U.S. patent application number 10/657184 was filed with the patent office on 2004-03-11 for optical information medium, making method, recording/reading method, and inspecting method.
This patent application is currently assigned to TDK CORPORATION. Invention is credited to Hirata, Hideki, Kato, Tatsuya, Komaki, Tsuyoshi.
Application Number | 20040047278 10/657184 |
Document ID | / |
Family ID | 26594704 |
Filed Date | 2004-03-11 |
United States Patent
Application |
20040047278 |
Kind Code |
A1 |
Komaki, Tsuyoshi ; et
al. |
March 11, 2004 |
Optical information medium, making method, recording/reading
method, and inspecting method
Abstract
In an optical information medium comprising a supporting
substrate, an information recording layer thereon, and a
light-transmitting layer wherein a recording/reading laser beam
enters the recording layer through the light-transmitting layer,
the light-transmitting layer is formed of a resin and has a tensile
strength at break of 5-40 MPa, a tensile elongation at break of
15-100%, and a tensile modulus of 40-1,000 MPa. The medium has
improved recording/reading characteristics when a laser beam
defines a beam spot having a small diameter of up to 300 .mu.m and
the medium is rotated at a high linear velocity of at least 8
m/s.
Inventors: |
Komaki, Tsuyoshi; (Chuo-ku,
JP) ; Hirata, Hideki; (Chuo-ku, JP) ; Kato,
Tatsuya; (Chuo-ku, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
TDK CORPORATION
Tokyo
JP
|
Family ID: |
26594704 |
Appl. No.: |
10/657184 |
Filed: |
September 9, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10657184 |
Sep 9, 2003 |
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09887434 |
Jun 25, 2001 |
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6667952 |
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Current U.S.
Class: |
369/275.2 ;
369/283; G9B/7.159; G9B/7.182; G9B/7.194 |
Current CPC
Class: |
G11B 2007/25708
20130101; G11B 2007/24314 20130101; G11B 7/26 20130101; G11B
2007/2431 20130101; G11B 7/24056 20130101; G11B 2007/25706
20130101; G11B 7/2585 20130101; G11B 2007/25715 20130101; G11B
7/256 20130101; G11B 2007/24316 20130101; G11B 2007/25716 20130101;
G11B 2007/24312 20130101; G11B 2007/2571 20130101; G11B 7/2534
20130101; G11B 7/2542 20130101; G11B 7/24067 20130101 |
Class at
Publication: |
369/275.2 ;
369/283 |
International
Class: |
G11B 007/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 26, 2000 |
JP |
2000-191787 |
Nov 29, 2000 |
JP |
2000-363891 |
Claims
What is claimed is:
1. An optical information medium comprising a supporting substrate,
an information recording layer thereon, and a light-transmitting
layer on the information recording layer wherein a recording or
reading laser beam enters the information recording layer through
the light-transmitting layer, said light-transmitting layer is
formed of a resin and has a tensile strength at break of 5 to 40
MPa, a tensile elongation at break of 15 to 100%, and a tensile
modulus of 40 to 1,000 MPa.
2. The optical information medium of claim 1 wherein said
light-transmitting layer has a thickness of 30 to 200 .mu.m.
3. An optical information medium of claim 1 wherein said
light-transmitting layer in an information recording region has a
birefringence in absolute value of up to 20 nm at a wavelength of
630 nm and a birefringence distribution breadth of up to 20 nm at a
wavelength of 630 nm.
4. An optical information medium of claim 1 wherein said
light-transmitting layer has a surface reflectivity of up to 10% at
the wavelength of the recording or reading laser beam.
5. An optical information medium of claim 1 wherein R/F is up to
10% wherein R is a residual error component of a focus error signal
at a linear velocity during recording or reading and F is a
peak-to-peak value of a focus sensitivity curve.
6. An optical information medium of claim 1 wherein said medium
satisfies Wt.ltoreq.1840e.sup.-0.04V wherein said
light-transmitting layer at its surface has a maximum waviness Wt
(in nm) and said medium is moved at a linear velocity V (in m/s)
during recording or reading, with the proviso that the recording or
reading laser beam defines on the surface of said
light-transmitting layer a beam spot having a diameter of up to 300
.mu.m.
7. The optical information medium of claim 1 which is to be
operated at a linear velocity of at least 8 m/s.
8. The optical information medium of claim 1 on which recording or
reading is performed by a system including an objective lens having
a numerical aperture NA and emitting a recording or reading beam
having a wavelength of .lambda. wherein .lambda./NA.ltoreq.780
nm.
9. An optical information medium comprising a supporting substrate,
an information recording layer thereon, and a light-transmitting
layer on the information recording layer wherein a recording or
reading laser beam enters the information recording layer through
the light-transmitting layer, said light-transmitting layer in an
information recording region has a birefringence in absolute value
of up to 20 nm at a wavelength of 630 nm and a birefringence
distribution breadth of up to 20 nm at a wavelength of 630 nm.
10. The optical information medium of claims 9 which is to be
operated at a linear velocity of at least 8 m/s.
11. The optical information medium of claim 9 on which recording or
reading is performed by a system including an objective lens having
a numerical aperture NA and emitting a recording or reading beam
having a wavelength of .lambda. wherein .lambda./NA.ltoreq.780
nm.
12. An optical information medium comprising a supporting
substrate, an information recording layer thereon, and a
light-transmitting layer on the information recording layer wherein
a recording or reading laser beam enters the information recording
layer through the light-transmitting layer, said light-transmitting
layer has a surface reflectivity of up to 10% at the wavelength of
the recording or reading laser beam.
13. The optical information medium of claim 12 which is to be
operated at a linear velocity of at least 8 m/s.
14. The optical information medium of claim 12 on which recording
or reading is performed by a system including an objective lens
having a numerical aperture NA and emitting a recording or reading
beam having a wavelength of .lambda. wherein .lambda./NA.ltoreq.780
nm.
15. An optical information medium comprising a supporting
substrate, an information recording layer thereon, and a
light-transmitting layer on the information recording layer wherein
a recording or reading laser beam enters the information recording
layer through the light-transmitting layer, R/F is up to 10%
wherein R is a residual error component of a focus error signal at
a linear velocity during recording or reading and F is a
peak-to-peak value of a focus sensitivity curve.
16. The optical information medium of claim 15 which is to be
operated at a linear velocity of at least 8 m/s.
17. The optical information medium of claim 15 on which recording
or reading is performed by a system including an objective lens
having a numerical aperture NA and emitting a recording or reading
beam having a wavelength of .lambda. wherein .lambda./NA.ltoreq.780
nm.
18. An optical information medium comprising a supporting
substrate, an information recording layer thereon, and a
light-transmitting layer on the information recording layer wherein
a recording or reading laser beam enters the information recording
layer through the light-transmitting layer, wherein said medium
satisfies Wt.ltoreq.1840e.sup.-0.04V wherein said
light-transmitting layer at its surface has a maximum waviness Wt
(in nm) and said medium is moved at a linear velocity V (in m/s)
during recording or reading, with the proviso that the recording or
reading laser beam defines on the surface of said
light-transmitting layer a beam spot having a diameter of up to 300
.mu.m.
19. The optical information medium of claim 18 wherein said
light-transmitting layer includes a light-transmitting sheet formed
of a resin and an adhesive layer which joins the light-transmitting
sheet to the supporting substrate side, said adhesive layer
comprising a cured product of a UV-curable resin and having an
average thickness of 0.5 .mu.m to less than 5 .mu.m.
20. The optical information medium of claim 18 wherein said
light-transmitting layer includes a light-transmitting sheet formed
of a resin and an adhesive layer which joins the light-transmitting
sheet to the supporting substrate side, said light-transmitting
sheet being constructed from a polycarbonate, polyarylate or cyclic
polyolefin by a casting technique.
21. The optical information medium of claim 18 which is to be
operated at a linear velocity of at least 8 m/s.
22. The optical information medium of claim 18 on which recording
or reading is performed by a system including an objective lens
having a numerical aperture NA and emitting a recording or reading
beam having a wavelength of .lambda. wherein .lambda./NA.ltoreq.780
nm.
23. A method for preparing the optical information medium of claim
18, in which said light-transmitting layer includes a
light-transmitting sheet formed of a resin and an adhesive layer
which joins the light-transmitting sheet to the supporting
substrate side, said adhesive layer being comprised of a cured
product of a UV-curable resin, said method comprising the step of
irradiating UV radiation to a coating of the UV-curable resin for
curing the resin to form said adhesive layer, the UV radiation
irradiated having an energy density of up to 1,000 mW/cm.sup.2.
24. In connection with an optical information medium comprising a
supporting substrate, an information recording layer thereon, and a
light-transmitting layer on the information recording layer,
wherein said light-transmitting layer has a birefringence in
absolute value of up to 20 nm at a wavelength of 630 nm and a
birefringence distribution breadth of up to 20 nm at a wavelength
of 630 nm, a recording or reading method wherein recording or
reading is performed by passing a recording or reading laser beam
to said information recording layer through said light-transmitting
layer.
