Abrading – Abrading process – Utilizing fluent abradant
Reexamination Certificate
2002-03-07
2003-06-24
Hail, III, Joseph J. (Department: 3723)
Abrading
Abrading process
Utilizing fluent abradant
C451S041000, C451S054000, C156S345120
Reexamination Certificate
active
06582279
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to data storage systems. More particularly, the present invention relates to an apparatus and method for reclaiming a disk substrate for use in a data storage device.
BACKGROUND
A typical data storage device includes a medium for storing data, typically in magnetic, magneto-optical or optical form, and a transducer used to write and read data respectively to and from the medium. A disk drive data storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the data storage disks at speeds typically on the order of several thousand or more revolutions-per-minute. Digital information, representing various types of data, is typically written to and read from the data storage disks by one or more transducers, or read/write heads, which are mounted to an actuator assembly and passed over the surface of the rapidly rotating disks.
In a typical magnetic disk drive, for example, data is stored on a magnetic layer coated on a disk substrate. Several characteristics of disk substrates significantly affect the areal density of a disk drive. One such characteristic that significantly affects the areal density of a disk drive is the uniformity of the surface of the disk substrate, i.e., the absence of substrate surface defects. It is generally recognized that minimizing the flyheight, i.e., the clearance distance between the read/write head and the surface of a data storage disk, generally provides for increased areal densities. It is also recognized in the art, however, that the smoothness of the surface of a data storage disk becomes a critical factor and design constraint when attempting to minimize the flyheight. A significant decrease in flyheight provided by the use of data storage disks having highly uniform recording surfaces can advantageously result in increased transducer readback sensitivity and increased areal density of the disk drive. The uniformity of disk substrate surfaces affects the uniformity of the recording surfaces because the layers sputtered onto the disk substrate, such as the magnetic layer, replicate any irregular surface morphology of the disk substrate.
Conventionally, disk substrates have been based upon aluminum, such as NiP coated Al/Mg alloy substrates. Coating the aluminum magnesium alloy with a nickel-phosphorus plate provides a harder exterior surface which allows the disk substrate to be polished and superfinished. A conventional superfinishing process and slurry is described in U.S. Pat. No. 6,236,542 to Hartog et al., which is assigned to the assignee of the present application. Typically, the Al/Mg—NiP substrate is superfinished to a smooth finish with a colloidal slurry, e.g., a pH adjusted aqueous slurry containing colloidal silica and/or colloidal alumina particles and an etching agent such as aluminum nitrate, prior to sputtering with thin film magnetic coatings. The colloidal alumina and silica slurries are then cleaned from the substrate by the general cleaning mechanisms of mechanical scrubbing, dispersion and etching. Surfactants and pH are generally used for dispersion cleaning, where the surfactant and pH act to separate the slurry particles from each other and from the substrate. Etching is generally accomplished by acids and acid soaps that erode or dissolve the substrate material beneath embedded slurry particles (under-cut) to release them from the substrate. Typical acids in use for NiP plated Al-based substrates include, for example, straight phosphoric acid, nitric acid, hydrofluoric acid-based soaps and phosphoric acid-based soaps. The straight acids generally have a pH less than 1 and the soaps generally have pH's above 1.
After cleaning, the substrates are sputtered with a series of layers, e.g., a chrome underlayer, a magnetic layer and a carbon protection layer. If residual slurry particles are left on the substrate or if there is galling to the relatively soft NiP layer, the sputtered layers replicate the irregular surface morphology, creating a bumpy surface on the finished disk. When the read/write head glides over the surface, it crashes into bumps created by the residual particles and/or damage that is higher than the glide clearance. This is known as a glide defect, which can ultimately cause disk drive failure. These bumps further cause magnetic defects, corrosion and decreased disk life. Thus, the residual slurry particles and/or damage needs to be removed from the superfinished substrate surface so that the substrate is as smooth as possible.
Unfortunately, aluminum-based substrates have relatively low specific stiffness, as well as relatively low impact and dent resistance. For example, the relatively low specific stiffness of the Al/Mg—NiP substrates (typically 3.8 Mpsi/gm/cc) makes this type of disk substrate susceptible to environmental forces which create disk flutter and vibration and which may cause the read/write head to impact and dent the disk substrate surface.
More recently, glass substrates have been used for disk drives in portable devices, such as laptop computers. Glass substrates have a higher impact and dent resistance than aluminum-based substrates, which is important in portable devices where the unit is subject to being bumped, dropped and banged around, causing the read/write head to bang on the disk substrate surface. Moreover, the specific stiffness of glass or glass-ceramic substrates (typically ≦6 or 7 Mpsi/gm/cc) is typically higher than that of aluminum-based substrates.
An additional benefit of glass is that it is easier to polish to and maintain as a smooth surface finish (as compared to NiP) than aluminum-based substrates. A smoother substrate allows the read/write head to fly closer to the disk, which produces a higher density recording. Glide height for some computer disk drives is on the order of 20 nanometers (about 200 Å) and less, which is an extremely small interface distance. Thus, the fact that glass substrates can be polished to smoother finishes makes an industry shift from Al-based substrate s to glass substrates desirable, not only for disk drives used in portable devices, but for disk drives used in stationary devices as well.
The surface uniformity of glass substrates can still present a problem, however, especially for low glide heights (typically ≦20 nanometers) and near contact recording. Just as with aluminum-based substrates, the surface of the glass substrate needs to be polished and superfinished with a slurry to provide an atomically smooth surface prior to sputtering. Such a conventional superfinishing polish process and slurry is also described in the above referenced U.S. Pat. No. 6,236,542 to Hartog et al. Typically, the glass substrate is superfinished to a smooth finish with a colloidal slurry, e.g., a pH adjusted aqueous slurry containing colloidal silica and/or colloidal alumina particles and an etching agent such as cerium sulfate, prior to sputtering with thin film magnetic coatings.
In this conventional superfinishing polish process colloidal silica particles attach to the surface being polished not only by the usual London dispersion forces, van der Waals forces and hydrogen bonding, but unlike NiP, also by molecular bonding even though the slurry has the usual stabilizing agents used in the colloidal silica to prevent the silica particles from sticking to each other (interparticle siloxane bonding), charge repulsion and/or steric stabilizers. Standard methods of scrubbing with soaps using polyvinyl alcohol (PVA) pads, ultrasonics or megasonics will not remove any significant percentage of such molecular bonded silica particles. Just as with aluminum-based substrates, if these particles are left in place on the glass substrate, glide defects occur that can ultimately cause disk drive failure. These glide defects further cause magnetic defects, corrosion and decreased disk life.
A less-than-optimal solution to this problem is to use stronger acid or base solutions than the cleaning soap, to etc
Fox Dennis L.
Hagan James A.
Hagen John Patrick
Hanson Paul Henry
Lewis Theresa Marie
Hitachi Global Storage Technologies - Netherlands B.V.
Johnson Grant A.
Nock James R.
Shakeri Hadi
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