Optical recording medium and method of preparing same

An optical recording medium having at least one aluminum nitride and silicon nitride composite dielectric layer is provided. The amount of aluminum nitride in the dielectric layer, x, is more than zero and less than or equal to about 95 mol %. The amount of silicon nitride in the layer is represented as (100-x) and is equal to or greater than about 5 and less than 100 mol %. The refractive index of the composite dielectric layer is between about 1.70 and 2.15. The composite dielectric layer is sandwiched between a transparent support such as a plastic transparent support and an optical recording layer adapted to be irradiated by laser light in order to read, write or erase information in order to protect the recording layer.

BACKGROUND OF THE INVENTION 
The present invention relates to optical recording media having composite 
dielectric layers and more particularly, to an improved optical recording 
medium having a composite dielectric layer formed of aluminum nitride and 
silicon nitride having a specific composition and refractive index. 
Information can be written onto prior art optical recording media by 
irradiation of condensed laser light onto a perpendicular magnetic 
recording layer in order to invert the magnetization of the recording 
layer. Alternatively, laser light irradiated onto the recording layer can 
be used to change the crystalline structure of the layer. Specifically, 
the crystalline structure can be changed from crystalline to amorphous, 
from amorphous to crystalline, from hexagonal to cubic, from cubic to 
hexagonal and the like by making holes, forming bubbles or otherwise 
transforming the recording medium using laser phase transition. 
Such optical recording media generally have plastic substrates or supports 
including polycarbonate, polymethylmethacrylate and epoxy resin supports 
with guide grooves formed therein. The plastic supports can be formed by 
injection molding in order to reduce cost and permit mass production. 
However, plastic supports are not completely satisfactory because they are 
susceptible to moisture absorption and are gas permeable. As a result, 
magneto-optic recording layers containing alloy films of rare earth metals 
and transition metals such as GdTbFe, TbFeCo, GdTbFeCo, DyFeCo, NdDyFeCo 
and NdDyFeCoTi are readily attacked by atmospheric moisture and gasses and 
the magnetic properties deteriorate significantly. 
Dielectric oxide layers are provided between the plastic supports and the 
magneto-optic recording layers thereon in order to improve corrosion 
resistance and overcome the disadvantages of the synthetic resin supports. 
The protective effect of such dielectric layers is not completely 
satisfactory because free oxygen in the dielectric oxide layer tends to 
oxidize the optical recording layer. 
Alternatively, non-oxide dielectric films such as silicon nitride, aluminum 
nitride and zinc sulfide have been formed on plastic supports. These 
non-oxide dielectric films do not completely adhere to the support and 
recording media provided on the dielectric films crack as a result. This 
method is therefore not suitable for practical use. 
Accordingly, it is desirable to provide a composite dielectric layer that 
overcomes the disadvantages of prior art dielectric layers. 
SUMMARY OF THE INVENTION 
Generally speaking, in accordance with the invention, an optical recording 
medium having an aluminum nitride and silicon nitride composite dielectric 
layer is provided. The amount of aluminum nitride in the dielectric layer, 
x, is more than zero and less than or equal to about 95 mol %. The amount 
of silicon nitride in the layer is represented as (100-x) and is equal to 
or greater than about 5 and less than 100 mol %. The refractive index of 
the composite dielectric layer is between about 1.70 and 2.15. The 
composite dielectric layer is sandwiched between a transparent support 
such as a plastic transparent support and an optical recording layer 
adapted to be irradiated by laser light in order to read, write or erase 
information in order to protect the recording layer. 
It is, therefore, an object of the invention to provide an improved 
composite dielectric layer. 
Another object of the invention is to provide a composite dielectric layer 
formed of aluminum nitride and silicon nitride and having a specific 
composition and refractive index. 
A further object of the invention is to provide a composite dielectric 
layer that adheres to a plastic support. 
Still another object of the invention is to provide a composite dielectric 
layer that does not oxidize the optical recording layer. 
Still a further object of the invention is to provide composite dielectric 
layer that protects a recording layer from deterioration in 
carrier-to-noise ratio and bit error rate over an extended period of time. 
Yet another object of the invention is to provide a composite dielectric 
layer that does not crack even after an extended period of time. 