25. In connection with an optical information medium comprising a
supporting substrate, an information recording layer thereon, and a
light-transmitting layer on the information recording layer having
a surface reflectivity of up to 10% at a recording or reading
wavelength, a recording or reading method wherein recording or
reading is performed by passing a recording or reading laser beam
to said information recording layer through said light-transmitting
layer.
26. A method for inspecting optical information media comprising a
supporting substrate, an information recording layer thereon, and a
light-transmitting layer on the information recording layer wherein
a recording or reading laser beam enters the information recording
layer through the light-transmitting layer, said method comprising
selecting those optical information media in which R/F is up to 10%
wherein R is a residual error component of a focus error signal at
a linear velocity during recording or reading and F is a
peak-to-peak value of a focus sensitivity curve.
Description
[0001] This invention relates to optical information media such as
read-only optical disks and optical recording disks, a method for
preparing the same, a method for performing recording or reading on
the same, and a method for inspecting the same.
BACKGROUND OF THE INVENTION
[0002] To record and store a vast quantity of information as
typified by moving image information, advanced optical information
media such as read-only optical disks and optical recording disks
are required to increase their recording density for increasing the
capacity. To meet such a demand, engineers have been engaged in the
research and development works targeting a higher recording
density.
[0003] One such approach relating to digital versatile disks (DVD)
is to shorten the wavelength of a recording/reading laser beam and
increase the numerical aperture (NA) of a recording/reading optical
system objective lens, thereby reducing the spot diameter of the
recording/reading laser beam. As compared with CD, DVD is
successful in achieving a recording capacity of 6 to 8 folds
(typically 4.7 GB/side) by changing the recording/reading
wavelength from 780 nm to 650 nm and the NA from 0.45 to 0.6.
[0004] For long-term recording of moving images of quality, an
attempt was recently made to achieve a recording capacity of at
least 4 folds of that of DVD, i.e., at least 20 GB/side, by
reducing the recording/reading wavelength to about 400 nm and
increasing the NA of the objective lens to about 0.85.
[0005] Increasing the NA, however, leads to a reduced tilt margin.
The tilt margin is a permissible tilt of an optical recording
medium relative to an optical system, which depends on the NA. The
tilt margin is in proportion to
.lambda./(t.multidot.NA.sup.3)
[0006] wherein .lambda. denotes the wavelength of recording/reading
beam and "t" denotes the thickness of a transparent substrate the
recording/reading beam enters. If the optical recording medium is
inclined or tilted relative to the laser beam, a wavefront
aberration (or coma) occurs. The coefficient of wavefront
aberration is represented by
(1/2).multidot.t.multidot.{n.sup.2.multidot.sin.theta..multidot.cos.theta.-
}.multidot.NA.sup.3/(n.sup.2-sin.sup.2.theta.).sup.-5/2
[0007] wherein n denotes the refractive index of the substrate and
.theta. is a tilt angle. It is appreciated from these formulae that
the tilt margin may be increased and the occurrence of comatic
aberration be suppressed by reducing the thickness "t" of the
substrate. In fact, the DVD design is such that a tilt margin is
secured by reducing the thickness of the substrate to about one
half (about 0.6 mm) of the thickness (about 1.2 mm) of the CD
substrate.
[0008] To record moving images of better quality for a longer
period of time, there has been proposed a structure allowing for
use of a thinner substrate. In this structure, a substrate of an
ordinary thickness is used as a supporting substrate for
maintaining rigidity, pits or a recording layer is formed on the
surface of the supporting substrate, and a light-transmitting layer
of about 100 .mu.m thick is formed thereon as a thin substrate.
Recording/reading beam reaches the pits or recording layer through
the light-transmitting layer. This structure can achieve a higher
recording density due to a higher NA because the substrate can be
made extremely thin as compared with the prior art. One typical
medium having such structure is disclosed in JP-A 10-289489. The
medium is described therein as having a light-transmitting layer of
photo-curable resin.
[0009] When the light-transmitting layer is formed of photo-curable
resins such as UV-curable resins, however, the media can deflect
due to shrinkage upon curing. Deflection can also occur when the
media are stored in a hot humid environment. Once the media
deflect, frequent errors can occur upon reading, and excessive
deflection can cause the media to be unreadable.
[0010] JP-A 8-194968 describes an optical disk having a protective
coat of resin. In this patent publication, use of the protective
coat having a tensile elongation at break of at least 15% prevents
the optical disk from deflection during storage in a hot humid
environment. It is not described in this patent publication that a
recording/reading beam is passed through the protective coat.
[0011] The inventors found that when a recording/reading beam is
passed to the recording layer through a light-transmitting layer
(or protective coat) of approximately 100 .mu.m thick, satisfactory
recording/reading characteristics are not obtained merely by
setting the tensile elongation at break of the light-transmitting
layer at 15% or higher. Problems arise particularly when the
diameter of a beam spot of a laser beam is reduced and recording
and reading is performed at a high linear velocity. The most
serious problem is that the focusing servo loses some stability.
Another problem is an increased birefringence.
[0012] By reducing the recording/reading wavelength, increasing the
NA of the objective lens to reduce the beam spot diameter, and
increasing the linear velocity during recording and reading, there
can be achieved a significant improvement in data transfer rate.
Even a data transfer rate of 100 Mbps or higher is possible. With
the start of the satellite digital broadcasting system at the end
of 2000, image information of high quality is now delivered to
home. A remarkable improvement in data transfer rate is thus
demanded for recording such image information.
[0013] However, the focusing servo stability must be improved
before the data transfer rate can be increased.
SUMMARY OF THE INVENTION
[0014] An object of the invention is to an optical information
medium comprising a supporting substrate, an information recording
layer thereon, and a light-transmitting layer thereon wherein a
recording or reading laser beam enters the recording layer through
the light-transmitting layer, in which recording/reading
characteristics are improved when the beam spot of a laser beam has
a small diameter and the linear velocity is high.
[0015] The above and other objects are achieved by the present
invention defined below.
[0016] (1) An optical information medium comprising a supporting
substrate, an information recording layer thereon, and a
light-transmitting layer on the information recording layer wherein
a recording or reading laser beam enters the information recording
layer through the light-transmitting layer,
[0017] said light-transmitting layer is formed of a resin and has a
tensile strength at break of 5 to 40 MPa, a tensile elongation at
break of 15 to 100%, and a tensile modulus of 40 to 1,000 MPa.
[0018] (2) The optical information medium of (1) wherein said
light-transmitting layer has a thickness of 30 to 200
[0019] (3) An optical information medium comprising a supporting
substrate, an information recording layer thereon, and a
light-transmitting layer on the information recording layer wherein
a recording or reading laser beam enters the information recording
layer through the light-transmitting layer,
[0020] said light-transmitting layer in an information recording
region has a birefringence in absolute value of up to 20 nm at a
wavelength of 630 nm and a birefringence distribution breadth of up
to 20 nm at a wavelength of 630 nm.
[0021] (4) An optical information medium comprising a supporting
substrate, an information recording layer thereon, and a
light-transmitting layer on the information recording layer wherein
a recording or reading laser beam enters the information recording
layer through the light-transmitting layer,
[0022] said light-transmitting layer has a surface reflectivity of
up to 10% at the wavelength of the recording or reading laser
beam.
[0023] (5) An optical information medium comprising a supporting
substrate, an information recording layer thereon, and a
light-transmitting layer on the information recording layer wherein
a recording or reading laser beam enters the information recording
layer through the light-transmitting layer,
[0024] R/F is up to 10% wherein R is a residual error component of
a focus error signal at a linear velocity during recording or
reading and F is a peak-to-peak value of a focus sensitivity
curve.
[0025] (6) An optical information medium comprising a supporting
substrate, an information recording layer thereon, and a
light-transmitting layer on the information recording layer wherein
a recording or reading laser beam enters the information recording
layer through the light-transmitting layer, wherein
[0026] said medium satisfies Wt.ltoreq.1840e.sup.-0.04V wherein
said light-transmitting layer at its surface has a maximum waviness
Wt (in nm) and said medium is moved at a linear velocity V (in m/s)
during recording or reading, with the proviso that the recording or
reading laser beam defines on the surface of said
light-transmitting layer a beam spot having a diameter of up to 300
.mu.m,
[0027] (7) The optical information medium of (6) wherein said
light-transmitting layer includes a light-transmitting sheet formed
of a resin and an adhesive layer which joins the light-transmitting
sheet to the supporting substrate side,
[0028] said adhesive layer comprising a cured product of a
UV-curable resin and having an average thickness of 0.5 .mu.m to
less than 5 .mu.m.
