Performance recording media for recordable element using silver reflector

An optical disk with improved stability and performance including a transparent substrate; and a recording layer over the substrate and a reflective layer formed of Ag over the recording layer and having its property changed by heat treatment for a time and temperature in a range of about 50.degree.-120.degree. C. so as to significantly improve window margin, reflectivity, jitter, and stability.

FIELD OF THE INVENTION 
The present invention relates to optical recording elements and methods of 
forming such elements with improved performance. 
BACKGROUND OF THE INVENTION 
Optical disks are becoming more and more prevalent for the use of recording 
information. One form of optical recording disks is called a CD-R or a 
recordable compact disk. The Photo CD is an example of this CD-R media. 
Typically, this type of disk has a transparent substrate, a recording 
layer formed on a substrate, and a reflective layer on the recording 
layer. The recording layer is essentially a photo absorption material made 
of mixture of some organic dye materials and is formed by spin coating. 
The recording materials used for CD-R applications have been described in 
U.S. Pat. Nos. 4,940,618; 5,604,004; 5,294,471; European Patent 
Application 0353393; and Canadian Patent 2,005,520. Commercial useful 
materials of the type described in these references have stringent 
requirements. One of these requirement is light stability. Since the Photo 
CD is a consumer product, it must be capable of withstanding extreme 
environment. The stability of the disk mainly depends on the nature of the 
recording layer and the reflector layer and their mutual interaction; and 
the protective overcoat. The above applications disclose phthalocyanine 
dye, metallized formazan dye and cyanine dye having excellent light 
stability. The reflecting layer is usually selected to be gold or a gold 
alloy because of its nobleness and high reflectivity. The CD-R 
specifications require that it has a high reflectivity of more than 60% 
similar to the compact disks. 
During recording, writing laser light passes through the plastic substrate 
and is focused on the dye recording layer which is heated to the 
decomposition temperature of the dye material. While the surface of the 
substrate is also heated to near the glass transition temperature of the 
substrate material. Then a small part of the dye material is decomposed 
and decomposition gas is generated in the photo-absorption layer. It 
causes the deformation of the recording layer as well as the dye/substrate 
interface. In those areas having the deformations or pits, the 
reflectivity is lower than in those areas not having the deformation which 
has passed through the recording layer is reflected back by the reflective 
layer and further enhances the process of forming the mark. Marks are 
formed either as a pit or as a change in the optical properties of the 
recording layer. In any event, the combination of some or all of these 
changes forms marks which can then be read back by the focused read laser 
beam. The record thus consists of marks of relatively low reflectivity on 
a background of relatively high reflectivity in relation to the read beam. 
Thin layer of gold is normally used as a main reflection material in the 
reflective layer. It is a noble metal with a very high stability and does 
not introduce problems into the recording stability. When other metals 
which have a high reflectivity such as aluminum, silver and copper are 
used instead of gold, they have a problem in that they are reactive and 
can form oxides or other corrosive layers. The recording stability of 
these types of disks varies over time and degrades. However, materials 
such as silver are much less expensive than gold, and it would be highly 
desirable to use them. Also silver reflector has about 5-7% higher 
reflectivity than gold. 
Jitter of a recorded feature is related to its ability of being detected 
without error during read back. Transitions from nominally identical 
recorded feature will not be read back precisely at the same time because 
of the slight variation in feature length and shape and system noise. This 
gives rise to a spread in detection time. A detection time window can read 
all these features if the distribution is so narrow as to lie completely 
within the time window. On the other hand, if the distribution is broad 
such that some of the transitions occur outside the window, they will 
result in a decoding error. Jitter is a measure of the overall noise and 
is the square root of the variance of the distribution of detection time 
commonly modeled as a Gaussian curve. The distribution of detection time 
may not be exactly centered in the timing window which will increase the 
probability of a decoding error even for a narrow distribution. The window 
margin (WM) is a derived parameter involving jitters and peak shifts of 
all recorded features. The lower the jitters and peak shifts, the higher 
is the Wm. The WM can be viewed as a figure of merit in that the discs 
with higher Wm has a greater probability of successful read back than the 
one with lower Wm. Also the disc with higher WM is expected to be read 
back by a wider variety of readers than the one with lower WM that is 
otherwise similar. 
