Method for manufacturing a composite oxide thin film

Abstract: The present invention provides a composite oxide thin film formed by energizing a work electrode and an opposite electrode immersed in a solution containing reactive components. Improvement of crystallinity is promoted by the use of water thermal conditions as compared with the conventional thin film forming methods, and it is possible directly to obtain a uniform composite oxide thin film having an excellent crystallinity at a relatively low temperature.

Description: The present invention relates to a composite oxide thin film, and more particularly, to a composite oxide thin film formed through an electrochemical reaction and a water thermal reaction. Composite oxide thin films are attracting the general attention as electronic materials for various applications and have already been industrialized or subjected to trial manufacture in different manners as materials for an inductor, a sensor, an optical component, a magnetic use and a superconducting application. There have conventionally been known, as such composite oxide thin films, those formed by physical evaporation as typically represented by sputtering and those formed by chemical evaporation as typically represented by CVD and MOCVD. These conventional composite oxide thin films based on vapour synthesis involve some problems to be solved. More specifically, these films based on vapour synthesis are defective in that they have a low rate of growth of the film and require consumption of much energy. In these methods, easy occurrence of nonuniform evaporation and reaction under a low partial oxygen pressure tend to cause much oxygen demand, leading to the possibility of being converted into semiconductors, thus needing annealing after film formation. During annealing, however, the substrate and the composite oxide thin film may react, or peel-off may be caused. The low insulation fracture voltage relative to the film thickness is another problem. In the case of the CVD method, a raw material of a high volatility must be used, but such a raw material is usually unstable and difficult to handle, with a very high cost. In addition to these vapour phase methods, there are known several thin film forming methods based on the liquid phase process, including, for example, a method for forming a dielectric thin film by causing an electro-chemical reaction through immersion of titanium or zirconium in a molten salt of barium or strontium (Japanese Patent Publication No. 43-2, 650), a method of immersing titanium in a molten salt (Japanese Patent Publication No. 44-13,455), and a method for forming a BaTiO 3 film through a chemical treatment in a strongly alkaline aqueous solution of barium (Japanese Patent Provisional Publication No. 60-116,119). In the methods using molten salt, however, it is necessary to employ a very high temperature and an expensive pressure vessel and contamination from the vessel is inevitable. It is furthermore difficult to precisely control the film thickness. In the case of chemical treatments, the defects include the low growth rate and the difficult control of the film thickness, and in addition, there is a concern about contamination from such mineralizers as sodium and potassium. In addition to those mentioned above, the organic metal application method is known. This method is however defective in that the thermal decomposition through firing of an organic metal compound applied to the substrate at a prescribed temperature causes a considerable shrinkage during the firing step and produces cracks in the resultant composite oxide thin film, and furthermore, evaporation and combustion of the organic components make it difficult to achieve a dense sinter. The reaction with the substrate during firing is another problem. US-A-3141798 describes the production of metal oxide coatings on aluminium based bodies. The metal oxide coating is made up of an oxide of calcuim, aluminium oxide and water of hydration of composition 3CaO.Al 2 O 3 .6-8H 2 O US-A-3751349 describes a method for electrochemically forming a coating of titanium-alkaline earth metal compound oxide on the surface of titanium used as one of the electrodes in an electrolytic process. Chemical Abstract 104366 Vol. 75, No. 16 relates to similar work. The coating produced comprises titanium oxide without barium or titanium oxide in the presence of barium depending on the conditions. The present invention was developed in view of the circumstances as described above and has an object to provide a new composite oxide thin film which solves the drawbacks of the conventional thin films, can be synthetically manufactured at a temperature lower than in the conventional manufacturing methods, is uniform and excellent in crystallinity, and easy to manufacture even in the case of a large-area film. To solve the above-mentioned problems, in one aspect, the present invention provides a process for manufacturing a composite oxide thin film comprising energizing a work electrode and an opposite electrode immersed in an aqueous solution containing reactive components through the reaction between said reactive components and said work electrode, said reaction being effected under heating at a pressure of at least the saturated vapour pressure. In another aspect, the present invention provides a method for producing a member coated with a composite oxide film, said method comprising treating