Abstract:
A method for texturing a substrate for a magnetic disk comprises abrading the substrate using nano-sized diamond particles (e.g. having a diameter less than or equal to 20 nm). A magnetic layer is then deposited over the substrate. Even when the texture is extremely smooth (e.g. less than about 2.2 Å Ra), the disk still exhibits good Hc and MrT orientation ratios, SNR and PW50.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention pertains to methods for texturing magnetic disk substrates and the resulting substrates. This invention also pertains to methods for manufacturing magnetic data storage media and the resulting media.  
           [0002]    Magnetic disks (e.g. disk  1  of FIG. 1) are manufactured by a) polishing a substrate; b) texturing the substrate; and c) depositing a set of layers on the substrate, e.g. by sputtering or other vacuum deposition technique. The substrate typically comprises an aluminum alloy  2  coated with an electroless-plated nickel phosphorus (“NiP”) alloy  3 . The layers can comprise one or more seed and/or underlayers  4  (e.g. Cr or a Cr alloy underlayer), one or more magnetic layers  5  (e.g. a Co alloy layer), and one or more protective overcoats  6  (e.g. one or more carbon protective overcoat layers). The texture typically comprises scratches in the circumferential direction (or generally in the circumferential direction) formed using polycrystalline or monocrystalline diamond particles having a size of about 0.05 to 0.5 μm in diameter. The texture typically accomplishes several functions, including facilitating the flying of a read-write head above the magnetic disk. However, more importantly, the texture causes the magnetic characteristics of the disk to become anisotropic. For example, the coercivity He of the magnetic layer is higher in the circumferential direction than the radial direction. Also, the magnetic remanence Mr is higher in the circumferential direction than the radial direction.  
           [0003]    It has been a trend in magnetic disk manufacturing that disks have been getting smoother and smoother. This is in part because it is necessary to permit the read-write head to fly closer and closer to the magnetic layer. Current disks have a Ra of about 3 to 5 Å as measured by an atomic force microscope (“AFM”). (Ra is a well-known measure of roughness.) Unfortunately, as disks become smoother, and as the Ra drops below 5 Å, the magnetic anisotropy of the disk drops. Further, other magnetic characteristics of the disk are degraded, e.g. the signal to noise ratio (“SNR”) and the pulse width PW50.  
           [0004]    In prior art diamond texturing, the diamond particles of texturing slurries have highly variable sizes. Because of this, the resulting texture scratches have highly variable sizes (i.e. variable widths and depths). This, in turn, can cause a) magnetic defects (e.g. because the effective flying height between the read-write head and magnetic layer increases at locations where the texture scratches are too deep); and b) reliability problems caused by the fact that deep scratches can make it difficult to passivate the disk. Further, the presence of oversized particles in the texturing slurry results in poor glide characteristics because excessively high ridges can be formed adjacent excessively deep scratches.  
         SUMMARY  
         [0005]    During a method in accordance with the present invention, “nano-sized” particles are used to texture a substrate. The particles are typically diamond, and typically have a diameter less than about 20 nm. In one embodiment, the particle diameters are less than about 10 nm. The particle diameters are typically greater than about 1 nm, and in one embodiment greater than about 2 nm. In one embodiment, they are between 3 and 8 nm. (The word “diameter” as used herein does not require that the particles are spherical.) The particles are typically monocrystalline. In one embodiment, they are used to form scratch lines having a density greater than 50 lines per micron.  
           [0006]    After texturing, one or more layers are deposited on the substrate, including a magnetic alloy layer (e.g. a Co or Fe based alloy layer) to thereby form a magnetic data storage medium.  
           [0007]    We have discovered that by using “nano-sized particles” (e.g. nano-sized diamond particles) for texturing a magnetic disk we can form extremely smooth textures without sacrificing such magnetic characteristics as anisotropy, SNR and PW50.  
