Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
2000-03-31
2003-03-25
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S064100, C428S064200, C428S065100, C428S690000, C428S900000
Reexamination Certificate
active
06537648
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a substrate for an information recording medium for use as a recording medium for an information processing apparatus, an information recording medium using the substrate, and a method of producing the substrate.
A magnetic disk is known as one of information recording media. The magnetic disk comprises a substrate and a thin film such as a magnetic layer formed thereon. As the substrate, use has been made of an aluminum substrate or a glass substrate. In recent years, in response to the demand for high-density recording, the glass substrate is used at an increasing ratio because a gap between a magnetic head (which is operable as a recording and/or reproducing head) and the magnetic disk can be small as compared with the aluminum substrate.
Generally, the glass substrate is produced through chemical strengthening in order to increase the strength so that the glass substrate is resistant against a shock when loaded into a magnetic disk drive. Alternatively, the surface of the glass substrate is heat treated to be crystallized so that a crystallized substrate improved in strength is produced. In addition, in order to lower a flying height of the magnetic head as low as possible, the surface of the glass substrate is polished with high precision. Thus, high-density recording is realized.
Not only the glass substrate, the magnetic head is also developed from a thin film head to a magnetoresistive head (MR head), further to a giant (large-sized) magnetoresistive head (GMR head) so as to meet the high-density recording.
However, there arises a problem that, even if the surface roughness Rmax (which is defined as a maximum height representative of a difference between a highest point and a lower point) or Ra (which is representative of a center-line-mean roughness) is reduced by high-precision polishing, the flying height of the magnetic head can not be lowered. The present inventors investigated the cause to find out that a microwaviness present on the surface of the substrate is responsible and that the microwaviness also affects a modulation of the magnetic disk.
Japanese Unexamined Patent Publication (JP-A) No. H08-147688 proposes a glass substrate for a magnetic disk, in which a surface waviness (Wa) and the surface roughness (Ra) satisfy the relationship Wa/Ra≦0.5 in order to reduce a glide height of the magnetic disk.
However, measurement of the surface waviness described in the above-mentioned publication uses a tracer-type surface roughness tester (Tencor) and is carried out over the length on the order of 100 &mgr;m along a straight line in a radial direction of the substrate. Thus, the measurement is performed only in a restricted local area and in a single direction. Thus, the above-mentioned measurement does not fully reflect the surface waviness of the whole substrate.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a substrate for an information recording medium and an information recording medium adapted to a high recording density, by adjusting each of a surface waviness (Wa) and a microwaviness (Ra′, wa) on the surface of the substrate to a level not exceeding a predetermined range, where each of the surface waviness (Wa) and the microwaviness (Ra′, wa) is measured by a measuring method capable of properly expressing the surface waviness (Wa) and the microwaviness (Ra′, wa) and has a correlation with a glide height or a modulation of a magnetic disk.
It is another object of this invention to provide a method of producing a substrate for an information recording medium, which is capable of adjusting each of a surface waviness (Wa) and a microwaviness (Ra′, wa) on the surface of the substrate to a level not exceeding the predetermined range.
It is still another object of this invention to provide a method of controlling the surface of a substrate for an information recording medium so as to adjust a gliding height of a head slider or a modulation of the information recording medium to a desired value.
This invention has been made in view of the above-mentioned objects and has the following structures.
(Structure 1)
A substrate for an information recording medium, the substrate having a microwaviness average height Ra′ not greater than 0.05 microinch as measured by a contactless laser interference technique for measurement points within a measurement region of 50 &mgr;m□-4 mm□ on a surface of the substrate, the microwaviness average height Ra′ being given by:
Ra
′
=
1
n
∑
i
=
1
n
&LeftBracketingBar;
x
i
-
x
_
&RightBracketingBar;
,
where xi represents a measurement point value of each measurement point, {overscore (x)} representing an average value of the measurement point values, n representing the number of the measurement points.
(Structure 2)
A substrate for an information recording medium, the substrate having a microwaviness period between 2 &mgr;m and 4 mm and a microwaviness average height Ra′ not greater than 0.05 microinch as measured by a contactless laser interference technique for measurement points in a measurement region on a surface of the substrate, the microwaviness average height Ra′ being given by:
Ra
′
=
1
n
∑
i
=
1
n
&LeftBracketingBar;
x
i
-
x
_
&RightBracketingBar;
,
where xi represents a measurement point value of each measurement point, {overscore (x)} representing an average value of the measurement point values, n representing the number of the measurement points.
(Structure 3)
A substrate for an information recording medium as described in structure 1 to 2, wherein the measurement region is smaller in area than a slider surface of a head slider of a recording and/or reproduction head.
(Structure 4)
A substrate for an information recording medium, the substrate having a disk-like shape, the substrate having a waviness period between 300 &mgr;m and 5 mm and a waviness average height Wa of 1.0 nm or less as measured by a contactless laser interference technique for measurement points in a measurement region surrounded by two concentric circles which is spaced from a center of a surface of the disk-shaped substrate by a predetermined distance, the waviness average height Wa being given by:
Wa
=
1
N
∑
i
=
1
N
&LeftBracketingBar;
X
i
-
X
_
&RightBracketingBar;
where Xi represents a measurement point value of each measurement point, {overscore (X)} representing an average value of the measurement point values, n representing the number of the measurement points.
(Structure 5)
A substrate for an information recording medium as described in any one of structures 1 through 4, wherein the substrate is a substrate for a magnetic recording medium.
(Structure 6)
A substrate for an information recording medium as described in any one of structures 1 through 5, wherein the substrate is made of a glass.
(Structure 7)
An information recording medium comprising a substrate described in any one of structures 1 through 6 and at least a recording layer formed on the substrate.
(Structure 8)
An information recording medium as described in structure 7, wherein the recording layer is a magnetic layer.
(Structure 9)
A method of producing a glass substrate for an information recording medium, the method comprising the steps of:
preparing a glass substrate having a flatness of 4 &mgr;m or less; and
polishing a principal surface of the glass substrate by the use of a soft polisher having a hardness not greater than 80 (Asker-C) and abrasive grains having an average grain size of 1.0 &mgr;m or less.
(Structure 10)
A method as described in structure 9, wherein:
the soft polisher has a hardness between 62 and 70 (Asker-C).
(Structure 11)
A method as described in structure 9 or 10, wherein:
the step of preparing the glass substrate is carried out by lapping of a glass material of a disk-like shape.
(Structure 12)
A method as described in any one of structures 9 through 11, wherein:
the polishing step is carried out with a work
Hata Genshichi
Miyamoto Takemi
Takahashi Kouji
Tomiyasu Hiroshi
Yokoyama Tomotaka
Hoya Corporation
Resan Stevan A.
Sughrue & Mion, PLLC
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