SUMMARY OF THE INVENTION 
Therefore it is an object of this invention to provide recording media with 
markedly improved stability and properties, particularly with respect to 
WM, jitter, nonlinearity, and reflectivity. 
This object is achieved by a recordable element with improved performance, 
comprising 
a) a transparent substrate; and 
b) a recording layer over the substrate and a reflective layer formed of Ag 
over the recording layer and having its property changed by heat treatment 
for a sufficient time and at a temperature in a range of about 
50-120.degree. C. so as to significantly improve window margin, 
reflectivity, jitter, and thermal stability. 
ADVANTAGES 
The media produced in accordance with the present invention have improved 
performance in, window margin, jitter, nonlinearity and reflectivity. 
The disk prepared according to the present invention has improved data 
stability. 
Silver metal is relatively inexpensive than gold and the disk can be 
manufactured at a substantially reduced cost.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a structure of a writable storage disk made in accordance with 
the invention. It includes a substrate 10 which is made of polycarbonate 
and a dye recording layer 20. The recording layer is deposited using a 
spin coating technique. On the recording layer 20 is provided a reflecting 
layer 30 which is formed of silver. The thickness of the reflecting layer 
generally is between 60 to 80 nm. FIG. 2 is a structure similar to FIG. 1 
but it has additional layer between the dye recording layer and the silver 
metal reflector of a metallic interlayer 25 consisting Pd, Au, Ni and Sn 
or a transparent dielectric interlayer consisting of Ge--C--H, Si--N and 
InSnSb--O and alloys. For a more detailed description of the use of 
interlayers, see commonly assigned U.S. patent application Ser. No. 
08/971,969, (docket 76,892) filed Nov. 17, 1997 entitled "Recordable 
Optical Disks With Metallic Interlayer" and U.S. patent application Ser. 
No. 08/971,740, (docket 76,936) filed Nov. 17, 1997 entitled "Recordable 
Optical Disks With Dielectric Interlayer." 
Thin films of silver reflector and other metals were deposited by sputter 
deposition using DC magnetron gun and metal targets in an argon 
atmosphere. Thin films of Ge--C--H were deposited by sputter depositing Ge 
target in Ar and CH.sub.4 atmosphere and Si--N films were prepared by 
sputter depositing Si target in Ar and N.sub.2 atmosphere. Similarly 
InSnSb--O were prepared by sputter depositing InSnSb target in an 
atmosphere with a mixtures of argon and oxygen. Then the UV-curable 
lacquer overcoat was spin coated on the reflecting layer. These disks were 
tested for reflectivity (R.sub.top), window margin (WM), jitters and other 
relevant parameters using Kodak PCD 600 6X writer/reader at 2.4 m/sec. 
Here, R.sub.top is reflectivity measured by PCD-600 as the reflectivity of 
the unwritten land. R.sub.top is substantially lower than the true 
reflectivity because of birefringence, groove structure of the disk and 
the influence of the neighboring marks and tracks. 
Several full structure disks were prepared using different combination of 
interlayers and the reflectors. Au is noble and provides a long term 
stability for the recording layer. Silver metal has higher reflectivity 
than the gold. The window margin which is a figure of merit for these 
disks shows a dramatic decrease in Window Margin for the disk with the 
silver reflector. The Window Margin (WM) for a disk with the gold 
reflector indicate almost no change. Thus CD-R disk using silver reflector 
does not provide adequate long term read stability of the written 
information. The reason for the degradation of the performance for the 
disk using silver reflector can be seen from the small (3T) and large 
(11T) mark jitter as function of aging time. The 3T mark jitter decreases 
while 11T mark jitter increases with the aging time. There also is a 
phenomenon of mark growth possibly due to the interfacial interaction 
between the silver metal and the dye recording layer. The improvement in 
the long term stability of the data was achieved by the use of interlayer. 
In this case the disk without and with interlayers have similar window 
margin and other performance parameters before the accelerated aging test. 
It has been found, quite unexpectedly, that the performance and the thermal 
stability of the disk using silver reflector dramatically improved when 
the disks were written after they were subjected to a heat treatment. As 
will be shown below, that the discs made using the present invention 
exhibit improvement, among other parameters, in jitter and WM and 
R.sub.top. 