           [0008]    In one embodiment, the slurry has generally uniform particle sizes (e.g. between 3 and 8 nm). Because of this, the texture lines formed by the slurry will have uniform widths and depths. This is important because it is desirable to avoid gouges (e.g. to avoid magnetic defect and passivation problems discussed above) and to avoid forming high ridges (e.g. to thereby avoid glide height problems). Further, the nano-sized diamond particles also result in a reduction of large low frequency texture lines. Also, the scratch size and density of scratch lines help one to control the magnetic layer grain size. (It is desirable to form a magnetic layer comprising small uniform grain sizes.)  
           [0009]    In one embodiment, the nano-sized particles are free abrasive particles, e.g. within a liquid (for example an aqueous slurry). In another embodiment, the nano-sized particles are fixed abrasive particles, e.g. bound to an abrasive cloth or other structure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 schematically illustrates in cross section a prior art magnetic disk.  
         [0011]    [0011]FIGS. 2A and 2B schematically illustrate in plan view and cross-section apparatus for texturing a magnetic disk substrate.  
         [0012]    [0012]FIG. 3 illustrates the relationship between MrT OR (the MrT orientation ratio) vs. roughness Ra for disks comprising substrates textured with conventional diamond particles and nano-sized diamond particles. FIG. 3 also illustrates the relationship between SNR and roughness Ra for disks comprising substrates textured with conventional diamond particles and nano-sized diamond particles.  
         [0013]    [0013]FIGS. 4A, 4B and  4 C are SEM photographs of prior art diamond texturing particles.  
         [0014]    [0014]FIGS. 5A and 5B are SEM photographs of nano-sized diamond particles used during a method in accordance with the present invention.  
         [0015]    [0015]FIG. 6A is an AFM photograph of a first substrate textured using prior art diamond texturing particles.  
         [0016]    [0016]FIG. 6B is a cross section AFM scan of the substrate of FIG. 6A.  
         [0017]    [0017]FIG. 6A′ is an AFM photograph of a second substrate textured using prior art diamond texturing particles.  
         [0018]    [0018]FIG. 6B′ is a cross section AFM scan of the substrate of FIG. 6A′.  
         [0019]    [0019]FIG. 7A is an AFM photograph of a substrate textured using nano-sized diamond texturing particles.  
         [0020]    [0020]FIG. 7B is a cross section AFM scan of the substrate of FIG. 6A.  
         [0021]    [0021]FIG. 8 illustrates a magnetic disk drive comprising a disk formed using a method in accordance with our invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]    [0022]FIGS. 2A and 2B illustrate apparatus  11  for texturing a substrate  12 . (Apparatus  11  is not novel in and of itself.) In FIGS. 2A and 2B, a motor (not shown) rotates substrate  12  while a sheet  13  of material (typically nylon) is pushed against substrate  12 . (Sheet  13  moves off of a supply reel  14 , around a roller  15 , and onto a take-up reel  16  (see arrow B).) Roller  15  urges sheet  13  against substrate  12 , e.g. with a force between 1 and 10 pounds. In one embodiment, the force is between 2.5 and 5 pounds. A slurry comprising nano-sized diamond particles (described below) is introduced between sheet  13  and substrate  12 . Simultaneously, sheet  13  and roller  15  reciprocate, moving back and forth in a direction A. (Alternatively, substrate  12  can reciprocate instead of sheet  13  and roller  15 .) The diamond particles form texture scratches that are non-random, and are in a generally circumferential direction in substrate  12 . Because of the motion of sheet  13  and roller  15  in the direction of arrow A, the resulting texture exhibits a cross-hatch. (The texture lines therefore intersect, e.g. at an angle between between 0 and 20 degrees, and in one embodiment a range of 2 to 6 degrees, although these values are merely exemplary.) In one embodiment, apparatus  11  is device model no. 1800, available from EDC Corporation located in California. The slurry flow rate is typically between 0.1 and 1 ml/second. The above-described apparatus and parameters are merely exemplary. Other types of texturing apparatus and parameters can also be used.  
         [0023]    In one embodiment, the slurry comprises a commercial coolant or lubricant and between 0.4 to 1 gram/liter of diamond particles, e.g. about 0.4 grams per liter of diamond particles. As mentioned above, the diamond particles can have a diameter from 2 and 8 nm.  
