High rigidity glass-ceramic substrate

Details High rigidity glass-ceramic substrate High rigidity glass-ceramic substrate

2000-05-23

2002-08-06

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Devitrified glass-ceramics

C501S010000, C428S690000, C428S690000

Reexamination Certificate

active

06429160

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a high rigidity glass-ceramic substrate and, more particularly, to a high rigidity glass-ceramic substrate for a magnetic information storage medium used, e.g., for a magnetic information storage device having a super flat substrate surface capable of coping with near contact recording or contact recording employed in the ramp loading system and capable also of coping with a high speed rotation of a magnetic information storage medium. The invention relates also to a magnetic information storage medium such as a magnetic disk which is provided by forming a film of an information storage medium on the glass-ceramic substrate.
In this specification, the term “magnetic information storage medium” means a magnetic 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 also other disk type magnetic information storage media which can be used in HDTV, digital video cameras, digital cameras, mobile communication devices etc. In this specification, the term “spiner” means at least one of (Mg and/or Zn)Al
2
O
4
, (Mg and/or Zn)
2
TiO
4
and a mixture in the form of a solid solution between these two crystals and the term “spinel solid solution” means a crystal in which other ingredient is mixed with spinel and/or a part of spinel is substituted by other ingredient.
Recent development of personal computers for multi-media purposes and digital video cameras and digital cameras requires handling of a large amount of data such as a moving picture and voice and, for this purpose, a magnetic information storage device of a large recording capacity is required. For increasing the recording density, there is a tendency in the art of a magnetic information storage medium toward reducing the size of a bit cell and thereby increasing the bit and track density. As a result, the magnetic head performs its operation in closer proximity to the disk surface. For coping with starting and stopping of a magnetic head which is operated in a near contact state or a contact state with respect to a magnetic information storage medium, a landing zone system has been developed according to which a specific part (an unrecorded part in a radially inside or outside portion of a disk) is processed for preventing stiction of the magnetic head to the disk.
In the current magnetic information storage device, the CSS (contact start stop) system is adopted according to which (1) the magnetic head is in contact with the magnetic information storage medium before starting its operation and, (2) when the magnetic head has started its operation, the magnetic head flies over the surface of the magnetic information storage medium. If the plane of contact between the magnetic head and the magnetic information storage medium is exceedingly in the state of a mirror surface, stiction of the magnetic head occurs resulting in unsmooth starting of rotation due to increased friction and damage to the surface of the magnetic information storage medium or the magnetic head. Thus, the magnetic information storage medium faces conflicting demands for lower flying height of the magnetic head accompanying increase in the storage capacity and prevention of stiction of the magnetic head to the surface of the magnetic information storage medium. As an answer to satisfy such conflicting demands, a ramp loading technique has been developed according to which the magnetic head in operation is completely in contact with the surface of the magnetic information storage medium but starting and stopping of the magnetic head are performed in an area outside of the surface of the magnetic information storage medium. Thus, there has been an increasing demand for a smoother surface of the magnetic information storage medium.
Developments are also in progress for transferring information at a higher speed by rotating a magnetic information storage medium at a higher speed. Since, however, a high speed rotation of a substrate for the magnetic information storage medium causes deflection and deformation in the substrate, the substrate is required to have a higher Young's modulus. Further, in addition to the currently used fixed type hard disks, magnetic information storage devices which use removable type hard disks and card type hard disks which require a high strength of the substrate are being considered and becoming feasible and application of the substrate to HDTV, digital video cameras, digital cameras and mobile communication devices is under way.
In the situation in which a high rigidity substrate material is required, an aluminum alloy substrate cannot provide a sufficient strength and, if thickness of the substrate is increased, it will make it difficult to realize a compact and light-weight design of the medium. For solving the problem inherent in the aluminum alloy substrate, known in the art are chemically tempered glasses such as alumino-silicate (SiO
2
—Al
2
O
3
—Na
2
O) glass proposed by Japanese Patent Application Laid-open Publication Nos. Hei 8-48537 and Hei 5-32431 etc.). These chemically tempered glasses, 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 chemically tempered phase causes aging when used for a long time, magnetic properties of the magnetic information storage medium are deteriorated. (3) Since the glass contains Na
2
O or K
2
O ingredient as an essential ingredient, these alkali ingredients diffuse in the film formed during the film forming process and thereby deteriorate magnetic properties of the magnetic information storage medium. For preventing this, a barrier coating over the entire surface for preventing diffusion of Na
2
O or K
2
O becomes necessary and this makes it difficult to produce the product in a stable manner at a low cost. (4) Chemical tempering is made for improving mechanical strength of the glass but this is based on utilization of tempering stress between the surface phase and the inside phase. Young's modulus of the chemically tempered glass is 830 GPa or below which is equivalent to ordinary amorphous glass and this poses limitation to application of the chemically tempered glass to a high speed rotation drive. Thus, the chemically tempered glasses are not sufficient as a substrate for a high recording density magnetic information storage medium.
Aside from the aluminum alloy substrates and chemically tempered substrates, known in the art are some glass-ceramic substrates. For example, glass-ceramic substrates disclosed in Japanese Patent Application Laid-open Publication No. Hei 9-35234 and EP0781731A1 have a Li
2
O—SiO
2
composition and contain lithium disilicate and &bgr;-spodumene crystal phases or lithium disilicate and &bgr;-cristobalite crystal phases. In the glass-ceramic substrates, however, no consideration or suggestion is made about relation between Young's modulus and specific gravity al all. Young's modulus of these glass-ceramics is 100 GPa at the maximum.
For improving such low Young's modulus, Japanese Patent Application Laid-open Publication No. Hei 9-77531 discloses a glass-ceramic of a SiO
2
—Al
2
O
3
−Mgo—ZnO—TiO
2
system and a rigid disk substrate for a magnetic information storage medium. This glass-ceramic contains a large quantity of spinel as a predominant crystal phase and also contains MgTi
2
O
5
and other crystal phases as sub-crystal phases and has Young's modulus of 96.5-165.5 GPa. In this material, the predominant crystal phase is only spinel represented by (Mg/Zn)Al
2
O
4
and/or (Mg/Zn)
2
TiO
4
and the sub-crystal phases are not limited to specific crystals but crystals of a broad range are mentioned. Further, this glass-ceramic contains a large amount of Al
2
O
3
and is different from the glass-ceramics of the present invention which contain a relatively small amount o

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