Ceramic substrate and preparation of the same

Abstract: A ceramic substrate which comprises a ceramic base material having porosity of not more than 5% and surface roughness R max of not larger than 0.1 µm, and an amorphous aluminum oxide film having thickness of 1 to 50 µm formed on a surface of said ceramic base material, has high reliabili­ ty and can be easily surface finished with high accuracy and can be used to form a thin film magnetic circuit.

Description: The present invention relates to a method for producing dense ceramic substrate having good workability and mechanical properties. A ceramic substrate is used widely, for example, as an insulation substrate for integrated circuits, since it has good insulation property and chemical stability. Recently, a thin film magnetic head which is used in a magnetic disk instrument is produced by forming a circuit on the ceramic substrate. For example, when the ceramic substrate is used as a substrate of the thin film magnetic head as described in U.S. Patent No. 4,660,114 (EP-A-153 141), its surface must be very smooth to such extent that surface roughness R max is about 0.02 µm so as to form the magnetic circuit. To this end, a dense ceramic base material which comprises aluminum oxide (Al₂O₃) and the like as a main component is polished to surface roughness R max of about 0.02 µm and coated with a ceramic film such as Al₂O₃ or SiO₂ so as to prepare a substrate, which is used to form a thin film magnetic circuit. However, even if the surface of the ceramic base material is polished to the surface roughness R max of not larger than about 0.02 µm, the surface smoothness of the base material has poor reliability and the ceramic film formed on the base material often has poor surface accuracy due to contamination of debris and presence of voids. In addition, the ceramic film cannot be surface finished sufficiently for this use because it cannot be easily machined and tends to suffer from peeling and faults during polishing. Therefore, the ceramic substrate cannot maintain the surface accuracy of the polished ceramic base material and can hardly be used to form the magnetic circuit having desired properties. EP-A-0 108 940 discloses a thin film magnetic head in which an aluminum oxide film is used for reducing the roughness of the surface. Furthermore, there is known from EP-A-0 013 363 a transducer head in which a permalloy layer is interposed between the base matherail and an amorphous aluminum oxide layer. An object of the present invention is to provide a method for producing a ceramic substrate which can be easily surface finished with desired surface accuracy for a thin film magnetic head and has high reliability. The invention is directed to a method for producing a ceramic substrate comprising at least one ceramic base material from the group of Al₂O₃, ZrO₂, SiO₂, SiC, Si₃N₄ and AlN and having a porosity of not larger than 1 % and a surface roughness R max of not larger than 0.05 µm and formed on a surface of said ceramic base material an amorphous aluminum oxide film, wherein said amorphous aluminum oxide film is formed by an ion plating method under a plasma state under the condition that the temperature of the base material is 100 to 500°C up to a thickness in the range of from 1 to 50 µm and a Vicker's hardness of not larger than 1000 kg/mm² and a surface roughness R max , due to machining by an Elastic Emission Machining Method of not larger than 0.02 µm. The ceramic base material comprises at least one selected from Al₂O₃, ZrO₂, SiO₂, SiC, Si₃N₄ and AlN₄ as a main component. More preferably, it is a densely sintered TiC-containing Al₂O₃ or TiC-containing TZP (tetragonal zirconia polycrystal). When usual ceramic films are used as the film formed on the ceramic base material, they cannot easily surface finished due to their high hardness as described above. When the amorphous aluminum oxide film is used according to the present invention, it has the same smooth surface as the base material due to uniform nucleation and it can be easily and accurately surface finished to surface roughness R max of not larger than 0.02 µm due to its softness. The hardness (Vickers hardness) of the amorphous aluminum oxide film is controlled to be not larger than 1,000 kg/mm², since then the surface finishing can easily be carried out. The amorphous aluminum oxide film is not necessarily a stoichiometric Al₂O₃ and it may contain oxygen defect and contaminant elements. The term "amorphous" used herein does not mean that no crystalline phase is contained, but the amorphous aluminum oxide film may contain a small amount of the crystalline phase, for example, which can not be identified by an X-ray analysis. The amorphous aluminum oxide film is prepared by the ion plating method