Metal working – Barrier layer or semiconductor device making – Barrier layer device making
Reexamination Certificate
1999-07-07
2001-05-01
Trinh, Michael (Department: 2822)
Metal working
Barrier layer or semiconductor device making
Barrier layer device making
C361S525000
Reexamination Certificate
active
06224639
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing an electrolytic capacitor using a valve metal such as aluminum, tantalum or the like and, more particularly, to a method of producing an electrolytic capacitor using a conductive polymer as a solid electrolyte.
2. Description of the Related Art
As a method of producing a conductive polymer to be turned into a solid electrolyte of an electrolytic capacitor by chemical oxidation polymerization, for example, a one-fluid method has hitherto been used. The one-fluid method introduces a mixed solution containing of a monomer to be turned into a conductive polymer by polymerization (hereinafter abbreviated to a “monomer solution”) and an oxidizing agent capable of polymerizing the monomer by oxidation (hereinafter abbreviated to an “oxidizing agent solution”) on a surface oxide layer of a porous valve metal to be turned into a capacitor, thereby to form a conductive polymer.
In the one-fluid method, however, the reaction is initiated immediately after the monomer and oxidizing agent were mixed and, in the case of a porous body having complicated pores such as tantalum sintered body, the reaction solution is deactivated before arriving the deep portion of pores and a conductive polymer is not formed at the center portion of a sintered body. As disclosed in Japanese Laid-open Patent Publication No. 6-310380, there is a method of introducing a mixed solution of a monomer and an oxidizing agent on an oxide layer of a porous body at low temperature and raising the temperature of the porous anode or mixed solution, thereby to make the amount of a conductive polymer formed at the center portion of the porous anode equal to the amount formed at the periphery of the surface. In the one-fluid method, however, since the polymerization reaction can not be completely terminated even when mixing the monomer solution with the oxidizing agent solution at low temperature, consumption of the monomer and oxidizing agent becomes larger.
Even when the monomer solution and oxidizing agent solution can be cooled to an ultra-low temperature and the polymerization reaction can be drastically inhibited, the cost of the production unit and running cost increase, which is uneconomical.
Accordingly, as disclosed in U.S. Pat. No. 4,697,001, there is generally used a two-fluid method of alternatively dipping a porous body to be turned into a capacitor in a monomer solution and an oxidizing agent solution without mixing the monomer solution and the oxidizing agent solution, that is, bringing the porous anode whose pores are impregnated with one of the monomer solution and oxidizing agent solution into contact with the other, thereby to form a conductive polymer in the inner spaces of the pores by polymerization.
However, when the porous anode impregnated with one solution among two solutions of the monomer solution and oxidizing agent solution is dipped in the other solution, the solution, with which the porous anode was impregnated, momentarily diffuses into the other solution on the external surface of the porous anode. Therefore, the conductive polymer is not easily adhered and formed on the external surface of the porous anode in comparison with the inner spaces of a lot of pores of the porous anode. When the external surface of the porous anode is not completely coated with the conductive polymer, a dielectric layer on the external surface is deteriorated by stress, resulting in large leak current. Therefore, a conductive polymer forming step must be repeated many times so as to obtain a capacitor with complete intrinsic capacity and small leak current by completely coating the dielectric layer formed on the internal surfaces of a lot of the pores and the external surface of the porous anode with the conductive polymer.
SUMMARY OF THE INVENTION
That is, an object of the present invention is to provide a method of producing an electrolytic capacitor with small leak current and high reliability, which can reduce the number of steps of forming a conductive polymer, thereby to improve the productivity.
The present invention provides a method of producing an electrolytic capacitor including a porous anode and a solid electrolyte made of a conductive polymer. The porous anode further includes a porous body and a dielectric layer, and the dielectric layer is formed on internal surfaces of a lot of pores of the porous body and an external surface of the porous body. The conductive polymer is formed on the surface of the dielectric layer by a chemical oxidation polymerization method with a monomer and an oxidizing agent.
According to a first aspect of the invention, the method of the present invention includes the step of forming a first conductive polymer portion in inner spaces of a lot of pores of the porous anode and the step of forming a second conductive polymer portion on the external surface of the porous anode, and a polymerization rate of the monomer in the step of forming the second conductive polymer portion is larger than that of the monomer in the step of forming the first conductive polymer portion.
Comparing the case of high polymerization rate of a monomer with the case of low polymerization rate of a monomer, in the case of high polymerization rate, the proportion of the amount of the conductive polymer formed on the external surface of the porous anode to the total amount of the conductive polymer becomes higher than in the case of low polymerization rate. Accordingly, by controlling the polymerization rate, it is possible to easily fill the inner spaces of a lot of the pores of the porous anode and coat the external surface of the porous anode with the conductive polymer in a separate step and the step of forming a conductive polymer can be composed of the first and second steps described above. Particularly, since the conductive polymer is preferentially formed on the external surface by making the polymerization rate in the step of forming the second conductive polymer portion larger than that in the step of forming the first conductive polymer portion, the number of polymerization of the conductive polymer can be more reduced than usual. Furthermore, the separate step makes it possible to completely fill the inner spaces of the pores of the porous anode and coat the external surface with the conductive polymer.
In the production method described above, the first conductive polymer is preferably formed after forming the second conductive polymer portion. As a result, coating properties on the external surface of the porous anode are improved, thereby making it possible to reduce the number of polymerization for forming the conductive polymer on the external surface of the porous anode.
In the production method described above, it is preferred that the first conductive polymer portion forming step is composed of two stages and the amount of the conductive polymer to be formed in the first stage is not more than 50% by volume of the amount of the conductive polymer to be formed in the whole pores, and that the second conductive polymer portion is subsequently formed and, furthermore, the rest of the first conductive polymer portion is formed in the pores in the second stage.
When coating the external surface of the porous anode with the conductive polymer after completely filling the inner spaces of the pores of the porous anode with the conductive polymer, since the pore portion of the porous anode contains a small number of voids into which the monomer solution or oxidizing agent solution penetrates, the amount of the monomer or oxidizing agent for previous impregnation is reduced. Therefore, the amount of the conductive polymer to be formed per one time is small and a large number of polymerization must be required. On the other hand, since the amount of the solution to be retained is large and the amount of the conductive polymer to be formed is also increased before completely filling the inner spaces of the pores of the porous anode with the conductive polymer, coating on the external
Hamada Takahiro
Hayashi Chiharu
Igaki Emiko
Tanahashi Masakazu
Tsuji Yasunobu
Matsushita Electric - Industrial Co., Ltd.
Trinh Michael
Wenderoth , Lind & Ponack, L.L.P.
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