WO1989005362A1

WO1989005362A1 - A magnetic disk with a high incidence chromium underlayer

Info

Publication number: WO1989005362A1
Application number: PCT/US1987/003130
Authority: WO
Inventors: Hudson A. Washburn; Philip L. Steiner; Eric K. Li
Original assignee: Akashic Memories Corp
Current assignee: Akashic Memories Corp
Priority date: 1987-12-07
Filing date: 1987-12-07
Publication date: 1989-06-15
Anticipated expiration: 1990-06-07

Abstract

A method and apparatus is provided for manufacturing a magnetic disk (11) which has a very high circumferential coercivity and anisotropy. The method is to sputter a chromium layer (5) onto a disk substrate (3) at a high angle of incidence to set up the seed characteristics of the later-deposited magnetic layer (7). The magnetic layer (7) is then sputter-deposited over the chromium layer (5) with a random incidence. Then the disk is finished by depositing a protective layer (9) over the magnetic layer (7). A shield arrangement is used to achieve the high angle of incidence for the chromium deposition.

Description

A MAGNETIC DISK WITH A HIGH

INCIDENCE CHROMIUM UNDERLAYER

Background of the Invention

This invention concerns a method of producing sputtered magnetic disks, and more particularly, magnetic disks having a chromium under l ayer sputter deposited at a high incidence angle. In the prior art, sputtered disks using a high incidence angle for the magnetic materials is known. For example, U.K. patents, No. 1,439,869 and No. 1,408,753, show a magnetic record member which has a substrate on which is deposited a metallic intermediate layer such as chromium. Subsequently, an iron-cobalt alloy is vapor deposited at an angle of incidence of about 60º to the substrate normal. The metallic undercoat is not obliquely deposited, however. In U.S. patent 4,426,265, a thin film magnetic disk is produced by first sputtering a chromium undercoat onto a substrate at an oblique angle of about 60º and then a magnetic layer such as an FeCoCr alloy is sputtered at about the same oblique angle. In each of these patents, the magnetic layer is deposited at high incidence and particular attention is paid to the role of pressure variations in achieving the desired magnetic properties and of using complex shielding arrangements to achieve adequate uniformity. The complex shielding employed in the prior patents tends to make them impractical for use in high throughput sputtering machines which usecylindrically symmetric sources such as the Varian Model MDP-1000 system. None of the prior art describes the role played by high incidence chromium in the undercoat in achieving the desired magnetic properties.

Summary of the Invention

In accordance with preferred embodiments of the invention, a method and apparatus is provided for manufacturing a magnetic disk which has a very high circumferential coercivity and anisotropy. The method is to sputter a chromium layer onto a disk substrate at a high angle of incidence, at least 45°, to set up the seed characteristics of the later-deposited magnetic layer. The magnetic layer is then sputter-deposited over the chromium layer with a random incidence. Then the disk is finished by depositing a protective layer over the magnetic layer. In one preferred embodiment, because of the presently available sputtering devices, the substrate with the high incidence chromium layer thereon is exposed to the air while it is transferred in preparation for sputtering of the magnetic and later layers. Hence, a flash coat of chromium is deposited over the high incidence chromium layer before the magnetic layer is deposited in order to eliminate any effects of oxidation which may have been caused by the exposure. Once the flash coat of chromium is deposited, the magnetic layer is begun within a short time thereafter, and without breaking vacuum to avoid any further oxidation. Also, the substrate is heated before deposition of the high incidence chromium and that temperature is maintained during that deposition. However, in another preferred embodiment, the substrate with the high incidence angle chromium layer is directly sputtered onto by the magnetic layer without exposure to the atmosphere.

A method of achieving this high angle of incidence for the chromium deposition is to use an annular shield which blocks sputtered material from inpinging on the substrate which does not have an angle of incidence greater than 45°.

Brief Description of the Drawings

Fig. 1 shows a. disk according to the invention.

Fig. 2 shows a conventional S-Gun source. Fig. 3 shows a disk standoff according to the invention.

