LAYERED MAGNETIC FILM STRUCTURE, METHOD OF ITS PRODUCTION AND APPLICATION IN A MAGNETIC HEAD

Abstract: A magnetic film of Fi-Si-Al alloy comprising a substrate (11) and a Fe-Si-Al alloy film (12), said substrate having a thermal expansion coefficient lower than that of said Fe-Si-Al alloy film and the amount of argon entrapped in said alloy film being controlled within the range of 0.01 to 0.3 wt. % so that internal stresses in said alloy film are substantially zeroed. Also disclosed are a method of making the magnetic film and a magnetic head utilizing the film.

Description: Technical Field This invention relates to layered magnetic film structures comprising Fe-Si-Al alloy film, and particularly comprising magnetic films of such alloy having internal stresses substantially zeroed and providing excellent magnetic properties, and a method of producing the same. Magnetic films of Fe-Si-Al alloy are suitable for use as magnetic cores for high density recording heads for high frequency use requiring a high S/N ratio, such as thin-film laminated magnetic heads used primarily as VCR and disk files and the like. Background Art With a recent marked improvment in the recording density in the field of magnetic recording, there has been an increasing need for a reduction in the track width for magnetic heads serving as electromagnetic transducer elements, intensified saturation magnetization of core materials, and an improvement in permeability in the high frequency range of core materials. Core members for a magnetic head have heretofore been fabricated by cutting ferrite or Fe-Si-Al material into a block and forming the block with grooves 3 for defining the track width 2 and grooves 4 for receiving coil windings by the use of a dicing blade, saw or the like, as shown in Fig. 5. However, in order to fabricate magnetic heads having narrow tracks less than 30 µm wide, it is necessary to reduce the spacings of the grooves 3 for defining the track width to extremely small dimensions, with the result that problems have arisen due to lack of precision of the track width 2 and irregularities in shape due to chipping in the portions of the track width 2. On the other hand, for metallic magnetic thin-film heads comprising metallic magnetic thin films such as conventional Fe-Si-Al alloy, such magnetic materials have been utilized which are laminated to a predetermined film thickness by stacking such metallic magnetic films 3 to 10 µm thick and non-magnetic insulation films about 0.5 µm thick in alternating relation. More specifically, in manufacturing such metallic magnetic thin-film heads, the laminated magnetic material may be fabricated by sputtering (see JP-A-58-100412, Sony k.k.) films on a substrate to a thickness corresponding to the track width wherein the process for defining the track width is eliminated, whereby the problems involved in fabricating magnetic heads from a block-type core material as described above may be resolved. However, it has been found through research and experiments conducted by the present inventors that metallic magnetic films formed by sputtering have quite high internal stresses and that the laminated metal thin-film magnetic head made from such multi-layered magnetic material composed of metallic magnetic films, that is magnetic core layers, exhibits poor magnetic properties due to magnetostriction. More specifically, it has been found that such magnetostriction increases magnetically anisotropic energy in correlation with the internal stresses within the magnetic material of Fe-Si-Al alloy which is a soft magnetic material and that such soft magnetic material exhibits undesirable properties such as increased coercive force (coercivity) and poor permeability, so that it may not produce high output magnetic heads. The common method of fabricating such metallic thin-film laminated magnetic heads involves sputtering a metallic magnetic thin film on a substrate of a non-magnetic material such as glass or the like, laminating such films alternately with insulating layers, and bonding another non-magnetic substrate onto said laminate to sandwich the laminate between the opposed substrates to thereby form a magnetic core. However, studies by the present inventors have revealed that the film formation by sputtering is accompanied by the disadvantage that the substrates on which the films have been sputtered are subject to warping due to internal stresses in the films, making