U.S. patent application number 11/759635 was filed with the patent office on 2009-01-15 for optical disk and optical disk apparatus.
This patent application is currently assigned to KABUSHIKI KAISHA TOSHIBA. Invention is credited to Yasuaki OOTERA.
Application Number | 20090016206 11/759635 |
Document ID | / |
Family ID | 32064022 |
Filed Date | 2009-01-15 |
United States Patent
Application |
20090016206 |
Kind Code |
A1 |
OOTERA; Yasuaki |
January 15, 2009 |
OPTICAL DISK AND OPTICAL DISK APPARATUS
Abstract
There is provided an optical disk which adopts a PRML (Partial
Response and Maximum Likelihood) method for reproduction of
recorded information and whose shortest pit has a conical shape
without a bottom surface. If the PRML method is adopted, since it
is not necessary to assure a large amplitude of a reproduction
waveform of the shortest pit, the pit may have a conical shape. The
conical pit enables recording with the dense shortest pit even in a
conventional original disk recorder and original disk exposure
process, thereby increasing a recording density in a dividing
direction.
Inventors: |
OOTERA; Yasuaki;
(Kawasaki-shi, JP) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
Tokyo
JP
|
Family ID: |
32064022 |
Appl. No.: |
11/759635 |
Filed: |
June 7, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10679273 |
Oct 7, 2003 |
7283459 |
|
|
11759635 |
|
|
|
|
Current U.S.
Class: |
369/275.4 ;
G9B/7.039; G9B/7.139 |
Current CPC
Class: |
G11B 7/24085 20130101;
G11B 7/261 20130101 |
Class at
Publication: |
369/275.4 ;
G9B/7.139 |
International
Class: |
G11B 7/24 20060101
G11B007/24 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 7, 2002 |
JP |
2002-294008 |
Claims
1. (canceled)
2. An optical disk comprising a reflecting film formed on a molded
substrate having pits indicative of information, the information
being read from the reflecting film side by using a laser beam, a
cross-sectional shape of the shortest pit is trapezoidal in the
molded substrate, and it is triangular on a surface of the
reflecting film.
3. The optical disk according to claim 2, wherein a bottom width x
of the shortest pit cross section in the molded substrate is as
follows: x=2dsin.theta. (within.+-.20%) wherein .theta. is a tilt
angle of a wall surface of the shortest pit in the molded
substrate, and d is a film thickness of the reflecting film.
4. The optical disk according to claim 2, wherein a pit cross
section of a pit other than the shortest pit has a trapezoidal
shape in both the molded substrate and the reflecting film
surface.
5. The optical disk according to claim 3, wherein a pit cross
section of a pit other than the shortest pit has a trapezoidal
shape in both the molded substrate and the reflecting film
surface.
6. A method of manufacturing an optical disk by forming a
reflecting film on a molded substrate having pits indicative of
information formed thereto, the shortest pit cross-sectional shape
before forming the reflecting film is trapezoidal and the shortest
pit cross-sectional shape after forming the reflecting film is
triangular.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of co-pending U.S.
application Ser. No. 10/679,273, filed Oct. 7, 2003, and for which
priority is claimed under 35 U.S.C. .sctn.121. This application is
based upon and claims the benefit of priority under 35 U.S.C.
.sctn. 119 from the prior Japanese Patent Application No.
2002-294008, filed Oct. 7, 2002, the entire contents of both
applications are incorporated herein by reference in their
entireties.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical disk, and more
particularly to a shape of an information pit formed on an optical
disk.
[0004] 2. Description of the Related Art
[0005] In an optical disk, pits are carved on a transparent molded
substrate at a part where data is recorded in advance, e.g., a
pre-formatted portion of an RAM disk or an ROM disk. Such a pit is
irradiated with a laser beam through a molded substrate from a
surface opposite to the surface on which the pit is formed, and
information is read.
[0006] Although the pit is formed with different sizes in
accordance with recording information, its size is a sub-micron
order. Forming a pit as small and accurate as possible is important
to increase a recording density of the optical disk.
[0007] For example, in a current DVD-ROM , the shortest pit length
(size in a circumferential direction) is 0.40 .mu.m, a depth is
approximately 100 nm, and a bottom forms a smooth conical
trapezoid. In the conventional DVD, a time length of each pit is
detected by slicing (binarizing) a reproduction waveform by a
predetermined threshold value, and reproduces information by
converting this length into data. However, in order to correctly
reproducing information by this method, the pit length must be
stably formed, and a signal amplitude which is sufficient for
enabling slicing must be obtained. Therefore, the shortest pit
shape must be conical trapezoid with a flat bottom, and this is one
factor determining a limit of the recording density. A bottom
circumferential size of the shortest pit is stipulated to, e.g.,
(0.2 to 0.25).times.(wavelength)/NA/1.14 .mu.m. Here, NA is a
numerical aperture of an object lens.
