Alloy target for manufacturing a magneto-optical recording medium

Abstract: An alloy target for making a magneto-optical recording medium by sputtering comprises an alloy containing 10 to 50 atom % of at least one rare earth element selected from among Sm, Nd, Gd, Tb, Dy, Ho, Tm and Er, with a balance consisting substantially of at least one transition metal selected from among Co, Fe and Ni. The alloy has a mixed structure composed of at least one phase of an intermetallic compound formed by the rare earth element and the transition metal and a phase of the rare earth element alone.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention : This invention relates to an alloy target which is suitable for use in the manufacture of a magneto-optical recording medium by sputtering. 2. Description of the Prior Art : A magneto-optical memory has been drawing a great deal of attention as it facilitates the erasure and re-recording of information. Single-crystal materials such as garnets, polycrystalline materials such as MnBi and PtCo, or amorphous materials such as alloys of rare earth elements and transition metals are usually employed for manufacturing a magneto-optical memory. The use of an amorphous alloy comprising a rare earth element or elements and a transition metal or metals, such as Tb-Fe-Co or Gd-Tb-Fe, provides a variety of advantages. For example, it enables the manufacture of a magneto-optical memory which requires only a small amount of energy for recording information, one which is free from any grain-boundary noise, or one which is large and yet is relatively easy to manufacture. A film of such an amorphous alloy is often formed by sputtering, i.e., by causing ions to collide with a target to form a film on a substrate positioned near it. There are known a variety of materials which can be used for preparing a target. One of them is disclosed in JP-A-6270550. It has a mixed structure composed of a phase of an inter-metallic compound of a rare earth element and a transition metal and a phase of a transition metal alone. This material has a number of advantages including (1) high cracking resistance and (2) uniform composition of a film. However, it also has a number of drawbacks including the following: (1) It can be sputtered only at a low speed; (2) As the angular distribution of the intermetallic materials which is sputtered differs from that of the transition metal, it is only possible to form a film which is greatly different in composition from the target; (3) Its permeability is so high that only a small magnetic flux leaks from the surface of the target, resulting in a low sputtering efficiency and thereby a low efficiency of use of the target, especially when a magnetron sputtering device is employed; and (4) The surface of the target has a rapidly changing shape which causes the formation of a film changing in composition with the lapse of time. EP 0288010 was published after the filing date of this patent and is therefore only relevant under the terms of Article 54(3) EPC. The document relates to a rare earth metal/iron group metal target for a magnetic-optical disk. An alloy comprising at least one rare earth metal and at least one transition metal in a composition range which permits the formation of a eutectic structure, such as 70.2 at.%Tb-Fe, is rapid quenched and atomised. This powder is mixed with transition metal powder, such as Fe powder, and sintered. The resulting disk has a micro-structure with large regions of transition metal alone, the transition metal being iron in the specific embodiments. It is therefore, an object of this invention to provide an improved alloy target which can be sputtered at high speed, enables the formation of a film which does not greatly differ in composition from the target, has a sufficiently low permeability to achieve a high efficiency of use, and enables the formation of a film not changing in composition with the lapse of time. This object is attained by an alloy target for making a magneto-optical recording medium by sputtering comprising an alloy containing 10 to 50 atom % of at least one rare earth element selected from the group consisting of Sm, Nd, Gd, Tb, Dy, Ho, Tm and Er, the balance being, apart from impurities, at least one transition metal selected from the group consisting of Co, Fe and Ni, said alloy having a mixed structure composed of regions of an intermetallic compound formed by said rare earth element and said transition metal and regions of said rare earth element alone and including no regions of said transition metal alone. The alloy target of this invention is uniform in composition of a film, has a high toughness, can be sputtered at a high speed, has a low permeability which permits it to achieve a high efficiency of use, and enables the formation of a film of which the composition does not substantially differ from the target, or does not change with the lapse of time. Other features and advantages of this invention will be apparent from the following detailed description. DETAILED DESCRIPTION OF THE INVENTION The alloy target of this invention contains 10 to 50 atom % of at lease one rare earth