BACKGROUND
The present technology relates to a recording medium,
such as an optical disc, and a method for manufacturing the
recording medium.
Optical discs, such as a compact disc (CD), a digital
versatile disc (DVD), and a Blu-ray® Disc, are widely used
for recording and reproducing music, images, and data.
In order to ship the discs to market, such optical
discs have a variety of information, such as the title of
recorded content, the disc type, the name of a manufacturer,
or a design pattern printed on the label-side surface
thereof opposite the recording surface onto which a laser
beam is emitted.
Japanese Unexamined Patent Application Publication No.
2001-14741 describes a technique for forming pits (a rough
surface portion) on the label-side surface of a transparent
resin substrate so as to display visible graphics or text.
In addition, Japanese Unexamined Patent Application
Publication Nos. 2007-179718 and 2007-226932 describe a
technique for processing the label-side surface of a
substrate into a satin surface.
SUMMARY
In order to display necessary information without
printing the information on the label-side surface, a
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difference in contrast between a rough surface and a mirror
surface can be used, as described in Japanese Unexamined
Patent Application Publication No. 2001-14741. However, if
this technique is applied to a high-density disc, such as a
Blu-ray disc, a recording/reproduction operation may be
deteriorated. For example, the quality of a tracking error
signal may be decreased. In addition, since display is
performed using a difference in contrast between a rough
surface and a mirror surface, it is necessary to ensure an
excellent visibility.
Accordingly, the present technology provides a
recording medium that provides excellent visibility and
recording/reproduction performance by forming the rough
surface portion into an appropriate shape rather than by
simply forming the rough surface portion.
According to an embodiment of the present technology, a
recording medium includes a substrate having a label-side
surface on one side and a recording layer for recording and
reproducing information on the other side. The label-side
surface of the substrate has a mirror surface portion and a
rough surface portion, a step having a step height of 10 ^m
or less is formed between the mirror surface portion and the
rough surface portion, and an average roughness of the rough
surface portion is between 10 fom and 2 pm, inclusive.
Visible information is formed on the label-side surface by
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using the mirror surface portion and the rough surface
portion. For example, the height of the step of the rough
surface portion is set to a value between 10 |im and 1 ^m
inclusive, and the average roughness of the rough surface
portion is set to about 5 \im.
According to another embodiment of the present
technology, a method for manufacturing a recording medium is
provided. The method includes generating a substrate having
one surface serving as a label-side surface and the other
surface having an irregularity pattern for forming a
recording layer, where the label-side surface includes a
mirror surface portion and a rough surface portion having a
step of 10 ^m or less formed therebetween, average roughness
of the rough surface portion is between 10 |am and 2 iJin
inclusive, and visible information is formed on the labelside
surface by using the mirror surface portion and the
rough surface portion, and forming, on the irregularity
pattern of the substrate, a layer structure including at
least a recording layer and a cover layer. For example, in
generating a substrate, the above-described rough surface
portion is generated so that the step height of the abovedescribed
rough surface portion is set to a value between 10
jjiii and 1 )jin, inclusive, and the average roughness of the
above-described rough surface portion is set to about 5 |im.
According to such technology, various information can
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be displayed on the label-side surface of the recording
medium by using the contrast between the rough surface
portion and the mirror surface portion. In addition, even
when the rough surface portion is formed, the rough surface
portion does not have an adverse impact on the recording and
reproduction performance.
According to the above-described technology, display
having an excellent visibility can be realized on the labelside
surface by using the rough surface portion and the
mirror surface portion. In addition, in manufacturing a
recording medium, an efficient printing process and cost
reduction can be provided. Furthermore, a recording medium
that does not have any adverse impact on the recording and
reproduction performance (the tracking performance) can be
provided.
BRIEF DESCRIPTION OF THE DRAWINGS
Figs. lA and IB illustrate a mirror surface portion and
a rough surface portion of an optical disc according to an
embodiment of the present technology;
Fig. 2 is a flowchart illustrating a process flow for
manufacturing the optical disc according to the embodiment;
Figs. 3A to 3D illustrate the process flow up to
stamper generation according to the embodiment;
Figs. 4A and 4B illustrate injection molding of a
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substrate according to the einbodiment ;
Figs. 5A to 5D illustrate a process flow for
manufacturing the optical disc according to the embodiment;
Figs. 6A to 6D illustrate a rough surface portion and a
tracking error signal; and
Fig. 7 illustrates a relationship between the rough
surface portion and the tracking error signal.
