U.S. patent application number 09/226701 was filed with the patent office on 2001-11-29 for optical disc and method for manufacturing same.
Invention is credited to FURUKI, MOTOHIRO, KASHIWAGI, TOSHIYUKI, NISHIDA, MASATO, SAKAMOTO, TETSUHIRO.
Application Number | 20010046203 09/226701 |
Document ID | / |
Family ID | 11558650 |
Filed Date | 2001-11-29 |
United States Patent
Application |
20010046203 |
Kind Code |
A1 |
NISHIDA, MASATO ; et
al. |
November 29, 2001 |
OPTICAL DISC AND METHOD FOR MANUFACTURING SAME
Abstract
An optical disc and its manufacturing method in which humps may
be prohibited from being formed on an outer rim area of the disc to
assure optimum surface properties of a light transmitting layer of
the disc to contribute to further increase in recording capacity.
On a substrate 2 of an optical disc 1 are sequentially formed a
recording portion 6 and a light transmitting layer 5. The light
falls on the light transmitting layer 5 to record and/or reproduce
information signals for a signal recording area 6a of the substrate
2. The radial distance D from the outermost rim of the substrate 2
to the signal recording area 6a is selected to be larger than the
radial width L of a hump 5a formed on the outer rim of the light
transmitting layer 5. The hump 5a has a height h from the surface
of the light transmitting layer 5 not larger than 70 .mu.m.
Inventors: |
NISHIDA, MASATO; (TOKYO,
JP) ; SAKAMOTO, TETSUHIRO; (CHIBA, JP) ;
KASHIWAGI, TOSHIYUKI; (TOKYO, JP) ; FURUKI,
MOTOHIRO; (TOKYO, JP) |
Correspondence
Address: |
William E Vaughn
Bell Boyd & Lloyd LLC
P O Box 1135
Chicago
IL
60690-1135
US
|
Family ID: |
11558650 |
Appl. No.: |
09/226701 |
Filed: |
January 7, 1999 |
Current U.S.
Class: |
369/280 ;
156/273.3; G9B/7.139; G9B/7.172; G9B/7.181; G9B/7.194 |
Current CPC
Class: |
G11B 7/2542 20130101;
B29L 2009/00 20130101; G11B 7/2533 20130101; G11B 2007/24304
20130101; G11B 7/2578 20130101; G11B 7/254 20130101; G11B 7/2433
20130101; G11B 7/2531 20130101; G11B 7/2534 20130101; G11B 7/2585
20130101; G11B 7/26 20130101; B29D 17/005 20130101; G11B 7/24
20130101; G11B 7/266 20130101 |
Class at
Publication: |
369/280 ;
156/273.3 |
International
Class: |
B32B 031/00; G11B
003/70; G11B 005/84; G11B 007/26 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 9, 1998 |
JP |
P10-003485 |
Claims
What is claimed is:
1. An optical disc in which a recording layer and a light
transmitting layer are sequentially formed on a substrate and in
which light is incident from the side of the light transmitting
layer to record and/or reproduce information signals for a signal
recording area of the recording layer, wherein a radial distance
from the outermost area of the substrate to said signal recording
area is selected to be larger than a radial width of a hump
produced on the outer rim of the light transmitting layer and
wherein the height of said hump from the surface of the light
transmitting layer is 70 .mu.m or less.
2. The optical disc according to claim 1 wherein the height of said
hump from the surface of the light transmitting layer is 20 .mu.m
or less.
3. The optical disc according to claim 1 wherein, with a radial
distance from the outermost rim to the signal recording area of
said substrate of D mm and a radial width of said hump formed at
the outer rim of the light transmitting layer of L mm, the relation
D-0.5.gtoreq.L mm is met.
4. The optical disc according to claim 1 wherein the radial width
of said hump 1 is 1.5 mm or less.
5. The optical disc according to claim 1 wherein the radial width
of said hump 1 is 1.0 mm or less.
6. A method for producing an optical disc in which a recording
layer and a light transmitting layer are sequentially formed on a
substrate and in which light is incident from the side of the light
transmitting layer to record and/or reproduce information signals,
comprising the steps of: coating, at the time of forming the light
transmitting layer, a UV curable resin on the portion of the
substrate carrying said recording layer; and rotating the substrate
carrying the UV curable resin coated thereon and concurrently
illuminating UV rays on said UV curable resin to cure said UV
curable resin.
7. The method for producing an optical disc according to claim 6
wherein said step of coating the UV curable resin drips the UV
curable resin on a substrate carrying the recording layer and
running the substrate in rotation to get the UV curable resin cured
on rotation; and wherein said step of curing the UV curable resin
runs the substrate carrying the UV curable resin coated thereon in
rotation at an rpm smaller than the rpm of rotation of the
substrate at the UV curable resin coating step and illuminates UV
rays on said UV curable resin.
8. The method for producing an optical disc according to claim 7
wherein said step of coating the UV curable resin drips the UV
curable resin on a substrate carrying the recording layer and
subsequently sets a light-transmitting plate on the UV curable
resin to get said UV curable resin stretched; and wherein said step
of curing the UV curable resin runs the substrate and the
light-transmitting plate in rotation and concurrently illuminating
UV rays on said UV curable resin; said light-transmitting plate
being peeled off after the end of curing of said UV curable
resin.
9. The method for producing an optical disc according to claim 6
wherein a cut-out is formed in the outer rim of said substrate,
with a spacing defined by said cut-out being used for accommodating
a hump of said UV curable resin.
