Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
1999-08-11
2001-05-29
Evans, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S064200, C428S064200, C428S064400, C428S913000, C430S270110, C430S495100, C430S945000
Reexamination Certificate
active
06238763
ABSTRACT:
TECHNICAL FIELD
The present invention relates to rewritable optical data storage media including magneto-optic disks useful in near-field, air-incident recording applications.
BACKGROUND INFORMATION
In magneto-optic recording, data is represented as a magnetized domain on a magnetizable recording medium such as a disk. Each domain is a stable magnetizable data site representative of a data bit. Data is written to the medium by applying a focused beam of high intensity light in the presence of a magnetic field. The disk typically includes a substrate, a magneto-optic recording layer, a reflective layer, and two or more dielectric layers.
In substrate-incident recording, the beam passes through the substrate before it reaches the recording layer. The reflective layer in a substrate-incident recording medium is formed on a side of the recording layer opposite the substrate. The reflective layer reflects the beam back to the recording layer, increasing overall exposure and absorption.
In near-field, air-incident recording, the beam does not pass through the substrate. Instead, the beam is incident on the recording layer from a side of the disk opposite the substrate. In an air-incident recording medium, the reflective layer is formed adjacent the substrate. A solid immersion lens (SIL) can be used to transmit the beam across an extremely thin air gap, and through the top of the recording medium to the recording layer. The SIL can be integrated with a flying magnetic head assembly. The air gap forms a bearing over which the flying head rides during operation. For near-field recording, the thickness of the air gap is less than one wavelength of the recording beam. Transmission of the beam is accomplished by a technique known as evanescent coupling.
For either substrate-incident or air-incident recording, the recording beam heats a localized area of the recording medium above its Curie temperature to form a magnetizable domain. The domain is allowed to cool in the presence of a magnetic field. The magnetic field overcomes the demagnetizing field of the perpendicular anisotropy recording medium, causing the localized domain to acquire a particular magnetization. The direction of the magnetic field and the resulting magnetization determine the data represented at the domain.
With beam modulation recording techniques, the magnetic field is maintained in a given direction for a period of time as the beam power is selectively modulated across the recording medium to achieve desired magnetizations at particular domains. According to magnetic field modulation (MFM) recording techniques, the beam is continuously scanned across the recording medium while the magnetic field is selectively modulated to achieve desired magnetization. Alternatively, the beam can be pulsed at a high frequency in coordination with modulation of the magnetic field.
To read the recorded data, the drive applies a lower intensity, plane-polarized read beam to the recording medium. Upon transmission through and/or reflection from the recording medium, the plane-polarized read beam experiences a Kerr rotation in polarization. The Kerr angle of rotation varies as a function of the magnetization of the localized area. An optical detector receives the read beam and translates the Kerr rotation angle into an appropriate bit value.
The amount of data storage capacity for a given magneto-optic disk depends on the spatial density of domains on the disk and the effective recording surface area of the disk. Greater spatial density results in more data per unit surface area. Greater recording surface area naturally results in greater storage capacity for a given spatial density. Recording surface area is limited, however, by disk size. Disk size has been limited in part by drive footprint requirements. Spatial density is limited primarily by the spot size of the drive laser. In other words, spatial density is a function of the ability of the drive to direct a beam to increasingly smaller domains in a consistent manner. Near-field, air-incident recording, in particular, has the potential to produce extremely small spot sizes using evanescent coupling and the resultant high numerical aperture, thereby providing increased spatial density and data storage capacity.
SUMMARY
The present invention is directed to a rewritable optical data storage disk having a substrate with an increased thickness that is in a range of approximately 2.3 to 2.6 mm. The increased thickness of the substrate enhances the flatness of the recording disk relative to a recording plane.
In particular, the increased thickness reduces process-induced surface variations such as warpage and tilt, and provides the disk with increased stiffness to resist deflection during use. The enhanced flatness enables data to be recorded on the disk in a consistent manner with greater spatial densities using techniques such as near-field, air-incident recording. The resulting disk thereby yields greater spatial density and data storage capacity.
An increased substrate thickness can adversely affect the cost and throughput of disk manufacture, and raise cost and performance issues with respect to disk drives. A substrate thickness in the range of approximately 2.3 to 2.6 mm, however, has been found to provide significantly enhanced flatness, rigidity, and resistance to warpage without excessive cost, throughput, or performance impact.
Flatness refers to the ability of the incident surface of the disk to maintain a substantially constant position relative to a recording plane on which the drive laser beam is focused. The “incident” surface refers to the surface of the disk through which the beam enters the disk. Deviation of the disk surface from the recording plane can impact the ability of the drive laser to focus on individual domains, particularly for higher spatial densities. This deviation is compounded by the fact that the disk is constantly spinning during use in a drive. In near-field applications, for example, it is expected that drives may rotate a disk at speeds on the order of 2400 to 3600 revolutions per minute (rpm). Consequently, the portion of the disk to which the recording head is directed is constantly and rapidly changing.
For near-field, air-incident recording, the size and focal plane of a recording spot is determined primarily by the thickness of the air bearing that separates the recording head and the disk surface. If the position of the disk surface is not uniform, the air bearing thickness can vary. Variation in the air bearing thickness can result in varying focus and spot size across the disk. In particular, the thickness of the air gap determines the amount of radiation received by the recording layer via evanescent coupling. Significant variation in spot size and focus can undermine the ability of the laser to consistently address extremely small domains. Also, excessive surface nonuniformity in the disk can cause acute changes in air bearing thickness for successive domains and resultant loss of tracking. In extreme cases, head crashes, i.e., physical contact of the head with the disk, can result. Thus, unacceptable flatness can lead to disk drive failure.
The surface of the disk can deviate from the recording plane for a number of reasons. The disk fabrication process, for example, can produce warpage and tilt in the disk. With thinner substrates, in particular, the effects of gravity and thermal gradients during the post-fill cooling phase can cause uneven densification and unbalanced thermal stresses at different areas of the disks. For example, once the mold is filled, portions of the disk closest to the mold surface will cool more quickly. The result is disk warpage and tilt.
According to its ordinary meaning, warpage refers to a curvature of the surface of the disk. For a warped disk, tilt can vary with both radial and angular position. Tilt refers to variation of the disk surface flatness relative to an ideal disk plane, and is represented by the following equation:
T=(∂z/∂r)
î ;
+(∂z/r∂&thgr;)
ĵ ;,
where
Evans Elizabeth
Imation Corp.
Levinson Eric D.
LandOfFree
Rewritable optical data storage disk having enhanced flatness does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Rewritable optical data storage disk having enhanced flatness, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Rewritable optical data storage disk having enhanced flatness will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2472713