Optical disk and method of manufacturing optical disk

An optical disk can provide a sufficiently wide power margin for a recording power even when the optical disk has a high-recording density. The optical disk has a transparent substrate on which a recording layer in which information can be rewritten by at least irradiation of laser beam, a dielectric layer and a thermal conduction control layer are sequentially deposited. The thermal conduction control layer comprises a first low thermal conductivity layer, a first high thermal conductivity layer, a second low thermal conductivity layer and a second high thermal conductivity layer sequentially deposited thereon.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical disk in which a recording layer 
and a dielectric layer are formed on a transparent substrate to rewrite 
information based on a rise in temperature generated by irradiation of 
laser beam and a method of manufacturing optical disk. 
Recently, in the field of recording medium or the like used as an external 
memory of computers, as an amount of information processed by computers or 
the like increases, there is an increasing demand for increasing a 
recording capacity of a recording medium used as an external memory. 
Further, there is an increasing demand that a recording medium can cope 
with an improvement of capability of central processing unit (CPU) of 
computer and a variety of applications. 
Therefore, it is very effective to use an optical disk capable of very 
high-density recording and which is excellent in portability as a 
recording medium for an external memory of large storage capacity. In 
particular, the use of rewritable optical disks effectively utilizing a 
magnetooptical effect or a crystal-amorphous phase transition phenomenon 
is very promising. 
Under the above-mentioned situations, a demand for making the optical disks 
become capable of high-density recording increases more and more. In order 
to improve a recording density, a wavelength of laser beam is shortened, 
and a so-called mark edge system is employed. 
It is requested that optical disks used as external memory of computer are 
able to record information in a recording power of wide range so as to 
prevent recording/reproducing characteristics from being affected due to 
various factors such as a difference of disk drives, lens smudged by dusts 
or the like or a difference of environmental temperature. 
However, in optical disks capable of high-density recording by laser beam 
with a short wavelength and the mark-edge recording system, a margin of 
recording power in which a satisfactory recording is possible is reduced 
due to deterioration of S/N (signal-to-noise ratio) and C/N 
(carrier-to-noise ratio) or thermal interference generated between 
adjacent codes. In particular, since the optical disk becomes capable of 
high-density recording, a recording mark length is fluctuated considerably 
due to an influence (thermal interference) of heat generated by mark 
immediately before the recording. 
In order to solve these problems, it is necessary to control a flow of heat 
generated immediately before the recording. Specifically, a recording 
layer should have a temperature characteristic which is easily heated and 
cooled when the recording layer is irradiated with laser beam upon 
recording. The optical disk requires such temperature characteristic as a 
characteristic for suppressing thermal interference. 
In order to solve the aforesaid problems, there are previously-proposed 
methods for controlling shape and size of a unit recording region (e.g., 
recording magnetic domain in a magnetooptical recording medium) by 
controlling flow of heat within the recording medium, e.g., the following 
methods (1) to (4): 
(1) To control a thermal conductivity of a reflecting film (see Japanese 
laid-open patent publication No. 2-152050); 
(2) To control a thermal conductivity of a protecting layer (UV protecting 
layer) (see Japanese laid-open patent publication NO. 4-337545 and 
Japanese laid-open patent publication No. 2-240846); 
(3) To provide two reflecting films thereby to control a thermal 
conductivity of the reflecting films (see Japanese laid-open patent 
publication NO. 5-342655); and 
(4) To deposit a dielectric film on the reflecting film (see Japanese 
laid-open patent publication No. 3-105742). 
However, the aforesaid previously-proposed methods (1) to (4) can improve 
any one of a characteristic in which a recording layer tends to be heated 
quickly, i.e., a temperature rise characteristic and a characteristic in 
which a recording layer tends to be cooled quickly, i.e., a temperature 
fall characteristic and cannot satisfy both of the temperature rise 
characteristic and the temperature fall characteristic. 
SUMMARY OF THE INVENTION 
In view of the aforesaid aspect, it is an object of the present invention 
to provide an optical disk which can provide a sufficiently wide power 
margin for a recording power even when the optical disk has a 
high-recording density. 