10. The method for producing an optical disc according to claim 6
further comprising the step of: running the substrate in rotation
at an rpm larger than the rpm of said substrate used for coating
said UV curable resin, said rotation step being carried out after
the step of coating the UV curable resin and before the step of
curing the UV curable resin.
11. The method for producing an optical disc according to claim 6
wherein said step of curing the UV curable resin includes a first
sub-step of running the substrate in rotation and illuminating UV
rays on only the UV curable resin coated on the signal recording
area of the recording layer; a second sub-step of running the
substrate in rotation at an rpm larger than the rpm at which the
substrate is run in rotation at said first step, for removing part
of the UV curable resin on an area other than said signal recording
area on an outer rim portion of the substrate; and a third
sub-step, subsequent to said second step, of running the substrate
in rotation at an rpm lower than the rpm with which the substrate
is run in rotation in said second step and concomitantly
illuminating UV rays at least on an area other than the signal
recording area for curing the UV curable resin.
12. The method for producing an optical disc according to claim 6
wherein said step of coating the UV curable resin is carried out
after illuminating UV rays on the area of said substrate other than
the signal recording area.
13. The method for producing an optical disc according to claim 6
wherein said step of coating the UV curable resin includes a first
sub-step of fitting said substrate inside of an annular auxiliary
substrate having an inner diameter substantially equal to the outer
diameter of an optical disc to be formed for supporting said
substrate and a second sub-step of dripping a UV curable resin onto
said auxiliary substrate and said substrate to get the UV curable
resin stretched on rotation by rotation of said auxiliary substrate
and said substrate; there being a further step subsequent to the UV
curable resin curing step for dismounting said auxiliary plate and
for removing the UV curable resin on said auxiliary substrate.
14. The method for producing an optical disc according to claim 6
wherein, following the step of coating the UV curable resin and the
step of curing the UV curable resin, with the use of a substrate
having an outer diameter larger than a desired value, part of the
substrate is removed to give a desired outer diameter 15.
15. The method for producing an optical disc according to claim 6
wherein, following the step of coating the UV curable resin, part
of the UV curable resin coated on the outermost rim of the
substrate is absorbed by a resin-absorbing material.
16. The method for producing an optical disc according to claim 6
wherein, following the step of coating the UV curable resin, part
of the UV curable resin coated on the outermost rim of the
substrate is sucked by suction means.
17. The method for producing an optical disc according to claim 6
wherein, following the step of coating the UV curable resin, part
of the UV curable resin coated on the outermost rim of the
substrate is blown off by a gas injection means.
18. The method for producing an optical disc according to claim 6
wherein said UV curable resin has a viscosity not less than 4000
cps.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to an optical disc having a substrate
and a recording layer and a light transmitting layer formed in this
order thereon and which is adapted for recording and/or reproducing
information signals on the recording layer by the light incident on
the light transmitting layer.
[0003] 2. Description of the Related Art
[0004] Among optical recording mediums for audio or video, adapted
for recording the various information thereon, there are, for
example, an optical disc on which information signals are
pre-recorded by embossed pits, a phase-change disc on which
information signals are written by exploiting phase changes of the
recording film, and a magneto-optical disc on which information
signals are written by exploiting the photomagnetic effect of the
recording film. With this type of the optical recording medium, a
recording layers, a light reflecting layer and a protective layer
are sequentially formed on a transparent substrate, and the laser
light is adapted to fall from the transparent substrate side for
recording and/or reproducing information signals on the recording
layer.
[0005] Meanwhile, with the conventional optical disc, the recording
density of the optical disc is determined by the minimum spot
diameter of a laser light source employed. That is, the smaller the
laser spot diameter, the higher is the recording density that can
be achieved. This laser spot diameter is proportionate to the
{fraction (.lambda./NA)} of the of the recording/reproducing
optical system, where .lambda. is the wavelength of the laser light
and NA is the numerical aperture of an objective lens. Thus, if
desired to realize high recording density of the optical disc, the
wavelength .lambda. of the laser light needs to be reduced to
increase the numerical aperture of the objective lens.
[0006] However, if the numerical aperture of the objective lens is
increased, coma aberration poses a problem, since coma aberration
is proportionate to [skew angle, that is the tilt angle of the
objective lens to the optical axis of the optical
disc].times.NA3.times.[thickness of the optical disc traversed by
the laser light]. For tackling with this problem of coma
aberration, a method of reducing the thickness of the transparent
substrate, as a disc substrate, is under consideration.
[0007] However, for the transparent substrate of the optical disc,
an injection-molded substrate of plastics, produced by the
injection molding method, is predominantly used. It is technically
difficult to fabricate the injection-molded substrate to an
extremely thin thickness and to high precision. The method for
improving the recording density of the conventional optical disc by
reducing the wavelength .lambda. of the laser light or by enlarging
the numerical aperture of the objective lens is, as it were,
approaching a technically unsurmountable limit level.
[0008] The present inventors have proposed an optical disc in
which, for possibly improving the recording density further, a
light reflecting layer, a recording layer and a light transmitting
layer are sequentially formed on a substrate and the laser light is
caused to fall from the light transmitting layer to record and/or
reproduce information signals on a signal recording area of the
signal recording layer.
[0009] In particular, since the optical disc is fabricated by
sequentially forming the light reflecting layer, recording layer
and the light transmitting layer on the substrate, the light
transmitting layer on which falls the laser light can be
manufactured to a thin thickness and to high accuracy. The result
is that the optical disc can sufficiently cope with the high NA of
the objective lens to improve the recording density.
