The present invention relates to a solid electrolytic capacitor, and more particularly to a solid electrolytic capacitor having an improved precoat between an oxide film and a solid electrolyte in the solid electrolytic capacitor.
In recent years, there was developed a solid electrolytic capacitor produced by forming a dielectric oxide film, which is hereinafter referred to as an anodic oxide film, on a porous member of a valve action metal such as aluminum by an anodic oxidation method, and then forming a conductive polymer layer as a solid electrolyte on the anodic oxide film. The solid electrolyte in this solid electrolytic capacitor has a conductivity 10 to 100 times higher than a solid electrolyte of manganese dioxide. Further, the solid electrolytic capacitor can have a low equivalent series resistance (ESR), and high-frequency characteristics are remarkably improved. Accordingly, this type of solid electrolytic capacitor is getting to be employed in various electronic devices for the purpose of removing high-frequency noise in small-sized devices.
Larger packing densities and higher speeds of electronic components increasingly require a smaller size, a larger capacity, and a lower ESR of a capacitor. Several attempts have been made to increase a capacitance per unit area. These include increasing an etching magnification of an aluminum substrate as a valve action metal, or increasing a volumetric efficiency, which is a ratio of a volume of a product and a volume of a capacitor by laminating capacitor elements.
The inventors have proposed a method for increasing capacitance by improving a substantial coverage of the conductive polymer layer to an anodic oxide film. This result seems to come from an improved adhesiveness between the anodic oxide film and a polypyrrole conductive polymer layer. This method is disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2005-159154, which is hereinafter referred to as Patent Document 1. According to this method, a surface of an aluminum substrate, which serves as an anode portion, is roughened by etching. An aluminum dielectric film or an anodic oxide film layer is formed on the roughened surface of the aluminum substrate in an aqueous solution containing ammonium adipate, phosphoric acid, ammonium phosphate, or the like. Then, the aluminum substrate is immersed in a polystyrene sulfonic acid solution and dried to form a thin film of polystyrene sulfonic acid as a precoat layer on a surface of the anodic oxide film. Thereafter, the aluminum substrate having the precoat layer formed is immersed in a solution containing a monomer for producing a conductive polymer, and then, is immersed in a solution containing protonic acid, metal halide, peroxide, or the like to form a conductive polymer film or an internal polymerization film layer, such as a polypyrrole film, a polythiophene film, or a polyoxy thiophene film. In case of, say, the polypyrrole layer, the precoat layer of polystyrene sulfonic acid is reacted with the polypyrrole layer to provide an improved adhesiveness between the anodic oxide film layer and the polypyrrole layer, leading an improved coverage of the conductive polymer layer to the anodic oxide film. Then, a plurality of conductive polymer films or external polymerization film layers, such as polypyrrole films, polythiophene films, or polyoxy thiophene films, are formed on the internal polymerization film layer by a chemical oxidation polymerization method. Then, a graphite layer and a silver paste layer are formed on the external polymerization film layers to provide a cathode portion.
The precoat layer of polystyrene sulfonic acid in the solid electrolytic capacitor disclosed in Patent Document 1 is very effective in improvement of a coverage. However, a polystyrene sulfonic acid is a water-soluble polymer compound. Although a portion of the precoat layer that has reacted with polypyrrole of the conductive polymer film formed on the precoat layer is not dissolved into water, a portion of the precoat layer that has not reacted with polypyrrole may be dissolved into water. A trace of polystyrene sulfonic acid dissolved into water has a pH of about 0.5 to about 1 and is thus strongly acid. The dissolved polystyrene sulfonic acid may have an adverse influence on the anodic oxide film and may cause an increase of a leaking current (LC) when the solid electrolytic capacitor is used at a high humidity.