Yet a further object of the invention is to provide a method of preparing 
an improved composite dielectric layer. 
Still other objects and advantages of the invention will in part be obvious 
and will in part be apparent from the specification. 
The invention accordingly comprises a product possessing the 
characteristics, properties, and the relation of components which will be 
exemplified in the product hereinafter described and the several steps and 
the relation of one or more of such steps with respect to each of the 
others, and the scope of the invention will be indicated in the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The recording medium of the invention has a dielectric layer having a 
refractive index between about 1.70 and 2.15 sandwiched between a 
transparent support and an optical recording layer. The amount of aluminum 
nitride, x, is more than zero and less than or equal to about 95 mol %. 
The amount of silicon nitride is (100-x) or equal to or greater than about 
5 and less than 100 mol %. 
Composite dielectric films having a refractive index of less than about 
1.70 are porous and cause deterioration of the optical recording medium. 
On the other hand, composite dielectric films having a refractive index of 
greater than about 2.15 can contain unreacted aluminum or silicon which 
causes the optical constant to change during accelerated aging 
(weatherability) testing and indicates that the optical recording medium 
will change over time. Thus, it is necessary for the refractive index of 
the composite dielectric layer to be between about 1.70 and 2.15. 
Furthermore, when the aluminum nitride content of the composition exceeds 
95 mol %, the film exhibits cracks during accelerated aging tests. On the 
other hand, when the silicon nitride content of the composition approaches 
100 mol %, the photo polymer (2P) solution used to adhere the substrates 
together peels after adhesion because silicon nitride layers develop 
pinholes and the 2P solution attacks the recording layer through the 
pinholes. 
The recording media and composite dielectric layers of the invention will 
be better understood with reference to the following examples. These 
examples are presented for illustration only and are not intended to be 
construed in a limiting sense. 
EXAMPLE 1 
An optical recording medium 8 having the structure shown in FIG. 1 in 
accordance with the invention was prepared as follows. A polycarbonate 
support 1 was grooved to a groove pitch of 1.6 .mu.m, a groove width of 
0.8 .mu.m and a groove depth of 600 .ANG.. A first aluminum 
nitride/silicon nitride composite dielectric film 2 having a thickness of 
1000 .ANG. was formed on the grooved side of polycarbonate support 1. An 
NdDyGdFeCoTi recording layer 3 having a thickness of 400 .ANG. was formed 
on composite dielectric film 2. A second aluminum nitride/silicon nitride 
composite dielectric film 4 having a thickness of 1000 .ANG. was formed on 
recording layer 3. Polycarbonate support 1, first dielectric layer 2, 
recording layer 3 and second dielectric layer 4 formed an optical 
transmission substrate 7. The grooved surface of optical transmission 
substrate 7 was adhered to a smooth polycarbonate support 6 using a photo 
polymer (2P) resin 5 to provide optical recording medium 8. 
Dielectric layers 2 and 4 were formed by sputtering using either alloy 
targets or sintered targets of aluminum and silicon as sputtering targets 
to form a composite dielectric layer of aluminum nitride and silicon 
nitride. Alloy targets were preferred unless the ratio of aluminum to 
silicon was not suitable for alloying. In those cases, sintered targets 
were used. 
The sputtering conditions were: 
______________________________________ 
Argon pressure 2.5 mTorr 
Partial nitrogen pressure 
0.5 mTorr 
Type of sputtering Reactive RF magnetron 
sputtering 
Power Constant at 500 W 
______________________________________ 
A variety of dielectric layer compositions were prepared. The ratio of 
aluminum to silicon in the compositions of the sputtering targets were as 
follows: 
______________________________________ 
Run Composition* 
______________________________________ 
(1) Al:Si = 0.1:99.9 mol % 
(2) Al:Si = 2:98 mol % 
(3) Al:Si = 10:90 mol % 
(4) Al:Si = 20:80 mol % 
(5) Al:Si = 30:70 mol % 
(6) Al:Si = 60:40 mol % 
(7) Al:Si = 80:20 mol % 
(8) Al:Si = 90:10 mol % 
(9) Al:Si = 95:5 mol % 
(10) Al:Si = 95.1:4.9 mol % 
(11) Al:Si = 96:4 mol % 
(12) Al:Si = 98:2 mol % 
______________________________________ 
*The precise composition of the dielectric layers resulting from the use 
of specific sputtering targets was not conclusively demonstrated. However 
it is assumed that the approximate ratio of aluminum nitride to silicon 
nitride in the resultant dielectric layers was approximately the same as 
the proportion of aluminum to silicon in the sputtering target. 