DETAILED DESCRIPTION OF EMBODIMENTS
Embodiments of the inventioii will now be described with reference to the
accompanying drawings, throughout which like parts are referred to by like
references.
Embodiments of the present technology are described below in
the following order:
1. Optical Disc
2. Disc Manufacturing Process Flow
3. Effect of Rough Surface Portion on Tracking
Operation and Visibility.
1. Optical Disc
Embodiments of the present technology are described
below with reference to a Blu-ray disc (BD) as an optical
disc serving as a recording medium. A BD is briefly
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described next. As a disc size, a BD has a diameter of 120
mm and a thickness of 1.2 mm. That is, in terms of the
external size, a BD has a size that is-the same as that of a
CD disc or a DVD disc. In addition, in order to record and
reproduce data, a blue laser (e.g., the wavelength X = 405
nm) is used. Furthermore, the optical system has a high NA
(e.g., NA = 0.85). Still furthermore, a narrow track pitch
(e.g., the track pitch = 0.32 )am) and high line density
(e.g., the recording line density = 0.112 |ain/bit) are
provided. Thus, a disc having a diameter of 12 cm and a
single-layer structure has a user data capacity of about 25
Giga byte (GB). If a further high-density recording
technique is employed, a capacity of about 27 GB can be
realized. In addition, a so-called multilayer disc having a
plurality of recording layers has been developed. A
multilayer disc has a user data capacity that is
substantially equal to the capacity per layer times the
number of layers.
Examples of a BD optical disc include a BD-ROM (Blu-ray
Disc Read Only Memory), a BD-R (Blu-ray Disc Recordable),
and a BD-RE (Blu-ray Disc Rewritable). A BD-ROM is a read
only optical disc. A BD-ROM stores information recorded in
a recording layer in the form of an embossed pit string,
A BD-R is a one time recordable disc that complies with
the Blu-Ray disc standard. A BD-RE is a rewritable disc
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that complies with the Blu-Ray disc standard. The disc
substrates of a BD-R and a BD-RE have substantially the same
structure. The disc substrates have only a groove preformed
therein and do not have a pit string. The track pitch of
the groove is 320 nm in almost all the areas including a
data recording area except for the area of the innermost
periphery PIC (Permanent Information & Control data) area.
The track pitch of the groove in the PIC area is 350 nm.
The disc substrates have a single spiral structure. In
addition, the depth of the groove is about 25 nm, in general.
The unique information, such as address information, is
recorded in the groove itself by wobbling.
According to the present exemplary embodiment, an
optical disc is one of a BD-R, a BD-RE, and a BD-ROM. In
addition, according to the present exemplary embodiment, one
of the surfaces of the substrate serves as a label-side
surface. A recording layer for recording and reproducing
information is formed on the other surface of the substrate.
The label-side surface includes a mirror surface portion and
a rough surface portion formed so as to be stepped upward
from the mirror surface portion by 5 |ain or less. Thus,
visible display is formed on the label-side surface by using
the mirror surface portion and the rough surface portion.
The visible display formed in this manner is illustrated in
Fig, lA. Fig. lA is a plan view of the label-side surface
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of the optical disc according to the present exemplary
embodiment. The label-side surface includes a rough surface
portion 6 and a mirror surface portion 7. For example, the
mirror surface portion 7 is provided so as to form text,
graphics, and a pattern. That is, in the example
illustrated in Fig. lA, the mirror surface portion 7 forms
the text "ABC" and "DEF". That is, users can recognize the
text using the contrast difference between the mirror
surface portion 7 and the rough surface portion 6. Note
that conversely, the text portion may be the rough surface
portion 6, and the other portion may be the mirror surface
portion 7.
Fig. IB is a schematic cross-sectional view of the
rough surface portion 6 and the mirror surface portion 7.
