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MAGNETIC RECORDING DESK WITH
METAL NITRIDE TEXTURING LAYER
This invention relates to a magnetic recording disk, in particular to a disk hang a textured surface, and to the process for making the disk. The invention also relates to a rigid disk drive incorporating such a disk and having an improved head-disk interface and data readback signal.
BACKGROUND OF THE INVENTION
In one type of rotating magnetic recording rigid disk drives, each of the read/write transducers (or heads) is supported on a carrier (or slider) that rides on a cushion or bearing of air above the surface of its associated disk when the disk is rotating at its operating speed. The slider has an air-bearing surface (ABS), typically in the form of a plurality of rails, and is connected to a linear or rotary actuator by means of a suspension. There may be a stack of disks in the disk drive with the actuator supporting a number of sliders. The actuator moves the sliders radially so that each head may access the recording area of its associated disk surface.
The slider in mis conventional disk drive is biased toward the disk surface by a small force from the suspension. The ABS of the slider is thus in contact with the disk surface from the time the disk drive is turned on until the disk reaches a speed sufficient to cause the slider to ride on the air bearing. The ABS of the slider is again in contact with the disk surface when the disk drive is turned off and the rotational speed of the disk fails below that necessary to create the air bearing. This type of disk drive is called a contact start/stop (CSS) disk drive. To provide wear resistance for the ABS in a CSS disk drive, a protective overcoat may be placed on the slider rails. IBM's U.S. Pat. No. 5,159,508 describes a slider with air-bearing rails having an amorphous carbon overcoat that is adhered to the rails by a silicon adhesion layer.
The magnetic recording disk in a CSS rigid disk drive is typically a thin film disk comprising a substrate, such as a disk blank made of glass, ceramic, glassy carbon or an aluminum-magnesium (AlMg) alloy with a nickelphosphorous (NiP) surface coating, and a cobalt-based magnetic alloy film formed by sputter deposition over the substrate. A protective overcoat, such as a sputter-deposited amorphous carbon film, is formed over the magnetic layer to provide corrosion resistance and wear resistance from the ABS of the slider. A liquid fluoroether lubricant is also maintained on the surface of the protective disk overcoat to prevent damage to the head and the disk during starting and stopping of the disk.
Protective carbon overcoats for thin film disks and slider air-bearing surfaces are well known. They are typically formed by sputter deposition from a graphite target, and are generally called protective carbon overcoats, "diamondlike" carbon overcoats, amorphous carbon overcoats, or, in the case of those overcoats formed by sputter deposition in the presence of a hydrogen-containing gas, hydrogenated carbon overcoats. Tsai et at. in "Structure and Properties of Sputtered Carbon Overcoats on Rigid Magnetic Media Disks". J. Vac. Science Technology A6(4), July/August 1988, pp. 2307-2314, describe such protective carbon overcoats and refer to them as amorphous "diamondlike" carbon films, the "diamondlike" referring to their hardness rather than their crystalline structure. IBM's U.S. Pat. No. 4,778, 582 describes a protective hydrogenated disk carbon overcoat formed by sputtering a graphite target in the presence
of Ar and hydrogen (H2). The carbon overcoats may also be formed by plasma-enhanced chemical vapor deposition (CVD) and may include nitrogen in addition to hydrogen, as described by Kaufman et al., Phys. Rev. B, Vol. 39, June
5 1989, p. 13053.
In addition to the magnetic layer and the protective overcoat, the thin film disk may also include a sputterdeposited underlayer, such as a layer of chromium (Cr) or a chromium-vanadium (CrV) alloy, between the substrate and
10 the magnetic layer and a sputter-deposited adhesion layer, such as a Cr. tungsten (W) or titanium (Ti) layer, between the magnetic layer and the protective overcoat.
To improve the wear resistance of the disk, as well as to maintain consistent magnetic properties, it is desirable to
15 make the disk surface as smooth as possible. However, a very smooth disk surface in a CSS disk drive creates what is called "stiction". This means that after the slider ABS has been in stationary contact with the disk for a period of time, the slider tends to resist translational movement or "stick" to
20 the disk surface. It is known that this "stiction" force can increase over time. Thus, the stiction force measured relatively soon after a CSS cycle is called "CSS stiction", while that measured several hours after a CSS cycle is called "rest stiction". Stiction is caused by a variety of factors, including
25 static friction and adhesion forces between the disk and slider created by the lubricant or by capillary condensation of atmospheric water vapor. Stiction in a CSS disk drive can result in damage to the head or disk when the slider suddenly breaks free from the disk surface when disk rotation is
30 initiated. Because the suspension between the actuator and the slider is relatively fragile to permit the slider to fly above the disk surface, sudden rotation of the disk can also damage the suspension.