"Aluminum nitride", as used herein, refers to mixtures of aluminum and 
nitrogen which occur in various complex compositions in an amorphous 
state. Specifically, some of the aluminum nitride is in the form of AlN 
and other portions are other combinations, mixtures and compounds of 
aluminum and nitrogen. Similarly, "silicon nitride" refers to 
combinations, mixtures and compounds of silicon and nitrogen and not 
specifically to Si.sub.3 N.sub.4. 
The photo polymer resin used to adhere optical transmission substrate 7 to 
ungrooved polycarbonate support 6 is an adhesive resin which is 
transformed from a monomer to a polymer when cured using ultraviolet 
light. The photo polymer resin solution (2P solution) has a low viscosity 
before curing and a higher viscosity after curing. When used with silicon 
nitride dielectric layers which do not adhere strongly to a substrate, the 
2P solution causes the dielectric layer to be peeled from the substrate. 
The media samples of Runs (1) to (12) were subjected to accelerated aging 
tests in a chamber maintained at a controlled temperature and humidity of 
60.degree. C. and 90% RH, respectively. The samples were examined after 
1000 hours to determine whether they exhibited cracks. The samples in Runs 
(10), (11) and (12) having aluminum to silicon proportions of 95.1:4.9 mol 
%; 96:4 mol %; and 98:2 mol % exhibited cracking after 1000 hours. 
For comparison, an optical recording medium having dielectric layers 
composed of 100% silicon was prepared and subjected to an accelerated 
aging test of the type described. This medium also exhibited cracks. As 
has been demonstrated, when the amount of aluminum nitride, x, is greater 
than 95 mol % or is not present at all, the dielectric layer exhibited 
cracking. Accordingly, the amount of aluminum nitride, x, should be more 
than zero and less than or equal to about 95 mol %. Conversely, the amount 
of silicon nitride, (100-x), should be equal to or greater than about 5 
and less than 100 mol %. The composite dielectric layers had a constant 
refractive index of 2.0. 
FIG. 2 shows the change in Bit Error Rate measured as a function of time 
when the samples were subjected to the accelerated aging test conducted at 
a constant temperature of 60.degree. C. and a relative humidity of 90% as 
described above. The ordinate of the graph shows the Bit Error Rate where 
a bit is 1.0 .mu.m in length and the abscissa shows the elapsed time. 
Curve 28 represents optical recording media having composite aluminun 
nitride/silicon nitride dielectric layers wherein the amount of aluminum 
was 0.1, 2, 10, 20, 30, 60, 80, 90 or 95 mol %. The optical recording 
medium represented by curve 20 had silicon nitride dielectric layers. The 
optical recording medium represented by curve 29 had a composite 
dielectric layer of aluminum nitride and silicon nitride wherein the 
proportion of aluminum in the target for preparing the medium was 95.1, 96 
or 98 mol %. 
The recording media prepared according to the invention and represented by 
curve 28 do not show a change in Bit Error Rate even after 5000 hours 
indicating reliability for over 50 years. Media formed from a target in 
which the proportion of aluminum is greater than about 95 mol % as 
represented by curve 29 exhibited cracks after 200 hours and were not 
suitable for practical use. Media prepared using silicon nitride 
dielectric layers represented by curve 20 began to exhibit cracks after 
100 hours. 
Inductively coupled plasma (ICP) analysis performed on media prepared 
according to the invention indicated that the dielectric layers have the 
same composition ratio as the sputtering targets from which they are 
formed. ICP is a method of analysis for specifying the type and amount of 
an element by the color and intensity of a flame. This is accomplished by 
burning a solution in which an experimental material is dissolved. 
EXAMPLE 2 
FIG. 3 shows the structure of another optical recording medium 38 which is 
similar to the structure of optical recording medium 8 shown in Example 1 
except for the support material. Recording medium 38 includes a 
polymethylmethacrylate (PMMA) grooved support 31 and a smooth 
polymethylmethacrylate support 36 in place of polycarbonate supports 1 and 
6 used in recording medium 8 of Example 1. Otherwise, recording medium 38 
is the same as recording medium 8 of Example 1. 
