Optical disc

An optical disc having a recording film formed on one surface of a plastics substrate transparent to light, a first protection film formed on the recording film, a moisture proof film formed on the other surface of the substrate, and a second protection film formed on the moisture proof film.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to optical discs, and more particularly to an 
optical disc for recording and reproducing data by a laser beam. 
2. Description of the Related Art 
Conventional counterparts of optical discs are constructed by forming a 
recording film 21 on one surface of a plastics substrate 20 and covering 
the recording film 21 with a protection film 22 as shown in FIG. 6. 
Since the recording film 21 generally consists of four or three layers, 
moisture will not permeate through the recording film 21 to the other side 
of the plastics substrate 20. Consequently, these optical discs tend to 
absorb or desorb moisture only at the other side of the plastics substrate 
20, i.e. at the incident side. Moisture absorption and desorption was 
caused by a partial variation in volume of the plastics substrate 20 to 
generate a warp thereof. 
The above warp of the optical discs cause the substrate surface to shift 
from the position perpendicular to optical axis of the laser beam incident 
for recording and reproducing data with the result that the incident light 
fails to strike signals, thereby causing the servo control to get 
disordered to degrade the resulting signal quality or to generate a track 
jump in servo control in extreme cases. 
Under these circumstances, there have been proposed standards or draft 
standards with regard to the warp of optical discs as shown in Table 1. 
TABLE 1 
______________________________________ 
Type of optical discs 
Level of warp 
______________________________________ 
Compact disc 10 mrad max. 
Write-once disc 5 mrad max. 
Magneto-optic disc 
5 mrad max. 
______________________________________ 
Compact discs have a relatively slow rotating speed practically ranging 
from 200 to 500 rpm, which can be sufficiently tracked by the tracking 
servo and the focus servo. Thus, another mild standard on warp is applied 
to the optical discs in comparison with the counterpart applied to other 
types of optical discs. 
Such discs as write-once discs and rewritable discs (magneto-optic discs) 
require an increase in the data transfer rate and hence must rotate at a 
high speed (1800 to 3600 rpm for example). For such discs, the warp level 
needs to be reduced as much as possible to meet with the tracking 
capabilities of the tracking servo and the focus servo. However, a single 
plate plastics substrate hardly reduces the warp level. Thus, single-plate 
discs have been laminated to each other back-to-back to provide a 
double-sided disc, thereby reducing the warp level. 
However, the overwriting technique has come to capture much attention 
recently to increase a demand for single-sided discs. That is because 
conventional magneto-optic discs involve the step of erasing old data 
prior to recording new data for rewriting data. Consequently, they need 
two rotations in total to rewrite data; the first rotation for erasing old 
data and the second rotation for recording new data. In contrast, the 
overwriting-technique permits erasing old data and recording new data in 
one rotation to obviate the conventional process of two rotations, thereby 
increasing the data transfer rate. 
There have been proposed so far several methods for overwriting, among 
which the magnetic field modulation method is one of the most promising 
ones. The magnetic field modulation method involves recording data by 
shifting the direction of the magnetic field while continuing the 
application of light onto the surface of optical discs whereas a data 
erasing dependent method (optical modulation method) involves recording 
data by intermittently applying light with the magnetic field kept to a 
certain direction. 
The magnetic field modulation method demands shifting the direction of the 
magnetic field at a high speed. Besides, actualizing a high-speed magnetic 
field modulation by minimizing the consumption of electric power in the 
electromagnet requires a minimization of the distance between the 
electromagnets and the recording film. However, with respect to 
double-sides discs, the presence of the substrate on the recording film as 
viewed from the side provided with an electromagnet inhibits the reduction 
of the distance between the recording film and the electromagnet, which 
has created an increased demand for single-side discs. 
Using a plastics substrate for single plate discs will naturally compel 
users to meet with the possibility of the warp of discs. The warp 
generated immediately after the manufacturing process can be suppressed to 
a level within the scope of the standard because of the advancement of the 
technique for forming the substrate, as well as the technique for forming 
the recording film and the protection film. On the other hand, it has been 
made clear that another kind of a warp has been generated in using discs. 
For example, the 35th Applied Physics Association Drafts, in spring 1988, 
describes on page 872 that it has been made clear that a change in the 
environmental conditions generates a transient warp of discs. For example, 
according to the document, a change in relative humidity from 90% R. H. to 
50% R. H. at 60% has changed the warp by 14 to 15 mrad at most. 
SUMMARY OF THE INVENTION 
The present invention provides an optical disc comprising a recording film 
formed on one surface of a plastics substrate transparent to light, a 
first protection film formed on the recording film, a moisture proof film 
formed on the other surface of the substrate, and a second protection film 
formed on the moisture proof film. 
