Optical information storage medium

An optical information storage medium comprising a transparent substrate and an information carrying layer positioned on one side of the substrate, wherein the information carrying layer contains at least 30 per cent by volume of tin and at least 15 per cent by volume of cobalt oxide. The information carrying layer may be formed directly on one surface of the substrate.

FIELD OF THE INVENTION 
The present invention relates to information storage materials and, more 
particularly, to an optical information storage medium for having 
information stored therein by means of a high-energy an optical, 
electrical or thermal radiation beams such as, typically a laser beam. 
BACKGROUND OF THE INVENTION 
An optical information storage medium is used for recording information in 
the form of a series of pits arranged to form a myriad of coaxial, spiral 
or linear information tracks provided in the record medium. Such an 
information storage medium is advantageous for implementing an information 
recording and reproducing disc of the mass storage type for its high 
storage density as well known in the art. 
An information storage medium of this nature is typically provided in the 
form of a multi-layer structure which consists of a transparent substrate 
and an information carrying layer of a low-melting point metal deposited 
on the substrate. As such a low-melting point metal is used, for example, 
tellurium or bismuth or any alloy containing one or both of these, as 
disclosed in Japanese Patent Specification No. 54-15483. Similar optical 
information storage materials are shown in, for example, U.S. Pat. Nos. 
3,971,874 and 4,188,214. 
From the viewpoint of providing a high degree of optical sensitivity, tin 
in particular is preferred as a low-melting point metal for use in such an 
information storage medium. A known information storage medium using tin 
as a low-melting point metal is however not fully acceptable for achieving 
information reproducing signals of satisfactory quality and has not been 
used for practical purposes. 
It is, accordingly, an object of the present invention to provide an 
improved optical information storage medium which is acceptable for 
achieving information reproducing signals of satisfactory quality. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided an optical 
information storage medium comprising a substantially transparent 
substrate and an information carrying layer provided on one side of the 
substrate, wherein the information carrying layer contains 30 to 85 per 
cent by volume of tin and 15 to 70 per cent by volume of cobalt oxide. In 
one preferred embodiment of the present invention, the information 
carrying layer may be formed directly on one surface of the substrate. 
An optical information storage medium proposed by the present invention is 
characterized, inter alia, in that tin is used in combination with cobalt 
oxide for forming the information carrying layer of the medium. The tin 
component of the information carrying layer is advantageous in that it 
provides a high degree of recording sensitivity for the information 
storage medium. If, however, the information carrying layer of an optical 
information storage medium were constructed solely of tin, then the 
information storage medium could not be used for practical purposes 
because of the poor surface state of the layer of tin. Such a poor surface 
state results from the particular crystal structure of a bulk of tin. When 
tin is used in combination with cobalt oxide for the formation of an 
information carrying layer and the proportion between the tin and cobalt 
components of such an information carrying layer is selected as herein 
specified, the information carrying layer has a significantly improved 
surface state which enables the information storage medium to provide 
excellent performance characteristics when used on an optical information 
recording and reproducing apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1 of the drawings, an optical information storage medium 
embodying the present invention comprises a substantially transparent 
substrate 10 and and information carrying layer 12 formed on one surface 
of the substrate 10. The transparent substrate 10 may be constructed of 
any desired material such as, for example, a syhthetic resin, glass, 
porcelain. As the synthetic resin may be used an acrylic resin such as 
polymethyl methacrylate; polycarbonate; polyetherimido; polysulfone; an 
epoxy resin; or polyvinyl chloride. The substrate 10 is in the form of a 
circular disc, an elongate tape or an otherwise contoured web. On the 
surface of such a substrate 10 may be provided a heat insulating layer or 
a smoothing film if desired, although such an additional layer or film is 
not shown in the drawings. 
In the embodiment of the present invention herein shown, the information 
carrying layer 12 provided on the transparent substrate 10 is formed of a 
mixture of tin and cobalt oxide (CoOx). While the information storage 
medium using such an information carrying layer 12 will provide 
satisfactory information recording capabilities, a third component 
substance may be added to the mixture of tin and cobalt oxide to provide 
further improved weather-proof ability and/or an accurately controlled 
reflectivity. Examples of such an additional substance include carbon, 
aluminum, silicon titanium, chromium, iron, cobalt, nickel, copper, zinc, 
gallium, germanium, selenium, zirconium, niobium, molybdenum, rhodium, 
palladium, silver, indium, antimony, tellurium, tantalum, tungsten, 
platinum, lead, bismuth, and gold. If desired, one or more of these 
substances may be used as a further additional component material of the 
information carrying layer 12. 