3J To avoid the stiction problem associated with CSS disk drives, some disk drives are of the "load/unload" type. In this type of drive, the slider is mechanically unloaded from the disk, typically by means of a ramp mat contacts the suspension when the actuator is retracted at power down,
^ and then loaded back to the disk when power is turned on and the disk has reached a speed sufficient to generate the air bearing. Even in load/unload disk drives, however, stiction can be a problem in the event of failure of the load/unload system
45 The more common solution to the stiction problem is to texture the disk. Typically, this is done by abrasive polishing of the disk substrate, which results in a texturing of the conforming layers deposited over the substrate. U.S. Pat. No. 5,108,781, assigned to Magnetic Peripherals, Inc.,
50 describes texturing (he disk substrate by laser heating to form a pattern of pits in the substrate surface. The disk overcoat replicates the texture of the substrate and reduces the stiction when the slider is resting on the disk overcoat. However, abrasive polishing and laser texturing of the
55 substrate adds to the disk manufacturing cost and complexity because it cannot be done in situ in the conventional sputter deposition process chamber.
IBM's U.S. Pat No. 5,053,250 describes an in-situ process for forming a textured underlayer on the disk substrate.
6o The '250 patent teaches the use of a low melting point metal material mat forms discontinuous liquid spheres as it is sputter deposited on a heated substrate. The magnetic layer and overcoat that are deposited over the solidified spheres follow this discontinuous topology, resulting in a textured
65 surface at the head-disk interface.
Texturing of the entire disk substrate, whether by abrasive polishing, laser texturing or an in-situ process, has the
additional disadvantage that the crystalline growth of the magnetic layer can be adversely affected if the texturing is not carefully controlled. This results in degraded magnetic properties, especially at high recording densities where a high signal-to-noise ratio (SNR) and a low soft error rate 5 (SER) are required. To avoid this problem, the texturing of the disk substrate may be limited to a nondata band, called the landing zone, where the slider is moved when the disk drive is stopped. The landing zone, which adds to the complexity of the drive electronics, is required to prevent the 10 substrate texturing from adversely affecting the magnetic properties of the disk in the data region.
As an alternative to texturing the substrate, texturing of the disk protective overcoat has been suggested. This can be accomplished by abrasive polishing or other mechanical 15 processes, or by chemical or laser etching as described in IBM Technical Disclosure Bulletin, October 1989, p. 264. Another type of overcoat "texturing", as described in IBM's U.S. Pat. No. 5,030.494, involves cosputtering the carbon with other material additives, such as tungsten carbide, to 20 form dusters of the additives that project above the relatively smooth carbon overcoat surface and present a discontinuous head-disk interface. These types of prior disk overcoat texturing techniques either involve additional complex and costly ex-situ process steps or result in an overcoat which is 25 not the preferred continuous film of amorphous carbon.
What is needed is a thin film magnetic recording disk that presents a continuous textured surface to the head carrier and that can be fabricated in situ using conventional processes. The disk must have reduced stiction and no degra- 30 dation in magnetic recording performance.
SUMMARY OF THE INVENTION
The invention is a thin film magnetic recording disk having a metal nitride texturing layer located between the 35 disk substrate and the top surface of the disk. In a preferred embodiment, the texturing layer comprises clusters of aluminum nitride (ATN) that are formed on the substrate under the underlayer. The A1N texturing layer is formed by sputtering an Al target in the presence of N2 gas. This results in 40 generally contiguous clusters of AIN with generally rounded upper surfaces being deposited on the substrate. The subsequently sputter-deposited underlayer, magnetic layer and disk overcoat replicate this surface, resulting in a textured disk surface at the head-disk interface. In an alternative 45 embodiment, the metal nitride texturing layer is formed above the magnetic layer, either directly on the magnetic layer or in the middle of the protective carbon overcoat The density and size of the AIN clusters in the texturing layer, and thus the texture of the completed disk at the head-disk 50 interface, are controlled by the amount of N2, the sputtering power and pressure, and the substrate temperature.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the J5 following detailed description taken in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a view in section of a schematic of a prior art ^ CSS disk drive.
FIG. 2 is an open top view of the prior art disk drive depicted schematically in FIG. 1.
FIG. 3 is a view in section illustrating the layers forming the thin film disk according to the embodiment of the present 65 invention where the texturing layer is formed on the disk substrate.
FIG. 4 is an atomic force microscope (AFM) micrograph of the topography of the outer surface of the thin film disk according to the embodiment of the present invention where the texturing layer is sputter deposited on the disk substrate.
