Dynamic magnetic information storage or retrieval – General processing of a digital signal – Data in specific format
Reexamination Certificate
1999-09-20
2002-07-16
Faber, Alan T. (Department: 2753)
Dynamic magnetic information storage or retrieval
General processing of a digital signal
Data in specific format
C360S055000, C360S135000, C360S077080, C360S077030, C360S078110
Reexamination Certificate
active
06421195
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to information storage systems that use magnetic disk media. More particularly, the invention relates to a disk drive that uses patterned magnetic disk media.
BACKGROUND
Magnetic recording disks are typically produced by depositing a thin magnetic recording layer on a suitable disk substrate. Data is generally written on the magnetic layer by a recording head that writes magnetic data bits in the magnetic layer while scanning the surface of the disk. To increase the magnetic recording density of the magnetic disk, the bit sizes in the magnetic layer need to be as small as possible.
Magnetic bit volumes in conventional crystalline magnetic media may include hundreds of magnetic grains or sets of grains. Grains and/or sets of grains are often surrounded by segregating materials that separate neighboring grains and reduce exchange-based magnetic coupling. In order to increase the areal density of disk drives the surface accessible dimensions of magnetic bit volumes must be reduced. This can generally be accomplished by reducing the average grain size while maintaining a critical number of grains in each bit volume to obtain adequate signal to noise ratios.
Depositing thinner magnetic layers often results in smaller grains and potentially higher areal densities of disk drives. However, it is well known in the art that a limiting factor for reducing the average grain size in a magnetic recording layer is the onset of superparamagnetism. This situation arises when the magnetic volumes of grains or groups of grains are thermally unstable, either at room temperature or at elevated temperatures. A magnetic layer with a large number of superparamagnetic magnetic grains is incapable of storing magnetic data for long periods of time.
Instead of reducing bit sizes by reducing the grain sizes, it has been suggested that patterning methods can be used to define bit boundaries and increase the bit densities of magnetic media. An early example of patterning magnetic media in a circumferential direction is disclosed in IBM Technical Disclosure Bulletin, Vol. 18 No. 15 October 1975, where a layer of a medium is patterned with circumferential magnetic and non-magnetic tracks alternating at different radial positions of a disk medium. Radial position, herein, denotes a position on a disk medium relative to the center the disk and radial direction refers to a direction that extends either inward or outward and that is substantially normal to a circumferential track. Circumferential position refers to a position along a circumferential track and circumferential direction refers a direction that extends along or follows a circumferential track.
Patterning a magnetic disk medium with magnetic and non-magnetic circumferential tracks results in sharp circumferential boundary definitions that can result in improved magnetic transitions. Further, because the magnetic circumferential tracks are separated by non-magnetic circumferential tracks, edge anomalies and cross-talk between data bits located in adjacent tracks is reduced. Other examples of patterning magnetic layers with circumferential recording tracks are described by Brady et al., in U.S. Pat. No. 5,571,591, and by Krounbi et al., in U.S. Pat. No. 4,935,278.
Patterning magnetic media in two-dimensions to form small isolated magnetic islands has also been described. For example, Fernandez et al. characterize isolated Co magnetic domains in “Magnetic Force Microscopy of Single-Domain Cobalt Dots Patterned Using Interference Lithography”, IEEE Trans. Mag., Vol. 32, pp. 4472-4474, 1996. By using interference lithography to pattern a resist coated silicon wafer followed by thermal evaporation of Co, isolated arrays of magnetic Co domains are generated. Krauss et al. in “Fabrication of Planar Quantum Magnetic Disk Structure Using Electron Beam Lithography, Reactive Ion Etching, and Chemical Mechanical polishing” J. Vac. Sci. Technol. B 13(6), pp. 2850-2852, Nov/Dec 1995, describe an etching processes to define domains followed by an electroplating step to isolate magnetic Ni domains.
Other methods of two-dimensional patterning of magnetic media are disclosed by Falcone et al., in U.S. Pat. No. 4,948,703. Falcone et al. describe a method of embossing a photo-polymer to pattern the surface of an optical disk and Chou et al., in “Imprint Lithography with
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-Nanometer Resolution”, Science, Vol. 275, Apr. 5, 1996, and U.S. Pat. No. 5,772,905 describes a method for embossing PMMA at elevated temperatures and pressures with a template to achieve high resolution patterning. Chou in “Patterned Magnetic Nanostructures and Quantized Magnetic Disks”, Proc. IEEE, Vol. 85, No 4, pp. 652-671, April 1997, further describes a method for making magnetic domains with ferromagnetic materials such as cobalt or nickel by electroplating a PMMA embossed surface. The magnetic material fills the depressions in the embossed PMMA surface and creates small magnetic domains.
The Methods of patterning magnetic media with circumferential tracks, described above, fail to take advantage of high-resolution optical techniques for patterning, such as optical interference lithography, which can rapidly pattern an entire disk. This is because optical interference lithography has a technical limitation of primarily being useful to perform patterning with linear dimensions and is an extremely difficult technique to adapt for creating patterns with curvature, such as is needed for circular or circumferential tracks.
One of the shortcomings of using optical interference lithography to pattern a medium in two dimensions is that it requires that the medium be patterned in a two step process. For example, in using optical interference lithography to pattern a magnetic medium, a first patterning step is performed to pattern the medium with a first set of linear patterning lines. Subsequently, in a second patterning step, a second set of linear patterning lines are made on the medium, preferably with the second set of lines extending in a direction orthogonal to the first set of patterning lines. The two step patterning process produces small magnetic islands that define the boundaries of the magnetic bits. Unfortunately, the process of patterning a second set of lines on the surface of a magnetic medium can alter the dimensions of the bits in the direction used during the first patterning step, thus introducing inconsistencies in the patterning process from disk-to-disk. These inconsistencies in bit dimensions from disk-to-disk make it difficult to consistently match the dimensions of a read head and write head to the bit dimensions. Therefore, to achieve good matching between bit and head dimensions, the head and bit sizes of a medium pattered by this method must be individually matched for each magnetic data storage system produced.
There are several advantages to patterning magnetic media. As already mentioned the well-defined boundaries of patterned bits can reduce cross talk between adjacent bits and provide sharper magnetic transition within the bits. Also, there is a potential to greatly increase the areal bit density by reducing the bit surface dimensions. Theoretically, pattering may be used to define very small individual bits that have sufficient magnetic volumes to be thermally stable.
Ideally, a magnetic disk storage system includes a patterned magnetic disk with bits which have patterned boundary dimensions closely and consistently matched with the dimensions of the read and write head. If the bits of the magnetic disk medium have dimensions that are smaller than the dimensions of the read sensor width, signal is lost. On the other hand, if the magnetic disk medium has bit dimensions that are larger than the dimensions of the write pole tip or read sensor width, the storage density of the magnetic medium is not fully utilized. Further, head/bit matching needs to reproducible from data storage system to data storage system.
A third shortcoming of magnetic storage systems and magnetic disk media disclosed in the prior art is that disk med
Rubin Kurt Alan
Terris Bruce David
Faber Alan T.
Lumen Intellectual Property Services
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