U.S. patent application number 13/187126 was filed with the patent office on 2011-11-10 for method for manufacturing optical disc master and method for manufacturing optical disc.
This patent application is currently assigned to SONY CORPORATION. Invention is credited to KATSUHISA ARATANI, AKIRA KOUCHIYAMA.
Application Number | 20110274895 13/187126 |
Document ID | / |
Family ID | 32089286 |
Filed Date | 2011-11-10 |
United States Patent
Application |
20110274895 |
Kind Code |
A1 |
KOUCHIYAMA; AKIRA ; et
al. |
November 10, 2011 |
METHOD FOR MANUFACTURING OPTICAL DISC MASTER AND METHOD FOR
MANUFACTURING OPTICAL DISC
Abstract
An optical disc master having a resist layer composed of a
resist material including an incomplete oxide of a transition metal
on a substrate, the oxygen content of the incomplete oxide being
smaller than the oxygen content of the stoichiometric composition
corresponding to a valence of the transition metal, wherein, (a)
the resist layer selectively exposed, according to a recording
signal pattern, to a light beam with an irradiation power that is
less than an irradiation threshold power at which exposure of the
resist starts, and (b) the resist layer is formed into a
predetermined irregular pattern.
Inventors: |
KOUCHIYAMA; AKIRA;
(Kanagawa, JP) ; ARATANI; KATSUHISA; (Chiba,
JP) |
Assignee: |
SONY CORPORATION
Tokyo
JP
|
Family ID: |
32089286 |
Appl. No.: |
13/187126 |
Filed: |
July 20, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12635314 |
Dec 10, 2009 |
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13187126 |
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10498044 |
Jun 8, 2004 |
7670514 |
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PCT/JP2003/012236 |
Sep 25, 2003 |
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12635314 |
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Current U.S.
Class: |
428/195.1 |
Current CPC
Class: |
Y10S 425/81 20130101;
G11B 7/261 20130101; Y10T 428/24802 20150115 |
Class at
Publication: |
428/195.1 |
International
Class: |
B32B 3/10 20060101
B32B003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 10, 2002 |
JP |
2002-297892 |
Claims
1. An optical disc master, comprising: a resist layer composed of a
resist material including an incomplete oxide of a transition metal
on a substrate, the oxygen content of the incomplete oxide being
smaller than the oxygen content of the stoichiometric composition
corresponding to a valence of the transition metal, wherein, the
resist layer selectively exposed, according to a recording signal
pattern, to a light beam with an irradiation power that is less
than an irradiation threshold power at which exposure of the resist
starts, and the resist layer is formed into a predetermined
irregular pattern.
Description
RELATED APPLICATION DATA
[0001] This application is a division of application Ser. No.
12/635,314, filed Dec. 10, 2009, which is a continuation of
application Ser. No. 10/498,044, filed, Jun. 8, 2004, which is the
United States National stage of PCT/JP2003/12236, filed Sep. 25,
2003, all of which are incorporated herein in their entireties to
the extent permitted by law. Priority is claimed to Japanese Patent
Application JP 2002-297892, filed in the Japanese Patent office on
Oct. 10, 2002.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method for manufacturing
a highly accurate optical disc master, and to a method for
manufacturing an optical disc produced by using the master.
[0003] Recently, recording media that record and store a wide
variety of information have been remarkably developing. In
particular, regarding a compact recording medium, as the recording
system is changed from a magnetic recording medium to an optical
recording medium, the recording capacity has been increasing from
an order of mega bytes (MB) to an order of gigabytes (GB).
[0004] The optical recording medium has changed from Compact
Disc.TM. (CD) to an optical disc in recent years. A read-only
optical disc, i.e., a digital versatile disc read-only memory
(DVD-ROM), being 12 cm in diameter has an information capacity of
4.7 GB on the single side. This disc can record images that
correspond to the recording for two hours in National Television
System Committee (NTSC) color television system.
[0005] However, as information and communication technology and
image processing technology have rapidly developed in recent years,
even the above optical disc requires a several fold recording
capacity relative to the present capacity. For example, a
next-generation optical disc, which is an extension of a digital
video disc being 12 cm in diameter, requires an information
capacity of 25 GB on the single side. This disc can record images
that correspond to the recording for two hours in the digital high
vision system.
[0006] The optical disc is composed of an optically clear
substrate, for example, polycarbonate. Fine irregular patterns such
as pits and grooves that represent information signals are formed
on one main surface on the substrate. A reflecting film, i.e., a
metal thin film composed of, for example, aluminum, is formed on
the fine irregular patterns. Furthermore, a protective film is
formed on the reflecting film.
[0007] In the above recording medium, minimizing the irregular
pattern can increase the recording density, and consequently, can
increase the recording capacity. A process for manufacturing an
optical disc, which relates to the minimizing of the irregular
pattern on the optical disc, will now be described with reference
to FIG. 10.
[0008] A resist layer 91 is uniformly formed on a substrate 90
(FIG. 10 (a)).
[0009] Subsequently, the resist layer 91 is selectively exposed
according to a signal pattern (FIG. 10 (b)). The resist layer 91 is
developed to produce a master 92 having a predetermined irregular
pattern thereon (FIG. 10 (c)). An example of the known method for
producing this master will now be described.
[0010] A glass substrate having a sufficiently smooth surface is
used as the substrate. The substrate is disposed on a rotatable
table. While the glass substrate is rotated at a predetermined
speed, a photosensitive resist, i.e., photo resist (organic resist)
is applied on the substrate. The glass substrate is further rotated
in order to spread the photo resist. Thus, the resist layer is
formed on the whole area by spin coating. Subsequently, the photo
resist is exposed with recording laser such that the photo resist
has a predetermined pattern. Thus, a latent image corresponding to
information signals is formed on the substrate. Then, the substrate
is developed with a developer to remove the exposed areas or the
unexposed areas of the photo resist. In this way, a resist master
is produced. The resist master 92 includes the glass substrate and
the photo resist layer formed thereon and having the predetermined
irregular pattern.
[0011] Then, a metallic nickel film is formed on the irregular
pattern of the resist master 92 by electroforming (FIG. 10 (d)).
The nickel film is lifted off from the resist master 92.
Subsequently, a predetermined process is performed to produce a
molding stamper 93 having the irregular pattern of the resist
master 92 (FIG. 10 (e)).
[0012] Polycarbonate, which is a thermoplastic resin, is molded by
injection molding using the molding stamper 93 to form a resin disc
substrate 94 (FIG. 10 (f)). The stamper is removed (FIG. 10 (g)),
and then a reflecting film 95 composed of an aluminum alloy (FIG.