under a plasma state in which the film is quickly formed. This deposition method using the plasma phase is used since the film can be formed at a low temperature at which crystallization can be inhibited, the temperature of the substrate is selected from a wide temperature range, and the resultant amorphous aluminum oxide film has good adhesion properties. Usually, as the temperature of the substrate increases, crystallinity of the film increases and workability of the film is degraded although adhesion of the film to the substrate is improved. Therefore, the upper limit of the temperature of the substrate is such temperature that crystalline aluminum oxide is not identified by the X-ray analysis. The temperature of the substrate is therefore 100 to 500°C. Although the amorphous aluminum oxide film could be surface finished by a conventional polishing or lapping, it is difficult to obtain repeatedly a fault-free film having the surface roughness R max of not larger than 0.02 µm. The thin amorphous aluminum oxide film having no fault and the surface roughness R max of not larger than 0.02 µm to be required for the thin film magnetic head is obtained, when a surface of a work piece is machined by an EEM (Elastic Emission Machining) method in which the surface of the work piece is machined by flowing slurry comprising abrasive grains without contacting the work piece with a rotating lapping tool in the slurry (cf. Mori and Yamauchi, The Journal of the Japan Precision Engineering Society, 51 [1], 12 (1985)). As the abrasive grains, fine powder comprising silicon oxide, aluminum oxide, silicon carbide, diamond, cubic system boron nitride and the like are used. The article diameter of the abrasive grains is smaller than desired surface roughness. The amorphous aluminum oxide film has a thickness of 1 to 50 µm. When the thickness is smaller than 1 µm, the surface finishing is difficult and the film tends to contain faults. When the thickness is larger than 50 µm, strain in the film increases and the film is expensive. The present invention is explained by following examples. Examples 1 to 11 Each base material comprising ceramic sintered body having ingredients shown in Table and a porosity of not more than 1% was polished to surface roughness R max of 0.05 µm by using a diamond paste. The base material had a diameter of 75 mm and a thickness of 4 mm. Then an amorphous aluminum oxide film was formed on a surface of each base material by a procedure shown in Table. The amorphous aluminum oxide film was formed at a substrate temperature of 300°C in an oxygen atmosphere of 5 x 10⁻⁴ Torr by an ion plating method using a vapor source of Al₂O₃ and CVD method. Each resultant amorphous aluminum oxide film was surface finished according to the EEM method by abrasing the film so as to reduce its thickness by 1 µm in slurry which comprised a dispersion of SiO₂ powder of an average particle size of 0.01 µm and was flowed by rotation of a lapping tool. The thickness (µm) and surface roughness R max (Å) of each amorphous aluminum oxide are shown in Table. Each resultant substrate in Examples according to the present invention has good surface accuracy. In contrast, when no amorphous aluminum oxide film is present (Example 9), the thickness of the amorphous aluminum oxide film is larger than 50 µm (Example 5) and the thickness of the amorphous aluminum oxide film is smaller than 1 µm (Example 10), the surface roughness R max is larger than 0.02 µm and the substrate is not suitable for the ceramic substrate for the thin film magnetic head. When the aluminum oxide film is crystalline (Examples 1 and 2), the surface finishing is difficult and the required surface accuracy cannot be obtained due to, for example, peeling of the film.

Claims: A method for producing a ceramic substrate comprising at least one ceramic base material from the group of Al₂O₃, ZrO₂, SiO₂, SiC, Si₃N₄ and AlN and having a porosity of not larger than 1 % and a surface roughness R max of not larger than 0.05 µm and formed on a surface of said ceramic base material an amorphous aluminum oxide film, wherein said amorphous aluminum oxide film is formed by an ion plating method under a plasma state under the condition that the temperature of the base material is 100 to 500°C up to a thickness in the range of from 1 to 50 µm and a Vicker's hardness of not larger than 1000 kg/mm² and a surface roughness R max , due to machining by an Elastic Emission Machining Method of not larger than 0.02 µm.

Inventors: IGARASHI TADASHI C O ITAMI WOR, YAMAKAWA AKIRA C O ITAMI WORKS, IGARASHI, TADASHI C/O ITAMI WORKS OF, YAMAKAWA, AKIRA C/O ITAMI WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1988
Main classification: H01F10
IPC codes: H01B17, H01F10, H01L23, H01F41, H05K1, G11B5, H05K3