[0010] The methods for forming the light transmitting layer on the
optical disc may be exemplified by a first method of sequentially
forming a light reflecting layer and a recording layer on a
substrate and subsequently affixing a resin sheet 100 .mu.m in
thickness on the recording layer via a transparent adhesive layer
several .mu.m in-between to form a light transmitting layer formed
by the resin sheet, and a second method of dripping a UV curable
resin on the recording layer and stretching the UV curable resin on
rotation and illuminating the UV rays on the substrate kept in a
stationary state to cure the resin to form the light transmitting
layer.
[0011] With the first method, the light transmitting layer exhibits
optimum thickness evenness. However, this light transmitting layer
is difficult to manufacture with respect to handling of the resin
sheet and double refraction and hence it has scarcely put to
practical use.
[0012] With the second method, a center opening in the substrate is
temporarily stopped and, in this state, a UV curable resin is
dripped onto this center hole and stretched on rotation. The
substrate is then kept in a stationary state and irradiated with
the UV rays to cure the UV curable resin to form the light
transmitting layer. Since this second method represents further
development of the technique of forming a protective film for a
conventional optical disc, such as compact disc (CD), and hence the
stock handling, know-how in coating and designing of the apparatus
so far developed can be utilized, it lends itself to mass
production.
[0013] However, with this second method, in which the UV curable
resin is stretched on rotation, the resin tends to be moved towards
the outer rim during the stretching process under the centrifugal
force, so that a protuberant portion tends to be produced in an
outer rim portion. Moreover, with this second method, since the UV
rays are illuminated on the substrate 101 coated with the UV
curable resin by rotational stretching, as the substrate is kept
stationary, the resin deposited on the outer rim portion is raised
and reverted towards the inner rim side under the surface tension
to form a humped portion 100 of a sizeable width, as a result of
which the light transmitting layer 102 presents a hump 100 on its
outer rim portion.
[0014] If the UV curable resin s coated to a film thickness of 100
.mu.m on the substrate of the same size as the conventional CD,
with the substrate being then rotated for stretching the resin and
then being halted to illuminate the UV rays to form the light
transmitting layer thereon, there is formed a humped portion in the
outer rim area having a radial width of 5 mm or more, thus
narrowing the signal recording area.
[0015] If the humped portion having a width sufficient to affect
the signal recording area is formed in this manner on the outer rim
area of the light transmitting layer, a sufficient signal recording
area cannot be obtained to render it impossible to improve the
recording capacity or to lead possibly to reduced recording
capacity. Specifically, for realizing the high recording capacity,
it is necessary to procure a signal recording area comparable to
that of the CD or DVD. In order to achieve this, the humped portion
formed on the outer rim of the light transmitting layer needs to be
of a width of not larger than 1.5 mm.
SUMMARY OF THE INVENTION
[0016] It is therefore an object of the present invention to
provide an optical disc that can be increased further in recording
capacity and that has a light transmitting layer of optimum surface
properties free to the utmost extent from humps on its outer rim
portion, and method for manufacturing such optical disc.
[0017] In one aspect, the present invention provides an optical
disc in which a recording layer and a light transmitting layer are
sequentially formed on a substrate and in which light is incident
from the side of the light transmitting layer to record and/or
reproduce information signals for a signal recording area of the
recording layer, wherein a radial distance from the outermost area
of the substrate to signal recording area is selected to be larger
than a radial width of a hump produced on the outer rim of the
light transmitting layer and wherein the height of hump from the
surface of the light transmitting layer is 70 .mu.m or less.
[0018] The optical disc of the present invention, in which the
width and the height of a hump formed at an outer rim of the light
transmitting layer following the manufacture of the disc are
limited as described above, has optimum surface properties by
having the light transmitting layer free from surface
irregularities as far as possible to provide a signal recording
area as broad as possible.
[0019] In another aspect, the present invention provides a method
for producing an optical disc in which a recording layer and a
light transmitting layer are sequentially formed on a substrate and
in which light is incident from the side of the light transmitting
layer to record and/or reproduce information signals, including the
steps of coating, at the time of forming the light transmitting
layer, a UV curable resin on the portion of the substrate carrying
recording layer and rotating the substrate carrying the UV curable
resin coated thereon and concurrently illuminating UV rays on UV
curable resin to cure UV curable resin.
[0020] With the optical disc manufacturing method according to the
present invention, UV rays are illuminated on the substrate,
carrying the UV curable resin, as the disc is run in rotation, thus
minimizing the risk of the UV curable resin on the outer rim of the
substrate reverting to the inner rim under centrifugal force to
produce a hump.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a cross-sectional view of a typical optical disc
embodying the present invention.
[0022] FIG. 2 is an enlarged cross-sectional view of an outer rim
of an optical disc embodying the present invention.
[0023] FIG. 3 is a cross-sectional view showing a step of coating
of a UV curable resin in the manufacturing method of an optical
disc embodying the present invention.
[0024] FIG. 4 is a cross-sectional view showing a step of
illuminating UV rays on a UV curable resin in the manufacturing
method of an optical disc embodying the present invention.
[0025] FIG. 5 is a cross-sectional view showing a step of coating
of a UV curable resin in the manufacturing method of an optical
disc embodying the present invention.
[0026] FIG. 6 is a cross-sectional view showing a step of setting a
glass plate on the coated UV curable resin in the manufacturing
method of an optical disc embodying the present invention.