Further preferably, a lubricant film is formed on the first protection film 
or an anti-static film is formed on the second protection film. 
Still further preferably, the moisture proof film is formed of either 
AlSiN, AlTaN, TaO, SiO or TiO.sub.2, its thickness is in the range of 1 to 
20 nanometers and a product of the refractive index and the thickness is 
within the range of not exceeding 42 nanometers. 
Still further preferably, the first protection film and the second 
protection film have a thickness of 2 to 20 micrometers and an 
approximately same thickness.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention has been made in view of the conventional problems 
mentioned above to provide an optical disc capable of reducing a transient 
warp of the disc as much as possible at environmental changes. 
The optical disc of the present invention comprises a recording film and a 
first protection film for protecting the recording film on one side of the 
plastics substrate. 
The transparent plastics substrate may be of any plastics material as long 
as it allows light to pass therethrough and resist deformation. Typical 
examples thereof include polycarbonate substrate. In addition, the 
substrate may be formed of such material as acrylic resin, epoxy resin, or 
the like. The thickness of the substrate usually ranges between 1.15 and 
1.25 mm. 
The recording film of the present invention may be any conventional 
recording film, which for example comprises a recording layer of a rare 
earth metal compound, dielectric layer of a metal nitride and a reflective 
layer of an aluminum. Typical examples include a four layer structure such 
as AlSiN/DyFeCo/AlSiN/Al, AlN/GdTbFe/AlN/Al, and AlN/DyFeCo/AlN/Al, and a 
three-layer structure such as SiN/TbFeCo/SiN and SiAlON/TbFeCo/SiAlON. The 
recording film generally has a thickness of 150 to 300 nm. The recording 
film is generally formed on the entire surface of the plastic substrate, 
but it may be formed on the part of the surface thereof. 
The first and the second protection film can be provided primarily for 
protecting the recording film. The film can be formed of such material as 
urethane acrylate ultra-violet curing resin, polyvinylidene chloride resin 
or polytrifluorochloroethylene resin. The film can be formed by a coating 
process. Each of these films has a thickness of 2 to 20 .mu.m. Preferably, 
the first and the second protection films have an approximately equal 
thickness. 
The moisture proof film can be formed of inorganic material such as AlSiN, 
AlTaN, TaO, SiO, TiO.sub.2, AlN, SiN, ZnS, Al.sub.2 O.sub.3, SiO.sub.2, 
and SiAlOH. Among the above-mentioned material, AlSiN, AlTaN, TaO, SiO, 
and TiO.sub.2 are particularly preferable. 
In spite of forming the moisture proof film on the surface of the 
substrate, sometimes it occurs to generate the transient warp of the 
optical disc such as 2 mrad, appear a moire phenomenon in the moisture 
proof film and consequently cause an error by the moire phenomenon for 
recording new data and erasing old data. 
The thickness of the moisture proof film shall be determined by 
characteristics of the material to be used, especially the refractive 
index thereof. We found that the moire phenomenon of the moisture proof 
film can be avoided when the thickness of the film is less than a result 
of 42 nm divided by the refractive index of the material. Specially, when 
AlSiN, for example, is used as the moisture proof film, its thickness is 
less than 20 nm, dividing 42 nm by 2.1 (refractive index of AlSiN), while 
its lower value is limited mainly due to convenience of film formation. 
Preferably, the thickness of AlSiN film is in the range of 1 to 20 nm, 
more preferably 5 to 15 nm, e.g., 5 nm, 10 nm, 15 nm. Similarly, suitable 
thickness of other materials as above-mentioned can be determined, 
utilizing the refractive indices of AlTaN, TaO, SiO and TiO.sub.2 being 
2.1, 2.1, 2.1 and 2.5, respectively. 
A lubricant film may be optionally provided on the first protection film. 
The lubricant film of the optical disc according to the present invention 
can be formed of material excellent in lubricative properties, such as, 
for example, fluoric resin. When using a floating type magnetic head, 
providing the lubricant film on the surface provided with the recording 
film improves lubricative properties between the floating type magnetic 
head and the optical magnetic head. The lubricant film has a thickness of 
about 2 .mu.m. 
An anti-static film may be optionally provided on the second protection 
film. The lubricant film and the anti-static film may not be provided. 
The anti-static film of the optical disc according to the present invention 
consists of a conductive filler mixed resins composition containing a 
transparent conductive filler. The anti-static film is provided to inhibit 
the deposition of dusts on the surface of the optical discs, and to 
inhibit generation of scratches thereon. Preferably, the anti-static film 
is formed of hard material with a relatively low surface resistivity. 