According to an aspect of the present invention, there is provided an 
optical disk which comprises a transparent substrate on which a dielectric 
layer, a recording layer in which information can be rewritten by at least 
irradiation of laser beam, and a thermal conduction control layer are 
sequentially deposited, wherein the thermal conduction control layer 
comprises a first low thermal conductivity layer, a first high thermal 
conductivity layer, a second low thermal conductivity layer and a second 
high thermal conductivity layer sequentially deposited thereon. 
According to another aspect of the present invention, there is provided a 
method of manufacturing an optical disk in which a dielectric layer, a 
recording layer in which information can be rewritten by at least 
irradiation of laser beam, and a thermal conduction control layer are 
sequentially laminated on a transparent substrate, the thermal conduction 
control layer comprising a first low thermal conductivity layer, a first 
high thermal conductivity layer, a second low thermal conductivity layer 
and a second high thermal conductivity layer sequentially deposited 
thereon. This method comprises the step of continuously discharging three 
layers of the first high thermal conductivity layer, the second low 
thermal conductivity layer and the second high thermal conductivity layer 
within the same sputtering chamber by use of the same target material 
while changing a flow rate of gas to thereby deposit the first high 
thermal conductivity layer, the second low thermal conductivity layer and 
the second high thermal conductivity layer by sputtering. 
According to still another aspect of the present invention, there is 
provided a method of manufacturing an optical disk having a transparent 
substrate on which a dielectric layer, a recording layer in which 
information can be rewritten by at least irradiation of laser beam, and a 
thermal conduction control layer are sequentially deposited, the thermal 
conduction control layer comprising a first low thermal conductivity 
layer, a first high thermal conductivity layer, a second low thermal 
conductivity layer and a second high thermal conductivity layer 
sequentially deposited thereon. This method is characterized in that four 
layers of the first low thermal conductivity layer, the first high thermal 
conductivity layer, a second low thermal conductivity layer and the second 
high thermal conductivity layer are deposited within the same sputtering 
chamber by the same target material by means of continuous bombardment of 
ionized gas molecules while only a flow rate of gas is varied. 
According to the present invention, since the optical disk includes the 
thermal conduction control layer and the thermal conduction control layer 
comprises the first low thermal conductivity layer, the first high thermal 
conductivity layer, the second low thermal conductivity layer and the 
second high thermal conductivity layer, the temperature characteristic of 
the recording layer can be recorded. Specifically, the first low thermal 
conductivity layer and the second low conductivity layer promote storage 
of heat in the recording layer thereby to enable a rapid rise in 
temperature to be realized, and the second high thermal conductivity layer 
promotes a diffusion of heat from the recording layer thereby to enable a 
rapid fall in temperature. Further, since the second low thermal 
conductivity layer is disposed between the first high thermal conductivity 
layer and the second high thermal conductivity layer, a temperature 
gradient in the film thickness direction can be controlled and the manner 
in which the temperature of the recording layer is changed with time can 
be controlled. 
Furthermore, when this optical disk is produced, according to the 
manufacturing method of the present invention, since the three layers of 
the first high thermal conductivity layer, the second low thermal 
conductivity layer and the second high thermal conductivity layer are 
deposited within the same sputtering chamber by the same target material 
under continuous bombardment of ionized gas (sputtering) while only the 
flow rate of gas is changed, the optical disk with the multi-layer 
structure can be produced by the conventional production line for 
producing optical disks formed of four layers of a dielectric layer, a 
recording layer, a dielectric layer and a reflecting layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An optical disk according to the present invention includes a transparent 
substrate on which a recording layer in which information can be rewritten 
by at least irradiation of laser beam, a dielectric layer and a thermal 
conduction control layer are deposited. The thermal conduction control 
layer comprises a first low thermal conductivity layer, a first high 
thermal conductivity layer, a second low thermal conductivity layer and a 
second high thermal conductivity layer laminated, in that order. 
The transparent substrate is made of a material for passing laser beam 
therethrough, e.g., a transparent resin such as polycarbonate or glass. 
Further, the optical disk has a protecting layer made of ultraviolet-curing 
resin formed on the opposite side of the transparent resin. 
Inventive examples of optical disk and a method of producing optical disk 
according to the present invention will be described. 
INVENTIVE EXAMPLE 1 
FIG. 1 is a schematic cross-sectional view illustrating an optical disk 
according to an embodiment of the present invention. 