[0027] FIG. 7 is a cross-sectional view showing the state in which
the UV curable resin is stretched on the entire surface of the
recording layer formed on the substrate.
[0028] FIG. 8 is a cross-sectional view showing a typical substrate
having a cut-out in its outer rim portion and which is used in the
manufacturing method of the optical disc embodying the present
invention.
[0029] FIG. 9 is a cross-sectional view showing a step of
illuminating deep UV only in the outermost rim portion of the
substrate carrying the recording layer in the manufacturing method
of the optical disc embodying the present invention.
[0030] FIG. 10 is a cross-sectional view showing a step of
illuminating UV rays only on the UV curable resin positioned on the
signal recording area in the manufacturing method of the optical
disc embodying the present invention.
[0031] FIG. 11 is a cross-sectional showing a step of exfoliating a
mask and rotating the substrate at an elevated speed following the
step shown in FIG. 10.
[0032] FIG. 12 is a cross-sectional view showing a step of mounting
a ring on the outer rim of the substrate in the manufacturing
method of the optical disc embodying the present invention.
[0033] FIG. 13 is a cross-sectional view showing a step of coating
a UV curable resin on a substrate carrying the ring and the
recording layer following the step of FIG. 12.
[0034] FIG. 14 is a cross-sectional view showing a step of
illuminating UV rays on the UV curable resin following the step
shown in FIG. 13.
[0035] FIG. 15 is a cross-sectional view showing a step of removing
the ring and the UV curable resin thereon following the step shown
in FIG. 13.
[0036] FIG. 16 is a cross-sectional view showing a step of forming
a light transmitting layer using a substrate larger in size than a
routine substrate as an optical disc manufacturing method embodying
the present invention.
[0037] FIG. 17 is an enlarged cross-sectional view showing an outer
rim portion of a conventional optical disc.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] Referring to the drawings, preferred embodiments of the
present invention will be explained in detail. FIG. 1 shows a
typical optical disc embodying the present invention.
[0039] An optical disc 1, embodying the present invention, has a
light reflecting layer 3, a recording layer 4 and a light
transmitting layer 5, layered in this order on a substrate 2. The
light reflecting layer 3 and the recording layer 4 make up a signal
recording layer 6. With the present optical disc 1, light is caused
to fall on the light transmitting layer 5 to record and/or
reproduce information signals for the signal recording layer 6.
[0040] On a major surface 2a of the substrate 2 are formed fine
crests and valleys, such as recording grooves or pre-grooves, for
recording signals, such as information signals. The substrate 2
preferably has a thickness of 0.3 to 1.2 mm. The materials of the
substrate 2 may be enumerated by plastics, including acrylic
resins, such as polycarbonate or polymethyl methacrylate (PMMA),
and glass. The substrate is molded by injection molding and by a
photopolymer (2P) method in the former and latter cases,
respectively.
[0041] The light reflecting layer 3, formed on the crests and
valleys on the major surface 2a of the substrate 2, operates not
only as a reflecting layer for reflecting the light transmitted
through the recording layer 4, but also as a heat sink layer for
preventing heat from being generated excessively in the recording
layer 4.
[0042] The light reflecting layer 3 is preferably formed of
elements metal, metalloids, semiconductor elements, which may be
used singly or in combination.
[0043] Most preferred is a material mainly composed of Al and
containing 0.4 to 0.8 wt % of Si, not more than 0.7 wt % of Fe,
0.15 to 0.40 wt % of Cu, not more than 0.15 wt % of Fe, 0.8 to 1.2
wt % of Mg, 0.04 to 0.35 wt % of Cr, not more than 0.25 wt % of Zn
and not more than 0.15 wt % of Ti. The light reflecting layer 3 is
formed as a thin film having a thickness of 50 to 200 nm.
[0044] The above material is preferred in that, if a phase-change
recording layer formed of a phase change material is layered as a
recording layer 4 on the light reflecting layer 3, the phase-change
recording layer is affected to a lesser extent by the crystallinity
of the light reflecting layer 3 or by the interface formed by the
crystal grains of the light reflecting layer 3, as a result of
which the phase change recording layer correctly reflects the
surface state of the substrate 2.
[0045] The methods for forming the light reflecting layer 3 of the
above-mentioned material on the substrate 2 may be enumerated by an
ion beam sputtering method, a dc sputtering method and an RF
sputtering method. Of these, the ion beam sputtering method is most
preferred.
[0046] The recording layer 4 is an optical recording layer for
which information signals can be written or erased by illumination
of the laser light. This recording layer may be formed of a
phase-change material undergoing reversible phase change between
crystal and amorphous phases or of a photomagnetic recording
material which loses coercivity on temperature increase beyond the
Curie temperature to undergo inversion of magnetization to the
direction of the external magnetic field.
[0047] As a photomagnetic recording layer, there is used a
perpendicular magnetic recording film exhibiting photomagnetic
characteristics, including Kerr effect or the Faraday effect, such
as an amorphous alloy thin film, exemplified by Tb--Fe--Co thin
film.
[0048] In the case of a read-only optical disc, a pre-set pattern
of lands and recesses corresponding to a signal pattern is formed
on the substrate 2. The recording area is completed by coating the
pattern of lands and recesses with the light reflecting layer
3.
[0049] The laser light falls on the light transmitting layer 5 at
the time of recording/reproduction of information signals. This
light transmitting layer 5 also operates as a protective layer to
prohibit the recording layer 4 from coming into contact with
corrosive factors, such as moisture.