The mirror surface portion 7 has a flat surface. In
contrast, the rough surface portion 6 is stepped upward from
the mirror surface portion 7 with a step height D and has a
rough surface. For example, the step height D is about 5 ^un,
and an average roughness Ra of the rough surface portion 6
is about 5 jim. Note that as illustrated in Fig. IB, the
step height D is defined as the distance between the
lowermost point of the rough surface portion 6 (the point
that is the closest to the mirror surface portion 7 in
level) and the mirror surface portion 7.
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2. Disc Manufacturing Process Flow
The process flow for manufacturing an optical disc
according to the present exemplary embodiment is described
below with reference to Figs. 2, Figs. 3A to 3D, Figs. 4A
and 4B, and Figs. 5A to 5D. Fig. 2 is a flowchart
illustrating the manufacturing process flow of the optical
disc. Each of Figs. 3A to 3D, Figs. 4A and 4B, and Figs. 5A
to 5D illustrates one of the processes for manufacturing the
optical disc. In this example, a BD-R or a BD-RE is
manufactured.
As illustrated in Fig. 2, in step FlOl, mastering is
performed first. That is, an exposure operation is
performed on a master using an exposure apparatus (a cutting
machine). For example, as illustrated in Fig. 3A, a laser
beam is emitted from an exposure head 51 onto a disc master
100 having a resist (e.g., an inorganic resist) applied
thereonto. In this way, the exposure operation is performed.
The disc master 100 placed on a turntable 50 is rotated by a
spindle motor 52. At that time, the turntable 50 is moved
in a slidable manner by a sliding mechanism (not shown). In -
this way, as illustrated in Fig. 7B, an exposure portion R
is formed. In this case, the laser beam is deflected in
accordance with a modulation signal based on, for example,
the address information using, for example, an acoustooptical
deflector (AOD), and the disc master 100 that is
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rotating is moved by the turntable 50 in a slidable manner.
Accordingly, the exposure portion R is formed into a
wobbling groove having a spiral pattern. Note that in the
case of a ROM disc, the exposure portion R is formed so as
to have a pit string pattern. Thereafter, a development
operation is performed on the disc master 100 illustrated in
Fig. 3B subjected to the exposure operation. Thus, as
illustrated in Fig. 3C, the disc master 100 having an
irregularity pattern in which the exposure portion is formed
as a concave portion is generated. The concave portion
serves as a groove of a final product of the optical disc.
Subsequently, in step F102, a stamper is generated.
For example, the irregularities of the disc master 100 are
transferred through a nickel electroforming process using
the disc master 100 to generate a stamper 101 (refer to Fig.
3D). In the irregularity pattern of the stamper 101, a
convex portion corresponds to the groove.
Subsequently, in step F103, substrate formation is
performed. In the substrate formation, display on the
label-side surface is also formed. Fig. 4A is a schematic
illustration of a molding tool used for forming the
substrate by injection molding. The molding tool has a
lower cavity 120 and an upper cavity 121. The lower cavity
120 includes a signal-side surface mirror 123. In addition,
the stamper 101 for transferring the groove is disposed on
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the signal-side surface mirror 123. The stamper 101 is
fixed by a stamper holder 122. Furthermore, the upper
cavity 121 includes a label-side surface mirror 124.
In Fig. 4B, a predetermined area of the label-side
surface mirror 124, for example, an area encircled by a
broken line, is enlarged and illustrated. As illustrated in
Fig. 4B, a rough surface portion 124a is formed in this area,
and the other portion is formed as a mirror surface portion
124b. For example, the label-side surface mirror 124 is
formed from a mirror master using an SUS-based material.
While the label-side surface mirror 124 is being produced,
one of the surfaces of the mirror master is subjected to
masking using a masking sheet having, for example, the text
and graphic pattern to be displayed on the label-side
surface. That is, the area corresponding to the mirror
surface portion 124b is masked, and the area corresponding
to the rough surface portion 124a is exposed. Thereafter, a
chemical etching process is performed on the master
subjected to masking in this manner. Thus, the exposed
portion is roughened. In this way, the label-side surface
mirror 124 having the exposed portion serving as the rough
surface portion 124a and the masked portion serving as the
mirror surface portion 124b is produced.
