# Evolution of Polymer Tantalum Capacitors
###### Abstract
F-Tech provides the highest chemical purity to the tantalum anodes, and thus eliminates the crystalline seeds and suppresses the crystallization of the amorphous matrix of the Ta\({}_{2}\)O\({}_{5}\) dielectric. There is special focus in F-Tech on reducing the oxygen content in the tantalum anodes, which increases during the vacuum sintering of the tantalum powder due to the diffusion of oxygen from the native oxide on the surface of tantalum particles into the bulk of the tantalum. Only sintering at temperatures equal or exceeding 1880 \({}^{\circ}\)C allows for oxygen to evaporate in a vacuum; however, most of the currently used tantalum powders would over shrink and totally lose their surface area at these high sintering temperatures. Reducing the oxygen content in tantalum anodes in F-Tech is typically performed by treatment of the vacuum sintered tantalum anodes in a deoxidizing atmosphere, such as  
Figure 1: Maximum working voltage and DC leakage per CV limit in polymer tantalum capacitors. Figure 2: Crystalline seeds in the amorphous matrix of the Ta\({}_{2}\)O\({}_{5}\) dielectric at the dielectric interface with a tantalum anode. magnesium vapor, followed by leaching of the magnesium oxide in a diluted solution of sulfuric acid. Deoxidizing is performed at temperatures significantly lower than the sintering temperature, which allows for the preservation of the surface area of the sintered tantalum anodes. Besides chemical purity, F-Tech also provides mechanical robustness to tantalum anodes, mainly through a strong mechanical bond between the sintered tantalum powder and the tantalum lead-wire that connects the anode with the external positive termination. In conventional technology, the tantalum lead-wire is inserted into the powder during pressing, and then the powder and the wire are sintered together at temperatures typically in the 1100-1600 \({}^{\circ}\)C range, depending on the powder charge and anode size. Mechanical stress during the manufacturing and testing of polymer tantalum capacitors can induce microscopic cracks in the oxide dielectric at the anode egress where the powder-wire bond is most vulnerable. These microscopic cracks may progress under additional stress during field application and cause catastrophic failures of the tantalum capacitors. In F-Tech, tantalum powder is pressed and sintered in a vacuum without the lead wire, and then the tantalum wire is resistance-welded to the anode surface in an inert atmosphere. The welding nugget with a strong powder-wire bond that forms at the anode egresses with this technology prevents the dielectric damage in this area under the mechanical stress. In addition to the improvements in the chemical composition and mechanical robustness of the tantalum anodes, F-Tech also incorporates special anodization conditions, preventing overheating that can trigger the crystallization of the amorphous matrix of the dielectric [5]. Another important improvement in the technology of Polymer Tantalum capacitors that helped increase the working voltage and reduce the DC leakage was the process of applying a PEDOT cathode on the surface of the Ta\({}_{2}\)O\({}_{5}\) dielectric. Originally, the PEDOT cathode was applied by multiple in-situ chemical reactions between the monomer and oxidant solution of iron (III) p-toluenesulfonate (FePTS). Although methanol and water washings were performed after every in-situ polymerization, it was practically impossible to fully wash out the by-products of the chemical reaction from the porous tantalum anodes. These byproducts, mainly iron, gradually accumulated in the PEDOT cathode and its interface with the Ta\({}_{2}\)O\({}_{5}\) dielectric, pinning the potential barrier at the dielectric/polymer interface. In an alternative technology, tantalum anodes with the oxide dielectric were dipped in the water-based dispersion of the pre-polymerized PEDOT particles (slurry PEDOT), followed by drying in air at room temperature and elevated temperatures [6,7].