Method and apparatus for producing a magnetic recording medium

Abstract: A method for producing a magnetic recording medium, which comprises forming, by sputtering, a thin undercoating layer and a thin cobalt alloy magnetic layer sequentially on a substrate, wherein an intermediate electrode is provided in the vicinity of the target to enclose at least 1/2 of the circumference of the sputter erosion end of the target, and at least one of the undercoating layer and the cobalt alloy magnetic layer is formed under such condition that a positive potential relative to the substrate and the grounded portion of the main body of the layer-forming apparatus, is applied to the intermediate electrode.

Description: The present invention relates to a method for producing a magnetic recording medium. Particularly, it relates to a method for producing a magnetic recording medium having a high coercive force. In recent years, coupled with the progress of the information treating techniques including computers, the level of requirements for high density recording tends to be increasingly high with respect to magnetic recording media such as magnetic disks used for external memory devices thereof. At present, it is common to employ a thin cobalt alloy layer formed epitaxially on a thin chromium undercoating layer by sputtering, as a magnetic layer for a magentic recording medium used as a longitudinal recording magnetic disk. Thus, with this thin cobalt alloy magnetic layer, it is required to impart a high coercive force as a magnetic property required for the high density recording. A number of reports have been made on such property (e.g. "New longitudinal recording media Co x Ni y Cr z from high rate static magnetron sputtering system", IEEE Trans. Magn. Mag-22, No. 5, (1986), 334; Japanese Unexamined Patent Publication No. 79233/1988; Japanese Unexamined Patent Publication No. 79968/1988). As reported in such reports, the coercive force of a thin cobalt alloy magnetic layer increases with the thickness of the chromium undercoating layer. However, when the thickness exceeds a certain upper limit value, the coercive force shows a saturation characteristic, and it becomes difficult to further increase the coercive force. For example, Japanese Unexamined Patent Publication No. 79968/1988 teaches that when the thickness of a thin chromium undercoating layer exceeds 1,500 Å, a saturation tendency is observed such that the coercive force of the magnetic layer no longer increases, and when the thickness of the undercoating layer is less than 1,500 Å, the coercive force tends to be remarkably low at a practically useless level. Further, the coercive force increases with a reduction of the thickness of the thin cobalt alloy layer. However, the reduction of the thickness of the cobalt alloy layer leads to a decrease in the reproduction output, and it is practically difficult to reduce the thickness beyond a certain level. Furthermore, the coercive force may be increased to some extent by selecting the sputtering conditions such as the pressure of the gas for forming or the substrate temperature during the formation of the magnetic layer, but such improvement is limited. It is also known to improve the coercive force of the magnetic layer by conducting sputtering under such condition that a negative bias voltage is applied to the substrate (The 35th Lecture Meeting of Applied Physics Association, Spring 1988, 29a-c-9,10). However, in a continuous layer-forming apparatus wherein the substrate moves, it is necessary to apply a negative potential to the moving substrate, whereby the apparatus is obliged to be complicated. Besides, it is necessary to apply the negative potential over a wide area i.e. not only in the vicinity of the target. Therefore, it is likely that an abnormal electric discharge takes place, which in turn leads to a damage to the layer-forming apparatus. Thus, this method has various problems as an industrial layer-forming method. It is therefore an object of the present invention to solve the above-mentioned conventional problems and to provide a method for producing a magnetic recording medium having a remarkably high coercive force. Under these circumstances, the present inventors have conducted extensive researches to improve the coercive force of the magnetic recording medium. As a result, they have discovered that the coercive force of the magnetic recording medium can be remarkably improved