Capacitor and process for production thereof

Abstract: Disclosed in this invention is a capacitor comprising a dielectric thin film (3) being sandwiched by two layers of thin film electrodes (1) directly or through interposition of a suitable thin film (2), and lead electrodes (21,22) connected to said thin film electrodes (1), said dielectric thin film (3) being a chemically adsorbed monomolecular film (4) or a chemically adsorbed monomolecular built-up film made from a chlorosilane surface active agent. Molecules having large dipoles (9,10) are incorporated in the molecules of the surface active agent to enhance the dielectric strength of the monomolecular film.

Description: BACKGROUND OF THE INVENTION This invention relates to a capacitor and a process for producing the same. More particularly, it pertains to a method for forming a dielectric thin film on a film electrode in the production of a capacitor. For forming a dielectric thin film in the production of capacitors, there have been generally employed a method in which a polyester resin or an epoxy resin is coated on each film electrode and a method in which a polyester film is held between the film electrodes. It has become apparent, however, that the dielectric films formed according to the conventional methods are poor in performance and unsatisfactory in reliability. For instance, it has been revealed that according to the conventional coating techniques, when the thickness of the coating film is reduced for enlarging the static capacity, although a satisfactory mechanical strength can be obtained, there arises a risk of forming many pinholes, which leads to a poor yield and greatly deteriorated environmental resistance and dielectric strength of the film. What is known from JP-A-62221103, is a capacitor wherein a dielectric thin film is sandwiched by two layers of thin film electrodes. The dielectric thin film is made from a chlorosilane surface active agent and is applied to the electrodes by chemical adsorption. In view of the above mentioned defects, the present inventions has for its object to provide a small-size high-performance thin film capacitor at low cost. According to one aspect of the invention, a capacitor comprises a thin film electrode and a dielectric thin film formed on one or both surfaces, respectively, of said thin film electrode directly or through interposition of a thin oxide film, and lead electrodes connected to said thin film electrode, said dielectric thin film being a chemically adsorbed monomomecular film made from a chlorosilane surface active agent or a chemically adsorbed multi-monomolecular film comprising two or more monomolecular films made from a chlorosilane surface active agent, said adsorption being effected by siloxane bonding, wherein the chlorosilane surface active agent is a compound in which chlorosilyl groups are bonded to liquid crystal molecules through a carbon chain. According to another aspect, the invention provides for a process for producing a capacitor, comprising a step of forming a single layer of monomolecular film on one or both surfaces of a metallic electrode-forming film by letting a chlorosilane surface active agent in a non-aqueous organic solvent be chemically adsorbed on the surface of the electrode-forming film either directly or by interposing therebetween a thin oxide film , and thereby effecting the adsorption by chemical siloxane bonding, wherein the chlorosilane surface active agent has a structure in which chlorosilyl groups are bonded to liquid crystal molecules through a carbon chain. Preferred embodiments of the invention are claimed in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustration of a thin film electrode for capacitor having a chemical adsorption monomolecular film formed thereon as a dielectric film. FIGS. 2 to 8 are diagrammatic representations of the steps shown by magnifying the encircled portion A in FIG. 1 to the molecular level. FIG. 9 is a schematic illustration of a process for continuously producing the thin film electrodes for capacitor having a dielectric film formed thereon. FIG. 10 is a perspective view of a capacitor produced in an embodiment of the present invention. FIG. 11 is a schematic sectional view of a capacitor produced in another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides a capacitor as defined in claim 1, wherein the electrode surface is coated, either directly or through a medium of an appropriate thin film, with a monomolecular chemically adsorbed film or a monomolecular built-up chemically adsorbed film composed of a chlorosilane surface active agent, and a process for producing such a capacitor. In the process according to the present invention, by use of chemical adsorption techniques, it is possible to form a thin film dielectric material having high moisture resistance and excellent dielectric strength and to provide a small-sized high-performance capacitor. Further, the capacitor of this invention can be produced with ease and at low cost since the production process merely comprises chemical adsorption of a chlorosilane surface active agent on substrates for thin film metallic