Data recording medium

The invention provides a data recording medium comprising: a disc-shaped substrate; a radiation-sensitive recording layer formed on one surface of the substrate; an oxide layer formed by oxidizing a surface of the said recording layer; and a protective layer formed on the oxide layer. The data recording medium of the invention prevents formation of cracks in the recording layer and/or protective layer and is improved in the manufacturing yield.

BACKGROUND OF THE INVENTION 
The present invention relates to a data recording medium. The data 
recording medium has a radiation-sensitive recording layer. A laser beam 
is focused on the radiation-sensitive recording layer to form a pit 
therein, thereby recording optical data. The optical data recorded in this 
manner can be read out. 
A conventional data recording medium is known wherein a recording layer 2 
of a metal such as Te, Bi, Mn or the like is formed on a substrate 1 of 
glass or a synthetic resin by vacuum deposition, sputtering or the like 
(FIG. 1). Another conventional data recording medium is known wherein an 
anticorrosive protective layer 3 is formed on the surface of the recording 
layer 2 (FIG. 2). The protective layer 3 is generally made of an organic 
or inorganic material which has resistance to corrosion. The protective 
layer of an inorganic material is formed on the recording layer into a 
thin film of less than 5,000 A in thickness, for example, by vacuum 
deposition, sputtering or the like of silicon dioxide, silicon monoxide, 
indium oxide or the like. On the other hand, the protective layer of an 
organic material is formed on the recording layer into a thin film of 
about 1 to 100.mu., for example, by dissolving in a solvent a polyvinyl 
chloride resin, styrene resin or the like to provide a solution of the 
resin and by applying the solution by a suitable means such as a spinner, 
a roll coater, an applicator or the like. 
However, when the protective layer of an inorganic material is formed on 
the recording layer by vacuum deposition or sputtering, cracks tend to 
form in the recording layer and/or protective layer after a certain period 
of time. On the contrary, when the protective layer of an organic material 
is formed in place of the protective layer of an inorganic material, the 
recording layer may flow during application or during curing steps of the 
protective layer of the organic material. For this reason, the formation 
of the protective layer of the organic material is cumbersome, and cracks 
tend to form after a certain period of time. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a data recording medium 
which prevents formation of cracks in a recording layer and/or a 
protective layer, and which is improved in a manufacturing yield and in 
the reliability. 
In order to achieve this object, there is provided according to the present 
invention a data recording medium comprising: 
a disc-shaped substrate; 
a radiation-sensitive recording layer formed on one surface of said 
substrate; 
an oxide layer formed by oxidizing a surface of said recording layer; and 
a protective layer formed on said oxide layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiment of the present invention will now be described 
with reference to the accompanying drawings. 
FIG. 3 is a sectional view of the main part of a data recording medium 
according to an embodiment of the present invention. Referring to FIG. 3, 
a data recording medium has a substrate 10, a recording layer 11 which is 
formed on the substrate 10 and on the surface of which is formed an oxide 
layer 11a, and a protective layer 12 covering the recording layer 11. The 
substrate 10 is made of glass or a synthetic resin and is of a disc-shape 
having a thickness of 0.2 to 2 mm and a diameter of 30 cm. In order to 
record or reproduce data on or from a recording layer 11 to be described 
later by focusing a laser beam thereon, the glass or synthetic resin as 
the material of the substrate 10 must transmit light. Examples of the 
synthetic resin may include a polyolefin such as polyethylene; polyvinyl 
chloride; a vinyl chloride-acetate copolymer; an ethylene-vinyl acetate 
copolymer, polycarbonate; polyacrylic acid; polymethacrylic acid; and a 
lower alkyl ester of polyacrylic acid or polymethacrylic acid. In order to 
obtain the substrate 21 of excellent transparency and greater mechanical 
strength, it is preferable to use polyacrylic acid, polymethacrylic acid 
or a lower alkyl ester thereof. 
The recording layer 11 is of an annular shape with the central hole (not 
shown) of the substrate 10 as the center. The recording layer 11 is a thin 
film made of a light-reflecting material. The light-reflecting material is 
required to reflect the laser beam and may be for example Te, Bi, or Al. 
