Optical recording medium

An optical recording medium comprising a recording layer on a substrate is more durable when an intermediate layer of a composition comprising a rare earth element oxide, silicon oxide, and silicon nitride is formed between the substrate and the recording layer.

BACKGROUND OF THE INVENTION 
This invention relates to optical recording media. 
For optical recording media of magneto-optical memory type, there are well 
known a number of materials for a recording layer thereof, for example, 
MnBi, MnAlGe, MnSb, MnCuBi, GdFe, TbFe, GdCo, PtCo, TbCo, TbFeCo, GdFeCo, 
TbFeO.sub.3, GdIG (gadolinium iron garnet), GdTbFe, GdTbFeCoBi, CoFe.sub.2 
O.sub.4, etc. These materials are deposited on transparent substrates of 
plastic material or glass as a thin film by any suitable thin-film forming 
techniques such as vacuum deposition or sputtering. The features common to 
these magneto-optical recording thin film layers are that the axis of easy 
magnetization is perpendicular to the film surface and that Kerr and 
Farady effects are great. 
Requirements imposed on such magneto-optical recording media are: 
(1) that the Curie point is of the order of 100.degree. to 200.degree. C. 
and the compensation point is close to room temperature, 
(2) that noise-inducing defects such as grain boundaries are relatively 
fewer, and 
(3) that a magnetically and mechanically uniform film is obtained over a 
relatively large area. 
In the light of these requirements, a great attention is recently drawn to 
amorphous perpendicular magnetizable thin films of rare earth 
element-transition metal among the above-mentioned materials. 
Magneto-optical recording media having such amorphous perpendicular 
magnetizable thin films of rare earth element-transition metal, however, 
have a storage problem. If the magnetic thin film layers are stored in 
contact with the ambient atmosphere, rare earth elements therein are 
preferentially erroded or oxidized by oxygen and moisture in the 
atmosphere, losing the necessary information recording and reproducing 
ability. The rotational angle available upon reading of recorded signals 
should be as large as possible in order to improve the S/N ratio. 
For this reason, most investigations are generally directed to those 
recording media of the construction having an intermediate layer disposed 
on a surface of a magnetic thin film layer adjacent to or remote from the 
substrate. The intermediate layer is provided for the purpose of imparting 
corrosion resistance or moisture proofness and adding a multiple 
interference effect or Farady effect to Kerr effect to increase the 
rotational angle. Known intermediate layers are vacuum deposited films of 
inorganic materials such as silicon monoxide, silicon dioxide, aluminum 
nitride, silicon nitride and zinc sulfide as well as resinous coatings 
(see Japanese Patent Application Kokai Nos. 58-80142 and 59-52443). 
However, these layers are insufficient in corrosion resistance or the 
like. 
It is also known to form an intermediate layer from a mixture of oxide and 
nitride. For example, Japanese Patent Application Kokai No. 60-145525 
discloses a mixture of Si.sub.3 N.sub.4 and SiO.sub.2, Si.sub.3 N.sub.4 
and SiO, or AlN and Al.sub.2 O.sub.3. 
These intermediate layers, however, are not satisfactory with respect to 
corrosion resistance, C/N (carrier-to-noise ratio), delamination, 
cracking, initial deformation of the medium like warpage, and film forming 
speed. There is a need for an optical recording medium having more 
improved properties. 
The same problem arises in an optical recording medium having a recording 
layer of the so-called phase conversion type. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide a novel and improved 
optical recording medium having improved durability, corrosion resistance, 
and C/N ratio. 
Another object of the present invention is to provide a novel and improved 
optical recording medium having a recording layer which is unlikely to 
deteriorate. 
A further object is to provide an optical recording medium wherein 
delamination, cracking and initial warpage are minimized. 
A still further object is to provide an optical recording medium of the 
type wherein information is recorded and reproduced with the use of heat 
and light of a laser beam. 
The present invention is directed to an optical recording medium comprising 
a substrate, a recording layer on the substrate, and an intermediate layer 
formed between the substrate and the recording layer. According to the 
feature of the present invention, the intermediate layer comprises a 
mixture of at least one rare earth element oxide, preferably the oxide of 
La and/or Ce, silicon oxide, and silicon nitride. 
In one preferred embodiment, a protective layer is disposed between the 
substrate and the intermediate layer and/or on the recording layer.

DETAILED DESCRIPTION OF THE INVENTION 
Two preferred embodiments of the optical recording medium of the present 
invention are illustrated in FIGS. 1 and 2. They have substantially the 
same structure except for the presence of a lower protective layer in FIG. 
2. Like reference numerals designate identical or corresponding parts 
throughout the figures. For brevity of description, the terms "upper" and 
"lower" are used in a normal sense as viewed in FIGS. 1 and 2. 
The optical recording medium according to the present invention is 
generally designated at 10 as comprising a substrate 12 having a pair of 
opposed major surfaces 11, 13 and an intermediate layer 26 on the upper 
surface 11 of the substrate. A recording layer in the form of a magnetic 
thin-film layer 18 is on the intermediate layer 26. The magnetic thin-film 
layer 18 has a pair of opposed major surfaces, that is, an upper surface 
17 disposed remote from the substrate 12 and a lower surface 19 disposed 
adjacent to the substrate 12. If desired, the medium may further include a 
protective layer 16 of vitreous material formed adjacent to the upper 
surface 17 of the magnetic thin-film layer 18 as shown in FIG. 1. It is 
also possible that a lower protective layer 14 of vitreous material be 
disposed between the substrate 12 and the magnetic thin-film layer 18, and 
an upper protective layer 16 of vitreous material be disposed on the upper 
surface 17 of the magnetic thin-film layer 18 as shown in FIG. 2. 