Fig. 4 shows a disk standoff in position over an S-Gun for achieving high incidence chromium.

Fig. 5 shows the geometry of the disk and standoff during sputtering.

Fig. 6 is a graph illustrating the relationship between coercivity in the radial direction and in the circumferential direction with increasing angles of incidence.

Detailed Description of the Invention

In accordance with preferred embodiments of the invention, shown in FIG. 1 is a cross-section of a disk 1 such as might be used in a Winchester type drive.

The disk is a multilayered structure having a substrate 3 on which is sputter—deposited a chromium layer 5, a magnetic layer 7, and a protective layer 9. In the preferred mode, the substrate is typically constructed of computer grade aluminum alloy 5086. However, other materials such as pure aluminum, glass, silicon, or a Nickel-Phosphorous plated surface layer on aluminium may also be used. The chromium layer is critical to the structure and acts to provide the seed characteristics for the later-deposited magnetic layer 7.

According to one method of the invention which is performed using a Varian Associates 3125 sputtering system, the substrate is first cleaned by standard techniques and is heated to a temperature of 200° C ± 50º. While maintaining that temperature, the sputtering of the chromium layer 5 is begun, the layer then being sputter-deposited in a vacuum at a high angle of incidence, typically higher than 45°, with respect to the normal to the substrate, preferably higher than 55º, more preferably higher than 60°, and apparently the closer to 90º the better.

The minimum desired thickness for the chromium layer is about 1000 A and the maximum thickness is about 4500 A, with the preferred thickness being about, 2000 A ± 20%.

To be effective, the chromium layer should be quite pure, and preferably should contain less than 0. 5% by weight of other materials and should be free of cracks and bubbles. Although the high angle of incidence can be produced by a number of methods, one approach is to use a sputtering source with a circular geometry such as a modified Varian Associates S-Gun which will be described later. With that modified source, and using a chromium grade A target such as that available from Varian Associates Specialty Metals Division, for a 95mm, outside diameter substrate, the chromium is deposited using an Argon gas pressure of 3 mTorr at a power rating of 3 kW for about 90 seconds. In the preferred embodiment, the magnetic layer 7 is typically CoNiCr, or CoNi. To achieve the desired magnetic properties, the angle of incidence for the deposition of the magnetic layer should be randomized, rather than at a high angle of incidence as was done for the chromium. This can be achieved using a standard S-Gun configuration while rotating the substrate on a planetary at a distance from the source as is known in the art. Although the distance is not critical, it is generally about 12 to 20 inches in the 3125. According to this method of the invention, as soon as the chromium layer is deposited, the substrate with the high incidence chromium layer thereon is mounted on the planetary in the 3125. (The 3125 is used since it is typically equipped to handle several sputtering sources within the same vacuum housing.) Once placed on the planetary, a thin layer, or flash coat, of chromium (typically more than 50 A but less than 500 A) is deposited to cover any oxide layer that may have formed when transferring the substrate from the S-Gun to the planetary. Then the magnetic layer is deposited on the flash coat of chromium. In this configuration, a desired nominal thickness of 500 A to 700 A of CoNiCr can be deposited in about three minutes when operating at a power level of 3.0 kW at an Argon pressure of 3 mTorr. For CoNi, a thickness of 450-650 A is obtained at these operating parameters.

