Optical information medium and manufacturing method therefor

An optical information mediumis made up of a transparent substrate, on which are formed a dye recording layer of at least one dyestuff, a reflective layer of metal, and a protective layer in sequence, wherein said protective layer is formed from a plurality layers of resin which are formed sequentially. Further, the reflective layer is made of silver, and the dye recording layer is made of a material which does not contain iodine ion. The protective layer contains a material for trapping molecules and/or ions which corrode the reflective layer. For instance, a metal powder or porous pigment is contained in the protective layer, which is active on or absorbs the molecules and/or ions corroding the reflective layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical information medium having a 
recording layer made of a dyestuff or coloring matter, such as a 
recordable CD (compact disk), i.e., so-called a CD-R, and in particular 
relates to an optical information medium provided with the dye recording 
layer, a reflective layer and a protective layer, one by one on a 
transparent substrate. 
2. Description of Related Art 
A disk of a single-plate type in conformity with an orange book, i.e., a 
CD-R, has, as shown for example in FIG. 3, a transparent substrate 1 of 
polycarbonate in the shape of a disk, having a center hole (not shown in 
FIG. 3), on which is provided a dye recording layer 2 of an organic 
dyestuff such as a cyanine dye or the like. On the dye recording layer 2 
is provided a reflective layer 3 of a metal film, and further on the 
reflective layer 3 is provided a protective layer 4 of a UV (ultraviolet) 
curable resin. On a main surface of the above transparent substrate 1, 
there is formed a guide groove 6 in a spiral shape as a tracking means, 
and the dye recording layer 2 is formed on the main surface having such 
the guide groove 6. 
Conventionally, with an optical information medium having the dye recording 
layer 2, such as the CD-R mentioned above, it is common to use a film of 
gold as the reflective layer 3. 
For the purpose of cost reduction of such an optical information medium 
having the dye recording layer 2, such as the CD-R mentioned above, it has 
been studied to apply a film of silver as the reflective layer 3 other 
than a gold film. However, silver is inferior in chemical stability to 
gold, therefore, there is a problem of corrosion due to hydrogen sulfide 
and so on during the long-term use thereof. 
As mentioned above, the protective layer 4 of the UV curable resin, which 
is provided on the reflective layer 3, is effective to protect it against 
shock or impact and to prevent the reflective layer 3 from damage thereon. 
However, it is not necessarily effective in preventing corrosive molecules 
and/or ions of such as hydrogen sulfide and so on from penetration when 
used as a barrier for protecting the reflective layer 3 from the corrosion 
thereof. This is mainly because the UV curable resin forming the 
protective layer does not fully show a perfect airtight property, 
therefore, the corrosive molecules and/or ions penetrate therethrough, as 
well as, it is conceivable that the corrosive molecules and/or ions 
penetrate into a side of the reflective layer 3 through defects 7, such as 
minute concave or pin-holes which are caused when the protective layer 4 
is formed. Further, forming the reflective layer 2 with silver showing a 
higher chemical activity, the dyestuff of the dye recording layer 2 easily 
reacts with the silver of the reflective layer 3, therefore, the 
reflective layer 2 and the reflective layer 3 easily deteriorate. 
SUMMARY OF THE INVENTION 
An object, in accordance with the present invention, for the problem in 
weather resistance of the reflective layer when it is formed with the 
silver film, in particular, the reflective layer of the optical 
information medium having a dye recording layer, such as the CD-R, is to 
provide an optical information medium having a reflective layer which 
shows a stability for a long time, thereby providing an optical 
information medium able to be used for a long term. 
According to the present invention, for achieving the object mentioned 
above, while a reflective layer 3 is formed with a silver film, a 
protective layer 4 is formed with a plurality of layers 4a, 4b of resin 
which are formed separately and sequentially, and further, the protective 
layer 4 formed with those resin layers 4a, 4b contains a material which 
traps molecules and/or ions which corrode the reflective layer 3. Thereby, 
the corrosive molecules and/or ions will not reach the reflective layer 3 
of the silver film. 
