The major trend for development of MLCCs with smaller sizes and larger capacities has led to the development of thinner layers of dielectric elements. Today, products are being mass-produced having dielectric elements of less than 1μm that are stacked in several hundred layers. The major issue in the development of thinner layers is the maintaining of insulation performance, and particularly the maintaining of insulation performance over long periods of time under high temperatures and high electric fields (referred to as "reliability" below) is needed.
Figure 1 shows a schematic diagram of an MLCC and an explanatory diagram of the parts of the ceramic structure. To achieve high reliability for MLCCs, it is important to understand the contribution to reliability and the role of each part of the ceramic structure. This will enable further strengthening of the strong parts or improvement of the weak parts.
Fig. 1 MLCC Schematic Diagram and Explanatory Diagram of the Parts of the Ceramic Structure
Figure 2 shows a comparison of the reliability for samples where the number of grain boundaries per element was changed by adjusting the number of grains per element. In this example, the number of grain boundaries refers to the quantity in the element thickness direction. This figure shows that samples with a large number of grain boundaries last a longer time until the insulation resistance is reduced, and thus have a higher reliability. This suggests that the grain boundary has an extremely high contribution to reliability, and it is important to obtain a certain number of grain boundaries per element in order to achieve sCompact High-Capacitance Device Applicationsufficient reliability.
This means that the development of thinner layers for dielectric elements requires smaller grain diameters for BaTiO3, which is the main raw material. However, as the grain diameter of BaTiO3 becomes smaller, the dielectric constant is reduced, and the desired capacitance will not be obtained. We then decided to investigate the reliability of the BaTiO3 grain interior. To do this, a fabrication process was devised so that samples were fabricated that had only a single grain in the thickness direction of the dielectric element. We confirmed that samples without grain boundaries in this element thickness direction had an extremely low reliability.
Therefore, to further improve reliability without relying solely on the grain boundary, we thought that it would be effective to modify the grain interior. In actuality, as shown in Fig. 3, using (Ba,Ca) TiO3 as the main raw material where a portion of the Ba in BaTiO3 was replaced with Ca enabled a significant improvement in reliability. To confirm the effect of modifying the grain interior, as described above, a (Ba,Ca) TiO3 sample without a grain boundary in the element thickness direction was fabricated for evaluation, and it was found that the reliability was improved. This means that, as we had intended, the modified grain interior resulted in higher reliability. Based on the changes in the lattice constant of various samples after the Ca replacement amount was changed and the changes in the activation energy resulting in insulation deterioration, it is surmised that this improvement in reliability by replacing with Ca is due to its suppressing of electromigration of oxygen vacancies, which is a cause of breakdowns, by lattice shrinkage due to the Ca replacement.
From this standpoint, we were successful in developing high-reliability materials that support thin layers for dielectric elements.
Fig. 2 Change in IR (Insulation Resistance) Over Time Under High Temperatures and High Electric Fields for MLCCs with Different Numbers of Grain Boundaries
Number of grain boundaries: a) 5.4 b) 6.6 c) 7.6 d) 8.1 Testing conditions: 150°C, 10kV/mm Composition: BaTiO3-Dy2O3-MgO-MnO-SiO2
Fig. 3 Change in IR (Insulation Resistance) Over Time Under High Temperature and High Electric Field Conditions for (Ba,Ca) TiO3-Based Material and BaTiO3-Based Material
Testing conditions: 150°C, 20kV/mm Composition: (Ba1-xCax) TiO3-Dy2O3-MnO-SiO2