it hard to provide uniform bonding throughout the bonding surfaces between the laminate and the substrates. In the course of studying soft magnetic films of Fe-Si-Al alloy and further thin-film laminated magnetic heads fabricated by laminating soft magnetic films of Fe-Si-Al alloy and non-magnetic insulating films alternately, the present inventors have discovered that the soft magnetic films deposited by sputtering on a substrate has internal stresses generated therein which adversely affect the soft magnetic films, resulting in degrading the properties of the magnetic head. Further analysis of the internal stresses generated in such soft magnetic films has revealed that such internal stresses arise from not only thermal stresses due to differences in thermal expansion coefficient between the substrates and the films, but also intrinsic stresses which are composed of compressive stresses generated in the films by the high energy particles sputtered from a target by Ar, impacting (peening) and penetrating into the film being deposited and of compressive stresses generated by the argon entrapped in the deposited films. Furthermore, upon studying and experimenting on methods for relieving the films of internal stresses, the inventors have discovered that compressive stresses ascribed to peening may be removed during the process of heating the deposited magnetic film up to the heat treatment temperature and that compressive stresses caused by entrapped argon can be controlled by the amount of argon entrapped in the deposited film of Fe-Si-Al alloy within the range from 0.01 to 0.3 percent by weight and using substrates having a thermal expansion coefficient lower than that of the Fe-Si-Al alloy film. The inventors have also found the following factors important in effectively producing magnetic films substantially free from internal stresses: (1) to employ a DC sputtering apparatus and apply RF bias to the substrate in order to control the amount of argon entrapped in the deposited film, (2) to use for a substrate a material having a thermal expansion coefficient lower than that of a film being deposited on the substrate, and (3) to subject a sputtered magnetic film of Fe-Si-Al alloy to heat treatment at a temperature of 450°C to 800°C. The present invention is based on these discoveries. Disclosure of the Invention Accordingly, it is an object of the invention to provide a layered magnetic film structure comprising Fe-Si-Al alloy film which obviates or mitigates the aforesaid problems with the prior art, and a method of fabricating such magnetic film. Another object of the invention is to provide a thin-film laminated magnetic head made from said magnetic film of Fe-Si-Al alloy which has good magnetic properties such as low coercivity and high permeability. The present invention is defined in the claims. Such layered magnetic film structures comprising Fe-Si-Al alloy film may be very effectively utilized as a magnetic film for a thin-film laminated magnetic head by laminating such alloy films alternately with non-magnetic insulating films. In a preferred embodiment of the invention the substrate may have a thermal expansion coefficient of 100 x 10⁻⁷ to 135 x 10⁻⁷ deg⁻¹, and the Fe-Si-Al alloy film is a composition having a high saturation magnetization and then containing 83 or more weight % of Fe and have a thermal expansion coefficient of 110 x 10⁻⁷ to 170 x 10⁻⁷ deg⁻¹. The thickness of the magnetic film is in the range of 1 µm to 20 µm. Crystallized glass (such as PEG 3120 C manufactured by Hoya Co., Ltd.) is suitable as a substrate which satisfies the aforesaid conditions. The film of Fe-Si-Al alloy to be deposited on the substrate may have a usual composition, but may preferably be a composition having a Fe content of 83 weight percent or more and a higher saturation magnetization as compared with ferrite. Suitable alloys include compositions of 85 wt. % Fe, 9.6 wt. % Si and 5.4 wt. % Al; 88 wt. % Fe, 7.7 wt. % Si and 4.3 wt. % Al; and 90 wt. % Fe, 7.8 wt. % Si and 2.2 wt. % Al, for example. If the amount of argon entrapped in the magnetic film does not exceed 0.01 wt. %, it would be insufficient to relieve tensile stresses caused by the heat treatment. On the contrary if the amount of entrapped argon exceeds 0.3 wt. %, compressive stresses higher than 10⁹ Pa would be generated in the deposited film. It is impossible to reduce such compressive stresses to substantially zero. As