Preferably, the material is more hard than HB pencils. Besides, still 
preferably the surface resistivity of the anti-static film usually does 
not exceed 10.sup.13 .OMEGA.. The process for preparing the anti-static 
film with the conductive filler mixed resins containing a transparent 
conductive filler comprises the steps of mixing a synthetic resin or a raw 
material thereof with the transparent conductive filler in a predetermined 
ratio, optionally adding a small amount of inorganic filler to be mixed 
with the mixture of the synthetic resin and the transparent conductive 
filler to improve the hardness thereof, and forming either the mixture of 
the synthetic resin and the transparent conductive filler or the mixture 
of the synthetic resin, the transparent conductive filler and the 
inorganic filler into a film on the substrate of the optical disc. 
Examples of the synthetic resin to be employed include urethane acrylate 
ultraviolet curing resin, acryl ultraviolet curing resin. The film is 
coated on the substrate by such methods as the spin coating method, the 
roll coating method, and the dip coating method. Depending on the kind of 
synthetic resin to be used, such means as ultraviolet ray irradiation, 
heating and cooling are used to cure the resin. Preferably, the film 
normally has a thickness of 1 to 20 .mu.m. 
Preferably, tin oxide is doped with phosphorus to provide a composition for 
use in a transparent conductive filler for incorporating both transparency 
and stability. In addition, tin oxide is doped with phosphorus by the 
coprecipitation sintering method from the liquid phase. Phosphorus to be 
doped can be obtained from phosphoric acid, sodium phosphate or the like. 
The dopant content in tin oxide preferably ranges between 3 and 7 wt. % 
with respect to the total amount of the conductive filler. Further 
preferably, the content of the conductive filler in the anti-static film 
ranges between 25 to 45 wt. % while the average particle diameter thereof 
does not exceed 0.15 .mu.m. 
EMBODIMENT 
Embodiment of optical discs according to the present invention will be 
detailed by way of the drawings, but the present invention is not limited 
by it. 
Embodiment 1 
Referring to FIG. 1, Reference Numeral 1 designates a plastics substrate 
made of 1.2 nm thick polycarbonate transparent to light. On one surface of 
the plastics substrate 1 is formed a recording film 2. When the recording 
film 2 serves as a magneto-optic recording film, the disc consists of four 
layers. In this particular Embodiment, there is provided on the plastics 
substrate 1 AlSiN/DyFeCo/AlSiN/Al in this order. Further, on this 
recording film 2, the first protection film 3 is formed. The first 
protection film 3 is formed by coating a polyurethane acrylate ultraviolet 
curing resin to a thickness of about 10 .mu.m. 
Reference numeral 4 designates a moisture proof-film formed on the other 
side of the plastics substrate 1. On the moisture proof film 4 a second 
protection film 5 is formed. The second protection film 5 is made of the 
same material as the first protection film 3. That is, polyurethane 
acrylate ultraviolet curing resin is used. The resin has a thickness of 
about 10 .mu.m. Both the first protection film 3 and the second protection 
film 5 may be formed to any thickness so long as they can offset the warp 
generated by the moisture absorption and desorption thereon. However, they 
are required to have an approximately equal thickness for the above 
reason. 
Five kinds of magneto-optic discs with the above construction respectively 
having a 5 nm thick, a 10 nm thick, a 20 nm thick, a 25 nm thick and a 80 
nm thick AlSiN moisture proof film were manufactured and observed under 
natural light from the side of the second protection film (moisture proof 
film) to examine how moire pattern appeared thereon. 
The examination clarified that no moire pattern appeared on the optical 
discs having 5 nm thick and 10 nm thick moisture proof films. Besides, it 
has been also clarified that a slight moire pattern detectable in 
attentive observation appeared on the optical discs having a 20 nm thick 
moisture proof film but the pattern was not conspicuous at all. On the 
other hand, moire patterns were observed on the optical discs having a 25 
nm thick and a 80 nm thick moisture proof film, and the moire pattern that 
appeared on the optical disc having a 80 nm thick moisture proof film was 
even more conspicuous than the disc having a 25 nm thick moisture proof 
film. 
Then, there was calculated a relation between the thickness of the moisture 
proof film 4 and the reflectance of light incident from the side of the 
second protection film 5 to clarify how the appearance of the moire 
pattern is affected by the reflectance with respect to moisture proof 
films 4 having a different thickness. The relation between the thickness 
of the moisture proof film 4 and the reflectance was calculated with 
respect to a three layer model having on a polycarbonate plastics 
substrate 1 an AlSiN moisture proof film 4 and a second protection film 
formed of polyurethane acrylate ultraviolet curing resin. 
Incidentally, the refractive indexes of the polycarbonate, AlSiN, 
polyurethane acrylate ultraviolet curing resin were to 1.58, 2.1 and 1.5 
respectively. The thickness of the second protection film was set to 10 
.mu.m. The light source used therein exhibited 780 nm, which commonly 
serves as a wavelength of a semiconductor laser widely used as a light 
source for optical discs. The reflectance was calculated by changing the 
thickness of the moisture proof film within the range of 200 nm. 