at least a part of said member as the work electrode in a process according to the invention. In the accompanying drawings: Fig. 1 is a sectional view illustrating an embodiment of the autoclave reaction apparatus applicable when forming the thin film of the present invention; Fig. 2 and 3 are chart diagrams illustrating the results of X-ray diffraction for an embodiment of the BaTiO 3 thin film of the present invention; Fig. 4 is a chart diagram illustrating the result of X-ray diffraction for the embodiment of the (Ba, Sr) TiO 3 solid-solution thin film of the present invention; and Fig. 5 is a chart diagram illustrating the result of X-ray diffraction for the embodiment of the BaFeO 2.9 thin film of the present invention. The work electrode comprises a reaction-active material such as metal, an alloy, an intermetallic compound, or an inorganic substance. In this case, the work electrode may be a single-body electrode or may be a composite or a multi-layer electrode, without any limitation in shape: it may be of a special shape having, for example, a cavity, and the possibility of forming a composite oxide thin film on the outer surface thereof or on the inner surface thereof is one of the features of the present invention. The work electrode may be formed on a substrate comprising inorganic materials, such as glass, ceramics, and organic polymers. Any arbitrary opposite electrode may be used. For the solution containing reactive components, any of various chemical compositions may be adopted. In general power should preferably be turned on under pressure and heating conditions in a pressure vessel. The thin film of the present invention may be manufactured, for example, in the apparatus as shown in Fig. 1. In this embodiment, in the apparatus having a heater (3) provided around an outer vessel (2) of an autoclave (1) and an inner vessel (4) such as one made of teflon® DuPont provided in the interior thereof, a work electrode (6) and an opposite electrode (%) are immersed in a solution (5) containing reactive components. A lid (8) is provided on the top of the outer vessel (2) to close the interior of the outer vessel (2). In such an apparatus, for example, with a work electrode (6) made of titanium and an opposite electrode (7) made or platinum, serving respectively as the anode and the cathode, a BaTiO 3 thin film can be formed on the surface or titanium by energizing the electrodes in a barium hydroxide solution. Any metal, alloy or inorganic substance such as aluminum, niobium, zirconium, hafnium, lead, tantalum or iron may be employed in place of titanium. The solution (5) may contain any reactive components reactive with the work electrode (6), including, for example, barium hydroxide, strontium hydroxide, calcium hydroxide, and lithium hydroxide. When a work electrode (6) made of a metal is used as the anode as described above, the metal of thin work electrode (6) forms an oxide or begins to be dissolved into the solution in the state of anodic oxidation, and reacts with the reactive components in the solution (5), and composite oxides are considered to be formed as a thin film. The temperature, the pressure and the applied electric current (DC or AC) in the formation of the film, varying with the reaction system, may be appropriately selected. For example, the temperature may be within the range of from 50°C to the critical point of water (374.2°C), and the pressure may be at least the saturated vapor pressure. In the case of lower temperatures, an autoclave is not necessary for the reaction. Now, the present invention will be described in more detail by means of the following examples. Example 1 A thin film was formed with the use of the apparatus shown in Fig. 1, under the following conditions: Solution : 0.5 N - Ba(OH) 2 ·8H 2 O, Work electrode : Ti (purity: 99.9%), Opposite electrode : Pt, Temperature : 200°C, Pressure : saturated vapor pressure 2.0 MPa, Electric current : 100 mA/cm 2 (DC). DaTiO 3 began to form on the surface of the work electrode. The relationship between the applied voltage and the treatment time is that the voltage shows a sudden initial rise, and immediately after that, a constant value, with no remarkable change thereafter. This is considered attributable to the tact that the growth of the film and dissolution through synthetic reaction of the thin film simultaneously proceed, resulting in equilibrium of speeds. The result of X-ray diffraction of the resultant thin film is illustrated in Fig. 2. The formed BaTiO 3 was of a single phase and had a satisfactory crystallinity. Example 2 A thin film was formed in the same manner as in the Example 1 with a reaction temperature of 100°C. The result of X-ray diffraction of the resultant BaTiO 3 thin film is illustrated in Fig. 3. Examples 3 to 5 Thin films were formed in the same manner as in the Example 1, with a concentration of 0.25 N of the solution and a current density of 50mA/cn 2 while changing the temperature from 200°C to 150°C and 100°C. The formation of the BaTiO 3 thin film brought about, after the lapse of 30 minutes, the following changes in weight of the work electrode: 200°C : 4.6 x 10 -6 g/(cm 2 ·minute) 150°C : 4.3 x 10 -6 g/(cm 2 ·minute) 100°C : 2.5 x 10 -6 g/(cm 2 ·minute) Example 6 A BaTiO 3 thin film was formed on a titanium sheet having a thickness of 1.0 mm by changing only the following conditions: Solution : 0.25N - Ba(OH) 2 ·8H 2 O, Temperature : 150 °C, Electric