Ichiro Fujiwara

Analysis of Carrier Traps in Si3N4 in Oxide/Nitride/Oxide for Metal/Oxide/Nitride/Oxide/Silicon Nonvolatile Memory.

The energy level, density and attempt to escape frequency of carrier traps in an Si3N4 film in tunnel oxide/nitride/oxide (ONO) multilayer for metal/oxide/nitride/oxide/silicon (MONOS) nonvolatile memory are investigated by discharging current transient spectroscopy (DCTS). To analyze the electrical properties of carrier traps observed through DCTS, a new model including the tunneling probability of the tunnel oxide film between the Si3N4 film and an Si substrate was proposed. As a result, the electron traps in the Si3N4 film, which are assumed to be related to the threshold voltage decay, i.e. data retention, were found for the first time. The energy level of the electron traps in the Si3N4 film in the ONO multilayer was 0.8–0.9 eV from the conduction band and the density was 1–5×1018 cm-3. The attempt to escape frequency of 2×1014 s-1 was also obtained. The energy level of the hole traps and its density were 0.8–0.9 eV from the top of the valence band and 1–4×1018 cm-3, respectively. The magnitude of the trap density obtained from DCTS shows good agreement with that obtained from memory hysteresis characteristics. These results indicate that their carrier traps are amphoteric traps.

In situ transmission electron microscopy analysis of conductive filament during solid electrolyte resistance switching

An in situ transmission electron microscopy (TEM) analysis of a solid electrolyte, Cu–GeS, during resistance switching is reported. Real-time observations of the filament formation and disappearance process were performed in the TEM instrument and the conductive-filament-formation model was confirmed experimentally. Narrow conductive filaments were formed corresponding to resistance switching from high- to low-resistance states. When the resistance changed to high-resistance state, the filament disappeared. It was also confirmed by use of selected area diffractometry and energy-dispersive x-ray spectroscopy that the conductive filament was made of nanocrystals composed mainly of Cu.

I-V measurement of NiO nanoregion during observation by transmission electron microscopy

Conduction measurements with simultaneous observations by transmission electron microscopy (TEM) were performed on a thin NiO film, which is a candidate material for resistance random access memories (ReRAMs). To conduct nanoscale experiments, a piezo-controlled TEM holder was used, where a fixed NiO sample and a movable Pt-Ir counter electrode were placed. After the counter electrode was moved to make contact with NiO, I-V measurements were carried out from any selected nanoregions. By applying a voltage of 2 V, the insulating NiO film was converted to a low resistance film. This phenomenon may be the “forming process” required to initialize ReRAMs. The corresponding TEM image indicated a structural change in the NiO layer generating a conductive bridge with a width of 30–40 nm. This finding supports the “breakdown” type forming in the so-called “filament model” of operation by ReRAMs. The inhomogeneity of resistance in the NiO film was also investigated.

0.13 µm Metal-Oxide-Nitride-Oxide-Semiconductor Single Transistor Memory Cell with Separated Source Line

A 0.13 µm metal (conductive gate) oxide nitride oxide semiconductor (MONOS) single transistor memory cell is proposed and demonstrated. The three main limiting factors and their solutions in a 0.13 µm MONOS single transistor memory cell with a separated source line are clarified. The first issue is the reduction in the margin of program inhibit voltage due to a short channel effect. This problem can be improved by applying a positive bias voltage to unselected wordlines. The second issue is the leakage current from unselected cells. This problem can be reduced by the use of a source bias read technique. The third issue is the read disturb of a selected cell. This problem can be improved with a thicker tunnel insulator without increase in program time. An experimental 0.13 µm MONOS memory cell is obtained with satisfactory results. A small cell size of about 6F2 (F: feature size) is proposed by the use of winding source lines.

High Density Charge Storage Memory with Scanning Probe Microscopy

We demonstrate an ultrahigh density erasable charge storage memory with a scanning probe microscope in an oxide-nitride-oxide-silicon (ONOS) system of semiconductor nonvolatile memory media. It is found that by applying a voltage pulse between the tip and the silicon substrate, charge carriers can be made to be recorded and erased in the ONOS media. It is also found that the locally trapped charges cause local changes of the surface potential. The locally trapped charges ( recorded bits) can be read efficiently by detecting the change of the surface potential, without contacting the ONOS media, with the scanning Maxwell-stress microscope (SMM). A pattern of a 4 ×3 array of recorded bits could be successfully fabricated. The size of the smallest recorded bit is about 100 nm in diameter. This value corresponds to about 63 Gb/in2 in recording density. Furthermore, it is found that the recorded bits can be read at up to a frequency range as high as 10 MHz with heterodyne detected SMM.

