U.S. patent application number 10/281353 was filed with the patent office on 2003-04-10 for master disk and method of manufacturing the same.
Invention is credited to Furuki, Motohiro, Takeda, Minoru.
Application Number | 20030067863 10/281353 |
Document ID | / |
Family ID | 14718303 |
Filed Date | 2003-04-10 |
United States Patent
Application |
20030067863 |
Kind Code |
A1 |
Takeda, Minoru ; et
al. |
April 10, 2003 |
Master disk and method of manufacturing the same
Abstract
There are provided an optical disk and a manufacturing method
thereof which can increase its recording capacity, e.g. to 15 GB or
higher. This optical disk may comprises an optical disk substrate 3
in which successive pits corresponding to a recording signal are
formed, a reflection film 4 formed in this optical disk substrate 3
on its surface that successive pits 2 are formed and a light
transmissive layer 5 formed on this reflection film 4. When a
recorded signal is read out, i.e. reproduced from the optical disk,
a signal recorded as successive pits is read out by irradiating of
short-wavelength laser light having a wavelength of 350 nm to 420
nm from the side of the light transmissive layer 5 formed on the
surface of the optical disk. Moreover, in this optical disk, when
the successive pits are observed from the side of the light
transmissive layer irradiated by reproducing laser light, the
successive pits contain pits having length and width ranging from
80 nm to 250 nm, and the reflection film has the film thickness
selected to be 20 nm or less, e.g. greater than 8 nm.
Inventors: |
Takeda, Minoru; (Tokyo,
JP) ; Furuki, Motohiro; (Tokyo, JP) |
Correspondence
Address: |
SONNENSCHEIN NATH & ROSENTHAL
P.O. BOX 061080
WACKER DRIVE STATION
CHICAGO
IL
60606-1080
US
|
Family ID: |
14718303 |
Appl. No.: |
10/281353 |
Filed: |
October 28, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10281353 |
Oct 28, 2002 |
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09720742 |
Apr 2, 2001 |
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6512735 |
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09720742 |
Apr 2, 2001 |
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PCT/JP00/02708 |
Apr 25, 2000 |
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Current U.S.
Class: |
369/275.4 ;
G9B/7.029; G9B/7.039; G9B/7.139; G9B/7.19; G9B/7.194 |
Current CPC
Class: |
G11B 7/243 20130101;
G11B 7/263 20130101; G11B 7/007 20130101; G11B 7/2585 20130101;
G11B 7/2534 20130101; G11B 7/2542 20130101; G11B 7/24 20130101;
G11B 7/261 20130101; G11B 7/24085 20130101; G11B 7/259 20130101;
G11B 7/2595 20130101; G11B 7/258 20130101; G11B 7/26 20130101 |
Class at
Publication: |
369/275.4 |
International
Class: |
G11B 007/24 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 26, 1999 |
JP |
11/117706 |
Claims
1. An optical disk comprising an optical disk substrate in which
successive pits corresponding to a recording signal are formed, a
reflection film formed in said optical disk substrate on its
surface that said successive pits are formed and a light
transmissive layer formed on said reflection film, wherein a signal
recorded as said successive pits is read out by irradiating of
laser light having a wavelength ranging from 350 nm to 420 nm,
characterized in that when the successive pits are observed from
the side of said light transmissive layer, said successive pits
contain pits having length and width ranging from 80 nm to 250 nm
and said reflection film has a film thickness selected to be 20 nm
or less.
2. An optical disk as claimed in claim 1, comprising a signal
recording film such as a phase-change film between the reflection
film and the light transmissive layer.
3. An optical disk as claimed in claim 2, wherein the reflection
film and/or signal recording film is formed as two or more
layers.
4. An optical disk as claimed in claim 1, wherein said reflection
film is made of one or more kinds of materials of aluminum, silver
and gold or two or more kinds of alloy materials thereof.
5. An optical disk as claimed in claim 2, wherein said reflection
film is made of one or more kinds of materials of aluminum, silver
and gold or two or more kinds of alloy materials thereof.
6. An optical disk as claimed in claim 3, wherein said reflection
film is made of one or more kinds of materials of aluminum, silver
and gold or two or more kinds of alloy materials thereof.
7. An optical disk as claimed in claim 1, wherein said reflection
film has a reflectance selected to be 15% or greater.
8. An optical disk as claimed in claim 2, wherein said reflection
film has a reflectance selected to be 15% or greater.
9. An optical disk as claimed in claim 3, wherein said reflection
film has a reflectance selected to be 15% or greater.
10. An optical disk as claimed in claim 4, wherein said reflection
film has a reflectance selected to be 15% or greater.
11. An optical disk as claimed in claim 5, wherein said reflection
film has a reflectance selected to be 15% or greater.
12. An optical disk as claimed in claim 6, wherein said reflection
film has a reflectance selected to be 15% or greater.