According to the present invention, the intermediate layer 26 is of a 
composition comprising the oxide of at least one rare earth element, 
silicon oxide, and silicon nitride. 
The rare earth element used herein includes all the elements chemically 
classified as rare earth elements, that is, Sc, Y, La through Sm, and Eu 
through Lu. At least one rare earth element is contained in the 
intermediate layer composition. Inclusion of lanthanum (La), cerium (Ce) 
or a mixture of La and Ce is preferred. The oxides of lanthanum and cerium 
are usually La.sub.2 O.sub.3 and CeO.sub.2. They usually take their 
stoichiometric composition, but may have a composition deviating 
therefrom. It suffices that either lanthanum oxide or cerium oxide or both 
lanthanum oxide and cerium oxide be present in the intermediate layer. 
When a mixture of lanthanum oxide and cerium oxide is present, the 
relative proportion is not critical. 
In addition to a primary rare earth element oxide selected from lanthanum 
oxide or cerium oxide or a mixture of lanthanum oxide and cerium oxide, 
the intermediate layer composition may contain less than about 10 atom % 
of the oxide of a secondary rare earth element such as Y and Er, the atom 
% being calculated as metal and based on the primary rare earth element. 
The intermediate layer composition may contain the oxides of other 
incidental elements such as Fe, Mg, Ca, Sr, Ba, and Al. For these 
incidental elements, Fe is present in an amount of less than about 10 at % 
and the remaining elements are present in a total amount of less than 
about 10 at %. 
The intermediate layer composition contains silicon oxide and silicon 
nitride in addition to the rare earth element oxide. Usually, silicon 
oxide is present in the form of SiO.sub.2 and SiO while silicon nitride is 
present in the form of Si.sub.3 N.sub.4. They may have a composition 
deviating from their stoichiometry. Preferably, the silicon oxide and 
silicon nitride are present at a molar ratio of from about 50:50 to about 
90:10 calculated as SiO.sub.2 and Si.sub.3 N.sub.4, respectively. The 
intermediate layer is generally in amorphous state. 
The intermediate layer 26 has a refractive index of from about 1.8 to about 
3.0, preferably from about 2.0 to about 2.5 at a wavelength of 800 nm. A 
layer with a refractive index of less than about 1.8 is insufficient in 
amplifying Kerr rotational angle to increase an output level. A refractive 
index of more than about 3.0 results in an output drop and a noise 
increase. 
The intermediate layer 26 preferably contains a rare earth element oxide 
and silicon compounds (oxide and nitride) such that the weight ratio of 
the total of rare earth element oxide to the total of silicon compounds 
and rare earth element oxide ranges from about 1:20 to about 1:2. Below 
this range, there are observable an output drop and a reduction of 
durability under high-temperature, high-humidity conditions. Beyond this 
range, there are observable a noise increase and a reduction of durability 
under high-temperature, high-humidity conditions. 
In the intermediate layer, the atomic ratio of O/N preferably ranges from 
about 0.2:1 to about 3:1. The medium is less durable under 
high-temperature, high-humidity conditions when the intermediate layer has 
an O/N atomic ratio of less than 0.2. The medium produces an output drop 
and tends to deteriorate with time when the intermediate layer has an O/N 
atomic ratio of more than 3. It will be understood that determination of 
such an atomic ratio can be made by a suitable spectral analysis such as 
Auger spectroscopy and EDA. 
The intermediate layer may have a graded concentration of oxygen and 
nitrogen in its thickness direction. It is preferred that the intermediate 
layer is oxygen rich on a side adjacent to the substrate and nitrogen rich 
on a side remote from the substrate. More specifically, the intermediate 
layer has an atomic ratio (O/N).sub.1 of from about 1.0 to about 100 near 
its lower surface adjacent to the substrate, and an atomic ratio 
(O/N).sub.u of from about 0.1 to about 2.0 near its upper surface remote 
from the substrate, with the ratio of (O/N)1/(O/N).sup.u ranging from 
about 1 to about 100. 
Another concentration gradient is also contemplated wherein both nitrogen 
and oxygen become rich on a side adjacent to the substrate. 
In combination with the intermediate layer 26, such an intermediate layer 
composition as mentioned above may be deposited on the magnetic thin-film 
layer 18 to form a protective layer 16. In this embodiment, the 
intermediate and protective layers 26 and 16 may have the same composition 
or different compositions falling within the specific composition of the 
present invention. 
It is desired to form the intermediate layer 26 by sputtering. The 
preferred target used in sputtering is a sintered mixture of a rare earth 
element oxide, preferably La.sub.2 O.sub.3 and/or CeO.sub.2, SiO.sub.2 and 
Si The rare earth element oxide, especially La.sub.2 O.sub.3 and/or 
CeO.sub.2, can be partially or entirely replaced by the oxide of a 
pyrophoric alloy such as Auer metal, Huber metal, Misch metal, and 
Welsbach metal. The composition of these pyrophoric alloys is shown in 
Table 1. 