Note that as illustrated in Fig. 4B, the rough surface
portion 124a is formed so as to have a rough surface that is
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stepped upward from the plane of the mirror surface portion
124b with the step height D. The step height D can be
controlled by the etching time. In addition, the roughness
of the rough surface portion 124a can be controlled by the
type of solvent or the type of additive agent being used.
Since the rough surface portion 124a is formed by etching,
the rough surface portion 124a is formed as a concave
portion of the label-side surface mirror 124.
Fig. 5A is a more schematic illustration of the molding
tool illustrated in Figs. 4A and 4B. As illustrated in Fig.
5A, through an injection molding process using a molding
tool in which the stamper 101 for transferring a groove and
the label-side surface mirror 124 having the rough surface
portion 124a formed thereon are disposed in this manner, a
substrate 1 of a disc is formed of, for example,
polycarbonate resin.
The molded substrate 1 is illustrated in Fig. 5B. That
is, the substrate 1 formed of polycarbonate resin has a
center hole 2 at the center thereof. In addition, a surface
of the substrate 1 onto which a recording and reproduction
laser beam is emitted has a groove pattern 3 formed by
transferring an irregularity pattern (a groove pattern) 101a
formed on the stamper 101 of the molding tool. Furthermore,
the label-side surface of the substrate 1 has the
irregularity pattern (the rough surface portion 124a/the
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mirror surface portion 124b) of the label-side surface
mirror 124 transferred thereonto. That is, the label-side
surface of the substrate 1 has the rough surface portion 6
and the mirror surface portion 7 formed thereon.
In step F104, formation of a layer structure including
a recording layer is performed on the disc substrate 1. As
illustrated in Fig. 5C, a recording layer 4 including a
recording material layer made of a phase-change material or
a photochromic material and a dielectric layer is formed on
the surface of the substrate 1 having a groove/land shape
formed thereon. In addition, as illustrated in Fig. 5D, a
light transmissive layer (a cover layer) 5 is formed on the
recording layer 4. For example, the light transmissive
layer 5 is formed using spin-coat of an ultraviolet curable
resin and emission of ultraviolet light to cure the
ultraviolet curable resin. In this way, a recordable
optical disc (a BD-R or a BD-RE) having a single layer (the
recording layer 4) can be produced. The optical disc has a
surface to which a recording and reproduction laser is
emitted on the light transmissive layer 5 side, and the
label-side surface of the optical disc has a display pattern,
such as predetermined text, graphic, and a predetermined
design pattern, formed using the rough surface portion 6 and
the mirror surface portion 7.
Although the disc substrate 1 is schematically
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illustrated in Figs. 5A to 5D, the thickness of the disc
substrate 1 is about 1.1 mm, and the thickness of the light
transmissive layer 5 is about 100 \xm, in reality. In some
cases, a hard coat layer is further formed on the light
.transmissive layer 5. If a disc including a plurality of
•- recording layers (e.g., a dual-layer disc or a quad-layer
disc) is produced, a plurality of recording layers are
• formed in formation of a layer structure performed in step
F105. For example, in the case of producing a dual-layer
disc, as illustrated in Fig. 5C, the recording layer 4 (a
first recording layer) is formed. Thereafter, a material
(an ultraviolet curable resin) for forming a spacer layer is
coated on the recording layer 4 by a spin coating process.
Subsequently, a groove pattern is transferred onto the
spacer layer using a stamper that is formed for a second
recording layer through the steps illustrated in Figs. 3A to
3D, In such a case, the stamper is pressed onto the uncured
.ultraviolet curable resin (the spacer layer), and
ultraviolet light is emitted to the ultraviolet curable
resin. Thereafter, the stamper is moved away. In this way,
a groove shape for the second recording layer is transferred.
Subsequently, the second recording layer including a
recording material layer made of a phase-change material or
a photochromic material and a dielectric layer is formed on
the surface having a groove/land shape formed thereon.
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Thereafter, a light transmissive layer 21 is formed on the
second recording layer. In this way, a dual-layer disc
including the first and second recording layers can be
produced. In order to produce a disc including more layers,
-> such as a triple-layer disc or a quad-layer disc, the abovedescribed
steps are repeated.
3. Effect of Rough Surface Portion on Tracking Operation and
Visibility
According to the present exemplary embodiment, as
illustrated in Fig. IB, the produced optical disc has the
label-side surface including the rough surface portion 6,
and information is displayed on the label-side surface using
the rough surface portion 6 and the mirror surface portion 7.