[0029] (8) The optical information medium of (6) or (7) wherein
said light-transmitting layer includes a light-transmitting sheet
formed of a resin and an adhesive layer which joins the
light-transmitting sheet to the supporting substrate side,
[0030] said light-transmitting sheet being constructed from a
polycarbonate, polyarylate or cyclic polyolefin by a casting
technique.
[0031] (9) The optical information medium of any one of (3) to (8)
which is to be operated at a linear velocity of at least 8 m/s.
[0032] (10) The optical information medium of any one of (3) to (9)
on which recording or reading is performed by a system including an
objective lens having a numerical aperture NA and emitting a
recording or reading beam having a wavelength of .lambda. wherein
.lambda./NA.ltoreq.780 nm.
[0033] (11) The optical information medium of any one of (3) to
(10) which is the optical information medium of (1) or (2).
[0034] (12) A method for preparing the optical information medium
of any one of (6) to (8), in which said light-transmitting layer
includes a light-transmitting sheet formed of a resin and an
adhesive layer which joins the light-transmitting sheet to the
supporting substrate side, said adhesive layer being comprised of a
cured product of a UV-curable resin,
[0035] said method comprising the step of irradiating UV radiation
to a coating of the UV-curable resin for curing the resin to form
said adhesive layer, the UV radiation irradiated having an energy
density of up to 1,000 mW/cm.sup.2.
[0036] (13) In connection with an optical information medium
comprising a supporting substrate, an information recording layer
thereon, and a light-transmitting layer on the information
recording layer, wherein said light-transmitting layer has a
birefringence in absolute value of up to 20 nm at a wavelength of
630 nm and a birefringence distribution breadth of up to 20 nm at a
wavelength of 630 nm,
[0037] a recording or reading method wherein recording or reading
is performed by passing a recording or reading laser beam to said
information recording layer through said light-transmitting
layer.
[0038] (14) In connection with an optical information medium
comprising a supporting substrate, an information recording layer
thereon, and a light-transmitting layer on the information
recording layer having a surface reflectivity of up to 10% at a
recording or reading wavelength.
[0039] a recording or reading method wherein recording or reading
is performed by passing a recording or reading laser beam to said
information recording layer through said light-transmitting
layer.
[0040] (15) A method for inspecting optical information media
comprising a supporting substrate, an information recording layer
thereon, and a light-transmitting layer on the information
recording layer wherein a recording or reading laser beam enters
the information recording layer through the light-transmitting
layer,
[0041] said method comprising selecting those optical information
media in which R/F is up to 10% wherein R is a residual error
component of a focus error signal at a linear velocity during
recording or reading and F is a peak-to-peak value of a focus
sensitivity curve.
FUNCTION AND RESULTS
[0042] In an optical information medium in which information is
read by way of a light-transmitting layer of about 100 .mu.m thick,
the present invention controls the tensile strength at break,
tensile elongation at break and tensile modulus of the
light-transmitting layer to specific ranges, respectively. The
light-transmitting layer having specific physical properties has a
reduced birefringence and a reduced birefringence distribution
breadth, and serves to reduce the deflection and axial runout of
the medium.
[0043] Because of the reduced deflection and reduced axial runout,
especially because of the minimized axial runout, the medium
undergoes a reduced axial runout acceleration when the linear
velocity is increased. As a result, the residual error component
(R) of a focus error signal at the increased linear velocity is 10%
or less of the peak-to-peak value (F) of a focus sensitivity curve,
whereby the focusing servo error at the increased transfer rate is
reduced.
[0044] It is noted that no direct correlation exists between the
deflection and the axial runout of the medium. A medium having a
large deflection quantity tends to have a large axial runout
quantity. However, a disk-shaped medium which has deflected like an
umbrella, for example, experiences little increase of axial runout.
On the other hand, a disk-shaped medium which has deflected while
being twisted will undergo a large quantity of axial runout even
when the deflection quantity measured is small.
[0045] Although it has not been proposed in the art to utilize R/F
as the criterion for judging the focusing servo performance, the
inventors have found that reducing R/F, specifically reducing R/F
to 10% or below provides a great contribution to a reduction of
jitter upon reading and to the prevention of writing errors. By
measuring R/F at the linear velocity used for recording or reading
(referred to as "operating linear velocity"), it can be judged
whether or not recording and reading with high reliability is
possible on the medium at the operating linear velocity. Therefore,
the measurement of R/F can be utilized in the inspection of the
medium. It is understood that the focusing servo does not fail even
at R/F in excess of 10%.
[0046] In the medium of the invention, the utilization efficiency
of light is increased on account of the reduced birefringence,
which results in an increased read signal output.
[0047] It is understood that for a resin layer such as a
light-transmitting layer, no direct correlation exists among
tensile strength at break, tensile elongation at break and tensile
modulus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 is a fragmentary cross-sectional view of an optical
information medium.
[0049] FIG. 2 is a graph of the maximum waviness Wt on the
light-incident surface of the medium versus the maximum linear
velocity Vmax at which the ratio R/F of the residual error
component (R) of focus error signals to the peak-to-peak value (F)
of a focus sensitivity curve do not exceed 10%.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0050] Referring to FIG. 1, there is illustrated one exemplary
construction of the optical information medium of the invention.
This optical information medium is a recording medium including a
supporting substrate 20, an information recording layer in the form
of a recording layer 4 on the supporting substrate 20, and a
light-transmitting layer 2 on the recording layer 4. A laser beam
for recording or reading is passed to the recording layer 4 through
the light-transmitting layer 2.
[0051] The invention is applicable to any type of recording layer.
Specifically, the invention is applicable to phase change recording
media, pit formation type recording media, and magneto-optical
recording media, for example. In general, additional layers such as
a dielectric layer and a reflective layer are provided on at least
one side of the recording layer for the purposes of protecting the
recording layer and achieving optical effects, although they are
omitted in FIG. 1. The invention is not limited to the recordable
type as in the illustrated embodiment, and may also be applicable
to the read-only type. In the latter case, the supporting substrate
20 having pits formed integrally therein is used, and a reflective
layer in the form of a metal film, metalloid film or multilayer
dielectric film is formed thereon. The pattern of pits is
transferred to the reflective layer whereby the reflective layer
constitutes the information recording layer.
[0052] Now the respective components of the inventive medium are
described in detail.
[0053] The supporting substrate 20 is provided to maintain rigidity
for the medium. The supporting substrate generally has a thickness
of 0.2 to 1.2 mm, preferably 0.4 to 1.2 mm and may be either
transparent or opaque. The supporting substrate 20 is usually
constructed of a resin like conventional optical recording media
although glass may also be used for the substrate. Grooves or guide
channels 21, which are generally formed in optical recording media,
are obtained by forming grooves in the supporting substrate 20 and
transferring the grooves to the layers deposited thereon. The
grooves 21 are (depressed) regions located closer to the incident
side of recording/reading laser beam, while strip-like raised
regions interposed between adjacent grooves serve as lands.
[0054] The light-transmitting layer 2 has a sufficient transparency
for laser beam to pass therethrough. The light-transmitting layer
preferably has a thickness in the range of from 30 .mu.m to 200
.mu.m, more preferably from more than 50 .mu.m to 200 .mu.m and
most preferably from 70 .mu.m to 150 .mu.m. If the
light-transmitting layer is thinner than the range, dust depositing
thereon can have detrimental optical effects. Under the situation
that the distance between the optical pickup and the medium is
reduced as a result of the increased NA, so that the optical pickup
can frequently contact the medium surface, a thinner
light-transmitting layer fails to provide a sufficient protective
effect against contact with the optical pickup. If the
light-transmitting layer is too thick, it may be difficult to
achieve a high recording density by an increase of NA. It is noted
that a thick light-transmitting layer can undergo substantial
shrinkage upon curing and as a result, the medium has a greater
deflection. However, since the light-transmitting layer used herein
has a low tensile modulus and a high tensile elongation at break,
it undergoes minimized deflection upon curing even if it is as
relatively thick as having a thickness of more than 50 .mu.m, and
even no less than 70 .mu.m. The deflection, even once incurred upon
curing, will mitigate with the lapse of time. This results in a
medium having minimized deflection and axial runout which has never
found in the art.
[0055] In one embodiment, the light-transmitting layer 2 has a
tensile strength at break of up to 40 MPa, preferably up to 35 MPa,
a tensile elongation at break of at least 15%, preferably at least
20%, and a tensile modulus (or modulus in tension) of up to 1,000
MPa, preferably up to 800 MPa. If the tensile strength at break is
too high, the layer may have a larger birefringence and larger
birefringence distribution breadth. If the tensile elongation at
break is too low, the medium may have a larger axial runout and
tends to deflect, especially during storage under severe
conditions, typically hot humid conditions. If the tensile modulus
is too high, the medium tends to deflect and may have a larger
axial runout.