The above observation of moire patterns and the calculation of the 
reflectance clarified that moire patterns appeared when the reflectance 
exhibited a large value whereas almost no moire patter appeared when the 
reflectance did not exceed about 8%. 
FIG. 2 shows the result of the calculation. The axis of abscissas 
represents the thickness of the AlSiN whereas the axis of ordinates the 
reflectance. 
Then, there was examined how the warp of optical discs related to the 
thickness of the moisture proof film 4 under condition of environmental 
changes. First, optical discs were allowed to stand at 34.degree. C. and 
90% R. H. Then, the humidity was reduced at a rate of 10% R. H./hr to set 
the environmental condition of 34.degree. C. and 30% R. H. in six hours 
followed by maintaining the environmental condition of 34.degree. C. and 
30% R. H. When the above environmental condition was kept, the warp level 
of optical discs was measured. 
Incidentally optical discs used in the measurement was the same as 
magneto-optic discs tentatively manufactured for the test except that the 
moisture proof films respectively have a 1 nm thick, a 5 nm thick, 25 nm 
thick and 80 nm thick AlSiN film. For comparison, the conventional type 
magneto-optic disc identical to the above was formed except that the 
moisture proof film 4 and the second protection film are formed. 
FIG. 3 shows a graph illustrating the result of the test. The axis of 
abscissa represents the passage of time from the initiation of the change 
in temperature whereas the axis of ordinates represents warp of optical 
discs. In all the embodiments of optical discs according to the present 
invention, the warp level of optical discs are limited to 2 mrad or less. 
It naturally follows from the above observation that the thickness of 1 nm 
or more in AlSiN film serving as a moisture proof film sufficiently 
inhibits the generation of warp of magneto-optical discs and that moire 
patterns cannot be observed from the side of the second protection film 5. 
Here, the lower limit value of 1 nm in the AlSiN film in the moisture proof 
film was set by confirming the effect of inhibiting a transient warp of 
magneto-optical discs. In the same manner, the upper limit value was set 
by confirming in the calculation of the above three layer model and actual 
observation of moire patterns that the reflectance of 8% or less obtained 
in the calculation does not produce almost no moire pattern appeared. FIG. 
2 shows that the thickness of 20 nm in the AlSiN film provides a 
reflectance of 8%. 
Setting the upper limit value will be detailed hereinafter. 
As mentioned above, when the calculated reflectance does not exceed 8%, 
almost no moire pattern appeared on optical discs tentatively manufactured 
for the test. In the above three layer model, the thickness of the 
moisture proof film 4 providing a reflectance of 8% varies with the 
refractive index of the moisture proof film 4. Here, the refractive index 
of the polycarbonate serving as a plastics substrate 1 and the refractive 
index of the polyurethane acrylate ultraviolet curing resin serving as a 
second protection film 5 were set to 1.58 and 1.5 respectively. In other 
words, the calculations allows for multiplex interferences Consequently, 
the reflectance changes along with the change in the value of the light 
wavelength generated by the diffractive index multiplied by the film 
thickness. In other words, obtaining the same reflectance requires the 
same light wavelength. As a result, when the moisture proof film 4 using 
AlSiN exhibiting a diffractive index of 2: 1 has an upper limit thickness 
of 20 nm, it also has a light wavelength of 42 nm. Besides, moisture proof 
films having a lower limit thickness of 1 nm has a light wavelength of 
about 2 nm. 
As explained above, setting the thickness of the moisture proof film 4 
within the scope of 2 to 42 nm enables the clarification of why no moire 
pattern cannot be observed in magneto-optical discs. 
Incidentally, the lower limit of the thickness of the moisture proof film 4 
is set to 1 nm which can be confirmed in the test. This minimum thickness 
of the moisture proof film 4 depends the accuracy of the measurement of 
the thickness (using a contact type film thickness meter, for example) and 
the thickness control capability of vapor deposition and a sputtering 
apparatus used in forming the moisture proof film 4. Consequently, such 
film thickness can be obtained easily with a machine that can accurately 
reproduce the moisture proof film having a thickness thinner than the 
lower limit value, or a thickness of 0.5 nm. It can be easily estimated 
from the test in FIG. 3 that the magneto-optic discs with a moisture proof 
film in such thickness inhibits or reduces a transient warp of the disc 
compared with discs with a moisture proof film in a thickness of 0, or 
discs with no moisture proof film. Consequently, the lower limit of the 
thickness of the moisture proof film exceed 0 nm excluding 0 nm. 