indicated hereinabove, the present inventors have discovered that it is of importance in fabricating the magnetic film of Fe-Si-Al alloy (1) to employ a DC sputtering apparatus and apply RF bias to the substrate, (2) to use for a substrate a material having a thermal expansion coefficient lower than that of a film being deposited on the substrate, and (3) to subject a sputtered magnetic film of Fe-Si-Al alloy to heat treatment at a temperature of 450°C to 800°C. As stated above, the amount of argon entrapped in the magnetic film is of critical importance in the present invention. Researches by the inventors have shown that when the film formation is performed by RF sputtering, a substantial quantity of argon amounting to as much as over 1 wt. % is entrapped in the film if RF bias is applied to the substrate, while about 0.7 to 0.9 wt. % of argon is entrapped in the film even with no RF bias applied, so that the amount of entrapped argon cannot be varied in a range effective in controlling the stresses. In addition application of RF bias in the RF sputtering process is impractical because it is difficult to control the phase between the two RF signals. When the film fabrication is carried out by DC sputtering, an amount of argon below 0.01 wt. % is entrapped in the film whether DC bias may be applied or not, so that it is impossible to vary the amount of entrapped argon in a range effective in controlling the stresses. It is possible to vary the amount of argon required of the soft magnetic film in a range effective in controlling the stresses in the film only by applying RF bias in the DC sputtering process according to the present invention. More specifically, the inventors have discovered that the amount of argon entrapped in the film may be controlled by adjusting the electric power of the DC glow, the RF electric power applied to the substrate holder, and the distance between the substrate and the target, whereby the magnitude of the compressive stresses caused by entrapped argon may be controlled. The compressive stresses due to the entrapped argon in the thus controlled amount are cancelled by the tensile stresses caused in the deposited film by the difference in thermal expansion between the substrate and the film, that is, by using a substrate having a thermal expansion coefficient lower than that of the film being deposited, and further the compressive stresses caused in the deposited film due to peening effect may be removed during the process of heating the film up to the heat treatment temperature, with the result that the soft magnetic film and the thin-film laminated magnetic head fabricated using such soft magnetic film will have their internal stresses substantially zeroed. Brief Description of the Drawings These and other objects and features of the present invention are more fully disclosed in the following specification with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of an embodiment of the thin-film laminated magnetic head comprising thin films of Fe-Si-Al alloy according to this invention; Fig. 2 is a diagrammatic view illustrating the successive steps of one embodiment of the method of fabricating a thin-film laminated magnetic head utilizing thin films of Fe-Si-Al alloy according to this invention; Fig. 3 is a schematic cross-sectional view of a DC magnetron sputtering apparatus used to produce a thin film of Fe-Si-Al alloy according to this invention and a thin-film laminated magnetic head; Fig. 4 is a graph showing the relationship between the internal stresses in a soft magnetic film of Fe-Si-Al alloy according to this invention, the amount of entrapped argon and the RF bias; and Fig. 5 is a perspective view of a core member from which a conventional magnetic head is made. Detailed Description of the Embodiments of the Invention Figs. 1 and 2 illustrate an embodiment of thin-film laminated magnetic head 10 and successive steps of a method of fabricating such magnetic head utilizing soft magnetic films of Fe-Si-Al alloy according to the present invention. Referring to Fig. 2, a non-magnetic substrate 11 such as crystallized glass or the like, is provided as shown in Fig. 2(A). A thin film of Fe-Si-Al alloy 12 is then deposited on the substrate 11 to a thickness of 1 to 20 µm under the conditions described hereinabove using a DC magnetron sputtering apparatus, the construction of which will be described later referring to Fig. 3. Next, a non-magnetic