Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Light polarizing article or holographic article
Reexamination Certificate
2001-04-25
2002-08-27
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Light polarizing article or holographic article
C264S001370
Reexamination Certificate
active
06440333
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical recording medium comprising a substrate having formed thereon a pattern of depressions and projections and a light-transparent layer formed on the substrate, and a method of manufacturing the optical recording medium.
2. Description of Related Art
As typical conventional optical recording media from or into which data is read or written using a light, ones shaped to have a generally disc-like form, so-called optical discs, are well known of which an optical disc substrate is formed from a synthetic resin such as polycarbonate or similar. The optical discs include a read-only type from which a signal already recorded therein can be read, a write-once type into which a signal can be recorded only once, and a rewritable one into which a signal can be recorded more than once.
The read-only type optical disc comprises an optical disc substrate having a pattern of microscopic depressions (so-called “pits”) formed on a surface thereof in the direction of track, and a reflective layer provided on the disc surface on which the pit pattern is formed. Namely, the optical disc has a signal recorded as a pattern of pits formed in the direction of a track. For reading a signal recorded as a bit pattern from this read-only type of optical disc, the disc is irradiated with a reading light from a surface thereof opposite to the surface on which the bit pattern is formed. The reading light is reflected at the reflective layer, and a return light (reflected reading light) is detected to read the signal.
Also in this optical disc, a diffracted light from the surface on which the pits are formed is detected for maintaining the spot of light from a source accurately on the track (so-called tracking).
On the other hand, the optical discs of the write-once and rewritable types comprise each an optical disc substrate having formed thereon concentric or spiral depressions (so-called groove) and projections (so-called lands) formed between adjacent grooves, recording layer provided on the substrate, and a reflective layer provided on the recording layer. In the optical discs of these types, a signal is written on the recording layer formed on the grooves and/or lands. Also in these write-once and rewritable optical discs, pit patterns as additional signals such as disc properties, addresses, etc. are formed between grooves adjacent to each other in the direction of track.
When these optical discs are played, a reading light is irradiated onto the recording layer formed on the lands and/or grooves from on a surface thereof opposite to a surface on which the pits and grooves are formed, thereby forming recording marks on the recording layer. For reading the optical discs of these types, a reading light is irradiated as in the read-only type optical disc, and a return light from the disc is detected.
Also in these write-once and rewritable types of optical discs, a return light reflected from the groove, for example, is detected for the purpose of tracking.
The read-only, write-once and rewritable optical discs (will be referred to as “optical disc” hereinunder) are formed by an injection melding using a stamper having formed thereon projections and depressions which will provide pits and grooves on the optical disc. More specifically, a synthetic resin such as polycarbonate is injection-molded to form a transparent substrate having pits and grooves formed thereon. A reflective layer, etc. are formed on the transparent optical disc substrate to form an optical disc. In the write-once and rewritable optical discs, a recording layer is formed between the transparent optical disc substrate and the reflective layer.
A recent tendency in this field of art is to decrease the diameter of the spot of light focused on the optical disc in order to attain a higher recording density. Generally, this object can be attained by using a light of shorter wavelength and an objective lens of a larger numerical aperture (NA).
In case the diameter of the spot of light focused on the optical disc is decreased for a higher recording density, however, the transparent optical disc substrate has to be increased in thickness. This is because as the NA of the objective lens is increased, the allowance of an aberration caused by a tilt angle, which the disc surface or plane forms with respect to the optical axis of an optical pickup, is smaller. Also, the thicker a disc portion through which a reading light is transmitted, the larger the aberration due to the tilt angle will become.
Therefore, the distance between a light-incident surface and signal layer of the optical disc is decreased to accommodate the decreased diameter of the spot of light focused on the optical disc.
In the above-mentioned optical disc, however, it is difficult to mold a pattern of depressions and projections on an optical disc substrate having a thickness of 0.3 mm or less, for example. Further, even if the molding is possible, the optical disc substrate is likely to warp, which will result in a difficulty in forming thereon layers such as recording layer, reflective layer, etc. Moreover, if the optical disc substrate having such a thickness, the user will not be able to easily handle an optical disc made from the optical disc substrate.
Therefore, there has so far been proposed an optical disc comprising a transparent disc substrate having a pattern of depressions and projections formed thereon, at least a reflective layer formed on the disc substrate, and a light-transparent layer formed above the reflective layer so that a light of a predetermined wavelength can be incident through the light-transparent layer to read and/or write data from and/or into the optical disc. This type of optical disc will be referred to as “read-from-rear optical disc” hereinunder for the convenience of description.
In this read-from-rear optical disc, a recording layer is formed between the reflective and light-transparent layers.
In the read-from-rear optical disc, the light-transparent layer can be formed very thin compared with the optical disc substrate having depressions and projections formed thereon. Thus, even with a light of a larger wavelength and an objective lens of a larger NA, read and write of a signal can be made accurately with respect to the optical disc without the above-mentioned problems.
Even in the aforementioned read-from-rear optical disc, however, the transparent optical disc substrate is formed by an injection molding using a stamper having formed thereon a pattern of projections and depressions which will provide pits and grooves on the optical disc substrate. To prepare the stamper, first a photoresist layer is formed over a glass substrate, and a predetermined area of the photoresist layer is exposed to a light, thereby providing a glass master having provided thereon a predetermined pattern of depressions and projections formed from the unexposed photoresist layer remaining on the glass substrate. Next, the glass master is plated with nickel or similar to form a stamper.
Therefore, to form a pattern of pits/grooves and lands on the transparent optical disc substrate with a high accuracy for this read-from-rear optical disc, it is necessary to form a pattern of projections and depressions with a correspondingly high accuracy and also replicate the pattern accurately to the optical disc substrate.
For smaller pits and grooves to attain a higher recording density, it is more difficult to replicate the pattern of projections and depressions on the stamper to the optical disc substrate with a high accuracy. For instance, for microscopic projections formed on one main surface of the transparent optical disc substrate, corresponding microscopic depressions shall be formed on the stamper. Therefore, a resin has to be filled or charged in such microscopic depressions on the stamper.
It is conceivable that when producing an optical disc, a resin of a lower viscosity could be filled into the microscopic depressions with a higher efficiency and a higher accuracy
Bell Boyd & Lloyd LLC
Sony Corporation
Vargot Mathieu D.
LandOfFree
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