Dynamic magnetic information storage or retrieval – General recording or reproducing – Thermomagnetic recording or transducers
Reexamination Certificate
2000-11-09
2002-04-23
Faber, Alan T. (Department: 2651)
Dynamic magnetic information storage or retrieval
General recording or reproducing
Thermomagnetic recording or transducers
C360S131000, C369S013140, C369S013410, C369S013510
Reexamination Certificate
active
06377414
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved method for patterning magnetic data/information recording, storage, and retrieval media as well as improved magnetic recording media obtained thereby. More specifically, the present invention relates to an improved method for patterning magnetic media in hard disk form such as are utilized in computer and computer-related applications.
BACKGROUND OF THE INVENTION
Magnetic media are widely utilized in various applications, particularly in the computer industry, and efforts are continually made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, of the magnetic media. Conventional magnetic thin-film media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are typically formed as “perpendicular” or “longitudinal” media depending upon the direction of magnetization of the grains. In this regard, “perpendicular” recording media have been found superior to the more common “longitudinal” media in achieving very high bit densities. However, as grain sizes decrease in order to achieve increased recording bit densities, e.g., to about 20 Gb/in
2
, effects arising from thermal instability, such as “superparamagnetism” are encountered. One proposed solution to the problem of thermal instability, including, inter alia, the so-called “superparamagnetic limit” encountered with ultra-high recording density magnetic recording media, is to increase the crystalline anisotropy, and thus the squareness of the bits, in order to compensate for the smaller grain sizes.
An alternative approach, however, to the formation of very high bit density magnetic recording media, is the formation of “patterned” media. Analogous to the situation with conventional polycrystalline thin-film magnetic media, both “longitudinal” and “perpendicular” types of patterned magnetic media have been developed, depending upon whether the magnetization direction is parallel or perpendicular to the media surface. When fabricated in disk form, such “patterned” media are readily adapted for use in conventional hard drives, with most of the drive design features remaining the same. Thus, hard drive-based “patterned” media technology would, in practice, comprise a spinning disk with a slider head flying above it in closely-spaced relation thereto, with read sensors or a read/write head that magnetizes and/or detects the magnetic fields emanating from the magnetic media.
According to a first approach, as exemplified by the Atomic Force Microscopy (“AFM”) approach of IBM (B. Terris et al.,
Data Storage,
August 1998, pp. 21-26), a sharp tip is utilized for scanning extremely close to the surface of a data/information storage medium. The tip is located at the end of a flexible cantilever, which deflects in response to changes in the force imposed on the tip during scanning. The force may arise from a variety of effects, including, inter alia, magnetic force. To date, only two types of AFM drives have been demonstrated, i.e., write-once/read-only and read-only. The former type of AFM drive, which provides write-once/read-only capability, utilizes a heated AFM tip for writing once by forming small indentations or pits in the surface of a substrate, e.g., of polycarbonate. Data is read by using the AFM tip to scan the thus-indented surface and sensing the changes in the force imposed on the AFM tip due to the presence of the indentations.
The latter type of AFM drive functions in a read-only mode, and data is initially written in the form of indentations (pits) which are created in the surface of a SiO
2
master by means of an electron beam. The data, in the form of the indentations, is then transferred, by replication, to a photopolymer-coated glass substrate, which photopolymer is cured by exposure to ultra-violet (UV) radiation to thereby form a surface topography representing the data. The data is then read from the cured photopolymer surface by scanning with the AFM tip to sense the changes in force thereat due to the indentations.
According to the second, lithographic approach, thin film processes such as are utilized in the fabrication of semiconductor integrated circuits including micron-sized features are adapted for making high aspect ratio, single column/bit, perpendicularly patterned media. According to one particular approach (M. Todorovic et al.,
Data Storage,
May 1999, pp. 17-20) designed to increase coercivity, hence stability of the individual columns, electroplated nickel (Ni) is utilized for forming the columns, and gallium arsenide (GaAs) and alumina (Al
2
O
3
) are employed as embedding media for the columns. The fabrication process starts with an electrically conductive GaAs substrate, on which thin layers of aluminum arsenide (AlAs) and GaAs are successively deposited, as by molecular beam epitaxy (“MBE”). Scanning electron beam lithography is then utilized to define the magnet patterns on the resin-coated sample. The patterns in the e-beam-exposed resin are developed utilizing an appropriate solvent system and then transferred, as by chemically-assisted ion beam etching (“CAIBE”), into the AlAs/GaAs layers. After pattern definition, the AlAs layer is converted into Al
2
O
3
by wet thermal oxidation. The thus-produced patterned layer acts as a mask for additional etching for extending the pattern of depressions perpendicularly into the GaAs substrate. The etched depressions in the Al
2
O
3
substrate are then filled with electroplated Ni. Overplated Ni “mushrooms” are then removed, as by polishing, to create a smooth surface for accommodating slider contact therewith.
Thus, the overall process sequence for forming such media requires successive, diverse technology steps for (1) MBE growth and mask deposition; (2) electron beam lithography; (3) chemically-assisted ion beam etching; (4) wet thermal oxidation; and (5) electroplating and polishing, e.g., chemical-mechanical polishing (“CMP”). The result is a complex and time-consuming fabrication process. Moreover, each of the above-described approaches for patterned media manufacture typically involves substantial capital investment for the process equipment, which together with the inherent process complexity, render them too costly for use in high product throughput magnetic disk media manufacture.
Yet another process for forming patterned magnetic media, which process is also useful in forming servo patterns on a magnetic media surface, is disclosed by D. S. Kuo in commonly assigned, co-pending U.S. patent application Ser. No. 09/130,657, filed Aug. 7, 1998, and is based upon the well-known property or phenomenon of magnetic films of exhibiting a decrease in coercivity (H
c
) with increase in temperature. Such decrease in H
c
, with increase in temperature is currently utilized to produce magnetic transitions in thermomagnetic materials, e.g., rare earth-transition metal (“RE-TM”) materials, such as terbium-iron (TbFe) films utilized in magneto-optical (“MO”) recording devices. Such devices typically employ a focussed laser beam for creating a “hot spot” on the RE-TM-based media surface, while simultaneous application of an external magnetic field is applied to the media to reverse the direction of local magnetization within the locally heated area.
Based upon this effect or phenomenon, Kuo has proposed, in the above-mentioned U.S. patent application, a method for forming patterned magnetic media, e.g., servo patterns in the surface of a magnetic recording layer. According to the process disclosed therein, instead of heating the magnetic media with a spot of focussed laser radiation, a focussed pattern (i.e., an image) of radiative energy (e.g., from a laser) is projected onto the surface of a magnetic recording film or layer, which film or layer has been subjected to a pre-alignment treatment by application of a strong magnetic field of a first polarity, to selectively heat and thus lower the coercivity H
c
of the magnetically pre-aligned film at the exposed areas. In order to generate a magn
Faber Alan T.
Seagate Technology LLC
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