[0008] As one conformation of an optical disk for coming
generation, for example, Jpn. Pat. Appln. KOKAI Publication No.
10-302310 proposes a mode of reading a signal through a cover layer
having a thickness of approximately 0.1 mm.
[0009] The optical disk having such a conformation is the same as a
conventional optical disk in that a reflecting film is formed on
irregularities provided to the molded substrate and this film is
irradiated with a laser beam in order to read a signal. In case of
this conformation, however, as different from a conventional
optical disk such as a CD or a DVD, the laser beam is transmitted
through a cover layer instead of the molded substrate, and the
reflecting film is irradiated with this laser beam.
[0010] The irregularities called the pit have a size of a
sub-micron order, and how correctly the small pit can be formed is
one important element which determines a signal quality and a
recording density of the optical disk.
[0011] In the conventional optical disk which reads a signal by
slicing a reproduction waveform by using a predetermined threshold
value and binarizing it, the pit must be formed in such a manner
that a reproduction waveform amplitude can be stably and largely
obtained even in case of the shortest pit. Therefore, since a
conical trapezoidal shape having a flat bottom surface must be
assured even in the shortest pit, the recording density of the
optical pit cannot be increased any further.
[0012] Further, like the prior art, when the shortest pit shape is
formed into a conical trapezoid having a bottom surface and a large
reproduction amplitude of a closest signal is assured, a difference
between the shortest pit and the second shortest pit becomes small,
and it is hard to discriminate a reproduction signal waveform of
the shortest pit and a reproduction signal waveform of the second
shortest pit. This becomes a serious factor of erroneous reading of
a signal.
[0013] Furthermore, in a cover layer type disk that a reflecting
film surface is irradiated with a laser beam instead of a substrate
through a cover layer and information is read, or a Blu-Ray disc,
when the shortest pit length is dense to the limit in order to
increase the recording density, forming a metallic reflecting film
may fill up the shortest pit in some cases. In such a case, there
occurs a problem that a reproduction signal is deteriorated.
[0014] It is, therefore, an object of the present invention to
provide a pit shape which can readily increase an information
recording density of an optical disk.
BRIEF SUMMARY OF THE INVENTION
[0015] A shape of the shortest pit is determined as a conical shape
having no bottom surface, and a PRML (Partial Response and Maximum
Likelihood) mode is used for reproduction of recording information.
When the PRML mode is adopted, since it is not necessary to assure
a large amplitude of a reproduction waveform from the shortest pit,
the pit may have a conical shape. When the conical pit can suffice,
the shortest pit can be dense and information can be recorded even
in a conventional original board recorder or original board
exposure process, thereby increasing a recording density in a
circumferential direction.
[0016] In a cover layer type disk, a bottom width in a molded
substrate having the shortest pit is determined as
2.times.(reflecting film thickness).times.sin (pit wall angle)
(within .+-.20%). By doing so, the pit can be prevented from being
filled up by formation of the reflecting film, while the pit has a
conical shape with a size suitable for increasing the density after
formation of the film.
[0017] In this manner, the bit bottom width is stipulated, the
conical trapezoidal pit is formed on the molded substrate, and the
film is formed thereon. As a result, the bottom surface is filled,
and the conical pit is formed. Consequently, the high-recording
density pit with the excellent asymmetry can be manufactured by
using a current original board production process while suppressing
irregularities in shape.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0018] The file of this patent contains at least one photograph
executed in color. Copies of this patent with color photographs
will be provided by the Patent and Trademark Office upon request
and payment of the necessary fee.
[0019] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate embodiments of
the invention, and together with the general description given
above and the detailed description of the embodiments given below,
serve to explain the principles of the invention.
[0020] FIG. 1A is a plane view showing a pit shape of a
conventional optical disk, and FIG. 1B is a cross-sectional
view;
[0021] FIG. 2A is a plane view showing a pit shape of an optical
disk according to a first embodiment of the present invention, and
FIG. 2B is a cross-sectional view;
[0022] FIG. 3 is a flow diagram of an optical disk manufacturing
method;
[0023] FIGS. 4A and 4B show reproduction waveforms of respective
pits;
[0024] FIG. 5 shows a reproduction waveform of an optical disk
having a density of 15 GB/plane class manufactured using a
conventional pit shape;
[0025] FIG. 6 shows a reproduction waveform of an optical disk
having a density of 15 GB/plane class manufactured using a pit
shape according to the present invention;
[0026] FIGS. 7A and 7B show pit shapes of an optical disk according
to a second embodiment of the present invention;
[0027] FIG. 8 is a cross-sectional view when a film is formed on
the shortest pit of the optical disk according to the present
invention; and
[0028] FIG. 9 is a block diagram showing a structure of an optical
disk apparatus according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Embodiments according to the present invention will now be
described in detail hereinafter with reference to the accompanying
drawings. The following describes embodiments according to the
present invention, and does not restrict an apparatus and a method
according to the present invention.