In accordance with the present invention, the tin component of the 
information carrying layer 12 accounts in bulk concentration for about 30 
per cent by volume or more of the information carrying layer 12 and the 
cobalt oxide component of the layer 12 accounts for about 15 per cent by 
volume of the information carrying layer 12 without respect to the 
chemical composition of the layer 12, for the reasons which will become 
apparent as the description proceeds. The bulk concentration of the 
remainder component or components of the information carrying layer 12 is 
preferably within the range of from about 10 per cent to 15 per cent of 
the total bulk of the layer 12 although a high concentration may be used 
depending upon the resultant physical properties of the layer 12. For 
providing satisfactory storage sensitivity and satisfactory quality of the 
signals to be produced from the information storage medium 10, the 
thickness of the information carrying layer 12 preferably ranges from 
about 100 angstroms to about 600 angstroms and more preferably from about 
200 angstroms to about 500 angstroms. 
For the recording of information in the information storage medium thus 
constructed, a series of pits are formed in the information carrying layer 
12 by selective irradiation of the layer 12 with, for example, a laser 
beam. The pits are arranged to form a multiplicity of coaxial or spiral 
tracks where the storage medium is provided in the form of a circular disc 
as herein assumed or along a plurality of linear information tracks where 
the storage medium is provided in the form of an elongated tape. In order 
that the information tracks formed by these pits be precisely located on 
the disc, the substrate 10 of the disc is ordinarily formed with coaxial 
or spiral guide grooves along which the laser beam is to be incident 
during reproduction of the information from the disc. The laser beam used 
for the information reproducing operation is collimated to have a cross 
sectional area approximately equal in diameter to the width of the guide 
grooves. When such a laser beam is focussed on one of the guide grooves, 
the beam is diffracted in the groove. If the spot of the beam incident on 
the substrate 10 happens to deviate radially from the particular groove, 
there results a change in the space distribution of the intensity of the 
diffracted beam. The focus servo system of the reproducing apparatus is 
arranged to detect such a change in the space distribution of light 
intensity and controls the beam to be correctly directed at the center 
line of the guide groove. The guide grooves used for ordinary optical 
information storage discs are usually about 0.3 micron to 1.2 micron wide 
and have depths which are approximately equal to one twelfth to one fourth 
of the wavelength of the laser beam used for the information reproducing 
operation. 
The features and advantages of an optical information storage medium 
according to the present invention will be more clearly appreciated from 
the following Examples of the invention. 
EXAMPLE I 
A centrally apertured transparent substrate of a polycarbonate formed with 
guide grooves was prepared which measured 15 mm in inside diameter, 130 mm 
in outside diameter and 1.2 mm in thickness. The substrate was placed 
within a vacuum deposition chamber which was evacuated to less than 
2.times.10.sup.-5 Torr. Within the chamber were also placed a 
resistance-heated boat (made of molybdenum) containing a charge of tin and 
an electron-beam heated crucible containing a charge of cobalt oxide 
(Co.sub.3 O.sub.4). Coevaporation techniques were used to have these 
substances concurrently deposited on the substrate while controlling the 
deposition rates of the charges with use of a quartz-crystal oscillator 
thickness monitor. The deposition process was terminated when it was 
observed that the deposit of the coevaporated tin and cobalt oxide reached 
a thickness of about 350 angstroms. An optical information storage disc 
including an information carrying layer of tin and cobalt oxide uniformly 
deposited on one surface of the transparent polycarbonate substrate was 
thus obtained. 
A number of such information storage discs were experimentally fabricated 
each following the procedure of Example I with the bulk concentration of 
cobalt oxide stepwise varied from 0 to 100 per cent by volume to the whole 
bulk of the information carrying layer. Tests (A) were conducted with 
these sample discs to determine the reflectivity of each of the discs to a 
laser beam of an 8300 angstrom wavelength. Tests (B) were further 
conducted to determine the noise levels of the raw or blank (viz., 
non-information carrying) sample discs with use of a laser beam of the 
same wavelength. A set of information was then written into each of the 
sample discs with a succession of pits formed in the information carrying 
layer of the disc along the guide grooves in the substrate by irradiation 
of the disc with a laser beam also having the wavelength of 8300 
angstroms. The signals used for the writing of the information has a 
frequency at 1.25 megahertz (duty 50%). Each of the discs was driven for 
rotation at a circumferential speed of 5.65 meters per second. Tests (C) 
were conducted with the resultant information-carrying sample discs to 
determine the carrier-to-noise ratios (C/N ratios) of the individual 
discs. 