[0056] Inversely, if the tensile strength at break of the
light-transmitting layer is too low, the desired effect of the
light-transmitting layer, that is, the effect of protecting the
information recording layer becomes insufficient, and the quantity
of deflection is rather increased. For this reason, the tensile
strength at break is at least 5 MPa, preferably at least 7 MPa. If
the tensile elongation at break of the light-transmitting layer is
too high, the light-transmitting layer is too soft and insufficient
in strength. Even when a surface layer having a higher strength is
formed thereon, no satisfactory protective effect is achievable.
For this reason, the tensile elongation at break is up to 100%,
preferably up to 80%. If the tensile modulus of the
light-transmitting layer is too low, the quantity of deflection is
rather increased and the light-transmitting layer becomes too soft.
For this reason, the tensile modulus is at least 40 MPa.
[0057] It is noted that the tensile strength at break, tensile
elongation at break and tensile modulus used herein is as
prescribed in JIS K-7127 (1989). Upon measurement, parameters are
set to:
[0058] specimen length: 60 mm,
[0059] specimen width: 10 mm,
[0060] distance between two gage marks: 40.+-.1 mm,
[0061] distance between grips: 44.+-.1 mm, and
[0062] separation rate: 30 mm/min,
[0063] and the remaining measurement conditions are as prescribed
in JIS K-7127 (1989). The described parameters differ from those of
JIS K-7127 (1989) because the size of the medium (usually a
diameter of about 12 cm for an optical disk) is taken into account
so that measurement may be made on the light-transmitting layer
peeled from the medium.
[0064] The adjustment of the tensile strength at break, tensile
elongation at break and tensile modulus in the above-defined ranges
makes it possible to reduce the birefringence and distribution
breadth thereof, to reduce the deflection of the medium immediately
after manufacture, to prevent the medium from deflecting during
storage in a hot humid environment, and to reduce the axial runout
of the medium.
[0065] More illustratively, it becomes easily possible that the
light-transmitting layer have a birefringence in absolute value of
up to 20 nm, especially up to 18 nm, at a wavelength of 630 nm. It
also becomes easily possible that the light-transmitting layer have
a birefringence distribution breadth of up to 20 nm, especially up
to 15 nm, in peak-to-peak value, at a wavelength of 630 nm. It is
noted that smaller birefringence and distribution breadth thereof
are more preferable. However, it is unnecessary to reduce the
birefringence below 2 nm and to reduce the distribution breadth of
birefringence below 2 nm since the effect is not remarkably
increased by remarkably reducing birefringence and distribution
breadth thereof. The birefringence and distribution breadth thereof
used herein are values in the information recording region. More
particularly, in the case of an optical disk, for example, they are
values in the region of the disk excluding the regions (inner and
outer peripheral regions) not serving as the information recording
region. It is noted that the birefringence and distribution breadth
thereof prescribed herein are measurements at a wavelength of 630
nm although the invention does not limit the wavelength of a
recording or reading beam to 630 nm. When the birefringence and
distribution breadth thereof at a wavelength of 630 nm are as low
as the above-defined ranges, effects including light utilization
efficiency-improving effect are achievable over a wide wavelength
region ranging from about 250 nm to about 900 nm.
[0066] Even at a high linear velocity at which the axial runout
acceleration becomes so high that focusing servo errors may
frequently occur, for example, a linear velocity of at least 8 m/s,
especially 10 to 35 m/s, the occurrence of focusing servo errors
can be fully suppressed. More particularly, provided that R is a
residual error component of a focus error signal and F is a
peak-to-peak value of a focus sensitivity curve, it becomes easily
possible that R/F be up to 10%, and especially up to 6%. It is
noted that smaller values of R/F are more preferable. However, it
is unnecessary to reduce the R/F below 0.1% since the effect is not
remarkably increased by remarkably reducing the R/F.
[0067] In a preferred embodiment of the invention, the
light-transmitting layer has a surface reflectivity of up to 10% at
a recording and reading wavelength. Setting the surface
reflectivity at 10% or lower is effective particularly at a high
linear velocity entailing a less efficient utilization of laser
energy, for example, at a linear velocity of at least 8 m/s,
especially 10 to 35 m/s. It is noted that smaller values of surface
reflectivity are more preferable. However, it is unnecessary to
reduce the surface reflectivity below 0.1% since the effect is not
remarkably increased by remarkably reducing the surface
reflectivity.
[0068] The advantages of the invention becomes more outstanding
when recording or reading is performed by a system including an
objective lens having a numerical aperture NA and emitting a
recording or reading beam having a wavelength of .lambda. wherein
.lambda./NA.ltoreq.780 nm, and especially .lambda./NA.ltoreq.680
nm. That is, the medium of the invention is more effective when a
recording/reading beam having a relatively short wavelength is
passed through an objective lens having a large numerical aperture.
It is noted that the medium is generally acceptable if satisfactory
recording/reading characteristics are available in the range of 400
nm.ltoreq..lambda./NA.
[0069] The invention is characterized in that the
light-transmitting layer has a tensile strength at break, a tensile
elongation at break and a tensile modulus in the above-defined
ranges, whereby the above-described benefits are achievable.
Therefore, the construction of the resin of which the
light-transmitting layer is formed and the method of forming the
light-transmitting layer are not critical. Included are a method of
applying a resin or a composition which will cure to form a resin,
followed by optional curing, and a method of joining a previously
formed resin sheet with a UV-curable adhesive or pressure-sensitive
adhesive. In order to obtain a light-transmitting layer having a
tensile strength at break, a tensile elongation at break and a
tensile modulus in the above-defined ranges, it is preferred that
the light-transmitting layer be formed by applying an active energy
radiation-curable resin by a spin coating technique, and exposing
the coating to active energy (actinic) radiation such as UV
radiation for curing.
[0070] The actinic radiation-curable resin composition used herein
generally contains at least one of mono- or polyfunctional
monomers, oligomers and polymers, a polymerization initiator,
photopolymerization initiator aid, polymerization inhibitor and
other additives. Such a composition may be selected, for example,
from the compositions for protective coat on high-density optical
disks described in the above-referred JP-A 8-194968. The preferred
composition used herein is one comprising at least a linear
difunctional oligomer having functional groups at opposite ends and
a monofunctional monomer. If the content or molecular weight of the
difunctional oligomer is too low, then the tensile elongation at
break after curing becomes small. As the ratio of the
monofunctional monomer to the difunctional oligomer increases, the
tensile strength at break can be reduced without a substantial loss
of the tensile elongation at break after curing. Also, the addition
of the monofunctional monomer improves the adhesion between the
light-transmitting layer and the surface on which it is formed. It
is noted that if the content of the monofunctional monomer is too
high, the tensile elongation at break after curing becomes small.
Therefore, the content and molecular weight of the difunctional
oligomer and the content of the monofunctional monomer may be
selected as appropriate depending on the physical properties of the
light-transmitting layer required in the present invention. It is
understood that such a composition may be selected from
commercially available ones.
[0071] Given below are examples of suitable oligomers and monomers
used in the actinic radiation-curable resin composition.
[0072] Suitable difunctional oligomers include polyester acrylates,
epoxy acrylates and urethane acrylates. The polyester acrylates are
available under the trade name of Aronix M-6200, Aronix M-6400X,
Aronix M-6410X and Aronix M-6420X from Toa Gosei Co., Ltd. The
epoxy acrylates are available under the trade name of Lipoxy
SP-1506, Lipoxy SP-1509, Lipoxy SP-1519-1, Lipoxy SP-1563, Lipoxy
VR-77, Lipoxy VR-60 and Lipoxy VR-90 from Showa Highpolymer Co.,
Ltd.; Biscoat 540 from Osaka Yukikagaku Co., Ltd.; Kayarad R-167
from Nippon Kayaku Co., Ltd.; Epoxy Ester 3002A, Epoxy Ester 3002M
and Epoxy Ester 8OMFA from Kyoeisha Yushi Co., Ltd.; and Nadecole
DM-851, Nadecole DA-811, Nadecole DM-811, Nadecole DA-721 and
Nadecole DA-911 from Nagase & Co., Ltd. The urethane acrylates
are available under the trade name of Art Resin UN-1000PEP, Art
Resin UN-9000PEP, Art Resin UN-9200A, Art Resin UN-2500, Art Resin
UN-5200, Art Resin UN-1102, Art Resin UN-380G, Art Resin UN-500 and
Art Resin UN-9832 from Negami Chemical Industrial Co., Ltd.; Aronix
M-1200 from Toa Gosei Co., Ltd.; and Chemlink 9503, Chemlink 9504
and Chemlink 9505 from Sartomer Co.