10 (h)) and a protective film 96 are formed on the irregular
surface of the resin disc substrate 94 to produce an optical disc
(FIG. 10 (i)).
[0013] As described above, in order to produce the fine irregular
pattern on the optical disc, the pattern is reproduced on the
substrate accurately and quickly by the use of the stamper on which
the fine irregular pattern is formed with high precision. In terms
of the precedent process, the precision of the fine irregular
pattern on the optical disc depends on the cutting process, i.e.,
the process in which the resist layer is exposed with laser to form
the latent image.
[0014] For example, according to the above read-only DVD (DVD-ROM)
having the information capacity of 4.7 GB, cut portions are formed
on the stamper such that a pit line (0.4 .mu.m in the minimum pit
length, 0.74 .mu.m in the track pitch) is formed in a spiral shape.
In order to form the cut portions, laser having the wavelength of
413 nm and an objective lens having the numerical aperture NA of
about 0.90 (for example 0.95) are used.
[0015] The minimum pit length P (.mu.m) to be exposed is
represented by following Formula (1):
P=K.lamda./NA (1)
wherein .lamda. (.mu.m) represents a wavelength of the light
source, NA represents a numerical aperture of the objective lens,
and K represents a proportionality constant.
[0016] The wavelength .lamda., of the light source and the
numerical aperture NA of the objective lens depend on the
specification of laser equipment, and the proportionality constant
K depends on the combination of the laser equipment and the resist
master.
[0017] When the optical disc having the information capacity of 4.7
GB is produced, the wavelength is 0.413 .mu.m, the numerical
aperture NA is 0.90, and the minimum pit length is 0.40 .mu.m.
Therefore, according to Formula (1), the proportionality constant K
is 0.87.
[0018] On the other hand, in order to meet the demand for the
optical disc having the information capacity of 25 GB, the minimum
pit length must be decreased to 0.17 .mu.m, and the track pitch
must be decreased to about 0.32 .mu.m.
[0019] In general, shortening the wavelength of the laser is
effective for nanofabrication of the irregular pattern (i.e., the
formation of submicron pits). As described above, in order to meet
the demand for the high-density optical disc having the information
capacity of 25 GB on the single side, the minimum pit length must
be decreased to about 0.17 .mu.m. In this case, if the
proportionality constant K is 0.87 and the numerical aperture NA is
0.95, the light source must include laser equipment having the
wavelength .lamda., of 0.18 .mu.m.
[0020] ArF laser having a wavelength of 193 nm has been developing
so that the laser is used as a light source for semiconductor
lithography for the next-generation. The above wavelength, i.e.,
0.18 .mu.m, is shorter than the wavelength of the ArF laser. An
exposure system that achieves an exposure with such a short
wavelength is very expensive because the exposure system requires
not only the special laser used as the light source, but also
special optical parts such as a special lens. Accordingly, the
above method for achieving nanofabrication, in which the wavelength
.lamda., during exposure is shortened and the numerical aperture NA
of the objective lens is increased in order to increase the optical
resolution, is not extremely suitable for producing inexpensive
devices. The reason is that, as the patterns become fine, the
existing exposure systems cannot be used and more expensive
exposure systems must be introduced instead. Accordingly, even if
the performance of the laser equipment in an exposure system is
improved, the increase of the recording capacity in the optical
disc is limited.
[0021] In a general present exposing step, organic resists such as
novolac resists and chemically amplified resists are exposed with
ultraviolet rays as the light source. The organic resists are
all-purpose and widely used in the photolithographic field.
[0022] Unfortunately, the patterns on the boundaries between the
exposed areas and the unexposed areas are not clear because of the
high molecular weight of the organic resists. Accordingly, in terms
of the precision, the organic resists cannot be used for the
nanofabrication of the optical disc having a high capacity level of
25 GB.
[0023] On the other hand, inorganic resists, in particular,
amorphous inorganic resists provide clear patterns on the
boundaries between the exposed areas and the unexposed areas
because the minimum structure unit of the inorganic resist is an
atomic level. Therefore, the inorganic resists are suitable for the
precise nanofabrication compared with the organic resists. The use
of the inorganic resists is promising to produce the optical disc
having a high capacity. For example, in a known nanofabrication
process, a resist material such as MoO.sub.3 or WO.sub.3 is exposed
with ion beam as the light source (see, for example, Nobuyoshi
Koshida, Kazuyoshi Yoshida, Shinichi Watanuki, Masanori Komuro, and
Nobufumi Atoda: "50-nm Metal Line Fabrication by Focused Ion Beam
and Oxide Resists", Jpn. J. Appl. Phys. Vol. 30 (1991) pp. 3246).
In other known process, a resist material composed of SiO.sub.2 is
exposed with electron beam as the light source (see, for example,
Sucheta M. Gorwadkar, Toshimi Wada, Satoshi Hiraichi, Hiroshi
Hiroshima, Kenichi Ishii, and Masanori Komuro: "SiO.sub.2/c-Si
Bilayer Electron-Beam Resist Process for Nano-Fabrication", Jpn. J.
Appl. Phys. Vol. 35 (1996) pp. 6673). Furthermore, a process has
been studied in which a resist material composed of chalcogenide
glasses is exposed with laser having the wavelength of 476 nm and
532 nm, and a mercury xenon lamp that radiates ultraviolet rays as
the light source (see, for example, S. A. Kostyukevych:
Investigations and modeling of physical processes in inorganic
resists for the use in UV and laser lithography", SPIE Vol. 3424
(1998) pp. 20).
[0024] As described above, when ion beam or electron beam is used
as the light source of the exposure, many kinds of inorganic resist
material can be used in combination. In addition, the fine
convergence of the electron beam or the ion beam allows the
irregular patterns to be minimized. However, an apparatus having
the electron beam or the ion beam as the irradiation source has a
complicated structure and is very expensive. Unfortunately, this
apparatus is not suitable for producing an inexpensive optical
disc.
[0025] In terms of the manufacturing cost, ultraviolet rays or
visible light, that is, light from, for example, laser equipment
installed in the existing exposure system, is preferably used.
However, a limited material of the inorganic resists can be
patterned to form the cut portions using ultraviolet rays or
visible light. Chalcogenide is the only material that can be
patterned using ultraviolet rays or visible light so far. The
materials of the inorganic resists other than chalcogenide transmit
ultraviolet rays or visible light, and barely absorb the light
energy. Accordingly, these materials are not suitable for the
practical use.
[0026] From an economical point of view, the use of the existing
exposure system and chalcogenide is a practical combination.
Unfortunately, chalcogenide includes materials that are harmful to
the human body, for example, Ag.sub.2S.sub.3, Ag--Ag.sub.2S.sub.3,
and Ag.sub.2Se--GeSe. Therefore, in terms of the industrial
production, the use of chalcogenide is difficult.