[0050] The light transmitting layer 5 is formed by forming a UV
curable resin on a signal recording portion 6 by a manufacturing
method of the present invention as will be explained
subsequently.
[0051] This light transmitting layer 5 is preferably of a thickness
of 3 to 177 .mu.m in consideration that the lower limit of the
thickness of the light transmitting layer 5 is determined in
dependence upon whether or not the protective function of the light
transmitting layer performing the role of protecting the recording
layer 4 or the light reflecting layer 3 can thereby be assured.
That is, the thickness of the light transmitting layer 5 not less
than 3 .mu.m is required in consideration of the reliability of the
optical disc and the effect of collision of the objective lens on
the surface of the light transmitting layer 5. On the other hand,
the maximum thickness of the light transmitting layer 5 of 177
.mu.m is desirable in consideration that the shorter wavelength of
the laser light is realized in future such that the blue laser
light thought to be promising in near future is to be coped with
rather than the currently used red laser light.
[0052] Therefore, the thickness t of the light transmitting layer 5
ranging between 3 .mu.m and 177 .mu.m is preferred.
[0053] The shape of the outer rim of the optical disc 1 of the
present invention, constructed as described above, is hereinafter
explained.
[0054] FIG. 2 shows, in a cross-sectional view, the outermost rim
of the optical disc 1 of the present invention to an enlarged
scale, without, however, showing the lands and recesses of the
signal recording portion 6. In this signal recording portion 6, an
area which actually is used for recording/reproducing the
information signals is indicated as a signal recording area 6a.
[0055] In particular, in the optical disc 1 according to the
present invention, a hump 5a formed on the outer rim of the light
transmitting layer 5 following the end of the manufacturing process
has a height h from the surface of the light transmitting layer 6
equal to 70 .mu.m or less.
[0056] For achieving high recording density of an optical disc, the
light transmitting layer 5 illuminated by the light is reduced in
thickness, as in the optical disc embodying the present invention,
or the numerical aperture NA of the objective lens of the optical
pickup is increased. That is, if, in the optical disc in which the
light is caused to fall from the side of the light transmitting
layer 5 for recording/reproducing the information signals, a higher
recording density is to be achieved, an optical pickup having its
objective lens of a higher NA value is to be used, as a result of
which the working distance between the optical pickup and the
optical disc is reduced.
[0057] Specifically, the working distance between the optical
pickup and the optical disc is approximately 100 .mu.m, depending
on the designing of the optical pickup. This working distance is
smaller by one digit of magnitude than that in a compact disc CD or
a digital versatile disc DVD. Thus, the optical disc is more
susceptible to collision against the optical pickup than the
conventional CD or DVD. For reducing the damage possibly inflicted
to the optical pickup by such collision, there is provided a
protector at a distal end of the usual optical pickup opposite to
the optical disc. This protector is mounted at a distance of
approximately 30 .mu.m from the optical pickup.
[0058] Therefore, the height of a hump, inevitably formed on the
readout surface of the optical disc for the reason pertinent to the
optical disc manufacture, needs to be suppressed to be 70 .mu.m or
less.
[0059] That is, with the optical disc embodying the present
invention, in which the height h of the hump 5a formed on the outer
rim of the light transmitting layer 5 is set so as to be not larger
than 70 .mu.m, it is possible to avoid the phenomenon of the
optical pickup and the optical disc colliding against each other
even if the objective lens has an enlarged numerical aperture NA to
meet the requirement for high recording density and thus the
distance between the optical disc and the optical pickup, that is
the working distance, is reduced. Therefore, the optical disc 1
embodying the present invention is able to sufficiently meet the
demand for the higher recording density.
[0060] As for the height h of the hump 5a of the light transmitting
layer 5, it is preferably not larger than approximately 20 .mu.m,
in consideration that contaminants such as dust and dirt tend to be
affixed to the surface of the optical disc. It is because the dust
and dirt tending to affect the recording/reproducing
characteristics are usually of a size of 30 to 40 .mu.m.
[0061] Also, with the optical disc according to the present
invention, the radial distance from the outermost rim of the
substrate 2 to the signal recording area 6a is larger than the
width in the radial direction of the hump 5a formed on the
outermost rim of the light transmitting layer. Thus, with the
optical disc 1, the signal recording area 6a can be enlarged
effectively to increase the recording capacity further.
[0062] It is now assumed that the distance from the outermost rim
of the substrate 2 to the signal recording area 6a is D mm and that
the width of the hump 5a formed a the outermost rim of the optical
disc is L mm.
[0063] Meanwhile, in the conventional compact disc, the diameter of
the optical disc and that of the signal recording area are
approximately 60 mm and approximately 58.5 mm, respectively. That
is, with the conventional CD, the distance D from the outermost rim
of the disc to the signal recording area 6a is approximately 1.5
mm.
[0064] Thus, for assuring a recording capacity of the inventive
optical disc equivalent to that of the conventional CD, the width L
of the hump 5a produced at the outermost rim of the light
transmitting layer 5 needs to be 1.5 mm or less.
[0065] In particular, since it is necessary with the optical disc
according to the present invention to set the signal recording area
6a so as to be broader to achieve the recording capacity larger
than that of the conventional CD, the distance D from the outermost
rim of the disc to the s6a needs to be approximately 1.0 mm.
[0066] Thus, with the optical disc, a larger recording capacity can
be achieved if the width L of the hump 5a at the outermost rim of
the light transmitting layer 5 is 1.0 mm or less.