by providing an intermediate electrode in the vicinity of the circumference of the target and forming a thin undercoating layer and/or a thin metal magnetic layer on a substrate by sputtering while applying a positive potential to the intermediate electrode relative to the grounded portion of the main body of the layer-forming apparatus. The present invention has been accomplished on the basis of this discovery. The present invention provides a method for producing a magnetic recording medium, which comprises forming, by sputtering, a thin undercoating layer and a thin cobalt alloy magnetic layer sequentially on a substrate, wherein an intermediate electrode is provided in the vicinity of the target to enclose at least 1/2 of the circumference of the sputter erosion end of the target, and at least one of the undercoating layer and the cobalt alloy magnetic layer is formed under such condition that a positive potential relative to the substrate and the grounded portion of the main body of the layer-forming apparatus, is applied to the intermediate electrode. The present invention also provides a sputtering apparatus for forming a thin layer on a substrate by sputtering, which comprises an intermediate electrode disposed in the vicinity of a target provided to face the substrate and having a shape to enclose at least 1/2 of the circumference of the sputter erosion end of the target and a power source to apply a positive potential to the intermediate electrode. Now, the present invention will be described in detail with reference to the preferred embodiments. In the accompanying drawings: Figure 1 is a diagrammatical view illustrating one embodiment of the sputtering apparatus of the present invention. Figure 2 is a detailed view of the embodiment shown in Figure 1. Figures 3(A) to 3(F) are cross sectional views illustrating the positions and shapes of intermediate electrodes. Figures 4 to 15 are diagrammatical and detailed views illustrating other embodiments of the sputtering apparatus useful for the operation of the present invention. As the substrate for the present invention, a disk substrate made of a metal, particularly aluminum or an aluminum alloy, is usually employed. It is usual to employ the one prepared by processing an aluminum substrate to a prescribed thickness, then treating the surface for specular finish, forming a first undercoating layer of a hard non-magnetic metal such as a Ni-P alloy by electroless plating or by anodic oxidation treatment, and then sputtering chromium as a second undercoating layer. As the substrate, it is also possible to employ the one prepared by sputtering chromium as an undercoating layer directly on an aluminum substrate treated for specular finish without forming the above-mentioned first undercoating layer. There is no particular restriction as to the sputtering condition for forming the chromium undercoating layer. It is possible to employ sputtering conditions commonly used for forming a usual chromium undercoating layer or sputtering conditions as described hereinafter for the formation of a thin magnetic layer. The thickness of the chromium undercoating layer is usually within a range of from 1,000 to 3,000 Å in view of the relation to the coercive force of the magnetic layer. However, when the magnetic layer is made of a Co-Cr-Ta alloy, a high coercive force can be obtained even when the thickness of the chromium undercoating layer is less than 1,000 Å, and in this case, the thickness may be within a range of from 50 to 1,000 Å. In the present invention, the thin chromium undercoating layer and/or the thin cobalt alloy magnetic layer is formed on the substrate by sputtering under the specific condition. Now, the method for forming this thin layer will be described in detail with reference to the drawings. Figure 1 is a diagrammatical view illustrating an embodiment of the sputtering apparatus of the present invention. In this Figure, reference numeral 1 indicates a target, and a substrate holder 2 is provided to face the target. A substrate 3 is mounted on the substrate holder 2. The substrate holder 2 is movable so that the substrate 3 can be continuously subjected to the layer-forming operation. An intermediate electrode 4 is disposed in a shape to enclose the space for sputtering discharge between this target 1 and the substrate 2, more particularly, in a shape to enclose at least 1/2 of the circumference of the sputter erosion end of