Optical recording medium

Details Optical recording medium Optical recording medium

2001-08-29

2004-03-16

Watko, Julie Anne (Department: 2652)

Dynamic information storage or retrieval

Storage medium structure

Layered

Reexamination Certificate

active

06707787

ABSTRACT:

RELATED APPLICATION DATA
The present application(s) claims priority to Japanese Application(s) No(s). P2000-261765 filed Aug. 30, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical recording medium having an optical recording layer for optically recording information and a method of producing the same.
2. Description of the Related Art
In recent years, in the field of information recording, research has been underway in various locations on optical information recording systems. Optical information recording systems have a number of advantages such as a capability of non-contact recording and reproduction, a capability of handling different forms of memories such as read only, write-once, and rewritable types. Broad applications from industrial use to consumer use can be envisioned for these as systems enabling realization of low cost, large size files.
A larger capacity of the optical recording media for the above various types of optical information recording systems (hereinafter also referred to as an “optical disk”) has been achieved mainly by shortening the wavelength of the laser light serving as the light source used in the optical information recording systems and using lenses having larger numerical apertures to reduce the spot size on the focal plane.
For example, in a CD (compact disk), the laser light wavelength is 780 nm, the numerical aperture (NA) of the lens is 0.45, and a capacity is 650 MB, while in a DVD-ROM (digital versatile disk—read only memory), the laser light wavelength is 650 nm, the NA is 0.6, and the capacity is 4.7 GB.
Furthermore, in the next generation optical disk systems, a larger capacity of 22 GB or more can be achieved by using an optical disk including an optical recording layer over which is formed a thin light transmitting protective film (cover layer) having a thickness of, for example, about 100 &mgr;m and making the laser light wavelength 450 nm or less and the NA 0.78 or more.
FIG. 1
is a schematic sectional view of the structure of an optical disk for the above next generation optical disk systems.
For example, an optical recording layer
11
comprised of an aluminum or other metal film or a phase change type thin film made of inorganic material or other and having irregularities of lands L and grooves G is formed on a substrate
10
comprised of a polycarbonate and having a thickness of about 1.1 mm.
The optical recording layer
11
is coated by, for example, an ultraviolet curing resin to form a light transmitting protective film
14
.
The above optical disk is a type of optical disk where laser light is focused on the optical recording layer
11
through the protective film
14
and the reflected light is read. By making it above type, a higher numerical aperture can be achieved.
The method of producing the above conventional optical disk having the above optical recording layer will be explained next.
First, a pattern of irregularities for the optical recording layer is formed by for example injection-molding on a substrate comprised for example of a polycarbonate and having a thickness of 1.1 mm, then aluminum etc. is deposited thereon by for example sputtering so as to form a reflection film having a pattern corresponding to the above pattern of irregularities, that is, the optical recording layer
11
.
Next, a liquid-state ultraviolet curing resin is supplied by spinning coating on the optical recording layer
11
from a dispenser while making the substrate
10
spin so as to form an ultraviolet curing resin film.
Next, ultraviolet light is irradiated on the ultraviolet curing resin film to cause the same to cure and form a protective film. By this, an optical disk having the structure shown in
FIG. 1
can be obtained.
However, in the above process of forming an ultraviolet curing resin film, there was the problem that the ultraviolet curing resin film for forming a protective film could not be formed uniformly on the substrate due to the high viscosity of the ultraviolet curing resin.
FIG. 2
is a schematic sectional view of the state in the step of forming the above ultraviolet curing resin film.
A not shown optical recording layer is formed on the substrate
10
. When coating an uncured film
14
a
of an ultraviolet curing resin thereon by spin coating, since the viscosity of the ultraviolet curing resin is high, the thickness of the uncured film
14
a
ends up becoming vastly different between near the center of the substrate (region expressed by x in the figure) and near the periphery (region expressed by y in the figure). It was thus difficult to obtain a uniform thickness.
Also, as explained above, at the time of curing the ultraviolet curing resin film by irradiating ultraviolet light, the cure shrinkage of the resin is large, so there was the problem that the optical disk produced ended up warping by a large degree. Also, a change in temperature or humidity also became a cause of warping because of the difference in the coefficient of expansion and water absorption of the protective film.
The amount of spherical aberration caused when the thickness of the above protective film deviates from its set value is proportional to NA
4
/&lgr; for a laser light wavelength &lgr; and a numerical aperture NA, so when shortening the wavelength of the laser light and raising the numerical aperture to increase the capacity as explained above, the characteristics required in the optical disk for keeping down spherical aberration become stricter and the uniformity-of the thickness of the protective film has to be improved.
Also, the amount of coma aberration occurring when warping (tangential or radial skew) of a disk occurs is proportional to NA
3
/&lgr;, so in the same way as above, when shortening the wavelength of the laser light and increasing the numerical aperture to increase the capacity, the characteristics required in the optical disk for keeping down coma aberration become stricter and warping of the disk has to be suppressed.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical recording medium capable of improving the uniformity of thickness of a protective film and suppressing warping and a method of producing the same.
To achieve the above object, an optical recording medium of the present invention comprises: a substrate; an optical recording layer formed on the substrate; and a light transmitting protective film formed at an upper layer of the optical recording layer; wherein the protective film includes a polymer sheet produced by melt casting and an adhesive layer for bonding the polymer sheet to the optical recording layer.
In the optical recording medium of the present invention, preferably, a surface formed in contact with the cast when the polymer sheet is produced by melt casting is arranged to be at a far side from the optical recording layer.
In the optical recording medium of the present invention, preferably, a residual solvent in the polymer sheet is not more than 0.3 wt %.
In the optical recording medium of the present invention, preferably, a birefringence in the in-plane direction of the polymer sheet is not more than 15 nm.
In the optical recording medium of the present invention, preferably, a difference of an average value of a refractive index in an in-plane direction of the polymer sheet and a refractive index in a direction perpendicular to the in-plane direction is not more than 0.0015.
In the optical recording medium of the present invention, preferably, the adhesive layer comprises a stack of a ultraviolet curing resin-based adhesive agent layer and a pressure-sensitive tackiness agent layer.
In the optical recording medium of the present invention, preferably, the polymer sheet comprises a polycarbonate.
The above optical recording medium of the present invention includes as a light transmitting protective film above the optical recording layer a polymer sheet produced by melt casting and an adhesive layer for bond

Affiliated with

Also associated with

Rating

Optical recording medium does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Optical recording medium, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical recording medium will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3275564