[0067] Also, with the present optical disc 1, the relation of
D-0.5.gtoreq.L mm is preferably met. That is, the difference
between the distance D from the outermost rim of the disc to the
signal recording area 6a and the width L of the hump 5a at the
outermost rim of the light transmitting layer 5 is preferably not
less than 0.5 mm. The reason is that, in this case, the hump 5a
affects the recording or reproducing characteristics to the least
extent to assure optimum recording/reproducing characteristics of
the optical disc 1.
[0068] That is, with the present optical disc 1, the signal
recording area can be increased to assure a larger recording
capacity. Also, with the present optical disc 1, in which the light
transmitting layer 5 is formed which has optimum surface properties
and which is thin in film thickness, the light falls from the side
of the light transmitting layer 5 for recording/reproducing
information signals for the signal recording portion 6, the high NA
of the objective lens and high recording density can be
sufficiently coped with. Moreover, the optical disc 1 has stable
recording/reproducing characteristics and hence high operational
reliability.
[0069] The method for fabricating the optical disc 1 constructed as
described above is hereinafter explained in detail.
[0070] For manufacturing the optical disc 1, substrate 2, having a
pre-set pattern of lands and recesses formed thereon in meeting
with the guide groove or the information signals, is fabricated by
an injection molding method.
[0071] Then, a light reflecting layer 3 is formed to a film
thickness of 150 nm on the substrate 2, by an ion beam sputtering
method, using a material mainly composed of Al and containing 0.4
to 0.8 wt % of Si, not more than 0.7 wt % of Fe, 0.15 to 0.40 wt %
of Cu, not more than 0.15 wt % of Mn, 0.8 to 1.2 wt % of Mg, 0.04
to 0.35 wt % of Cr, not more than 0.25 wt % of Zn and not more than
0.15 wt % of Ti. This film-forming method by ion beam sputtering
gives an optical disc having signal properties better than those of
the method by dc sputtering.
[0072] On the light reflecting layer 3, thus formed, a first
protective film of a dielectric material composed of a mixture of
ZnS and SiO.sub.2, a film of a phase-change material, composed of
GeSbTe, and a second protective film of a dielectric material,
composed of ZnS and SiO2, are sequentially layered to complete the
recording layer 4.
[0073] The film thicknesses of the first protective film, film of
the phase-change material and the second protective film are 20 nm,
25 nm and 100 nm, respectively.
[0074] Finally, a UV curable resin is formed on the recording layer
4 by the spin coating method, now to be explained, to form the
light transmitting layer 5 with a film thickness of 0.1 mm, to
fabricate the optical disc 1.
[0075] For forming the light transmitting layer 5 by the optical
disc manufacturing method according to the present invention, a
spin coating device, shown for example in FIG. 3, is used. Although
the light transmitting layer 5 is formed on the substrate 2
carrying the recording layer 4, this recording layer 4 is not shown
in FIG. 3.
[0076] With the optical disc manufacturing method according to the
present invention, the substrate 2, carrying the recording layer 4,
is arranged on a turntable 10 rotatably supported on a center
rotation supporting member 11. The substrate 2 is arranged on the
turntable 10 so that the recording layer 4 as a film-forming
surface is opposite to the substrate surface contacted with the
turntable 10. With the center opening portion of the substrate 2
being stopped by a lid member 12, the substrate 2 is run in
rotation a an rpm of 800 in the direction indicated by arrow A in
the drawing and, under this condition, the UV curable resin is
dripped in the direction indicated by arrow B in the drawing onto
the lid member 12 disposed at the center of the substrate 2. The
substrate 2 is then set into rotation for stretching the UV curable
resin over the recording layer 4 of the substrate 2.
[0077] Then, in the optical disc manufacturing method of the
present invention, the rpm by the rotation supporting member 11 is
reduced to, for example, 400. As the substrate is rotated in this
condition further, UV rays are illuminated on the resin coated on
the recording layer 4 of the substrate 2. Finally, the UV curable
resin is cured to form the light transmitting layer 5.
[0078] The rpm for illumination of UV rays is preferably of the
order of 40 to 50% or 40 to 60% of the rpm for stretching of the UV
curable resin by rotation if the latter is approximately 1000 or
less or not less than 1000, respectively.
[0079] In the conventional spin coating method, the UV rays are
illuminated while the substrate 2 is stationary. However, with the
optical disc manufacturing method according to the present
invention, the UV rays are illuminated while the substrate 2 is
kept in rotation.
[0080] With the optical disc manufacturing method according to the
present invention, as described above, the light transmitting layer
is manufactured by the spin coating method by dripping the UV
curable resin on the recording layer 4 of the rotating substrate 2
for stretching the resin and by illuminating the UV rays on the UV
curable resin, with the substrate 2 being kept in rotation at a
reduced rpm.
[0081] That is, the optical disc manufacturing method according to
the present invention is a method of illuminating the UV rays on
the rotating substrate to cure the resin. Specifically, the UV
curable resin is stretched on rotation on the recording layer 4 on
the substrate rotating at an rpm of 800 and subsequently the UV
rays continue to be illuminated on the substrate 3 at a reduced 400
rpm.
[0082] An experimental example is now shown in which the optical
disc is manufactured using an optical disc embodying the present
invention.
[0083] First, a substrate 120 mm in diameter was prepared by
injection molding. On this substrate was dripped a UV curable resin
2200 cps in viscosity and stretched on rotation at 810 rpm to apply
the UV curable resin to a thickness of 100 .mu.m.