Namely, an optical information medium, for achieving the object mentioned 
above, comprises a transparent substrate 1, on which are formed a dye 
recording layer 2 of at least one dyestuff, a reflective layer 3 of metal, 
and a protective layer 4, in sequence. Here, it is characterized in that 
said protective layer 4 comprises a plurality layers 4a, 4b of resin which 
are formed sequentially. Further, said reflective layer 3 is made of 
silver or an alloy mainly containing silver. 
In such an optical information medium, since the protective layer 4 is 
formed with the plural resin layers 4a, 4b which are formed separately and 
sequentially, then, even if the concave and/or pin-holes occur in the 
resin layers 4a, 4b, those defects are filled up with other resin films 
4a, 4b, as a result, the defects never communicate between outside air and 
the reflective layer 3. Thereby, the probability that the corrosive 
molecules and/or ions of the outside air reaching the reflective layer 3 
of the silver film is greatly reduced in comparison with that in the 
conventional arts. Accordingly, the corrosion of the reflective layer 3 
hardly occurs. 
Further, according to experiments made by the inventors of the present 
invention, it is found that the reaction between the dyestuff material for 
forming the dye recording layer 2 and the silver forming the reflective 
layer 3 accelerates in the presence of ions of iodine. Therefore, it is 
preferable that the iodine ion contained in the dye recording layer 2 is 
equal to or less than 1,000 ppm in weight %. More preferably, the iodine 
ion in the dye recording layer 2 is equal to or less than 100 ppm in 
weight %. 
Moreover, it is preferable that the protective layer 4 contains material 
for trapping molecules and/or ions which corrode the reflective layer 3. 
For example, metal particles for reacting with the molecules and/or ions 
which corrode the reflective layer 3, or porous pigments for absorbing the 
molecules and/or ions which corrode the reflective layer 3 are contained 
in the protective layer 4, thereby, the corrosive molecules and/or ions 
trying to enter into the reflective layer 3 through the protective layer 4 
are trapped within the protective layer 4. 
For those reasons, with the optical information medium according to the 
present invention, if the reflective layer is formed of a silver film, the 
reflective layer 3 hardly deteriorates, even in an atmosphere including 
the corrosive molecules and/or ions. Accordingly, it is possible to obtain 
an optical information medium which is usable or practicable for a long 
time with stability, while achieving the cost reduction thereof, in 
particular, the cost of the metal material for forming the reflective 
layer 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, detailed explanation of the embodiments according to the 
present invention will be given by referring to the attached drawings. 
FIG. 1 shows the view of a CD-R, as an optical information medium, from a 
reverse surface side of a surface upon which a reproducing light beam is 
incident, wherein a transparent substrate 1 is shown in a lower portion of 
the same figure. Further, FIG. 2 shows the diagrammatic view of the 
section of the CD-R mentioned above. 
On the surface of a transparent substrate 1 of polycarbonate resin and so 
on, a guide groove 6 for use in tracking is formed in a spiral shape, and 
further on it is formed a dye recording layer 2 of a cyanine dye or the 
like. On the dye recording layer 2, there is formed a reflective layer 3 
of silver or an alloy mainly containing silver, and further on it is 
formed a protective layer 4 of a UV curable resin. The protective layer 4 
is formed with a plurality of resin films 4a, 4b which are formed 
separately and sequentially to each other. In the embodiment shown in the 
figure, they are constructed with two (2) layers of the resin films 4a, 
4b. 
For the transparent substrate 1 in a shape of a plate for use in the 
optical information medium, a resin material is used which has a high 
transparency and a refractive index within a range from 1.4 to 1.5 with 
respect to a laser beam, and has a superior resistance against impact or 
shock. In more detail, a polycarbonate, polyolefin, and acryl can be 
listed for it, for instance, however, it should not be restricted only to 
those mentioned. 
The transparent substrate 1 is formed, for instance, by using resin 
materials and by means of an injection molding, etc. Upon the surface of 
the transparent substrate 1 is formed the guide groove 6 of the spiral 
shape, however, it can be substituted with a means of another shape or 
configuration for use in the tracking guide. Such a tracking guide means 
can be formed by use of a stamper, in general, through a conventional 
method. 