TABLE 1 
__________________________________________________________________________ 
Alloy Component, % by weight 
designation 
Fe Zn 
Ce La Y Er Mg Sn 
Pb Cd 
__________________________________________________________________________ 
Auer metal 
35 -- 
35 24 4 2 -- -- 
-- -- 
Huber metal 
-- -- 
85 -- -- -- 15 -- 
-- -- 
Misch metal 
-- -- 
40-60 
20-40 
balance 
-- -- 
-- -- 
Welsbach metal 
30 -- 
70 -- -- -- -- -- 
-- -- 
Welsbach metal 
No. 1 30 -- 
60 balance -- -- 
-- -- 
Welsbach metal 
No. 1A -- -- 
57 -- -- -- 3 -- 
-- 40 
Welsbach metal 
No. 2 -- -- 
67 -- -- -- 3 30 
-- -- 
Welsbach metal 
No. 3 -- 30 
67 -- -- -- 3 -- 
-- -- 
Welsbach metal 
No. 4 -- -- 
67 -- -- -- 3 -- 
30 -- 
__________________________________________________________________________ 
Any other suitable gas phase film-forming technique may be chosen, for 
example, chemical vapor deposition (CVD), evaporation, and ion plating. 
The intermediate layer may contain impurities such as argon and nitrogen 
which are introduced from the film-forming atmosphere. In addition, such 
elements as Fe, Ni, Cr, Cu, Mn, Mg, Ca, Na, and K can be present as 
impurities. 
The intermediate layer 26 has a thickness of from about 300 to about 
3,000.ANG., preferably from about 500 to about 2,000.ANG.. A thickness of 
less than 300.ANG. results in a reduced output and poor weatherability. 
Sensitivity and productivity are reduced with a thickness in excess of 
3,000.ANG.. 
In the preferred embodiment of the present invention, the protective layer 
14 is formed between the substrate 12 and the intermediate layer 26. That 
is, the protective layer 14 is on the upper surface 11 of the substrate 
12. 
Any desired material can form the protective layer 14 as long as it is of 
vitreous nature. Preferred materials are described below. 
(I) A first vitreous material consists essentially of silicon oxide, an 
alkali metal oxide, and aluminum oxide or boron oxide. Preferably, the 
content of silicon oxide ranges from about 40 to about 60% by weight and 
the content of alkali metal oxide ranges from about 0.5 to about 10% by 
weight, more preferably from about 1.0 to about 10% by weight. 
(II) A second vitreous material consists essentially of silicon oxide, an 
alkali metal oxide, aluminum oxide or boron oxide, and a metal oxide 
represented by M(II)O wherein M(II) is a divalent metal. Preferably, the 
content of silicon oxide ranges from about 40 to about 60% by weight and 
the content of alkali metal oxide ranges from about 0.5 to about 10% by 
weight, more preferably from about 1.0 to about 10% by weight. 
(III) A third vitreous material consists essentially of silicon oxide, 
aluminum oxide or boron oxide, and a metal oxide represented by M(II)O 
wherein M(II) is a divalent metal. Preferably, the content of silicon 
oxide ranges from about 40 to about 60% by weight and the content of 
divalent metal oxide ranges from about 10 to about 50% by weight. 
(IV) A fourth vitreous material consists essentially of silicon oxide, an 
alkali metal oxide, and aluminum oxide and boron oxide. Preferably, the 
content of silicon oxide ranges from about 40 to about 60% by weight and 
the content of alkali metal oxide ranges from about 0.5 to 1 about 0% by 
weight, more preferably from about 1.0 to 10% by weight. 
(V) A second vitreous material consists essentially of silicon oxide, an 
alkali metal oxide, aluminum oxide and boron oxide, and a metal oxide 
represented by M(II)O wherein M(II) is a divalent metal. Preferably, the 
content of silicon oxide ranges from about 40 to about 60% by weight and 
the content of alkali metal oxide ranges from about 0.5 to about 10% by 
weight, more preferably from about 1.0 to 10% by weight. 
(VI) A third vitreous material consists essentially of silicon oxide, 
aluminum oxide and boron oxide, and a metal oxide represented by M(II)O 
wherein M(II) is a divalent metal. Preferably, the content of silicon 
oxide ranges from about 40 to about 60% by weight and the content of 
divalent metal oxide ranges from about 10 to about 50% by weight. 
Outstanding durability is achieved particularly when vitreous materials (I) 
through (VI), especially (I), (II), (IV), and (V) are used with the 
contents of certain components falling within the above-defined ranges. 
In these vitreous materials (I) through (VI), silicon oxide is usually 
present in the form of SiO.sub.2 The alkali metal oxides include Li.sub.2 
O, Na.sub.2 O, K.sub.2 O, Rb.sub.2 O, Cs.sub.2 O, and Fr.sub.2 O, with the 
lithium oxide, sodium oxide and potassium oxide being preferred. They may 
be used alone or in admixture of two or more. Boron oxide and aluminum 
oxide are usually present in the form of B.sub.2 O.sub.3 and Al.sub.2 
O.sub.3, respectively. The metal oxides M(II)O include BaO, CaO, MgO, ZnO, 
PbO, SrO, etc., with the barium oxide, calcium oxide and strontium oxide 
being preferred. They may be used alone or in admixture of two or more. 
Most preferred compositions for vitreous materials (I) through (VI) are 
described below. 
In (I), the content of boron oxide or aluminum oxie ranges from about 30 to 
59.5% by weight. 
In (II) and (III), the content of boron oxide or aluminum oxide ranges from 
about 30 to 59.5% by weight. The boron or aluminum oxide can be partially 
or entirely replaced by divalent metal oxide M(II)O such that the content 
of M(II)O ranges from about 10 to 50%, especially from about 10 to 35% by 
weight based on the total weight of the layer. 