         [0024]    As mentioned above, the size and spacing of the texture marks depend at least in part on the size of the diamond particles. Using particles having sizes greater than or equal to 2 nm helps ensure a certain minimum grain size in the subsequently formed magnetic layer. (If the grains are too small, their magnetization state may be thermally unstable. However, in other embodiments, diamond particles less than 2 nm in size can be used.)  
         [0025]    Description of Types of Nano-Diamond Particles Used with the Invention  
         [0026]    In one embodiment, the particles can be formed using a method as described by Vereschagin et al. in U.S. Pat. No. 5,861,349, incorporated herein by reference. Such particles are available from Ultradiamond Technologies, Inc. of Somerville, Mass. (e.g. product no. UD90). Alternatively, the particles can be of the type available from PlasmaChem of Mainz, Germany. They are typically formed at a high temperature and pressure with an explosion. In one embodiment, the diamond particles comprise about 90% or more of diamond, with some ash and/or oxidatable carbon making up the remainder.  
         [0027]    Manufacture of a Magnetic Disk Comprising a Substrate Textured Using the Present Invention  
         [0028]    After texturing, one or more underlayers, magnetic layers and protective overcoats are applied to the disk. In one embodiment, the underlayers and magnetic layers can be as described in U.S. patent application Ser. No. 10/075,123, filed by Bertero et al. and incorporated herein by reference or U.S. Pat. No. 6,150,015, issued to Bertero et al. and also incorporated herein by reference. The overcoat layers can be as described in U.S. patent application Ser. No. 09/604,490, filed by Lairson et al. or German patent document DE 101 30 942 A1, each being incorporated herein by reference. However, the specific processes described by Bertero and Lairson are merely exemplary, and other processes could also be used. For example, other disk manufacturing processes in which one or more ferromagnetic layers (e.g. Co or Fe based magnetic layers) are deposited by a vacuum deposition process (e.g. sputtering) can be used. Also, other types of layers can be deposited on the substrate during the disk manufacturing process.  
         [0029]    Properties of a Magnetic Disk Using a Substrate Textured with Nano-Diamond Particles  
         [0030]    A magnetic disk manufactured using nano-sized diamond particles exhibits several surprising and unique characteristics. As mentioned above, prior to the present invention, substrates were textured using diamond particles having a diameter between 0.05 and 0.5 μm. FIG. 3 illustrates the relationship between the MrT orientation ratio (MrT OR) for disks comprising substrates manufactured using such particles. (MrT equals magnetic remanence times thickness of the magnetic layer.) Curve  22  shows the MrT OR for disks manufactured using diamond particles in accordance with the prior art, whereas curve  24  shows the MrT OR for disks manufactured using nano-sized diamond particles (in this case having diameters between 3 and 8 nm with an average diameter of 5 nm). As can be seen, when using the prior art texturing particles (curve  22 ), MrT OR dropped precipitously when the Ra dropped below about 2.2 Å. Thus, one skilled in the art, using these particles, would be led to believe that one could not use a Ra less than 2.2 Å without sacrificing MrT OR. However, as demonstrated in curve  24 , we have discovered that when using nano-sized diamond particles, one can maintain MrT OR high (e.g. about 1.6) even for Ra&#39;s of 1.4 to 1.5 Å. This is an important and surprising discovery.  
         [0031]    [0031]FIG. 3 also shows that for a magnetic disk textured with the prior art diamond particles, when the Ra dropped below 2.2 Å, the SNR dropped precipitously. (See curve  26 .) Surprisingly, when one textures the substrates with nano-sized diamond particles, even at a Ra of about 1.4 to 1.5 Å, the SNR remained high—about 0.45 dB. (See curve  28 .) Again, this result was completely unexpected.  
         [0032]    [0032]FIGS. 4A, 4B and  4 C are SEM photographs of prior art diamond texturing particles at 60,000 ×, 300,000 × and 500,000 × magnification, respectively, whereas FIGS. 5A and 5B are SEM photographs of nano-sized diamond texturing particles used during a texturing method in accordance with the invention at magnifications of 60,000 and 500,000 ×, respectively. As can be seen, the prior art particles are larger than the nano-sized particles. Further, there is considerable size variation in the prior art particles, and the prior art particles have numerous jagged and irregular facets. This leads to highly variable and undesirable scratch characteristics.  