In particular, according to the present exemplary embodiment,
the step height D of the rough surface portion 6 with
respect to the mirror surface portion 7 is less than or
equal to 10 ^m and is preferably between 10 ^m and 1 jam,
inclusive. In addition, the average roughness Ra of the
rough surface portion 6 is between 10 ^m and 2 ^im, inclusive.
It is desirable that the average roughness Ra be set to
about 5 ^m.
The rough surface portion 6 is provided in order to
display information on the label-side surface. Accordingly,
it is necessary that the step height D of the step of the
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rough surface portion 6 and the roughness of the rough
surface portion 6 be determined so that excellent visibility,
that is, a certain contrast difference between the rough
surface portion 6 and the mirror surface portion 7 is
obtained. In addition, if the structure of the rough
surface portion 6 is not appropriate, a tracking error
signal may be disturbed when a laser beam for recording and
reproduction is emitted to the recording layer 4.
Fig. 6A illustrates an optical disc having the rough
surface portion 6 configured so that the step height D = 15
pm and the average roughness Ra = 15 ^m. Fig. 6B
illustrates a tracking error signal TE obtained from a laser
beam reflected by the recording layer 4 of such an optical
disc. In an area indicated by "X" in Fig. 6B, a disturbance
of the tracking error signal TE is observed. The area X
corresponds to an area where the rough surface portion 6 is
formed on the label-side surface. Such a disturbance of the
tracking error signal TE occurs because the rough surface
portion 6 having a high roughness causes the substrate to
largely contract, and if a step having the step height D is
formed, the contraction has an adverse impact on the shape
of the groove formed on the opposite surface of the
substrate from the rough surface portion 6.
In contrast. Fig. 6C illustrates an optical disc having
the rough surface portion 6 configured so that the step
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height D = 5 pm and the average roughness Ra = 5 jjm. Fig.
6D illustrates a tracking error signal TE obtained from a
laser beam reflected by the recording layer 4 of such an
optical disc. In this case, any disturbance of the tracking
error signal TE is not observed. Accordingly, it can be
seen that a disturbance of the tracking error signal TE
occurring during recording and reproduction can be prevented
(or can be reduced to an allowable range) by appropriately
setting the step height D of a step of the rough surface
portion 6 when the rough surface portion 6 is formed on the
label-side surface.
Therefore, the present inventors conducted an
experiment to examine a variety of structures of the rough
surface portion 6. Fig. 7 illustrates the residual error in
the tracking error signal TE observed when the step height D
and the average roughness Ra of the rough surface portion 6
are changed. Note that in the case of a BD-R and the 6x
linear speed, it is necessary that the residual error be
less than or equal to 20 nm at a position at which the
radius is 40 mm. That is, since a BD has a track pitch of
320 nm, an offset of tracking of 20 nm or more is not
acceptable. The 6x speed linear is chosen since the 6x
linear speed is currently the highest linear speed of a BD.
At that time, only the values in an area encircled by a
broken line in Fig. 7 satisfy the above-described conditions.
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That is, the step height D is less than or equal to 10 pm,
and the average roughness Ra is less than or equal to 10 ^m.
Therefore, the rough surface portion 6 acceptable in
consideration of the effect on the tracking error signal TE
has a step height D of 10 ^m or less and an average
roughness Ra of 10 |jm or less.
The visibility is discussed next. If the average
roughness Ra of the rough surface portion 6 is 2 urn or less,
the visibility is decreased. The reason for this is that as
the average roughness Ra is made closer to 0 pm, the rough
surface portion 6 becomes more similar to a mirror surface.
Accordingly, the contrast difference between the rough
surface portion 6 and the mirror surface portion 7 decreases.
In contrast, if the rough surface portion 6 has a certain
level of fineness (e.g., about 5 fim) , the rough surface
portion 6 relatively whitens. Thus, the contrast difference
between the rough surface portion 6 and the mirror surface
portion 7 occurs, and the visibility increases. In addition,
the step height D is determined through an etching process.
