Compositions: coating or plastic – Coating or plastic compositions – Polishes
Reexamination Certificate
1999-06-04
2001-02-27
Marcheschi, Michael (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Polishes
C051S308000, C051S309000, C252S079100, C510S165000, C510S397000, C438S692000, C438S693000
Reexamination Certificate
active
06193790
ABSTRACT:
POLISHING COMPOSITION
The present invention relates to a polishing composition suitable for finish polishing of the surface of substrates for memory hard disks, i.e. magnetic disks to be used for memory devices used for computers, etc., in the production of such memory hard disks. More particularly, the present invention relates to a polishing composition which is useful for finish polishing of disk substrates (hereinafter referred to as “substrates”) to be used for memory hard disks represented by Ni-P disks, Ni-Fe disks, aluminum disks, boron carbide disks and carbon disks and which is applicable to a production technique, whereby it is possible to prevent formation of micro protrusions, micro pits and other surface defects and to provide a polishing rate not lower than conventional polishing compositions, and at the same time, it is possible to obtain an excellent finished surface useful for magnetic disk devices with high capacities and high recording densities.
Memory hard disks to be used for magnetic disk devices as memory media for computers, etc., tend to be small in size and large in capacity year after year, and magnetic media are changing from conventional coating type media to thin film media by means of a sputtering method, a plating method or the like.
Substrates which are most widely used at present, are ones having electroless Ni-P plating applied to a blank material. Here, a blank material is one obtained by shaping an aluminum or other base plate as a base material for a substrate, by lathe processing by means of a diaturn for the purpose of providing parallelism or flatness, by lapping by means of a PVA grindstone prepared by binding SiC abrasive material, or by other methods. However, by the above-mentioned various shaping methods, it is not possible to completely remove relatively large waviness. Accordingly, when electroless Ni-P plating is applied to such a blank material, a film will be formed along such waviness, whereby the resulting substrate will also have the waviness. Polishing is carried out for the purpose of removing such waviness of the substrate and smoothing the surface.
Along with the increase in the capacity of memory hard disks, the area recording density is increasing at a rate of a few tens % per year. Accordingly, the space on a memory hard disk occupied by a predetermined amount of information recorded, tends to be very narrow, and the magnetic force required for recording tends to be weak. Accordingly, with a recent magnetic disk device, it is required to reduce the head flying height which is the space between the magnetic head and the memory hard disk, and the head flying height has now been reduced to a level of not higher than 0.02 &mgr;m.
Further, so-called texturing may be applied to provide a concentric texture to a substrate after polishing, for the purpose of preventing sticking to the memory hard disk of a magnetic head for reading and writing information, or preventing a possibility that the magnetic field on a memory hard disk becomes non-uniform due to a texture in one direction which is different from the rotational direction of the memory hard disk, formed by polishing the substrate surface. Recently, light texturing is carried out to reduce the texture provided on the substrate for the purpose of further reducing the flying height of the head, and a non-textured substrate is now being employed which has no texturing applied. A technique to support such reduction of the flying height of the magnetic head has been developed, and reduction of the flying height of the head is progressing more than ever.
A magnetic head flies along the shape of the surface of a memory hard disk which is rotated at a very high speed. If there is a micro protrusion of a level of a few &mgr;m on the memory hard disk surface, so-called “head crush” is likely to occur, and the head crushes against the protrusion, whereby the magnetic head and/or the magnetic medium on the surface of the memory hard disk may be damaged, thus leading to a failure of the magnetic disk device or an error in reading or writing information.
On the other hand, if pits are present on a memory hard disk, information can not be completely written, thus leading to omission of information or failure in reading out of information, so-called “bit falls”, and causing an error.
Here, “pits” are dents initially present in the substrate or dents formed on the substrate surface by polishing, and micro pits are dents having a diameter of less than about 10 &mgr;m, among them.
Accordingly, it is important to reduce the roughness on the substrate surface, and at the same time, it is necessary to completely remove relatively large waviness, micro protrusions or pits and other surface defects, in the step before forming a magnetic medium i.e. in the polishing step.
For the above-described purpose, it has been heretofore common to carry out finishing by a single polishing operation by means of a polishing composition (hereinafter referred to as a “slurry” from its nature) comprising an aluminum oxide or other various abrasives and water, and various polishing accelerators. For example, JP-B-64-436 and JP-B-2-23589 disclose a polishing composition for memory hard disks, prepared by adding and mixing aluminum nitrate, nickel nitrate or nickel sulfate as a polishing accelerator to water and aluminum oxide, to form a slurry. Further, JP-B-4-38788 discloses an acidic polishing composition for aluminum magnetic disks, which is prepared by adding gluconic acid or lactic acid as a polishing accelerator and colloidal alumina as a surface-modifier, to water and fine powder of alumina abrasive. Still further, JP-A-7-216345 discloses a polishing composition comprising water, an alumina abrasive and a polishing accelerator, wherein the polishing accelerator comprises a molybdenate and an organic acid.
However, with any one of the above polishing compositions, it has been very difficult to satisfy all of the requirements for removing relatively large waviness and surface defects on a substrate surface, finishing to reduce the surface roughness to a very low level in a predetermined period of time and preventing formation of micro protrusions, micro pits and other surface defects, in a single step of polishing. Accordingly, a polishing process comprising at least two steps has now been studied.
To carry out a polishing process in two steps, it is the main purpose of polishing in the first step to remove the relatively large waviness, large pits and other surface defects on the substrate surface, i.e. to carry out fairing or reshaping. Accordingly, a polishing composition is required which has a high ability to process and mend the above-mentioned waviness or surface defects with little formation of deep scratches which can not be removed by finish polishing in the second step, rather than to reduce the surface roughness.
The purpose of polishing in the second step i.e. finish polishing is to minimize the surface roughness of the substrate. For this purpose, it is important for a polishing composition to be able to reduce the surface roughness and to prevent formation of micro protrusions, micro pits and other surface defects, rather than to have a high ability to process or mend a large waviness or surface defects as required for polishing in the first step. Further, from the viewpoint of the productivity, it is also important that the polishing rate is high. As far as the present inventors are aware, it is possible to obtain a substrate surface having small surface roughness in polishing in the second step by a conventional two step polishing, but the polishing rate is very low and inadequate for practical production, or it has been difficult to prevent formation of micro protrusions, micro pits or other surface defects.
The degree of surface roughness is determined by the process for preparing the substrate, the final memory capacity as the memory hard disk, and other conditions, and depending upon the degree of the desired surface roughness, a polishing process comprising more than two steps may
Fujimi Incorporated
Marcheschi Michael
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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