13. An optical disk manufacturing method comprising the steps of:
producing an optical disk manufacturing master disk for forming
successive pits by exposure corresponding to a recording signal
using laser light having a wavelength ranging from 200 nm to 370
nm; producing an optical disk substrate in which successive pits
containing pits having length and width both ranging from 80 nm to
250 nm are formed by transferring said successive pits of said
master disk; and forming a reflection film having a film thickness
of 20 nm or less in said optical disk substrate on its surface that
said successive pits are formed.
14. An optical disk manufacturing method as claimed in claim 13,
wherein said reflection film is made of one or more kinds of
materials of aluminum, silver and gold or alloy materials
containing these materials.
15. An optical disk manufacturing method as claimed in claim 13,
wherein said reflection film has a reflectance selected to be 15%
or greater.
Description
TECHNICAL FIELD
[0001] The present invention relates to an optical disk which can
increase its recording density and a manufacturing method
thereof.
BACKGROUND ART
[0002] A conventional optical disk, e.g. DVD (Digital Versatile
Disc) is formed such that, as its cross section is schematically
shown in FIG. 6, a reflection film 102 having a thickness of
several tens nanometers, e.g. thickness of 50 nm is formed on a
light transmissive disk substrate 101 having a signal recording
portion 100 on which successive pits are formed and a protection
layer 103 made of an organic material having a thickness of, e.g.
about 10 .mu.m, covers the surface of the reflection film.
[0003] To read out a signal from this DVD, reproducing laser light
105 is irradiated on the signal recording portion through an
objective lens 104 from the side of the light transmissive disk
substrate 101, and pits in the signal recording portion 100, is
detected i.e. recorded data is read out by returned light of the
reproducing laser light.
[0004] In the case of ordinary DVD, since the disk substrate has a
thickness of 0.6 mm and a signal is reproduced through this disk
substrate 101, the numerical aperture N. A. of an objective lens
that a reproducing pickup includes is restricted to about 0.6.
[0005] By the way, the size of a spot of reproducing light is in
proportion to a ratio .lambda./N. A. between a wavelength (of the
reproducing laser light 105 and an N. A. of the objective lens 104.
In the conventional ordinary DVD, the wavelength of the reproducing
light is 650 nm; the N. A. is 0.6; and one side of the disk has a
recording capacity of 4.7 GB.
[0006] For example, consider an optical disk which is reproduced by
reproducing laser light having a wavelength k of 400 nm through an
objective lens having a numerical aperture N. A. of 0.85. Then, the
recording capacity of one side of this optical disk can simply be
calculated to be 25 GB based on the proportion of this disk to the
above-mentioned conventional DVD.
[0007] However, the recording capacity thus calculated is obtained
in consideration of only the characteristics of the reproducing
pickup. In actual practice, the pit size of the optical disk should
also be made minute and made with high accuracy.
[0008] An ordinary optical disk manufacturing method is as follows.
As shown in FIG. 7, on a glass disk 106 having a diameter of about
200 mm and a thickness of several millimeters and whose surface was
precisely polished, there is formed a photoresist layer 107 having
a film thickness of about 0.1 .mu.m, in which a photoresist
sufficiently sensitive to a wavelength of a recording laser light
source 107 of a laser cutting apparatus is uniformly
spin-coated.
[0009] This photoresist layer 107 is subjected to exposure. When
this exposure is carried out, there is employed a pattern exposure
in which laser light 109 having a wavelength of 413 nm from a
recording laser light source 108 made of, e,g, a Kr laser, is
on/off-modulated by an acousto-optic modulator, i.e. AOM 110 in
response to a recording signal and focused to irradiate on the
photoresist layer 107 through an expander 111 and an objective lens
112, making this laser light spot scan the photoresist layer 107 in
a spiral fashion to form latent images of pits and grooves.
[0010] Thereafter, when this photoresist layer 107 is developed by
an alkaline developer, the exposed portion is dissolved, and there
is formed, as shown in FIG. 8, a master disk 121 in which an uneven
pattern 120 comprising pits and grooves is formed on the
photoresist layer 107 coated on the disk 106.
[0011] Then, on this uneven pattern 120 of this master disk 121, as
shown in FIG. 8, there is deposited a metal layer 122 having a
thickness of about 300 (m by sequentially effecting nonelectrolytic
plating and electroplating of nickel (Ni) in such a manner as to
fill this uneven pattern. Thereafter, this metal layer 122 is
peeled from the master disk 121, and a stamper 123 having an
inverted version of the uneven pattern 120 of the master disk 121
is obtained from the metal layer 122 thus peeled.
[0012] This stamper 123 is disposed within, e,g, an injection
molding die for injection molding to produce the optical disk
substrate 101 made of polycarbonate (PC) or the like, as shown in
FIG. 9.
[0013] On this optical disk substrate 101, there are transferred
uneven patterns of the stamper 123, i.e. there are formed pits and
grooves corresponding to the uneven pattern of the master disk,
thus causing the signal recording portion 100 shown in FIG. 6 to be
formed.
[0014] In this optical disk substrate 101 on its surface that the
signal recording portion 100 is formed, there is deposited, as
shown in FIG. 9, the reflection film 102 shown in FIG. 6 by
sputtering using aluminum (Al) target 124 for example. Further, the
protection film 103 is formed on this reflection film.
