Compositions: coating or plastic – Coating or plastic compositions – Polishes
Reexamination Certificate
2000-08-28
2002-07-23
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Polishes
C252S079100, C252S079200, C051S308000, C051S309000, C051S307000
Reexamination Certificate
active
06423125
ABSTRACT:
The present invention relates to a polishing composition suitable for final polishing of the surface of memory hard disks in the production of such memory hard disks i.e. magnetic disk substrates (hereinafter referred to simply as “substrates”) to be used for memory devices useful for e.g. computers. More particularly, the present invention relates to a polishing composition which is capable of preventing formation of fine pits, microprotrusions and other surface defects with little surface residues in the final polishing in the process for producing various substrates represented by e.g. Ni—P disks, Ni—Fe disks, aluminum disks, boron carbide disks and carbon disks and which, at the same time, is capable of polishing such substrates to provide excellent polished surfaces useful for memory hard disks with high capacities and high recording densities. Memory hard disks which are one of memory devices for e.g. computers, tend to be small in size and large in capacity year after year, and a substrate which is most widely employed at present, is one having electroless Ni—P plating applied to a blank material. Here, the blank material is one having an aluminum or other base material for a substrate shaped by lathe processing by means of a diamond grinding wheel, by lapping by means of a PVA grinding wheel prepared by solidifying SiC abrasive material, or by other methods, for the purpose of imparting parallelism or flatness.
However, by such shaping methods, it is not possible to completely remove a relatively large waviness. And, the electroless Ni—P plating will be formed also along such waviness of the blank material, and the waviness tends to remain in the substrate. Polishing is carried out for the purpose of removing the waviness of the substrate and making the surface flat.
Further, along with the trend for high capacity of the memory hard disk, the recording density is being improved at a rate of a few tens % every year. Accordingly, a space on a memory hard disk occupied by a predetermined quantity of information recorded, tends to be increasing narrow, and the magnetic force required for recording tends to be weak. Accordingly, recently, it is required to minimize the flying height of the head, which is a space between the magnetic head and the memory hard disk. At present, the flying height of the head is reduced to a level of at most 1.0 microinch (about 0.025 &mgr;m).
Further, so-called texturing may sometimes be applied to form concentrically circular scorelines on the substrate after polishing, for the purpose of preventing sticking of a magnetic head for writing and reading information to a memory hard disk or preventing formation of scorelines in a certain direction different from the rotational direction of the memory hard disk on the substrate surface by polishing whereby the magnetic field on the memory hard disk tends to be non-uniform. Recently, for the purpose of further reducing the flying height, it has been proposed to carry out light texturing to form thinner scorelines on the substrate, or to use a non-textured substrate having no scorelines without carrying out texturing. A technology for supporting such a trend for a low flying height of a magnetic head, has been developed, and the trend for low flying height of a magnetic head has been further advanced.
The magnetic head flies along the surface shape of the memory hard disk which is rotating at a very high speed. If pits of a few &mgr;m are present on the surface of the memory hard disk, it may happen that information may not completely be written on the disk, thus leading to missing of information or failure in recording information, so-called “bit error”, which causes an error.
Here, “pits” may be dents initially present in the substrate or dents formed on the substrate surface by polishing. Fine pits are dents with a diameter of less than about 10 &mgr;m, among them.
Accordingly, it is important to reduce the surface roughness of the substrate and at the same time necessary to completely remove a relatively large waviness, microprotrusions, pits and other surface defects, in the polishing process as a step prior to forming a memory hard disk.
For such a purpose, it has heretofore been common to employ single polishing finish by means of a polishing composition (hereinafter referred to also as a “slurry” from its nature) comprising aluminum oxide or other various abrasives, water and various polishing accelerators. For example, JP-B-64-436 and JP-B-2-23589 disclose a polishing composition for a memory hard disk, which is made into a slurry by adding and mixing e.g. aluminum nitrate, nickel nitrate or nickel sulfate as a polishing accelerator to water and aluminum hydroxide. Further, JP-B-4-38788 discloses an acidic polishing composition for an aluminum magnetic disk, which comprises water, a fine powder of alumina abrasive material, gluconic acid or lactic acid as a polishing accelerator, and colloidal alumina as a-surface improver.
However, with any one of such polishing compositions, it has been very difficult to satisfy, by single step polishing, all requirements to remove the surface defects or the relatively large waviness on the substrate surface, to reduce the surface roughness to a very low level in a predetermined period of time and to prevent formation of microprotrusions, fine pits and other surface defects. Accordingly, a polishing process comprising two or more steps, has been studied.
The desired degree of surface roughness is determined depending upon the process for producing the substrate, the final recording capacity as a memory hard disk and other conditions. Depending upon the desired degree of surface roughness, a polishing process may be employed which comprises more than two steps.
When the polishing process is carried out in two steps, polishing in the first step will be intended mainly for removing a relatively large waviness, large pits and other surface defects on the substrate surface, i.e. for adjusting the shape. Accordingly, a polishing composition which has a large correcting ability against the above-mentioned waviness or surface defects while minimizing deep scratches which may not be removed by the final polishing in the second step, rather than reducing the surface roughness, is required. Therefore, for the purpose of increasing the stock removal rate, an abrasive having a relatively large particle size is employed as the abrasive material in the composition.
Polishing in the second step i.e. the final polishing, is intended for minimizing the surface roughness of the substrate. Accordingly, the polishing composition is required to be able to minimizing the surface roughness and to prevent formation of microprotrusions, fine pits or other surface defects, rather than to have a large correcting ability against the large waviness or surface defects, which is required for the polishing in the first step.
Recently, in order to reduce the processing cost, an improvement has been made in the processing of the blank material by means of a PVA whetstone, and it is designed to reduce the surface roughness of the blank material prior to the use of a polishing composition, thereby to bring the quality, such as the surface roughness or waviness, of the substrate prior to polishing to a level of the quality after the polishing in the first step. If such processing is carried out, the polishing in the first step will be unnecessary, and only the so-called finish polishing may be necessary.
Therefore, as a means to reduce the surface roughness of the substrate, it has been common to employ an abrasive having a relatively small particle size as the abrasive material in the polishing composition or to use a polishing composition containing a surfactant, irrespective of polishing of the first step or the second step. For example, JP-A-5-32959 (prior art 1) discloses a polishing composition comprising water, an alumina abrasive and a fluorine type surfactant, JP-A-5-59351 (prior art 2) discloses a polishing composition for a metal material, which comprises water, an alumina abrasive
Ishibashi Tomoaki
Sugiyama Hiroyasu
Yokomichi Noritaka
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