In (IV), the total content of boron oxide and aluminum oxide ranges from 30 
to 59.5% by weight. In (IV) to (VI), it is desired that boron oxide is 
present in an amount of 1.0 to 40% by weight and aluminum oxide is present 
in an amount of 3.0 to 45% by weight. 
In (V), the total content of boron oxide and aluminum oxide ranges from 10 
to 49.5% by weight and the content of divalent metal oxide M(II)O ranges 
from 10 to 49.5% by weight. 
In (VI), the total content of boron oxide and aluminum oxide ranges from 10 
to 50% by weight. 
The protective layer 14 disposed adjacent to the substrate 12 and formed as 
described above has a thickness of from about 300 to about 1,000.ANG., 
more preferably from 400 to 800.ANG.. 
Formation of the protective layer 14 can be carried out by a variety of gas 
phase film-forming techniques as used for the intermediate layer 26. 
Instead of or in addition to the protective layer 14 of a vitreous 
material, another protective layer 16 of a similar material may be formed 
on a surface of the magnetic thin-film layer 18 remote from the substrate 
12 as shown in FIGS. 1 and 2. Like the lower protective layer 14 of 
vitreous material, the upper protective layer 16 may be formed of any 
desired vitreous material properly selected from materials (I) through 
(VI) mentioned above. The lower and upper protective layers 14 and 16 
usually have the same composition, but may have different compositions if 
desired. The upper protective layer 16 of vitreous material may be formed 
by the same or similar technique as described for the lower protective 
layer 14. The upper protective layer 16 preferably has a thickness of 
about 300 to about 3,000.ANG., more preferably from 500 to 2,000.ANG.. 
The magnetic thin-film layer 18 is a layer in which signals indicative of 
information are magnetically recorded using a modulated thermal beam or 
modulated magnetic field and the recorded signals are reproduced through 
magnetic-to-optical signal conversion. 
The magnetic thin-film layer is generally formed from alloys containing 
rare earth elements and transition metals by conventional techniques such 
as sputtering and vacuum deposition to produce an amorphous film to an 
ordinary thickness. The rare earth elements and transition metals used 
include all the members of their chemically defined classes. The preferred 
rare earth elements are gadolinium (Gd) and terbium (Tb), and the 
preferred transition metals are iron (Fe) and cobalt (Co). The magnetic 
thin-film layer is preferably comprised of 65 to 85 atom percents of Fe 
and Co in total and the balance essentially of rare earth metals, 
especially Gd and/or Tb. Preferred combinations are TbFeCo, GdFeCo, 
GdTbFeCo, etc. The magnetic thin-film layer may contain less than about 10 
atom % of Cr, Al, Ti, Pt, Si, Mo, Mn, V, Ni, Cu, Zn, Ge, Au, etc. The 
magnetic thin-film layer may also contain less than about 10 atom % of an 
additional rare earth element such as Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, 
Dy, Ho, Er, Tm, Yb, Lu, etc. 
The magnetic thin-film layers are preferably 100 to 
10,000 A thick. 
The material of which the recording layer is made also includes materials 
of phase conversion type, for example, 
Te--Se, Te--Se--Sn, Te--Ge, Te--In, Te--Sn, Te--Ge--Sb--S, Te--Ge--As--Si, 
Te--Si, Te--Ge--Si--Sb, Te--Ge--Bi, Te--Ge--In--Ga, Te--Si--Bi--Tl, 
Te--Ge--Bi--In--S, Te--As--Ge--Sb, Te--Ge--Se--S, Te--Ge--Se, 
Te--As--Ge--Ga, Te--Ge--S--In, Se--Ge--Tl, Se--Te--As, Se--Ge--Tl--Sb, 
Se--Ge--Bi, Se--S (see Japanese Patent Publication No. 54-41902 and 
Japanese Patent No. 1004835), 
TeO.sub.x (Te dispersed in tellurium oxide as described in Japanese Patent 
Application Kokai No. 58-54338 and Japanese Patent No. 974257), 
TeO.sub.x +PbO.sub.x (see Japanese Patent No. 974258), 
TeO.sub.x +VO.sub.x (see Japanese Patent No. 974257), 
chalcogens, for example, Te and Se base materials such as Te--Tl, 
Te--Tl--Si, Se--Zn--Sb, Te--Se--Ga, and TeN.sub.x, 
alloys capable of amorphous-crystal transformation such as Ge--Sn and 
Si--Sn, 
alloys capable of color change through crystal structure transformation 
such as Ag--Zn, Ag--Al--Cu, and CuAl, and 
alloys capable of grain size change such as In--Sb. 
The recording layer may be formed by any desired dry coating technique 
including evaporation, sputtering, and ion plating. The recording layer 
generally has a thickness of from about 20 nm to about 1 .mu.m. 
The substrate 12 for use in the optical recording medium according to the 
present invention is generally formed of glass or a resinous material. 
Typical resins include acrylic resins, polycarbonate resins, epoxy resins, 
and olefinic resins such as polymethylpentene. Preferred among these 
resins are polycarbonate resins because of their durability, especially 
resistance to warpage. 
The polycarbonate resins used herein may be aliphatic polycarbonates, 
aromatic-aliphatic polycarbonates and aromatic polycarbonates, with the 
aromatic polycarbonates being particularly preferred. Polycarbonates 
derived from bisphenols are preferred because of melting point, 
crystallinity and ease of handling. The most preferred is a bisphenol-A 
polycarbonate. The polycarbonate resin preferably has a number average 
molecular weight of from about 10,000 to 15,000. 
The substrate 12 preferably has a refractive index of from about 1.55 to 
1.59 at a wavelength of 830 nm. Since recording is generally carried out 
through the substrate, the transmittance of recording or reading-out light 
is preferably 86% or higher. 