On the other hand, the upper limit of the moisture proof film 4 was set to 
20 mm or less, which produces a reflectance of 8% or less, on the basis of 
the observation of moire patterns on the disc actually manufactured. 
However, FIG. 2 shows that the range of the thickness of the moisture 
proof film producing a reflectance of 8% also occurs between 120 nm and 
200 nm. This has resulted from the fact that the reflectance in a cycle of 
wavelength/(2 X refractive index) takes the same value owing to the light 
interference in the AlSiN film serving as the moisture proof film 4. In 
FIG. 2, the reflectance varies in a cycle of a film thickness of 186 nm 
obtained by substituting the wavelength of 780 nm and the refractive index 
of 2.1 in the AlSiN film. Consequently, the film thickness producing a 
reflectance of 8% or less occurs in three ranges; 1 to 20 nm, 120 nm to 
206 nm and 306 nm to 392 nm. 
However, it is apparent that the thickness of the moisture proof film 4 
should be advantageously in the most thin range possible, namely in the 
range of 1 to 20 nm considering time and cost required for manufacturing 
the film. 
In the above embodiment, the first protection film 3 and the second 
protection film need not be made of the same material in the same 
thickness. A transient warp of the disc can be inhibited by setting the 
material and the thickness inhibits a transient warp in such a manner that 
the warp of the moisture absorption and desorption can be offset to each 
other. For example, the first protection film 3 may be formed of 
polyurethane ultraviolet curing resin whereas the second protection film 5 
may be formed of acryl hard coat resin 
Embodiment 2 
Embodiment 2 of the optical disc according to the present invention is 
constructed in the same manner as Embodiment 1 of the magneto-optic disc 
except that an anti-static film (having a thickness of about 4 .mu.m) 
formed of acryl hard coat resin mixed with a conductive filler is formed 
on a second protection film formed on a moisture proof film. Incidentally, 
the AlSiN moisture proof film has a thickness of about 10 nm in terms of 
the light wavelength. 
Embodiment 2 of the optical disc will be detailed as follows with reference 
to FIG. 4. 
Referring to FIG. 4, Reference Numeral 1 designates a plastics substrate 
formed of 1.2 mm thick polycarbonate transparent to light. On one surface 
of this plastics substrate is formed a recording film 2. When the 
recording film 2 is formed of a magneto-optical recording film, the 
optical disc consists of four layers. This particular Embodiment of the 
optical disc is formed of AlSiN/DyFeCa/AlSiN/Al as viewed from the side of 
the plastics substrate 1. Further, on this recording film 2, the first 
protection film 3 is formed. The first protection film 3 is formed by 
coating polyurethane acrylate ultraviolet curing ray resin to a thickness 
of 10 .mu.m. 
Reference Numeral 4 designates the moisture proof film formed on the 
opposite side of the plastics substrate 1. On this moisture proof film 4 
was formed the second protection film 5. The second protection film 5 was 
formed of material same as the first protection film 3. That is, 
polyurethane acrylate ultraviolet curing resin was used and it had a 
thickness of about 6 .mu.m. On the second protection film 5 the 
anti-static film 6 formed of acryl hard coat resin mixed with a conductive 
filler was formed to a thickness of about 4 .mu.m. 
No moire pattern was observed when viewing the magneto-optical disc from 
the side of the anti-static film 6. Besides the warp level exhibited 2 
mrad or less. 
This magneto-optic disc was provided with a anti-static film 6 on the 
surface of the side light strikes for recording, erasing and reproducing 
data, thereby making it very difficult for dusts and small particles that 
tend to scatter light to deposit onto the surface of the optical disc. 
This has largely reduced the possibility that the focusing servo or the 
tracking servo might get disordered in the cycle of recording, erasing and 
reproducing data to degrade the recording signals or reproducing signals 
or to generate the track jump. 
Besides the level of the transient warp in Embodiment 2 exhibited almost 
the same value as Embodiment 1 of the magneto-optic disc. This is because 
polyurethane arylate ultraviolet curing resins serving as the second 
protection film 5 had the same degree of moisture absorption and 
desorption as conductive filler mixed acrylic hard coat resins serving as 
the anti-static film 6 and because the thickness of the combination of the 
second protection film 5 and the anti-static film 6 was set to about 10 
.mu.m identical to the thickness of the first protection film 3. For 
reference, it is to be noted that increasing only the second protection 
film 5 from 6 .mu.m to 10 .mu.m has increased somewhat the level of warp. 
Embodiment 3 
Embodiment 3 of the optical disc according to the present invention was 
produced by forming a lubricant film formed on fluoric resin with 
excellent lubricative properties to a thickness of about 2 .mu.m on the 
first protection film provided on the recording film of the 
magneto-optical shown in Embodiment 2. 