The dye recording layer 2 is formed through spin coating of an organic 
dyestuff, such as a cyanine dye or the like, on a main surface of the 
transparent substrate 1 on which the tracking means as mentioned is formed 
and thereafter drying it. In this case, the organic dyestuff which is used 
contains iodine anion at a weight % equal to or less than 1,000 ppm. 
The reflective layer 3 is formed through forming the silver film on the 
organic dyestuff layer 13. In more detail, the silver film is coated by 
means of a vacuum evaporation method or a sputtering method, thereby 
forming the reflective layer 3 of the metal film. 
The optical information medium comprises a portion, in which the 
information which is optically readable through the laser beam is 
recorded, and this means, for instance, a layer from and into which the 
information is reproduced and recorded optically by irradiating the laser 
beam thereon, or a surface of the substrate or other surface(s) thereof 
relating to the recording and reproducing of the information. For example, 
in the case of the above-mentioned optical information medium which is 
shown in FIGS. 1 and 2, the recording and reproduction of the information 
is possible by means of the dye recording layer 2 which is formed on the 
transparent substrate 1 and the reflective layer 3 which is formed 
thereon. 
The method for recording and/or reproduction is an optical one, and it is 
in general by use of a laser beam. Such a recording and/or reproduction is 
conducted from one side surface of the optical information medium, and in 
more detail, by means of irradiating the laser beam on the surface of the 
transparent substrate 1 and so on. However, no optical recording and/or 
reproduction of the information is conducted from the other surface 
thereof. 
In the case where the laser beam is used as a light for recording and 
reproducing, it is common to use a laser beam having a wavelength of from 
770 to 830 nm, however, it is also possible to use a laser beam having a 
wavelength other than that. 
Further, it is also possible to provide a layer other than the dye 
recording layer 2 and the reflective layer 3 which are shown in FIG. 2. 
For example, a layer for improving the bonding property can be provided 
for the purpose of increasing the reliability, other than recording of the 
information. Also in FIG. 2, the dye recording layer 2 is coated and 
adhered on the transparent substrate 1 directly, however, there can be a 
case where another layer is provided therebetween. 
The protective layer 4 is a layer for protecting the information recording 
portion from physical and mechanical obstruction received from a side 
opposing the transparent substrate 1, therefore, it is provided at the 
side opposite to the transparent substrate 1. 
As mentioned previously, the protective layer 4 is formed with a plurality 
of resin layers 4a, 4b which are formed separately and sequentially. Here, 
separately and sequentially means that, after a solid resin layer 4a is 
formed by applying the resin material and then curing it, a separate resin 
layer 4b is formed on it. In more detail, the UV curable resin is applied 
on the surface of the reflective layer 3 or on the other layer formed 
thereon by means of spin coating, and then an Ultraviolet (UV) ray is 
irradiated thereon to cure it. After being formed on the resin layer 4a, 
the other resin layer 4b is formed on the resin layer 4a in the same 
manner. In the embodiment shown in the figure, the resin layer is of two 
(2) layers 4a, 4b, however, the protective layer can be formed of more 
than the two resin layers 4a, 4b. 
The thickness is several pm for each of the resin layers 4a, 4b, and that 
of the protective layer as a whole is from 10 to 20 .mu.m. It is 
preferable to mix a material as an additive, which easily reacts with the 
corrosive molecules and/or ions, such as hydrogen sulfide and sulfur 
dioxide, into the resin layers 4a, 4b forming the protective layer 4. As 
the additive, for example, metal particles, in particular, particles of 
silver are added into the resin material. Thereby are trapped the 
corrosive molecules and/or ions mentioned above, trying to penetrate 
through the protective layer 4 into the reflective layer 3, therefore, it 
is possible to inhibit them from reaching the reflective layer 3. 
The amount of addition of the material showing a reactive property with the 
corrosive molecules and/or ions mentioned above, with respect to that of 
the resin material, is, for example, in a range from 1 to 50 weight % to 
the total weight of the resin material and the additive. This is because, 
if it is less than 1 weight % in the addition amount, it is ineffective in 
trapping the corrosive molecules and/or ions. While, if exceeding 50 
weight % in the addition amount, it loses its property of fluidity as a 
resin paint, thereby making it difficult to form the resin layers 4a, 4b 
therewith. The particles of the additive is equal to or less than 10 .mu.m 
in the diameter thereof. 
