Takeki Ninomiya

Highly reliable TaOx ReRAM and direct evidence of redox reaction mechanism

Highly reliable TaOx ReRAM has been successfully demonstrated. The memory cell shows stable pulse switching with endurance over 109 cycles, sufficient retention exceeding 10 years at 85degC. TaOx exhibits stable high and low resistance states based on the redox reaction mechanism, confirmed by HX-PES directly for the first time. An 8 kbit 1T1R memory array with a good operating window has been fabricated using the standard 0.18 mum CMOS process.

Demonstration of high-density ReRAM ensuring 10-year retention at 85°C based on a newly developed reliability model

A new oxygen diffusion reliability model for a high-density bipolar ReRAM is developed based on hopping conduction in filaments, which allows statistical predication of activation energy. The filament in the active cells is confirmed by EBAC and TEM directly for the first time. With optimized filament size, a 256-kbit ReRAM with long-term retention exceeding 10 years at 85°C is successfully demonstrated.

Conductive Filament Scaling of

The retention model of a bipolar ReRAM considering the percolative paths in a conductive filament is proposed. We demonstrate, for the first time, that the control of oxygen vacancy concentration in a conductive filament is the key for ensuring data retention including tail bits. To improve the retention property under low-current operation, the size of the conductive filament must be scaled down while keeping the density of oxygen vacancy high enough. Based on this concept, we demonstrate both low-current operation and sufficient retention results exceeding 500 h at 150°C, which correspond to more than 10 years at 85°C.

{\rm TaO}_{\rm x}

We demonstrate for the first time that the density of oxygen vacancy in a conductive filament plays a key role in ensuring data retention. We achieve very good retention results up to 100 hours at 150°C even under the low current operation due to the scaling of conductive filament size while retaining sufficiently high density of oxygen vacancy.

Bipolar ReRAM for Improving Data Retention Under Low Operation Current

We investigate, for the first time, the expansion of resistive random access memory (ReRAM) conductive filaments during pulse cycles, which may cause retention failure after cycling endurance. We find that filament size becomes larger gradually because of oxygen diffusion from the region surrounding a filament during reset operations. To achieve long-term use of ReRAM while avoiding filament expansion, it is the key to control both an electric power and a pulsewidth input at a switching operation. We successfully demonstrate good data retention even after endurance of 100-k cycles with an optimized reset pulse.

Conductive filament scaling of TaO x bipolar ReRAM for long retention with low current operation

Resistive RAM (ReRAM) has been recently developed for applications that require higher speed and lower voltage than Flash memory is able to provide. One of the applications is micro-controller units (MCUs) or SoCs with several megabits of embedded ReRAM. Another is solid-state drives (SSDs) where a combination of higher-density ReRAM and NAND flash memory would achieve high-performance and high-reliability storage [1], suitable for server applications for future cloud computing. ReRAM is attractive for several reasons. First, it operates at high speed and low voltage. Second, it enables high density due to the simple structure of the resistive element (RE) [2]. Third, it is immune to external environment such as magnetic fields or radiation, since the resistive switching is based on the redox reaction [3].

Improvement of Data Retention During Long-Term Use by Suppressing Conductive Filament Expansion in

A retention model for both the high resistance state and low resistance state of the bipolar ReRAM is developed. Degradation of resistance is caused by the oxygen vacancy profile in filament changing due to oxygen diffusion.

{\rm TaO}_{x}

The post-cycling data retention of filamentary operated resistive random access memory (ReRAM) can be improved by minimizing conductive filament expansion during pulse cycling. We find that filament size gradually grows with increasing pulse cycles due to oxygen diffusion from the region surrounding each filament. To achieve long term use of ReRAM while suppressing filament expansion, the key is to control both electric power and pulse width input during switching. We minimize CF expansion based on this concept and demonstrate long data retention even after 106 pulse switchings under optimized reset conditions.

Bipolar-ReRAM

Characteristics and their origin of a conductive filament in TaOx ReRAM are investigated. The results of systematic experimentation demonstrate that the formation of a small conductive filament with high density of oxygen vacancies, achieved by controlling the oxygen content of the resistance-switching material and forming/set current, is the key to achieving low-current switching combined with long retention.

Filament scaling forming technique and level-verify-write scheme with endurance over 107 cycles in ReRAM

Taking advantage of electron hopping between oxygen vacancies in filaments, ReRAM switching is caused by oxygen vacancy migration. We have developed an oxygen diffusion retention model, based on this switching mechanism, for both typical bits and outlier bits. Degradation of resistance of typical bits is due to the oxygen vacancy profile in the filament changing during oxygen diffusion, and the retention failure of outlier bits is caused by the critical percolation path being broken within the filament during oxygen diffusion.