[0015] This protection film 103 is generally made of a
ultraviolet-curing resin cured by irradiating ultraviolet rays
after the ultraviolet-curing resin is applied to the reflection
film 102 by spin-coating so as to have a uniform thickness.
[0016] Since the limit of the numerical aperture of the objective
lens 112 is generally about 0.9, the optical disk thus obtained
after the master disk was produced by the pattern exposure effected
by laser light having a wavelength of 413 nm will have successive
pits formed thereon, which have the shortest pit length of 0.4
.mu.m and the track pitch of 0.74 .mu.m. In addition, the width of
the pit, i.e. the length along the radial direction of the disk is
about 0.35 .mu.m which is half of the track pitch.
[0017] Due to such restrictions imposed upon the pit size when it
is made minute and with high precision, the pit size cannot be made
so minute and made with high such precision that an optical disk
having a recording capacity of, e,g, 15 GB or more, in particular
25 GB can be obtained by the conventional pattern exposure using
the laser light having the wavelength of 413 nm.
DISCLOSURE OF INVENTION
[0018] The present invention provides an optical disk and a method
of manufacturing the same which can provide a high-density
recording, i.e. which can provide the aforementioned recording
capacity of, e.g. 15 GB or higher recording capacity of, e.g. 25
GB.
[0019] An optical disk according to the present invention may
comprises an optical disk substrate in which successive pits
corresponding to a recording signal are formed, a reflection film
formed in this optical disk substrate on its surface that the
successive pits are formed and a light transmissive layer formed on
this reflection film.
[0020] When a recorded signal is read out, i.e. reproduced from
this optical disk, a signal recorded as the successive pits is read
out from the optical disk by irradiating short-wavelength laser
light having a wavelength of 350 nm to 420 nm from the side of the
light transmissive layer formed on the surface of the optical
disk.
[0021] Moreover, when this optical disk is seen from the side of
the light transmissive layer irradiated by reproducing laser light,
the successive pits contain pits having length and width ranging
from 80 nm to 250 nm, and the thickness of the reflection film is
selected to be 20 nm or less, e.g. 8 nm greater.
[0022] The optical disk manufacturing method according to the
present invention is the manufacturing method of producing the
above described optical disk according to the present invention,
comprising the steps of producing an optical disk manufacturing
master disk for forming successive pits using laser light having a
wavelength ranging from 200 nm to 370 nm for exposure in response
to a recording signal, producing an optical disk substrate in which
the successive pits containing pits having length and width both
ranging from 80 nm to 250 nm are formed by transferring the
successive pits of this master disk and forming a reflection film
having a film thickness of 20 nm or less in this optical disk
substrate an its surface that the successive pits are formed.
BRIEF DESCRIPTION OF DRAWINGS
[0023] FIG. 1 is a schematic cross-sectional view of an example of
an optical disk according to the present invention.
[0024] FIG. 2 is an enlarged cross-sectional view of pits of an
optical disk according to the present invention.
[0025] FIG. 3 is a diagram showing measured results of values of
bottom jitter of a reproduced signal obtained when a film thickness
of an Al reflection film is varied as a parameter.
[0026] FIG. 4 is a diagram of arrangement of an example of a laser
cutting apparatus.
[0027] FIG. 5 is a diagram of optical paths showing an auto focus
optical system of an example of a laser cutting apparatus.
[0028] FIG. 6 is a cross-sectional view of an optical disk
according to the prior art.
[0029] FIG. 7 is a diagram of arrangement of a laser cutting
apparatus for producing a master disk to manufacture an optical
disk according to the prior art.
[0030] FIG. 8 is a diagram for explaining the manner in which a
stamper is produced from the optical disk manufacturing master
disk.
[0031] FIG. 9 is diagram for explaining a method of manufacturing
an optical disk.
BEST MODE FOR CARRYING OUT THE INVENTION
[0032] An optical disk 1 according to the present invention
comprises, as a schematic cross-sectional view of its example is
shown in FIG. 1, an optical disk substrate 3 having a thickness of
1.1 mm for example, in which successive pits containing pits 2
corresponding to a recording signal are formed, a reflection film 4
formed in this optical disk substrate 3 on its surface that the
pits 2 are formed and a light transmissive layer 5 formed on this
reflection film 4.
[0033] When a recorded signal is read out, i.e. reproduced from
this optical disk 1, a signal recorded as the successive pits is
read out from the optical disk by irradiating short-wavelength
laser light having a wavelength of 350 nm to 420 nm from the side
of the light transmissive layer 5 formed on the surface of the
optical disk.
[0034] Moreover, when this optical disk is seen from the side of
the light transmissive layer 5 irradiated by reproducing laser
light, the successive pits contain the pits 2 having length and
width ranging from 80 nm to 250 nm. The reflection film 4 is made
of one or more kinds of materials of aluminum (Al), silver (Ag) and
gold (Au) or two or more kinds of alloy materials of them. The
thickness of the reflection film is selected to be 20 nm or less,
and a reflectance of the reflection film is selected to be 15% or
greater.