FIG. 4 is a graph showing Bit Error Rate as a function of time when 
recording media 38 were subjected to an accelerated aging test at 
60.degree. C. and 90% relative humidity as described in Example 1. Curve 
43 shows the results of accelerated aging testing of an optical recording 
medium 38 having dielectric layers 32 and 34 containing between about 0.1 
and 95 mol % of aluminum. The Bit Error Rate for these media began to 
increase after about 4,000 hours had passed. 
Curve 44 shows the results of accelerated aging testing when dielectric 
layers 32 and 34 contained more than about 95 mol % aluminum. These media 
began to exhibit cracks after 1,000 hours had elapsed. Curve 45 shows the 
results of accelerated aging testing on an optical recording media having 
dielectric layers 32 and 34 formed only of silicon nitride. These media 
began to exhibit cracks after only 60 hours. 
Media 38 deteriorated sooner than media 8 of Example 1 due to the use of 
polymethylmethacrylate supports 31 and 36. However, in spite of the use of 
polymethylmethacrylate supports, reliability for up to 40 years is ensured 
when the media were prepared using dielectric layers in accordance with 
the invention. 
EXAMPLE 3 
Optical recording media having the structure 8 shown in FIG. 1 and prepared 
as described in Example 1 were prepared except that the refractive index 
of dielectric layers 2 and 4 was evaluated using a sputtering target 
contained 20 mol % of aluminum and 80 mol % of silicon. The refractive 
index of dielectric layers 2 and 4 was adjusted by changing the argon 
pressure and partial pressure of nitrogen during sputtering. An 
accelerated aging test was conducted at 60.degree. C. and 90% relative 
humidity as described in Example 1. 
FIG. 5 is a graph showing the relationship between Kerr rotation angle 
measured from the substrate side of the optical recording media as a 
function of time. Kerr rotation angle is shown as the ratio of Kerr 
rotation angle at time t, .theta..sub.kr(t) to Kerr rotation angle at time 
0, .theta..sub.kr(0). Kerr rotation angle at time t is the angle after 
time t has elapsed. Kerr rotation angle at time 0 is the angle when the 
film has just been completed. 
Curve 55 shows the results for optical recording media having composite 
dielectric layers Al.sub.20 Si.sub.80 N with refractive indices, n, of 
2.15, 2.01, 1.90, 1.85, 1.80 and 1.70, respectively. Curve 56 shows the 
results for an optical recording medium using composite dielectric layers 
with refractive indices, n, of 2.24. Curve 57 shows the results for an 
optical recording medium using composite dielectric layers having 
refractive indices, n, of 2.31. Curve 58 shows the results for optical 
recording media using composite dielectric layers having refractive 
indices, n, of 1.69 or 1.65. Curve 59 shows the results for optical 
recording media using composite dielectric layers having refractive 
indices, n, of 1.63 or 1.60. 
As can be seen from the graph, optical recording media prepared in 
accordance with the invention having a refractive index between about 1.70 
and 2.15 do not change even after 5,000 hours have passed. On the other 
hand, optical recording media represented by curves 56 and 57 having 
refractive indices of greater than 2.15 change after the initial time 
period of the accelerated aging test, i.e. after 10 to 30 hours, and the 
ratio of .theta..sub.kr(t) /.theta..sub.kr(0) decreases as a result. 
Dielectric layers having refractive indices of greater than 2.15 contain 
unreacted aluminum or silicon which change into stable oxides during the 
accelerated aging test. As a result, .theta..sub.kr(t) changes and affects 
the optical recording properties of the media, i.e. the carrier-to-noise 
(C/N) ratio. 
The optical recording media represented by curves 58 and 59 having a 
refractive index of less than 1.70 began to change after about 100 hours 
and .theta..sub.kr(t) approached 0. This is due to the fact that a 
dielectric layer having a refractive index of less than about 1.70 
exhibits porous film formation and moisture or oxygen enters the film 
during the accelerated aging test. As a result, the optical recording 
layer deteriorates and .theta..sub.kr(t) changes. The carrier-to-noise 
ratio is also affected. 
