Shigeo Ohnishi

TiO2 anatase nanolayer on TiN thin film exhibiting high-speed bipolar resistive switching

The surface oxidized layer of a TiN barrier metal thin film grown on a Pt electrode was used as a resistive switching material. The fabricated memory cell shows bipolar resistive switching on a nanosecond order. A TiO2 anatase layer of about 2.5nm thick on TiN thin film was characterized by high-resolution scanning transmission electron microscopy. The results suggested that the high-speed resistive change was derived from the Mott transition in the TiO2 anatase nanolayer, and the obtained results could relate to the formation of filament paths previously reported in binary transition metal oxide thin films exhibiting resistive switching.

High-Temperature Etching of PZT/Pt/TiN Structure by High-Density ECR Plasma

Submicron patterning technologies for the PZT/Pt/Ti/TiN/Ti structure with a spin on glass (SOG) mask were demonstrated using a high-density ECR plasma and a high substrate temperature above 300° C. A 30%-Cl2/Ar gas was used to etch a lead zirconate titanate (PZT) film. No deposits remained, which resulted in an etched profile of more than 80°. A 40%-O2/Cl2 gas was used to etch a Pt film. The etching was completely stopped at the Ti layer. 30-nm-thick deposits remained on the sidewall. They were removed after dipping in hydrochloric acid. The etched profile of a Pt film was more than 80°. The Ti/TiN/Ti layer was etched with pure Cl2 gas. The size shift from the SOG mask was less than 0.1 µ m. Interdiffusion between SOG and PZT was not detected by transmission electron microscopy and energy dispersive X-ray spectroscopy (TEM-EDX) analysis.

Controllability of Electrical Conductivity by Oxygen Vacancies and Charge Carrier Trapping at Interface between CoO and Electrodes

Recently, the role of resistance random access memory (RRAM) is becoming extremely important in the development of nonvolatile memories. RRAM works by changing the resistance of the transition metal oxide contained in RRAM after the application of a sufficiently high voltage, however, this switching mechanism has not been fully clarified. In this study, by performing first principles calculations based on the density functional theory, we first investigate the change in the property of bulk CoO resulting from oxygen vacancies and charge carrier trapping in the vicinity of the oxygen vacancies. Next, we perform calculations for slab models of CoO in contact with Ta, W, and Pt electrodes and hence investigate the effects of oxygen vacancies at the interface between the CoO layer and the electrode layer. On the basis of the obtained results, we conclude that W is the most suitable electrode material compared with Ta and Pt.

Realization of the Switching Mechanism in Resistance Random Access Memory™ Devices: Structural and Electronic Properties Affecting Electron Conductivity in a Hafnium Oxide–Electrode System Through First-Principles Calculations

The resistance random access memory (RRAM™) device, with its electrically induced nanoscale resistive switching capacity, has attracted considerable attention as a future nonvolatile memory device. Here, we propose a mechanism of switching based on an oxygen vacancy migration-driven change in the electronic properties of the transition-metal oxide film stimulated by set pulse voltages. We used density functional theory-based calculations to account for the effect of oxygen vacancies and their migration on the electronic properties of HfO2 and Ta/HfO2 systems, thereby providing a complete explanation of the RRAM™ switching mechanism. Furthermore, computational results on the activation energy barrier for oxygen vacancy migration were found to be consistent with the set and reset pulse voltage obtained from experiments. Understanding this mechanism will be beneficial to effectively realizing the materials design in these devices.

CoOx-RRAM memory cell technology using recess structure for 128Kbits memory array

This paper presents the process integration and device technology for the Resistance RAM(RRAM) memory array using a CoOx film and a recess structure as a resistor, which is capable of low voltage, high speed and low current operation. The resistance of the CoOx film and its uniformity are strongly dependent on the film quality, which is optimized by controlling the O2 gas flow rate during the film deposition. We demonstrate the basic write and read operation of the 128Kbits memory array by developing the novel process integration technology and optimizing the test algorism.

Studies of Ir–Ta–O as High Temperature Stable Electrode Material and Its Application for Ferroelectric SrBi2Ta2O9 Thin Film Deposition

An Ir–Ta–O/Ta/Si structure with Ir–Ta–O as electrode and Ta as diffusion barrier layer on silicon substrate has been fabricated. The Ir–Ta–O film was deposited by reactive sputtering using separate Ir and Ta targets in oxygen ambient. Annealing results performed from 500–1000°C in oxygen ambient showed that the Ir–Ta–O film exhibited extraordinary high temperature stability. This film showed good conductivity and integrity even after 5 min annealing at 1000°C. No destructive peeling and hillock formation were observed. By using this film as bottom electrode for depositing ferroelectric SrBi2Ta2O9 thin film at 800°C, good ferroelectric properties were achieved.

In Situ Rapid Thermal Nitridation of Collimated Titanium by Physical Vapor Deposition as a Blanket Tungsten Barrier

A very thin bilayered titanium nitride/titanium barrier was obtained from in situ rapid thermal nitridation (RTN) for collimated Ti by physical vapor deposition for blanket tungsten plug. Ti with in situ RTN results in fine barrier properties compared with those by RTN with exposure to air after Ti deposition. Furthermore, this thinner film without the re-entrant profile at the contact shoulder was effective for complete contact filling at 0.25 µm diameter with the aspect ratio of 4. This is one of the barrier technologies applicable to quarter-micron devices.

Integration of SBT thin film into MFMOS structure for one transistor memory applications

The SBT thin film has been successfully integrated into a MFMOS structure for one-transistor memory applications. A new high temperature stable electrode, IrMO, that consists of Ir, metal, and O was used as the bottom electrode. This new electrode can maintain good conductivity and integrity even after very high temperature (800-1000/spl deg/C) oxygen ambient annealing. Therefore the SBT film can be deposited on the bottom electrode using MOD methods and annealed at high temperature (700-800/spl deg/C) without any hillock and peeling problems. Excellent ferroelectric properties were achieved. The thermal stability of the bottom electrode on thin gate oxide (35 /spl Aring/) was also very stable. Good C-V characteristics maintained even after the SBT capacitor formation. The etching damage to the SBT capacitor stack can be minimized by optimizing the etching conditions. No polarization loss and just slightly Ec increase was observed after etching of the SBT capacitor stack. The aluminum oxide based passivation layer was successfully integrated into this device with excellent passivation properties. Finally, the memory window of the MFMOS device will be presented.

Highly Stable Ir-Ta-O Electrode for Ferroelectric Material Deposition

Ir-Ta-O composite bottom electrode has extraordinary high temperature stability. It can maintain good conductivity and integrity even after 5min annealing at 1000 °C in oxygen ambient. The thermal stability of Ir-Ta-O on different substrates has been studied. It shows that Ir-Ta-O is also very stable on Si and SiO 2 substrates. No hillock formation and peelings of the bottom electrode were observed after high temperature and long time annealing in O 2 ambient. SEM, TEM, XRD, and AES have been used to characterize the Ir-Ta-O film and the interfaces between Ir-Ta-O bottom electrode and Si or SiO 2 substrate. The composition and conductivity changes of the electrode during oxygen ambient annealing and the interdiffusion issue will be discussed. Furthermore, Ir-Ta-O/SiO 2 /Si capacitor with 30A gate oxide was fabricated and the C-V and I-V characteristics were measured to confirm the stability of Ir-Ta-O on thin gate oxide.