[0035] If a thickness t of the light transmissive layer 5 is
selected between 10 .mu.m to 177 .mu.m, e.g. 100 .mu.m (0.1 mm),
then a pickup which uses as reproducing laser light
short-wavelength laser light, e.g. violet laser light by a
GaN-based laser having a short wavelength, e.g. a wavelength of 350
nm to 420 nm as and which also uses an objective lens having a high
numerical aperture, e.g. 0.85 will be able to ensure an allowance
of disk skew, i.e. a so-called skew margin.
[0036] In other words, according to the optical disk of the-present
invention, because the optical disk is arranged so that a signal is
not read out by irradiating reproducing laser light from the side
of the thick optical disk substrate having the thickness of 0.6 mm
for example as in the past, but a signal is read out by irradiating
reproducing laser light from the side of the light transmissive
layer 5 having a considerably thin thickness of 0.1 mm for example,
it is possible to use an objective lens having a numerical aperture
of 0.85 for example, and a laser spot can be reduced in size, which
in turn increases the recording density of an optical disk.
[0037] Unfortunately, if the pits are made minute as described
above, then when the same reflection film is formed as in the
ordinary conventional manner, a signal of good quality cannot be
reproduced from an optical disk.
[0038] The reason for this is as follows. If an EFM (Eight to
Fourteen Modulation) signal is recorded as successive pits having a
shortest pit length of 220 mm and a track pitch of 410 nm and
having about 15 GB, as the recording capacity in one side of the
optical disk, then when a conventional Al reflection film having a
thickness of about 30 nm is formed, the insides of the pits are
filled with this reflection film by an amount corresponding to the
thickness of this reflection film so that the pit size when the
pits are seen from the side of the light transmissive layer 5
cannot be set at the above-mentioned target size.
[0039] As shown in FIG. 2, for example, if the reflection film 4
has a cross-section with a tapered angle .theta. relative to the
major surface 3a of the optical disk substrate 3 on which the pit 2
is formed and this reflection film 4 having a uniform thickness T
is formed on the wall surface, the bottom surface and the major
surface of the pit 2 by sputtering, for example, then an effective
length B of the pit when the pits are observed from the side of the
light transmissive layer 5 irradiated by reproducing laser light
after the reflection film was formed can be calculated based on the
film thickness T of the reflection film 4 and the length A of the
bottom surface of the pit as:
B=A-2.multidot.T.multidot.tan(.theta./2)
[0040] The tapered angle .theta. generally falls within a range of
approximately 40.degree. to 80.degree.. Moreover, the length A of
the bottom surface of the pit is considerably small in the shortest
pit due to the tapered angle .theta.. Thus, if the depth of the
pit, for example, is assumed to be 90 nm and the tapered angle
.theta. is assumed to be 60.degree., then the above-mentioned
length will be about 120 nm in the track direction and about 100 nm
in the radial direction of the disk.
[0041] Accordingly, if the reflection film has a film thickness of
30 nm and over, then the value of the effective pit size B will be
85 nm in the track direction and 65 nm in the radial direction of
the disk. Thus, the value of the effective pit size will
unavoidably decrease to approximately 1/3 of the above-mentioned
proper pit size.
[0042] However, the longest pit having the length about 3.7 times
as long as the shortest pit length can also cause a similar pit
reduction effect. In this case, the reduction ratio of the pit
length in the track direction will be about 75% relative to the
proper pit length. If the pit length deviates from the proper size
and the imbalance of the shortest and longest pit lengths occurs,
then a reproduced signal is affected by such deviation and
imbalance so that jitters will greatly increase.
[0043] In contrast, according to the above-mentioned optical disk
of the present invention, the jitters can be prevented from
increasing by making the thickness of the reflection film 4 equal
to or less than 20 nm.
[0044] Thus, in the optical disk according to the present invention
can avoid the disadvantage that the successive pits each pit being
of very small size equal to or less than 250 nm are filled with the
reflection film 4 and so the reproduced signal is deteriorated,
when recorded data is reproduced from the optical disk by
irradiating reproducing laser light from the side of the reflection
film 4 formed on the signal pits.
[0045] FIG. 3 shows measured results of values of bottom jitter of
a reproduced signal obtained when the thickness of the Al
reflection film as parameter is varied to 15 nm, 20 nm and 30 nm
respectively, using the optical disk in which the successive pits
by the EFM signal equivalent to the actual recording density of 15
GB are formed.
[0046] In this case, the optical disk had such a structure that a
signal is read out of the optical disk by irradiating laser light
from the side of the light transmissive layer 5, and the film
thickness of the light transmissive layer 5 was selected to be 100
(m. In this case, although the reproducing optical system used the
wavelength of 532 nm, the N. A. was selected to be 0.94. Moreover,
the horizontal axis in FIG. 3 represents the asymmetry of the
reproduced signal, and the vertical axis thereof represents the
value of jitter.