[0073] Examples of the monofunctional monomer include benzyl
acrylate, benzyl methacrylate, butoxyethyl acrylate, butoxyethyl
methacrylate, butane diol monoacrylate, cyclohexyl acrylate,
cyclohexyl methacrylate, dicyclopentanyl acrylate, dicyclopentanyl
methacrylate, alicyclically modified neipentyl glycol acrylate,
dicyclopentenyl acrylate, dicyclopentenyloxyethyl acrylate,
dicyclopentenyloxyethyl methacrylate, 2-ethoxyethyl acrylate,
2-ethoxyethyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl
methacrylate, glycidyl acrylate, glycidyl methacrylate,
2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate,
2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, isobornyl
acrylate, isobornyl methacrylate, isodecyl acrylate, isodecyl
methacrylate, isooctyl acrylate, isooctyl methacrylate, lauryl
acrylate, lauryl methacrylate, 2-methoxyethyl acrylate,
methoxy-diethylene glycol methacrylate, methoxyethylene glycol
acrylate, morpholine acrylate, phenoxyhydroxypropyl acrylate,
phenoxyethyl acrylate, phenoxyethyl methacrylate, phenoxydiethylene
glycol acrylate, EO-modified phthalic acid acrylate, EO-modified
phthalic acid methacrylate, stearyl acrylate, stearyl methacrylate,
tetrahydrofurfuryl acrylate, tetrahydrofurfuryl methacrylate, vinyl
acetate, and N-vinylpyrrolidone.
[0074] The preferred actinic radiation-curable resin composition
used herein has a relatively high viscosity, typically in the range
of about 1,000 to 30,000 centipoise at 25.degree. C. In contrast,
the above-referred JP-A 8-194968 discloses that the composition
preferably has a viscosity of 5 to 300 centipoise at 25.degree.
C.
[0075] The above-referred JP-A 8-194968 describes that the
protective coat on the optical disk has a tensile elongation at
break of at least 15%, which is effective for restraining the
occurrence of deflection of the optical disk during storage in a
hot humid environment. The range of tensile elongation at break
described in said patent publication overlaps the range defined in
the present invention. However, since the protective coat-forming
composition described in said patent publication differs in
viscosity from the composition used in the present invention as
mentioned just above, it is not believed that the
light-transmitting layer resulting from curing of the composition
described in said patent publication has the physical properties of
the light-transmitting layer defined in the present invention. It
is merely described in said patent publication that a protective
coat of 1 to 50 .mu.m thick is formed in order to protect the
recording portion from corrosion, but not that a recording and
reading beam enters the recording portion through the protective
coat. It is thus evident that said patent publication does not
intend to reduce the birefringence of the protective coat. In
contrast, it is important for the present invention to reduce the
deflection quantity of the medium to a remarkably low level and to
reduce the birefringence and distribution breadth thereof to
remarkably low levels in order that a recording/reading beam of a
relative short wavelength enter the recording layer through the
thin light-transmitting layer of about 100 .mu.m thick. The
above-referred patent publication describes that the deflection is
reduced, but does not refer to axial runout and axial runout
acceleration. It is substantially impossible to form a
light-transmitting layer having a thickness in excess of 50 .mu.m
using a composition having a low viscosity as described in said
patent publication.
[0076] The light-transmitting layer used herein may be a laminate
of two or more resin layers. One exemplary laminate type is a
structure in which an inner layer having a low tensile strength at
break and a high tensile elongation at break and a surface layer
having higher wear resistance than the inner layer lying on the
inner layer, the surface layer constituting the surface of the
light-transmitting layer. This structure allows the surface layer
to be thin because satisfactory wear resistance is obtained even
when the surface layer is considerably thinner than the inner
layer. Then, the requirement relating to physical properties is not
so rigorous for the material of which the surface layer is formed.
Accordingly, for the material of which the surface layer is formed,
a relatively free choice may be made of a resin having good wear
resistance among numerous resins. Understandably, the surface layer
desirably has a higher tensile strength at break than the inner
layer, specifically a tensile strength at break of more than 40
MPa.
[0077] It is preferred that the inner and surface layers in the
above-described structure be cured products of actinic
radiation-curable resin compositions as mentioned above. For the
composition used to form the inner layer, one that will have a low
tensile strength at break, a high tensile elongation at break and a
low tensile modulus after curing is selected. For the composition
used to form the surface layer, on the other hand, the proportion
of polyfunctional oligomer and/or polyfunctional monomer is
preferably set relatively high whereby the surface layer has a
higher hardness. A relatively high proportion of monofunctional
monomer improves the adhesion between the surface layer and the
inner layer.
[0078] In the two-layer structure of surface and inner layers, the
surface layer preferably has a thickness of 0.1 to 10 .mu.m, more
preferably 0.3 to 5 .mu.m. If the surface layer is too thin, the
protective effect may become insufficient. Inversely, if the
surface layer is too thick, it may become difficult to control the
physical properties of the entire light-transmitting layer so as to
fall within the range specified herein.
[0079] It is noted that the inner layer may have a multilayer
structure consisting of two or more plies.
[0080] In the embodiment wherein the light-transmitting layer has a
multilayer structure formed of actinic radiation-curable resins, it
is customary that a lower layer-forming composition is applied and
cured before an upper layer-forming composition is applied and
cured. To improve the adhesion between lower and upper layers, a
procedure of laying the upper layer on the lower layer which has
been semi-cured, and completely curing all the layers at last may
be employed.
[0081] As previously described, the ratio R/F of th residual error
component R of a focus error signal to the peak-to-peak value F of
a focus sensitivity curve can be utilized for the inspection of the
optical information medium. It is noted that the same medium can
have different R/F ratios depending on the operating linear
velocity. Seeking for the parameter to be controlled in accordance
with the operating linear velocity in order that R/F fall within
the preferred range, the inventors have found that R/F is
substantially affected by the maximum waviness on the surface of
the light-transmitting layer. More specifically, it has been found
that the relationship:
Wt.ltoreq.1840e.sup.-0.04V
[0082] should preferably be met, provided that the
light-transmitting layer at its surface has a maximum waviness Wt
(in nm) and the medium is rotated at a linear velocity V (in m/s)
during recording or reading. Note that e is the base of natural
logarithm. By forming the light-transmitting layer so that the
maximum waviness Wt may fall in the above range in accordance with
the operating linear velocity V, there can be realized a medium
that entails minimized focusing servo errors. It is noted that
smaller Wt is more preferable, but the effect is not remarkably
improved by reducing Wt to a remarkably low level. Then, in most
cases, it suffices that Wt be:
Wt.ltoreq.10 nm.
[0083] It is noted that the maximum waviness Wt used herein is
prescribed in ANSI B46.1. Upon measurement of the maximum waviness
Wt of the light-transmitting layer, parameters are set to:
[0084] high-pass filter: 0.2 mm,
[0085] low-pass filter: 2 mm,
[0086] evaluated length: 20 mm,
[0087] reference length: 16 mm,
[0088] probe pressing force: 10 .mu.g,
[0089] probe: diamond stylus with curvature radius 12.5 .mu.m,
and
[0090] measuring time: 25 sec, and the remaining measurement
conditions are as prescribed in ANSI B46.1.
[0091] The influence of Wt on the above-described R/F differs with
the beam spot diameter of a recording or reading laser beam. The
beam spot diameter used herein is the diameter of the spot that a
laser beam defines on the incident surface (light-transmitting
layer surface). In a situation wherein the beam spot diameter is
large, the R/F is not so increased even when Wt has a relatively
large value. Then the relationship of Wt to V prescribed in the
present invention stands under the condition that the beam spot
diameter is not greater than 300 .mu.m, especially not greater than
200 .mu.m. However, if the beam spot diameter is too small, the
influence of surface roughness of the light-transmitting layer
becomes substantial. Then the control of Wt according to the
invention is preferably applied to the situation wherein the beam
spot diameter is 10 .mu.m or larger. The control of Wt is effective
where the light-transmitting layer is thin as in the medium of the
invention, because the beam spot diameter on the surface of the
light-transmitting layer is small. It is noted that the beam spot
diameter D on the surface of the light-transmitting layer is
represented by
D=2tan{sin.sup.-1(Na/n)}.multidot.t+1.22 .lambda./NA
[0092] wherein the laser beam has a wavelength .lambda., the laser
beam irradiating optical system includes an objective lens having a
numerical aperture NA, and the light-transmitting layer has a
refractive index "n" at the wavelength .lambda. and a thickness
"t." When the beam spot is elliptic, the beam spot diameter is the
diameter in the longitudinal direction of recording track.
[0093] In the disk-shaped medium, the maximum waviness Wt on the
light-transmitting layer surface is not correlated to the
deflection and torsion of the medium that are incurred by the
above-described mechanism. It is noted that the measurement of Wt
sometimes becomes impossible if the medium undergoes too much
deflection and torsion.
[0094] Since an appropriate value of the maximum waviness Wt varies
with the operating linear velocity V, the maximum waviness on the
surface of the light-transmitting layer may be controlled in
accordance with the operating linear velocity. Where it is
necessary for the maximum waviness to conform to a high linear
velocity, for example, a linear velocity of at least 8 m/s,
especially 10 to 35 m/s, the light-transmitting layer is preferably
constructed as described below.