It is possible to add such an additive only into the resin layer 4b at the 
surface side, but it is more effective to add it into all of the resin 
layers 4a, 4b forming the protective layer 4. 
Further, in place of the material having the property of reacting with the 
corrosive molecules and/or ions, it is also possible to add into the 
protective layer 4 a material which shows a property of absorbing the 
corrosive molecules and/or ions. For that purpose, there can be listed a 
porous inorganic pigment powder made of, e.g., silica, activated carbon 
(charcoal), talc, mica, calcium carbonate, titanium oxide, zinc white (or 
zinc flower), colloidal silica, carbon black, Indian red, and so on. 
The amount of addition of the material showing the property of absorbing 
the corrosive molecules and/or ions as mentioned above, with respect to 
that of the resin material, is in a range from 2 to 20 weight % to the 
total weight of the resin material and the additive, for example. This is 
because, if it is less than 2 weight % in the addition amount, it is 
inadequate in the effect of trapping the corrosive molecules and/or ions. 
While, if exceeding 20 weight % in the addition amount, it loses its 
property of fluidity as the resin paint, thereby making it difficult to 
form the resin layers 4a, 4b therewith. The particles of the additive are 
equal to or less than 10 .mu.m in the diameter thereof. 
It is also possible to add the additive only into the resin layer 4b at the 
surface side, however, it is more effective to add it into all of the 
resin layers 4a, 4b forming the protective layer 4. 
As mentioned previously, since the protective layer 4 is formed with the 
plurality of resin layers 4a, 4b, even if defects 7a and 7b, such as a pin 
hole or the like, are caused in the lower resin layer 4a and the upper 
resin layer 4b, as shown in FIG. 2 for instance, those defects 7a and 7b 
are filled up with the other resin layer 4a, 4b as far as they are located 
differently to each other. As a result of this, the surface of the 
reflective layer 3 is sealed from outside air. Further, by mixing the 
additive for trapping the corrosive molecules and/or ions into the resin 
layers 4a, 4b, those molecules and/or ions are trapped in the protective 
layer 4, and do not reach the reflective layer 3. 
The protective layer 4 is preferably made of a resin having a superior 
shock or impact resistance. For instance, the hardness of the protective 
layer 4 is preferably within a range from 2H to 7H/Grass of the hardness 
of lead of a pencil, for example. Further, it is preferable that the 
thermal deformation temperature of it is equal to or higher than 
80.degree. C., and more preferably equal to or higher than 100.degree. C. 
With respect to the thickness of the protective layer 4, it is preferably 
within a range from 10 to 20 .mu.m, and it can be formed with the 
plurality of layers of materials being different from each other. 
Next, examples and a comparison will be explained by showing specific 
values thereof. 
EXAMPLE 1 
On the transparent substrate 1 of the polycarbonate, formed with the guide 
groove 6, a solution of the cyanine dye was applied through spin coating 
and was cured so as to form the dye recording layer 2. However, the 
solution of the cyanine dye was used here in which the iodine anion is not 
included. On the surface of the dye recording layer 2 was sputtered the 
silver to form the reflective layer 3. Further, on the reflective layer 3, 
the UV curable resin was applied through spin coating, and it was 
optically cured by irradiating the UV ray thereon, thereby forming a 
transparent first resin layer 4a of a thickness of 5 .mu.m. After the 
first resin layer 4a was cured, the UV curable resin was also applied 
through spin coating on the first protective layer 4a and was optically 
cured by irradiating the UV ray thereon, thereby forming a transparent 
second resin layer 4b of a thickness of 5 .mu.m. 
Among five-hundred (500) units of optical information mediums made in such 
manner, eighty (80) units of them were sampled at random, and further 
among of those, twenty (20) units of them were measured with a block error 
rate (BLER) thereof. An average value of those results is shown in the 
column 0 hour in Table 1. 