[0027] As described above, the optical disc having a high recording
capacity cannot be manufactured with the existing exposure system
so far.
[0028] In order to solve the above problems, it is an object of the
present invention to provide a method for manufacturing an optical
disc master and a method for manufacturing an optical disc having a
higher recording capacity. In the method for manufacturing an
optical disc master, expensive irradiation equipment having, for
example, electron beam or ion beam is not used, instead, a safe
resist material suitable for precise nanofabrication and the
existing exposure system are used.
SUMMARY OF THE INVENTION
[0029] As described above, completely oxidized transition metals
(i.e., complete oxides of transition metals) such as MoO.sub.3 or
WO.sub.3 have been used as resist materials for electron beam
exposure or ion beam exposure. However, these oxides are clear to
ultraviolet rays and visible light, and barely absorb the light.
Accordingly, these oxides are not suitable for the nanofabrication
that employs ultraviolet rays or visible light as the light source
for exposure.
[0030] As a result of intensive study, the present inventors have
found the following phenomena: A slight shift of oxygen content
from the stoichiometric composition of the transition metal oxides
dramatically increases the absorption of ultraviolet rays or
visible light. Absorbing ultraviolet rays or visible light changes
chemical properties of the transition metal oxides. Therefore, the
metal oxides can be applied to the resist material, and to a method
for producing an optical disc master. In other words, the
proportionality constant K is improved in the above Formula (1),
thereby decreasing the minimum pit length P.
[0031] A method for manufacturing an optical disc master according
to the present invention is based on the above fact. The method for
manufacturing an optical disc master includes the steps of forming
a resist layer composed of a resist material including an
incompletely oxidized transition metal (i.e., incomplete oxide of a
transition metal) on a substrate, the oxygen content of the
incomplete oxide being smaller than the oxygen content of the
stoichiometric composition corresponding to the valence of the
transition metal; selectively exposing the resist layer according
to a recording signal pattern; and developing the resist layer to
form a predetermined irregular pattern.
[0032] According to a method for manufacturing an optical disc of
the present invention, an optical disc master is used to produce
the disc having an irregular pattern thereon, the master being
produced by the steps of forming a resist layer composed of a
resist material including an incomplete oxide of a transition metal
on a substrate, the oxygen content of the incomplete oxide being
smaller than the oxygen content of the stoichiometric composition
corresponding to the valence of the transition metal; selectively
exposing the resist layer according to the recording signal
pattern; and developing the resist layer to form the predetermined
irregular pattern.
[0033] The above incomplete oxide of a transition metal is defined
as a compound wherein the oxygen content of the oxide is shifted to
be smaller than the oxygen content of the stoichiometric
composition corresponding to a valence of the transition metal. In
other words, the oxygen content of the incomplete oxide of a
transition metal is smaller than the oxygen content of the
stoichiometric composition corresponding to the valence of the
transition metal.
[0034] If the incomplete oxide includes a plurality of kinds of
transition metals, one kind of transition metal atoms that have a
crystal structure are partly replaced with other transition metal
atoms. In this case, the determination of the incomplete oxides
depends on the fact if the oxygen content of the oxide is smaller
than the oxygen content of the stoichiometric compositions of the
plurality of kinds of the transition metal.
[0035] According to the present invention, since the incomplete
oxide of a transition metal used as the resist material absorbs
ultraviolet rays or visible light, the resist can be exposed
without using a special light source for exposure, such as electron
beam or ion beam. Furthermore, since the incomplete oxide of a
transition metal has a low molecular weight, the boundaries between
the unexposed areas and the exposed area are clear compared with an
organic resist having a high molecular weight. Accordingly, the use
of the incomplete oxide of a transition metal provides a highly
precise resist pattern.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 includes drawings illustrating a manufacturing
process of an optical disc according to a method for manufacturing
the optical disc of the present invention.
[0037] FIG. 2 is a schematic view of an exposure system used in a
method for manufacturing an optical disc master according to the
present invention.
[0038] FIG. 3 is a characteristic graph showing a relationship of
an irradiation power of the light source used for the exposure and
a difference of the etching rate between an exposed area and an
unexposed area in the case where a resist layer composed of a
resist material according to the present invention is exposed.
[0039] FIGS. 4A to 4C are characteristic graphs showing an example
of an irradiation pattern in the exposing step. FIGS. 4A and 4B
show examples of irradiation pulses, and FIG. 4C shows an example
of continuous light.
[0040] FIGS. 5A to 5D are schematic sectional views of the
principal parts illustrating a bilayer resist process. FIG. 5A
illustrates a step of forming a first resist layer and a second
resist layer, FIG. 5B illustrates a step of patterning the first
resist layer, FIG. 5C illustrates a step of etching the second
resist layer, and FIG. 5D illustrates a step of removing the first
resist layer.
[0041] FIG. 6 is an SEM image of a resist layer composed of an
incomplete oxide of tungsten (W) after the development.
[0042] FIG. 7 is an SEM image of a resist layer composed of an
incomplete oxide of tungsten (W) and molybdenum (Mo) after the
development.
[0043] FIG. 8 is an SEM image of a pit pattern formed on the
surface of an optical disc having a recording capacity of 25 GB
that was produced in Example 2.
[0044] FIGS. 9A to 9C show evaluation results of signals in the
optical disc having the recording capacity of 25 GB that was
produced in Example 2.
[0045] FIG. 10 includes drawings illustrating a known manufacturing
process of an optical disc.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0046] An embodiment of a method for manufacturing an optical disc
according to the present invention will now be described in detail
with reference to the drawings.
[0047] The summary of a manufacturing process according to the
method for manufacturing an optical disc of the present invention
will now be described with reference to FIG. 1.
[0048] A resist layer 102 composed of a predetermined inorganic
resist material is uniformly deposited on a substrate 100 by
sputtering (a step of forming a resist layer, FIG. 1 (a)). The
material of the resist layer 102 will be described later in detail.
A predetermined interlayer 101 may be formed between the substrate
100 and the resist layer 102 to improve the exposure sensitivity of
the resist layer 102. FIG. 1 (a) illustrates this case. Although
the resist layer 102 may have any thickness, the resist layer 102
preferably has a thickness of 10 to 80 nm.
[0049] Subsequently, the resist layer 102 is selectively exposed
according to a signal pattern with an exposure system having
existing laser equipment (a step of exposing the resist layer FIG.
1 (b)). The resist layer 102 is developed to prepare a master 103
having a predetermined irregular pattern thereon (a step of
developing the resist layer FIG. 1 (c)).