the target 1 in the vicinity of the target 1. Numeral 5 indicates a power source for sputtering, which is connected to the target 1 and the intermediate electrode 4. Numeral 6 indicates a power source for the intermediate electrode, which is connected to the grounded portion of the main body of the layer-forming apparatus and the intermediate electrode 4. Numeral 7 indicates the sputter erosion portion of the target 1. The sputtering power source 5 and the intermediate electrode power source 6 are preferably direct current power sources. However, RF power sources may also be employed. As the sputtering apparatus, a usual DC magnetron sputtering apparatus or RF magnetron sputtering apparatus may be employed. Figure 2 is a detailed view of the sputtering apparatus of Figure 1. Numeral 8 indicates a magnet for magnetron, numeral 9 a cathode of the DC magnetron, numeral 10 the main body of the layer-forming apparatus, and numeral 11 the grounded portion of the main body 10 of layer-forming apparatus. Figures 3(A) to 3(F) are cross sectional views illustrating the positions and shapes of intermediate electrodes 4 of the present invention. In each Figure, the electrode 4 is in a shape to enclose at least 1/2 of the circumference of the sputter erosion end of the target 1. Preferably, at least a part of the intermediate electrode 4 is located between a position distanced by 1/3, preferably 1/4, of the distance between the surface of the target and the surface of the substrate, from the surface of the target in a direction towards the substrate, and a position distanced by 30 mm, preferably 20 mm, from the surface of the target in a direction away from the substrate. Further, the intermediate electrode 4 is located between the circumference of the sputter erosion end of the target and a position distanced by 200 mm, preferably 150 mm, more preferably 120 mm, from said circumference outwardly in the plane of the target, in a shape to enclose at least 1/2 of the circumference of the sputter erosion end of the target, preferably the entire circumference of the target. It is particularly preferred that the intermediate electrode 4 is located between a position distanced by 30 mm from the surface of the target in a direction towards the substrate and a position distanced by 20 mm from the surface of the target in a direction away from the substrate, and between the circumference of the target and a position distanced by 120 mm from said circumference outwardly in the plane of the target in a shape to enclose the entire circumference of the target. If the location of the intermediate electrode 4 is outside the above ranges, the effects for improving the coercive force of the magnetic recording medium tend to be low. As the shape of the intermediate electrode 4, a shape to enclose at least 1/2 of the circumference of the target 1, such as a ring shape, a cylindrical shape or a sheet shape, is employed. If the intermediate electrode encloses less than 1/2 of the circumference of the target 1, the effects for improving the coercive force tend to be low, such being undesirable. The intermediate electrode is made usually of a metal such as stainless steel, aluminum or copper. Here, the sputter erosion end of the target is meant for a boundary between the sputtered area (sputter erosion portion) and the non-sputtered area when the surface of the target has been sputtered. For the purpose of the present invention, the circumference of this boundary is defined as the circumference of the sputter erosion end of the target. In a case of a flat plate target, the sputter erosion portion will have a ring shape having an inner circumference and an outer circumference. This outer circumference is regarded as the circumference of the sputter erosion end of the target in the present invention. The position of the sputter erosion end is determined by the size of the target and the position of the magnet for magnetron. In a case of a sputtering apparatus wherein the surface of the target is covered with a shield plate, the position of the sputter erosion end is determined by the size of such shield plate. As the target 1, a cobalt alloy containing cobalt as the main component such as Co-Cr, Co-Cr-X, Co-Ni-X, Co-W-X wherein X is at least one element selected from the group consisting of Li, Si, B, Ca, Ti, V, Cr, Ni, As, Y, Zr, Nb, Mo, Ru, Rh, Ag, Sb, Hf, Ta, W, Re, Os, Ir, Pt, Au, La, Ce, Pr, Nd, Pm, Sm and Eu, may be