Abrading – Abrading process – Glass or stone abrading
Reexamination Certificate
2001-06-28
2002-08-06
Hail, III, Joseph J. (Department: 3723)
Abrading
Abrading process
Glass or stone abrading
C451S559000
Reexamination Certificate
active
06428396
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of producing an information recording medium for use as a recording medium for an information processing apparatus and to a method of producing a substrate for the information recording medium. This invention also relates to the information recording medium for use as the recording medium and to the substrate for the information recording medium.
A magnetic disk is known as one of information recording media for use as a recording medium for an information processing apparatus. The magnetic disk comprises a substrate and a thin film such as a magnetic layer formed thereon. With a recent demand for an increase in recording density, a flying height which is a distance between the magnetic disk and a magnetic head has a very small value, for example, equal to 30 nm or less. As a consequence, it is desired to provide a substrate having a surface high in flatness and smoothness. The substrate having a surface high in flatness and smoothness can be obtained by appropriately selecting a polishing pad used in a polishing step or by reducing a particle size of abrasive grains used in the polishing step.
In addition to the improvement in flatness and smoothness of the surface of the substrate, the magnetic head is improved from a thin film head to a magnetoresistive head (MR head) and a giant magnetoresistive head (GMR head) utilizing an anisotropic magnetoresistance in order to meet the increase in recording density.
As described above, the high flatness of the surface of the magnetic disk is essential and indispensable in order to achieve a low flying height required to increase the recording density. In case where the MR head is used, the surface of the magnetic disk is required to be high in flatness and smoothness in view of thermal asperity also. The thermal asperity is a phenomenon such that, if a protrusion is present on the surface of the magnetic disk, the MR head is affected by the protrusion to generate heat and, as a consequence, the MR head becomes unstable in resistance value to cause malfunction in electromagnetic conversion.
Thus, in order to reduce the flying height of the magnetic head and to prevent occurrence of the thermal asperity, the demand for the high flatness and the high smoothness of the surface of the magnetic disk is more and more increasing day by day.
However, at the present stage, the increase in recording density of the magnetic disk can not be achieved only by polishing the surface of the substrate with high precision. Even if high-precision polishing is performed, protrusions may thereafter be formed on the substrate due to presence of foreign matters. In this event, the high flatness and the high smoothness of the magnetic disk can not be achieved. In fact, the removal of the protrusions due to presence of the foreign matters is already performed. However, the protrusions on the substrate, which are very small and need not be removed in the past, cause a serious problem at a present level of the increase in recording density.
If the thin film such as the magnetic layer is deposited on the substrate with the protrusions of the type attached to the surface of the substrate, protrusions are formed on the surface of the magnetic disk to become a factor inhibiting the reduction in flying height of the magnetic head and the prevention of occurrence of the thermal asperity (prevention of occurrence of a recording or a reproducing error).
Likewise, if the thin film such as a recording layer is deposited on the substrate with the protrusions of the type attached to the surface of the substrate, protrusions are formed on the surface of the information recording medium to become a factor causing a defect such as the recording or the reproducing error.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a method of producing a substrate for an information recording medium, which is capable of suppressing and preventing a protrusion from being formed on a substrate to act as a factor inhibiting a reduction in flying height of a magnetic head and a prevention of occurrence of thermal asperity and/or as a factor causing a defect in an information recording medium.
It is another object of this invention to provide a method of producing an information recording medium using the above-mentioned substrate.
It is still another object of this invention to provide a substrate for an information recording medium which is capable of suppressing the influence of protrusions acting as a factor inhibiting a reduction in flying height of a magnetic disk and a prevention of occurrence of thermal asperity and/or as a factor causing a defect in the information recording medium.
It is a further object of this invention to provide an information recording medium using the above-mentioned substrate.
The present inventors found out that, even if a substrate is washed and dried in a washing/drying step, some protrusions may be left without being diminished and that, if a thin film such as a magnetic layer is deposited on the substrate with the protrusions attached to the surface of the substrate, protrusions are formed on the surface of a magnetic disk to become a factor inhibiting a reduction in flying height of a magnetic head and a prevention of occurrence of thermal asperity.
It has also been found out that the protrusions contain at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr present in a cleaning liquid or an atmosphere.
The present inventors have investigated the reason why the protrusions containing the above-mentioned element or elements are attached to the substrate. As a consequence, it has been found out that, if the surface of the substrate is washed or rinsed with water and thereafter dried, the protrusions are left on the substrate at local spots where the water is deposited and then dried. It has also been found out that some of the protrusions containing the above-mentioned element or elements can not be removed by various washing techniques and can only be removed by polishing the substrate again.
As the water used in the washing/drying step, use is generally made of filtered water, DI water (deionized water), and the like. The washing/drying step is carried out in a clean atmosphere within a clean room or in an atmospheric air. The water or the atmosphere mentioned above contains C, Al, Si, Fe, Cu, Zn, Zr, and the like. It has been found out that the water containing such element or elements, in combination with the surface condition (hydrophobic nature) of the substrate, is left in particular regions to form the protrusions. Even if the water containing such element or elements has a very small amount on the order of ppb, the protrusions are formed on the surface of the substrate.
It has been found out that the product defects due to presence of the protrusions containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr can be avoided by determining and controlling the content of such element or elements contained in the water used in the washing/drying step and causing the protrusions so that the product defects are avoided. This leads to the completion of this invention.
Furthermore, investigation has been made about how the protrusions are formed. As a result, it has been found out that the size (height) of the protrusions is closely related to the wettability of the surface of the substrate before washing. Specifically, if the surface of the substrate before washing is inferior in wettability (i.e., hydrophobic), water droplets of the cleaning liquid locally stay on the surface of the substrate and are dried. In this event, those elements contained in the water or the atmosphere concentrate to the particular regions to form the protrusions. On the other hand, if the surface of the substrate before washing is superior in wettability (i.e., hydrophilic), the water droplets of the cleaning liquid spread over the surface of the substrate and are dried. In this event, those elements contained in the water or the atmosphere do not concentrate to the particular regions but are dispers
Isono Hideki
Kawai Hisao
Takeda Hiroshi
Hail III Joseph J.
Hoya Corporation
Shakeri Hadi
LandOfFree
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