Coating processes – Optical element produced
Reexamination Certificate
2001-06-07
2004-02-10
Beck, Shrive P. (Department: 1762)
Coating processes
Optical element produced
C427S240000, C427S282000, C427S407100, C427S425000, C118S052000, C118S320000, C118S504000, C118S505000
Reexamination Certificate
active
06689415
ABSTRACT:
This invention relates to a method and apparatus for preparing optical information media such as read-only optical disks and optical recording disks.
BACKGROUND OF THE INVENTION
To record and store a vast quantity of information as typified by moving image information, advanced optical information media such as read-only optical disks and optical recording disks are required to increase their recording density for increasing the capacity. To meet such a demand, engineers have been engaged in the research and development works targeting a higher recording density.
One such approach relating to digital versatile disks (DVD) is to shorten the wavelength of a recording/reading laser beam and increase the numerical aperture (NA) of a recording/reading optical system objective lens, thereby reducing the spot diameter of the recording/reading laser beam. As compared with CD, DVD is successful in achieving a recording capacity of 6 to 8 folds (typically 4.7 GB/side) by changing the recording/reading wavelength from 780 nm to 650 nm and the NA from 0.45 to 0.6.
Increasing the NA, however, leads to a reduced tilt margin. The tilt margin is a permissible tilt of an optical recording medium relative to an optical system, which depends on the NA. The tilt margin is in proportion to
&lgr;/(
d·NA
3
)
wherein &lgr; denotes the wavelength of Recording/reading beam and “d” denotes the thickness of a transparent substrate the Recording/reading beam enters. If the optical recording medium is inclined or tilted relative to the laser beam, a wavefront aberration (or coma) occurs. The coefficient of wavefront aberration is represented by
(½)·
d·{n
2
·sin &thgr;·cos &thgr;}·
NA
3
/(
n
2
−sin
2
&thgr;)
−
{fraction (5/2)}
wherein n denotes the refractive index of the substrate and &thgr; is a tilt angle. It is appreciated from these formulae that the tilt margin may be increased and the occurrence of comatic aberration be suppressed by reducing the thickness “d” of the substrate. In fact, the DVD design is such that a tilt margin is secured by reducing the thickness of the substrate to about one half (about 0.6 mm) of the thickness (about 1.2 mm) of the CD substrate.
To record moving images of better quality for a longer period of time, there has been proposed a structure allowing for use of a thinner substrate. In this structure, a substrate of an ordinary thickness is used as a supporting substrate for maintaining rigidity, pits or a recording layer is formed on the surface of the supporting substrate, and a light-transmitting layer of about 0.1 mm thick is formed thereon as a thin substrate. Recording/reading beam reaches the pits or recording layer through the light-transmitting layer. This structure can achieve a higher recording density due to a greater NA because the substrate can be made extremely thin as compared with the prior art. Media having such structure are disclosed in JP-A 10-320859 and 11-120613.
The provision of a light-transmitting layer of approximately 0.1 mm thick allows for use of an objective lens having a large numerical aperture NA, say about 0.85.
The light-transmitting layer of approximately 0.1 mm thick can be formed, for example, by a spin coating technique. The spin coating technique generally involves feeding a resin onto the surface of a disk substrate secured on a rotating table, and rotating the disk substrate for spreading the resin by a centrifugal force. Since the disk substrate is formed with a center hole which is utilized for mounting the disk on a drive, the resin cannot be fed to the center of rotation (or the center of the disk substrate), but to an annular band equidistantly spaced apart from the center of rotation. As the position of resin feed is spaced apart from the center of rotation, the resulting coating or light-transmitting layer has a greater thickness variation in a radial direction.
To reduce the radial thickness variation of the light-transmitting layer, several proposals were made to close the center hole of the disk substrate with suitable plug means such as a plate member, disk member, plug or cap so that the resin can be fed near the center of the plug means, that is, the center of rotation. See JP-A 10-320850, 10-249264, 10-289489, 11-195250, and 11-195251.
However, the techniques of JP-A 10-320850, 10-249264, and 11-195250 are difficult to practice in the industry because it is not described how to remove the plug means such as the plate member or cap at the end of spin coating.
In contrast, JP-A 10-289489 describes that at the end of spin coating, the plug means such as the disk member is removed by punching out or magnetic attraction using an electromagnet. However, the punching step or the removal by magnetic attraction is a cumbersome operation. The plug means is removed at a high acceleration, causing a disturbance to the resin coating.
The above-referenced JP-A 11-195251 describes a plug means of the structure having a circular cap and an integral support at the center thereof. The support allegedly facilitates attachment/detachment and alignment of the plug means. The support described therein is a hollow cylinder having at least one discharge port or consists of a plurality of rods. Once a resin is introduced into the interior of the hollow cylinder or the region circumscribed by the rods, the disk substrate is rotated together with the plug means whereby a resin layer is formed on the disk substrate. Finally the plug means can be readily removed.
With this plug means used, spin coating is carried out by causing the resin to flow out of the discharge port in the hollow cylinder or the gaps between the rods. Therefore, the resin is retained or dammed by the support wall (region other than the discharge port) or the rods. The once dammed resin can flash over the disk substrate at incidental timing, frequently forming asperities on the coating. Also the side of the plug means to be in contact with the resin has a complex shape and a large contact area, which is inconvenient at washing the plug means. If some resin is left on the surface of the plug means, the coating frequently becomes uneven. Table 1 of the above-referenced JP-A 11-195251 reports the thickness variations of the coating formed when the outer diameter of the hollow cylinder ranges from 4 mm to 16 mm. It is evident from the test results that the thickness variation of the coating depends on the outer diameter of the hollow cylinder. The larger the outer diameter, the greater becomes the thickness variation. More specifically, even if the resin is fed to the interior of the hollow cylinder, the coating start position is not coincident with the center of rotation, but the outer perimeter of the hollow cylinder. With the relatively high viscosity of the resin taken into account, it is difficult to reduce the outer diameter of the hollow cylinder below 4 mm. The method of this patent is thus very difficult to minimize the thickness variation of the resin coating.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method and apparatus for preparing an optical information medium comprising a disk-shaped supporting substrate, an information recording layer thereon, and a resin-based light-transmitting layer thereon by which recording/reading laser beam is transmitted, the method and apparatus being capable of minimizing the thickness variation of the light-transmitting layer without significant complication of the method and apparatus.
In one aspect, the invention provides an apparatus for preparing an optical information medium which comprises a disk-shaped supporting substrate having a center hole, an information recording layer thereon, and a resin-based light-transmitting layer on the information recording layer by which recording/reading laser beam is transmitted. The apparatus includes a rotating table for holding and rotating the supporting substrate having the information recording layer borne thereon. A plug means for closing the center hole includes a disk member for closing the center hole and an integral support sha
Hirata Hideki
Komaki Tsuyoshi
Yamaga Kenji
Beck Shrive P.
Jolley Kirsten Crockford
TDK Corporation
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