Narrow distribution of threshold Voltage in 4-Mbit MONOS memory-cell array with F-N channel write and direct/F-N tunneling erase operation as a single transistor structure

This paper describes the narrow and nonspreading distribution of threshold voltage in metal-oxide-nitride-oxide semiconductor (MONOS) memory cell array with Fowler-Nordheim (F-N) channel write operation and direct/F-N tunneling erase operation as a single transistor structure. We fabricated a 4-Mbit MONOS memory test chip using 0.25-/spl mu/m technology. The gate length of the memory cell was shrunk to 0.18 /spl mu/m. The distribution of threshold voltage for many operations were evaluated. The range of threshold voltage distribution is small, within 0.5 V in 12-14 V for programming and -8.5 to -9 V for erasing. It was also narrow for program/erase cycles up to 10/sup 4/ and after exposure to temperatures of 300/spl deg/C for 17 h and 150/spl deg/C for 304 h. These characteristics of narrow Vth distribution represent advantages of the MONOS memory device both for nonverify operation in program/erase mode and for low supply voltage operation in read mode. Another advantage is that no anomalous leak cell or tail bit is evident in the data retention result, demonstrating high reliability. The MONOS memory device is a promising candidate for use in cheaper and more scalable gate length fabrication processes compared with floating gate for highly reliable embedded applications.

Visualization of Conductive Filament during Write and Erase Cycles on Nanometer-Scale ReRAM Achieved by In-Situ TEM

The paper shows clear evidence that in-situ transmission electron microscopy (TEM) can be used as a powerful tool to analyze ReRAM operation. Reproducible resistive switching of 100k cycles in 30- or 70-nm Cu-Te CBRAMs was achieved for the first time during in-situ TEM observation. A TEM sample of the CBRAM cells was processed by a focused ion beam method. The formation and rupture of a Cu filament was observed and analyzed in the TEM with energy dispersive x-ray (EDX) mapping. Since the overshoot current at the resistive switching was efficiently suppressed by a MOSFET placed in the TEM holder, stable and reproducible ReRAM switching operations were achieved in the TEM.

Friction Force Microscopy Study of the Langmuir-Blodgett Films with Different Molecular Structures

The microscopic frictional properties of Langmuir-Blodgett (LB) films were investigated with a friction force microscope (FFM). At first, the microscopic frictional properties of LB films with different molecular structures were investigated. It is found that the friction coefficient of a tilt LB film are smaller than that of Ba arachidate. It is suggested that the friction coefficient of the LB film which alkyl chains are tilted with a large limiting area is smaller than that of the LB film with a small limiting area. Second, molecular level friction properties of behenic acid LB films were investigated. The molecular resolution image of the friction force is obtained. It is found that the FFM image shows the two dimensional regularity and the period of the regularity is 0.48 nm and 0.52 nm respectively. The period of the regularity in the FFM image is in fair agreement with that of the atomic force microscope (AFM) image. The molecular simulation of the friction force is carried out using local density functional method for LB film model system. It is found that the period of the regularity of the lateral force is consistent with that of the potential energy. This indicates that the calculated results support the experimental results.

The observation of "Conduction Spot" on NiO resistance RAM

We succeeded to observe the “conduction spot” (CS) in the capacitor structure ReRAM, which includes a conductive filament. In this study, we used NiO prepared by thermal oxidation at high temperature as 800°C. It requires a forming process by extra high voltage, which partly removes the top electrode from the resistance switched area. These experiments enabled us to observe the conductive filament directly in CS on NiO ReRAM by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From SEM images, CSs seem to be produced by some kind of breakdown, but we confirmed the reproducible resistance switching at least 50 cycles after the CS generation. By means of energy dispersive X-ray spectroscopy (EDX) with TEM observations, drastic oxygen reduction was recognized in local area within CS of NiO films. Moreover, the CS area depended on the injection power for forming. These experimental data suggest that miniaturization of ReRAM will be achieved by reducing the injection power for forming. * Tel: +81-011-706-6457, E-mail:y-taka@nano.ist.hokudai.ac.jp