As described above, it is desirable that the step height D
be less than or equal to 10 jim. In terms of visibility, if
the step height D is grater than a certain value, the rough
surface portion 6 is easily recognizable from the mirror
surface portion 7.
For the reason noted above, the step height D of the
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step of the rough surface portion 6 is set to a value less
than or equal to 10 ^m and, preferably, to a value between
10 ^m and 1 pm, inclusive. In addition, the average
roughness Ra of the rough surface portion 6 is set to a
value between 10 |4m and 2 ^iin inclusive, and, preferably, to
about 5 ^m. As described above, the rough surface portion 6
is formed on the label-side surface by transferring the
rough surface portion 124a of the label-side surface mirror
124 onto the label-side surface when the substrate is
generated in step F103 illustrated in Fig. 2 (refer to Figs.
5A and 5B). Accordingly, if the concave portion of the
rough surface portion 124a of the surface of the label-side
surface mirror 124 has a concave step having a height of 10
|am or less and the average roughness Ra is between 10 ^ and
2 |am inclusive, the optical disc according to the present
exemplary embodiment can be produced. In order to form such
a rough surface portion 124a, the etching time and the type
of solvent for the label-side surface mirror 124 can be
controlled. Thereafter, by forming the rough surface
portion 6 of the disc substrate 1 using the label-side
surface mirror 124, an optical disc having information
displayed on the label-side surface and an excellent
recording and reproduction performance (a tracking
performance) can be provided.
In addition, for an optical disc according to the
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present exemplary embodiment, label printing is not
necessary. Accordingly, the manufacturing cost can be
advantageously reduced, and a printing process is not
necessary.
While the above exemplary embodiment has been described
with reference to a BD-R, the present technology is
applicable to a Blu-ray disc, such as a BD-RE and a BD-ROM,
and an optical disc other than a Blu-ray disc. That is, the
present technology is applicable to an optical disc formed
from a substrate having a label-side surface on one side and
a recording layer for recording and reproducing information
on the other side. In particular, the present technology is
suitable for a high-density optical disc that has a density
higher than or equal to that of a Blu-ray disc having a
track pitch of 0.32 pm or less. Furthermore, the present
technology is applicable to another type of recording medium,
such as a card recording medium, in addition to a discshaped
recording medium.
The present technology contains subject matter related
to that disclosed in Japanese Priority Patent Application JP
2011-009383 filed in the Japan Patent Office on January 20,
2011, the entire contents of which are hereby incorporated
by reference.
It should be understood by those skilled in the art
that various modifications, combinations, sub-combinations
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and alterations may occur depending on design requirements
and other factors insofar as they are within the scope of
the appended claims or the equivalents thereof.

WHAT IS CLAIMED IS:
1. A recording medium comprising:
a substrate having a label-side surface on one side and
a recording layer for recording and reproducing information
on the other side;
wherein the label-side surface of the substrate has a
mirror surface portion and a rough surface portion, a step
having a step height of 10 pm or less is formed between the
mirror surface portion and the rough surface portion, and an
average roughness of the rough surface portion is between 10
^im and 2 |jin inclusive, and wherein visible information is
formed on the label-side surface by using the mirror surface
portion and the rough surface portion.
2. The recording medium according to Claim 1, wherein
the step height of the rough surface portion is between 10
pm and 1 jxm inclusive, and the average roughness of the
rough surface portion is about 5 [ua.
3. The recording medium according to Claim 2, wherein
the recording layer has a groove pattern for recording
information.
4. A method for manufacturing a recording medium,
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comprising:
generating a substrate having one surface serving as a
label-side surface and the other surface having an
irregularity pattern for forming a recording layer, the
label-side surface including a mirror surface portion and a
rough surface portion having a step of 10 \m. or less formed
therebetween, an average roughness of the rough surface
portion being between 10 |jm and 2 )im inclusive, visible
information being formed on the label-side surface by using
the mirror surface portion and the rough surface portion;
and
forming, on the irregularity pattern of the substrate,
a layer structure including at least a recording layer and a
cover layer,
5. The method according to Claim 4, wherein the rough
surface portion is formed so that the step height of the
rough surface portion is between 10 ^m and 1 jam, inclusive,
and an average roughness of the rough surface portion is
about 5 )jin.
Dated this 13/01/2012