Naoki Banno

Electronic transport in Ta2O5 resistive switch

The authors examined the electronic transport of a solid electrolyte resistive switch. Using element analysis and the temperature dependence of its electronic transport, they deduced that the conductive path is composed of Cu metal precipitated in the solid electrolyte film by an electrochemical reaction. Furthermore, they observed Coulomb blockade phenomena at 4K when the switch was in the off state. Their observations and experimental results suggest that the metallic conductive path consists of metallic islands separated by tunneling barriers and that switching between the on and off states originates from modulation in the tunneling barriers.

Diffusivity of Cu Ions in Solid Electrolyte and Its Effect on the Performance of Nanometer-Scale Switch

A novel solid-electrolyte nonvolatile switch that we previously developed for programmable large-scale-integration circuits turns on or off when a conducting Cu bridge is formed or dissolved in the solid electrolyte. Cu⁺ ion migration and an electrochemical reaction are involved in the switching process. For logic applications, we need to adjust its turn-on voltage (<i>V</i> _ON), which was too small to maintain the conductance state during logic operations. In this paper, we clarified that <i>V</i> _ON is mainly affected by the rate of Cu⁺ ion migration in the solid electrolyte. Considering the relationship between the migration rate and <i>V</i> _ON, we replaced the former electrolyte, Cu_2-alphaS, with Ta₂O₅, which enabled us to appropriately adjust <i>V</i> _ON with a smaller Cu⁺ ion diffusion coefficient.

Polymer Solid-Electrolyte Switch Embedded on CMOS for Nonvolatile Crossbar Switch

A polymer solid-electrolyte (PSE) switch has been embedded in a 90-nm-node CMOS featuring a forming-less programming and extremely high on/off ratio of 105. A fast programming of 10 ns is also demonstrated for 50-nmΦ 1 k-b array by introducing the PSE switches integrated with a fully logic compatible process below 350°C. A high free volume in the PSE is supposed to result in the smooth formation of the Cu bridge without destroying the electrolyte, thereby also resulting in forming-less programming and high breakdown voltage. High disturbance reliability (T50; 50% fail) is extracted to be over 10 years at operation condition. The improved switching characteristics enable us to accurately program the crossbar circuit in a practical scale (32 × 32) without cell transistors. The developed switch is a strong candidate for realizing a low-power and low-cost nonvolatile programmable logic.

Off-state and turn-on characteristics of solid electrolyte switch

We have investigated off-state and turn-on characteristics of a Ta2O5-based solid-electrolyte switch, the resistance of which changes when the metallic current path is formed in the solid-electrolyte. The turn-on voltages are found to vary widely even when the switches are in an off-state with similar resistance. The variation is induced by the residual metal with different shapes that remains in the solid-electrolyte after a switch is turned off. The residual metal with a sharp point enhances the electrochemical reaction, resulting in the turn-on voltage lowering. We also developed a screening scheme to reduce the variation of the turn-on voltages.

Programmable cell array using rewritable solid-electrolyte switch integrated in 90nm CMOS

Programmable devices such as SRAM-based FPGAs have the major challenges of power consumption and circuit area due to the excessive standby leakage current and the threshold voltage variation in highly scaled SRAM. Back-end-of-line (BEOL) device, which is integrated in the interconnect layers, is attractive for reducing the performance gap between FPGA and cell-based ASIC [1–4]. In this paper, we demonstrate the fundamental operations of a programmable cell array and a 32×32 crossbar switch using a nonvolatile and rewritable solid-electrolyte switch (nanobridge or NB). A 72% reduction in chip-area compared with that of a standard-cell-based design is achieved on a 90nm CMOS platform.

First demonstration of logic mapping on nonvolatile programmable cell using complementary atom switch

Reconfigurable nonvolatile programmable logic using complementary atom switch (CAS) is successfully demonstrated on a 65-nm-node test chip. Various logics are realized by synthesizing RTL codes and mapping the configurations into CAS-based programmable cell array. Each cell includes the two 4-input LUTs, 19×16 crossbar switch, and 368-b CAS. The CAS integrated over CMOS reduces the cell area by 78% compared to a conventional SRAM-based design.

Solid-electrolyte nanometer switch

We have developed a solid-electrolyte nonvolatile switch (here we refer as NanoBridge) with a low ON resistance and its small size. When we use a NanoBridge to switch elements in a programmable logic device, the chip size (or die cost) can be reduced and performance (speed and power consumption) can be enhanced. Developing this application required solving a couple of problems. First, the switching voltage of the NanoBridge (∼ 0.3 V) needed to be larger than the operating voltage of the logic circuit (> 1 V). Second, the programming current (> 1 mA) needed to be suppressed to avoid large power consumption. We demonstrate how the Nanobridge enhances the switching voltage and reduces the programming current.

Highly scalable nonvolatile TiOx/TaSiOy solid-electrolyte crossbar switch integrated in local interconnect for low power reconfigurable logic

A fully logic-compatible, nonvolatile crossbar switch using a novel dual-layer TiOx/TaSiOy solid-electrolyte, “NanoBridge”, has been developed for the first time, which is scalable to 50 nm and beyond and keeps the extremely low ON-resistance of ≪100 Ω. A key breakthrough is the dual-layer solid-electrolyte, in which TiOx works as an oxygen absorber as well as a superior ionic conductor, thus improving the yield, ON/OFF resistance ratio (≫106) and cycling endurance (≫103). The highly scalable 4 × 4 crossbar switch composed of NanoBridge integrated in a local Cu interconnect of a standard CMOS is successfully configured without select transistors. The nonvolatile solid-electrolyte, crossbar switch is a promising switch element for low power and low cost reconfigurable logic.

Improved Off-State Reliability of Nonvolatile Resistive Switch With Low Programming Voltage

A complementary atom switch (CAS) is proposed to realize low programming voltage and high off-state reliability for crossbar switch application. Two atom switches with bipolar operation are connected in series with opposite direction, in which the two atom switches work as a single element. The two off-state atom switches in the CAS complementarily divide voltage stress, greatly enlarging the off-state lifetime. The CAS is embedded in Cu BEOL on a 65-nm-node CMOS platform without degrading the CMOS and interconnect performances. The CAS using two atom switches is one of the candidates for realizing energy-efficient nonvolatile programmable switches.

Highly reliable, complementary atom switch (CAS) with low programming voltage embedded in Cu BEOL for Nonvolatile Programmable Logic

A novel complementary atom switch (CAS) embedded in Cu BEOL has been developed to realize low programming voltage of 2V and extremely high disturbance reliability of OFF-state (T0.1>10 years at 1V, 125°C). The decrement of solid-electrolyte density successfully reduces the programming voltage down to 2V. Two bipolar resistive-change elements such as atom switches are connected in series with opposite direction, in which the two OFF-state elements complementarily divide the voltage stress, greatly enlarging the OFF-state lifetime. The highly reliable, complementary atom switch is a promising device for energy efficient, Nonvolatile Programmable Logic (NPL).