Optical recording medium

Details Optical recording medium Optical recording medium

1999-08-03

2001-07-10

Evans, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064400, C428S064500, C428S064600, C428S457000, C428S913000, C430S270120, C430S495100, C430S945000, C369S283000, C369S288000

Reexamination Certificate

active

06258433

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical recording medium such as optical disks having a half-transmissive film.
As demands for an optical recording medium, such as optical disks, of higher density and smaller size increase, optical disks such as digital video disks (DVD) that allow two types of signals to be read from one side of the disk have been developed and are now available in the market. Such a one-sided disk has a double layer structure of a total-reflection film and a half-transmissive film.
Below, one example of a conventional one-sided, read-only dual layer DVD is described with reference to
FIGS. 9A and 9B
.
FIG. 9A
illustrates an actual construction of the disk while
FIG. 9B
is a conceptualized view of same. A first resin substrate
21
is made of, for example, polycarbonate (PC) or polymethyl methacrylate (PMMA). A half-transmissive film
22
of Au is deposited on a backside of the resin substrate
21
by sputtering or the like. A second resin substrate
23
is made of PC, PMMA or the like, and a total-reflection film
24
of Al deposited on a front surface of the second resin substrate
23
by means of sputtering or the like. These resin substrates
21
,
23
, on which respective layers have been formed, are bonded together with a transparent adhesive
25
made of a UV-curable photo-polymer or the like, thereby forming a one-sided double layer disk. A laser beam
26
of
650
nm wavelength is shown and a a reflected beam
27
are shown. The laser beam
26
passes through the resin substrate
21
and is reflected by the Au half-transmissive film
22
. A reflected beam
28
is produced when the laser beam
26
passes through the resin substrate
21
, the Au half-transmissive film
22
, the adhesive layer
25
, and is reflected by the Al total-reflection film
24
.
The one-sided, read-only double layer disk constructed as described above works as explained below. In this double layer disk, the reflectivity of the Au half-transmissive film
22
is set to be equal to the reflectivity of the Al total-reflection film
24
relative to the wavelength of the laser light
26
for reproduction of data, whereby reproduction signals can be read equally from the two layers. For this reason, the reflective layer on the side on which the laser beam
26
is incident must be a half-transmissive film, and its reflectivity is generally set at 25 to 30% so as to equalize the reflected light
27
and
28
.
The Au half-transmissive film
22
in the prior art exhibits the reflectivity characteristics required thereof, having a complex index of refraction of 0.19 and absorption coefficient of 3.5. Half-transmissive films are also demanded to be reliable not to change in outer appearance or reflectivity, due oxidization and corrosion, even in an environment under severe temperature and humidity conditions: The above described Au half-transmissive film
22
is also satisfactory in this regard, as it is made of a material which is remarkably resistant to oxidization and corrosion.
However, in the one-sided double layer disk described above, the use of Au for the half-transmissive film
22
, which costs 20 to 30 times more than other metal or dielectric materials deposited by means of sputtering, raises the cost of disks considerably. If, by way of example, the price of Au is 1400 yen per gram, the price of the half-transmissive film 2 per one disk is 3.5 yen.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide an optical recording medium which can be manufactured at a remarkably reduced cost by employing low-cost materials for the half-transmissive film.
To accomplish the above object, the present invention provides an optical recording medium having a half-transmissive film wherein the half-transmissive film comprises an oxide of Cu, whereby reflectivity characteristics similar to those of Au are achieved. Moreover, the use of an oxide deters promotion of oxidization, thereby avoiding changes in appearance or reflectivity due to oxidization and corrosion, and allowing production cost to be remarkably reduced due to the low material cost.
The half-transmissive film may comprise Cu mixed with a corrosion-resistant material, whereby reflectivity characteristics similar to those of Au are achieved. The use of a corrosion-resistant material added to Cu deters promotion of oxidization, thereby avoiding changes in appearance or reflectivity due oxidization and corrosion, and allowing production cost to be remarkably reduced due to the low material cost.
Preferably, the above corrosion-resistant material comprises one or a plurality of elements selected from a group consisting of Si, Mg, Mn, Cr, Sn, Al, and Be, and is added in a proportion of 25% or less so that the reflectivity of the half-transmissive film is 25 to 30% at 650 nm wavelength. It should be noted that the aforementioned reflectivity is defined to be a proportion of an amount of light reflected by the half-transmissive film relative to an amount of light incident onto the optical recording medium.
Alternatively, the half-transmissive film may comprise a stack of layers including an upper layer of a corrosion-resistant material and a lower layer of any of elements selected from a group consisting of Cu, Ag, and Al, whereby reflectivity characteristics similar to those of Au are achieved. The corrosion-resistant material of the upper layer deters the promotion of oxidization, thereby avoiding changes in the appearance or reflectivity due to oxidization and corrosion, and allowing production cost to be remarkably reduced due to the low material cost.
In the above structure, it is preferable that the lower layer has a thickness such that the reflectivity of the half-transmissive film is 25 to 30% at 650 nm wavelength, and the corrosion-resistant material of the upper layer comprises one or a plurality of elements selected from a group consisting of Si, Mg, Mn, Cr, Sn, Al, and Be, and has a thickness 30% or less of the thickness of the lower layer.
Furthermore, the present invention provides an optical recording medium having a half-transmissive film wherein the half-transmissive film comprises ZnS.SiO
2
, whereby reflectivity characteristics similar to those of Au are achieved. Moreover, the use of an oxide deters the promotion of oxidization, thereby avoiding changes in the appearance or reflectivity due to oxidization and corrosion, and allowing production cost to be remarkably reduced due to the low material cost.
The half-transmissive film may comprise a nitride of Ti, whereby reflectivity characteristics similar to those of Au are achieved. Moreover, the use of a nitride deters the promotion of oxidization, thereby avoiding changes in the appearance or reflectivity due to oxidization and corrosion, and allowing production cost to be remarkably reduced due to the low material cost.
Alternatively, the half-transmissive film may comprise an alloy of Au and Cu or a compound consisting of a plurality of elements including Au and Cu, whereby reflectivity characteristics similar to those of Au are achieved. Moreover, the use of Au deters the promotion of oxidization, thereby avoiding changes in the appearance or reflectivity due to oxidization and corrosion, and since the mixing proportion of Au is reduced, production cost is remarkably decreased due to the low material cost.


REFERENCES:
patent: 5645908 (1997-07-01), Shin
patent: 5679429 (1997-10-01), Hintz
patent: 5702792 (1997-12-01), Iida
patent: 5766717 (1998-06-01), Kaneko
patent: 5817389 (1998-10-01), Ono

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