[0047] As is evident from FIG. 3, when the film thickness of the Al
reflection film is 30 nm as in the prior-art optical disk, the
value of the bottom jitter increases approximately up to 10%, which
makes the quality of a signal unsatisfactory. However, when the
film thickness is made 20 nm or less, the value of jitter becomes
approximately 8%. When the film thickness is decreased down to 15
nm, a satisfactory value of jitter on a 6% level can be
achieved.
[0048] However, if the film thickness is simply decreased
progressively in order to prevent the shortest pit from being
filled with the reflection film, then the reflectance of the
optical disk substrate 3 is lowered with the result that the S/N of
reproduced signal will deteriorate. From this point of view, the
film thickness should preferably be selected to be 8 nm or
greater.
[0049] The table 1 shows the dependence of the reflectance of
reading laser light (wavelength is 407 nm) at the Al reflection
film surface on the film thickness of the Al reflection film.
1 TABLE 1 Film thickness (nm) 40 30 20 15 8 5 of Al reflection film
Reflectance (%) 88 82 67 43 15 8
[0050] From the above description, it can be seen that the optical
disk having a high-recording capacity of 15 GB or greater in which
the film thickness falls within the range of 8 nm to 20 nm and the
reflectance is made 15% or greater is able to provide a reproduced
signal of good quality.
[0051] Moreover, as described above, the reflection film 4 of the
optical disk according to the present invention can be made of, in
addition to Al which is widely utilized, metal materials which have
a high reflectance at a thin film thickness, such as Au (gold) and
Ag (silver), or alloy materials of two or more kinds of these
metals, or metal (alloy) materials in which Ti (titanium) and the
like is added to these respective materials.
[0052] Furthermore, the optical disk according to the present
invention can be formed as the so-called data repeatedly recordable
type of optical disk by arranging a signal recording film such as a
phase-change film made of, e,g, GeSb, Te or the like, between the
reflection film 4 and the light transmissive layer 5.
[0053] Further, the optical disk according to the present invention
can be modified to as an optical disk having the so-called
multilayer structure by forming two or more layers of both the
reflection film 4 and the signal recording film, or two or more
layers of only the signal recording film of the present
invention.
[0054] For example, by laminating the signal recording films each
having the successive pits through reflection films having a
required reflectance, an optical disk can be formed, in which
recorded signals are reproduced from the respective signal
recording films by a suitable method such as focusing reproducing
laser light on the respective signal recording films in
reproduction.
[0055] A method of manufacturing an optical disk according to the
present invention will be described next. This manufacturing method
is to obtain the optical disk according to the present invention
described above, including successive pits which contain pits
having length and width both ranging from 80 nm to 250 nm.
[0056] In the optical disk manufacturing method according to the
present invention, the optical disk is manufactured by the process
of producing a master disk to manufacture an optical disk in which
successive pits are formed by exposure corresponding to a recording
signal using laser light having a wavelength ranging from 200 nm to
370 nm, producing an optical disk substrate having successive pits
containing pits having length and width both ranging from 80 nm to
250 nm by transferring successive pits of this master disk and
forming a reflection film having a film thickness of 20 nm or less
in this optical disk substrate on its surface that the successive
pits are formed.
[0057] The exposure process to produce the master disk in the
manufacturing method according to the present invention, takes
place using the so-called laser cutting apparatus. An example of
this laser cutting apparatus will be described below with reference
to a schematic diagram of FIG. 4.
[0058] Although this laser cutting apparatus uses short-wavelength
recording laser light, its fundamental arrangement can be based on
the ordinary conventional laser cutting apparatus.
[0059] This apparatus is provided with a recording laser light
source 20 which can generate laser light having a wavelength of 266
nm for example. This recording laser light source 20 comprises a
solid-state laser 21, a phase modulator 22, an external resonator
23 and an anamorphic optical system 24.
[0060] The solid-state laser 21 comprises a YAG (yttrium aluminum
garnet) laser (wavelength is 1064 nm), for example, and an SHG
(Secondary Harmonic Generator) for generating laser light having a
wavelength of 532 nm by converting the laser light from the
above-mentioned laser to that of double-wave. Then, laser light
from this solid-state laser 21 is introduced through the phase
modulator 22 to the external resonator 23. This external resonator
23 includes a wavelength converting optical crystal 25 made of,
e,g, BBO (.beta.-BaB.sub.2O.sub.4) crystal having a sufficiently
high light transmissivity up to the far-ultraviolet ray region for
further converting the above laser light into laser light of
double-wave having a wavelength of 266 nm as well as an optical
resonator forming a predetermined resonator length by mirrors
M.sub.1 to M.sub.4, for example. As illustrated, the mirrors
M.sub.1 and M.sub.2 are formed of mirrors having necessary
reflectance and transmissivity. The mirrors M.sub.3 and M.sub.4 are
formed of mirrors having a reflectance of, e,g, nearly 100%.