The protective layer 9 is preferably carbon and can be sputtered according to methods well known in the art using a standard S-Gun configuration within the 3125 or using a planar magnetron. The preferred thickness of the carbon layer is about 600 A, which can be achieved with the standard S-Gun configuration i n about 20 minutes using a power level of 1.2 kW at an Argon pressure of 8 mTorr. In another preferred embodiment, the high incidence chromium layer is initially sputter-deposited as before in a modified S-Gun in the 3125. Then the substrate with the high incidence chromium layer thereon is transferred to a Varian Associates MDP-1000 Disk Sputtering System, described in detail in U.S. Patent 4,500,407. The MDP-1000 also employs a plurality of sources with circular geometry but uses an M-Gun design such as described in U.S. Patents 4,500,408 and 4,500,409. The sources in the MDP-1000 are typically arranged in pairs, hereinafter called stations, all connected to a common vacuum chamber, in addition to having individual vacuum systems for each pair of sources. In this second embodiment, the substrate with the high incidence chromium layer thereon is placed between a pair of the circular sources, and both sides are simultaneously coated with a flash coat of chromium sputter-deposited at a random incidence angle to cover any oxide which might have formed, the thickness of this flash coat being about the same as before. The substrate is then transferred within the vacuum housing to another sputtering station where a magnetic layer is deposited simultaneously on both sides of the substrate and then to a succession of other stations where a protective overcoat is sputtered over the magnetic layer on both sides of the substrate. By using the MDP-1000, vacuum is maintained throughout the transfer of the substrate from one pair of sources to the next so that in the typical use with a short traverse time between stations (typically less than 80 seconds) oxide does not form to any significant degree between the flash coat of chromium and the deposition of the magnetic layer. To achieve high quality disks with the MDP-1000 with this process, the pressure in the main vacuum chamber is first reduced to less than 2x10^-7 Tory, and preferably to below 1x10^-7 Torr, as measured on a Varian 860 cold cathode gauge, and the system is checked to ensure that the following readings are obtained on a residual gas analyzer such as Inficon Model 017-450-G1: O₂ (mass 32) less than 0.3x10^-9, H₂O (mass 18) less than 15x10^-9 , N₂ less than 2.0x10^-9 , and H₂ (mass 2) less than 10.0x10^-9

Similarly, the individual sputtering stations are pumped by their individual cryopumps to achieve a gas pressure therein of less than 5x10^-7 Torr or lower, and all cryopumps are stabilized at a temperature of less than 15° K. Standard procedures are used to ensure adequate cooling water to the station sputtering magnetrons; and gas lines for nitrogen, argon, and compressed air, are monitored to ensure adequate pressure. At the first station in the MDP-1000, the disk with the high incidence chromium is preheated for about 35 seconds to a temperature of 200º C to 330° C to help cause desorption of any adsorbed gases and to precondition the disk for the direct sputtering of the flash coat of chromium to follow.

The flash coat of chromium is sputter-deposited in the second station of the apparatus using an argon flow rate of 20 rnl/min. ± 10% and argon pressure of less than 15 mTorr, with a cathode voltage of 550 volts and a target current of 11 amps. The typical deposition rate is about 3000 A/min.., and the sputtering time is generally about 2 seconds, to achieve a nominal 100 A for the flash coat of chromium. Other voltages, currents and times can be used to give the same thickness of chromium. A typical target for the chromium deposition is part number 02000-24-000-00A, available from Varian Associates, which is of Grade A purity (≥ 99.5% chromium by weight), having less than 250 parts per million of other materials therein, other than oxygen. The magnetic layer is then sputter deposited in the next station, using an argon flow rate of 17ml /rain. ± 10%, and an argon pressure of less than 15 mTorr, with a cathode voltage of 550 volts and a target current of 2.4 amps ± 5%. The sputtering time is typically about 22 seconds and the preferred deposition rate is about 1090 A/min. For a CoNi target, this results in a cobalt-nickel layer having a preferred nominal thickness of about 400 A. A typical target for nickel-cobalt deposition is part number 03100-27-003-300 available from Varian Associates, having nominal nickel and cobalt atomic percentages of 20% and 80% respectively, with an admixture of impurities of less than 200 parts per million. A typical target for CoNiCr has a preferred composition of 72% cobalt, 18% nickel, and 10% chromium (atomic percentages), and using the same process parameters with a sputtering time of 30 seconds results in a magnetic layer having a thickness of about 550 A.

The carbon deposition is typically performed in three successive stages in separate stations, since the carbon deposition rate is much lower than the metal deposition rates, only about 500 A/min. In all three carbon deposition stations, the typical cathode voltage is 600 volts, the target current is 4.1 amps ± 10%, and the total sputtering time at each station is about 25 seconds to achieve a total preferred nominal thickness of 600 A. Again, the argon pressure is maintained below 15 mTorr in the stations, and the argon flow rate is maintained at about 20ml/min. A typical target for the carbon deposition is Spectrographic grade carbon, having a purity of 99.999%, e.g., available from Sputtering Materials, Inc. in Santa Clara, California.