[0030] FIG. 1A is a plane view showing a pit shape of a
conventional optical disk; FIG. 1B, a cross-sectional view; FIG.
2A, a plane view showing a pit shape of an optical disk according
to a first embodiment of to the present invention; FIG. 2B, a
cross-sectional view; FIG. 3, a flow diagram of an optical disk
manufacturing method; FIG. 4, a reproduction signal waveform chart
of each pit; FIG. 5, a reproduction waveform chart of an optical
disk having a density of 15 GB/plane class manufactured using a
conventional pit shape; and FIG. 6, a reproduction waveform chart
of an optical disk having a density of 15 GB/plane class
manufactured using a pit shape according to the present
invention.
[0031] In the optical disk, usually, the disk on which pits such as
shown in FIG. 1 are recorded is read by using a laser beam, and the
information is reproduced. The laser beam enters the optical disk
in a direction indicated by an arrow in FIG. 1B, and the
information is read based on an intensity of a reflected light
beam.
[0032] In this embodiment, although it is determined the disk has a
diameter of 120 mm and a thickness of 1.2 mm (lamination of two
substrates each having a thickness of 0.6 mm) and it is an ROM disk
dedicated to reproduction, the present invention is not restricted
thereto, it is possible to adopt a disk having a cover layer of 0.1
mm attached to a substrate of 1.1 mm, and it includes a pre-pit
portion of an RAM disk.
[0033] In FIG. 1, reference numeral 10 denotes the shortest pit;
11, another pit; 12, a depth of the shortest pit; and 13, a depth
of another pit. In FIG. 2, reference numeral 20 designates the
shortest pit; 21, another pit; 22, a depth of the shortest pit; and
23, a depth of another pit.
[0034] A method of manufacturing a disk having such pits will now
be described hereinafter with reference to FIG. 3. First, as an
original board, a glass original board 31 having a surface polished
and cleansed is used (ST 1). A photoresist 32 is applied to the
glass original board surface (ST 2), and this surface is exposed by
using a laser beam, thereby recording information (ST 3). Then, the
photoresist on the exposed glass original board is etched,
irregularities of pits are formed (ST 4). This glass original board
is subjected to plating processing, thereby producing a stamper 33
(ST 5). This stamper 33 is used as a die, and a resin (generally,
polycarbonate) molded plate 34 is manufactured by injection molding
(ST 6). Thereafter, a reflecting film or a recording film is formed
on the molded plate 34 (ST 7), and this plate is attached to
another molded plate 35 manufactured in the similar manner (ST 8),
thereby bringing the optical disk to completion. In this
embodiment, a thickness of each of the molded substrates 34 and 35
is 0.6 mm.
[0035] In the conventional optical disk, a pit bottom surface is
formed flat even in case of the shortest pit as shown in FIG. 1.
That is because an amplitude of a reproduction signal from the
shortest pit tends to become very small and binarization is
impossible by slicing a reproduction waveform by a predetermined
threshold value as it is. Using a conical trapezoid in this manner
can increase the amplitude slightly and enable slicing. In this
case, only a pit length of the shortest pit is set longer than a
theoretical value. For example, when a pit corresponding to a 2 T
code (T: a length corresponding to a reference clock cycle) is the
shortest pit, a pit length of the 2 T code is set longer than 2/3
of a pit length of, e.g., a 3 T code. That is, only the pit of the
2 T code has a length which is not proportionate to a code
value.
[0036] When manufacturing a disk based on steps such as shown in
FIG. 3, a depth of the shortest pit depends on a film thickness of
the photoresist 32, a tilt angle of a wall surface of the pit
depends on an intensity distribution in a laser spot and a
characteristic and an etching condition of the photoresist 32. A
depth of the pit is restricted to .lamda./4n (.lamda.: a
reproduction wavelength, n: a refractive index of a substrate) in
case of an optically ROM in order to obtain a reproduction signal,
and a tilt angle of the pit is approximately 40 degrees in the
current process technology. In this case, in order to flatten the
bottom surface of the shortest pit as shown in FIG. 1, the length
of the shortest pit cannot be greatly reduced. Therefore, it can be
understood that the recording density in the disk circumferential
direction is not very high.