An optical disk, which is generally depicted by reference numeral 20 in 
FIG. 1, is a magnetooptical disk from which recorded information can be 
read out owing to a magnetooptical effect. The magnetooptical disk 20 has 
a transparent substrate 1 on which there are sequentially deposited a 
dielectric layer 2, a recording layer 3, a first low thermal conductivity 
layer 11, a first high thermal conductivity layer 12, a second low thermal 
conductivity layer 13, a second high thermal conductivity layer 14 and a 
protecting layer 5. A thermal conduction control layer 4 comprises four 
layers of the first low thermal conductivity layer 11, the first high 
thermal conductivity layer 12, the second low thermal conductivity layer 
13 and the second high thermal conductivity layer 14. 
This optical disk 20 is manufactured as follows. 
The dielectric layer 2 formed of SiN film with a thickness of 100 nm is 
deposited on the transparent substrate 1 made of a polycarbonate resin by 
sputtering, and the recording layer formed of TbFeCo film with a thickness 
of 20 nm is deposited on the dielectric layer 2 by sputtering. 
The first low thermal conductivity layer 11 formed of SiN film with a 
thickness of 30 nm is deposited on the recording layer 3 by sputtering, 
the first high thermal conductivity layer 12 formed of Al film with a 
thickness of 30 nm is deposited on the first low thermal conductivity 
layer 11 by sputtering, the second low thermal conductivity layer 13 
formed of ZnS-SiO.sub.2 film with a thickness of 30 nm is deposited on the 
first high thermal conductivity layer 12 by sputtering, and the second 
high thermal conductivity layer 14 formed of Al film with a thickness of 
60 nm is deposited on the second low thermal conductivity layer 13 by 
sputtering, in that order, thereby forming the thermal conduction control 
layer 4. 
Then, the protecting layer 5 made of an ultraviolet-curing resin with a 
thickness of 5 .mu.m is formed on the thermal conduction control layer 4 
by spin coat, thereby forming the optical disk 20, i.e., the 
magnetooptical disk. 
At that time, a thermal conductivity of the SiN film forming the first low 
thermal conductivity layer 11 is 0.03J/(cm.multidot.sec.multidot.deg), and 
a thermal conductivity of the ZnS-SiO.sub.2 film forming the second low 
thermal conductivity layer 13 is 0.007J/(cm.multidot.sec.multidot.deg) 
which is smaller than a thermal conductivity of 
0.13J(cm.multidot.sec.multidot.deg) of the TbFeCo film forming the 
recording layer 3. 
A thermal conductivity of the Al film forming the first and second high 
thermal conductivity layers 12, 14 is 2.3J/(cm-sec-deg) which is larger 
than that of the TbFeCo film forming the recording layer 3. 
INVENTIVE EXAMPLE 2 
FIG. 2 is a schematic cross-sectional view illustrating an optical disk, 
i.e., magnetooptical disk according to another embodiment of the present 
invention. 
An optical disk, which is generally depicted by reference numeral 30 in 
FIG. 2, has a transparent substrate 1 on which there are sequentially 
deposited a dielectric layer 2, a recording layer 3, a first low thermal 
conductivity layer 11, a first high thermal conductivity layer 12, a 
second low thermal conductivity layer 13 and a second high thermal 
conductivity layer 14. Then, the first low thermal conductivity layer 11, 
the first high thermal conductivity layer 12, the second low thermal 
conductivity layer 13 and the second high thermal conductivity layer 14 
constitute a thermal conduction control layer 4. 
As compared with the optical disk 20 shown in FIG. 1, the optical disk 30 
has no protecting layer deposited thereon at the opposite side of the 
transparent substrate 1. 
This optical disk 30 is produced as follows. 
The dielectric layer 2 formed of SiN film with a thickness of 100 nm is 
deposited on the transparent substrate 1 made of a polycarbonate resin by 
sputtering, and the recording layer 3 formed of TbFeCo film with a 
thickness of 20 nm is deposited on the dielectric layer 2 by sputtering. 
