INVESTIGATING PRE-BREAKDOWN CURRENTS IN POLYMER TANTALUM CAPACITORS

Poly(3,4-ethylenedioxythiophene) (PEDOT), a conducting polymer, has been used since the 1990s for the cathode of tantalum capacitors, which have a tantalum anode, and a layer of tantalum pentoxide as the dielectric. Such capacitors are referred to as Polymer Ta capacitors. The first method, an in situ polymerization technique, used to deposit the polymer resulted in capacitors with a significant leakage current and breakdown voltages near 50 V. The second method, a pre-polymerization (pre-poly) technique, resulted in capacitors that have a much lower leakage current and a higher breakdown voltage than the in situ capacitors. In this thesis, an accurate measurement technique for dielectric leakage current, also referred to as the pre-breakdown current, was established for capacitors. Current versus time measurements at constant voltage were performed at several voltages and the results were compiled to obtain the current-voltage (I-V) characteristics of both in situ and pre-poly types of capacitors. These characteristics were then modeled and analyzed, which led to the conclusion that the pre-breakdown current is controlled by the Poole-Frenkel mechanism in the in situ capacitors and by both the Poole-Frenkel and Schottky mechanisms in the pre-poly capacitors. Current versus time measurements were also performed at various temperatures to obtain the activation energy for the current in the capacitors and to verify the leakage mechanisms. Results suggest the presence of shallow 0.15 eV traps in the dielectric of the in situ capacitor and deeper 0.75 eV traps in that of the pre-poly capacitor. Additionally, pre-poly capacitors also have a 0.54 eV Schottky barrier that limits the electrons from being emitted into the dielectric from the electrode. Both the deep trap levels and the Schottky barrier explain the lower leakage current and higher breakdown voltage observed in the pre-poly capacitors as compared to the in situ capacitors.