Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
1999-02-22
2002-05-14
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C428S432000, C428S701000, C428S702000, C501S004000, C501S007000, C501S010000
Reexamination Certificate
active
06387509
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a glass-ceramic substrate for a magnetic information recording medium used for recording various types of information and, more particularly, to a magnetic information recording disk suitable for perpendicular recording having a high heat resisting property which will enable film forming at a high temperature and also annealing at a high temperature, super flatness of the surface of the substrate, and a low degree of elution of alkali ingredients from the substrate during film forming and annealing processes conducted at a high temperature. In this specification, the term “magnetic information recording medium” means a magnetic information recording medium in the form of a disk and includes fixed type hard disks, removable type hard disks and card type hard disks used respectively for so-called hard disks for personal computers and other magnetic information recording medium in the form of a disk which can be used for storage of data in, e.g., digital video cameras and digital cameras.
In recently developed magnetic information recording devices which handle data of a large amount such as data of a moving image or voice which is necessitated by developments of personal computers for multi-media purposes and digital video cameras and digital cameras, recording density of a magnetic information recording medium has reached an extremely high recording density exceeding 1 Gb/in
2
.
Conventional magnetic information recording devices mostly employ the longitudinal magnetic recording system according to which magnetization (i.e., recording) is made in the longitudinal direction. In these conventional devices, however, wavelength of recording, track width and film thickness of a recording medium are reduced as the recording density increases and this causes decrease in magnetic energy of a magnetic material constituting 1 bit resulting in decrease in the signal output level and increase in the signal-to-noise ratio. When the recording density exceeds 10 Gb/in
2
, there arises a further problem that such a small magnetization gives rise to thermal instability and thereby causes difficulty in the magnetic recording.
In contrast, according to the perpendicular magnetic recording system in which the easy axis of magnetization is perpendicular, the bit size can be significantly reduced and, by capability of having a desired film thickness of a magnetic recording medium (five-fold to ten-fold of the film thickness used in the case of the longitudinal recording system), reduction of the diamagnetic field and an advantageous result caused by the shape magnetic anisotropy can be expected. Thus, according to the perpendicular recording system, the decrease in the recording energy and the problem of thermal instability which are caused in increasing the recording density in the conventional longitudinal recording system can be eliminated and a significant improvement in the recording density over the longitudinal recording system can be realized. For these reasons, recording density of 20 Gb/in
2
or over can be easily achieved and study for realizing recording density of 100 Gb/in
2
has already been started.
Since magnetization is made in the perpendicular direction with respect to the surface of a recording medium in the perpendicular recording system, a recording medium having the easy axis of magnetization extending in the perpendicular direction is employed instead of a recording medium used in the longitudinal recording system which has the easy axis extending in the longitudinal direction. The most hopeful candidate for a perpendicular magnetic film is a barium ferrite film and other materials which are considered suitable for the perpendicular magnetic film are alloy films including films of Co-&ggr;Fe
2
O
3
alloy, Co alloys, Fe alloys and Ni alloys.
A substrate suitable for the recording medium used for the new technique which is different from the conventional longitudinal recording system is required to have the following properties:
(1) An oxide medium such as barium ferrite requires film forming at a high temperature for minimization and growth in the perpendicular direction of crystal grains of the magnetic substance. Further, a recent study has revealed that there is a case where a recording medium should be annealed at a high temperature of 500° C. to 900° C. Therefore, the substrate material must stand such a high temperature without causing deformation of the substrate or change in the surface roughness.
(2) As the recording density increases in the perpendicular recording system, there is an increasing tendency toward lowering of the glide height of a magnetic head to 0.025 &mgr;m or below and there is also a tendency toward employing the near contact recording system or the contact recording system. On the other hand, for utilizing the surface of a recording medium effectively as a data zone, the ramp loading system which does not provide the landing zone as opposed to the currently employed recording system which provides both the landing zone and the data zone attracts the attention of the industry. For coping with these tendencies, the data zone on the disk surface or the entire surface of the substrate must be formed in a super flat surface for enabling such low glide height or even contact recording of the magnetic head.
(3) A substrate for a perpendicular magnetic information recording medium must be free of crystal anisotropy, foreign matters and impurities which adversely affect crystals of the material formed into a film. The substrate must also have a dense, uniform and fine texture.
(4) As the recording density increases in a perpendicular magnetic information recording medium, a magnetic film of a higher precision and a finer texture is required. If Na
2
O, K
2
O or an ingredient having an OH group is included in the material of the recording medium, ion of such ingredient diffuses during the film forming process resulting in increased coarseness of grains of the film and deterioration in orientation. The substrate therefore should not substantially contain such ingredients. Further, the substrate should not substantially contain PbO which is undesirable from the viewpoint of environment protection.
(5) The substrate should have sufficient chemical durability for standing rinsing and etching with various chemical reagents.
(6) As the recording density increases in a perpendicular magnetic information recording medium, the bit density and track density increase. As the size of the bit cell is reduced, a coefficient of thermal expansion of the substrate has a great influence. Therefore, a coefficient of thermal expansion within a temperature range from −50° C. to +600° C. must be within a range from −10×10
−7
/° C. to 80×+10
−7
/° C.
As a material for a magnetic disk substrate, aluminum alloy has been conventionally used. The aluminum alloy, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process. As a result, the aluminum alloy substrate is not sufficient in flatness and smoothness as a substrate for the perpendicular magnetic information recording medium. Further, since the aluminum alloy is a soft material and tends to be easily deformed, it is difficult to form into a thin disk. Furthermore, a head crash is likely to occur during a high speed rotation of the disk due to flexion of the disk which will damage the recording medium. Therefore, the aluminum alloy is not a material which can sufficiently cope with the tendency toward the high recording density. Moreover, since the heat resisting temperature of the aluminum alloy during the film forming process which is the most important factor as the substrate for the perpendicular recording system is only 300° C. or below, film forming at a temperature above 500° C. or annealing at a temperature within a range from 500° C. to 900° C. will cause thermal deformation in the substrate so that application of the aluminum alloy to a substrate
Goto Naoyuki
Nakajima Kousuke
Hedman & Costigan ,P.C.
Jones Deborah
Kabushiki Kaisha Ohara
McNeil Jennifer
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