employed. As such target 1, an alloy comprising Co as the main component and Cr and Ta, is preferably employed. This Co-Cr-Ta alloy preferably comprises from 70 to 95 atomic % of Co, from 5 to 20 atomic % of Cr and from 0.1 to 10 atomic % of Ta. To produce a magnetic recording medium in accordance with the method of the present invention by means of the sputtering apparatus as shown in Figure 1, firstly the above-mentioned substrate 3 is attached to the substrate holder 2 of the apparatus, and then sputtering is conducted by using the above-mentioned Cr or Co alloy target 1 in the presence of a rare gas such as argon (Ar). In such case, sputtering is conducted under such condition that a positive potential relative to the substrate and the grounded portion of the main body of the sputtering apparatus, for example, a voltage of at most 1,000 V, preferably from 50 to 500 V, more preferably from 100 to 400 V, is applied to the intermediate electrode 4, to form a thin chromium undercoating layer or a thin cobalt alloy magnetic layer on the substrate 3. In the present invention, the sputtering conditions may be those commonly employed for forming a magnetic layer of a magnetic recording medium. For example, sputtering can be conducted under such conditions that the inner pressure of the evacuated chamber (background pressure) is at most 1 x 10⁻⁶ Torr, the pressure of a rare gas such as argon is from 0.5 x 10⁻³ to 2 x 10⁻² Torr, preferably from 1 x 10⁻³ to 5 x 10⁻³ Torr, and the substrate temperature is at least 150°C, preferably from 200 to 300°C. The thickness of the thin magnetic alloy layer formed by such sputtering is preferably at such a level that the product (Br·t) of the residual magnetic flux density (Br) and the thickness (t) of the thin magnetic alloy layer would be from 300 to 700 G·µm. Figures 4 to 13 are diagrammatical and detailed views of other embodiment of the sputtering apparatus suitable for the operation of the present invention. Figures 14 and 15 are diagrammatical and detailed views illustrating still another embodiment of the sputtering apparatus useful for the operation of the present invention, in which reference numeral 12 is a bias power source to apply a negative bias voltage to the substrate holder 2. To produce a magnetic recording medium in accordance with the method of the present invention by means of the sputtering apparatus as shown in Figures 14 and 15, firstly, the above-mentioned substrate 3 is mounted on the substrate holder 2, and sputtering is conducted by using the above-mentioned chromium or cobalt alloy target 1 in the presence of a rare gas such as Ar. In such case, sputtering is conducted under such condition that a positive potential relative to the substrate and the grounded portion of the main body of the sputtering apparatus, for example, a potential of at most 1,000 V, preferably from 50 to 500 V, more preferably from 100 to 400 V, is applied to the intermdiate electrode 4. More preferably, sputtering is conducted in such state that a negative bias voltage is applied to the substrate holder 2, i.e. a voltage of e.g. at least -1,000 V, preferably from -50 to -500 V, more preferably from -100 to -400 V, is applied to the substrate holder 2 by the bias power source 12, to form a thin chromium undercoating layer or a thin cobalt alloy magnetic layer on the substrate 3. A high performance magnetic layer having a high coercive force is obtained by the thin magnetic metal layer of a cobalt alloy formed by sputtering under such condition that the intermediate electrode is provided in the vicinity of the circumference of the target and a positive potential relative to the substrate and the grounded portion of the main body of the layer-forming apparatus is applied to the intermediate electrode. Now, the present invention will be described in further detail with reference to Examples and Comparative Examples. However, it should be understood that the present invention is by no means restricted to such specific Examples. In the following Examples and Comparative Examples, symbols a to e are used to identify the position and the shape of the intermediate electrode as shown in Figures. In Figures, b indicates the distance from the circumference of the target to the intermediate electrode. However, in the following Examples and Comparative Examples, the sputter erosion end of the target and the circumference of the target are the same, and b therefore indicates