One of the important features of the invention is the heat treatment of 
recording elements. Heat treatment can be accomplished by any number of 
well known techniques. For example, a plurality of discs can be mounted on 
a spindle, and that spindle and discs can be placed in an air oven. The 
temperature of the air oven is preselected and the discs are heat treated 
for a predetermined period of time. Thereafter, the spindle and discs are 
removed from the oven and are cooled in air until they reach room 
temperature. Benefits are generally improved at more elevated temperatures 
and longer periods of time. For example, it has been found that, at 
temperature of 70.degree. C. or greater for time periods of 6 hours or 
more, significant improvements are made. The temperature and the time of 
heat treating are limited by the plasterizing of polycarbonate substrates 
so that the geometry of the disc and the groove structure in the disc, are 
not significantly altered. 
The long term stability of the recorded data was examined by subjecting 
these to accelerated aging tests which were carried using two conditions: 
95.degree. C. dry heat; and 80.degree. C. and 85% RH wet conditions. The 
original mark were read back after the incubation test at periodic 
intervals. 
The discs were tested using an automated tester including a Kodak PCD 600 
writer/reader. The recording and read back were performed at the constant 
linear velocity of 2.4 m/s. The following parameters were determined: 
reflectivity (R.sub.top), optimum recording power (ORP), written 
contrasts; and nonlinearity NL (maximum difference of peak shifts among 
all features) Wm and jitter. In the CD recording scheme there are 18 
features--nine of them are made by laser exposures and are called marks 
and the rest nine are features created by not exposing to laser beam and 
are called lands. Their exposure times are expressed in multiples of 
detection window width (115.7 ns). The 3T, 4T, 5T . . . 11T marks are made 
by exposing to laser beam for 3 detection widths, 4 detection widths, 5 
detection widths, and so on. The marks are correspondingly designated as 
3M, 4M, 5M . . . 11M with 3M being the smallest and 11M being the largest 
of the marks. Similarly there are nine lands designed as -3L, 4L, 5L . . . 
11L S3M, S4M, etc. are the jitters of the corresponding marks. S3L, S4L, 
etc. are the jitters of the corresponding lands between marks. The 
contrasts (C.sub.3 and C.sub.11) and jitters (S3M, S11M and S3L, S11L) 
presented in the examples are of the smallest and largest features. 
Several full structure disks as shown in FIG. 1 were prepared in quick 
sequence. The thickness of the Ag reflector was about 80 nm and were 
overcoated with an UV-cured protective lacquer overcoat. Performance of a 
typical control disk is shown in column 2 of the Table 1. 
TABLE 1 
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Comparison of the Disk Performance Before and After Heat Treatment 
Recording Performance 
Performance 
Parameter as deposited after heat treatment 
______________________________________ 
ORP(mW) 6.9 6.9 
Rtop(%) 64.1 65.6 
NL(ns) 43.8 38.2 
S3M(ns) 8.9 7.6 
S3L(ns) 11 9.5 
S11M(ns) 8 6.4 
S11L(ns) 10 8.4 
WM(%) 35 43 
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The above control disk then was subjected to treatment consisting of 
heating to 85.degree. C. in air oven for 16 hours followed by normal 
cooling in air. The disk performance was then determined by writing a new 
band adjacent to original band. The disk performance after the heat 
treatment is included in column 3 of the Table 1. 
It is clear that the heat treated disk has much better performance. Thus 
because of the modification effected by the present invention, there occur 
significant improvements in R.sub.top, contrasts, NL, jitters and Wm. 
The improvement in disk performance is a function of heating temperature 
and time. Longer time is required if the heat treatment is done at a low 
temperature, while shorter time is required if the heat treatment is done 
at high temperature. The temperature and the time of heat treating are 
limited by the plasterizing of polycarbonate substrates so that the 
geometry of the disc and the groove structure in the disc, are not 
significantly altered. Heat temperatures can be selected from 50.degree. 
to 120.degree. C. and the time of treatment can vary from 1 to 24 hours. 
FIG. 3 shows the improvement in the window margin of the disks as a 
function of heating temperature. These disks were subjected to heating at 
respective temperatures for 16 hours in dry air oven. It is evident that 
because of the heat treatment there occur significant improvements window 
margin. Similarly, improvement in R.sub.top, contrasts, NL, and jitters 
were observed. 
FIG. 4 shows comparison of jitter of a disk before and after heat 
treatment. The performance of the disk was measured just after the 
deposition. The jitters of mark and land are shown in FIG. 4. Then it was 
subjected to treatment of heating to 95.degree. C. in air oven for 16 
hours followed by normal cooling in air. The performance of this disks was 
then determined by writing/reading another band adjacent to original band. 