FIG. 5 is a view in section illustrating the layers farming the thin film disk according to the embodiment of the present invention where the texturing layer is formed over the magnetic layer.
FIG. 6 is an atomic force microscope (AFM) micrograph of the topography of the outer surface of the thin film disk according to the embodiment of the present invention where the texturing layer is sputter deposited over the magnetic layer.
DETAILED DESCRIPTION OF THE
Referring to FIG. 1. there is illustrated in sectional view a schematic of a prior art CSS disk drive. The disk drive comprises a base 10 to which are secured a disk drive motor 12 and an actuator 14, and a cover 11. The base 10 and cover
11 provide a substantially sealed housing for the disk drive. A gasket 13 is located between base 10 and cover 11 and a small breather port (not shown) equalizes pressure between the interior of the disk drive and the outside environment.
A magnetic recording disk 16 is connected to drive motor
12 by means of hub 18 to which it is attached for rotation by the drive motor 12. A lubricant film 40 is maintained on the surface of disk 16. The substrate for disk 16 is typically made of glass, ceramic, glassy carbon or an AlMg alloy having a textured NiP surface coating. The texturing of the substrate, which is most commonly done by abrasive polishing, causes a replication of the textured topography in the subsequently deposited underlayer, magnetic layer, and protective overcoat so that the overcoat presents a textured outer surface at the head-disk interface. In CSS disk drives, the substrate may be textured only in a nondata region referred to as the landing zone, where the slider 20 is moved when the disk drive is stopped.
A read/write head or transducer 25 is formed on the trailing end of a carrier, such as an air-bearing slider 20. Transducer 25 may be an inductive read and write transducer or an inductive write transducer with a magnetoresistive (MR) read transducer. The slider 20 is connected to the actuator 14 by means of a rigid arm 22 and a suspension 24. The suspension 24 provides a biasing force that urges the slider 20 toward the surface of the recording disk 16.
During operation of the disk drive, the drive motor 12 rotates the disk 16 at a constant speed, and the actuator 14, which is typically a linear or rotary voice coil motor (VCM), moves the slider 20 generally radially across the surface of the disk 16 so that the read/write head may access different data tracks on disk 16.
FIG. 2 is a top view of the interior of the disk drive with the cover 11 removed, and illustrates in better detail the suspension 24 that provides a force to the slider 20 to urge it toward the disk 16. The suspension may be a conventional type of suspension, such as the well-known Watrous suspension, as described in IBM's U.S. Pat. No. 4,167,765. This type of suspension also provides a gimbaled attachment of the slider which allows the slider to pitch and roll as it rides on the air bearing. The data detected from disk 16 by the transducer 25 is processed into a data readback signal by signal amplification and processing circuitry in the integrated circuit chip 15 located on arm 22. The signals from transducer 25 travel via flex cable 17 to chip 15, which sends its output signals via cable 19.
The thin film disk 5* according to the present invention is illustrated in section in FIG. 3. When used in place of the prior art disk 16 shown in FIGS. 1 and 2, disk 50 results in a disk drive with an improved head-disk interface but without adverse effects on SNR and SER. The disk SO comprises a glass disk blank as substrate 51, a texturing layer 52 formed directly on glass substrate 51, a Cr underlayer 54 formed on the substrate 51, a CoPtCr alloy magnetic layer 56 formed on the Q underlayer 54, a protective overcoat 60 formed on the magnetic layer 56, and a lubricant film 58 on the protective overcoat 60. Alternative substrates usable with the present invention may be made of silicon (Si), silicon-carbide (SIC), ceramic, glassy carbon or an AlMg disk blank with a NiP surface coating. The texturing layer 52 comprises clusters of aluminum nitride (AIN) that have generally spherically-shaped or rounded surfaces. Underlayer 54 and magnetic layer 56 are formed by conventional sputter deposition. The disk overcoat 60 may be formed of any conventional disk overcoat material. However, in the preferred embodiment, the overcoat 60 is hydrogenareal essentially amorphous carbon. The overcoat 60 may also be doped with nitrogen.
Disks as shown in FIG. 3 were fabricated using smooth glass substrates (average surface roughness Ra=5A) in a DC magnetron sputtering system having multiple sputtering chambers. The glass substrates were first washed using detergent and distilled water. As part of the normal manufacturing process, the substrates were heated to 180° C. (+70° C). The substrates are heated to improve the later deposition of the cobalt alloy magnetic layer. In the first chamber, containing a commercially available target of aluminum (99.99% purity), argon (Ar) and nitrogen (Ni) gases were introduced. The Ar/N2 volumetric flow rate ratio was approximately 5:1, and is preferably in the range of 10:1 to 3:1. The DC sputtering power was 500 watts and is preferably in the range of 200-1000 watts. Sputtering pressure was maintained in the range of 5-20 mTorr. During this sputtering step, the nitrogen gas reacts with the Al from the sputtering target and clusters consisting essentially of the compound AIN are formed on the glass substrate. The clusters are generally contiguous so that the texturing layer 52 can be described as an AIN layer having rounded bumps over its top surface. The thickness of the texturing layer 52 is controlled by controlling the deposition time. In the preferred embodiment, the texturing layer 52 has a mean thickness in the range of 100-300A.