Moreover, one mirror, e.g. mirror M.sub.3 can be moved and adjusted
by an electromagnetic actuator 26 having a so-called VCM (Voice
Coil Motor) structure for example, thereby allowing the resonator
length to be controlled. Then, light passing through the mirror
M.sub.1, for example, is detected from this resonator by a
photodetector 27 such as a photodiode PD, and the actuator 26 is
controlled by the output of this photodiode. Thus, servo-control is
effected so as to provide an optimum resonator length, i.e.
resonance wavelength, and laser light having a wavelength of 266 nm
based on high output and stable continuous wave oscillation
wavelength can be obtained. Then, laser light derived from the
external resonator 26 is reshaped on its beam shape by the
anamorphic optical system 24. In this manner, high-output and
stable continuous oscillation laser light 50 of several tens of
milliwatts having a wavelength of 266 nm can be derived from the
recording laser light source 20.
[0061] Then, the laser light 50 obtained from this recording laser
light source 20 is split by a beam splitter 28, for example. One
part of the laser light is supplied to a photodetector 29 such as a
photodiode, in which the power or the like of the laser light 50 is
monitored.
[0062] The other part of the laser light, which is split by the
beam splitter 28, is focused by a condenser lens 30 and then
introduced into a modulator 31 such as the AOM where it is
modulated in response to the recording signal. The laser light thus
modulated is introduced through a collimator lens 32 and a beam
splitter 33 into a beam expander 36 comprising lenses 34 and 35,
and expanded by this beam expander 36 and introduced into an
objective lens 37 as a beam spot of a diameter several times as
large as its entrance pupil diameter. Reference numeral 40 denotes
a mirror for directing the laser light from the beam expander 36
toward an objective lens 37.
[0063] The laser light thus converged by the objective lens 37 is
irradiated on a resist disk 39, which is installed onto a turn
table 38 by an air spindle rotating with high accuracy, for
obtaining an optical disk manufacturing master disk.
[0064] This resist disk 39 rotates about the center axis as the
turn table 38 rotates. This resist disk 39 has such a structure
that a photoresist layer photosensitive to the wavelength of the
laser light 50 is previously coated on a substrate forming the
master disk, e.g. a glass disk.
[0065] Then, the laser light 50 which is turned on/off in response
to the recording signal by the above-mentioned modulator 31, i.e.
exposure laser light is irradiated on the photoresist layer of this
resist disk 39 with a spot size of 0.3 .mu.m or less.
[0066] On the other hand, the laser cutting apparatus is provided
with a moving optical table 41 which is movable in the direction
along the radial direction of the turn table 38. On this moving
optical table, there is mounted, e.g. the beam expander 36 and an
auto focus optical system to be described below although not
shown.
[0067] In this manner, as this moving optical table 41 is moved and
the turn table 38 is rotated, exposure laser light scans the
photoresist layer of the resist disk 39 in a spiral fashion or an
annular fashion for example.
[0068] On the other hand, a part of laser light which passes
through the above-mentioned collimator lens 32, and is split by the
beam splitter 33 is detected by a photodetector 42 such as a
photodiode and thus the modulated laser light is monitored.
[0069] Returned light of exposure laser light from the disk 39
passes through the beam splitter 33 and extended optical path by
mirrors 43, 44, 45 or the like for example and is converged by a
condenser lens 46, thereby causing exposure laser light to be
monitored, e.g. by a CCD (Charge Coupled Device) type of monitor
camera 47 for monitoring exposure laser light.
[0070] Then, the objective lens 37 is arranged to constantly be
focused on the photoresist layer of the resist disk 39 under the
control of focusing servo.
[0071] An optical system of an auto focus servo means for executing
this focusing is located on the aforementioned moving optical table
41. FIG. 5 shows a schematic arrangement of an example of the
optical system of this auto focus servo means. The objective lens
37 is supported in such a manner that it may very slightly be moved
in the optical axis direction by an actuator 60 having a VCM
structure for example.
[0072] In this case, the optical system comprises an auto focus
laser light source 61, optical lenses 62, 63, mirrors 64, 65 and a
position detection device (PSD) 66.
[0073] The laser light source 61 can be comprised of a
semiconductor laser having a wavelength of 680 nm, to which a high
frequency superposition of a frequency 400 MHz and a pulse duty 50%
is applied.
[0074] Laser light 67 from this laser light source 61 is made
skewed relative to the optical axes of the optical systems of the
lenses 62, 63 and irradiated on the resist disk 39 through the
objective lens 37. Returned light of such laser light is detected
by a position detection device (PSD) 66 through the mirror 65, and
the actuator 60 is controlled by the detected output so as to move
the objective lens 37 in its optical axis direction for focusing
control.
[0075] Since the optical system of the focusing servo means thus
arranged does not use a polarized beam splitter PBS and a polarized
optical system such as a quarter-wave plate QWP or the like as in
the conventional ordinary focusing servo, the above optical system
is not limited by the numerical apertures of these optical elements
and therefore the skew angle of the laser light incident on the
objective lens 37 can be increased sufficiently. Specifically, a
large opening angle can be formed between outgoing laser light 67a
incident on the objective lens 37 from the laser light source 61
and returned light from the focusing surface of the resist disk 39
after passing through the objective lens 37, i.e. incoming laser
light 67b, whereby the outgoing laser light 67a and the incoming
laser light 67b can be separated from each other completely and the
focusing state can be detected reliably by the position detection
device 66, thus allowing a focusing servo signal to be obtained
without fail.
