Takeshi Takagi

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.

An 8 Mb Multi-Layered Cross-Point ReRAM Macro With 443 MB/s Write Throughput

Nonvolatile memories with fast write operation at low voltage are required as storage devices to exceed flash memory performance. We develop an 8Mb multi-layered cross-point ReRAM macro with 443MB/S write throughput (64b parallel write per 17.2ns cycle), which is almost twice as fast as existing methods, using the fast-switching performance of TaOχ ReRAM and the following three techniques to reduce the sneak current in bipolar type cross-point cell array structure in an 0.18μm process. First, memory cell and array technologies reduce the sneak current with a newly developed bidirectional diode as a memory cell select element for the first time. Second, we use a hierarchical bitline (BL) structure for multi-layered cross-point memory with fast and stable current control. Third, we implement a multi-bit write architecture that realizes fast write operation and suppresses sneak current. This work is applicable to both high-density stand-alone and embedded memory with more stacked memory arrays and/or scaling memory cells.

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}

The electroforming and the resistance-switching behaviors in magnetite, Fe3O4, by the application of an appropriate electric field are demonstrated on a lateral device with multiple electrodes. By means of this device, both the location and the nature of the change in Fe3O4 are specified from the electrical measurements and Raman spectroscopy. The switching phenomenon is caused in maghemite, γ-Fe2O3, which is formed by oxidation of Fe3O4, near an interface of an anode. The authors argue that the switching motion is originated in a redox reaction between the Fe3O4 and γ-Fe2O3.

Bipolar ReRAM for Improving Data Retention Under Low Operation Current

A novel iron oxide (Fe-O) ReRAM is proposed and its high-speed resistance-switching of 10 ns is demonstrated. The switching mechanism is confirmed as a redox reaction between Fe₃O₄ and y-Fe₂O₃. Based on this model, we have achieved long-retention characteristics by introducing Zn atoms to suppress the reduction process.

Electroforming and resistance-switching mechanism in a magnetite thin film

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.

Fast switching and long retention Fe-O ReRAM and its switching mechanism

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

In this letter, we propose a novel SiGe channel heterostructure dynamic threshold metal oxide semiconductor (DTMOS) and demonstrate its superiority over conventional Si-DTMOS. The introduction of a SiGe layer for the channel is very effective for reducing the threshold voltage in spite of keeping impurity doping level at the body region. Therefore, a low threshold voltage and a large body effect factor can be achieved simultaneously. The SiGe HDTMOS with highly doped body exhibits two times higher transconductance, 1.4 times higher saturation current, and better short channel immunity than that of the control Si-DTMOS with lightly doped body of which threshold voltage is nearly the same.

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

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].