Direct current-voltage failure in lead magnesium niobate-based multilayer ceramic capacitors
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Resistance measurement, P–E hysteresis measurement, and transmission electron microscope and energy dispersive analysis of x-rays (TEM-EDAX) analysis were used to study the resistance failure of lead magnesium niobate-based multilayer ceramic capacitors (MLCC) under dc voltage. It was found that the failure rate of MLCC with 1/9 Pd/Ag internal electrodes was 10 times that of MLCC with 3/7 Pd/Ag electrodes after the temperature–humidity–bias test (THB). Voltage shifts of hysteresis loops showed that an internal bias field between electrodes of MLCC was formed after THB test. Ag diffusion from electrodes into the ceramics during cofiring was examined through TEM-EDAX analysis. It was also found that the degraded specimens could be partially restored after storing under natural condition. On the basis of these results, the failure mechanism was established that oxygen vacancies induced by Ag diffusion accumulated under the external bias field, which increased the concentration of electronic defects, thereby resulting in the resistance failure of MLCC.
I. INTRODUCTION
With the rapid development of surface-mounting technology, multilayer ceramic capacitors (MLCC) are widely used in many electronic circuits for their high volumetric ratio and reliability. As one of the most important material systems in MLCC industry, lead magnesium niobate (PMN)-based relaxor ferroelectrics have been paid much attention and been intensively studied for their low sintering temperatures, high dielectric constants, and broad maxima.1–3 In general, 3/7 Pd/Ag alloys are adopted as internal electrodes of MLCC. However, increasing interest has been developed in the low-Pd electrode technique in MLCC fabrication due to the demand for cost lowering. It was found that electrical properties of some PMNbased MLCC deteriorated under working conditions for a few days and even local melting of some MLCC has been observed, which damaged the reliability of the whole electronic circuits. Usually, the resistance and capacitance of MLCC decreased while the loss factor increased with the degradation, namely dc-voltage failure. As accepted, the electrical failure of MLCC could be all ascribed to the resistance degradation. Many studies have been dedicated to understanding the failure mechanism and reliability improvement of MLCC. It was generally considered that the resistance a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 4, Apr 2002
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failure originated from defects in the MLCC, maybe external flaws induced by fabrication technique, or internal ones. As to the latter, it referred to the nature of materials themselves and the possible mismatch between them, for example, nonstoichiometry of the ceramics and interactions between electrodes and ceramics. The temperature–humidity–bias (THB) test was generally adopted to study the failure mechanism, and much work has been carried out. There were several hypotheses attempting to explain this failure.4–7 Ling
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