[0076] By making the optical system, as it were, a non-polarized
auto focus optical system, the skew angle of the laser light 67a
incident on the objective lens 37 can be increased as much as
possible, and the value of the height of laser light incidence on
the objective lens can also be increased sufficiently. Accordingly,
an optical gain expressed by an equation proportional to the
above-described height of laser light incidence on the objective
lens can also be increased remarkably as compared with the
conventional auto focus optical system, which can largely
contribute to improvements of the servo characteristics of the auto
focus optical system.
[0077] Specifically, on the position detection device in the auto
focus optical system, there exists, in addition to original
exposure laser light to be detected, which has returned through the
objective lens after being reflected on the surface of the exposure
photoresist layer, slightly expanded laser light (hereinafter
referred to as noise laser light) which has not reached the surface
of the photoresist layer, but is reflected on the rear surface of
the objective lens, i.e. on the surface of the opposite side of the
surface of the objective lens facing the photoresist layer. This
noise laser light exerts a bad influence upon the operation of the
auto focus servo as a background noise component of the detected
output of the position detection device.
[0078] Then, when this noise laser light interferes with original
returned light, which should originally be detected, from the
photoresist layer to cause interference fringes, the servo
characteristics are greatly deteriorated and so the occurrence of
such interference fringes has a serious effect. In general, since
laser light upon which the high frequency is not superposed has a
coherence length of several tens of centimeters, an optical path
difference between returned light, which should originally be
detected, from the photoresist layer and noise laser light caused
by reflected light from the rear surface of the objective lens
nearly falls within this range. Therefore, it is unavoidable that
the interference fringes occur in the position detection
device.
[0079] This interference fringes move frequency on the position
detection device as the objective lens moves very slightly on the
optical axis of the objective lens, thereby making the position
detection signal of the original returned laser light inaccurate.
In actual practice, if the auto focus servo is operated under the
condition that the interference fringes occur, then the servo will
oscillate frequently. As a result, it is difficult to maintain a
normal auto focus operation.
[0080] In contrast, when the above-mentioned laser light source 61
upon which the high frequency of 400 MHz is superposed is used,
because the coherence length is decreased sufficiently, it is
possible to prevent the original returned (incoming) laser light
67b and the noise laser light caused by the reflected light from
the rear surface of the objective lens from interfering with each
other, thus enabling, the occurrence of the interference fringes to
be avoided. In other words, because only the laser light 67b that
should originally be detected is projected onto the position
detection device 66, the spot position of cutting laser light
relative to the photoresist layer can be detected with accuracy.
Actually, in the case of the above-mentioned arrangement, it was
confirmed that the auto focus servo hardly oscillates and so the
normal auto focus servo operation can be maintained.
[0081] The laser cutting apparatus using the above auto focus
optical system described with reference to FIG. 5 can realize the
extremely stable and highly-accurate auto focus servo operation.
Thus, this laser cutting apparatus can constantly and stably
execute the cutting of the high-recording density optical disk with
high productivity.
[0082] Therefore, it is possible to produce an optical disk
manufacturing master disk for obtaining an optical disk substrate
having successive pits with a recording density of 15 GB by this
laser cutting apparatus.
[0083] An example of the method of producing this master disk using
the above-mentioned laser cutting apparatus will be described below
in detail.
[0084] Initially, there is prepared a glass disk which serves as a
substrate to produce a master disk, having a diameter of about 200
nm and a thickness of several millimeters and whose surface is
polished with high precision. There is then prepared the resist
disk 39 in which the photoresist layer made by spin-coating the
photoresist uniformly a in thickness of about 0.1 .mu.m sensitive
to laser light with a wavelength of far-ultraviolet ray region
wavelength: 266 nm) of the above-mentioned recording laser light 50
is formed on the surface polished with high precision.
[0085] Next, by the laser cutting apparatus described with
reference to FIGS. 4 and 5, the recording laser light 50 is focused
on the resist disk 39 by the objective lens 37 having a high N. A
of about 0.9 as a spot of 0.3 .mu.m or less in size. In this case,
the laser light 50 scans the resist disk 39 in the spiral fashion
or in the annular fashion as mentioned before while turning on/off
the laser luminous flux in response to the recording signal by the
AOM modulator 31 for example, thereby forming the latent images of
the uneven pattern of the successive pits containing pits in which
its length in the track direction and its width in the disk radial
direction are both in the range of 80 nm to 250 nm (exposure
process). The track pitch of the successive pits is selected
between 150 nm and 450 nm.
[0086] If the resist disk 39 on which the latent images of the pits
or the groove-like pattern are formed as described above is
immersed in an alkaline developer and the exposed portion of the
photoresist is dissolved, then the uneven patterns of the
successive pits containing pits in which its length in the track
direction and its width in the disk radial direction are both in
the range of 80 nm to 250 nm can be obtained on the resist disk 39
(development process).
