Takao Kanehara

Crystallization of Sr0.7Bi2.3Ta2O9+α Thin Films by Chemical Liquid Deposition

By chemical liquid deposition in which an alkoxide with a carbon number of 4 or smaller was used as raw material, a Sr0.7Bi2.3Ta2O9+α (SBTO) thin film was fabricated for use as a ferroelectric memory device for the purpose of decreasing the temperature of crystallization and improving surface morphology. The crystallization process was also examined. Crystallization began when the film was heat treated in oxygen at 650° C. When it was heat treated at higher than 700° C, it showed ferroelectric properties, and the squareness (remanent polarization/saturation polarization) of its hysteresis loop was improved at 800° C. A film heat treated at temperature 650° C was a cluster of fine particles, and a film heat treated at 800° C was a cluster of large particles. A film heat treated at 700° C was a mixture of fine particles and large particles. Therefore, it is concluded that the alkoxide with a carbon number of 4 or smaller as raw material enable the lowering of the heat-treatment temperature and improvement of the surface morphology of SBTO thin films.

Preparation of MgO Protective Layer for AC‐Type Plasma Display Panel by Means of Screen‐Printing

We studied how to form an MgO protective layer on an ac-type plasma display panel (PDP) using a screen-printing method. In our study, we focused on high luminous efficiency and low drive voltage. To raise the luminous efficiency, it is necessary to limit the discharge current. The firing voltage (V t ), which is the voltage at which all cells start to discharge, and the sustaining voltage (V s ), which is the minimum voltage required for all cells to sustain discharge, have no clear relationship to luminous efficiency. To attain both a higher luminous efficiency and a lower drive voltage, a thinner more crystalline layer is necessary. We found that we could prepare an MgO layer by screen-printing and sintering fine vapor-deposited MgO powders of different grain sizes with an MgO liquid binder. An MgO protective layer prepared by this method provided both a low drive voltage and superior luminous efficiency 2.8 times higher than that of a sputtered layer. The screen-printing method thus enables larger ac-PDPs to be produced at lower cost

Effects of H2 Sintering and Pt Upper Electrode on Metallic Bi Content in Sr0.9Bi2.1Ta2O9 Thin Films for Ferroelectric Memories Prepared by Sol-Gel Method

The effects of Pt electrode and H2 sintering on the metallic Bi content of Sr0.9Bi2.1Ta2O9 (SBT) thin films for ferroelectric memories were studied using X-ray photoelectron spectroscopy (XPS). Oxidic Bi in SBT films is reduced to metallic Bi by H2 sintering. The degree of reduction depends on the structure of the SBT film. The SBT film with a fluorite structure is more difficult to reduce than that with a Bi-layered structure. Pt upper electrode formation also leads to an increase of metallic Bi content. Both H2 sintering and the Pt upper electrode work synergistically. In the combination of the pure Pt upper electrode and H2 sintering, over 80% of Bi on the SBT film surface was metallic. Pt films of 5 nm thickness on the SBT film recrystallized during the heat treatment and were broken up into particles. Pt electrode coverage was about 20% after the 2nd annealing at 800°C for 30 min in O2 atmosphere. Electrical properties were significantly affected by the presence of metallic Bi; in particular, SBT films with higher metallic Bi content showed higher leakage current density. The metallic Bi content must therefore be suppressed to obtain SBT films with low leakage current density.

Orientation Control of Sr0.7Bi2.3Ta2O9+α Thin Films by Chemical Liquid Deposition

The orientation of Sr 0.7 Bi 2.3 Ta 2 O 9+α (SBT) films, which are layer-type bismuth compounds, was controlled by varying the Sr-source. In this paper, the effect of crystal orientation on film characteristics is described. The crystal orientation of the SBT ferroelectric films did not affect the surface morphology, leakage current or fatigue characteristics, but it did affect the shape of the hysteresis loop (polarization) and the window value of the C-V characteristics when the films were connected to a metal-oxide-semiconductor (MOS) diode. Although a complete c-axis orientation film with a stoichiometry of SrBi 2 Ta 2 O 9 shows no spontaneous polarization in general, the highly c-axis orientated Sr 0.7 Bi 2.3 Ta 2 O 9+α film in this study showed some spontaneous polarization. The polarization values are larger than expected by considering orientation alone. A deviation from stoichiometry resulted in an increase made in the polarization along the c-axis. Therefore, control of the crystal orientation and composition of SBT films is quite an important factor in actual applications.