[0095] In the embodiment wherein the light-transmitting layer is
formed by forming a coating containing a UV-curable resin by a spin
coating technique and exposing the coating to UV radiation for
curing, the spin coating conditions must be controlled such that no
asperities may form on the coating. To this end, it is recommended,
for example, that the viscosity of the coating solution is
controlled such that no bubbles may be introduced into the coating,
or that the coating solution is prepared such that no gel may form
in the coating solution.
[0096] In the other embodiment wherein the light-transmitting layer
consists of a light-transmitting sheet formed of a resin and an
adhesive layer for joining the light-transmitting sheet to the
supporting substrate side, the adhesive layer containing a cured
product of a UV-curable resin, it is preferred that the adhesive
layer have an average thickness of from 0.5 .mu.m to less than 5
.mu.m, and especially from 1 .mu.m to 3 .mu.m. Too thin an adhesive
layer may absorb a less amount of UV radiation, with which few
active species are formed. Also, when curing is performed in air,
too thin an adhesive layer leads to a more amount of contact air
per unit volume of the adhesive layer, which tends to deactivate
active species in the case of resins susceptible to oxygen
inhibition (e.g., radical polymerization resins) or resins
susceptible to moisture inhibition (e.g., cation polymerization
resins). Therefore, the adhesive layer is rather restrained from
curing, resulting in an insufficient bonding force. Further, too
thin an adhesive layer is difficult to form as a uniform layer.
Inversely, if the adhesive layer is too thick, the adhesive layer
has a greater thickness distribution and the maximum waviness Wt on
the surface of the light-transmitting sheet overlying the adhesive
layer becomes larger. It is noted that the average thickness of the
adhesive layer is the thickness at a position spaced from the
center of the optical disk by one half of the disk radius.
[0097] Next, it is described how to form the adhesive layer.
[0098] In forming the adhesive layer, a UV-curable resin or a
solution thereof is applied to the entire surface on the supporting
substrate side (the surface of the recording layer 4 in FIG. 1) to
form a coating. To reduce the thickness distribution of the
adhesive layer and coating variation, it is preferred that the
coating is formed by diluting the UV-curable resin with a solvent
to form a resin solution and applying this resin solution. The
solvent used in diluting the resin is not critical, and any
suitable one may be selected from a variety of solvents such as
alcohol, ester, cellosolve and hydrocarbon solvents as long as it
does not attack the supporting substrate and the light-transmitting
sheet.
[0099] The resin solution preferably has a viscosity of less than
10 centipoise, more preferably 4 to 6 centipoise. Too high a
viscosity may make it difficult to reduce the thickness
distribution of the adhesive layer. Inversely, too low a viscosity
may make it difficult to form a uniform adhesive layer. The resin
solution preferably has a solid concentration of 10 to 50% by
weight, and more preferably 20 to 40% by weight. Too low a solid
concentration may make it difficult to form a uniform adhesive
layer. Inversely, too high a solid concentration may make it
difficult to reduce the thickness distribution of the adhesive
layer and to reduce the maximum waviness Wt on the surface of the
light-transmitting sheet.
[0100] The technique of forming the coating is not critical. There
may be employed any of spin coating, spray coating, roll coating,
screen coating, die coating, curtain coating, and dip coating
techniques. Notably, the spin coating technique has the tendency
that the adhesive layer becomes thicker toward the outer periphery
of the optical disk; and the roll coating and die coating
techniques have the tendency that the adhesive layer differs in
thickness between the leading side and the trailing side. The
effects resulting from the control in thickness of the adhesive
layer become more enhanced with these techniques. Of these coating
techniques, the spin coating technique is preferred because it
facilitates to form a uniform adhesive layer, causes no damage to
the medium on account of non-contact coating, and reduces the
surface roughness of the adhesive layer.
[0101] After the coating is formed, a light-transmitting sheet is
placed thereon, preferably under a subatmospheric pressure. The
subatmospheric pressure is below 1 atm., preferably 0.3 atm. or
lower, more preferably 0.1 atm. or lower. The placement of a
light-transmitting sheet under a subatmospheric pressure prevents
bubbles from being introduced into the adhesive layer, thus
preventing any tracking servo failure caused by bubbles.
[0102] Once the light-transmitting sheet is placed on the coating,
the coating is cured by exposing it to UV radiation. UV exposure
may use customary high-pressure mercury vapor lamps. Curing may be
effected under the subatmospheric pressure or after the vacuum is
relieved back to air. Curing under the subatmospheric pressure can
alleviate oxygen inhibition and moisture inhibition upon
curing.
[0103] When the adhesive layer is formed by UV curing, the energy
density of UV radiation is preferably set to 1,000 mW/cm.sup.2 or
less, and especially 600 mW/cm.sup.2 or less. By setting the energy
density of UV radiation within this range, the maximum waviness Wt
on the surface of the light-transmitting sheet bonded to the
adhesive layer can be reduced. Even when the adhesive layer has a
thickness of 5 .mu.m or more, for example, Wt can be suppressed
fully low. It is noted that since too low an energy density of UV
radiation requires a too long time for curing and results in
short-cure, the energy density of UV radiation is preferably at
least 5 mW/cm.sup.2.
[0104] Suitable materials of which the light-transmitting sheet is
made are polycarbonates, polyarylates and cyclic polyolefins.
[0105] The polycarbonates used herein are not critical. For
example, aromatic polycarbonates of the conventional bisphenol type
are useful. Polycarbonate sheets prepared by casting to be
described later are commercially available under the trade name of
Pure Ace from Teijin Co., Ltd.
[0106] The polyarylates are polyesters of dihydric phenols with
aromatic dicarboxylic acids. The polyarylates used herein are
amorphous ones, with polycondensates of bisphenol A with
terephthalic acid being especially preferred. The polyarylates tend
to develop birefringence due to the inclusion of aromatic rings
like the polycarbonates, but are more resistant to heat than the
polycarbonates. Polyarylate sheets prepared by casting to be
described later are commercially available under the trade name of
Elmec from Kaneka Corp.
[0107] The cyclic polyolefins used herein are preferably highly
transmissive to light. Such light transmissive cyclic polyolefins
are amorphous cyclic polyolefins prepared from norbornene compounds
as the starting material. These cyclic polyolefins are also
resistant to heat. Commercially available cyclic polyolefins are
useful in the practice of the invention. Commercially available
cyclic polyolefins are Arton by JSR Co., Ltd., Zeonex by Nippon
Zeon Co., Ltd., and Apel by Mitsui Chemical Co., Ltd. Arton and
Zeonex are available in the form of sheets. Arton and Zeonex are
prepared by effecting ring-opening polymerization of norbornene
monomers, followed by hydrogenation. In particular, Arton starts
with a norbornene monomer having an ester group on a side chain and
is readily soluble in solvents. Arton is advantageous in that it
can be formed into sheets by casting. Other advantages of Arton are
a high bond strength with the adhesive layer because it is highly
bondable with organic materials, and less electrostatic charging
leading to less dust attachment.
[0108] It is not critical how to prepare the light-transmitting
sheet. Since the light-transmitting sheet used herein is thin, it
is difficult to form the sheet by conventional injection molding.
Consequently, techniques capable of forming resins into sheets such
as solvent casting and melt extrusion are preferable. The
especially preferred technique is solvent casting. The casting
technique is disclosed, for example, in JP-B 3-75944 as the
technique capable of forming flexible disk substrates having
improved transparency, birefringence, flexibility, surface
precision and film thickness uniformity. In the practice of the
invention, the casting technique is preferably utilized in forming
the light-transmitting sheet.
[0109] The process of forming the light-transmitting sheet by the
casting technique involves (1) dissolving resin pellets such as
polycarbonate pellets in a suitable solvent such as methylene
chloride, acrylonitrile or methyl acrylate, (2) thoroughly
agitating, deaerating and filtering the solution and then
continuously flowing the solution on a mold having a high surface
precision through a die, (3) evaporating the solvent by passing
through a drying furnace, and (4) continuously winding up the sheet
into a roll.
[0110] The light-transmitting sheet formed by the casting technique
has a reduced birefringence since it has experienced less tension
during manufacture, as compared with the sheet formed by a
conventional melt extrusion technique which inevitably develops a
distribution of birefringence in a stretched direction. Also, the
casting technique can form a sheet of uniform thickness having an
excellent surface state by properly controlling the rate of
evaporation of the solvent. Additionally, the casting technique
eliminates flaws by die lines as found in the melt extruded sheet.
Additionally, the sheet formed by the flow casting technique has
the advantage of reduced maximum waviness Wt on its surface, for
example, a fully reduced value of Wt even when the adhesive layer
has a thickness of at least 5 .mu.m.
[0111] It can be confirmed by an isotropic pattern of birefringence
whether or not a particular light-transmitting sheet has been
formed by the casting technique. The same can also be confirmed by
qualitative analysis of the residual solvent in the sheet by gas
chromatography or the like.