[0050] Then, a metallic nickel film is formed on the irregular
pattern of the master 103 by electroforming (FIG. 1 (d)). The
nickel film is lifted off from the master 103, and then a
predetermined process is performed to produce a molding stamper 104
having the irregular pattern of the master 103 (FIG. 1 (e)).
[0051] Polycarbonate, which is a thermoplastic resin, is molded by
injection molding using the molding stamper 104 to form a resin
disc substrate 105 (FIG. 1 (f)). Subsequently, the stamper is
removed (FIG. 1 (g)), a reflecting film 106 composed of, for
example, an aluminum alloy (FIG. 1 (h)) and a protective film 107
having a thickness of about 0.1 mm are formed on the irregular
surface of the resin disc substrate 105 to produce an optical disc
(FIG. 1 (i)). The steps of manufacturing the optical disc using the
resist master (i.e., the master having the resist thereon) may be
performed with a known art.
[0052] [Resist Materials]
[0053] The resist material used for the resist layer 102 is
composed of incomplete oxide of a transition metal. The incomplete
oxide of a transition metal is defined as a compound wherein the
oxygen content of the oxide is shifted to be smaller than the
oxygen content of the stoichiometric composition corresponding to a
valence of the transition metal. In other words, the oxygen content
of the incomplete oxide of a transition metal is smaller than the
oxygen content of the stoichiometric composition corresponding to
the valence of the transition metal.
[0054] A chemical formula MoO.sub.3 will now be described as an
example of the incomplete oxide of a transition metal. The
oxidation state of the chemical formula MoO.sub.3 is converted into
a composition ratio of Mo.sub.1-xO.sub.x. When the value x is 0.75
(i.e., x=0.75), the compound is a complete oxide. When the value x
is represented by 0.ltoreq.x.ltoreq.0.75, the compound is an
incomplete oxide in which the oxygen content of the compound is
smaller than the oxygen content of the stoichiometric
composition.
[0055] Some transition metals can form its oxides that have
different valences. In this case, if the actual oxygen content of
an oxide is smaller than the oxygen content of the stoichiometric
composition corresponding to the valence of the transition metal,
the compound is defined as an incomplete oxide according to the
present invention. For example, the oxides of molybdenum (Mo)
include not only the above trivalent oxide (MoO.sub.3), which is
the most stable compound, but also a monovalent oxide (MoO). In
this case, the oxidation state is converted into a composition
ratio of Mo.sub.1-xO.sub.x. When the value x is represented by
0.ltoreq.x.ltoreq.0.5, the compound is an incomplete oxide in which
the oxygen content of the compound is smaller than the oxygen
content of the stoichiometric composition. The valence of the
transition metal oxide can be determined with a commercially
available analytical instrument.
[0056] The incomplete oxide of the transition metal absorbs
ultraviolet rays or visible light. Irradiating ultraviolet rays or
visible light changes chemical properties of the incomplete oxide
of the transition metal. Consequently, as described later in
detail, in spite of an inorganic resist, the exposed areas and the
unexposed areas of the resist have different etching rates in the
developing step. That is, the resist has selectivity. Furthermore,
according to the resist material composed of the incomplete oxide
of the transition metal, since the size of the microparticle of the
resist film material is small, the pattern on the boundaries
between the exposed areas and the unexposed areas become clear.
Accordingly, the resolution can be improved.
[0057] Since the property of the resist material composed of the
incomplete oxide of the transition metal depends on the degree of
the oxidation, the optimum degree of the oxidation must be
appropriately selected. If the oxygen content of the incomplete
oxide of the transition metal is considerably smaller than that of
the stoichiometric composition of the complete oxide, some
disadvantages arise. For example, the exposing step requires a high
irradiation power and the developing step takes a long time.
Preferably, the oxygen content of the incomplete oxide of the
transition metal is slightly smaller than that of the
stoichiometric composition of the complete oxide.
[0058] Examples of the transition metal used as the resist material
include Ti, V, Cr, Mn, Fe, Nb, Cu, Ni, Co, Mo, Ta, W, Zr, Ru, and
Ag. Preferably, Mo, W, Cr, Fe, and Nb are used. More preferably, Mo
and W are used in terms of considerable chemical change by
irradiation of ultraviolet rays or visible light.
[0059] According to the present invention, the incomplete oxide of
the transition metal may be an incomplete oxide of a first
transition metal. The incomplete oxide of the first transition
metal may further include a second transition metal. The incomplete
oxide of the first transition metal may further include a plurality
of kinds of transition metals. The incomplete oxide of the first
transition metal may further include at least one element other
than transition metals. According to the present invention, in
particular, the incomplete oxide of the transition metal preferably
includes a plurality of kinds of metal elements.
[0060] If the incomplete oxide of the first transition metal
further includes a second transition metal, or further includes at
least three transition metals, the first transition metal atoms
that have a crystal structure are partly replaced with other
transition metal atoms. In this case, the determination of the
incomplete oxides depends on the fact if the oxygen content of the
oxide is smaller than the oxygen content of the stoichiometric
compositions of the plurality of kinds of the transition
metals.
[0061] Examples of the element other than transition metal include
at least one element selected from the group consisting of, for
example, Al, C, B, Si, and Ge. The use of at least two kinds of
transition metals in combination or adding at least one element
other than transition metal decreases the crystal grain size of the
incomplete oxides of the transition metal. Accordingly, the
boundaries between the exposed areas and unexposed areas become
clearer, thereby significantly improving the resolution. The
exposure sensitivity can be also improved.
[0062] The above resist material can be produced in an atmosphere
including argon and oxygen by sputtering using a target containing
the predetermined transition metal. For example, the content of the
oxygen is 5% to 20% of the total gas flow introduced in a chamber
at a normal sputtering gas pressure (1 to 10 Pa).
[0063] [Method for Producing Optical Disc Master]
[0064] A method for manufacturing an optical disc master, the
method being a backbone of the method for manufacturing the optical
disc, will now be described in detail.
[0065] An embodiment of the method for manufacturing the optical
disc master according to the present invention, for example,
includes the steps of forming a resist material composed of an
incomplete oxide of a transition metal on a substrate to form a
resist layer; selectively exposing the resist layer; and developing
the resist layer to produce a master having a predetermined
irregular pattern thereon, as described above. Each step will now
be described in detail.
[0066] [Step of Forming Resist Layer]
[0067] A resist layer composed of an incomplete oxide of a
transition metal is formed on a substrate having a sufficiently
smooth surface. Examples of the method include a deposition by
sputtering in an atmosphere including argon and oxygen with a
sputtering target composed of the transition metal. In this case,
changing the content of oxygen gas in vacuum can control the degree
of oxidation of the incomplete oxide of the transition metal. When
incomplete oxides of transition metals including at least two kinds
of transition metals are deposited by sputtering, the substrate is
constantly rotated on the different kinds of sputtering targets to
mix the plurality of kinds of transition metals. The mixing ratio
of the transition metals is controlled by individually changing the
sputtering power.
