Ryutaro Yasuhara

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 physical analytic formula based on Stochastic Differential Equation was successfully developed to describe intrinsic ReRAM variation. The formula was proved useful for projecting scaled ReRAM memory window and resistance distribution after long-term retention, verified by testing 40 nm 2-Mbit ReRAM. The formula also centered on practical and quantitative filament characterization.

{\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

Instead of wide distributed resistance for a single bit, we introduce non-fluctuating physical parameters, filament diameter and packing factor (corresponding to oxygen vacancy concentration) to describe ReRAM bit. The quantitative 3D percolation model is developed based on direct observation of the filament structure and hopping conduction, which is confirmed with ultra-low temperature (30 K) measurement. Moreover, we provide a simulation method to obtain quantitative filament diameter, packing factor and to do the prediction of the resistance distribution after retention, which is verified with experiment.

Distribution projecting the reliability for 40 nm ReRAM and beyond based on stochastic differential equation

ReRAM is increasingly being developed for applications that require higher speeds and lower voltages than flash memory. We have found TaOx to have high performance and high reliability. However one of the phenomena observed in ReRAM is that each resistance after Set and Reset varies during every cycle. To stabilize resistive switching, the key is to limit these variations in resistance. In ReRAM, a conductive filament (CF) is created by the forming pulse. Resistive switching in the CF is based on reduction and oxidization using this voltage pulse. This paper reviews a hopping percolation model which we have proposed for the switching process, and this paper proposes an automatic forming circuit using our newly-developed externally-scalable forming pulse (ESF) scheme. In this CF model, conductive paths show different conductivities caused by the formation of different percolation networks that link hopping sites. Larger CFs show greater variation in resistance due to the many possible combinations of percolation networks. This makes it important to develop a forming technique that limits CFs to their optimal size. Forming is based on dielectric breakdown, so the pulse width ranges over approximately three orders. The automatic forming circuit detects, bit by bit, whether forming is over, and stops the forming pulse after a specified period. A forming pulse is then generated, using an external clock, to cover the range of pulse widths. This allows the filament size to be controlled to ensure it is uniform for all of the bits in the circuit, at the cost of only a small area overhead.

Conductive Filament Expansion in TaOx Bipolar Resistive Random Access Memory during Pulse Cycling

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.