EXAMPLE
[0112] Examples of the invention are given below by way of
illustration and not by way of limitation.
Example 1
[0113] Optical recording disk samples were fabricated as
follows.
[0114] There were furnished disk-shaped supporting substrates
(polycarbonate, diameter 120 mm, thickness 1.2 mm) having grooves
formed therein. The grooves had a depth of .lambda./6 as expressed
by optical path length at the wavelength .lambda. of 405 nm. In the
land-groove recording system, the recording track pitch was 0.3
.mu.m. On the grooved surface of the substrates, a reflective layer
of Al.sub.98Pd.sub.1Cu.sub.- 1 (atomic ratio) was formed by
sputtering.
[0115] On the surface of the reflective layer, a second dielectric
layer of 20 nm thick was formed by sputtering an Al.sub.2O.sub.3
target.
[0116] Next, on the surface of the second dielectric layer, a
recording layer of 12 nm thick was formed by sputtering an alloy
target of phase change material. The recording layer had the
composition of Sb.sub.74Te.sub.18(Ge.sub.7In.sub.1) in atomic
ratio.
[0117] Next, on the surface of the recording layer, a first
dielectric layer of 130 nm thick was formed by sputtering a target
of 80 mol % ZnS-20 mol % SiO.sub.2.
[0118] Next, on the surface of the first dielectric layer, an inner
layer of 97 .mu.m thick was formed by spin coating a UV-curable
resin (SSP50U10 by Showa Highpolymer Co., Ltd., viscosity 1,900
centipoise at 25.degree. C.) and exposing the coating to UV
radiation. Then another UV-curable resin (MH-7361 by Mitsubishi
Rayon Co., Ltd.) was spin coated on the inner layer and exposed to
UV radiation, forming a surface layer of 3 .mu.m thick. This
resulted in a light-transmitting layer of 100 .mu.m thick.
[0119] The recording layer of the thus fabricated optical recording
disk sample was initialized or crystallized by means of a bulk
eraser. Using Biref 126P by Dr. Schenk, the birefringence of the
light-transmitting layer in the information recording region (the
region ranging from a radius 23 mm to a radius 58 mm of the disk
sample) was measured while a laser beam with wavelength 630 nm was
directed to the recording layer from the light-transmitting layer
side. The maximum in absolute value of birefringence and the
difference between the maximum and the minimum (distribution
breadth) were determined. The results are shown in Table 1.
[0120] Also the surface reflectivity of the light-transmitting
layer of this sample was measured by the following process. First,
the light-transmitting layer was formed on the surface of a glass
substrate having the same size as the supporting substrate by the
above-described procedure. The light-transmitting layer was
stripped from the glass substrate. The reflectivity of the
light-transmitting layer at a wavelength of 405 nm was measured
using a 45 .degree.absolute specular reflectivity meter attached to
a spectrophotometer MPS-2000 by Shimadzu Corp. The results are
shown in Table 1.
[0121] This disk sample was mounted on an optical recording medium
tester. Signals were recorded in lands and grooves under the
following conditions.
[0122] laser wavelength: 405 nm
[0123] numerical aperture NA: 0.85
[0124] linear velocity: 11.4 m/s
[0125] recording signals: 1-7 modulation signals (shortest signal
length 2T)
[0126] Then the recorded signals were read out, during which a
jitter was determined. The results are shown in Table 1. The jitter
is a clock jitter determined by analyzing the read signals by a
time interval analyzer and computing according to
.sigma./Tw (%)
[0127] wherein Tw is the window margin. If the jitter is up to 13%,
errors fall within the permissible range. To provide satisfactory
ranges of various margins, the jitter is desirably up to 10%, and
more desirably up to 9%.
[0128] The ratio of a residual error component of a focus error
signal to a peak-to-peak value of a focus sensitivity curve was
determined as follows. The sample was mounted on a measuring device
where it was rotated at a linear velocity of 11.4 m/s. Next, with
the focusing servo kept inoperative, focus error signals were
detected while the distance between the sample and the optical
pickup was changed. A focus sensitivity curve was obtained by
plotting the focus error signal output as a function of the
positional change of the sample. The focus sensitivity curve is
known as S-shaped curve and described, for example, in "Optical
Disk Technology," Feb. 10, 1989, Radio Gijutsu K.K., page 81. From
the focus sensitivity curve, the peak-to-peak value of focus error
signal outputs, that is, the difference between the peak value of
positive outputs and the peak value of negative outputs was
determined. Next, the focusing servo was made operative, the output
peak-to-peak value of residual error components of focus error
signals was measured. It is noted that in this measurement, the
focusing servo depended on the knife edge method. From the thus
obtained peak-to-peak value F of the focus sensitivity curve and
the peak-to-peak value R of the residual error component of focus
error signals, R/F was computed. The results are shown in Table
1.
[0129] The focusing servo method used in determining the ratio of
the residual error component of a focus error signal to the
peak-to-peak value of a focus sensitivity curve is not particularly
limited and may be selected from the knife edge method, astigmatism
method and the like.
[0130] Next, the light-transmitting layer of the sample was cut by
a cutter knife to rectangular strips of 60 mm.times.10 mm. Using
Tensilon Model TRM-100 by Orientec K.K., the test strip was
measured for tensile strength at break, tensile elongation at break
and tensile modulus under the measuring conditions prescribed in
JIS K-7127 (1989) and in the present invention. The results are
shown in Table 1. It is noted that when the test strips were cut
out, the dielectric layers, recording layers and reflective layer
remained stuck to the test strips, but the attachment of such
layers to the test strips had no influence on the measurements of
tensile strength at break, tensile elongation at break and tensile
modulus.
[0131] The sample was also measured for deflection and axial
runout, using an instrument LM1200 by Ono Sokki K.K. After the
sample was held for 50 hours in an atmosphere of 80.degree. C. and
RH 80%, similar measurement was repeated. The results are shown in
Table 1.
Example 2
[0132] An optical recording disk sample was fabricated as in
Example 1 except that a light-transmitting layer of 100 .mu.m thick
was formed by spin coating a UV-curable resin (B8 by Nippon Kayaku
Co., Ltd., viscosity 5,000 centipoise at 25.degree. C.) and
exposing the coating to UV radiation. The sample was determined for
various properties as in Example 1. The results are shown in Table
1.
Example 3
[0133] An optical recording disk sample was fabricated as in
Example 1 except that a light-transmitting layer of 100 .mu.m thick
was formed by spin coating a UV-curable resin (SSP50U14 by Showa
Highpolymer Co., Ltd., viscosity 4,000 centipoise at 25.degree. C.)
and exposing the coating to UV radiation. The sample was determined
for various properties as in Example 1. The results are shown in
Table 1.
Comparative Example 1
[0134] An optical recording disk sample was fabricated as in
Example 1 except that a light-transmitting layer of 100 .mu.m thick
was formed by spin coating a UV-curable resin (No. 303-2 by Showa
Highpolymer Co., Ltd., viscosity 4,000 centipoise at 25.degree. C.)
and exposing the coating to UV radiation. The sample was determined
for various properties as in example 1. The results are shown in
Table 1.
Comparative Example 2
[0135] An optical recording disk sample was fabricated as in
Example 1 except that a light-transmitting layer of 100 .mu.m thick
was formed by spin coating a UV-curable resin (T695/UR740 by
Ciba-Nagase Co., Ltd., viscosity 2,100 centipoise at 25.degree. C.
and exposing the coating to UV radiation. The sample was determined
for various properties as in Example 1. The results are shown in
Table 1.
Comparative Example 3
[0136] An optical recording disk sample was fabricated as in
Example 1 except that a light-transmitting layer of 100 .mu.m thick
was formed by spin coating a UV-curable resin (SD318 by Dainippon
Ink & Chemicals, Inc., viscosity 140 centipoise at 25.degree.
C. and exposing the coating to UV radiation. The sample was
determined for various properties as in Example 1. The results are
shown in Table 1.
Comparative Example 4
[0137] There was furnished the UV-curable resin composition
described in Example 3 of JP-A 8-194968. This UV-curable resin
composition had the following recipe:
1 Component Parts by weight EPA-1 (Nippon Kayaku Co., Ltd.) 20
MANDA (Nippon Kayaku Co., Ltd.) 40 THF-A (Kyoei Yusi Co., Ltd.) 10
R-561 (Nippon Kayaku Co., Ltd.) 30 Darocur BP (Ciba Specialty
Chemicals) 3 DMBI (Nippon Kayaku Co., Ltd.) 1 Irgacure 651 (Ciba
Specialty Chemicals) 3
[0138] and had a viscosity of 64 centipoise at 25.degree. C. An
optical recording disk sample was fabricated as in Example 1 except
that a light-transmitting layer was formed using this UV-curable
resin composition. The energy density of UV radiation was 2,000
mW/cm.sup.2. Since this UV-curable resin composition had a low
viscosity, a light-transmitting layer could not be formed to a
thickness in excess of 50 .mu.m. Thus the light-transmitting layer
had a thickness of 50 .mu.m. The sample was determined for various
properties as in Example 1. The results are shown in Table 1.