[0068] In order to deposit the resist layer composed of the
incomplete oxide of the transition metal, as described above, the
sputtering may be performed in an atmosphere containing oxygen with
the metal target. Alternatively, the sputtering may be performed in
argon atmosphere as usual with a target composed of the incomplete
oxide of the transition metal that has the predetermined oxygen
content.
[0069] Furthermore, in addition to sputtering, the resist layer
composed of the incomplete oxide of the transition metal can be
readily deposited by vapor deposition.
[0070] Examples of the substrate include glass; a plastic such as
polycarbonate; silicon; alumina-titanium carbide; and nickel.
[0071] Although the resist layer may have any thickness, the resist
layer may have a thickness of, for example, 10 to 80 nm.
[0072] [Step of Exposing Resist Layer]
[0073] The substrate after deposition of the resist layer
(hereinafter referred to as resist substrate 1) is disposed on a
turntable 11 of an exposure system shown in FIG. 2 such that the
face having the resist layer thereon is placed on the upper
side.
[0074] The exposure system includes a beam source 12 that emits
light, such as laser, to expose the resist layer. The beam source
12 irradiates the resist layer of the resist substrate 1 with
laser. The laser is focused on the resist layer through a
collimator lens 13, a beam splitter 14, and an objective lens 15.
According to this exposure system, reflected light from the resist
substrate 1 is converged on a split photo detector 17 through the
beam splitter 14 and a converging lens 16. The split photo detector
17 detects the reflected light from the resist substrate 1,
generates a focus error signal 18 based on the detection result,
and sends the focus error signal 18 to a focus actuator 19. The
focus actuator 19 controls the position of the objective lens 15 in
the vertical direction. The turntable 11 includes a feeding
attachment (not shown in the figure) to change the exposing
position of the resist substrate 1 precisely. According to this
exposure system, the exposure or the focusing is performed while a
laser driving circuit 23 is controlling the beam source 12 based on
a data signal 20, a reflected light intensity signal 21, and a
tracking time difference signal 22. Furthermore, a spindle motor
control system 24 is disposed at the central axis of the turntable
11. The spindle motor control system 24 determines an optimal
revolution speed of a spindle based on the radial position of the
optical system and a desired linear velocity, thus controlling a
spindle motor.
[0075] In an exposing step where a known organic resist is used as
a resist layer, the focusing to the resist layer is not performed
with the exposing light source itself. The reason is as follows:
The chemical property of the organic resist is continuously changed
by exposure. Therefore, even though light for focusing is faint,
the resist layer composed of the organic resist is unnecessarily
exposed by the irradiation. Accordingly, additional light source
that emits light having a wavelength to which the organic resist is
not sensitive, for example, a red light source that emits light
having a wavelength of 633 nm, is prepared to perform the focusing.
As described above, since the exposure system used for the known
organic resist uses two light sources that emit light having
different wavelength, the exposure system requires an optical
system that can perform wavelength division. Unfortunately, the
exposure system requires a very complicated optical system and the
cost of the exposure system is increased. Furthermore, in the
exposure system used for the known organic resist, the resolution
by the focus error signal, which is used for controlling the
position of the objective lens in the vertical direction, is
proportional to the wavelength of the light source (for example,
wavelength: 633 nm) used for the detection. Accordingly, the
resolution is not as high as a resolution accomplished by the light
source used for the exposure. Unfortunately, a precise and stable
focusing cannot be performed.
[0076] On the other hand, according to the inorganic resist
material of the present invention, the chemical property of the
resist changes very rapidly in the exposure. FIG. 3 shows the
relationship of an irradiation power of a light source used for the
exposure and a difference of the etching rate (i.e., contrast)
between an exposed area and an unexposed area. When the irradiation
power is less than an irradiation threshold power P0 at which the
exposure starts, even repeated irradiation does not cause
unnecessary exposure. Accordingly, the focusing can be performed
with the exposing light source itself at an irradiation power
smaller than the P0. According to the method for manufacturing the
optical disc master of the present invention, the exposure system
does not require an optical system that performs wavelength
division, thereby decreasing the cost of the exposure system.
Furthermore, since a highly precise focusing that corresponds to
the wavelength in exposure can be achieved, an accurate
nanofabrication can be performed. The resist material of the
present invention, which is an inorganic resist, is not sensitized
with faint light having an irradiation power smaller than the
irradiation threshold power P0. Therefore, unlike the process in
which the known organic resist is used, cutting ultraviolet light
in room lighting is not necessary.
[0077] As described above, the focusing is performed with light
having the irradiation power smaller than the irradiation threshold
power P0, and the turntable 11 is then moved at a desired radial
position. In this case, the optical system including such as the
objective lens 15 is fixed in position in the longitudinal
direction, whereas the turntable 11 is moved in order to change the
exposure position of the resist substrate 1. Alternatively, of
course, the turntable 11 having the resist substrate 1 thereon may
be fixed, whereas the position of the optical system may be
changed.
[0078] Subsequently, the beam source 12 radiates laser on the
resist layer, and the turntable 11 is rotated at the same time in
order to expose the resist layer. In this exposing step, in order
to form a fine irregular latent image, the turntable 11 is
continuously moved in the radial direction of the resist substrate
1 by a small pitch, while the turntable 11 is kept rotating. For
example, in order to produce a recording disc, a spiral pregroove
is formed as the fine irregular latent image. In order to produce
an optical disc, irregular pits representing information data, and
a meandering pregroove are formed as the fine irregular latent
image. In order to produce a disc that has concentric tracks, for
example, a magnetic hard disc, the turntable 11 or the optical
system is moved not continuously but stepwise.
[0079] According to the conditions described above, irradiation
pulses or continuous light having a desired power, which is larger
than or equal to the irradiation threshold power P0, is irradiated
in order on the resist layer from a desired position of the resist
substrate 1 corresponding to the pits or the pregroove based on the
information data. Thus, the exposure is performed. FIGS. 4A and 4B
show examples of irradiation pulses, and FIG. 4C shows an example
of continuous light.
[0080] According to the resist material of the present invention
composed of an incomplete oxide of a transition metal, the chemical
property of the resist is changed by irradiation of ultraviolet
rays or visible light having a power larger than or equal to the
irradiation threshold power P0. Consequently, the exposed areas and
the unexposed areas of the resist have different etching rates in
an alkali or an acid. That is, the resist has selectivity.
[0081] A low irradiation power can form a short and narrow pit.