electrodes. Examples In the following, embodiments of the dielectric film used in this invention or precursor material thereof, a continuous process for making electrodes and possible electrode arrangements are described with reference to FIGS. 1 to 11. As shown in FIG. 1, on a film 1 made of a metal (such as Al, Sn, Cu, stainless steel or an alloy containing said metal(s)) designed to serve as a thin film electrode for capacitor and formed by suitable means such as rolling, there is formed a monomolecular dielectric film 3 made of a chlorosilane surface active agent by having said chlorosilane surface active agent adsorbed on said metallic film 1 by a chemical adsorption method either directly or with the intermedium therebetween of a suitable thin film such as a natural oxide film 2. Instead of forming said natural oxide film 2, an electrolytic oxide film may be formed on said metallic film surface by means of electrolytic oxidation or a thin film of an oxide such as Al₂O₃, SnO₂, Cu₂O, or Fe₂O₃ may be formed on said metallic film surface to a thickness of several thousands of angstroms by suitable means such as sputtering deposition. This enables obtainment of a capacitor with even higher performance. Chemical adsorption in the above operation can be performed in the following way. A hydrophilic metallic film 1 is dried well. Then a solution is prepared by dissolving a chlorosilane surface active agent, e.g., CH₂ = CH-(CH₂) n -SiCl₃ (n represents an integer preferably from 10 up to about 25 for best handling), in an 80:12:8 mixture (by weight ratio) of n-hexadecane, tetrachlorocarbon and chloroform to a concentration of 2 x 10⁻³ to 5 x 10⁻² mol/l, and said metallic film 1 is dipped in this solution at room temperature for about one hour. In this process, if an oxide is formed on the surface of said metallic film, it is hydrophilic, and -OH groups are contained in the film surface. Consequently, there takes place a dehydrochlorination reaction between -SiCl₃ groups and -OH groups to form bonding of siloxane (-SiO-), and as a result, as shown in FIG. 2, a dielectric film 3 made of a monomolecular chemically adsorbed film 4 is formed, by the action of said chlorosilane surface active agent, on the surface of the monolayer metallic film 1 to a thickness of 20 - 30 Å. In FIG. 2, numeral 5 denotes a vinyl group. It is possible to further strengthen and stabilize the monomolecular film 4 by crosslinking (indicated by 6 in FIG. 3) the vinyl groups (CH₂ = CH-) 5 juxtaposed in the film surface by irradiating it with energy beams such as electron rays, X-rays, γ-rays, ultraviolet rays or ionic rays at a dose of 2 to 5 Mrad in an inert gas such as helium. If a fluorine-containing surface active agent such as CF₃-(CF₂) n -(CH₂) m -SiCl₃ (m and n are the integers the sum of which is preferably from about 10 to about 25 for best handling quality) or CF₃-(CF₂) n -(CH₂) m -O-SiCl₃ (m and n are the integers the sum cf which is preferably from about 10 to about 25 for best handling quality) is used as said chlorosilane surface active agent as shown in FIG. 4, there can be obtained a fluorine-containing adsorption film which is improved in dielectric constant. In FIG. 4, numeral 7 denotes a fluorine atom. When it is desired to obtain a film with a slightly greater thickness, a metallic film formed with a single layer of monomolecular film 4 shown by FIG. 2 is dipped in a THF solution of diborane in a concentration of 1 mol/l at room temperature and then further dipped in a 30% H₂O₂ aqueous solution of sodium hydroxide (NaOH) in a concentration of 0.1 mol/l at room temperature for about one hour to add the hydroxyl groups (OH) 8 to the vinyl groups in the monomolecular film surface as shown in FIG. 5. Thereafter, by repeating the chemical adsorption step using the same reaction solution as used in the above-described adsorbing operation and the OH group addition step, it is possible to obtain a monomolecular built-up film having a desired thickness for use as a dielectric film such as shown in FIG. 6. Also, in the step of adding hydroxyl groups 8, when radiation such as electron rays, X-rays, γ-rays or ionic rays are applied at a dose of 1-10 Mrad in an atmosphere of an inert gas such as oxygen, nitrogen or water vapor instead of performing the oxidation using a chemical, the active groups such as OH groups, amino groups (NH₂) or imino groups (NH) can be added to the portion of vinyl groups, so that by repeating the chemical adsorption step using the same reaction solution and the addition step by irradiation of energy beams, it is possible to obtain a required film thickness for use as a dielectric film. The similar effect can be obtained by conducting a plasma treatment in an active gas atmosphere at 100 W for about 10 minutes instead of applying radiation as in the above embodiment. Increase of dielectric constant of the monomolecular chemically adsorbed film is effected by using as adsorbing reagent a chlorosilane surface active agent having a chemical structure with liquid crystal molecules as molecules with large dipoles at a part of the linear carbon chain of the chlorosilane surface active agent. By using