In general, the substrate is of disk shape although it may have another 
shape such as tape and drum. The substrate is of normal dimensions. 
The surface 11 of such a disk-shaped substrate 12 on which the magnetic 
thin-film layer 18 is formed may be provided with a tracking channel. The 
channel has a depth of about .lambda./8n, especially from .lambda./6n to 
.lambda./12n wherein n is the refractive index of the substrate. The 
channel has a width of about 0.4 to 2.0 .mu.m. The substrate may also be 
formed with a pit for addressing purpose. 
Those portions of the magnetic thin-film layer which are located on the 
bottom of the channels constitute recording tracks where writing and 
reading light beams are preferably directed from the lower surface of the 
substrate. With this construction, the reading C/N ratio is improved and a 
control signal of a greater magnitude is available for tracking. 
The protective layer 16 is usually disposed on the magnetic thin-film layer 
18 as previously described. The protective layer 16 may be of the same 
composition as defined for the intermediate layer. Alternatively, the 
protective layer 16 may be formed from a variety of vitreous materials, 
such as SiO, SiO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2, Si.sub.3 N.sub.4, 
AlN, TiN, SiC, ZnS and a mixture thereof. The protective layer 16 
generally has a thickness of from about 300 to 3,000.ANG.. 
The optical recording medium of the present invention may further include 
an organic protective coating layer 20 which is formed on the magnetic 
thin-film layer 18 with or without the intervening protective layer 16. 
The material of which the organic protective coating layer 20 is formed 
includes a variety of well-known organic materials. Preferably the organic 
protective coating layer 20 is a coating of a radiation-curable compound 
cured with radiation such as electron radiation and ultraviolet radiation. 
Illustrative radiation-curable compounds include monomers, oligomers and 
polymers having contained or incorporated in their molecule groups capable 
of crosslinking or polymerizing upon exposure to radiation, for example, 
acrylic double bonds as given by acrylic and methacrylic acids having an 
unsaturated double bond capable of radical polymerization in response to 
an ionization energy and esters thereof, allyl double bonds as given by 
diallyl phthalate, and unsaturated bonds as given by maleic acid and 
maleic derivatives. The radiation-curable monomers used herein are those 
compounds having a molecular weight of less than 2,000 and the oligomers 
are those compounds having a molecular weight of 2,000 to 10,000. 
The radiation-curable compounds having unsaturated double bonds which may 
be used as oligomers and monomers in the present invention include 
styrene, ethylacrylate, ethylene glycol diacrylate, ethylene glycol 
dimethacrylate, diethylene glycol diacrylate, diethylene glycol 
methacrylate, 1,6-hexaneglycol diacrylate, and 1,6hexaneglycol 
dimethacrylate. More preferred are pentaerythritol tetraacrylate (and 
methacrylate), pentaerythritol triacrylate (and methacrylate), 
trimethylolpropane triacrylate (and methacrylate), trimethylolpropane 
diacylate (and methacrylate), polyfunctional oligoester acrylates (e. g., 
Aronix M-7100, M-5400, M-5500, M-5700, M-6250, M-6500, M-8030, M-8060, 
M-8100, etc., available from Toa Synthetic K.K.), acryl modified products 
of urethane elastomers (e.g., Nippolane 4040 available from Nippon 
Polyurethane K.K.), and the derivatives thereof having a functional group 
such as COOH incorporated therein, acrylates and methacrylates of phenol 
ethylene oxide adducts, compounds having a pentaerythritol fused ring 
represented by the following general formula and having an acryl or 
methacryl group or epsilon-caprolactone-acryl group attached thereto: 
##STR1## 
for example, compound wherein m=1, a=2, and b=4 (to be referred to as 
special pentaerythritol condensate A, hereinafter), compound wherein m=1, 
a=3, and b=3 (to be referred to as special pentaerythritol condensate B, 
hereinafter), compound wherein m=1, a=6, and b=0 (to be referred to as 
special pentaerythritol condensate C, hereinafter), and compound wherein 
m=2, a=6, and b=0 (to be referred to as special pentaerythritol condensate 
D, hereinafter), and special acrylates represented by the following 
general formulae: 
##STR2## 
The radiation-curable oligomers include polyfunctional oligo-ester 
acrylates as represented by the following general formula: 
##STR3## 
wherein R.sub.1 and R.sub.2 are alkyl and n is an integer, and 
acryl-modified urethane elastomers, and derivatives thereof having such a 
functional group as COOH incorporated therein. 
Also employable are radiation-curable resins which are prepared by 
modifying thermoplastic resins to be radiation sensitive. 
Illustrative radiation-curable resins are thermoplastic resins having 
contained or incorporated in their molecule groups capable of crosslinking 
or polymerizing upon exposure to radiation, for example, acrylic double 
bonds as given by acrylic and methacrylic acids having an unsaturated 
double bond capable of radical polymerization and esters thereof, allyl 
double bonds as given by diallyl phthalate, and unsaturated bonds as given 
by maleic acid and maleic derivatives. 
The thermoplastic resins which can be modified into radiation-curable 
resins include vinyl chloride copolymers, saturated polyester resins, 
polyvinyl alcohol reins, epoxy resins, phenoxy resins, cellulosic 
derivatives, etc. 
Other examples of the resins which can be modified to be radiation curable 
include polyfunctional polyester resins, polyether ester resins, polyvinyl 
pyrrolidone resins and derivatives (e.g., PVP-olefin copolymers), 
polyamide resins, polyimide resins, phenol resins, spiroacetal resins, and 
acrylic resins comprising as a polymerization component at least one acryl 
or methacryl ester having a hydroxyl group. 