2 TABLE 1 Birefringence at Tensile Tensile wavelength strength
elongation Tensile 630 (nm) Surface Deflection Axial runout at
break at break modulus Distribution R/F Jitter reflectivity (.mu.m)
(.mu.m) (MPa) (%) (MPa) Maximum breadth (%) (%) (%) Initial Aged
Initial Aged Example 1 32 68 76 15 8 4.53 8.8 8.0 -74.8 -116.3 55.8
62.4 Example 2 17 55 200 16 11 4.38 8.6 8.5 -96.7 -123.8 34.9 31.8
Example 3 24 18 784 16 7 4.80 9.0 10.0 -68.4 -110.5 48.6 42.1
Comparative 44* 17 1044* 30* 25* 8.72 12.3 11.0* -186.2 -284.6
111.6 108.6 Example 1 Comparative 4* 16 24* 19 16 6.50 11.5 9.0
-141.1 -248.4 87.4 79.7 Example 2 Comparative >50* 5* >2000*
54* 60* 10.70* 15.0 11.0* -265.7 -430.8 180.7 194.1 Example 3
Comparative 65* 4* 1810* 30* 24* 10.30* 12.0 11.0* -223.5 -286.3
150.5 174.3 Example 4 *outside the specific range
[0139] Data in Table 1 demonstrate the effectiveness of the
invention. Those samples whose light-transmitting layer has a
tensile strength at break, tensile elongation at break and tensile
modulus within the specific ranges of the invention have the
advantages of a low birefringence, a small distribution breadth of
birefringence, less focus errors, a reduced jitter, a reduced
deflection, a reduced axial runout, and little exacerbation of
deflection and axial runout during hot humid storage.
[0140] By contrast, the advantages of the invention are lost when
at least one of tensile strength at break, tensile elongation at
break and tensile modulus is outside the range of the
invention.
[0141] A sample was fabricated as in Example 1 except that the
inner layer was omitted and the surface layer was formed to a
thickness of 100 .mu.m and served as the light-transmitting layer.
The light-transmitting layer of this sample was found to have a
tensile strength at break of more than 50 MPa.
Example 4
[0142] Sample No. 1
[0143] An optical recording disk sample was fabricated as in
Example 1 except that the light-transmitting layer was formed by
the following procedure.
[0144] The light-transmitting layer was constructed by first spin
coating a UV-curable resin SK5110 (Sony Chemical Co., Ltd.) onto
the surface of the first dielectric layer to form a resin layer.
Then in vacuum (below 0.1 atm.), a polycarbonate sheet (thickness
100 .mu.m, birefringence 15 nm) as a light-transmitting sheet was
placed on the resin layer. The polycarbonate sheet was formed of
Pure Ace (Teijin Co., Ltd.) by a casting technique as previously
mentioned. This polycarbonate had a glass transition temperature of
145.degree. C. and a molecular weight of about 40,000. With the
light-transmitting sheet rested thereon, the disk was rotated at
6,000 rpm for 10 seconds to spin off the extra resin. After the
vacuum was relieved back to air, UV radiation was irradiated to
cure the resin layer, obtaining a light-transmitting layer of 103
.mu.m thick.
[0145] As the UV source, a Xe flash lamp by Eye Graphic Co. was
used. UV radiation was irradiated 8 passes at an output of 120 J.
The energy density of UV radiation could not be measured (beyond
5,000 mW/cm.sup.2), using Uvicure Plus of ETI Instrumentation
Products.
[0146] Sample No. 2
[0147] An optical recording disk sample was fabricated by the same
procedure as Sample No. 1 except that a high-pressure mercury vapor
lamp of Ushio Electric Co., Ltd. (3 kW type, radiation energy
density 300 mJ/cm.sup.2) was used as the UV source and the energy
density of UV radiation was 600 mW/cm.sup.2.
[0148] Sample No. 3
[0149] An optical recording disk sample was fabricated by the same
procedure as Sample No. 1 except that a UV-curable resin T695/UR621
of Nagase Chemtec Co., Ltd. was used in forming the adhesive layer,
Multi-Light of Ushio Electric Co., Ltd. (250 W type) was used as
the UV source, and the energy density of UV radiation was 50
mW/cm.sup.2.
[0150] Sample No. 4
[0151] An optical recording disk sample was fabricated by the same
procedure as Sample No. 1 except that a light-transmitting layer of
100 .mu.m was formed by spin coating a UV curable resin (B8 by
Nippon Kayaku Co., Ltd., viscosity 5,000 centipoise at 25.degree.
C. onto the surface of the first dielectric layer, and exposing the
coating to UV radiation. The UV exposure conditions were the same
as the curing conditions for the adhesive layer of sample No.
2.
[0152] Sample No. 5 (comparison)
[0153] Sample No. 5 is a DVD-RAM (one side 2.6 GB type) by TDK
Corporation which is structured such that a recording/reading laser
beam reaches through a polycarbonate substrate of 0.6 mm thick.
[0154] Sample No. 6 (comparison)
[0155] Sample No. 6 is a DVD-RAM (double side 5.2 GB type) by TDK
Corporation which is structured such that a recording/reading laser
beam reaches through a polycarbonate substrate of 0.6 mm thick.
[0156] Evaluation
[0157] The laser beam-incident surface of each sample was measured
for roughness parameters (center line average roughness Ra, maximum
roughness Rt, center line waviness Wa and maximum waviness Wt) as
prescribed in ANSI B46.1. These roughness parameters were measured
at a position of radius 40 mm on the laser beam-incident surface.
The measurement direction was tangential to the cicumferential
direction of the sample, and the measurement length was 20 mm. The
measurement was repeated 5 times, and an average was computed. The
conditions under which the maximum waviness Wt was measured was the
previously-described conditions prescribed in the present
invention. The results are shown in Table 2.
[0158] For each sample, the ratio R/F of the residual error
component (R) of focus error signals to the peak-to-peak value (F)
of a focus sensitivity curve was determined while the linear
velocity was changed from low to high. The highest linear velocity
Vmax at which R/F did not exceed 10% was determined. The results
are shown in Table 2. For sample Nos. 1 to 4, the measuring
conditions included a laser wavelength of 405 nm and a numerical
aperture NA of 0.85; and for sample Nos. 5 and 6, the measuring
conditions were the same as in DVD-RAM, that is, included a laser
wavelength of 635 nm and a numerical aperture NA of 0.60.
3 TABLE 2 Sample Vmax Ra Rt Wa Wt No. (m/s) (nm) (nm) (nm) (nm) 1
7.3 1.3 21.1 293 1400 2 10.8 1.3 18.5 273 1240 3 12.5 1.3 17.5 258
1170 4 28.9 1.1 9.1 144 636 5 24.1 1.0 9.0 414 1780 (comparison) 6
8.4 1.2 11.4 568 2440 (comparison)
[0159] It is evident from Table 2 that the Vmax differs among
distinct samples and that the Vmax is not correlated to Ra and Rt,
but to Wa and Wt. FIG. 2 is a graph in which the linear velocity V
is on the abscissa and the maximum waviness Wt is on the ordinate.
In the graph of FIG. 2, Wt is plotted as a function of Vmax for
each sample, and the curve represented by
Wt=1840e.sup.-0.04V
[0160] is depicted.
[0161] To operate each of the samples having Wt plotted in FIG. 2
in such a manner that the R/F might be up to 10%, the operating
linear velocity V should be below the Vmax of that sample. In FIG.
2, all the samples have Wt plots above the curve represented by
Wt=1840e.sup.-0.04V. Therefore, when operated at a linear velocity
V satisfying Wt.ltoreq.1840e.sup.-0.04- V, all the samples show an
R/F of up to 10%.
[0162] It is noted that in FIG. 2, sample Nos. 5 and 6 which are
commercial DVD-RAM disks show a higher Vmax than sample No. 1,
despite their remarkably high Wt as compared with sample No. 1.
Then, they can be operated at a remarkably higher linear velocity
than the linear velocity V limited by Wt.ltoreq.1840e.sup.0.004V.
This is because the R/F measuring conditions for sample Nos. 5 and
6 are the same as the reading conditions for DVD-RAM. That is,
control of Wt is effective in sample Nos. 1 to 4 in which the beam
spot on the beam-incident surface has a diameter of about 120
.mu.m, whereas control of Wt is unnecessary in sample Nos. 5 and 6
in which the beam spot on the beam-incident surface has a diameter
as large as about 490 .mu.m.
[0163] Japanese Patent Application Nos. 2000-191787 and 2000-363891
are incorporated herein by reference.
* * * * *