Embodiment 3 will be detailed as follows with reference to FIG. 5. 
Referring to FIG. 5, Reference Numeral 1 designates a plastics substrate 
formed of transparent polycarbonate having a thickness of 1.2 mm. On one 
surface of the plastics substrate 1 is formed a recording film 2. The 
recording film for use in a magneto-optic disc consist of four layers. In 
this particular Embodiment, the four layers are formed in the order of 
AlSiN/DyFeCo/AlSiN/Al from the side of the plastics substrate 1. The first 
protection film 3 is formed by coating a polyurethane acrylate ultraviolet 
curing resin to a thickness of about 8 .mu.m whereas the lubricant film 7 
is formed of fluoric resin to a thickness of about 2 .mu.m. 
Reference numeral 4 designates a moisture proof film formed on the other 
side of the plastics substrate 1. In this particular Embodiment, AlSiN 
film with a refractive index of 2.1 is formed to a thickness of about 5 
nm. On this moisture proof film 4 is formed the second protection film 5. 
The second protection film 5 uses the same material as the first 
protection film 3, namely polyurethane acrylate ultraviolet curing resin 
having a thickness of about 6 .mu.m. On the second protection film 5 is 
formed the anti-static film 6 formed of acrylate hard coat resin mixed 
with a conductive filler to a thickness of about 4 .mu.m. 
No moire pattern was observed when the magneto-optic disc was observed from 
the side of the anti-static film 6. The level of a transient warp was 
examined in the similar manner as Embodiment 1 to show 2 mrad or less. 
Besides, the lubricant film 7 was provided on the surface of the side 
provided with this particular Embodiment of the magneto-optical disc 
according to the present invention. Consequently, using a floating type 
magnetic head permitted improvement in the lubrication between the 
floating type magnetic head and the magneto-optical disc. 
In other words, the floating type magnetic head is disposed to record and 
erase data on the recording medium film 2 with a gap of several to 15, 16 
.mu.m kept therebetween. The above gap is kept by taking a balance between 
the pressure by a suspension spring that works to press the floating type 
magnetic head to the recording film and the floating force generated by 
the air stream of high speed rotation of the magneto-optic disc to 
separate-the floating type magnetic head from the recording film 2. 
When using such conventional floating type magnetic head, the floating type 
magnetic head may be broken until it stops depending on time required for 
reaching to the predetermined number of rotations from the start of the 
rotation of the magneto-optic disc and the predetermined number of 
rotations at the end of the rotation of the disc. This is caused by the 
adsorption of the floating type magnetic head and the optical magnetic 
head when contact-start-stop (CSS) method is adopted in which the floating 
head and the optical magnetic head contact each other. 
However, the magneto-optic disc provides a lubricant film on the recording 
film to improve lubricative properties between the floating type magnetic 
head and the magneto-optic disc, thereby inhibiting breakage of the 
floating type magnetic head because of adsorption of the two heads. 
Embodiment 4 
Embodiment 4 of the optical disc according to the present invention was 
constructed in the same manner as Embodiment 1 except that AlTaN was used 
as a moisture proof film 4. AlTaN film used therein had a refractive index 
of about 2.1 approximately equal to AlSiN film used in Embodiment 1. 
In the same manner as Embodiment 1, five magneto-optic discs were 
tentatively manufactured respectively having a 5 nm thick, a 10 nm thick, 
a 20 nm thick, 25 nm thick and a 80 nm thick AlTaN moisture proof film to 
be examined from the side of the moisture proof film 4. The examination 
showed that no moire pattern was observed on the magneto-optic discs 
having a 5 nm thick and a 10 nm thick moisture proof film. On the other 
hand, a light moire pattern was observed on magneto-optic discs having a 
20 nm thick moisture proof film 4 with much attention, with nearly 
inconspicuous degree. Besides, moire pattern was observed on magneto-optic 
discs having a 25 nm thick and a 80 nm thick moisture proof film 4. The 
moire pattern was more conspicuous on the magneto-optic disc having a 80 
nm moisture proof film than the disc having a 25 nm thick moisture proof 
film. 
Further, in the same manner as Embodiment 1, four magneto-optic discs were 
tentatively manufactured respectively having a 1 nm thick, a 5 nm thick, a 
25 thick and a 80 nm thick AlTaN moisture proof film 4. Consequently, the 
level of warp in the magneto-optic disc according to the present invention 
was limited to 2 mrad or less in the same manner as Embodiment 1. 
As mentioned above, setting the thickness of the AlTaN moisture proof film 
to 1 to 20 nm inhibited the generation of a transient warp of 
magneto-optic disc and further allows making the moire pattern 
unobservable from the side of the second protection film 5. 