an adsorption reagent prepared by adding trichlorosilyl groups, through a carbon chain, to the molecules of liquid crystal such as nematic liquid crystal or other ferroelectric liquid crystals such as azomethine type, azoxy type or ester type, as the chlorosilane surface active agent having said liquid crystal molecules, it is possible to form a monomolecular film such as shown in FIG. 7 or FIG. 8. (In the representations of FIGS. 7 and 8, R1 and R2 each represents a substituent.) In FIG. 7, numeral 9 denotes a nematic liquid crystal portion, and in FIG. 8, numeral 10 denotes a ferroelectric liquid crystal portion. Further, in FIGS. 3 to 8, numerals 4A through 4F denote monomolecular films having different chemical structures. In this process, if chemical adsorption of the adsorption reagent containing the liquid crystal molecules is conducted after said monomolecular built-up film has been formed, there can be obtained a monomolecular built-up film containing the liquid crystal molecules in the outermost surface. Also, when R1 or R2 in the molecule of the chlorosilane surface active agent for chemical adsorption in FIG. 7 or 8 is defined to be a vinyl group, acetylene group or cyano group, it is possible to obtain a monomolecular dielectric built-up film containing the liquid crystal molecules in the whole structure of the monomolecular film by using the above-mentioned monomolecular film build-up method in which chemical adsorption is performed by treatments with a chemical agent or by irradiation. Further, since the dielectric film thus obtained has a structure in which the dipoles in the molecules constituting each layer of monomolecular adsorption film are arranged in order, the obtained dielectric film can have a very high dielectric constant. According to a process where the above-described steps are conducted continuously, as shown in FIG. 9, a metal ingot 11 (e.g., Al or Al-containing alloy) is rolled into a film by a rolling mill 12, and by using a capstan 13, the film is subjected to the successive treatments in an adsorption tank 14, a washing cistern 15, an electron ray irradiator 16, an adsorption tank 17, a washing cistern 18 ..., and lastly, after a desired number of layers of monomolecular film were obtained by repeatedly conducting said successive treatments, the thin film electrode for capacitor 20 comprising the thus formed dielectric film is taken up on a drum 19. This process can be conducted continuously. In case the natural oxide film is not sufficiently formed on the metallic film surface immediately after rolling, it needs to set, in the rear of the rolling mill, a tank containing an aqueous solution of dilute nitric acid or such for dipping the film therein for a short period of time. It is possible to add a step or steps before the adsorption step, such as a step for electrolytically oxidizing said metallic film surface or a step for sputter-depositing a thin film of an oxide such as SiO₂. As for the adsorbing reagent, CH₂ = CH-(CH₂) n -SiCl₃ was used in the above-described embodiment, but it is also possible to use a reagent in which the vinyl group is replaced by acetylene or cyano group, or the trichlorosilyl group is replaced by dichlorosilyl or monochlorosilyl group, or a reaction group is allowed to coexist with said groups. Lastly, 2 sheets of thin film electrode formed with said dielectric film are placed one on the other and rolled up as shown in FIG. 10, or 2 sheets of thin film electrode are laminated and press-bonded as shown in FIG. 11, with lead wires 21 and 22 being connected to the respective sheets to complete a capacitor, wherein a second or another electrode may be deposited directly on the dielectric film 3 by sputtering. If an electrolyte prepared by dissolving water or boric acid in an amount of a few percents in glycerin or ethylene glycol is held between the electrodes each or having a coat of a monomolecular film or monomolecular built-up film, there can be obtained an electrolytic capacitor. The present invention provides a capacitor characterized by the features of claim 1, wherein the electrode surface is coated, either directly or indirectly by interposing a suitable thin film, with a chemically adsorbed film or a chemically adsorbed monomolecular built-up film composed of a chlorosilane surface active agent, and a process for producing such a capacitor defined by the features of claim 11, so that according to this invention it is possible to easily form an extremely thin, high-density faultless dielectric film having a high dielectric constant on the surface of a metallic thin film electrode. Also, since this dielectric film is bonded to the electrode surface through a strong siloxane bond, a small-sized high-performance capacitor with excellent moisture resistance and dielectric strength can be offered at notably low cost. Further, since the dielectric film formed by said chemical adsorption method is incorporated with a chemical structure having e.g., nematic liquid crystal molecules or ferroelectric liquid crystal molecules as molecules with large dipoles , it is possible to control the dielectric constant of the dielectric film.

