Local impedance and microchemical analysis of electrical heterogeneities in multilayer electroceramic devices
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We present an experimental methodology for locating and studying local failure sites in multilayer electroceramic devices at the submicron-length scale. In particular, the inhomogeneous degradation of multilayer ceramic capacitors is studied using a judicious combination of scanning electron microscopy (SEM), local-probe electrical measurements, focused ion beam (FIB) extraction, and transmission electron microscopy (TEM). Voltage-contrast SEM permits the identification of regions of different electrical potential within degraded multilayer devices. The local impedance from specific regions is measured in situ between a tungsten probe and the internal device electrodes, while impedance spectra are extracted for more detailed analysis. Because implementation occurs in dual-beam FIB/SEM, these locally defective sites can be extracted and thinned to electron transparency for further investigation by TEM. In this study, degraded sites in BaTiO3 multilayer capacitors are found to be associated with local oxygen deficiencies in BaTiO3, as measured by electron energy loss spectroscopy.
I. INTRODUCTION
The success of an electronic device emerging from the research laboratory into commerce can often be limited by defects at the submicron level, which may be linked to the fabrication of raw materials and/or to thermochemical reactions during processing or use. In particular for multilayer dielectric devices, local defect sites in a few layers could dominate the total electrical characteristics as a consequence of the parallel connection of each dielectric layer. The ability to identify and understand the local behavior underpinning heterogeneous failure mechanisms becomes increasingly important in the development of miniaturized electronic materials and devices. For example, in state-of-the-art base metal electrode (BME) multilayer ceramic capacitors (MLCCs) it is invaluable to understand the local failure mechanisms that degrade device resistance to optimize the capacitor processing and thereby enable higher volumetric efficiency. While the macroscopic electrical characterization of capacitive materials in the time and frequency domain and the time-dependent evolution of these properties has provided insight into grain-boundary and interfacial phenomena that contribute to the degradation process,1–9
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0443 J. Mater. Res., Vol. 22, No. 12, Dec 2007
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such holistic characterization techniques do not fully illuminate the possible material inhomogeneities that can lead to localized failures. In most capacitive devices, the failure may not be associated with the intrinsic properties of the dielectric material but may be due to locally defective sites that could arise from impurities or other processing issues. These sites can collectively determine the failure of the device and, more broadly, the processing yields. There is, therefore, a significant need to develop characterization te
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