Stock material or miscellaneous articles – Circular sheet or circular blank – Recording medium or carrier
Reexamination Certificate
1999-09-24
2001-11-06
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Circular sheet or circular blank
Recording medium or carrier
C428S064600, C428S064900, C428S065100, C428S065100, C428S336000, C428S690000, C428S900000
Reexamination Certificate
active
06312781
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for forming a protective film on a disc-shaped recording medium.
2. Description of the Related Art
The photomagnetic recording system is such a recording system in which a recording layer formed of a magnetic material is partially raised in its temperature to higher than the Curie point or a temperature compensation point to reduce the coercivity, and in which a recording magnetic field is applied from outside for inverting the direction of magnetization of the recording layer to record information signals. This photomagnetic recording system is put to practical use in an optical filing system, an external storage device for a computer or in a device for recording the audio or video information.
The magneto-optical disc for recording by the above-described photomagnetic recording system may be exemplified by a magneto-optical disc having a recording layer of a thin magnetic film formed on a transparent substrate including plastics, such as polycarbonate, or glass. This recording layer is made up of a magnetic layer for recording information signals thereon, a dielectric film and a recording layer. The magnetic film is a thin magnetic film made up of, for example, a rare earth-transition metal alloy amorphous thin film having an easy axis perpendicular to the film surface and a large photomagnetic effect. On the reflective film, layered on the uppermost layer of the recording layer, a protective film formed of an ultra-violet ray curable resin, is usually deposited for preventing corrosion or damage to the recording layer.
As the magneto-optical disc, there is also a magneto-optical disc of a double-plate structure, in addition to the above-described single-plate type structure. With the magneto-optical disc of a double-plate structure, two magneto-optical discs are bonded together so that the recording layer sides or the substrate sides face each other. With the magneto-optical disc of-the double-plate structure, since signals are recorded independently on the respective recording layers of the discs, the recording capacity is twice that of the magneto-optical disc of the single-plate structure. Moreover, the magneto-optical disc of the double-plate structure is of a symmetrical structure relative to the bonding surface, and hence has a merit that the substrate is less liable to be warped or otherwise deformed against changes in temperature or humidity than the magneto-optical disc of the single-plate structure.
The photomagnetic recording system for recording on the magneto-optical disc is roughly classified into an optical modulation system of recording signals by modulating the light and a magnetic field modulation system of recording signals by modulating the recording magnetic field.
Of these, the magnetic field modulation system is a system of recording signals on the recording layer by inverting the recording magnetic field at a high speed during signal recording in a state of illuminating the light. Researches in this system are going on energetically since this system enables facilitated overwriting as well as high recording density and high speed accessing.
In this magnetic field modulation system, a magnetic field is applied during signal recording/reproduction by a magnetic head which generates the magnetic field in the recording layer. Since this magnetic head needs to be inverted speedily during signal recording, the magnetic field as strong as that of the above-described optical modulation system cannot be generated.
The magnitude of the magnetic field applied to the magneto-optical disc by the magnetic head is inversely proportionate to the distance between the magneto-optical disc and the magnetic head. That is, the magnitude of the magnetic field applied to the magneto-optical disc becomes smaller as the distance between the magneto-optical disc and the magnetic head becomes larger. Therefore, with the magnetic head, the distance between it and the magneto-optical disc needs to be reduced in effecting signal recording.
With the magneto-optical disc of the single-plate structure, this problem is addressed by providing an optical pickup device for illuminating the laser light on one surface of the magneto-optical disc and by providing the magnetic head on the opposite surface of the magneto-optical disc.
For recording/reproducing the magneto-optical disc of the double-plate structure, the optical pickup device is unified with the magnetic head and the optical pickup device and the magnetic head thus unified together are arranged on both sides of the magneto-optical disc.
With this unified optical pickup device-magnetic head system, the laser light is directly illuminated on the recording layer, without interposition of the substrate. Thus, the material of the substrate of the magneto-optical disc may be opaque without having to be transparent.
Therefore, the magneto-optical disc of the double-plate structure has an additional merit that Al, for example, may be used as the substrate material to prevent the substrate from warping. The magneto-optical disc may be classified into the single-plate structure and the double-plate structure. On these magneto-optical discs, protective films are usually formed for preventing corrosion of the recording layer. This protective film is usually formed by the spin coating method.
For forming the protective film, a disc substrate
20
, having the protective film formed thereon, is set on the turntable, and rotated at a reduced speed by a spindle motor. A UV curable resin
21
is supplied in a toroidal pattern along an inner peripheral area
20
a
of the recording layer of the disc substrate
20
. The UV curable resin
21
is applied up to the outer rim by the centrifugal force produced by rotating the disc substrate
20
at an elevated speed for coating the entire surface of the disc substrate
20
with the UV curable resin
21
. For obtaining a sufficient protective effect after illumination of the UV rays during the subsequent step, this thickness of the protective film is formed to approximately 15 &mgr;m.
However, the protective film, formed by supplying the UV curable resin
21
from the inner rim
20
a
of the disc substrate
20
as described above, tends to be thicker in film thickness as the outer rim if the disc substrate
20
is approached.
However, the protective film formed by the spin coating is varied with viscosity of the UV curable resin, rpm of the magneto-optical disc or the rotating time. However, if the protective film is applied from the inner rim
20
a
of the disc substrate
20
and the rotating velocity as well as the rotating time is changed for coating the entire surface of the disc substrate to a uniform film thickness, the disc substrate becomes thicker in film thickness at an outer rim than at the inner rim
20
a
of the disc substrate
20
.
FIG. 2
shows the film thickness distribution when the protective film is formed with the viscosity of the UV curable resin being varied from 500 cps through 140 to 37 cps. In
FIG. 2
, the ordinate and the abscissa denote the film thickness of the protective film and the radial position on the disc substrate
20
, respectively. Also, in
FIG. 2
, characteristics A denote the relation between the film thickness of a protective film formed by a UV curable resin of 500 cps in viscosity and the radial position on the disc substrate
20
, while characteristics B and C denote the relation between the film thickness of a protective film formed by a UV curable resin of 140 cps in viscosity and the radial position on the disc substrate
20
and that between the film thickness of a protective film formed by a UV curable resin of 37 cps in viscosity and the radial position on the magneto-optical disc, respectively.
In measuring the relation between the film thickness and the radial position on the disc substrate, shown in
FIG. 2
, the UV curable resin was supplied in a toroidal pattern at a radial position of approximately 17 mm from the center of the disc substrate
2
Kiliman Leszek
Sonnenschein Nath & Rosenthal
Sony Corporation
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