[0087] In this way, there is produced the optical disk
manufacturing master disk on which the uneven pattern in accordance
with the pattern of the photoresist layer is formed.
[0088] Then, an Ni (nickel) thin film having a film thickness of
several hundreds angstroms is deposited on this master disk by
sputtering or nonelectrolytic plating. The metal layer is then
formed on this thin film serving as a conductive film in
electroplating, and this metal layer is peeled in the same way as
described with reference to FIG. 8, thus making in an Ni stamper
having a thickness of about 300 .mu.m to be produced. The rear
surface of this Ni stamper is then polished and the end face
thereof is worked (stamper manufacturing process).
[0089] Next, this Ni stamper is disposed within a molding die and
the injection molding of, e.g. polycarbonate (PC) or the like is
performed. Thus, the optical disk substrate 3 having the diameter
of 120 mm for example, made of a plastic material shown in FIG. 1
is produced as a replica of the Ni stamper.
[0090] Onto the signal recording portion of the optical disk
substrate 3 thus produced, there is transferred the uneven pattern
based on the successive pits and grooves recorded by the
above-mentioned cutting and containing the pits in which its length
in the track direction and its width in the disk radial direction
both fall within the range of 80 nm to 250 nm (transfer
process).
[0091] Subsequently, by the sputtering apparatus, an Al reflection
film 4 having a film thickness of 20 nm or less, e.g. 15 nm is
formed on the surface of the signal recording portion side where
the pits or the groove-like pattern of the optical disk substrate 3
are formed reflection film forming process).
[0092] Further, on this metal reflection film 4, there is cured and
formed the light transmissive layer 5 having the thickness of about
0.1 mm by spin-coating of ultraviolet-curing resin and irradiation
of ultraviolet rays (light transmissive layer forming process). In
this manner, there is finished the optical disk 1 according to the
present invention shown in FIG. 1.
[0093] It is desirable that the spot diameter of the reproducing
laser light 6 of the high-recording density optical disk 3
according to the present invention manufactured by the
above-mentioned manufacturing method according to the present
invention is selected in the range of 200 nm to 500 nm.
[0094] Incidentally, concrete shapes and structures of the
respective portions shown in the above-mentioned embodiment
illustrate only an example of modes for carrying out the invention.
It will be appreciated that the technical scope of the present
invention should not be limitedly interpreted from these concrete
shapes and structures.
[0095] As described above, the optical disk according to the
present invention comprises the optical disk substrate in which the
successive pits corresponding to the recording signal are formed,
the reflection film formed in the optical disk substrate on its
surface that the successive pits are formed and the light
transmissive layer formed on the reflection film, wherein the
signal recorded as the successive pits is read out of the optical
disk by irradiating the laser light from the side of the light
transmissive layer. When the successive pits are observed from the
side of the transmissive layer, the successive pits contain the
pits having length and width both ranging from 80 nm to 250 nm and
the film thickness of the reflection film is selected to be 20 nm
or less. Thus, even when the successive pits of very small size 250
nm or less are cut, the pits can be prevented from being filled
with the reflection film so that the reproduced signal will not
deteriorate. Therefore, it is possible to obtain the high-recording
density optical disk of excellent quality.
[0096] Moreover, since the reflection film is made of one or more
kinds of materials of aluminum, silver, gold or alloy materials
containing these materials, by using an optimum material as the
material of the reflection film for reflecting the laser light, it
is possible to obtain the satisfactory reflection characteristics
as the reflection film of the high-recording density optical
disk.
[0097] Furthermore, since the reflectance of the reflection film is
selected to be 15% or greater, it is possible to reliably read out
recorded information from the successive pits.
[0098] Moreover, in the optical disk manufacturing method for
manufacturing the optical disk by transferring the successive pits
formed on the master disk by exposure in response to the recording
signal onto the optical disk substrate, because the optical disk
manufacturing method according to the present invention comprises
the steps of exposing by the laser light having the wavelength of
200 nm or more, to form the successive pits containing the pits
having length and width both ranging from 80 nm to 250 nm
transferring the successive pits formed on the master disk onto the
optical disk substrate and forming the reflection film having the
film thickness of 20 nm or less in the optical disk substrate on
its surface that the successive pits are transferred, even when the
successive pits of very small size less than 250 nm or less are
cut, the pits can be prevented from being filled with the
reflection film so that the reproduced signal will not deteriorate.
Thus, it is possible to manufacture the high-recording density
optical disk of good quality.
[0099] Moreover, since the reflection film is made of one or more
kinds of materials of aluminum, silver, gold or alloy materials
containing these materials and the optimum material can be used as
the material of the reflection film for reflecting the laser light,
it is possible to manufacture the high-recording density optical
disk in which the reflection film has the satisfactory reflection
characteristics.
[0100] Furthermore, since the reflectance of the reflection film is
selected to be of 15% or higher, it is possible to manufacture the
high-recording density optical disk in which the recorded
information can be reliably read out from the successive pits.
* * * * *