However, an excessively low irradiation power gets close to the
irradiation threshold power P0, and therefore, prevents the stable
pattern formation. Accordingly, the exposure must be appropriately
performed with an optimal irradiation power.
[0082] The present inventors have verified that the exposure using
a red semiconductor laser that emits light having a wavelength of
660 nm and a mercury lamp that emits light having peaks at
wavelengths of 185 nm, 254 nm, and 405 nm provides the resist
material of the present invention with the selectivity, and this
process can form a fine pit pattern.
[0083] [A Step of Developing Resist Layer]
[0084] Subsequently, the resist substrate 1 having the exposed
pattern as described above is developed to produce a resist master
used for producing an optical disc. The resist master has a fine
irregular surface including the pits or the pregroove corresponding
to the predetermined exposure pattern.
[0085] The step of developing includes a wet process using, for
example, an acidic solution or an alkaline solution. This process
provides the resist layer with selectivity. The step of developing
may be appropriately changed depending on, for example, the
intended use, the application, and the device and equipment.
Examples of the alkaline developer include a solution of
tetramethylammonium hydroxide; and solutions of inorganic alkali
such as KOH, NaOH, and Na.sub.2CO.sub.3. Examples of the acidic
developer include hydrochloric acid, nitric acid, sulfuric acid,
and phosphoric acid. The present inventors have verified that in
addition to the wet process, a dry process such as plasma etching,
i.e., reactive ion etching (RIE) can be also used for the
development in which the kinds of the gas and the mixing ratio of a
plurality of gases are controlled.
[0086] A method for controlling the exposure sensitivity will now
be described. Take an example where the oxidation state of a
transition metal oxide represented by chemical formula WO.sub.3 is
converted into a composition ratio of W.sub.1-xO.sub.x. When the
value x is represented by 0.1.ltoreq.x.ltoreq.0.75, a high exposure
sensitivity can be achieved. When the value x is 0.1, this value is
a critical value in which, for example, the exposing step requires
a high irradiation power and the developing process takes a long
time disadvantageously. When the value x is in the range of about
0.4 to about 0.7, the highest exposure sensitivity can be
achieved.
[0087] Take an example where the oxidation state of a transition
metal oxide represented by chemical formula MoO.sub.3 is converted
into a composition ratio of Mo.sub.1-xO.sub.x. When the value x is
represented by 0.1.ltoreq.x.ltoreq.0.75, a high exposure
sensitivity can be achieved. When the value x is 0.1, this value is
a critical value in which, for example, the exposing step requires
a high irradiation power and the developing process takes a long
time disadvantageously. When the value x is in the range of about
0.4 to about 0.7, the highest exposure sensitivity can be
achieved.
[0088] Furthermore, take an example where the oxidation state of a
transition metal oxide represented by chemical formula MoO is
converted into a composition ratio of Mo.sub.1-xO.sub.x. When the
value x is represented by 0.1.ltoreq.x.ltoreq.0.5, a high exposure
sensitivity can be achieved. When the value x is 0.1, this value is
a critical value in which, for example, the exposing step requires
a high irradiation power and the developing process takes a long
time disadvantageously.
[0089] A high exposure sensitivity of the resist material, for
example, advantageously decreases the irradiation power during
exposure and decreases the exposing time corresponding to the pulse
width and the linear velocity. However, excessively high exposure
sensitivity disadvantageously causes unnecessary exposure during
focusing, and causes an adverse effect in the exposure due to the
lighting environment in the processing room. Accordingly, optimal
exposure sensitivity is appropriately selected depending on the
application. In order to control the exposure sensitivity of the
resist material according to the present invention, the oxygen
content of the material is increased or decreased; alternatively, a
second transition metal is effectively added to an incomplete oxide
of a first transition metal. For example, adding molybdenum (Mo) to
W.sub.1xO.sub.x can improve the exposure sensitivity by about
30%.
[0090] Furthermore, in addition to the change of the resist
material composition, the selection of the substrate material and
pretreatments for exposure on the substrate can also control the
exposure sensitivity. The dependency of the substrate material to
the exposure sensitivity was investigated using quartz, silicon,
glass, and a plastic (polycarbonate). As a result, the exposure
sensitivity depended on the substrate material, more specifically;
the highest exposure sensitivity was achieved with the plastic,
subsequently, glass, quartz, and silicon, in this order. This order
corresponds to the order of the thermal conductivity. A substrate
having a small thermal conductivity achieves high exposure
sensitivity. The reason is as follows: The use of the substrate
having a small thermal conductivity significantly increases the
temperature of the resist material during exposure. Subsequently,
as the temperature increases, the chemical property of the resist
material is significantly changed.
[0091] Examples of the pretreatments for exposure include a
formation of an interlayer disposed between the substrate and the
resist material, a heat treatment, and ultraviolet irradiation.
[0092] In particular, when a substrate having a large thermal
conductivity, for example, silicon wafer composed of single crystal
silicon is used, an interlayer having a relatively small thermal
conductivity is preferably formed on the substrate to appropriately
improve the exposure sensitivity. The reason is that the interlayer
enhances the thermal storage in the resist material during
exposure. Examples of the material of the interlayer having a small
thermal conductivity include amorphous silicon, silicon dioxide
(SiO.sub.2), silicon nitride (SiN), and alumina (Al.sub.2O.sub.3).
The interlayer may be formed by sputtering or other vacuum
depositions.
[0093] UV curable resin layer having a thickness of 5 .mu.m was
formed on a quartz substrate by spin coating. Ultraviolet rays were
then irradiated to cure the liquid resin. The exposure sensitivity
in the above substrate was higher than that in the untreated quartz
substrate. This is also because the thermal conductivity of the UV
curable resin is as low as a plastic.
[0094] Other pretreatments for exposure, for example, a heat
treatment and ultraviolet irradiation can also improve the exposure
sensitivity. Although the effect is not perfect, these
pretreatments allow the chemical property of the resist material of
the present invention to be changed at some level.
[0095] As described above, appropriate choices of the resist
material composition, the developing condition, and the substrate
material can express functions of the resist composed of an
incomplete oxide of a transition metal, and having various
properties. Furthermore, in order to expand the application of the
resist material, a bilayer resist process (i.e., a process using
bilayer resist) is very useful. The outline of the bilayer resist
process will now be described with reference to FIGS. 5A to 5D.
[0096] Referring to FIG. 5A, a first resist layer 30 is composed of
an incomplete oxide of a transition metal according to the present
invention. Before the deposition of the first resist layer 30, a
second resist layer 32 is deposited on a substrate 31. The
selectivity of the material of the second resist layer 32 and the
selectivity of the incomplete metal oxide of the transition metal
in the first resist layer 30 are significantly different.