The first low thermal conductivity layer 11 formed of SiN film with a 
thickness of 30 nm is deposited on the recording layer 3 by sputtering, 
the first high thermal conductivity layer 12 formed of Al film with a 
thickness of 20 nm is deposited on the first low thermal conductivity 
layer 11 by sputtering, the second low thermal conductivity layer 13 
formed of SiN film with a thickness of 50 nm is deposited on the first 
high thermal conductivity layer 12 by sputtering and the second high 
thermal conductivity layer 14 formed of Al-Ti film is deposited on the 
second low thermal conductivity layer 13 by sputtering, in that order. 
Thus, these four layers constitute the thermal conduction control layer 4 
and the optical disk 30 is formed. 
At that time, a thermal conductivity of the SiN film forming the first low 
thermal conductivity layer 11 and the second thermal low thermal 
conductivity layer 13 is 0.03J(cm.multidot.sec.multidot.deg) which is 
smaller than a thermal conductivity of 
0.13J/(cm.multidot.sec.multidot.deg) of the TbFeCo film forming the 
recording layer 3. 
A thermal conductivity of the Al film forming the first high thermal 
conductivity layer 12 is 2.3J/(cm.multidot.sec.multidot.deg), and a 
thermal conductivity of the Al-Ti film forming the second high thermal 
conductivity layer 14 is 1.2J/(cm.multidot.sec.multidot.deg). These 
thermal conductivities are larger than that of the TbFeCo film forming the 
recording layer 3. 
COMATIVE EXAMPLE 
FIG. 3 is a schematic cross-sectional view illustrating a comparative 
example of an optical disk which is compared with the optical disk based 
on the magnetooptical disk according to the present invention. 
An optical disk, which is generally depicted by reference numeral 50 in 
FIG. 3, has a transparent substrate 51 on which there are sequentially 
deposited a first dielectric layer 52, a recording layer 53, a second 
dielectric layer 54, a reflecting layer 55 and a protecting layer 56 made 
of an ultraviolet-curing resin. 
This optical disk 50 is produced as follows. 
As shown in FIG. 3, the first dielectric layer 52 formed of SiN film with a 
thickness of 100 nm is deposited on the transparent substrate 51 made of a 
polycarbonate resin or the like by sputtering, and the recording layer 53 
formed of TbFeCo film with a thickness of 20 nm is deposited on the first 
dielectric layer 52 by sputtering. The second dielectric layer 54 formed 
of SiN film with a thickness of 30 nm is deposited on the recording layer 
53 by sputtering, the reflecting layer 55 formed of Al film with a 
thickness of 50 nm is deposited on the second dielectric layer 54 by 
sputtering and the protecting layer 56 made of an ultraviolet-curing resin 
with a thickness of 10 .mu.m is deposited on the reflecting layer 55 by 
spin coat, thereby the optical disk 50 being formed. 
Power margins of these optical disks for recording power were measured. The 
recording conditions were such that a wavelength of recording laser beam 
was 680 nm, numerical aperture N.A. of optical system was 0.55, linear 
velocity was 6.4 m/sec and that a clock of one channel was 40 nsec. A 
random signal based on (1, 7) RLL recording system was recorded by 
light-emission of comb-shaped pulse. Information was recorded with 
recording powers being changed and bit error rates were measured. 
FIG. 4 shows measured results. FIG. 4 is a graph showing a relationship 
between recording power Pw (mW) and bit error rate bitER. A study of FIG. 
4 reveals that a range of recording power Pw in which the bit error rate 
becomes less than 10.sup.-4 in the respective inventive examples became 
wider than that of the comparative example and the power margin was 
expanded. 
It is considered that these measured results are based on a difference of 
heat characteristics in the recording layer. In order to examine heat 
characteristics more in detail, a manner in which a temperature is changed 
with time was examined. 
In order to form unit recording pits of the same size, the recording layer 
was heated and cooled, and the manner in which temperatures of the 
recording layers of the optical disks in the inventive examples and the 
comparative example are changed with time was measured. FIG. 5 shows 
measured results. A study of FIG. 5 reveals that temperatures in the 
recording layers of the optical disks according to the inventive examples 
rise and fall quickly, i.e., the recording layers tend to be heated and 
cooled quickly as compared with the recording layer of the optical disk 
according to the comparative example. 
Speeds at which the temperature of the recording layer rise and fall when 
information is recorded in actual practice are faster as compared with 
heating and cooling conditions in FIG. 5. Due to the conditions shown in 
FIG. 5, differences of temperature characteristics of the recording layers 
in the inventive examples and the comparative example become remarkable. 
