Youichi Mizuno

Base-Metal Electrode-Multilayer Ceramic Capacitors: Past, Present and Future Perspectives

Multilayer ceramic capacitor (MLCC) production and sales figures are the highest among fine-ceramic products developed in the past 30 years. The total worldwide production and sales reached 550 billion pieces and 6 billion dollars, respectively in 2000. In the course of progress, the development of base-metal electrode (BME) technology played an important role in expanding the application area. In this review, the recent progress in MLCCs with BME nickel (Ni) electrodes is reviewed from the viewpoint of nonreducible dielectric materials. Using intermediate-ionic-size rare-earth ion (Dy2O3, Ho2O3, Er2O3, Y2O3) doped BaTiO3 (ABO3)-based dielectrics, highly reliable Ni-MLCCs with a very thin layer below 2 µm in thickness have been developed. The effect of site occupancy of rare-earth ions in BaTiO3 on the electrical properties and microstructure of nonreducible dielectrics is studied systematically. It appears that intermediate-ionic-size rare-earth ions occupy both A- and B-sites in the BaTiO3 lattice and effectively control the donor/acceptor dopant ratio and microstructural evolution. The relationship between the electrical properties and the microstructure of Ni-MLCCs is also presented.

Dielectric Properties of BaTiO3-Based Ceramics under High Electric Field.

The dielectric properties under a high electric field (ac-field) of BaTiO3-based ceramics with core grains, shell grains and core-shell grains were compared with those of multilayer ceramic capacitors (MLCCs) with these three kinds of grains. The MLCCs with the X7R specification had a core-shell structure, and the relative dielectric permittivity (er) of the dielectric layers in the MLCCs increased with increasing ac-field. Similar behavior was observed in the MLCCs consisting of only cores, indicating that the core predominantly determined the dielectric properties of MLCCs under high ac-fields. The dielectric properties of MLCCs and ceramic plates consisting of only shell grains showed that the shell was the relaxor ferroelectrics. A slight change in the shell composition yielded a large shift of the peak temperature of er. The shell improved the temperature stability of er at low temperatures under low ac-fields. In a ceramic plate with relatively large BaTiO3 grains (approximately 3 µm), the maximum er was observed at a moderate ac-field, which was explained from the electric displacement vs electric field hysteresis curves of ferroelectric BaTiO3. The MLCCs and ceramics plates with fine BaTiO3 grains (0.4 to 0.5 µm) showed similar dielectric behavior to the MLCC with the core-shell structure. The size effect of BaTiO3 played an important role in determining the temperature stability of er. For future MLCCs with very thin dielectric layers, a microstructure with fine BaTiO3 grains and grain boundary layers of the shell was proposed.

INFLUENCE OF THE MICROSTRUCTURE EVOLUTION ON ELECTRICAL PROPERTIES OF MULTILAYER CAPACITOR WITH NI ELECTRODE

The influence of the microstructure evolution on electrical properties as a parameter of the firing temperature was studied for materials in the BaTiO3(BT)–MgO–Ln2O3 (Ln=Ho and Dy) system. The sintering behavior and the formation of the core-shell structure were dependent on the kind of doped rare earth elements. The stability of the core-shell structure and electrical properties of Dy doped specimens against the firing temperature were much lower than those of the Ho doped specimens. Especially, the Dy doped disk specimens fired at more than 1320°C, in which the core-shell grains were destroyed as judged by the differential scanning calorimetry (DSC) measurement and the transmission electron microscopy (TEM) observation, exhibited characteristic electrical properties. The electrical properties of the Ho doped multilayer capacitor (MLC) specimen were superior to those of the Dy doped one. It was found that the microstructure had a definite influence on the electrical properties, such as the temperature dependence of the dielectric constant and the capacitance aging behavior under an unloaded field.