[0097] Subsequently, as shown in FIG. 5B, the first resist layer 30
is exposed and is then developed to form a pattern thereon.
[0098] Then, the second resist layer 32 is etched under a high
selective etching condition by using the pattern of the first
resist layer 30 as a mask. As shown in FIG. 5C, the pattern of the
first resist layer 30 is copied on the second resist layer 32.
[0099] Finally, the first resist layer 30 is removed. Thus, as
shown in FIG. 5D, the patterning of the second resist layer 32 is
completed.
[0100] In order to apply the present invention to the bilayer
resist process, for example, the substrate is composed of quartz,
the second resist layer is composed of a transition metal such as
Cr, and the etching is performed by RIE, i.e., plasma etching with
a chlorofluorocarbon gas. In this case, the difference of the
selectivity between the incomplete oxide of the transition metal in
the first resist layer 30 and the second resist layer 32 becomes
approximately the largest.
[0101] As described above, the above resist material composed of an
incomplete oxide of a transition metal is used in the method for
manufacturing an optical disc master of the present invention.
Accordingly, even though the resist is composed of an inorganic
material, the resist can be advantageously exposed with ultraviolet
rays or visible light. This property is absolutely different from
that of the known inorganic resist: Since the known inorganic
resist is optically clear to ultraviolet rays or visible light, the
ultraviolet rays or the visible light cannot be used as the light
source for exposure. Subsequently, an expensive exposure system
that uses, for example, electron beam or ion beam is essential to
exposure the known inorganic resist.
[0102] Since the use of the ultraviolet rays or visible light
achieves a high imaging speed, the time required for the exposure
can be significantly decreased, compared with a known method for
producing an optical disc master in which the electron beam is used
as the light source in order to expose the known inorganic
resist.
[0103] The use of the inorganic resist material composed of an
incomplete oxide of a transition metal provides clear patterns at
the boundaries between the exposed areas and the unexposed areas,
thus achieving a highly precise nanofabrication. Furthermore, since
the focusing can be performed with the exposing light source itself
in the exposing step, a high resolution can be achieved.
[0104] As described above, according to the method for
manufacturing an optical disc master of the present invention, the
proportionality constant K in the formula P=K.lamda./NA is
decreased in order to achieve the nanofabrication. Unlike the known
method in which the wavelength .lamda., in exposure is shortened
and the numerical aperture NA of the objective lens is increased to
achieve the nanofabrication, the method of the present invention
can perform more precise patterning with the existing exposure
system. Specifically, according to the present invention, the
proportionality constant K can be decreased to less than 0.8, and a
minimum fine patterning cycle f of a workpiece can be decreased as
follows:
f<0.8.lamda./NA
[0105] According to the present invention, the existing exposure
system can be used without further improvement. Therefore, the
present invention inexpensively provides an optical disc master on
which a more precise nanofabrication is performed.
EXAMPLES
[0106] Examples according to the present invention will now be
described with reference to the experimental results.
Example 1
[0107] In Example 1, a master having a resist thereon (i.e., resist
master) used for producing an optical disc was actually produced
using a resist material composed of an incomplete oxide of
trivalent tungsten (W).
[0108] A resist layer composed of an incomplete oxide of tungsten
was uniformly deposited by sputtering on a glass substrate having a
sufficiently smooth surface. The sputtering was performed with a
sputtering target composed of tungsten element in an atmosphere
containing argon and oxygen. The content of oxygen gas was changed
in order to control the degree of oxidation of the incomplete oxide
of tungsten.
[0109] The composition of the deposited resist layer was analyzed
with an energy dispersive X-ray spectrometer (EDX). When the
composition ratio was represented by W.sub.1-xO.sub.x, the value x
was 0.63. The thickness of the resist layer was controlled to be 40
nm. The wavelength dependence of the refractive index was measured
by spectroscopic ellipsometry.
[0110] The substrate having the deposited resist layer thereon,
i.e., resist substrate, was disposed on the turntable of the
exposure system shown in FIG. 2. While the turntable was rotated at
a desired revolution speed, laser having less than the irradiation
threshold power was irradiated on the resist layer. The actuator
controlled the position of the objective lens in the vertical
direction to focus on the resist layer.
[0111] Subsequently, the turntable was moved at a desired radial
position with the feeding attachment attached to the turntable,
whereas the optical system was fixed. Based on the information
data, irradiation pulses corresponding to the pits were irradiated
on the resist layer to expose the resist layer. In the exposing
step, the turntable was continuously moved in the radial direction
of the resist substrate by a small pitch, while the turntable was
kept rotating. The wavelength in the exposure was 405 nm and the
numerical aperture NA of the exposing optical system was 0.95. The
linear velocity in the exposing step was 2.5 m/s and the
irradiation power was 6.0 mW.
[0112] After the exposure, the resist substrate was developed by a
wet process with an alkaline developer. In this developing step,
the resist substrate was submerged in the developer and ultrasonic
waves were applied in order to etch the resist uniformly. After the
development, the substrate was sufficiently washed with purified
water and isopropyl alcohol and was dried by, for example, blowing
air to finish the process. The alkaline developer was a solution of
tetramethylammonium hydroxide and the developing time was 30
minutes.
[0113] FIG. 6 is a scanning electron microscope (SEM) image of the
resist pattern after the development. Referring to FIG. 6, the pits
correspond with the exposed areas. The exposed areas formed hollows
relative to the unexposed area on the resist layer. That is, the
resist material composed of the incomplete oxide of tungsten was a
positive resist. In the resist layer composed of the incomplete
oxide of tungsten, the etching rate of the unexposed area was
smaller than that of the exposed areas. Accordingly, the unexposed
areas of the resist layer had a thickness almost the same as the
thickness after the deposition. On the other hand, the exposed
areas of the resist layer were removed by etching. Consequently,
the surface of the glass substrate was exposed at the exposed
areas.
[0114] The smallest pit of the four pits shown in FIG. 6 has the
width of 0.15 .mu.m and the length of 0.16 .mu.m. Accordingly, the
method for manufacturing the optical disc master wherein the resist
material of the present invention is used, can significantly
improve the resolution relative to the known method with an organic
resist, in which the expected pit width is 0.39 .mu.m. Furthermore,
FIG. 6 shows that the pit has a very clear edge.
[0115] The experimental results also showed that the width and the
length of the pit formed after developing depended on the
irradiation power and the pulse width of the light source for
exposure.
Comparative Example 1
[0116] In Comparative Example 1, a resist master used for producing
an optical disc was actually produced using a resist material
composed of a complete oxide of tungsten, i.e., WO.sub.3.