current : 13 mA/cm 2 , Time : 80 minutes. A silver electrode was vapor-deposited onto the surface of the resultant BaTiO 3 thin film to evaluate dielectric constant characteristics. It had a capacity of approximately 70 nF, tan δ = 15% and ε = 300 (on the assumption of 0 ≃ 0.1 µm). Example 7 A treatment was conducted, with the use of the apparatus as shown in Fig. 1, under the following conditions: Solution : 0.5N - Ba(OH) 2 ·8H 2 O, Electrode : both work and opposite electrodes made of metallic titanium, Temperature : 200°C, Pressure : saturated vapor pressure 2 MPa, Voltage : AC, constant voltage of 20 V, 50 Hz. After the lapse of approximately ten minutes, BaTiO 3 , formed on the surfaces of the both electrodes. The resultant thin films showed X-ray diffraction patterns similar to that shown in Fig. 2, permitting confirmation of a single phase and an excellent crystallinity. Example 8 A metal Ti was deposited on a surface of pyrex glass substrate in a vapor phase deposition process by an RF sputtering method. The Ti film formed by the above process is used as work electrode. A thin film comprising composite oxide was formed in the same manner as in Examples 1 and 2. The formed thin film has a high density and a brightness. It shows several different color tones, such as blue, violet, gold corresponding to different treatments. Peeling of the thin film was not observed in a treatment of cutting by a shape knife. Example 9 A thin film was formed in the same manner as in Example 1 and 2, using a Ti deposition film on a surface of polyphenylene sulfide (PPS) film by a process of RF sputtering method. Under the condition of 100 ∼ 180°C temperature, BaTiO 3 thin film was formed. Example 10 An SrTiO 3 thin film was formed on a titanium sheet having a thickness of 0.2 mm, by changing only the following conditions: Solution : 1 N- Sr(OH) 2 ·8H 2 O, Temperature : 200°C, Electric current : 50 mA/cm 2 , Time : 60 minutes. An SrTiO 3 thin film having a satisfactory crystallinity was obtained. Example 11 A mixed solution of 0.5N - Sr(OH) 2 ·8H 2 O and 0.5N - Ba(OH) 2 ·8H 2 O was employed as the reaction solution, and a thin film was formed under the same conditions as in the Example 8. The X-ray diffraction of the resultant thin film is illustrated in Fig. 4. It was confirmed that the thin film thus obtained was a uniform (Ba, Sr)TiO 3 solid-solution film in which BaTiO 3 and SrTiO 3 were not separated, Example 12 An LiNbO 3 film was formed under the following conditions: Reaction solution : 1 N - LiOH, Work electrode : Nb (purity: 99.9%), Temperature : 200°C, Pressure : 1.8 MPa, Electric current : 68 mA/cm 2 . After the lapse of approximately 18 minutes, LiNbO 3 was formed on the surface of the work electrode. Example 13 A thin film was formed using an iron sheet as the work electrode under the following conditions: Solution : 0.5 N - Ba(OH) 2 -NaOH, Work electrode : Fe (purity: 99.9%) , Opposite electrode : Pt, Temperature : 200°C, Pressure : saturated vapor pressure, Current density : 18 mA/cm 2 . Formation of a BaFeO 2.9 film with a satisfactory crystallinity was confirmed from the X-ray diffraction pattern shown in Fig. 5. No BaFeO 2.9 was produced when electricity was not turned on. According to the present invention, as described above in detail, improvement of crystallinity is promoted by the use of water thermal conditions as compared with the conventional thin film forming methods, and it is possible to obtain a uniform composite oxide thin film having an excellent crystallinity directly at a relatively low temperature. A large-area thin film can thus easily be manufactured.

Claims: A process for manufacturing a composite oxide thin film comprising energizing a work electrode and an opposite electrode immersed in an aqueous solution containing reactive components through the reaction between said reactive components and said work electrode, said reaction being effected under heating at a pressure of at least the saturated vapour pressure. A process as claimed in claim 1 wherein said heating is effected at a temperature between 50°C and the critical point of water (374.2°C). A process as claimed in claim 1 or claim 2 wherein said heating is effected at a temperature between 100°C and the critical point of water (374.2°C). A process as claimed in any one of the preceding claims wherein said work electrode comprises a metal, an alloy, an intermetallic compound or an inorganic substance or comprises an inorganic substrate. A process as claimed in any one of the preceding claims wherein the work electrode comprises aluminium, niobium, zirconium, hafnium, lead, tantalum, iron or titanium. A process as claimed in any one of the preceding claims wherein the work electrode is an anode. A process as claimed in any one of the preceding claims comprising immersing the work electrode in an aqueous solution of one or more of barium hydroxide, strontium hydroxide, calcium hydroxide and lithium hydroxide. A method for producing a member coated with a composite oxide film, said method comprising treating at least a part of said member as the work electrode in a process as claimed in any one of the preceding claims.

Inventors: EUL YOO SEUNG, ISHIZAWA NOBUO, YOSHIMURA MASAHIRO, EUL, YOO SEUNG, ISHIZAWA, NOBUO, YOSHIMURA, MASAHIRO
Applicants: JAPAN RES DEV CORP, RESEARCH DEVELOPMENT CORPORATION OF JAPAN
Application year: 1990
Main classification: C25D11
IPC codes: C25D9, C01B13, C25D11, C01G23