insulating film 13 such as SiO₂, Al₂O₃ or the like is deposited on the magnetic film 12 to a thickness of 0.03 to 0.5 µm using a RF magnetron sputtering apparatus similar to that used to deposit the magnetic film 12. These steps are repeated to laminate a required number of magnetic films 12 and non-magnetic insulating films 13 alternately to form a film laminate 14 as shown in Fig. 2(C). The thickness and number of stacked magnetic films 12 and non-magnetic insulating films 13 are selected appropriately so that the thickness of the film laminate 14 is equal to the track width W shown in Fig. 1. A bonding material 15 is then deposited on the film laminate 14 by conventional sputtering process as shown in Fig. 2(D), and another substrate 16 which is made of the same material as that of which the first substrate 11 is made is bonded onto the bonding material 15, as shown in Fig. 2(E). The bonding material 15 may be bonding glass such as GA-120, FH-11 manufactured by Nippon Electric Glass Co., Ltd. or 1990 available from Corning Corp., and preferably B₂O₃-SiO₂-Al₂O₃ type bonding glass. The thus produced laminated film structure 17 is cut through the thickness thereof into a pair of core half blocks 18 and 19 as shown in Fig. 2(F). Preferably the cut is made at an azimuth angle ϑ. Then, at least one of the core halves, the core half 18 in the illustrated embodiment, is formed with grooves 20 (Fig. 1) for receiving a coil winding, followed by polishing the mating surfaces 18 a and 19 a of the core half blocks 18 and 19 and depositing non-magnetic gap spacers such as SiO₂ or the like on the polished mating surfaces as by sputtering process, as shown in Fig. 2(F). The two core half blocks 18 and 19 are then bonded together at the mating surfaces 18 a , 19 a as shown in Fig. 1. Finally, R-grinding operation to form tape slide surfaces and other forming operations and coil winding operation are carried out to fabricate a thin-film laminated magnetic head 10. The invention will now be described with respect to examples. Example 1 Fig. 3 schematically illustrates a DC magnetron sputtering (RF bias applying) apparatus such as model SPF-210 manufactured by ANELVA Corp. The DC sputtering apparatus 30 was provided with a cathode 32 and an electrically isolated substrate holder 34 connected with a high voltage DC power source 31 and a RF bias power source 33, respectively. Disposed on the cathode 32 was a target 35 while a substrate 11 was disposed on the holder 34. The apparatus is further provided with an exhaust port 36 through which the apparatus was evacuated to a vacuum by a vacuum pump (not shown) and an inlet port 37 through which argon gas was introduced into the apparatus. The target 35 was hot-pressed plate of 4 inches in diameter and 4 mm in thickness composed of 10.5 wt. % Si, 5.5 wt. % Al and Fe for the balance. The substrate 11 was a sheet of crystallized glass (PEG 3120 manufactured by Hoya Co., Ltd.) having a thermal expansion coefficient of 120 x 10⁻⁷ deg⁻¹ and a diameter of 2 inches which was polished to a surface roughness of 150 Å(15 nm). The target 35 and substrate 11 were spaced from each other by a distance of 45 mm. The pressure of argon gas was 4 x 10⁻³ Torr, the electric input power 500 W, the substrate temperature 60°C, and the film deposition rate 0.4 µm/min. Under the foregoing operating conditions, a film of Fe-Si-Al alloy was deposited on the substrate to a thickness of 4 µm while RF electric power was applied to the holder varying in the range of 0 to 0.6 KV in terms of the plate voltage of the oscillating tube for use with a RF power source. The thus formed soft magnetic film was then heat treated at a temperature of 650°C for one hour. The thus prepared film of Fe-Si-Al alloy had a thermal expansion coefficient of 110 x 10⁻⁷ to 170 x 10⁻⁷ deg⁻¹, and the relation between the internal stresses in said soft magnetic film, the amount of argon entrapped and the RF electric power is as shown in Fig. 4. It will be seen from the graph of Fig. 4 that there is a direct proportional relationship between the amount of entrapped argon and the compressive stresses and that it is possible to change the internal stresses from tensile to compressive stresses since the amount of entrapped argon is in proportion to the plate voltage of the RF power oscillating tube. In this example, when the plate voltage was 0.35 KV, the internal stresses in the deposited film were below ±1 x 10⁸ Pa, and the film provided magnetic properties such as coercivity of 0.16 Oe (1 Oe = 79.6 A / m) and effective specific initial permeability of 2000. Example 2 A film 12 of Fe-Si-Al alloy was deposited on a substrate 11 to a thickness of 4.7 µm using the same DC magnetron sputtering apparatus, substrate and target 35 as used in Example 1 and under the same operating conditions. The thus formed film of Fe-Si-Al alloy had a thermal expansion coefficient of 110 x 10⁻⁷ to 170 x 10⁻⁷ deg⁻¹, and the relation between the internal stresses in said soft magnetic film, the amount of argon entrapped in the film and the RF electric power is as shown as in Fig. 4. In this example when the plate voltage of the oscillating tube was 0.35 KV, the internal stresses in the deposited film were below ±1 x 10⁸ Pa. An insulating film 13 was then formed on the Fe-Si-Al alloy film 12. The formation of the insulating film was performed using the same magnetron sputtering apparatus as used to form the Fe-Si-Al alloy film but with the apparatus connected to a RF power source and with a plate of SiO₂ of 4 inches in diameter and 5 mm in thickness used as a target. The target and the substrate having the magnetic film deposited thereon were spaced from each other by a distance of 45 mm. The pressure of argon was 4 x 10⁻³ Torr, the input electric power 300 W, the substrate temperature 60°C and the deposition rate 0.1 µm/min. The SiO₂ film was deposited in a thickness of 0.3 µm on the magnetic film formed on the substrate under these conditions. The aforesaid steps of depositing the magnetic film 12 and insulating film 13 were repeated four times to produce a film laminate 14 having a total thickness of 20 µm. A bonding material 15 which was FH-11 made by Nippon Electric Glass Co., Ltd. in this example was deposited on said film laminate 14 as by conventional sputtering process, and then another substrate 16 made of the same material as the first substrate 11 was applied to the bonding material 15 to fabricate a laminated film structure 17. Subsequently, the laminated film structure 17, that is, the magnetic films 12 of the film laminate 14, are subjected to heat treatment at 650°C for one hour. The thus formed Fe-Si-Al alloy/SiO₂ laminated soft magnetic film system had internal stresses therein below ±1 x 10⁸ Pa. It was determined by experiments that the thus formed laminated soft magnetic film had internal stresses below ±1 x 10⁸ Pa for the total thickness of 3 to 40 µm. With regard to the magnetic properties the laminated film structure 17 provided a coercivity of 0.18 Oe and a specific initial permeability of 2000. This laminated film structure was worked on according to the process shown in Fig. 2(F) to fabricate a VCR magnetic head as shown in Fig. 1 having its track width extending in the direction of the film thickness. This magnetic head was evaluated for its read/write characteristics. The parameters of the heads, the measuring conditions and the maximum outputs at various frequencies are shown in the following Tables 1, 2 and 3, respectively. From the Table 3 it is to be appreciated that in comparison at the standardized output at 5 MHz the maximum reproducing output of the Fe-Si-Al alloy thin-film laminated magnetic head according to the present invention is about 3 times as great as that of the conventional single crystal Mn-Zn ferrite head used for the VHS video cassette recorder. The method for introducing argon gas is not limited to the method illustrated in the examples, but argon gas may be ion implanted directly into a film being sputtered, or alternatively the combination of vacuum deposition and ion implantation methods or any other suitable method may be used. Further, it is to be understood that the thin film of Fe-Si-Al alloy may also be effectively utilized for thin-film inductors, magnetic shielding films and the like. As is explained hereinabove, the thin film of Fe-Si-Al alloy substantially zeroes internal stresses of a metallic magnetic film, thereby providing a significantly increased initial permeability, and may be very effectively employed as a magnetic film for a high performance thin-film laminated magnetic head or other magnetic devices having a greatly increased reproducing output as compared with the prior art. Further, according to the method of this invention, such thin film of Fe-Si-Al alloy, and hence a thin-film laminated magnetic head may be very conveniently manufactured.