the distance from the circumference of the sputter erosion end of the target to the intermediate electrode. In the apparatus used in Examples 1 to 49 (Figures 1 and 2 and Figures 4 to 13), the substrate holder 2 is grounded via the main body 10 of the layer-forming apparatus. EXAMPLES 1 TO 7 AND COMPARATIVE EXAMPLE 1 By means of the apparatus as shown in Figures 1 and 2, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 1,000 Å) as the undercoating layer and a Co-Cr-Ta alloy target 1 under such condition that a positive potential as identified in Table 1 relative to the substrate was applied to the intermediate electrode 4 in the presence of Ar gas at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 4 x 10⁻³ Torr and at a substrate temperature of 210°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (400 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 1. From these Examples, it has been found that when sputtering is conducted under such condition that a positive potential relative to the substrate is applied to the intermediate electrode, the coercive force of the magnetic disk thereby obtained is substantially improved. Referring to Figures 1 and 2, the position and the shape of the intermediate electrode were as follows: a = 15 mm, b = 50 mm, c = 52 mm, d = 3 mm, e = 83 mm, Shape: a shape to enclose the entire circumference of the target. Potential applied to intermediate electrode (V) Coercive force (Oe) Br·t (G·µm) Comparative Example 1 0 798 400 Example 1 50 818 400 Example 2 100 870 400 Example 3 150 972 400 Example 4 200 1038 400 Example 5 250 1130 400 Example 6 300 1199 400 Example 7 350 1234 400 EXAMPLES 8 TO 12 AND COMPARATIVE EXAMPLE 2 By means of the apparatus as shown in Figures 4 and 5, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 1,000 Å) formed as the undercoating layer by sputtering and a Co-Cr-Ta alloy target 1 under such condition that a potential as identified in Table 2 relative to the substrate 3 was applied to the intermediate electrode 4 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (550 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 2. Referring to Figures 4 and 5, the position and the shape of the intermediate electrode were as follows: a = 3 mm, b = 5 mm, c = 3 mm, d = 43 mm, e = 83 mm, Shape: a shape to enclose the entire circumference of the target. Magnetic layer Voltage applied to intermediate electrode (V) Coercive force (Oe) Comparative Example 2 Co₈₆-Cr₁₂-Ta₂ 0 950 Example 8 " 50 1100 Example 9 " 100 1160 Example 10 " 150 1260 Example 11 " 200 1350 Example 12 " 300 1380 EXAMPLES 13 TO 18 AND COMPARATIVE EXAMPLE 3 By means of the apparatus as shown in Figures 4 and 5, sputtering was sequentially conducted by using an aluminum substrate 3 and a chromium target and a Co-Cr-Ta alloy target under such condition that a potential as identified in Table 3 relative to the substrate 3 was applied to the intermediate electrode 4 at the background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C to sequentially form a chromium layer (thickness: 1,000 Å) and an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (550 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 3. Cr undercoating layer Co-Cr-Ta magnetic layer Coercive force (Oe) Voltage applied to intermediate electrode (V) Voltage applied to intermediate electrode (V) Br·t (G·µm) Comparative Example 3 0 0 550 950 Example 13 0 100 " 1160 Example 14 100 100 " 1230 Example 15 0 200 " 1350 Example 16 200 200 " 1390 Example 17 0 300 " 1380 Example 18 300 300 " 1410 EXAMPLES 19 TO 24 AND COMPARATIVE EXAMPLE 4 By means of the apparatus as shown in Figure 6, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 1,000 Å) formed as the undercoating layer by sputtering and a Co-Cr-Ta alloy target 1 under such condition that a potential as identified in Table 4 relative to the substrate 3 was applied to the intermediate electrode 4 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (550 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 4. The position and the shape of the intermediate electrode were as follows: a = 3 mm, b = 5 mm, c = 71 mm, d = 100 mm, e = 83 mm, Shape: a shape to enclose the entire circumference of the target. Magnetic layer Voltage applied to intermediate electrode (V) Coercive force (Oe) Comparative Example 4 Co₈₆-Cr₁₂-Ta₂ 0 950 Example 19 " 50 1140 Example 20 " 100 1210 Example 21 " 150 1280 Example 22 " 200 1340 Example 23 " 300 1400 Example 24 " 400 