Shown in FIG. 2 is a standard Varian Associates S-Gun sputtering source. It is made up of a disk-shaped anode 11, a concave annular cathode target 13 and an annular magnetic assembly 15. Also typically included is a ground shield 17, a removable plasma confinement shield 19, a water cooling jacket 21 and an anode cooling cavity 23. To alter the configuration of the apparatus for sputtering high angle chromium for magnetic disks according to the method of the invention, a disk standoff 25 is constructed, typically of stainless steel. The standoff 25 is made up of four portions as illustrated in FIG. 3. The first portion is a sputtering shield 27 which restricts the angle of incidence of the sputtered material onto the disk. In the preferred configuration, shield 27 is typically constructed of 0.1 in. stainless steel and has an outer diameter of 5.6 in. and an inner diameter L1 typically of about 2.0 in. for a 95mm or 100mm diameter disk to be sputtered.

The second portion is a retaining ring 30 which holds the shield 27 in place inside the plasma shield 19 of the sputtering source as shown in FIG. 4.

The third portion is a standoff ring 29 having an inner diameter of 5.80 in. which is constructed typically of 0.06 in. to 0.10 in. thick stainless steel attached to retaining ring 30. The height D of standoff ring 29 is typically 0.4 in. The retaining ring 30 typically extends below standoff ring 29 a distance of about 0.1 in.

The fourth portion is a disk holder 26 which is constructed of a stainless steel annular ring having an outer diameter of 6.0 in. and a machined inner diameter L2 with a chamferred edge 28, the chamfer typically being about 30º. The distance L2 varies depending on the size of the disk being sputtered. For example, for a 100mm diameter disk, L2 is about 3.90 in. Also, the typical disk has a chamferred edge which matches the 30° edge 28, thus assisting in precise centering of the disk over the concentric elements 27, 28, 29, and 30.

FIG. 5 illustrates how the incidence angle for the chromium deposition is achieved using the sputtering shield 27. Essentially the cathode sputtering target 13 depletes primarily from the region 51. The shield 27 tends to block the path of the sputtered material from the target to the closest side of the disk as illustrated by the dotted line 53. The argon sputtering gas will cause some scattering of the sputtered chromium atoms. However, at argon presures of 3 to 15mTorr the average angle of incidence will be close to that determined by the dotted lines 54 and 55. Thus, as a practical matter, substantially only the high incidence material is incident on the disk. To achieve the desired results when using a 100mm diameter disk 11 having a 25mm diameter center hole 57, the typical geometry uses a distance L3 of 15 to 20mm and a distance L3 + L4 from the used portion 51 of the target to the disk of about 38mm. For standard S-Gun targets, the radii L5 and L6 are typically about 44.5mm and 64mm, respectively.

FIG. 6 shows the results of an experiment designed to show the results of increasing angles of incidence for the chromium deposition. As illustrated, there is a significant difference in the coercivity Hc in the circumferential direction compared to the radial direction, with that anisotropy and the absolute magnitude of the coercivity in the circumferential direction increasing dramatically for increasing angles of incidence for the chromium deposition. Such high values of the circumferential coercivity and am sotropy are very important in achieving a high quality recording medium.

As can be seen, the circumferential coercivity begins to increase at an angle A of about 30º and begins to rise much more steeply between 40º and 60º. Although it is not shown in FIG. 6, angles above 60º appear to be even better. Hence, in the preferred mode, the angle A should be greater than about 45º, preferably greater than 55º, an more preferably above 60º to achieve the desired circumferential coercivity and the desired am sotropy.

Other measures of recording quality also illustrate significant improvement using the method of the invention. For example, using the first process, on a disk with a 95mm outside diameter and a 25mm inside diameter with the sputtering being done entirely in the Varian 3125, yields a track average amplitude 2FID of 0.55mV and a resolution of 76% for the disk with a high incidence chromium layer as opposed to a track amplitude 2FID of 0.45mV and a resolution 70% when the chromium layer has a random incidence.