Further, sixty (60) units of the sampled optical information mediums were 
disposed under a gaseous atmosphere in which hydrogen sulfide of 10 ppm 
was mixed into a vaporous atmosphere of a humidity of 95% and a 
temperature at 70.degree. C. Twenty (20) units of them were picked up 
every time after 50 hours, 100 hours and 500 hours after starting the 
disposal thereof, and were measured with the block error rate (BLER) 
thereof, in the same manner mentioned above. The average values of those 
results are shown in the columns 50 hour, 100 hour and 500 hour in Table 
1, respectively. 
EXAMPLE 2 
Optical information mediums were made or prepared in the same manner as 
Example 1 mentioned above, except that a solution of the UV curable resin 
was used, into which silica powder equal to or less than 10 .mu.m in the 
diameter thereof was mixed and dispersed, when forming the second resin 
layer 4b. However, the addition amount of the silica powder to the total 
weight of the silica powder and the UV curable resin was 20 weight %. 
Among five-hundred (500) units of those optical information mediums made in 
such manner, eighty (80) units of them were sampled at random, and they 
were tested under an atmospheric condition identical to that mentioned 
above and were measured with the block error rate (BLER) thereof, at every 
time after starting the disposal thereof. The average values of those 
results are shown in each column of times in Table 1, respectively. Here, 
0 hour means the block error rate (BLER) which is measured without 
conducting the atmospheric test. 
EXAMPLE 3 
Optical information mediums were made or prepared in the same manner as 
Example 1 mentioned above, except that the second resin layer 4b was 
formed by applying a hydrophilic UV curable resin ink (hydrophilic UV 
curable resin ink for printing labels), into which silver powder equal to 
or less than 10 .mu.m in diameter thereof was mixed and dispersed, through 
a screen printing and cured with the UV ray. Here, the addition amount of 
the silver powder to the total weight of the silver powder and the UV 
curable resin was 20 weight %. 
Among five-hundred (500) units of those optical information mediums made in 
such manner, eighty (80) units of them were sampled at random, and they 
were tested under an atmospheric condition identical to that mentioned 
above and measured with the block error rate (BLER) thereof at every time 
after starting the disposal thereof. The average values of those results 
are shown in the column of times in Table 1, respectively. Here, 0 hour 
means the block error rate (BLER) which is measured without conducting the 
atmospheric test. 
Comparison 
Optical information mediums were made or prepared in the same manner as 
Example 1 mentioned above, except that the protective layer 4 having a 
thickness of 10 .mu.m was formed at one time, instead of forming the first 
resin layer 4a and the second resin layer 4b separately, and that a 
solution was used which contained the iodine anion in an amount of 5 
weight % as the cyanine dye therein. 
Among five-hundred (500) units of those optical information mediums made in 
such manner, eighty (80) units of them were sampled at random, and they 
were tested under an atmospheric condition identical to that mentioned 
above and were measured with the block error rate (BLER) thereof, at every 
time after starting the disposal thereof. The average values of those 
results are shown in each column of times in Table 1, respectively. Here, 
0 hour means the block error rate (BLER) which was measured without 
conducting the atmospheric test. 
TABLE 1 
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Test Embodiment 
Embodiment Embodiment 
Hour(s) 1 2 3 Comparison 
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0 hour 1 cps 1 cps 1 cps 1 cps 
50 hours 9 cps 1 cps 1 cps 30 cps 
100 hours 80 cps 1 cps 1 cps 200 cps 
500 hours 300 cps 1 cps 1 cps 900 cps 
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As is apparent from the results mentioned above, with the optical 
information medium, in which the protective layer 4 was formed of two 
layers of the resin layers 4a, 4b, in comparison with the optical 
information medium which had the protective layer formed from a single 
resin layer, the increase in the block error rate was small or almost 
zero, when it was disposed in an atmosphere of hydrogen sulfide under a 
high temperature and high humidity. This is mainly because of the 
deterioration in signal characteristics due to the corrosion by the 
hydrogen sulfide in the reflective layer 3 of the silver film. Further, 
with Example 2 and Example 3 in which the additives for trapping the 
hydrogen sulfide were added into the protective layer 4, almost no 
increase could be recognized in the block error rate thereof, when the 
optical information medium was disposed in an atmosphere of hydrogen 
sulfide under a high temperature and high humidity.