Claims: A layered magnetic film structure comprising a substrate (11) and Fe-Si-Al alloy film (12), characterized in that said substrate has a thermal expansion coefficient lower than that of said Fe-Si-Al alloy film and that the amount of argon entrapped in said alloy film is controlled within the range of 0.01 to 0.3 wt. % so that internal stresses in said alloy film are substantially zeroed. A magnetic film structure as claimed in claim 1, characterized in that said substrate (11) has a thermal expansion coefficient of 100 x 10⁻⁷ to 135 x 10⁻⁷ deg⁻¹, and said Fe-Si-Al alloy film (12) is a composition containing 83 wt. % or more Fe and having a high level of saturation magnetization, the thermal expansion coefficient of the alloy film being 110 x 10⁻⁷ to 170 x 10⁻⁷ deg⁻¹. A magnetic film structure as claimed in claim 1 or claim 2, characterized in that said substrate is made of crystallized glass, and said Fe-Si-Al alloy film is a composition of 83 to 94 wt. % Fe, 4 to 11 wt. % Si and 2 to 6 wt. % Al. A method of producing a layered magnetic film structure comprising Fe-Si-Al alloy film, characterized by using a DC sputtering apparatus to apply RF bias to a substrate and to deposit a Fe-Si-Al alloy film on said substrate, said alloy film having a thermal expansion coefficient higher than that of said substrate, and the amount of argon entrapped in said alloy film being controlled within the range of 0.01 to 0.3 wt. % so as to substantially zero the internal stresses in said alloy film. A method as claimed in claim 4, characterized in that said substrate has a thermal expansion coefficient of 100 x 10⁻⁷ to 135 x 10⁻⁷, and said Fe-Si-Al alloy film is a composition containing 83 wt. % or more Fe and having a high level of saturation magnetization, the thermal expansion coefficient of the alloy film being 110 x 10⁻⁷ to 170 x 10⁻⁷ deg⁻¹. A method as claimed in claim 4 or claim 5, characterized in that said substrate is made of crystallized glass, and said Fe-Si-Al alloy film is a composition of 83 to 94 wt. % Fe, 4 to 11 wt. % Si and 2 to 6 wt. % Al. A thin-film laminated magnetic head comprising a pair of opposed substrates (11 and 16), and alternately laminated Fe-Si-Al alloy films and non-magnetic insulating films (12 and 13) between said substrates, characterized in that said substrates have a thermal expansion coefficient lower than that of said Fe-Si-Al alloy films and that the amount of argon entrapped in said alloy films is controlled within the range of 0.01 to 0.3 wt. % so that internal stresses in said alloy films are substantially zeroed. A thin-film laminated magnetic head as claimed in claim 7, characterized in that said substrates have a thermal expansion coefficient of 100 x 10⁻⁷ to 135 x 10⁻⁷ deg⁻¹, and said Fe-Si-Al alloy films are each a composition containing 83 wt. % or more Fe and having a high level of saturation magnetization, the thermal expansion coefficient of the alloy films being 110 x 10⁻⁷ to 170 x 10⁻⁷ deg⁻¹. A thin-film laminated magnetic head as claimed in claim 7 or claim 8, characterized in that said substrates are made of crystallized glass, and said Fe-Si-Al alloy films are each a composition of 83 to 94 wt. % Fe, 4 to 11 wt. % Si and 2 to 6 wt. % Al.

Inventors: MORI, TAIICH, SAITO, KAZUHIRO
Applicants: NIPPON MINING COMPANY LIMITED
Application year: 1986
Main classification: H01F10
IPC codes: H01F41, C22C38, H01F10, G11B5, C23C14