The jitters of this heat treated disk are included in FIG. 4. It is clear 
that because of the modification effected by the present invention there 
occurs significant decrease in the jitters of mark and land. 
The another important advantage of the invention is that the disks prepared 
by following the present invention exhibited higher stability than that 
without the heat treatment. FIG. 5 shows the window margin as a function 
of accelerated aging time for four disks, A, B, C, and D using Ag 
reflector. Disk A is a control disk which did not undergo heat treatment. 
Disks B, C and D were subjected to a treatment consisting of heating at 
70.degree., 85.degree. and 95.degree. C., respectively, in air ovens for 
16 hours followed by normal cooling in air. These disks then were 
subjected to accelerated aging test at 80.degree. C./85% RH and their 
performance was measured after periodic intervals. The data for all disks 
are shown in FIG. 5. It shows that disks B, C, and D have much improved 
stability as compared to disk A. Thus recording elements made in 
accordance with the present invention have significantly superior data 
stability than the control disk. 
Stability of the CD-R disk using Ag reflector was improved using 
interlayers of metals and dielectrics between the reflector and the dye 
recording layer as described in the commonly assigned U.S. patent 
application Ser. No. 08/971,969, (docket 76,892) filed Nov. 17, 1997 
entitled "Recordable Optical Disks With Metallic Interlayer" and U.S. 
patent application Ser. No. 08/971,740, (docket 76,936) filed Nov. 17, 
1997 entitled "Recordable Optical Disks With Dielectric Interlayer." 
Before the accelerated aging test, performance (in particularly WM, 
jitters) of the disks using interlayers are similar to the disks without 
interlayer. It was found that the performance and stability of the disks 
with Ag reflector and interlayers can further be improved by following the 
present invention. FIG. 6 shows the window margin as a function of 
accelerated aging time for two disks A and B using Ag reflector and 0.5 nm 
Pd interlayer. Disk A is control disk which did not go any treatment. 
While Disk B is similar in structure as of Disk A but was subjected to a 
treatment consisting of heating at 85.degree. C., in air ovens for 24 
hours followed by normal cooling in air. Both disks A and B then were 
subjected to accelerating aging tests at 80.degree. C./85% RH and 
performance was measured after periodic intervals. As seen from FIG. 6, at 
the zero aging time, it is clear that because of the modification effected 
by the present invention there occur significant improvements in Wm. 
Similarly the WM of the heat treated disk B is significantly higher than 
that of control disk A, for an accelerated aging time of six weeks, 
indicating that disks B has much improved stability as compared to disk A. 
Thus, recording elements made in accordance with the present invention 
have significantly superior stability than the control disk. 
Similarly FIG. 7 shows the WM as a function of accelerated aging time for 
two disks using 3 nm Sn interlayer. Disk A is a control disk and disk B 
was subjected to a heat treatment for 16 hours at 85.degree. C. After 
measuring their performance, both disks A and B then were subjected to 
accelerating aging tests at 80.degree. C./85% RH and performance was 
measured after periodic intervals. Again disk B shows much higher 
recording stability than the disk A. 
Thus the recording elements made in accordance with the present invention 
have significantly superior performance with respect to WM, R.sub.top, NL, 
and jitters, and much improved recording stability. 
To further improve the extended stability of the disks prepared following 
this invention, stable alloys of silver metal reflector such as Ag--Pd, 
Ag--Cu, and Ag--Ni can be used instead of pure silver. The alloying 
additions of 2-5% Pd, Cu, or Ni into Ag metal were found to increase the 
environmental stability of the silver reflector. Moreover, stability of 
the disk structure can further be improved by protecting silver metal or 
silver alloys from corrosion/oxidation using a thin protective layer of 
metal, such as Pd, Cr, Ni etc. or a thin layer of dielectric, such as 
Ge--C--H, Si--N and InSnSb--O on the reflector layer before overcoating 
the disk with an UV-curable lacquer. 
The invention has been described in detail with particular reference to 
certain preferred embodiments thereof, but it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention. 
______________________________________ 
TS LIST 
______________________________________ 
10 substrate 
20 dye recording layer 
25 interlayer 
30 reflecting layer 
40 UV-curable lacquer protective overcoat 
______________________________________