1410 EXAMPLES 25 AND 26 AND COMPARATIVE EXAMPLE 5 By means of the apparatus as shown in Figure 7, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 1,000 Å) formed as the undercoating layer and a Co-Cr-Ta alloy target 1 under such condition that a potential as identified in Table 5 was applied to the intermediate electrode 4 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (550 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 5. The position and shape of the intermediate electrode were as follows: a = 3 mm, b = 102 mm, c = 58 mm, d = 3 mm, e = 83 mm, Shape: a shape to enclose the entire circumference of the target. Magnetic layer Voltage applied to intermediate electrode (V) Coercive force (Oe) Comparative Example 5 Co₈₆-Cr₁₂-Ta₂ 0 950 Example 25 " 100 1200 Example 26 " 200 1290 EXAMPLES 27 AND 28 AND COMPARATIVE EXAMPLE 6 By means of the apparatus as shown in Figure 8, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 1,000 Å) formed as the undercoating layer by sputtering and a Co-Cr-Ta alloy target 1 under such condition that a potential as identified in Table 6 relative to the substrate 3 was applied to the intermediate electrode 4 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (550 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 6. The position and the shape of the intermediate electrode were as follows: a = 3 mm, b = 5 mm, c = 58 mm, d = 100 mm, e = 83 mm, Shape: a shape to enclose the entire circumference of the target. Magnetic layer Voltage applied to intermediate electrode (V) Coercive force (Oe) Comparative Example 6 Co₈₆-Cr₁₂-Ta₂ 0 950 Example 27 " 150 1260 Example 28 " 200 1320 EXAMPLES 29 TO 31 AND COMPARATIVE EXAMPLE 7 By means of the apparatus as shown in Figure 9, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 1,000 Å) formed as the undercoating layer by sputtering and a Co-Cr-Ta alloy target 1 under such condition that a potential as identified in Table 7 relative to the substrate 3 was applied to the intermediate electrode 4 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (550 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 7. The position and the shape of the intermediate electrode were as follows: a = 17 mm, b = 10 mm, c = 13 mm, d = 13 mm, e = 83 mm, Shape in Examples 29 and 30: a shape to enclose the entire circumference of the target, Shape in Example 31: a shape to enclose 60% of the circumference of the target. Magnetic layer Voltage applied to intermediate electrode (V) Coercive force (Oe) Comparative Example 7 Co₈₆-Cr₁₂-Ta₂ 0 950 Example 29 " 200 1150 Example 30 " 300 1080 Example 31 " 200 1050 EXAMPLE 32 AND COMPARATIVE EXAMPLE 8 By means of the apparatus as shown in Figure 10, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 1,000 Å) formed as the undercoating layer by sputtering and a Co-Cr-Ta alloy target 1 under such condition that a potential as identified in Table 8 relative to the substrate 3 was applied to the intermediate electrode 4 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (550 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 8. The position and the shape of the intermediate electrode were as follows: a = 13 mm, b = 53 mm, c = 3 mm, d = 52 mm, e = 83 mm, Shape: a shape to enclose the entire lower circumference of the target. Magnetic layer Voltage applied to intermediate electrode (V) Coercive force (Oe) Comparative Example 8 Co₈₆-Cr₁₂-Ta₂ 0 950 Example 32 " 200 1300 COMPARATIVE EXAMPLES 9 AND 10 By means of the apparatus as shown in Figure 11, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 1,000 Å) formed as the undercoating layer by sputtering and a Co-Cr-Ta alloy target 1 under such condition that a potential as identified in Table 9 relative to the substrate 3 was applied to the intermediate electrode 4 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (550 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 9. The position and shape of the intermediate electrode were as follows: a = 17 mm, b = 10 mm, c = 13 mm, d = 13 mm, e = 83 mm, Shape: a shape to enclose 30% of the circumference of the target. Magnetic layer Voltage applied to intermediate electrode (V) Coercive force (Oe) Comparative Example 9 Co₈₆-Cr₁₂-Ta₂ 0 950 Comparative Example 10 " 200 990 EXAMPLES 33 TO 38 AND COMPARATIVE EXAMPLES 11 TO 15 By means of the apparatus as shown in Figures 4 and 5, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer with a thickness as identified in table 10 formed as the