[0084] As the substrate was kept rotating at 400 rpm, UV rays were
irradiated thereon for 25 seconds to cure the resin to form the
light transmitting layer to produce the ultimate optical disc.
[0085] The optical disc, thus prepared, had the hump with a width L
of approximately 3 mm on the outer rim of the light transmitting
layer.
[0086] The UV curable resin with a viscosity of 4500 cps was
dripped on a similar substrate and coated thereon to a thickness of
100 .mu.m on stretching by rotation at 1200 rpm. The substrate, run
in rotation at 500 rpm, was irradiated with UV rays for 25 sec to
cure the resin to form the light transmitting layer to produce the
ultimate optical disc.
[0087] The optical disc, thus prepared, had the hump with a width L
of approximately 2 mm on the outer rim of the light transmitting
layer.
[0088] Thus, with the optical disc manufacturing method according
to the present invention, in which the UV curable resin is cured as
the substrate is kept in a rotating state, it is possible to
suppress the phenomenon of the UV curable resin from reverting from
the outer rim side to the inner rim side to produce a hump by
surface tension as far as possible.
[0089] Thus, with the present optical disc manufacturing method,
such optical disc 1 can be manufactured which has the light
transmitting layer 5 of optimum surface properties and an
effectively broad signal recording area 6a and which can realize a
further increased recording capacity.
[0090] Meanwhile, a glass plate may be set on the UV curable resin
dripped prior to illuminating the UV rays prior to illumination of
UV rays.
[0091] Specifically, a UV curable resin 14 is dripped on the
substrate 2 set on the turntable 10, as shown in FIG. 5. At this
time, the substrate 2 is kept in a stationary state.
[0092] Then, a glass plate 15 is set on the UV curable resin 14, as
shown in FIG. 6. At this time, the substrate 2 is run in rotation
along with the glass plate 15 to stretch the UV curable resin 14 on
the entire surface of the recording layer 4.
[0093] When the UV curable resin 14 is stretched in this manner on
the entire surface of the recording layer 4 on the substrate 2, UV
rays 13 are illuminated on the substrate 2, kept in rotation along
with the glass plate 15, to permit the UV curable resin 14 to be
cured to form the light transmitting layer 5. Any excess UV curable
resin 14 is whirled off to prevent as much as possible the UV
curable resin from being formed as a hump on the outer rim portions
of the substrate 2.
[0094] Finally, the glass palte 15 is peeled off from the light
transmitting layer 5 to produce the optical disc 1.
[0095] By setting the glass plate 15 in this manner on the UV
curable resin, the light transmitting layer 5 with optimum surface
properties can be produced, because the smooth surface properties
of the glass plate 15 are directly transferred to the produced
light transmitting layer 5.
[0096] For producing the optical disc according to the present
invention, it is also possible to mold the outer rim of the
substrate 2 with a cut-out 2a and to use this substrate 2 to
perform the steps of coating the UV curable resin and the rotating
and curing step as shown in FIGS. 3 and 4. If the height H of the
substrate 2 is 1.2 mm, the cut-out 2a may have a radial width W of
1.2 mm and an angle of 45.degree., as shown in FIG. 8.
[0097] By using the substrate 2 having the cut-out 2a pre-formed on
its outer rim, the light transmitting layer can be produced which
can effectively and readily suppress the generation of a humped
portion on the outer rim of the substrate. With the present method,
the cut-out 2a can be formed during the molding the substrate 2 by
a method similar to the method of forming the guide groove or the
crests or valleys corresponding to the information signals during
molding of the substrate 2. The cutout 2a may also be formed by
etching following molding of the usual substrate 2.
[0098] In manufacturing the optical disc according to the present
invention, UV rays may be illuminated on the outermost rim of the
substrate 2 before dripping the UV curable resin. Specifically, the
portion of the substrate 2 excluding its outermost rim portion is
covered by a mask 16, before proceeding to drip the UV curable
resin, and the so-called UV rays (UV rays of short wavelength
ranging between 200 and 300 nm) are illuminated on the outermost
rim of the disc. The outermost rim denotes an area lying radially
outwardly of the signal recording area and specifically the
outermost area of the substrate 2 and/or the recording unit 6.
[0099] The mask 16 is peeled off and subsequently the steps of
coating the UV curable resin and rotation/curing are carried out to
form the ultimate light transmitting layer 5.
[0100] With this method, since the UV rays of shorter wavelength
are previously illuminated on the outermost portion of the
substrate 2, in which the hump is liable to be formed, in order to
keep an optimum wetting properties of this outermost rim portion,
it is possible to suppress the formation of the hump in the
outermost rim of the light transmitting layer 5 to the maximum
extent possible to form the light transmitting layer 5 of optimum
surface properties.
[0101] In the optical disc manufacturing method according to the
present invention, the substrate 2 coated with the UV curable
resin, obtained on dripping the UV curable resin and stretching it
on rotation, may be run in rotation at an elevated rpm for a short
time duration prior to illumination of UV rays. That is, the UV
curable resin is dripped on the substrate 2 and stretched on
rotation, after which the rotation supporting member 11 is run in
rotation at an elevated speed for short time duration. After
running the substrate 2 in this manner for short time at an
elevated rpm, the UV rays are illuminated as shown in FIG. 4 to
perform the rotating curing step for the UV curable resin to form
the light transmitting layer 5.
[0102] By running the substrate 2 in rotation at an elevated rpm
before illumination of UV rays, it is possible to whirl off excess
UV curable resin collected on the outer rim portion by the
centrifugal force on stretching by rotation.
