[6] T. Ninomiya et al., “Conductive Filament Scaling of TaOx Bipolar ReRAM for Long Retention with Low Current Operation,” VLSI. Tech. Dig., T73-74 (2012).