undercoating layer by sputtering and a Co-Cr-Ta alloy target 1 under such a condition that a potential as identified in Table 10 relative to the substrate 3 was applied to the intermediate electrode 4 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (550 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 10. EXAMPLES 39 AND 40 AND COMPARATIVE EXAMPLES 16 TO 22 A magnetic disk was prepapred in the same manner as in Example 33 except that a Co-Ni-Cr alloy target was used instead of the Co-Cr-Ta alloy target and a 70 atomic % Co-20 atomic % Ni-10 atomic % Cr magnetic layer (550 G·µm) was formed on the substrate at the voltage as shown in Table 10. The coercive force was measured, and the results are shown in Table 10. EXAMPLES 41 AND 42 AND COMPARATIVE EXAMPLES 23 AND 24 By means of the apparatus as shown in Figures 4 and 5, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 2,000 Å) formed as the undercoating layer by sputtering and a cobalt alloy target 1 having the composition as identified in Table 11 under such condition that a potential as identified in Table 11 relative to the substrate 3 was applied to the intermediate electrode 4 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form a magnetic layer as identified in Table 11 (550 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 11. Target or Magnetic layer Voltage applied to intermediate electrode (V) Br·t (G·µm) Coercive force (Oe) Comparative Example 23 Co₈₆-Cr₁₂-Ta₂ 0 550 1000 Example 41 " 200 " 1320 Comparative Example 24 Co₇₀-Ni₂₀-Cr₁₀ 0 " 850 Example 42 " 200 " 1190 EXAMPLES 43 TO 46 AND COMPARATIVE EXAMPLE 25 By means of the apparatus as shown in Figure 12, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 2,000 Å) formed as the undercoating layer and a Co-Cr-Ta alloy target 1 under such condition that a potential as identified in Table 12 was applied to the intermediate electrode 4 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (400 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 12. The position and the shape of the intermediate electrode were as follows: a = 1 mm, b = 1 mm, c = 5 mm, d = 56 mm, e = 200 mm, Shape: a shape to enclose the entire circumference of the target. EXAMPLES 47 AND 48 AND COMPARATIVE EXAMPLES 26 AND 27 Sputtering was conducted in the same manner as in Example 43 except that the Co-Cr-Ta alloy was changed to a Co-Cr-Ag alloy or a Co-Ni-B alloy and the voltage applied to the intermediate electrode was changed as shown in Table 12. The results are shown in Table 12. Magnetic layer Voltage applied to intermediate electrode (V) Coercive force (Oe) Comparative Example 25 Co₈₆-Cr₁₂-Ta₂ 0 940 Example 43 " 100 1380 Example 44 " 200 1500 Example 45 " 300 1550 Example 46 " 400 1570 Comparative Example 26 Co₈₆-Cr₉-Ag₅ 0 750 Example 47 " 250 1100 Comparative Example 27 Co₆₇-Ni₃₀-B₃ 0 890 Example 48 " 250 1240 EXAMPLE 49 AND COMPARATIVE EXAMPLE 28 By means of the RF magnetron sputtering apparatus as shown in Figure 13, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 2,000 Å) formed as the undercoating layer by sputtering and a Co-Cr-Ta alloy target 1 under such condition that a potential as identified in Table 13 relative to the substrate 3 was applied to the intermediate electrode 4 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (400 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 13. The position and the shape of the intermediate electrode were as follows: a = 1 mm, b = 1 mm, c = 185 mm, d = 56 mm, e = 200 mm, Shape: a shape to enclose the entire circumference of the target. Magnetic layer Voltage applied to intermediate electrode (V) Coercive force (Oe) Comparative Example 28 Co₈₆-Cr₁₂-Ta₂ 0 850 Example 49 " 100 1210 EXAMPLES 50 TO 59 AND COMPARATIVE EXAMPLES 29 TO 32 By means of the apparatus as shown in Figures 14 and 15, sputtering was conducted by using an aluminum substrate 3 having a thin chromium layer (thickness: 2,000 Å) formed as the undercoating layer and a Co-Cr-Ta alloy target 1 under such condition that a potential as identified in Table 14 was applied to the intermediate electrode 4 and a voltage from the bias power source 12 as identified in Table 14 was applied to the substrate 3 at a background pressure of at most 1 x 10⁻⁶ Torr, at an argon gas pressure of 2 x 10⁻³ Torr and at a substrate temperature of 250°C, to form an 86 atomic % Co-12 atomic % Cr-2 atomic % Ta magnetic layer (400 G·µm) on the substrate. The coercive force of the magnetic disk thus