Takafumi Okamoto

Electric field concentration in the vicinity of the interface between anode and degraded BaTiO3-based ceramics in multilayer ceramic capacitor

The electric field distribution of degraded dielectric layers in multilayer ceramic capacitors (MLCCs) was investigated by Kelvin probe force microscopy (KFM) to clarify the insulation degradation mechanism in MLCCs. For the degraded dielectric layers, the electric field was found to be concentrated near the anodes. This concentration easily moved to the opposite side with a reversal of the applied voltage of 5 V (13 kV/cm) during KFM measurement at room temperature. On the other hand, electric field concentrations and electric field distributions did not change in fresh MLCCs, indicating that the electric field concentrations easily transfer near higher-potential interfaces between electrodes and ceramics only in degraded MLCCs. These facts suggest that Schottky barriers would be formed in degraded MLCCs. The KFM technique discussed in this work is a very useful tool for measuring the surface potential and helps clarify the local electric field concentration near the electrodes.

Insulation degradation behavior of multilayer ceramic capacitors clarified by Kelvin probe force microscopy under ultra-high vacuum

We investigated surface potential images on the cross section of degraded multilayer ceramic capacitors (MLCCs) by Kelvin probe force microscopy measured under a dc bias voltage in ultra-high vacuum. A highly accelerated lifetime test (HALT) was conducted to obtain degraded MLCCs. The high energy resolution of the present measurement allows us to observe the step-like voltage drops on dielectric layers of as-fired MLCCs. The step-like voltage drops disappear on the dielectric layers of degraded MLCCs, indicating that the resistance at grain boundaries declines with the progress of insulation degradation. Furthermore, the electric field concentrations near the electrodes are clearly observed under forward and backward bias. The discussion based on energy band diagrams suggests that the electric field concentrations near electrodes are attributable to energy barrier formed at the interface between electrode and dielectrics. In particular, the electric field concentration at cathode in HALT measured under ba...

Alkaline-Earth and Rare-Earth Elements and Oxygen Vacancy in BaTiO3: Analyses by First-Principles Calculations and EXAFS

We performed first-principles calculations to examine the interaction among rare-earth (RE), alkaline-earth (AE) elements and oxygen vacancy (VO) in BaTiO3, in order to clarify the combined effects of VO trapping by two different elements. It was found that there is a synergistic effect of VO trapping by RE at Ba site and AE at Ti site, so that the co-doping is effective to improve insulating reliability of BaTiO3. We also verified that the local atomic structures around dopants obtained by calculations well agree with that obtained experimentally by extended X-ray absorption fine structure (EXAFS) analyses. The present work is the first one to clarify the structural environment around dopants including VO by both theoretical and experimental approaches.

Nanoscale characterization of ferroelectric materials by scanning probe microscope under ultrahigh vacuum

The downsizing trend in multilayer ceramic capacitors (MLCCs) will reach a limit as long as a conventional way of materials development continues. Creation of advanced materials and precise control of materials properties will be a key technology to break through the current situation. To address this issue, scanning probe microscopy under ultrahigh vacuum (UHV-SPM) is employed to clarify materials properties from nanoscale perspective. Kelvin probe force microscopy allows us to obtain surface potential mapping on degraded dielectrics with highly spatial resolution, which is promising to consider degradation mechanism of MLCCs. Piezoresponse force microscopy enables not only to evaluate size dependent properties of ferroelectric nanoislands but also to investigate temperature dependence of domain images in Sn doped strontium titanate ceramics. Characterization of ferroelectrics based on UHV-SPM will open a new way to design future MLCCs.

Change in Electric Field Distribution in Non-Reducing BaTiO3 Based-Dielectric Layer Loaded at High Temperature

The electric field distributions in loaded dielectric layers of multilayer ceramic capacitors were investigated at several stages of insulation degradation for the load, using Kelvin probe force microscopy. The electric field distribution was found to be different at each stage of loaded time. Initially, the electric field was concentrated near the cathode, indicating that the insulation resistance near the anode decreased. Then, following the homogeneous distribution shown for an intermediate stage, the electric field eventually concentrated near the anode. This change indicates how insulation degradation occurs locally; this change can plausibly be explained by a hole density increase.

The Kelvin Probe Force Microscopy Measurement of Degraded Multilayer Ceramic Capacitors with Ni Inner Electrode

Electric potential mapping of degraded dielectric layers of multilayer ceramic capacitors (MLCCs) was carried out using Kelvin probe force microscopy in order to clarify their degradation mechanism under conditions of an accelerated lifetime test condition. In the cross sections of the degraded and as-prepared dielectric layers, a significant electric field concentration was found in the vicinity of the anode of the degraded dielectric layer, in contrast to a homogeneous concentration found throughout layers of the samples before the accelerated lifetime test.