[0117] A resist layer composed of the complete oxide of tungsten
was deposited by sputtering on a glass substrate. According to the
analytical result by the EDX, when the composition ratio of the
deposited resist layer was represented by W.sub.1-xO.sub.x, the
value x was 0.75. By the way, the analytical result of electron
diffraction by a transmission electron microscope showed that
before the exposure, the crystal state of the incomplete tungsten
oxide (WO) was amorphous.
[0118] This resist layer was exposed with the same irradiation
power as in Example 1 or a sufficiently strong irradiation power.
However, the selectivity in the resist layer was 1 or less, and the
desired pit pattern was not formed. Since the complete oxide of
tungsten was optically clear to the light source for exposure, the
complete oxide of tungsten barely absorbed the light. The small
absorption could not chemically change the resist material.
Example 2
[0119] In Example 2, a resist master used for producing an optical
disc was actually produced using a resist material composed of an
incomplete oxide of trivalent tungsten and trivalent molybdenum
according to the manufacturing process shown in FIG. 1. Then, the
optical disc was finally manufactured. The operating process will
now be described with reference to FIG. 1.
[0120] Firstly, an interlayer 101 composed of amorphous silicon and
having a thickness of 80 nm was uniformly deposited on a substrate
100 that is a silicon wafer by sputtering. Subsequently, a resist
layer 102 composed of an incomplete oxide of tungsten (W) and
molybdenum (Mo) was uniformly deposited on the substrate by
sputtering (FIG. 1 (a)). The sputtering was performed in argon
atmosphere with a sputtering target composed of the incomplete
oxide of tungsten and molybdenum. According to the analytical
result of the deposited resist by the EDX, the ratio of the
tungsten and molybdenum in the deposited incomplete oxide of
tungsten and molybdenum was 80:20, and the oxygen content of the
incomplete oxide was 60 atomic percent. The resist layer had the
thickness of 55 nm. The analytical result of electron diffraction
by the transmission electron microscope showed that before the
exposure, the crystal state of the above incomplete oxide (WMoO)
was amorphous.
[0121] Regarding the step of exposing of the resist layer and the
subsequent steps, all conditions except for the exposing condition
were the same as in Example 1. Thus, a resist master 103 used for
producing the optical disc was produced (FIG. 1 (b) and FIG. 1
(c)). The exposing condition in Example 2 was as follows: [0122]
Wavelength in exposure: 405 nm [0123] Numerical aperture NA of
exposing optical system: 0.95 [0124] Modulation: 17 PP [0125] Bit
length: 112 nm [0126] Track pitch: 320 nm [0127] Linear velocity in
exposing step: 4.92 m/s [0128] Irradiation power in exposure: 6.0
mW [0129] Writing system: Simple writing system the same as
phase-change disc
[0130] FIG. 7 is an SEM image showing an example of the resist
pattern after the development of the resist master used for
producing the optical disc. The resist material composed of the
incomplete oxide of tungsten and molybdenum was a positive resist.
Referring to FIG. 7, the pits correspond with the exposed areas.
The exposed areas formed hollows relative to the unexposed area on
the resist layer. The pit length (diameter) was about 130 nm. In
other words, this pit length (diameter) was smaller than 170 nm
(0.17 .mu.m), which was required for the minimum pit length in the
high density optical disc having the recording capacity of 25 GB on
the single side. Furthermore, the resist pattern included
identically shaped pits with a constant pitch of 300 nm in the pit
line direction and with a constant pitch of 320 nm in the track
direction. This result showed that the pits were stably formed in
this Example.
[0131] Then, a metallic nickel film was formed on the irregular
pattern of the resist master by electroforming (FIG. 1 (d)). The
nickel film was lifted off from the resist master. Subsequently, a
predetermined process was performed to produce a molding stamper
104 having the irregular pattern of the resist master (FIG. 1
(e)).
[0132] Polycarbonate, which was a thermoplastic resin, was molded
by injection molding using the molding stamper to form a resin disc
substrate 105 (FIG. 1 (f)). The stamper was removed (FIG. 1 (g)),
and then a reflecting film 106 composed of an aluminum alloy (FIG.
1 (h)) and a protective film 107 having a thickness of 0.1 mm were
formed on the irregular surface of the resin disc substrate to
produce an optical disc having a diameter of 12 cm (FIG. 1 (i)).
The above steps of manufacturing the optical disc using the resist
master were performed according to the known art.
[0133] FIG. 8 is an SEM image showing an example of a pit pattern
formed on the surface of the above optical disc. Referring to FIG.
8, pits that were formed on the optical disc corresponded to an
actual signal pattern including, for example, pits having the
length of 150 nm and linear pits having the width of 130 nm. This
result showed that the optical disc had the recording capacity of
25 GB.
[0134] Subsequently, the optical disc was read out under the
following condition. The RF signals were converted into eye
patterns to evaluate the signals. FIGS. 9A to 9C show the results
of the signal evaluation. [0135] Tracking servo: Push-pull method
[0136] Modulation: 17 PP [0137] Bit length: 112 nm [0138] Track
pitch: 320 nm [0139] Linear velocity in readout: 4.92 m/s [0140]
Irradiation power in readout: 0.4 mW
[0141] The jitter value of an eye pattern (FIG. 9B) generated by
performing conventional equalization on an untreated readout eye
pattern (FIG. 9A) was 8.0%. The jitter value of an eye pattern
(FIG. 9C) generated by performing limit equalization on the
untreated readout eye pattern (FIG. 9A) was 4.6%. These jitter
values were sufficiently small for practical use of the optical
disc as a ROM disc having the recording capacity of 25 GB.
[0142] The photolithographic technology according to the present
invention including the steps from the formation of the resist
layer to the development may be applied to produce various devices
such as semiconductor devices, e.g. a dynamic random access memory
(DRAM), a flash memory, a central processing unit (CPU), and an
application specific integrated circuit (ASIC); magnetic devices,
e.g. magnetic head; display devices, e.g. a liquid crystal device,
an electroluminescence (EL) device, and a plasma display panel
(PDP); and optical devices, e.g. an optical recording medium and a
light modulation device.
[0143] As described above, according to the method for producing an
optical disc master of the present invention, the resist layer is
composed of an incomplete oxide of a transition metal that absorbs
ultraviolet rays or visible light. Accordingly, an existing
exposure system having an exposing light source that emits
ultraviolet rays or visible light can be used in order to expose
the resist layer. Furthermore, since the resist material composed
of the incomplete oxide of the transition metal has a small
molecular size, the developed resist layer has a superior edge
pattern, thus achieving a highly precise patterning.
[0144] According to the method for manufacturing an optical disc
using the optical disc master described above, an optical disc
having the recording capacity of 25 GB class can be produced with
the existing exposure system.
* * * * *