Claims: A capacitor comprising a thin film electrode (1) and a dielectric thin film (3) formed on one or both surfaces, respectively, of said thin film electrode (1) directly or through interposition of a thin oxide film (2) , and lead electrodes (21, 22) connected to said thin film electrode (1), said dielectric thin film being a chemically adsorbed monomomecular film (4E, 4F) made from a chlorosilane surface active agent or a chemically adsorbed multi-monomolecular film comprising two or more monomolecular films made from a chlorosilane surface active agent, said adsorption being effected by siloxane bonding, wherein the chlorosilane surface active agent is a compound in which chlorosilyl groups are bonded to liquid crystal molecules through a carbon chain. The capacitor according to Claim 1, wherein said carbon chain is linear. The capacitor according to Claim 2, wherein said carbon chains comprises 10 to 25 carbon atoms. The capacitor according to any preceding Claim, wherein the chlorosilane surface active agent has at one molecular end thereof a substituent selected from a vinyl group, an acetylene group or a cyano group and at the other end molecular thereof a chlorosilyl group. The capacitor according to any preceding Claim, wherein the liquid crystal molecules are nematic liquid crystal molecules. The capacitor according to any of Claims 1 to 4, wherein the liquid crystal molecules are ferroelectric liquid crystal molecules. The capacitor according to Claim 6, wherein the ferroelectric liquid crystal is azomethine type, azoxy type or ester type. The capacitor according to any preceding Claim, wherein CH₂ = CH - (CH₂) n - SiCl₃ (wherein n is an integer) is used as the chlorosilane surface active agent. The capacitor according to any preceding Claim, wherein a pair of opposing electrodes is provided and an electrolyte is held therebetween. The capacitor according to any preceding Claim, wherein the metallic electrode-forming film (1) is made of Al or an Al-containing alloy. A process for producing a capacitor, which comprises a step of forming a single layer (4) of monomolecular film on one or both surfaces of a metallic electrode-forming film (1) by letting a chlorosilane surface active agent in a non-aqueous organic solvent be chemically adsorbed on the surface of the electrode-forming film (1) either directly or by interposing therebetween a thin oxide film (2), and thereby effecting the absorption by chemical siloxane bonding, wherein the chlorosilane surface active agent has a structure in which chlorosilyl groups are bonded to liquid crystal molecules through a carbon chain. The process according to Claim 11, wherein a metallic electrode-forming film (1) is made of Al or an Al-containing alloy. The process according to Claims 11 or 12, wherein said hydrocarbon chain is linear. The process according to Claim 13, wherein said carbon chain comprises 10 to 25 carbon atoms. The process according to Claim 13 or 14, wherein the linear carbon chain is terminated at one end thereof with a substituent selected from a vinyl group, an acetylene group or a cyano group and at the other end thereof with a chlorosilyl group. The process according to any of Claims 11 to 15, wherein the liquid crystal molecules are nematic liquid crystal molecules. The process according to any of Claims 11 to 15, wherein the liquid crystal molecules are ferroelectric liquid crystal molecules. The process according to Claim 17, wherein the ferroelectric liquid crystal is azomethine type, azoxy type or ester type. The process according to Claim 15, further compring a step of reacting said substituent by a treatment with a chemical agent for adding a hydroxyl group and then effectuating chemical adsorption of said chlorosilane surface active agent, said step being performed after the step of forming a single layer of monomolecular film by having said chlorosilane surface active agent chemically adsorbed on the metallic electrode surface. The process according to Claim 15, wherein the reaction of the substituent is performed by application of radiation or a plasma treatment, and the process includes a step of effecting chemical adsorption of the chlorosilane surface active agent. The process according to any of Claims 11 to 20, wherein CH₂ = CH - (CH₂) n - SiCl₃ (n being an integer) is used as the chlorosilane surface active agent.

Inventors: MINO NORIHISA, OGAWA KAZUFUMI, MINO, NORIHISA, OGAWA, KAZUFUMI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: H01G4
IPC codes: G02F1, H01G4, C08F20, H01B3, C08F30