Therefore, the thermal interference under high laser power can be 
suppressed. 
With respect to the magnetooptical disks according to the inventive example 
2 and the comparative example, amounts in which the disks are warped with 
time were measured. 
The warp of 3.3-inch optical disks was measured under promoted aging 
condition at a temperature of 80.degree. C. with a humidity of 85%. The 
warped amounts of optical disks were measured by measuring disk skew 
angles. 
FIG. 6 shows measured results obtained when the disk skew angles were 
changed with time. A study of FIG. 6 shows that the optical disk according 
to the inventive example 2 has a small inclination angle, i.e., small 
warped amount of optical disk because the protecting layer made of an 
ultraviolet-curing resin is not formed on the optical disk according to 
the inventive example 2. 
A method of manufacturing an optical disk according to the present 
invention, i.e., an example in which the three layers of the first high 
thermal conductivity layer, the second low thermal conductivity layer and 
the second high thermal conductivity layer in the thermal conduction 
control layer were formed by successive sputtering will be described. 
INVENTIVE EXAMPLE 3 
According to the inventive example 3, an optical disk based on a 
magnetooptical disk with a structure similar to that of the inventive 
example 1 shown in FIG. 1 is produced as follows. 
A dielectric layer 2 formed of SiN film with a thickness of 80 nm was 
deposited on the transparent substrate 1 made of a polycarbonate resin by 
sputtering. A target material was silicon and sputtering conditions were 
such that a flow rate of argon gas was 85 sccm, a flow rate of nitrogen 
gas was 15 sccm, a gas pressure was 3 mtorr and an applied power was 1 kW. 
Then, a recording layer 3 formed of TbFeCo film with a thickness of 20 nm 
was deposited on the dielectric layer 2 by sputtering. A target material 
was TbFeCo alloy, and sputtering conditions were such that a flow rate of 
argon gas was 80 sccm, a gas pressure was 3 mtorr and an applied power was 
1.2 kW. 
Subsequently, a first low thermal conductivity layer 11 formed of SiN film 
with a thickness of 30 nm was deposited on the recording layer 3 by 
sputtering. A target material was silicon, and sputtering conditions were 
such that a flow rate of argon gas was 85 sccm, a flow rate of nitrogen 
gas was 15 sccm, a gas pressure was 3 mtorr and an applied power was 1 kW. 
Further, a first high thermal conductivity layer 12 formed of Al film was 
deposited on the first low thermal conductivity layer 11 by sputtering. A 
target material was Al, and sputtering conditions were such that a flow 
rate of argon gas was 80 sccm, a gas pressure was 2.5 mtorr and an applied 
power was 1 kW. 
After a time period during which the thickness of the Al film becomes about 
25 nm had been passed, in order to deposit the second low thermal 
conductivity layer 13, discharge was not stopped, and nitrogen gas was 
added to argon gas and flowed at a flow rate of 21 sccm. At that time, a 
gas pressure in the sputtering was 3 mtorr. 
In this manner, the second low thermal conductivity layer 13 formed of AlN 
film was deposited. After a time period during which a thickness of the 
second low thermal conductivity layer 13 becomes about 50 nm had been 
passed, in order to deposit a second high thermal conductivity layer 14, 
discharge was not stopped, only the nitrogen gas was interrupted, and the 
second high thermal conductivity layer 14 formed of Al film with a 
thickness of 150 nm was deposited. 
Specifically, in the inventive example 3, the three layers of the first 
high thermal conductivity layer 12, the second low thermal conductivity 
layer 13 and the second high thermal conductivity layer 14 were laminated 
by successive sputtering. Only when the second low thermal conductivity 
layer 13 was formed, nitrogen gas was flowed in addition to the argon gas. 
Furthermore, a protecting layer made of an ultraviolet-curing resin with a 
thickness of 10 .mu.m was formed on the second high thermal conductivity 
layer 14 by spin coat and thereby the optical disk 30 was formed. 
At that time, a thermal conductivity of the SiN film forming the first low 
thermal conductivity film 11 was 0.03J/(cm.multidot.sec.multidot.deg) and 
a thermal conductivity of the AlN film forming the second low thermal 
conductivity layer 13 was 0.006J/(cm.multidot.sec.multidot.deg) which is 
smaller than a thermal conductivity of 
0.13J/(cm.multidot.sec.multidot.deg) of the TbFeCo film forming the 
recording layer 3. 