Reduction of Process-induced Damage and Improvement of Imprint Characteristics in SrBi2Ta2O9 Capacitors by Postmetallization Annealing

We investigated the effect of process damage induced after capacitor etching procedures in the process of ferroelectric random access memory (FeRAM) fabrication for SrBi2Ta2O9 capacitors. We found that this damage was suppressed by postmetallization annealing (400 ?C, 30 min in O2) and that imprint characteristics were improved by the annealing, because active elements such as hydrogen and water induced during contact hole formation on tungsten plugs and first-metal formation are adsorbed effectively by the annealing before they penetrate into these capacitors.

Effects of bottom electrode conditions on ferroelectric properties of SrBi2Ta2O9 thin films

The effect of adhesion layers under the bottom electrodes on ferroelectric characteristics of SrBi2Ta2O9 (SBT) thin films synthesized by sol-gel method was investigated. The remanent polarization (2Pr) of SBT films was found to be smaller for the tantalum oxide (TaOx) adhesion layer than for the titanium oxide (TiOx) adhesion layer, whereas the coercive voltage (2Vc) was larger for the TaOx adhesion layer than for the TiOx adhesion layer. The difference was due to a larger c-axis distribution of crystals in the SBT film on Pt with the TaOx layer than with the TiOx layer. Moreover, the imprint characteristics of SBT capacitors on the TaOx layer were better than those on the TiOx layer because the 2Vc in SBT capacitors was larger for the TaOx layer than for the TiOx layer, and the polarization stability against the hysteresis shift was better for the TaOx layer than for the TiOx layer.

Effects of Storage Conditions on Imprint Characteristics in SrBi2Ta2O9 Capacitors

The effects on the imprint characteristics of SrBi2Ta2O9 capacitors of various storage conditions were investigated. It was observed that the hysteresis shifts due to imprint degradations were smaller in the case in which the both electrodes of the capacitors were connected after increasing the temperature of the capacitors that were polarized at room temperature than in the case in which the electrodes were not connected. The acceleration ratio (Racc) of the slope of the hysteresis shift in the capacitors without connected electrodes to that with connected electrodes is 1.35 at 125 °C in device-size capacitors, and the ratio is much less dependent on temperature than the hysteresis shift. Moreover, it was also found that Racc was smaller in the case in which the process-induced damage located near the interfaces was larger.

Dependences of Process-Induced Damage on Imprint Characteristics in SrBi2Ta2O9 Capacitors

We investigated hysteresis shifts in SrBi2Ta2O9 capacitors during high-temperature storage in an attempt to understand the mechanism underlying imprint degradation. We found that the activation energy (Ea) of imprint degradation is derived from the temperature dependence of the hysteresis shift, but is almost independent of the size of the capacitors (about 0.2 eV). On the other hand, Ea is strongly dependent on the size of contact areas on the upper electrodes of the capacitors (0.1 to 0.2 eV). This result suggests that imprint degradation is mainly due to the characteristics of ferroelectric-electrode interfaces, and is not due to the edges of the capacitors. We reached this conclusion because the dependence on the contact areas is related to process-induced damage near the upper electrode.

Effect of MgO Liquid Binder on Film Properties of Screen-printed MgO Protective Layer for AC-PDP.

The effect of liquid binder on the film properties of screen-printed MgO film for the protective layer for AC-PDP has been studied. The liquid binder is a stabilized magnesium diethoxide solution which changes to MgO after firing and binds the MgO particles. After drying, the MgO film thickness increases with increasing MgO liquid binder content. However, after firing, the MgO film thickness decreases with increasing MgO liquid binder content, because the MgO powder content decreases with increasing liquid binder content. When liquid binder content is less than 10 wt%, surface roughness of the MgO film increases and film density decreases with increasing liquid binder content. This is caused by a gas generated by the liquid binder. When liquid binder content is more than 10 wt%, the surface roughness of the MgO film decreases and film density increases with increasing liquid binder content. This is caused by the liquid binder combining powders. Therefore, it is concluded that the optimization of liquid binder content is very important for improving film properties.