element selected from among Sm, Nd, Gd, Tb, Dy, Ho, Tm and Er, with a balance consisting essentially of at least one transition metal selected from among Co, Fe and Ni. No material containing less than 10 atom %, or more than 50 atom %, of a rare earth element or elements would be able to form a film having good magneto-optical properties when sputtered. The target of this invention may contain unavoidable impurities, such as Ca, Si, C, P, S and Mn. The alloy target of this invention has a mixed structure composed of at least one region of an intermetallic compound and a region consisting solely of a rare earth element. The latter region does not only act as binding phase for the former regions which are brittle, but also is effective for achieving a high sputtering speed and enabling the formation of a film which does not substantially differ in composition from the target. In this connection, it is preferable that the rare earth element regions have a size not exceeding 500 microns, contain at least 5% by volume thereof. They may be particulate, angular or columnar, or may take any other shape without limitation in particular. It is imperative that the target be essentially free from any regions consisting solely of a transition metal. The presence of any such regions would result in a lowering of efficiency with which the target can be used for sputtering, and in the formation of a film changing in composition with the lapse of time. There is no particular limitation to the number shape or size of the regions of the intermetallic compound. The invention will now be described more specifically with reference to a few examples thereof, together with a process which can be used for manufacturing the alloy target of this invention EXAMPLE 1 A powder of Tb₄O₇ having an average particle diameter not exceeding three microns, a powder of Gd₂O₃ having an average particle diameter not exceeding three microns, a powder of iron having a particle size not exceeding 200 mesh (as determined by a Tyler standard sieve; hereinafter referred to simply as "mesh"), a powder of cobalt having a particle size not exceeding 200 mesh, metallic calcium having a particle size not exceeding 4 mesh and anhydrous calcium chloride having a grain size not exceeding 100 mesh, each having a purity of at least 99.9% by weight, were mixed together carefully to produce three target samples of Tb-Fe, Tb-Fe-Co and Gd-Tb-Fe alloys, respectively, as shown in TABLE 1. The amount of each powder as employed for producing each sample is shown in TABLE 1. The mixture was placed in a reaction vessel of the stainless steel construction and was heated to a temperature of 1000°C in about an hour in a stream of high-purity argon gas. It was held at that temperature for five hours and was thereafter cooled to an ordinary room temperature, whereby the mixture formed a lump. It was put in five liters of water and was thereby broken to yield a slurry. A supernatant suspension of Ca(OH)₂ was removed from the slurry by decantation. These steps of operation were repeated to yield an alloy powder. The alloy powder was contacted with dilute acetic acid having a pH of 4.5, washed in water and ethanol, and dried at a temperature of 50°C in a vacuum having a pressure of 1 x 10⁻² Torr. The powder had an average particle diameter of 60 microns. Five grams of a powder of metallic terbium (Tb) having a purity of at least 99.9% by weight and an average particle diameter of 20 microns were mixed with 55 g of the above described alloy powder. The mixture was sintered under heat and pressure in a graphite mold having an inside diameter of 130 mm in a vacuum having a reduced pressure of 5 x 10⁻⁵ Torr. A pressure of 150 kg/cm² was applied to the powder until it was heated to a temperature of 1000°C and a pressure of 250 kg/cm² was thereafter applied thereto, while it was held at that temperature for an hour. Then, the sintered alloy product was cooled to an ordinary room temperature and removed from the mold. A visual inspection of each sintered product did not reveal any crack thereon. No crack was found in the interior of any product, either, as a result of X-ray inspection. The composition, relative density and structure of each sample are shown in TABLE 2. The size and amount (% by volume) of the regions consisting solely of the rare earth element in each sample were determined by cutting. No regions consisting solely of a transition metal were found in any of the samples. The total amount of the rare earth element or elements in the surface of each sintered product was quantitatively analyzed by EPMA and the difference thereof was obtained. The results are shown in TABLE 3. Each of the alloy targets which had been prepared as hereinabove described and had a diameter of 130 mm and a thickness of 4.5 mm was used to form a film having a thickness of 3000 Å by sputtering on a substrate of soda glass. The sputtering was carried out by employing argon gas having a pressure of 6 x 10⁻⁵ Torr and an electric power of 4 W/cm². The sputtering operation could be continued with high stability. After sputtering, each target was inspected for cracks again as hereinabove