[0103] As a method for manufacturing the optical disc according to
the present invention, it is also possible to illuminate UV rays
first on the UV curable resin in the signal recording area, run the
substrate at an elevated speed and to illuminate UV rays on the UV
curable resin in an area other than the signal recording area.
[0104] That is, after the UV curable resin is stretched by rotation
on the substrate 2, masks 17, 18 are applied to the UV curable
resin lying on the UV curable resin in an area outwardly of the
signal recording area 6a and in a center area of the substrate 2
and UV rays are illuminated on the substrate 2 kept in a rotating
state. Since the area of the substrate 2 outwardly of the signal
recording area 6a is coated by the mask 17, only the UV curable
resin which later becomes the signal recording area 6a is
illuminated and cured by the UV rays.
[0105] The mask 17 is then peeled off and the substrate 2 then is
run in rotation at an elevated rpm. This whirls off excess UV
curable resin lying on the outer rim of the substrate.
[0106] The rotation of the substrate 2 is further continued as the
rpm of the rotation supporting member 11 is decreased to perform
the rotation curing of the UV curable resin shown in FIG. 4 to form
the light transmitting layer 5.
[0107] After illuminating the UV curable resin on the signal
recording area 6a, the substrate is run in rotation at an elevated
rpm UV curable resin 14 to whirl off excess UV curable resin and UV
rays are illuminated on the UV curable resin 14 on the outer rim to
produce the light transmitting layer 5 having of optimum surface
properties easily and effectively. This method enables the excess
UV curable resin on the outer rim to be whirled off more
selectively than is possible with the above-described method of
running the substrate in rotation at an elevated speed to whirl off
excess resin from the outer rim without applying masks following
stretching by rotation of the UV curable resin.
[0108] As the optical disc manufacturing method according to the
present invention, it is also possible to use a ring 20 having the
same inner diameter as the outer diameter of the manufactured
optical disc and the substantially same thickness as the thickness
of the optical disc. In this method, the substrate 2 is fitted in
the center hole of the ring 20.
[0109] As the substrate 2 is run in rotation along with the ring 20
in the direction indicated by arrow A in FIG. 13, a UV curable
resin 21 is dripped to stretch the resin on rotation.
[0110] The rotation supporting member 11 is kept rotating at a
reduced rpm to run the substrate 2 along with the ring 20 to
illuminate UV rays on the UV curable resin 21 to cure the
resin.
[0111] Finally, the ring 20 is dismounted to remove the UV curable
resin formed thereon to complete the optical disc 1.
[0112] In this manner, the ring 20 is first fitted on the outer rim
of the substrate 2, and UV curable resin is coated and cured on the
ring 20 and the substrate 2. The ring 20 is then dismounted and the
UV curable resin thereon is removed to manufacture the optical disc
of optimum surface properties freed of surface irregularities, such
as humps, to the maximum extent possible. Since the UV curable
resin on the outer rim liable to produce surface humps is not used
in the present method as the light transmitting layer, the optical
disc produced is of optimum surface properties.
[0113] With the optical disc manufacturing method according to the
present invention, it is possible to use a substrate having an
outer diameter slightly larger than the outer diameter of the usual
substrate. Specifically, a substrate 22 having a diameter slightly
larger than the outer diameter of the usual substrate is used and,
after a recording portion is formed thereon, the substrate 22 is
kept in rotation to allow the UV curable resin to be cured as shown
in FIGS. 3 and 4 to produce a light transmitting layer 23 having a
diameter slightly larger than the outer diameter of the usual
substrate shown in FIG. 16. Finally, the light transmitting layer
23 and an excess outer rim 25 of the substrate 22 are cut off to
give the same size as the usual substrate size, as indicated by
arrow c in FIG. 16, to produce the optical disc 1.
[0114] With the present method, in which a light transmitting layer
of a larger diameter is previously formed and an excess portion of
the light transmitting layer on the outer rim is cut off following
curing on rotation, the UV curable resin on the outer rim portion
liable to form humps is not used as the light transmitting layer,
this realizing optimum surface properties.
[0115] With the optical disc manufacturing method according to the
present invention, it is also possible to get the UV curable resin
stretched by rotation on the entire surface on the substrate 2, to
wipe off excess UV curable resin on the outermost rim portion by a
resin-absorbing material, such as cloth, before curing the resin,
and to then perform the rotating curing step of the UV curable
resin, as shown in FIG. 4.
[0116] With the optical disc manufacturing method according to the
present invention, it is also possible to get the UV curable resin
stretched by rotation on the entire surface on the substrate 2, to
suck excess UV curable resin on the outermost rim portion by a
vacuum pump etc, before curing the resin, and to then perform the
rotating curing step of the UV curable resin, as shown in FIG.
4.
[0117] With the optical disc manufacturing method according to the
present invention, it is also possible to get the UV curable resin
stretched by rotation on the entire surface on the substrate 2, to
blow off excess UV curable resin on the outermost rim portion by
nitrogen blowing etc, before curing the resin, and to then
illuminate UV rays on the rotating substrate 2, as shown in FIG.
4.
[0118] In the optical disc manufacturing method according to the
present invention, it is desirable to use the UV curable resin of
low surface tension since this permits the UV curable resin on the
outer rim area to revert t the inner rim area to suppress formation
of humps as far as possible.
[0119] Also, in the optical disc manufacturing method according to
the present invention, it is desirable to use a UV curable resin of
higher viscosity.
* * * * *