Besides, in Embodiments 2 and 3, an magneto-optical disc was temporarily 
manufactured having a moisture proof film 4 to confirm the similar effect 
produced in the magneto-optic disc. 
Embodiment 5 
Embodiment 5 of the optical disc according to the present invention was 
manufactured by using a TaO moisture proof film 4 in a magneto-optic disc 
having a structure shown in Embodiment 1. The TaO moisture proof film used 
in this particular Embodiment has a refractive index of 2.1 which is 
almost the same as AlSiN film. 
In the same manner as Embodiment 1, five magneto-optic discs were 
tentatively manufactured respectively having a 5 nm thick, a 10 nm thick, 
a 20 nm thick, a 25 nm thick and a 80 nm thick moisture proof film 4. 
Observation of moire patterns from the side of the moisture proof film 4 
made clear that no moire pattern was observed on the magneto-optic discs 
respectively having a 25 nm thick and a 80 nm thick moisture proof film. 
Much clearer moire pattern was observed on the optical disc having a 80 nm 
moisture proof film. 
Further, in the similar way as Embodiment 1, four optical discs were 
tentatively manufactured having a TaN film respectively having a 1 nm 
thick, a 5 nm thick, a 25 nm thick, and a 80 nm thick moisture proof film 
to examine the level of warp at environmental changes similar to the one 
described in Embodiment. The examination clarified that the level of warp 
in the magneto-optic disc in this Embodiment was inhibited to 2 mrad or 
less in the similar way as Embodiment 1. 
Thus, setting the: thickness of TaO moisture proof film 4 in the range of 1 
to 20 nm inhibits a transient warp of the magneto-optic disc and further 
allows making the moire pattern unobservable from the side of the second 
protection film 5. 
Besides, a magneto-optic disc having TaO moisture proof film using the 
structure of Embodiment 2 and 3 was tentatively manufactured to confirm 
that it can produce the similar effect. 
Embodiment 6 
Embodiment 6 of the magneto-optic disc having a SiO moisture proof film was 
manufactured in the same manner as Embodiment 1 except that SiO film was 
used as a moisture proof film 4. 
The refractive index of the SiO film used in the Embodiment exhibited 2.1 
approximately identical to AlSiN film in Embodiment 1. 
In the same manner as Embodiment 1, five magneto-optic discs were 
tentatively manufactured respectively having a 5 nm thick, a 10 nm thick, 
a 20 nm thick, a 25 nm thick and a 80 nm thick SiO moisture proof film to 
prove that no moire pattern was observed-on the magneto-optic discs 
respectively having a 5 nm thick and a 10 nm thick moisture film. In 
addition, a slight moire pattern was observed on the magneto-optic disc 
with a 20 nm thick moisture proof film 4 when observed with much 
attention. On the other hand., moire patterns were observed on 
magneto-optic discs respectively having a 25 nm and 80 nm thick moisture 
proof film. The moire pattern was more clearly observed on the 
magneto-optic disc with a 80 nm thick moisture proof film. 
Further, in the same manner as Embodiment 1, four magneto-optic discs were 
tentatively manufactured with a SiO moisture proof film 4 respectively 
having a thickness of 1 nm, 5 nm, 25 nm and 80 nm to examine the level of 
transient warp at environmental changes. The examination made clear that 
the level of warp was limited to 2 mrad or less. 
Thus, setting the thickness of the TiO.sub.2 moisture proof film 4 to 1 to 
15 nm inhibits a transient warp of the magneto-optic disc and further 
allows making the moire pattern unobservable from the side of the second 
protection film 5. 
Besides, a magneto-optic disc was tentatively manufactured also in the same 
manner as Embodiments 2 and 3 having a SiO moisture proof film 4 to 
confirm the same effect-can be realized. 
Embodiment 7 
Embodiment 7 of the magneto-optic disc according to the present invention 
was tentatively manufactured in the same manner as Embodiment 2 except 
that TiO.sub.2 was used as a moisture proof film 4. The TiO.sub.2 used 
therein has refractive index higher than any other material mentioned 
above, exhibiting about 2.5. 
In the same manner as Embodiment 1, five magneto-optic discs were 
manufactured respectively having a 5 nm thick, a 15 nm thick, a 20 nm 
thick, a 25 nm thick and a 80 nm thick TiO.sub.2 moisture proof film to 
examine them from the side of the moisture proof film 4. The examination 
showed that no moire pattern was observed on the magneto-optic discs 
having a 5 nm thick and a 15 nm thick moisture proof film. In addition, a 
slight moire pattern was observed on the magneto-optic disc having a 20 nm 
thick moisture proof film 4 with much attention. On the other hand, moire 
patterns were observed on magneto-optic discs respectively having a 25 nm 
and a 80 nm thick moisture proof film. The moire pattern was more clearly 
observed on the magneto-optic disc with a 80 nm thick moisture proof film. 