The organic protective coating layer 20 of radiation-cured compound has a 
thickness of about 0.1 to 30 .mu.m, preferably about 1 to 10 .mu.m. Films 
of less than 0.1 .mu.m thick are difficult to produce as a uniform film, 
less moisture proof in a highly humid atmosphere, and thus insufficient to 
improve the durability of the magnetic thin-film layer 18. Films in excess 
of 30 .mu.m thick are practically unacceptable because their shrinkage 
during curing causes the recording medium to be warped and cracks to occur 
in the protective layer. 
The protective layer may be formed by coating an appropriate composition by 
any well-known coating methods such as spinner coating, gravure coating, 
spray coating, and dipping. The conditions under which the film is coated 
may be suitably chosen by taking into account the viscosity of the polymer 
component in the composition, the substrate surface state, and the 
intended coating thickness. 
These radiation-curable resins may be cured by any of various well-known 
methods using electron or ultraviolet radiation. 
For electron radiation curing, a radiation accelerator is preferably 
operated at an accelerating voltage of 100 to 750 kV, more preferably 150 
to 300 kV to generate radiation having a sufficient penetrating power such 
that the object is exposed to a radiation dose of 0.5 to 20 megarad. 
When curing is effected with ultraviolet radiation, a photo polymerization 
sensitizer may be added to the radiation curable compounds as mentioned 
above. 
The photo polymerization sensitizers used herein may be selected from 
well-known sensitizers. Examples of such sensitizers include benzoins such 
as benzoin methyl ether, benzoin ethyl ether, .alpha.-methylbenzoin, 
.alpha.-chlorodeoxybenzoin, etc.; ketones such as benzophenone, 
acetophenone, bis(dialkylamino)benzophenones; quinones such as 
anthraquinone and phenanthraquinone; and sulfides such as benzyl sulfide, 
tetramethylthiuram monosulfide, etc. The photo polymerization sensitizers 
may be added in amounts of 0.1 to 10% by weight based on the resin solids. 
For exposure to ultraviolet radiation there may be used UV lamps such as 
xenon discharge lamps and hydrogen discharge lamps. 
The optical recording medium 10 of the present invention further includes a 
protective plate 24 disposed on the organic protective coating layer 20 
through an adhesive layer 22. More particularly, the protective plate 24 
is used when the medium is of single side recording type wherein recording 
and reproducing operations are carried out only from the lower surface 13 
of the substrate 12 which is free of a magnetic thin-film layer. 
The protective plate 24 may be of a resinous material. Since the resinous 
material of the protective plate need not be transparent, a variety of 
resins may be used, for example, thermoplastic resins such as 
polyethylene, polyvinyl chloride, polystyrene, polypropylene, polyvinyl 
alcohol, methacrylic resin, polyamide, polyvinylidene chloride, 
polycarbonate, polyacetal, and fluoro resin; and thermosetting resins such 
as phenol resin, urea resin, unsaturated polyester resin, polyurethane, 
alkyd resin, melamine resin, epoxy resin, and silicone resin. 
It is also possible to form the protective plate 24 from an inorganic 
material such as glass and ceramics. The shape and dimensions of the 
protective plate 24 are approximately the same as the substrate 12. 
The protective plate 24 is bonded to the organic protective coating layer 
20 through the adhesive layer 22. The adhesive layer 22 may be of a 
hot-melt resin adhesive and has a thickness of about 1 to 100 .mu.m. 
Instead of the protective plate 24, two sets of substrate 12, intermediate 
layer 26, magnetic thin-film layer 18, protective layer 14/16, and organic 
protective coating layer 20 may be prepared and bonded with an adhesive 
layer such that the magnetic thin-film layers may be opposed to each 
other. This results in a double sided recording type medium wherein 
recording and reproducing operations can be carried out from the outside 
surfaces of both the substrates. 
The lower surface 13 of the substrate 12 and the top surface of the 
protective plate 24 which is remote from the magnetic thin-film layer 18 
are preferably covered with suitable protective coatings. Such coatings 
may be formed from the same material as previously described for the 
organic protective coating layer 20. 
The optical recording medium of the present invention having an 
intermediate layer of a specific composition formed between a substrate 
and a recording layer is satisfactorily durable and resistant to 
corrosion. It can reproduce an output with a high C/N ratio and a 
minimized error rate. 
EXAMPLES 
In order that those skilled in the art will better understand the practice 
of the present invention, examples of the present invention are given 
below by way of illustration and not by way of limitation. 
Example 1 
An optical recording disk having the structure as shown in FIG. 1 was 
prepared by molding a disk-shaped substrate 12 from a bisphenol-A 
polycarbonate of optical disk grade to a diameter of 13 cm and a thickness 
of 1.2 mm. A sintered mixture of La.sub.2 O.sub.3, SiO.sub.2, and Si.sub.3 
N.sub.4 as a target was sputtered on the substrate to form an intermediate 
layer 26 having a thickness of 800.ANG.. The composition of the 
intermediate layer reported in Table 2 is calculated by the data of 
chemical analysis on the basis of the stoichiometric composition of the 
oxides and nitrides indicated in the table heading. The refractive index 
of the intermediate layer at 800 nm is also reported in Table 2. The 
refractive index was changed by a choice of source composition and 
sputtering conditions including gas pressure and gas composition. 
