Asymmetric Conduction and Stability of Polymer Tantalum Capacitors

Conduction asymmetry and DC leakage (DCL) current variations with time were investigated in Polymer Hermetically Sealed (PHS) Ta capacitors. The hermetic design facilitates the control of the atmosphere inside these Ta capacitors, in particular the amount of moisture, and maintains it during long-term testing. These conditions were utilized to determine the dominant mechanisms responsible for DCL degradation and failures at normal and reverse polarities. The presence of moisture was found to have a profound effect on the DCL and breakdown voltage (BDV) of these devices, depending on the polarity. For humid capacitors, the DCL magnitude and stability were significantly improved for normal polarity, which was attributed to moisture acting as a plasticizer increasing polymer chain mobility while decreasing relaxation time of the interface dipoles. However, at reverse polarity DCL and stability degraded over time. This was attributed primarily to H+ ion diffusion toward the Ta/Ta2O5 interface and eventually oxygen migration into the Ta anode. In dry capacitors the reverse polarity leakage was minimal due to the absence of the creation of H+ ions, while at normal polarity DCL and breakdown were significantly degraded compared to humid devices under similar conditions.