The thickness of the recording layer 11 is less than 5,000 A, preferably 
200 to 2,000 A. The method for forming the recording layer 11 on the 
surface of the substrate 10 may be selected freely only if the recording 
layer 11 of small thickness can be uniformly formed. The method, 
therefore, may be a deposition method such as vacuum deposition, 
sputtering or ion deposition. The dense oxide layer 11a is formed on the 
surface of the recording layer 11 by oxidizing this surface. The method 
for oxidizing the surface of the recording layer may include radiation 
with ultraviolet rays or infrared rays, heating, standing in the ambient 
atmosphere, or the like. The oxide layer 11a is preferably 20 to 500 A 
thick. Among the oxidizing methods mentioned above, radiation with 
ultraviolet rays or standing in a dry air are preferred since the dense 
oxide layer is easy to form thereby. If the dense oxide layer is too thin, 
favorable effects may not be obtained. On the other hand, if the dense 
oxide layer is too thick, the contrast of the recording bit is difficult 
to control. 
The protective layer 12 is formed so as to cover the oxide layer 11a of the 
recording layer 11 and to form a concentric ring having the central hole 
(not shown) of the substrate 10 as a center. The protective layer 12 may 
therefore be made of an organic material such as polystyrene, an epoxy 
resin, a polycarbonate resin, a vinyl chloride-acetate copolymer, an 
ethylenevinyl acetate copolymer, and a silicone resin. The protective 
layer 12 of an organic material as described above can be formed by 
preparing a solution of the organic material in an organic solvent such as 
benzene, toluene, or the like, applying the solution on the surface of the 
oxide layer 11a of the recording layer 11, and removing the organic 
solvent. Said protective layer 12 may be formed by using said resin of 
uncured state without said solvent. The protective layer 23 may 
alternatively be made of an inorganic material such as SiO.sub.2, SiO, 
In.sub.2 O.sub.3, SnO.sub.2, MgO, ZnO, ZnO.sub.2, Al.sub.2 O.sub.3, 
CeO.sub.2, TiO.sub.2, MgF.sub.2, CaF.sub.2 and CeF.sub.3. The protective 
layer 12 of the inorganic material is formed on the recording layer by 
vacuum deposition, sputtering or the like. 
An embodiment of the manufacturing method of the data recording medium of 
the construction as described above will now be described with reference 
to FIG. 4. FIG. 4 is a schematic view showing an embodiment of the method 
of manufacturing the data recording medium of the present invention. 
First, the recording layer 11 of Te is formed to a thickness of 500 A by 
vacuum deposition on the surface of the substrate 10 of an acrylics. 
Thereafter, the whole structure is fed to a heating section by a supply 
belt 15 at a rate of 5 m/min. The heating section has an arranging 
distance L (50 cm) in which three focusing type ultraviolet lamps 14 of 80 
W/cm are arranged. In this manner, the oxide layer 11a of about 100 A 
thickness as shown in FIG. 3 is formed on the surface of the recording 
layer 11. A styrene resin is coated on the surface of the recording layer 
11 to a thickness of 20 .mu.m by spinner coating to form the protective 
layer 12 as shown in FIG. 3. As a result, the recording layer 11 of Te and 
of 400 A thickness is formed on the surface of the substrate 10 of the 
acrylics. The surface layer of the recording layer 11 is oxidized to form 
the oxide layer 11a (TeO.sub.x) of 100 A thickness. The protective layer 
12 of a styrene resin is formed thereover. Therefore, since the oxide 
layer 11a is formed on the surface of the recording layer 11, the 
recording layer 11 will have good mechanical strength. The formation of 
the microcracks in the recording layer 11 during formation of the 
protective layer 12 may therefore be prevented. 
Although the present invention has been described with reference to a 
particular embodiment thereof, the present invention is by no means 
limited to this. It is to be understood that various other modifications 
and changes may be made within the spirit and scope of the present 
invention. 
In summary, according to the present invention, an oxide layer is formed on 
the surface of a recording layer, so that the recording layer has better 
mechanical strength and formation of cracks in the recording layer during 
the formation of the protective layer is prevented. Accordingly, the data 
recording medium provides advantages such as improvements in the 
manufacturing yield and the reliability.