Further in the same manner as Embodiment 1, four magneto-optic discs were 
tentatively manufactured respectively having a 1 nm thick, a 5 nm thick, a 
25 nm thick and a 80 nm thick moisture proof film to examine the level of 
the transient warp at environmental changes. The examination showed that 
the warp level of the Embodiment of the magneto-optic disc was limited to 
2 mrad or less. 
Thus, setting the thickness of the TiO.sub.2 moisture- proof film within 
the range of 1 through 15 nm inhibits the transient warp of the disc and 
further allows making the moire pattern unobservable from the second 
protection film 5. Here, in order to set the light wavelength of 42 nm 
obtained by multiplying the refractive index and the film thickness, the 
thickness of the TiO.sub.2 film with a refractive index of 2.5 is set to 
about 17 nm. The Embodiment discloses that moire pattern was observed on 
the magneto-optic disc with a 15 nm thick TiO.sub.2 moisture proof film. 
However, it also discloses almost no moire pattern was observed on 
magneto-optic discs with a 17 nm or less thick moisture proof film. 
Besides, in Embodiments 2and 3, magneto-optic discs were tentatively 
manufactured with a TiO.sub.2 moisture proof film 4 to confirm that the 
discs have the same effect. 
The effect and the superiority will be detailed with respect to moisture 
proof films formed on AlSiN, AlTaN, TaO, SiO and TiO.sub.2. 
Firstly, the above materials are formed on both or one sides of the 
magneto-optic recording layer in the recording film 4 to inhibit the 
oxidation of a thin film formed of rare earth transition metal alloy such 
as DyFeCo, GdTbFe, TbFeCo, TbDyFeCo, GdDyFeCo, GdDyFe and the like. They 
are suitable for an oxidation protection film 4 for obtaining long-term 
durability. 
Secondly, as mentioned above, AlSiN, AlTaN, TaO, SiO, TiO.sub.2 are formed 
on both or one sides of the magneto-optic layer in the recording layer 2. 
Consequently, directly using a thin film manufacturing device such as a 
sputtering apparatus or the like for forming a magneto-optic disc to form 
the moisture proof film 4, which facilitates the manufacturing process 
thereof. 
Thirdly, particularly with respect to the magneto-optic disc having a 4 
layer recording film whose structure is shown in the above Embodiment, 
AlSiN film constituting a first and a second transparent film serves as a 
antioxidant film as well as a magneto-optic enhancement layer for 
improving the signal quality. It is known that heightening this 
enhancement effect requires an increase in the refractive index of the 
above film. Thus, AlSiN, AlTaN, TaO, SiO and TaO.sub.2 with a relatively 
high refractive index of about 2 are suitable for it, despite the 
transparency of the film. 
Incidentally, a thin film formed of AlSiN, AlTaN, TaO, SiO and TiO.sub.2 is 
formed by a normal sputtering apparatus. However, changing the sputtering 
conditions allows the control of the refractive index of these materials 
within the range of about 1.7 to 2.6. 
For example, AlSiN and AlTaN are respectively used as an AlSi-made alloy 
target or AlTa alloy-made target, which is formed with a reactant 
sputtering method using N.sub.2 or N.sub.2 +Ar as a sputtering gas. 
Changing the pressure and the flow rate of this sputtering gas allows the 
control of the refractive index of these materials within the above range. 
By the way, as mentioned above, when using AlSiN, AlTaN, Tao, SiO, 
TiO.sub.2 as a transparent film for use in the recording film 2 of the 
magneto-optic disc the refractive index should be advantageously raised to 
heighten the enhancement effect. Thus, in Embodiment 1, the refractive 
index was set to a rather high level of 2.1 to coincide the refractive 
index of AlSiN serving as-the moisture proof film 4 with the counterpart 
of the above transparent film. Such procedure allowed a continuous 
formation of the transparent film of the recording film 2 and the moisture 
proof film 4 in the same manufacturing apparatus under the same 
manufacturing conditions to realize a high productivity. 
Further, the present invention provides a very thin film having a thickness 
of 42 nm in terms of light wavelength to make the moire pattern invisible 
and to inhibit the transient warp, thereby making the manufacturing cost 
and time. 
In the above Embodiment, polycarbonate was used as a plastics substrate 1. 
The plastics substrate 1 can be formed of such materials as acrylic resin 
and epoxy resin in addition to polycarbonate. 
As disclosed above, the optical disc of the present invention accompanied 
with few transient element at environmental changes has the effect of 
making obscure moire patterns seen from the side which light strikes. 
Besides, optical discs according to the present invention is typified by a 
magneto-optic disc which includes a compact disc, a write-once disc, and a 
photochromic type disc using as a recording film photochromic material.