Visualization of Localized Degradation in Ni/BaTiO₃-Based Multilayer Ceramic Capacitors Under Electric Fields by In-Situ STEM

Ni/BaTiO₃-based multilayer ceramic capacitors (MLCCs) have been widely used in mobile communication, aerospace, and electric vehicle applications. For commercially available MLCCs, ‘reliability’, defined as the ability to maintain performance from the measurement of voltage until dielectric breakdown, is one of the important properties. Previous research has employed the highly accelerated lifetime test (HALT), which involves applying a DC voltage to MLCCs at high temperatures and monitoring the resistance changes over time to figure out the mechanism of degradation. These studies have indicated that oxygen vacancies play a crucial role in the overall degradation behavior of MLCCs. However, variations in resistance among individual MLCCs during HALT suggest that degradation primarily occurs in localized regions. According to the weakest link theory, degradation in localized areas accelerates, leading to rapid dielectric breakdown.<br/>In this study, infrared optical beam induced resistance change (IR-OBIRCH) was applied to identify potential degradation regions with low resistance on the surface of pre-breakdown MLCCs with applied voltage at high temperatures. These identified regions were subsequently cross-sectioned using focused ion beam (FIB) techniques to apply IR-OBIRCH again on the cross-section sample to pinpoint further locally degradation regions in three dimensions. Following the process, locally degraded area has been securely contained within a (S)TEM specimens using FIB to conduct in-situ biasing STEM. Four-dimensional scanning transmission electron microscopy (4D-STEM) experiments were performed to measure the deflection of a transmission beam with a small convergence angle of 60 μrad, depending on the applied voltage, to determine the electric field strength within the specimen. Subsequently, the chemical composition distribution of dopants and the distribution of oxygen vacancies were analyzed using STEM electron energy loss spectroscopy (STEM EELS) and energy-dispersive X-ray spectroscopy (EDS). The analysis revealed that the regions identified as degraded by IR-OBIRCH did not exhibit differences in grain size or the number of grain boundaries when compared to normal regions. However, these degraded regions demonstrated a significantly smaller electric field than the non-degraded regions. This phenomenon is hypothesized to result from changes in conductivity attributed to the distribution of oxygen vacancies, which arise from dopant segregation. This study aims to figure out the mechanisms underlying the formation of locally degraded areas by observing the distribution of oxygen vacancies, dopants, grain size, grain boundaries, and electric field distribution under in-situ conditions.