described. No crack was found in any of the targets. Each sample was examined with respect to the following items: (1) Sputtering speed: The speed at which the film had been formed was calculated from its thickness and the time which had been required for forming it; (2) Efficiency of use of the target: The weight loss of the target was measured when the target of which the thickness had been reduced as a result of a long time of use had a minimum thickness of 0.5 mm; (3) Difference in composition between the target and the film: The total amounts of the rare earth elements in the target and the film were quantitatively analyzed by EPMA and their difference was obtained; (4) Permeability of the target: The permeability of a sample which had been cut off the target was determined by a vibrating sample magneto meter; and (5) Variation of film composition with time: Difference of the total amount of the rare earth element in the film obtained when sputtering had been continued for each of 5, 20, 40 and 60 hours was determined. The results are shown in TABLE 3. EXAMPLE 2 An alloy was prepared by melting 48.6 g of metallic terbium (Tb), 44.2 g of iron and 7.2 g of cobalt, all having a purity of at least 99.9% by weight, by arc discharge. Toluene was added to the alloy as a solvent and the mixture was crushed in a vibrating mill to form an alloy powder having an average particle diameter of 60 microns. Five grams of a powder of metallic terbium having an average particle diameter of 20 microns and a purity of at least 99.9% by weight were mixed with 55 g of the above described alloy powder. Then, the processes of EXAMPLE 1 were repeated for preparing a sintered alloy target and forming a film by sputtering. No crack was found in the target by either visual or X-ray inspection. The composition, relative density and structure of the target are shown as Sample No.4 in TABLE 2 above. The results of examination with respect to the various items as hereinabove set forth are shown in TABLE 3. COMPARATIVE EXAMPLE 29.1 g of a powder of metallic terbium having an average particle diameter of 20 microns and 30.6 g of a powder of an Fe-Co alloy having an average particle diameter of 300 microns, each of which had been prepared by a melting and rapid cooling process and had a purity of at least 99.9% by weight, were mixed in a ball mill with methyl alcohol employed as a solvent and the resulting mixture was dried in a vacuum. The mixed powder was sintered under heat and pressure in a graphite mold having an inside diameter of 130 mm. It was sintered in a vacuum having a reduced pressure of 5 x 10⁻⁵ Torr. A pressure of 300 kg/cm² was applied to the powder until it was heated to a temperature of 700°C, and the powder was held at that temperature for an hour. Then, the pressure was lowered to 10 kg/cm² and the compacted powder was heated again to a temperature of 900°C. After it had been held at that temperature for 20 minutes, it was cooled to an ordinary room temperature under no load and was thereafter removed from the mold to yield a sintered alloy target. No crack was found therein. Its composition, relative density and structure are shown as sample No. 5 in TABLE 2. It did not contain any phase composed solely of a rare earth element, but contained 7% by volume of a phase composed solely of a transition metal and having a size of about 60 microns. The target was used for forming a film by repeating the process which had been employed in EXAMPLE 1. The results of examination with respect to the various items as hereinbefore set forth are shown in TABLE 3 (Sample No. 5).

Claims: An alloy target for making a magneto-optical recording medium by sputtering comprising an alloy containing 10 to 50 atom % of at least one rare earth element selected from the group consisting of Sm, Nd, Gd, Tb, Dy, Ho, Tm and Er, the balance being, apart from impurities, at least one transition metal selected from the group consisting of Co, Fe and Ni, said alloy having a mixed structure composed of regions of an intermetallic compound formed by said rare earth element and said transition metal and regions of said rare earth element alone and including no regions of said transition metal alone. An alloy target as set forth in claim 1, wherein the said impurities comprise at least one element selected from the group consisting of Ca, Si, C, P, S and Mn. An alloy target as set forth in claim 1 or claim 2, wherein said alloy contains at least 5% by volume of said regions of said rare earth element alone. An alloy target as set forth in claim 1, 2 or 3, wherein each said region of said rare earth element alone has a size up to and including 500 microns. An alloy target as set forth in claim 1, 2, 3 or 4, wherein said regions of said rare earth element alone are particulate, angular or columnar in shape.

Inventors: MORIMOTO TOSHIO CENTRAL RES LA, NATE TATSUO CENTRAL RES LAB SU, MORIMOTO, TOSHIO CENTRAL RES.LAB.SUMITOMO METAL, NATE, TATSUO CENTRAL RES.LAB.SUMITOMO METAL
Applicants: SUMITOMO METAL MINING CO, SUMITOMO METAL MINING COMPANY LIMITED
Application year: 1988
Main classification: C23C14
IPC codes: H01F10, C23C14, H01F41, G11B11