[0037] On the other hand, in the optical disk for coming
generation, it has been examined that information is reproduced by
using a mode called a PRML (Partial Response and Maximum
Likelihood) method in place of reading a signal by a binarization
method based on slicing of a reproduction waveform like the prior
art. This mode converts a reproduction signal from each pit into
multiple values based on its waveform and amplitude level. In case
of this mode, it is desirable that the reproduction signal from the
shortest pit is small so that it can be recognized as a signal from
the shortest pit. Therefore, it is not necessary to form a pit
bottom surface by setting only the pit of the shortest code longer
than a value which is proportionate to the code and increase the
signal amplitude as described above. On the contrary, since the
PRML reads information on the amplitude level, centers of the
reproduction signal amplitudes from the respective pits must match
with each other. That is, the PRML cannot be applied unless the
asymmetry (which will be described later) is close to 0.
[0038] Base on these points, the pit shape which realizes a
high-recording density suitable for the PRML is the first
embodiment according to the present invention shown in FIG. 2. A
characteristic of this pit shape lies in that the shortest pit does
not have a flat bottom surface but has a conical shape and its
depth 22 is smaller than a depth 23 of another pit. An inclination
of a wall surface of the shortest pit is equal to that of the
conventional pit shown in FIG. 1.
[0039] The conventional shortest pit shape which is a conical
trapezoidal shape has a limit in reducing the shortest pit length
since the tilt angle of the pit wall surface is gentle. However, if
this conical shape is adopted, even the current original board
process technology can greatly reduce the shortest pit length,
thereby considerably increasing the recording density.
[0040] Since the shortest pit has a conical shape and its bottom
surface is not flat, a reproduction amplitude from the shortest pit
becomes small. Further, irregularities in pit size (mainly a depth
22) caused due to unstableness of the exposure process slightly
become large. However, these phenomena which are serious drawbacks
in reproduction by the slice method hardly become shortages when
using the PRML method. Conventionally, there is no optical disk
product having a conical pit nor a concept of reading the conical
pit by using the PRML method.
[0041] In this embodiment, since the bottom surface of the shortest
pit is not formed, the asymmetry of the reproduction waveform can
approximate 0. FIG. 4A shows a reproduction signal waveform
obtained when reproducing a pit having a conventional shape, and
FIG. 4B shows a reproduction signal waveform obtained when
reproducing a pit having a shape according to the present
invention. In FIGS. 4, reference character W2T denotes a
reproduction signal waveform of a 2 T code pit (shortest pit); W3T,
a reproduction signal waveform of a 3 T code pit; WMT, a
reproduction signal waveform of a longest code pit; A2T, an
amplitude of a 2 T code pit reproduction signal; AMT, an amplitude
of a longest code pit reproduction signal; L2T, a central level of
a 2 T code pit reproduction signal waveform; LMT, a central level
of a reproduction signal waveform of a longest code pit; and D, a
level difference (LMT-L2T) obtained by subtracting the central
level L2T from the central level LMT.
[0042] Although the asymmetry can be defined with respect to the
pit reproduction signal of each code, it is determined as the
asymmetry of the 2 T code which is most important in the present
invention. That is, here, the asymmetry is determined as a value
(D/AMT) obtained by dividing the level difference D by the
reproduction signal waveform amplitude AMT of the longest code
pit.
[0043] Assuming that an area where the pit is formed is a pit area
and an area where no pit is formed is a mirror area, the mirror
area has a high reflected light level than that of the pit area
(see FIGS. 1 and 2). In the prior art as shown in FIG. 4A, although
the asymmetry has a large value, this is because the pit length of
the conventional shortest pit is set longer than a value which is
in proportion to the code and the central level of the reproduction
signal waveform of the shortest pit is lower than that of a pit of
any other code.
[0044] As shown in FIG. 4B, in regard to the reproduction signal
waveform of the pit having the shape according to the present
invention, the reproduction signal amplitude A2T of the shortest
pit is smaller than that of the prior art shown in FIG. 4A, and the
asymmetry (D/AMT) also becomes small. That is because the shortest
pit has a conical shape and the reflected light level in the
vicinity of the area where the shortest pit is formed is higher
than that of the prior art.
