Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
1999-03-02
2002-05-07
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C428S064200, C428S064400, C428S065100, C428S065100, C428S690000, C428S900000, C501S004000, C501S068000, C501S069000, C501S072000
Reexamination Certificate
active
06383645
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a glass-ceramic substrate for an information storage medium and, more particularly, to a glass-ceramic substrate for an information storage medium such as a magnetic disk made of a glass-ceramic having improved super flatness of a surface of the substrate, a high Young's modulus and a low specific gravity capable of coping properly with a high speed rotation, and a range of coefficient of thermal expansion matching with coefficients of thermal expansion of constituent elements of the information storage medium. The invention relates also to a method for manufacturing the same and also to an information storage medium using this glass-ceramic substrate. In this specification, the term “information storage medium” means an information storage 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 storage of information in a network and other information storage medium in the form of a disk which can be used for storage of data in, e.g., digital video cameras and digital cameras.
Recent development of personal computers for multi-media purposes and digital video cameras and digital cameras which requires handling of a large amount of data has necessitated a magnetic information storage device of a large recording capacity. As a result, for increasing the recording density, there is a growing tendency in a magnetic information storage medium toward increasing in bit and track density and reducing the size of a bit cell. In conformity with the reduction in the size of the bit cell, a magnetic head performs its operation in closer proximity to the surface of a disk. As the magnetic head performs its operation in a near-contact state or contact state against the disk surface, technical development of a landing zone system has become important as a technique for starting and stopping a magnetic head. According to this system, a sticking prevention processing such as texturing is made in a specific zone of a disk (e.g., a radially inward or outward unrecorded portion of a disk) and starting and stopping of the magnetic head are performed in this zone which is called “landing zone”.
In the current magnetic information storage device, the CSS (contact start stop) system is generally employed according to which a magnetic head is in contact with a surface of a magnetic information storage medium before starting and is lifted from the surface of the medium when the head has started its operation. If the surface of the medium on which the magnetic head contacts is exceedingly of a mirror surface, stiction takes place between the surface of the medium and the magnetic head with resulting difficulty in smooth starting of rotation of the medium due to increased friction and occurrence of damage to the surface of the medium. Thus, a substrate for a magnetic information storage medium must satisfy two conflicting requirements for a lower glide height of a magnetic head accompanying increased storage capacity and prevention of sticking of the magnetic head on the surface of the medium. For satisfying these conflicting requirements, the landing zone system has been developed and, aside from the landing zone system, development of a ramp loading system is under way according to which a magnetic head is completely in contact with the surface of a medium except for starting and stopping of the magnetic head when the magnetic haed is moved away from the surface of the medium. Accordingly, a current requirement for a substrate for a magnetic information storage medium is a smoother surface.
A technical development is under way for a higher speed transfer of information by a higher speed rotation of a magnetic information storage medium used for a magnetic information storage device. As the number of revolution of a medium increases, deflection and deformation of the medium occur and this gives rises to a requirement for a higher Young's modulus. Further, in addition to the conventional fixed type hard disks, information storage media such as a removable type hard disks and card type hard disks have been proposed and put into practice and application of digital video cameras and digital cameras for various uses have been started.
Known in the art of magnetic disk substrate materials is aluminum alloy. The aluminum alloy substrate, however, has projections or spot-like projections and depressions on the substrate surface during polishing due to various defects of the material and, therefore, is not sufficient as a substrate for a high recording density storage medium in flatness and smoothness. Besides, since aluminum alloy is a soft material and has a low Young's modulus and surface hardness, vibration of the substrate takes place during a high speed rotation of the medium with resulting deformation of the medium. Difficulty also arises in making the information storage medium thinner. Further, damage of the medium by contact with a head is liable to occur. Thus, the aluminum alloy substrate cannot sufficiently cope with the requirements for a high speed recording.
As materials for overcoming the above problems of the aluminum alloy substrate, known in the art are chemically tempered glasses such as soda-lime glass (SiO
2
—CaO—Na
2
O) and alumino-silicate glass (SiO
2
—Al
2
O
3
—Na
2
O). These materials, however, have the following disadvantages: (1) Since polishing is made after the chemical tempering process, the chemically tempered layer is seriously instable in making the disk thinner. (2) Since the glass contains Na
2
O as an essential ingredient, the glass has the problem that the film forming characteristics of the medium is deteriorated and, for preventing diffusion of Na
2
O, it becomes necessary to apply a barrier coating over the entire surface of the substrate. This prevents stable production of the product at a competitive cost.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are som glass-ceramic substrates. For example, the glass-ceramics of a SiO
2
—Li
2
O—MgO—P
2
O
5
system disclosed in U.S. Pat. No. 5,626,935 containing lithium disilicate (Li
2
O.2SiO
2
) and &agr;-quartz (&agr;-SiO
2
) as main crystal phases is an excellent material as a material textured over the entire surface in which, by controlling the grain diameter of globular crystal grains of &agr;-quartz, the conventional mechanical texturing or chemical texturing can be omitted and the surface roughness after polishing (Ra) can be controlled within a range from 15 Å to 50 Å. This glass-ceramic, however, cannot sufficiently cope with the requirement for the low glide height necessitated by the rapidly increasing recording density which requires the surface roughness (Ra) of 9 Å or below, preferably 6 Å or below. Further, no discussion or suggestion about a coefficient for thermal expansion has been made in this patent.
Japanese Patent Application Laid-open Publication No. Hei 9-35234 discloses a magnetic disk substrate made of a glass-ceramic of a SiO
2
—Al
2
O
3
—Li
2
O system having predominant crystal phases of lithium dislicate (Li
2
O.2SiO
2
) and &bgr;-spodumene (Li
2
O.Al
2
O
3
.4SiO
2
). This glass-ceramic has a composition which contains a relatively large amount of Al
2
O
3
ingredient and in which growth of SiO
2
crystals such as &agr;-quartz (&agr;-SiO
2
) and &agr;-cristobalite (&agr;-SiO
2
) is extremely restricted. The center line mean surface roughness of this glass-ceramic after polishing is defined as 20 Å or below but the center line mean surface roughness disclosed in examples is a rough one of 12 Å-17 Å which fails to reach the above described desired surface roughness and, therefore, this glass-ceramic cannot cope sufficiently with the requirement for the low glide height of a magnetic head. Further, since this glass-ceramic requires a high temperature of 820° C. to 920° C. for crystallization which prevents a l
Goto Naoyuki
Ishioka Junko
Kawashima Yasuyuki
Hedman & Costigan ,P.C.
Kabushiki Kaisha Ohara
McNeil Jennifer
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