A magnetic thin-film layer 18 of Tb 21 at %-Fe 68 at %-Co 7 at %-Cr 4 at % 
alloy was formed on the intermediate layer 26 by sputtering to a thickness 
of 800.ANG.. The target used in sputtering was an iron (Fe) target having 
Tb, Co and Cr chips rested thereon. 
A protective layer 16 of borosilicate glass was formed on the magnetic 
thin-film layer 18 to a thickness of 1,000.ANG.. 
The protective layer 16 was coated with a radiation-curable coating 
composition by spinner coating to form an organic protective coating layer 
20. The coating composition contained 100 parts by weight of a 
multifunctional oligo-ester acrylate and 5 parts by weight of a light 
sensitizer. The coating of the composition was exposed to UV for 15 
seconds into a cured film. 
In this way, a series of samples designated Nos. 101 to 107 were prepared 
as shown in Table 2. 
Example 2 
A disk sample No. 108 was prepared by repeating the procedure of Example 1 
except that the target used to form the intermediate layer by sputtering 
was a mixture of CeO.sub.2, SiO.sub.2, and Si.sub.3 N.sub.4. 
Example 3 
A disk sample No. 109 was prepared by repeating the procedure of Example 1 
except that the target used to form the intermediate layer by sputtering 
was a mixture of the oxide of Misch metal (MM) having the composition 
shown in Table 1, SiO.sub.2, and Si.sub.3 N.sub.4. 
Comparative Example 1 
A film consisting of SiO.sub.2 and/or Si.sub.3 N.sub.4 was formed as the 
intermediate layer 26 by reactive sputtering to a thickness of 800.ANG.. 
The remaining structure was the same as Example 1. The resulting samples 
are designated sample Nos. 110 to 112. 
Comparative Example 2 
Disk sample No. 113 was prepared by repeating the procedure of Example 1 
except that the intermediate layer was omitted. 
These samples were measured for the following properties. 
(1) Initial C/N 
The initial C/N (carrier-to-noise ratio) of a sample expressed in dB was 
measured under the following conditions. 
Rotating speed: 4 m/sec. 
Carrier frequency: 1.0 MHz 
Resolution: 30 KHz 
Recording power: 3-6 mW at 830 nm 
Reproducing power: 1 mW at 830 nm 
(2) Durability 
After a sample was kept for 1,000 hours at 60.degree. C. and RH, it was 
observed for a change of bit error rate, film spalling, and a change of 
appearance. The bit error rate was measured by recording an NRZ signal 
under the conditions mentioned in (1). The initial bit error rate was 
3.times.10.sup.-6. 
The results are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Intermediate Layer 
Initial 
Durability 
Composition (wt %) 
Refractive 
C/N Bit error 
Appearance 
Sample No. 
La.sub.2 O.sub.3 
SiO.sub.2 
Si.sub.3 N.sub.4 
index (dB) 
rate (.times. 10.sup.-6) 
change 
__________________________________________________________________________ 
101 
(Example 1) 
20 20 60 2.4 56 3.0 
102 
(Example 1) 
30 20 50 2.2 55 3.0 
103 
(Example 1) 
40 30 30 2.1 55 3.0 
104 
(Example 1) 
10 30 60 2.3 56 4.0 
105 
(Example 1) 
20 50 30 2.0 54 3.0 
106 
(Example 1) 
10 30 60 2.9 52 4.0 
107 
(Example 1) 
20 60 20 1.8 52 3.0 
108 
(Example 2) 
CeO.sub.2 
30 10 60 2.3 54 4.0 
109 
(Example 3) 
MM 
30 40 30 2.0 54 4.0 
110 
(Comparative 
0 50 50 1.9 52 10 some 
Example 1) spalling 
111 
(Comparative 
0 0 100 2.2 52 10 spalling 
Example 1) pinholes 
112 
(Comparative 
0 100 
0 1.6 47 20 spalling 
Example 1) pinholes 
113 
(Comparative 
-- -- -- -- 47 -- many 
Example 2) pinholes 
__________________________________________________________________________ 
Example 4 
An optical recording disk having the structure as shown in FIG. 2 was 
prepared by molding a disk-shaped substrate 12 from a bisphenol-A 
polycarbonate of optical disk grade to a diameter of 13 cm and a thickness 
of 1.2 mm. A protective layer 14 of a vitreous material having the 
composition shown in Table 3 was formed on the substrate by sputtering to 
a thickness of 1,000.ANG.. Then a sintered mixture of La.sub.2 O.sub.3, 
SiO.sub.2, and Si.sub.3 N.sub.4 as a target was sputtered on the 
protective layer 14 to form an intermediate layer 26 having a thickness of 
800.ANG.. The composition of the intermediate layer reported in Table 3 is 
calculated by the data of chemical analysis on the basis of the 
stoichiometric composition of the oxides and nitrides indicated in the 
table heading. The refractive index of the intermediate layer at 800 nm is 
also reported in Table 3. The refractive index was changed by a choice of 
source composition and sputtering conditions including gas pressure and 
gas composition. 
A magnetic thin-film layer 18 of Tb 21 at %-Fe 68 at %-Co 7 at %-Cr 4 at % 
alloy was formed on the intermediate layer 26 by sputtering to a thickness 
of 800.ANG.. The target used in sputtering was an iron (Fe) target having 
Tb, Co and Cr chips rested thereon. 
A protective layer 16 of a vitreous material was formed on the magnetic 
thin-film layer 18. The composition and thickness of the upper protective 
layer 16 are the same as those of the lower protective layer 14 for each 
sample. 