Similarly, m the second process, i.e. first depositing the high angle chromium layer m an S-Gun in the 3125 and then depositing the magnetic layer and the overcoat in the MDP-1000, yields a track amplitude

2FID of 0.75mV and a resolution of 74%. This is far better than using a chromium layer deposited at random incidence in the 3125 and then completing deposition in the MDP-1000 which yields a track amplitude 2FID of

0.51mV and a resolution of only 65%. (These measurements of track amplitude and resolution were obtained using a standard head having a monolithic Mn-Zn ferrite slider with a nominal gap length of 25.4 micrometers, a nominal gap of 1.1 micrometers having a nominal inductance of 14 microhenries in air at 1MHz, at a nominal flying height of 0.41 micrometers at the innermost radius of the head flight zone.)

Those skilled in the art will understand that there are many variations in the above processes to achieve the high angle of incidence of the chromium layer and the observed thicknesses of each of the layers generally. For example, the deposition times and gas pressures can be varied. Other methods or devices may also be used to achieve a high angle of incidence for the chromium layer. For example, an appropriate design change to sputter high incidence chromium within the MDP-1000 without breaking vacuum, can eliminate the step of providing a flash coat to cover oxide formed on the high incidence chromium layer when transferring the substrate to the MDP-1000 from the 3125. This design change to sputter high incidence chromium can be accomplished by providing an annular shield in the M-Gun source much like the shield used in Varian 3125.

Claims

C la ims

1. A method of manufacturing a magnetic disk comprising: sputtering a chromium layer onto a substrate at an angle of incidence greater than 45º; sputtering a magnetic layer with random incidence over said chromium layer; and depositing a protective layer over said magnetic layer.

2. The method of claim 1 wherein said substrate is heated before sputtering said chromium layer.

3. The method of claim 1 wherein after sputtering of said chromium layer, the disk is exposed to air.

4. The method of claim 3 wherein after being exposed to air, a second chromium layer is sputtered onto said chromium layer.

5. The method of claim 1 wherein the angle of incidence is greater than 55º.

6. The method of claim 1 wherein the angle of incidence is greater than 60º.

7. The method of claim 1 wherein the magnetic layer comprises CoNiCr.

8. The method of claim 1 wherein the magnetic layer comprises CoNi.

9. An apparatus for controlling angle of incidence of sputtered material onto an annular substrate when using a source and a target with circular geometry, comprising: a circular shield in the shape of an annular ring, the inner diameter of which is smaller than all portions of the target diameter where sputtering occurs; and holding means for holding said substrate concentric with said target and for holding said shield between the target and substrate to be sputtered, said holding means locating the shield and the substrate such that sputtered material is blocked which does not have an angle of incidence greater than 45º.

10. A magnetic disk comprising: substrate means having a circular periphery for providing a supporting planar surface; a chromium layer sputtered onto said substrate means at an angle of incidence greater than 45º relative to a normal through the geometric center of said surface of said substrate; and a magnetic layer sputtered with random incidence over said chromium layer.

11. A magnetic disk as in claim 10 further comprising a protective layer over said magnetic layer.

12. A magnetic disk as in claim 10 wherein said angle of incidence is greater than 55º.

13. A magnetic disk as in claim 10 wherein said angle of incidence is greater than 60º.

14. A magnetic disk as in claim 10 wherein the magnetic layer comprises CoNiCn.

15. A magnetic disk as in claim 10 wherein the magnetic layer comprises CoNi.

17. A magnetic disk as in claim 15 further comprising a protective layer of carbon over said magnetic layer.

18. A magnetic disk as in claim 17 wherein said angle of incidence is greater than 55º.

19. A magnetic disk as in claim 14 further comprising a protective layer of carbon over said magnetic layer.

20. A magnetic disk as in claim 19 wherein said angle of incidence is greater than 55º.