A thermal conductivity of the Al film forming the first and second high 
thermal conductivity layers 12 and 14 is 
2.3J/(cm.multidot.sec.multidot.deg), and larger than that of the TbFeCo 
film forming the recording layer 3. 
Incidentally, the Al target may be added with a material such as titanium 
so long as an added material does not change heat characteristics greatly. 
Other method of manufacturing an optical disk according to the present 
invention, i.e., an example in which four layers for forming the thermal 
conduction control layer, i.e., the first low thermal conductivity layer, 
the first high thermal conductivity layer, the second low thermal 
conductivity layer and the second high thermal conductivity layer are 
deposited by successive sputtering will be described below. 
INVENTIVE EXAMPLE 4 
In the inventive example 4, an optical disk based on a magnetooptical disk 
with a structure similar to that of the inventive example 1 shown in FIG. 
1 was manufactured as follows. 
A dielectric layer 2 formed of AlN film with a thickness of 80 nm was 
deposited on a transparent substrate 1 made of a polycarbonate resin or 
the like by sputtering. A target material was Al, and sputtering 
conditions were such that a flow rate of argon gas was 80 sccm, a flow 
rate of nitrogen gas was 21 sccm, a gas pressure was 3 mtorr and an 
applied power was 1 kW. 
Then, a recording layer 3 formed of TbFeCo film with a thickness of 20 nm 
was deposited on the dielectric layer 2 by sputtering. A target material 
was TbFeCo alloy, and sputtering conditions were such that a flow rate of 
argon gas was 80 sccm, a gas pressure was 3 mtorr and an applied power was 
1.2 kW. 
Subsequently, a first low thermal conductivity layer 11 formed of AlN film 
was deposited on the recording layer 3 by sputtering. A target material 
was Al, and sputtering conditions were such that a flow rate of argon gas 
was 80 sccm, a flow rate of nitrogen gas was 21 sccm, a gas pressure was 3 
mtorr and an applied power was 1 kW. 
After a time period during which the thickness of the AlN film becomes 
about 30 nm had been passed, in order to deposit a first high thermal 
conductivity layer 12, discharge was not stopped and only nitrogen gas was 
interrupted. At that time, a sputtering gas pressure was 2.5 mtorr. 
In this manner, the first high thermal conductivity layer 12 formed of Al 
film was deposited. After a time period during which the thickness of the 
Al film becomes about 25 nm had been passed, in order to deposit a second 
low thermal conductivity layer 13, discharge was not stopped, nitrogen gas 
was flowed at a flow rate of 21 sccm in addition to the argon gas. At that 
time, a sputtering gas pressure was 3 mtorr. 
As described above, the second low thermal conductivity layer 13 formed of 
AlN film was deposited. Then, after a time period during which the 
thickness of the AlN film becomes about 50 nm had been passed, in order to 
deposit a second high thermal conductivity layer 14, discharge was not 
stopped and only nitrogen gas was interrupted, whereafter the second high 
thermal conductivity layer 14 formed of the Al film with a thickness of 
150 nm was deposited. 
Specifically, in the inventive example 4, the four layers of the first low 
thermal conductivity layer 11, the first high thermal conductivity layer 
12, the second low thermal conductivity layer 13 and the second high 
thermal conductivity layer 14 were deposited by successive sputtering. 
Only when the first and second low thermal conductivity layers 11 and 13 
are deposited, nitrogen gas was flowed in addition to argon gas. 
Furthermore, a protecting layer 5 made of an ultraviolet-curing resin with 
a thickness of 10 .mu.m was deposited on the second high thermal 
conductivity layer 14 by spin coat, thereby forming the optical disk 20. 
At that time, a thermal conductivity of the AlN film forming the first and 
second low thermal conductive layers 11, 13 is 
0.006J/(cm.multidot.sec.multidot.deg) and smaller than a thermal 
conductivity of 0.13J/(cm.multidot.sec.multidot.deg) forming the recording 
layer 3 of the TbFeCo film forming the recording layer 3. 