Effect of Mn Addition on dc-Electrical Degradation of Multilayer Ceramic Capacitor with Ni Internal Electrode

The effect of Mn addition on the microstructure and electrical properties, especially on the dc-electrical degradation, of the X7R-type multilayer ceramic capacitor with Ni internal electrode (Ni-MLCC) with thin active layers was investigated. As the amount of Mn increased, grain growth was suppressed, and the temperature characteristic (TC) curve was flattened. I–V characteristic measurements revealed that nonlinearity coefficient (α) at a high electric field of more than 10 V/µm was decreased, and the lifetime during the highly accelerated lifetime testing (HALT) under 20 V/µm was improved, as the Mn content increased. It was found that Mn addition caused the change of the electrical properties of the grain boundary (GB). The effect of Mn on dc-electrical degradation during HALT was investigated by introducing impedance measurement at elevated temperatures from the microstructural view point. The roles of Mn on dc-electrical degradation during HALT were proposed.

Effect of Occupational Sites of Rare-Earth Elements on the Microstructure in BaTiO3

The effect of rare-earth elements such as La, Ho and Yb on the microstructure in BaTiO3 (ABO3) with various Ba/Ti ratios was studied. The grain size changed markedly depending on the Ba/Ti ratio in the case of La- and Yb-doped BaTiO3. On the other hand, in the case of Ho-doped BaTiO3, the grain size changed gradually in a wide range of Ba/Ti ratio. It is considered that Ho ions occupy both A- and B-sites, depending on the Ba/Ti ratio, while La and Yb ions occupy A- and B-sites, respectively. The solubility, the microstructure and the resistivity of rare-earth elements and Mg-substituted BaTiO3 were also investigated. It was also confirmed that La ions occupied A-sites, Yb ions occupied B-sites and Ho ions occupied both A- and B-sites. It appears that the drastic change of the resistivity of Ho–Mg-substituted BaTiO3 is due to the change in the predominant occupational site of Ho ions.

Effects of Microstructure on the Curie Temperature in BaTiO3–Ho2O3–MgO–SiO2 System

In response to the demand for multilayer ceramic capacitors (MLCCs) with stable capacitance in a wide temperature range, a material with high Curie temperature (Tc) has recently been being developed. In this study, we investigated the effects of microstructure on the Tc for the BaTiO3–Ho2O3–MgO–SiO2 system with various Ho and Si contents. As the Ho/Si ratio increased, the secondary phase (Pyrochlore) increased; further, the tetragonality of the BaTiO3 phase at 125 °C increased, and the Tc shifted toward higher temperatures. A transmission electron microscope equipped with energy dispersive X-ray spectrometer (TEM–EDS) analysis revealed that the core-shell structure is the key to understand this Tc shift: a thin shell with a high concentration of Ho was the most promising microstructure for a high-Tc material in this composition system. We discussed the mechanism of the Tc shift from the viewpoints of both microstructure and crystal structure.

Improved reliability predictions in high permittivity dielectric oxide capacitors under high dc electric fields with oxygen vacancy induced electromigration

This paper attempts to improve upon the range of applicability and predictability of the empirical highly accelerated lifetime testing (HALT) equation that has been traditionally used to estimate time dependent breakdown strength performance in multilayer ceramic capacitors (MLCC) and integrated thin film capacitor structures. The present and traditional HALT equation shows evidence of being limited in thin dielectric layers under high fields, for example, in high capacitance MLCCs. When the traditional HALT equations are applied to MLCCs with higher operating electric fields, there are often field dependent voltage acceleration factors resulting in ambiguous data analysis. Here, we introduce a physical model to account for a critical ionic space charge accumulation preceded by the ionic hopping or electromigration of oxygen vacancies leading to an ultimate increase in leakage current typical of dielectric resistance degradation. Mean time to failure degradation data on experimental capacitors indicates superior predictions with the new non-linear equation than with the traditional HALT equation to provide more accurate and simpler testing in future components. It is further noted that this approach may be applicable to many capacitive devices that operate under a high bias and can have ionic space charge accumulation at interfaces prior to breakdown.This paper attempts to improve upon the range of applicability and predictability of the empirical highly accelerated lifetime testing (HALT) equation that has been traditionally used to estimate time dependent breakdown strength performance in multilayer ceramic capacitors (MLCC) and integrated thin film capacitor structures. The present and traditional HALT equation shows evidence of being limited in thin dielectric layers under high fields, for example, in high capacitance MLCCs. When the traditional HALT equations are applied to MLCCs with higher operating electric fields, there are often field dependent voltage acceleration factors resulting in ambiguous data analysis. Here, we introduce a physical model to account for a critical ionic space charge accumulation preceded by the ionic hopping or electromigration of oxygen vacancies leading to an ultimate increase in leakage current typical of dielectric resistance degradation. Mean time to failure degradation data on experimental capacitors indicates s...

