Setsu Kotsuji

Resistance Controllability of

The controllability of resistance in the set (low resistance) and reset (high resistance) states of Ta2O5/TiO2 resistive random access memory (ReRAM) was investigated to achieve low-voltage and multilevel operation. Since resistance in a set state tended to decrease due to the history effect of the lowest resistance, multilevel operation with a controlled set resistance was found to be difficult to achieve. On the other hand, the reset resistance could be controlled accurately by varying the reset voltage. The switching mechanism, the tunnel barrier of which is regenerated to cut off the filament, obtains the high repeatability of the reset resistance. As a result, a four-level storage ReRAM was successfully demonstrated with multireset level operation. In addition, the set voltage was found to strongly depend on reset resistance. Preventing reset resistance from exceeding 1 G¿ achieved a low-set-voltage operation below 5 V.

\hbox{Ta}_{2} \hbox{O}_{5}/\hbox{TiO}_{2}

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.

Stack ReRAM for Low-Voltage and Multilevel Operation

The Effect of the bottom electrode of ReRAM with a Ta2O5/TiO2 stack on noise and retention was investigated. The current fluctuation due to complex random telegraph noise (RTN) resulted in errors in the read-out with multi-level operation. We clarified that the Ti diffusion into the TiO2 layer from the bottom electrode increased the trap density and thus degraded current stability. The increased density caused the minority bit in the reset state to fail under a high temperature stress. The use of a stack with a Ru or Pt electrode with controlled Ti diffusion resulted in low noise and high thermal stability (≫190°C).

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

We investigated the conduction mechanism of a Ta2O5/TiO2-stacked resistance random access memory (ReRAM) device and found that its highly resistive state can be attributed to tunnel barriers induced in the filament, since single-electron tunneling phenomena was observed in the current–voltage characteristics at low temperatures and the resistance depended only slightly on temperature. We also found that the largest tunnel barrier, whose resistance is more than 1000 times larger than the second largest one, is located at the interface between the Ta2O5 layer and TiO2 layer and that variation in the resistance was caused by variation in the tunnel barrier width.

Effect of bottom electrode of ReRAM with Ta 2 O 5 /TiO 2 stack on RTN and retention

We propose a new approach for improving the operating margin of Ta2O5/plasma oxidized TiO2 stacked unipolar ReRAM. It was found that the reset voltage (switching from low resistance state to high resistance state) can be minimized by using local minimum against the resistance of the low resistance state. In addition, weakening the plasma oxidation condition reduced the power consumption and the variation of reset voltage. Excellent operating margin and more than 105 switching cycle times was successfully demonstrated using the integrated device.

Physical Model for Reset State of Ta2O5/TiO2-Stacked Resistance Random Access Memory

We investigated the effect of ReRAM-stack asymmetry on read disturb immunity. Stacking stoichiometric Ta<inf>2</inf>O<inf>5</inf> and ultrathin TiO<inf>2</inf> led to bipolar switching property. Filament (conduction path) penetrated both Ta<inf>2</inf>O<inf>5</inf> and TiO<inf>2</inf> layer. Because single Ta<inf>2</inf>O<inf>5</inf> film has no switching property, the resistance was not changed under positive bias on Ta<inf>2</inf>O<inf>5</inf>-side electrode. Under negative bias, the resistance of the filament near TiO<inf>2</inf>-side electrode increases because of anodic oxidation. A high read-disturb immunity were achieved by using the 1T1R ReRAM of this stack. These results can be attributed to the asymmetric switching behavior of the stoichiometric Ta<inf>2</inf>O<inf>5</inf>/ultrathin-TiO<inf>2</inf> stack.

A new approach for improving operating margin of unipolar ReRAM using local minimu m of reset voltage

We characterized how positive and negative bias temperature instabilities (PBTI and NBTI) occur in HfSiON gate stacks. The PBTI was confirmed to be suppressed by using amorphous (a-) HfSiON instead of crystallized (c-) HfSiON. The a-HfSiON reduced the capture cross-section and lowered the density of electron traps, which explains the suppression of the PBTI. The different trap parameters for a-HfSiON and c-HfSiON suggest that the electron traps of these structures have different origins. The PBTI of a-HfSiON gates occurred through electron trapping without generation of interface traps, while the NBTI of a-HfSiON gates occurred through generation of interface traps and positive oxide charges. Furthermore, it was found that the NBTI of a-HfSiON gates also involves electron trapping. Additionally, the subthreshold slope decreased under positive BT stress. We attribute these characteristic BTI behaviors of HfSiON gates to the influence of charge traps that are present within the HfSiON bulk.

Effect of ReRAM-stack asymmetry on read disturb immunity

We have developed low-stress chromium nitride (CrN) films as hardmasks for x-ray absorber etching. The stress in the CrN films is 3 MPa and its distribution (gradient) is less than 10 MPa in a 25×25 mm area. In addition, the CrN film is electrically conductive (1.4 Ω/□). We have fabricated 0.10 μm line-and-space patterns in 0.4-μm-thick tantalum germanide using a 75-nm-thick CrN hardmask. The results demonstrate that a sputtered CrN film is an excellent hardmask material for x-ray mask fabrication.

Influence of Charge Traps within HfSiON Bulk on Positive and Negative Bias Temperature Instability of HfSiON Gate Stacks

The characteristics of several elements for binary tantalum alloys, such as crystal structure, x‐ray absorption, and dry‐etching properties were systematically investigated. As a result, TaGe, TaSi, TaRe, and TaTi were selected as potential candidates for x‐ray mask absorbers. We deposited these Ta alloys using conventional magnetron sputtering. The stress in the TaX films was controlled more precisely than in Ta films. It was found that TaGe was one of the most suitable materials based on x‐ray absorption, stress control, and fine pattern fabrication.

Low-stress sputtered chromium–nitride hardmasks for x-ray mask fabrication

Memory-state [low-resistance state (<i>R</i>_L) and high-resistance state (<i>R</i>_H)] dependence of random telegraph noise (RTN) of Ta₂O₅/TiO₂ resistive random access memory is investigated. The conduction mechanism of both memory states and a limit of resistance controllability are also investigated to clarify the difference in the RTN mechanism of both states. The boundary between the <i>R</i>_L and <i>R</i>_H states was found at 5-20 kΩ , and the conduction mechanism much depended on the memory state. The noise also depended on the memory state. The noise amplitude in the <i>R</i>_H state was larger than that in the <i>R</i>_L state. In the <i>R</i>_H state, a tunnel barrier was generated to cut off a conduction path (filament), and traps inside the tunnel barrier were supposed to increase the noise amplitude. Moreover, the composition of the following degraded the noise distribution in the <i>R</i>_H state: 1) capture and emission of charges with traps and 2) instability of these traps against the bias temperature stress.