A thermal conductivity of the Al layer forming the first and second high 
thermal conductivity layers 12 and 14 is 
2.3J/(cm.multidot.sec.multidot.deg) and larger than that of the TbFeCo 
film forming the recording layer 3. 
Similarly to the inventive example 3, the Al target may be added with a 
material such as titanium so long as the material doe not change heat 
characteristics greatly. 
Power margins of these optical disks according to the inventive examples 3, 
4 and the comparative example for recording power were measured. The 
recording conditions were such that a wavelength of recording laser beam 
was 680 nm, a numerical aperture N.A. of optical system was 0.55, linear 
velocity was 6.4 m/sec and that a clock of one channel was 40 nsec. A 
random signal based on (1, 7) RLL recording system was recorded by 
light-emission of comb-shaped pulse. Information was recorded with 
recording powers being changed and bit error rates were measured. 
Measured results are shown in FIG. 7. FIG. 7 is a graph showing a 
relationship between a recording power Pw (mW) and a bit error rate bitER. 
A study of FIG. 4 reveals that a range of recording power Pw in which the 
error rate becomes less than 10.sup.-4 in the respective inventive 
examples became wider than that of the comparative example and the power 
margin was expanded. 
As described above, it is possible to form an optical disk whose power 
margin is large as compared with the conventional optical disk. 
As described above, if the manufacturing method according to the present 
invention is applied, then four layers of the first low thermal 
conductivity layer, the first high thermal conductivity layer, the second 
low thermal conductivity layer and the second high thermal conductivity 
layer are continuously deposited by use of the same target material while 
only the flow rate of gas is varied. Thus, the four layers can be 
continuously deposited within the same sputtering chamber. Therefore, the 
optical disk according to the present invention can be produced by the 
sputtering chambers of the same number as that of the conventional optical 
disk having four layers (structure having four layers deposited between 
the transparent substrate and the protecting layer). 
While the recording layer is formed of a magnetic layer from which a 
magnetooptical signal can be obtained owning to a magnetooptical effect as 
described above so far, the optical disk and the manufacturing method 
according to the present invention can be applied to a so-called phase 
change-type optical disk in which the recording layer is formed of a layer 
such as a GeSbTe layer in which information can be recorded by the phase 
change generated with irradiation of laser beam. 
The optical disk according to the present invention includes the recording 
layer and the thermal conduction control layer in which the first low 
thermal conductivity layer, the first high thermal conductivity layer, the 
second low thermal conductivity layer and the second high thermal 
conductivity layer are sequentially deposited. Therefore, even when the 
recording density is increased, the thermal interference can be suppressed 
and the margin of recording power can be expanded. 
If the optical disk according to the present invention is such optical disk 
in which a thickness of a protecting layer made of an ultraviolet-curing 
resin is reduced or such protecting layer is not provided, then an amount 
in which the optical disk is warped due to the change of environmental 
conditions, in particular, the change of temperature can be decreased. 
According to the manufacturing method of the present invention, of the 
thermal conduction control layer, the three layers of the first high 
thermal conductivity layer, the second low thermal conductivity layer and 
the second high thermal conductivity layer or the four layers of the first 
low thermal conductivity layer, the first high thermal conductivity layer, 
the second low thermal conductivity layer and the second high thermal 
conductivity layer are continuously deposited by the same target material 
while only the flow rate of gas is changed. Therefore, the three or four 
layers can be continuously deposited by the same sputtering chamber. 
Optical disks of multi-layer structure can be produced by the same 
apparatus as the apparatus for forming the conventional disk with four 
layers. 
Specifically, there are required sputtering chambers of the number 
corresponding to the number of layers. When the optical disk having the 
six layers shown in FIG. 1 is produced, there are required six sputtering 
chambers. However, if the manufacturing method according to the present 
invention is applied, the optical disk can be manufactured by the three or 
four sputtering chambers. Therefore, the optical disk including six layers 
can be manufactured by the conventional deposition line having four 
sputtering chambers for forming the optical disk comprising four layers. 
Having described preferred embodiments of the invention with reference to 
the accompanying drawings, it is to be understood that the invention is 
not limited to those precise embodiments and that various changes and 
modifications could be effected therein by one skilled in the art without 
departing from the spirit or scope of the invention as defined in the 
appended claims.