         [0033]    As mentioned above, FIGS. 5A and 5B are SEM photographs of nano-sized diamond particles used to texture magnetic disks in accordance with the present invention. As can be seen, the particles are small, have roughly the same size. As discussed above, the small size and uniformity of particles permits one to form small, uniform scratch marks.  
         [0034]    FIGS.  6 A and  6 A′ are AFM photographs of first and second substrates textured using prior art diamond particles. FIGS.  6 B and  6 B′ are AFM cross section scans of the substrates of FIGS.  6 A and  6 A′, respectively. FIGS. 7A and 7B are an AFM photograph and cross section scan, respectively, of a substrate textured using nano-sized diamond particles. Table I below lists various parameters relating to the surface finish of the substrates of FIGS.  6 ,  6 ′ and  7 .  
                                                   TABLE I                           (Distances in angstroms)                AFM   Distance between mean   Distance between mean       Substrate   Ra   surface and peaks   surface and valleys                        3.9   16.8   32.0       FIG. 6A′,   2.8   14.9   23.9       6B′           1.6   8.2   13.2                  
 
         [0035]    As can be seen, the scratch lines of the substrate of FIGS. 7A and 7B are shallower and more regular compared to the scratch lines of FIGS. 6A, 6B,  6 A′ and  6 B′. As mentioned above, this has the following advantages: a) the effective flying height of a read-write head with respect to the magnetic layer is reduced; b) there are fewer or no deep gouges for causing magnetic defects; c) the substrate of FIGS. 7A and 7B is easier to passivate; d) there are fewer or no ridges for a read-write head to collide with; and e) the texture grooves of FIGS. 7A and 7B facilitate forming smaller magnetic grains.  
         [0036]    In one embodiment, the texture scratch lines formed using a method in accordance with the invention have a density greater than or equal to 50 per micron and less than or equal to 150 per micron. In one embodiment, the scratch density is between about 50 and 120 per micron, e.g. about 80 per micron. (Typically, the minimum scratch density is inversely proportional to the particle diameter, whereas the maximum scratch density is inversely proportional to {fraction (1/10)} of the particle diameter.)  
       Industrial Application  
       [0037]    Magnetic disks are incorporated into disk drives, e.g. disk drive  30  of FIG. 8. Referring to FIG. 8, disk  32  is mounted on a spindle  33 , which is rotated by a motor  34 . A pair of read-write heads  35   a,    35   b  are held on suspensions  36   a,    36   b,  which in turn are mounted on an actuator  38  for moving heads  35   a,    35   b  over the various tracks of disk  32 . During use, heads  35   a,    35   b  “fly” above disk  32 , and are used to read data from or write data to disk  32 . (It will be appreciated that both sides of disk  32  are textured, and the various layers used to manufacture a magnetic disk are formed on both sides of disk  32 , although in other embodiments, one can texture and deposit these layers on only one side of disk  32 .)  
         [0038]    While the invention has been described with respect to specific embodiments, those skilled in the art will appreciate that changes can be made in form and detail without departing from the spirit and scope of the invention. For example, different types of substrates can be textured using the above-described techniques. Thus, the substrate can be an aluminum alloy disk coated with a nickel phosphorus alloy. However, materials other than NiP and aluminum alloys can be used (e.g. glass or glass ceramic substrates), and substrate shapes other than disks can be used (e.g. for media that is not disk-shaped). Different types of texturing apparatuses, with different parameters, can also be used. Different sized particles (e.g. having a size from 0.5 to 20 nm) can be used.  
         [0039]    While the method of the present invention can be used to manufacture disks used in longitudinal magnetic recording, the method can also be used to form disks used in vertical recording. (As mentioned above, controlling the scratch sizes can be used to control or influence the magnetic layer grain size. This is useful in both longitudinal and vertical recording.) The method of the present invention can also be used to form isotropic media. While the texturing particles are typically diamond, other hard materials can also be used. Accordingly, all such changes come within the invention.