Table 1 demonstrates the results of these tests A, B and C, wherein the 
figures in the column under the heading ""Noise Level" indicate the 
results of the tests B and thus refer to the noise levels of the sample 
discs (numbered 1/1 to 1/10) as determined before information had been 
written into the discs which are herein referred to as raw or black discs. 
In Table 1, furthermore, the rightmost column under the heading "Note" 
refers to the results of evaluation of the discs tested and indicate 
whether the sample discs are acceptable or "OK" within the purview of the 
present invention or the discs are unacceptable or "NO" within the purview 
of the invention. For example, the sample discs numbered 1/3 to 1/8 have 
been evaluated to be acceptable while the sample discs numbered 1/1, 1/2, 
1/9 and 1/10 have been evaluated to be unacceptable. In table 1, the 
thickness of each information carrying layer 12 is 350 angstroms. 
From the results of the tests A shown in Table 1 it will be seen that there 
is a tendency that the reflectivities of the discs decrease as the bulk 
concentrations of the cobalt oxide component in the information carrying 
layer increases. When the bulk concentration (100 per cent minus Co.sub.3 
O.sub.4 concentration) of the tin component in the information carrying 
layer is less than 30 per cent by volume, the reflectivity of the disc is 
short of 15 per cent. In this instance, the focus servo system of the 
reproducing apparatus could not operate properly to have the laser beam 
correctly focussed at the center line of the guide groove. On the other 
hand, the results of the tests B shown in Table 1 indicate that the noise 
levels of the raw or blank discs decrease abruptly with addition of cobalt 
oxide to the bulk of tin and are acceptable when the bulk concentration of 
the cobalt oxide component in an information carrying layer is higher than 
15 per cent. From the results of the tests C, it is seen that the 
recording sensitivities of discs as represented by the C/N ratios of the 
discs deteriorate when the bulk concentration of the cobalt oxide 
component is less than 15 per cent and the bulk concentration of the tin 
component is less than 30 per cent. Thus, the results of the tests A, B 
and C show that excellent performance characteristics can be achieved by 
an information storage medium according to the present invention. 
TABLE 1 
______________________________________ 
C.sub.03 O.sub.4 
Concen- Reflec- Noise 
tration tivity Level C/N 
Sample No. 
(%) (%) (dB) (dB) Note 
______________________________________ 
1/1 0 44 -50 less than 
NO 
48 
1/2 10 42 -53 less than 
NO 
48 
1/3 15 46 less than 
more than 
OK 
-70 48 
1/4 22 43 less than 
more than 
OK 
-70 48 
1/5 35 28 less than 
more than 
OK 
-70 48 
1/6 45 21 less than 
more than 
OK 
-70 48 
1/7 60 17 less than 
more than 
OK 
-70 48 
1/8 70 15 less than 
more than 
OK 
-70 48 
1/9 75 16 less than 
more than 
NO 
-70 48 
1/10 100 9 less than 
more than 
NO 
-70 48 
______________________________________ 
Sample discs were further fabricated with the thickness of the information 
carrying layer varied from one of the discs to another. Table 2 shows the 
results of the tests A, B and C. 
TABLE 2 
______________________________________ 
C.sub.03 O.sub.4 
Concen- Thick- Reflec- 
Noise 
Sample 
tration ness tivity 
Level C/N 
No. (%) (.ANG.) (%) (dB) (dB) Note 
______________________________________ 
2/1 27 250 26 less than 
more than 
OK 
-70 48 
2/2 20 400 45 less than 
more than 
OK 
-70 48 
______________________________________ 
EXAMPLE II 
A transparent substrate was prepared and was placed within a vacuum 
deposition chamber as in Example I. Within the chamber were also placed a 
resistance-heated boat containing a charge of tin and an electron-beam 
heated crucible containing a charge of cobalt oxide (Co.sub.2 O.sub.3). 
Coevaporation techniques were used to have these substances concurrently 
deposited on the substrate. 
Table 3 shows the results of tests A, B and C. 