The formation of the rounded AIN clusters is dependent on the substrate temperature, the sputtering pressure and power and the Ar/N2 volumetric flow rate ratio. It has been discovered that for the specific Intervac brand sputtering system used if power is below approximately 200 watts, the AIN layer has no texturing; and if the power is above approximately 700 watts, the outer surface of the AIN layer is no longer in the desired shape of rounded bumps, but has generally random projections with discontinuous peaks and valleys.
Following the deposition of the texturing layer 52, the disks were moved to the next successive sputtering chambers where the 500-1000A Cr underlayer 54 and the 300-700A CoPtCr magnetic layer 56 were formed in an Ar-only atmosphere. Finally, the disks were moved to the final sputtering chamber where the hydrogenated carbon overcoat 60 was formed to a thickness of 100-250A in an Ar-H2 atmosphere.
While die disks were made in a sputtering system with separate isolated sputtering chambers, it is also possible to
make the disks using an in-line system where there is only a single vacuum chamber and the disks are moved past the different sputtering targets in succession. In this type of system, such as the commercially available Ulvac and
5 Leybold brand systems, the N2 gas is added only in the region of the Al sputtering target.
FIG. 4 is an atomic force microscope (AFM) micrograph of the topography of the outer surface of the carbon overcoat 60 of the disk with the texturing layer 52 formed directly on
10 the glass substrate 51. As is apparent, the outer surface of the overcoat 60 has replicated the texturing provided by the clusters of AIN. FIG. 4 shows that the in-plane spacing of the tops of the clusters is on the order of 0.1 microns (lOOOA) which is significantly greater than the thickness of the
15 cobalt-based alloy magnetic layer 56.
The SNR measured at 3000 flux reversals/mm for disks made with varying amounts of N2 gas in the sputtering chamber showed a generally linear increase from approximately 32.5 dB to approximately 35 dB as N2 was increased
20 from 0 to 20% of the Ar-N2 gas mixture. These values indicate that die addition of the N2 to the Al favorably controls the crystallographic preferred orientation and grain structure of the Cr underlayer and the CoPtCr magnetic layer. X-ray diffraction analysis of the Cr layer and the
25 CoPtCr magnetic layer deposited over the AIN texturing layer shows a substantial decrease in the  preferred orientation in the Cr layer and thereby the  preferred orientation in the CoPtCr magnetic layer. Furthermore, a strong (10.0) peak was observed in the CoPtCr magnetic
30 layer, indicating the alignment of the C-axis in the plane of the magnetic layer which enhances the recording performance.
The measured SER for the disks showed significant
35 improvement at high linear recording densities (4000-6000 flux reversals/mm) compared with conventional mechanically textured disks. This improvement is due to the absence of the so-called "texture-induced noise" present in mechanically textured disks.
40 The density and size of the AIN clusters can be controlled by the amount of N2, the sputtering power and pressure, and the substrate temperature. At a given sputtering power, the duster size decreases with increasing N2. The cluster density is controlled by substrate temperature, sputtering pressure.
45 and the surface energy of die substrate. For example, an increase in substrate temperature reduces the density of the dusters. In the above experimental examples, the substrates were heated. However AIN clusters have been successfully formed on substrates without the application of heat. While
50 the process was described for disks with glass substrates, the AIN clusters have also been successfully formed on substrates of ceramic (e.g., Memcor brand substrate from Corning Glass), glassy carbon (e.g., the amorphous carbon substrate available form Kobe Precision), semiconductor grade
55 Si wafer, and AlMg with a NiP surface coating. For each of these substrates different process parameters of N2 concentration, sputtering power and pressure, and substrate temperature must be experimentally selected to deposit the AIN clusters at the desired size and density.
60 Thin film disks were also fabricated as described above but with the texturing layer 52 formed either directly on the magnetic layer 56 or, as shown in the embodiment of FIG. 5, on a first carbon layer 61 of 50A thickness with a second carbon layer 62 of 100A thickness being deposited directly
65 on the texturing layer 52. In the embodiment of FIG. 5, the carbon overcoat thus includes first carbon layer 61, texturing layer 52, and second carbon layer 62. The process is the