obtained was measured by a sample vibrating type magnetometer, and the results are shown in Table 14. The position and the shape of the intermediate electrode were as follows: a = 1 mm, b = 1 mm, c = 5 mm, d = 56 mm, e = 200 mm, Shape: a shape to enclose the entire circumference of the target. Voltage applied to intermediate electrode (V) Voltage applied from bias power source (V) Coercive force (Oe) Comparative Example 29 0 0 940 Example 50 100 0 1380 Example 51 200 0 1500 Example 52 300 0 1550 Example 53 400 0 1570 Comparative Example 30 0 -100 1380 Example 54 100 -100 1550 Example 55 200 -100 1590 Example 56 300 -100 1610 Comparative Example 31 0 -200 1560 Example 57 100 -200 1580 Example 58 200 -200 1600 Comparative Example 32 0 -300 1580 Example 59 100 -300 1600

Claims: A method for producing a magnetic recording medium, which comprises forming, by sputtering in a sputtering apparatus, a thin undercoating layer and a thin cobalt alloy magnetic layer sequentially on a substrate (3), characterised in that an intermediate electrode (4) is provided in the vicinity of the target (1) to enclose at least 1/2 of the circumference of the sputter erosion end (7) of the target (1), and at least one of the undercoating layer and the cobalt alloy magnetic layer is formed under such condition that a positive potential relative to the substrate and the grounded portion of the main body of the sputtering apparatus, is applied to the intermediate electrode (4). The method according to Claim 1, wherein at least a part of the intermediate electrode is located between a position distanced by 1/3 of the distance between the surface of the target and the surface of the substrate, from the surface of the target in a direction towards the substrate, and a position distanced by 30 mm from the surface of the target in a direction away from the substrate. The method according to Claim 1 or 2, wherein the intermediate electrode is located between the circumference of the sputter erosion end of the target and a position distanced by 200 mm from said circumference outwardly in the plane of the target. The method according to Claim 1, 2 or 3, wherein the undercoating layer is a thin chromium layer. The method according to any one of Claims 1 to 4, wherein the cobalt alloy magnetic layer is a thin cobalt-chromium alloy magnetic layer, a thin cobalt-nickel alloy magnetic layer or a thin cobalt-chromium-tantalum alloy magnetic layer. The method according to Claim 1, 2 or 3, wherein the cobalt alloy magnetic layer is a thin cobalt-chromium tantalum alloy magnetic layer, and the undercoating layer is a chromium layer having a thickness of from 50 to 3,000 Å. The method according to any one of Claims 1 to 6, wherein a positive potential of at most 1,000 V is applied to the intermediate electrode. The method according to any one of Claims 1 to 7, wherein a negative bias voltage relative to the grounded portion of the main body of the layer-forming apparatus is applied to the substrate. A sputtering apparatus for forming a thin layer on a substrate by sputtering, which comprises an intermediate electrode disposed in the vicinity of a target provided to face the substrate and having a shape to enclose at least 1/2 of the circumference of the sputter erosion end of the target, and a power source to apply a positive potential to the intermediate electrode with respect to the substrate and the grounded portion of the main body of the sputtering apparatus. The sputtering apparatus according to Claim 9, wherein at least a part of the intermediate electrode is located between a position distanced by 1/3 of the distance between the surface of the target and the surface of the substrate, from the surface of the target in a direction towards the substrate, and a position distanced by 30 mm from the surface of the target in a direction away from the substrate. The sputtering apparatus according to Claim 9 or 10, wherein the intermediate electrode is located between the circumference of the sputter erosion end of the target and a position distanced by 200 mm from said circumference outwardly in the plane of the target.

Inventors: ICHIKAWA MASATOSHI, KOMIZO KAZUNAGA, SAKUMA TAKESHI, YAMAGUCHI YUKIO, YOKOYAMA FUMIAKI, ICHIKAWA, MASATOSHI, KOMIZO, KAZUNAGA, SAKUMA, TAKESHI, YAMAGUCHI, YUKIO, YOKOYAMA, FUMIAKI
Applicants: MITSUBISHI CHEM IND, MITSUBISHI KASEI CORPORATION
Application year: 1989
Main classification: G11B5
IPC codes: G11B5