Effect of Reverse Bias Stress on Leakage Currents and Breakdown Voltages of Solid Tantalum Capacitors

摘要

The majority of solid tantalum capacitors are produced by high-temperature sintering of a fine tantalum powder around a tantalum wire followed by electrolytic anodization that forms a thin amorphous Ta_2O_5 dielectric layer and pyrolisis of manganese nitrite on the oxide to create a conductive manganese dioxide electrode. A contact to tantalum wire is used as anode terminal and to the manganese layer as a cathode terminal of the device. This process results in formation of an asymmetric Ta - Ta_2O_5 - MnO_2 capacitor that has different characteristics at forward (positive bias applied to tantalum) and reverse (positive bias applied to manganese cathode) voltages. Reverse bias currents might be several orders of magnitude larger than forward leakage currents so I-V characteristics of tantalum capacitors resemble characteristics of semiconductor rectifiers. Asymmetric I-V characteristics of Ta - anodic Ta_2O_5 systems have been observed at different top electrode materials including metals, electrolytes, conductive polymers, and manganese oxide thus indicating that this phenomenon is likely related to the specifics of the Ta - Ta_2O_5 interface. There have been multiple attempts to explain rectifying characteristics of capacitors employing anodic tantalum pentoxide dielectrics: (1) Analysis of photoeffects and variations of capacitance with applied voltage allowed Sasaki [1] to conclude that the oxide film can be presented as a p-i-n junction. A thin n-type layer is formed at the Ta interface due to excess of Ta atoms, and a p-type layer is formed at the external oxide surface due to excess of oxygen atoms. (2) Vermileya [2] suggested that rectification in Ta - Ta_2O_5 systems occurs at flaws in the oxide layer by at least two mechanisms, one involving the opening of physical holes and the other an electronic junction that is formed by electrodes with different contact potentials resulting in an asymmetric barrier.