Effect of milling process on core-shell microstructure and electrical properties for BaTiO3-based Ni-MLCC

Abstract The effect of the process parameter in the milling process on the microstructural evolution and the electrical properties of multilayer ceramic capacitor (MLCC) samples were investigated in the BaTiO 3 (BT)–Ho 2 O 3 –MgO system. The microstructure for MLCC samples fired at 1320°C was dependent on the degree of damage for BT given by the milling process, judging from the field emission scanning electron microscopy observation and differential scanning calorimetry measurement (DSC). The mean grain size ( D 50 ) determined from the chemically etched samples decreased as the damage increased. The endothermic peak of DSC profile at around 125°C was broadened and the peak area decreased as the damage increased. Furthermore, the electrical properties were dependent on the degree of damage. The dielectric constant for MLCC samples decreased and the peak of dielectric constant at around room temperature shifted to a higher temperature as the damage increased. It was found that the MLCC sample showed the small leakage current and long mean lifetime as the degree of damage increased.

Electric Conduction of Thin-Layer Ni-Multilayer Ceramic Capacitors with Core–Shell Structure BaTiO3

The electric conduction mechanism for multilayer ceramic capacitors with Ni internal electrodes (Ni-MLCCs) was investigated, utilizing impedance spectroscopy (IS) and thermally stimulated current (TSC) measurement techniques. A modified 4RC equivalent circuit model was proposed to analyze the IS data for the Ni-MLCCs. This model revealed that electrode/ceramics interfaces (E/C-I) and grain boundaries (GBs) have a Schottky type conduction mechanism controlling the leakage behavior at low electric field. The Schottky barrier height at E/C-I and surface level height at GB were calculated being 1.43 and 1.06 eV, respectively. The Ni-MLCCs showed a tunneling conduction occurs with high dc electric fields of more than 10 V/µm. The onset electric field for the tunneling conduction shifted toward high electric fields as the Mn content of the capacitors increased. TSC measurements revealed that a low Mn content resulted in high mobile oxygen vacancies concentration in the Ni-MLCCs. Mn also played a role in preventing oxygen vacancies from migrating to cathode electrodes, which resulted in a long lifetime for the Ni-MLCCs.

Possibility of Cofiring a Nickel Inner Electrode in a (Na0.5K0.5)NbO3–LiF Piezoelectric Actuator

Multilayer (Na0.5K0.5)NbO3 (NKN) ceramics are considered promising candidates for lead-free piezoelectric actuators. The possibility of cofiring a nickel inner electrode in an NKN–LiF ceramic was investigated by evaluating the electrical properties and microstructures. The fabricated Ni inner electrode multilayer actuator sintered in a reduced atmosphere exhibited comparable dielectric properties to those of bulk NKN ceramics. The electric-field-induced strain was approximately 210 pm/V. Analysis based on the Rayleigh model showed that the extrinsic non-180° domain wall motion was suppressed in the multilayer structure. The microstructure of the NKN ceramic was not affected by cofiring with nickel, and the NKN/Ni interface was both compositionally and structurally sharp. These results imply that the NKN–LiF ceramic can be cofired with nickel without any deterioration of its properties.