Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse...
Reexamination Certificate
1999-09-22
2001-10-09
Dinh, Tan (Department: 2651)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
C428S064200
Reexamination Certificate
active
06301200
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to magneto-optic recording. More specifically, the present invention relates to a reflector layer for a magneto-optic (MO) disc.
BACKGROUND OF THE INVENTION
In magneto-optic recording, data is represented as a magnetic domain in a magnetizable recording medium such as a disc. 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 disc 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 disc 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 laser beam. Transmission of a portion 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 a recording medium having perpendicular anisotropy above its Curie temperature to form a magnetic domain. The area is allowed to cool in the presence of a magnetic field to orient the growing domain. The magnetic field is strong enough to overcome the demagnetizing field of the 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 light intensity modulation (LIM) 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 the 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 rotation in polarization. The angle of rotation varies as a function of the magnetization of the localized area. An optical detector receives the read beam and translates the rotation angle into an appropriate bit value.
SUMMARY OF THE INVENTION
Increasing storage density is one of the main objectives of data storage system manufacturers because it results in lower cost per data unit for the consumer, allows greater storage capacity within a standard drive geometry, and can lead to new smaller drive formats. High data storage density and low cost currently drive the highly competitive data storage business.
One technique for increasing the storage density of optical recording media is to reduce the spot size of the light beam incident on the recording medium. The full-width-half-maximum (FWHM) of the focused light spot used for reading the storage media must be reduced to read back smaller marks. Thin film performance and engineering issues emerge both as a result of the reduced focused light spot area and as a result of the methods used for achieving the reduced light spot.
One problem that results from going to a smaller FWHM, for example, less than about 0.35 &mgr;m, is excessive heating of the media. Because the minimum laser read power is limited by system considerations such as laser and detector shot noise, reducing the FWHM leads to larger light power densities at the media surface. Increasing the light power density at the media surface becomes a critical problem when the stored data is corrupted at the higher temperatures associated with the higher power density.
Methods for achieving a smaller FWHM include both the use of an optical system with a very high numerical aperture (NA), and the use of near field optical techniques. Either of these methods may necessitate the use of an air incident medium. In systems that use air-incident media, the reading laser beam is incident onto the thin film side of the optical storage disc. Thus, the thin films in the layered stack on the disc are in reverse sequence from those in conventional substrate-incident MO recording media.
Reversing the sequence of layers in the stack has implications for several aspects of the media performance. One important aspect is the magnetic field sensitivity of the recording layer in the recording medium. The field sensitivity of the recording layer depends critically on the surface conditions, especially roughness, of the layer onto which it is deposited. When the layer sequence is reversed using conventional MO stack materials and processes, the field sensitivity is usually degraded (i.e., less sensitive in a magnetic field modulation writing scheme).
The present inventors have discovered a layer for an optical storage medium that includes: 1) materials selected to minimize the difficulties that result when increased power density is applied at the media surface; 2) surface structure designed to minimize the difficulties when field sensitivity is an issue; and 3) optical properties designed to maximize signal. This layer may be incorporated into a recording medium that has reduced susceptibility to high light intensity effects. This medium addresses the thermal requirements, as well as the associated magnetic and optical requirements, of a high-density optical storage system.
In one embodiment, the present invention is an air incident recording medium for use in an air-incident optical data storage system. An air incident medium includes an optical stack with layers in the following order: a substrate, a reflector layer, a magneto-optic recording layer, and a protective layer. The air incident medium of the present invention includes a reflective layer with an electrical conductivity greater than about 5.0×10
4
(&OHgr;-cm)
−1
. The reflective layer in the air incident medium of the present invention has an index of refraction (n) of less than about 0.5 at the wavelength used to read the medium. The reflector layer has a root mean square (RMS) surface roughness less than about 1.0 nm.
The reflective layer in the air incident medium of the present invention may be made of any material that has any of: (1) an electrical conductivity greater than about 5.0×10
4
(&OHgr;-cm)
−1
, (2) an index of refraction (n) of less than about 0.5 at the wavelength used to read the medium; or (3) a root mean square (RMS) surface roughness less than about 1.0 nm. Materials preferred for use as the reflective layer of the present invention include metallic compounds and their alloys such as, for example, copper, gold, silver, and aluminum. Copper and its alloys are particularly preferred.
This reflective layer provides improved read power sensitivity and field sensitivity in small FWHM applications in air-incident optical recording systems.
In another embodiment, the present invention is an air incident optical storage system in which data are record
Aspen Frank E.
Sexton Joseph H.
Willson Richard F.
Dinh Tan
Imation Corp.
Levinson Eric D.
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