FIG. 6 is a graph showing the relationship between coercivity of optical 
recording media as a function of time. The ratio of coercivity Hc(t) which 
is the coercivity after time t has passed to coercivity Hc(0) which is the 
coercivity when the film has just been completed is plotted as a function 
of time t. 
Curve 60 represents recording media having dielectric layers with 
refractive indices, n, of 2.15, 2.01, 1.90, 1.85, 1.80, 1.70, 2.24 and 
2.31. Curve 61 represents optical recording media having dielectric layers 
with refractive indices, n, of 1.69 and 1.65. Curve 62 represent optical 
recording media with dielectric layers having refractive indices, n, of 
1.63 or 1.60. As can be seen, the coercity of recording media with 
dielectric layers whose refractive index, n, is less than 1.70 change with 
time because the dielectric layers form porous films which cause the 
magneto-optic recording layer to deteriorate. This is particularly true 
where the magneto-optic recording layer includes more richly transition 
metal elements than that under the condition of compensation composition. 
On the other hand, the coercivity of media with dielectric layers having 
refractive indices, n, of greater than about 1.70 are stable for extended 
periods of time. However, media having refractive indices, n, of greater 
than 2.15 are not suitable for use as dielectric layers for the reasons 
explained in connection with FIG. 5. None of the media used in this 
Example exhibited cracks even after 5,000 hours of accelerated aging 
testing. 
The composite dielectric layers tested were formed using targets consisting 
of Al.sub.20 Si.sub.80 and the refractive indices were changed as 
indicated. When sputtering targets formed from Al.sub.0.1 Si.sub.99.9 ; 
Al.sub.2 Si.sub.98 ; Al.sub.10 Si.sub.90 ; Al.sub.20 Si.sub.80 ; Al.sub.30 
Si.sub.70 ; Al.sub.60 Si.sub.40 ; Al.sub.80 Si.sub.20 ; Al.sub.90 
Si.sub.10 and Al.sub.95 Si.sub.5 mol % were used, similar results were 
obtained. Accordingly, it can be seen that dielectric layers having 
refractive indices between about 1.70 and 2.15 are most suitable for use 
as protective layers. 
The invention is not limited to the Examples discussed. Other polymer 
supports can be used in place of the polycarbonate and 
polymethylmethacrylate supports shown and can include epoxy resin 
supports, amorphous polyolefin supports and glass supports. Similar 
effects are obtained regardless of the type of support used. 
In addition, the optical transmission substrate and ungrooved support can 
be adhered to each other by adhesives other than the photo polymer 
adhesive cured by ultraviolet light described. The disk can be adhered as 
an air sandwich wherein the optical transmission substrate and ungrooved 
support are adhered only at their respective edges with air in between. 
The optical recording layers can include NdDyGdFeCoTi or magneto-optic 
recording layers such as TbFeCo, GdTbFeCo and the like. The invention can 
also be applied to phase transition type optical recording media in 
addition to the magneto-optic recording media shown. Finally, the Examples 
show formation of a composite dielectric layer by nitrogen reactive 
sputtering using alloy or sintered targets of aluminum and silicon. 
Alternatively, co-reactive sputtering using an aluminum target and a 
silicon target can be performed. Sintered targets of aluminum nitride and 
silicon nitride can also be used. Alternatively, the dielectric layers can 
be formed by deposition methods. 
Optical recording media prepared in accordance with the invention do not 
deteriorate over extended periods of time. Specifically, such media do not 
exhibit cracks and do not experience deterioration in the carrier-to-noise 
ratio or the Bit Error Rate for periods of up to about 50 years. The 
reliability of recording media over extended periods of time is thereby 
greatly improved. 
It will thus be seen that the objects set forth above, among those made 
apparent from the preceding description, are efficiently attained and, 
since certain changes may be made in carrying out the above method and in 
the composition set forth without departing from the spirit and scope of 
the invention, it is intended that all matter contained in the above 
description and shown in the accompanying drawings shall be interpreted as 
illustrative and not in a limiting sense. 
It is also to be understood that the following claims are intended to cover 
all of the generic and specific features of the invention herein described 
and all statements of the scope of the invention which, as a matter of 
language, might be said to fall therebetween.