The protective layer 16 was coated with a radiation-curable coating 
composition by spinner coating to form an organic protective coating layer 
20. The coating composition contained 100 parts by weight of a 
multifunctional oligo-ester acrylate and 5 parts by weight of a light 
sensitizer. The coating of the composition was exposed to UV for 15 
seconds into a cured film. 
In this way, a series of samples designated Nos. 201 to 207 were prepared 
as shown in Table 3. 
Example 5 
A disk sample No. 208 was prepared by repeating the procedure of Example 4 
except that the target used to form the intermediate layer by sputtering 
was a mixture of CeO.sub.2, SiO.sub.2, and Si.sub.3 N.sub.4. 
Example 6 
A disk sample No. 209 was prepared by repeating the procedure of Example 4 
except that the target used to form the intermediate layer by sputtering 
was a mixture of the oxide of Misch metal (MM having the composition shown 
in Table 3, SiO.sub.2, and Si.sub.3 N.sub.4. 
Comparative Example 4 
A film consisting of SiO.sub.2 and/or Si.sub.3 N.sub.4 was formed as the 
intermediate layer 26 by reactive sputtering to a thickness of 800.ANG.. 
The remaining structure was the same as Example 4. The resulting samples 
are designated sample Nos. 210 and 211. 
Comparative Example 5 
Disk sample No. 212 was prepared by repeating the procedure of Example 4 
except that the intermediate layer was omitted. 
These samples were measured for the following properties. 
(1) Initial C/N 
The initial C/N (carrier-to-noise ratio) of a sample expressed in dB was 
measured under the following conditions. 
Rotating speed: 4 m/sec. 
Carrier frequency: 1.0 MHz 
Resolution: 30 KHz 
Recording power: 3-6 mW at 830 nm 
Reproducing power: 1 mW at 830 nm 
(2) Durability 
A thermal cycling test was carried out on a sample according to IEC-2-38 
over a temperature range of from --10.degree. C. to +65.degree. C. at a 
relative humidity of 93%. This accelerated test was continued until the 
bit error rate reached twice the initial. The durability of the sample is 
expressed by the duration of the test continued. Under the conditions, 
spalling and cracking in the film largely contributed to an increase of 
bit error rate. 
After a sample was kept for 1,000 hours at 60.degree. C. and 90% RH, it was 
observed for a change of bit error rate, film spalling, and a change of 
appearance. 
The bit error rate was measured by recording an NRZ signal under the 
above-mentioned conditions. 
(3) Corrosion resistance 
A high-temperature, high-humidity aging test was carried out. That is, a 
sample was stored at 70.degree. C. and 90% RH. This accelerated test was 
continued until the bit error rate reached twice the initial. The 
corrosion resistance of the sample is expressed by the duration of the 
test continued. Under the conditions, occurrence of pinholes largely 
contributed to an increase of bit error rate. 
The results are shown in Table 3. 
3 TABLE 3 
Protective Layers 14, 16 Intermediate Layer Corrosion Composition (wt 
%) Composition (wt %) Refractive C/N Durability resistance Change of 
Sample No. SiO.sub.2 Al.sub.2 O.sub.3 B.sub.2 O.sub.3 Na.sub.2 O K.sub.2 
O BaO CaO MgO ZnO La.sub.2 O.sub.3 SiO.sub.2 Si.sub.3 N.sub.4 index (dB) ( 
hr.) (hr.) appearance 
201 (Example 4) 53 42 -- 3.0 2.0 -- -- -- -- 20 20 60 2.4 56 &gt;800 
&gt;1000 202 (Example 4) 48 -- 47 3.0 2.0 -- -- -- -- 30 20 50 2.2 55 &gt;800 
&gt;1000 203 (Example 4) 55 -- 30 2.0 1.0 8.0 4.0 -- -- 40 30 30 2.1 55 
&gt;800 &gt;1000 204 (Example 4) 53 25 17 3.0 2.0 -- -- -- -- 10 30 60 2.3 56 
&gt;800 &gt;1000 205 (Example 4) 48 6 12 1.0 1.0 20.0 10.0 -- 2.0 20 50 30 2.0 
54 &gt;800 &gt;1000 206 (Example 4) 45 -- 20 1.0 0.5 22.0 11.5 -- -- 10 30 60 
2.8 52 &gt;800 &gt;1000 207 (Example 4) 53 25 17 3.0 2.0 -- -- -- -- 20 60 20 
1.8 51 &gt;800 &gt;1000 208 (Example 5) 54 7 13 0.5 0.5 21.0 -- -- 4.0 
30CeO.sub.2 10 60 2.3 54 &gt;800 &gt;1000 209 (Example 6) 48 6 12 1.0 1.0 20.0 
10.0 -- 2.0 30 MM 40 30 2.0 53 &gt;800 &gt;1000 210 (Comparative 48 -- 19 9.0 
6.0 5.0 9.0 4.0 -- -- 50 50 1.9 52 800 700 some spalling Example 4) 211 
(Comparative 48 6 12 1.0 1.0 20.0 12.0 -- -- -- 100 -- 1.6 48 600 1000 
some spalling Example 4) 212 (Comparative 48 -- 19 9.0 6.0 5.0 9.0 4.0 
-- -- -- -- -- 47 800 600 some spalling Example 5) 105 (Example 1) -- 
-- -- -- -- -- -- -- -- 20 50 30 2.0 54 600 600 spalling 
The results of Examples 1 to 6 show the effectiveness of the present 
invention. 
Similar results were obtained with phase conversion type recording layers 
of Te--Ge, TeO.sub.x, and Te--Se. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.