TABLE 3 
______________________________________ 
C.sub.02 O.sub.3 
Concen- Thick- Reflec- 
Noise 
Sample 
tration ness tivity 
Level C/N 
No. (%) (.ANG.) (%) (dB) (dB) Note 
______________________________________ 
3/1 20 250 20 less than 
more than 
OK 
-70 48 
3/2 25 350 28 less than 
more than 
OK 
-70 48 
3/3 50 450 24 less than 
more than 
OK 
-70 48 
______________________________________ 
EXAMPLE III 
A transparent substrate was prepared and was placed within a vacuum 
deposition chamber as in Example 1. Within the deposition chamber were 
also placed a resistance-heated boat containing a charge of tin, a first 
electron-beam heated crucible containing a charge of cobalt oxide 
(C.sub.03 O.sub.4) and a second electron-beam heated crucible cohtaining a 
charge of chromium. Coevaporation techniques were used to have these 
substances concurrently deposited on the substrate to bulk concentrations 
of 60 per cent, 35 per cent and 5 per cent by volume for the tin, cobalt 
oxide and chromium components, respectively. As optical information 
storage disc was thus obtained, including an information carrying layer of 
tin, cobalt oxide and chromium uniformly deposited on one surface of the 
transparent acryl disk substrate. The thickness of the information 
carrying layer was about 400 angstroms. Information was written into this 
disc whereupon tests C were conducted with the resultant 
information-carrying disc to determine the C/N ratio of the disc as in 
Example 1. The results of the tests showed that the disc was acceptable 
with the C/N ratio of the disc determined to be more than 48 decibels. 
EXAMPLE IV 
A transparent substrate was prepared and was placed within a vacuum 
deposition chamber as in Example 1. Within the deposition chamber were 
also placed a resistance-heated boat containing a charge of tin, a first 
electron-beam heated crucible containing a charge of cobalt oxide 
(C.sub.03 O.sub.4) and a second electron-beam heated crucible containing a 
charge of cobalt. Coevaporation techniques were used to have these 
substances concurrently deposited on the substrate to bulk concentrations 
of 60 per cent, 35 per cent and 5 per cent by volume for the tin, cobalt 
oxide and cobalt components, respectively. A single-layer optical 
information storage disc was thus obtained, including an information 
carrying layer of tin, cobalt oxide and cobalt uniformly deposited on one 
surface of the transparent polycarbonate disc substrate. The thickness of 
the information carrying layer was about 300 angstrons. Information was 
written into this disc whereupon tests C were conducted with the resultant 
information-carrying disc to determine the C/N ratio of the disc as in 
Example 1. The results of the tests showed that the disc was acceptable 
with the C/N ratio of the disc determined to be more than 48 decibels. 
As will have been understood from the foregoing description an optical 
information storage medium according to the present invention basically 
comprises a substantially transparent substrate 10 and an information 
carrying layer 12 positioned on one side of the substrate, wherein the 
information carrying layer 12 contains 30 to 85 per cent by volume of tin 
and at least 15 to 70 per cent by volume of cobalt oxide. 
The tin component of the information carrying layer 12 is advantageous in 
that it provides a high degree of recording sensitivity for the 
information storage medium. If, however, the information carrying layer is 
constructed solely of tin, the resultant information storage medium could 
not be used for practical purposes because of the poor surface state of 
the layer of tin, such a poor surface state resulting from the particular 
crystal structure of a bulk of tin. When tin is used in combination with 
cobalt oxide for the formation of the information carrying layer of an 
information storage medium and the proportion between the tin and cobalt 
components of such an information carrying layer is selected as herein 
specified, the information carrying layer has a significantly improved 
surface state which enables the information storage medium to provide 
excellent performance characteristics when used on an optical information 
recording and reproducing apparatus. 
The foregoing excellent performance characteristics of the present 
invention were not deteriorated after locating those samples in the 
environment of high temperature of 70.degree. C. and high moisture of 80% 
for a period of 200 hours. 
That test shows that the optical information recording medium of the 
present invention has a good weather resistance. 
In view of this excellent weather resistance, the present invention is 
superior to the material of the copending Patent Application entitled 
"Optical Information Storage Medium", filed on Aug. 8, 1986 and assigned 
to the same assignee, in which an optical information carrying layer 
contains at least 30 per cent by volume of tin and at least 15 per cent by 
volume of nickel oxide.