[0045] Furthermore, since the signal amplitude of the shortest pit
is small in accordance with the pit length, it is easy to
discriminate the shortest pit (e.g., 2 T) and the second shortest
pit (e.g., 3 T). Thus, conical shape of the shortest pit is
advantageous in PRML reproduction. According to the pit shape of
the present invention, the asymmetry can readily approximate +0.10
or lower (preferably within a range of .+-.10). Defining a value
obtained by dividing the shortest pit reproduction signal amplitude
A2T by the maximum amplitude AMT (A2T/AMT) as a resolution, a
resolution of not more than 15% can be easily realized in the
present invention. It is to be noted that, in the present
invention, the 2 T code pit is preferably constantly formed into a
conical shape in order to facilitate identification of the 2 T code
pit (shortest pit) and the 3 T code pit. That is, when the shortest
pit (2 T) is formed in proportion to the code (3 T, 4 T, . . .),
the shortest pit is always formed into a conical shape even if the
shortest pit has a flat bottom portion. In such a case, a
circumferential length of the shortest pit (2 T) is not in
proportion to the pit lengths of other codes.
[0046] FIGS. 5 and 6 respectively show results of manufacturing
disks of 15 GB/plane class with the pit shape according to the
prior art and the pit shape according to the present invention by
trial and reproducing information.
[0047] FIG. 5 shows a signal reproduction waveform of an optical
disk having the conventional pit shape (FIG. 1), and the jitter
obtained by the slice method is 12.6% since the signal amplitude
A2T of the shortest pit (2 T) is large, but reproduction by the
PRML method cannot be performed because of the large asymmetry and
an error rate cannot be measured. On the other hand, FIG. 6 shows a
pit shape according to the present invention (FIG. 2), and the
jitter is 13.4% which is relatively bad since the shortest signal
amplitude A2T is small, but the asymmetry is good and an error rate
is 4.times.10-6 in reproduction by PRML, which is a level making
practicable.
[0048] As described above, in the optical disk that information is
reproduced by the PRML method, using the pit shape according to the
present invention can increase the recording density in a disk
tangential direction even in the conventional original board
process, and the asymmetry is also improved, thereby enhancing the
error rate.
[0049] A second embodiment according to the present invention will
now be described. FIGS. 7 are plane views showing a pit shape of an
optical disk according to the present invention, and FIG. 8 is a
cross-sectional view when forming a film on the shortest pit of the
optical pit according to the second embodiment of the present
invention.
[0050] In FIG. 7A, reference numeral 50 denotes the shortest pit,
and 51 designates another pit. In FIG. 7B, reference numeral 52
denotes a molded substrate; 53, a reflecting film; x, a shortest
bit bottom width on the molded substrate. In the present invention,
a target is an optical disk which is of a type that a laser beam
enters from a side opposite to the molded substrate 52 as indicated
by an arrow in FIG. 7B (e.g., a disk which is of a surface
recording type or a type that information is read through a cover
layer of approximately 0.1 mm or an L1 layer in a conventional
two-layer DVD disk). That is, an incident direction of the laser
beam is an opposite direction as compared with the optical disk
shown in FIG. 1 or 2.
[0051] In the conventional optical disk, since the recording
density is low, the bottom width x of the shortest pit is
sufficiently wider than a film thickness of the reflecting film 53.
Therefore, there occurs no problem even if the reflecting film is
formed on the bit, the film is irradiated with the light from the
direction indicated by an arrow and a signal is read. However, in
case of the high-density optical disk for coming generation, when
using the PRML mode in signal reproduction or when using a code
series that the shortest pit is 2 T in particular, since the
recording line density is very high, the bottom width x of the
shortest pit approximates 0 in the current original board
manufacturing process technology. In this case, the shortest pit is
filled by forming the film, and reading the signal from the
direction indicated by an arrow deteriorates the reproduction
signal.
[0052] On the contrary, when the bottom width of the shortest pit
is set larger than a value which is in proportion to the code, this
becomes disadvantageous for increasing the density, and
discrimination of the shortest pit and the second shortest pit
becomes difficult because of the extremely large signal from the
shortest pit, thereby degrading the asymmetry. In particular, since
the asymmetry is important when using the PRML mode in signal
reproduction, the pit length of the shortest pit cannot be set
larger than a value which is in proportion to the code in the light
of this point.
[0053] For the reasons mentioned above, the shortest pit must be
set as small as possible in a range which does not fill the
shortest pit and does not lead to degradation in the signal. Thus,
in the present invention, as shown in FIG. 8, the bottom width x of
the shortest pit is determined as x=2.times.d.times.sin.theta.
(within .+-.20%) (d: a thickness of the reflecting film, 0: a wall
angle of the pit).
[0054] That is, in the optical disk according to this embodiment,
the cross-sectional shape of the shortest pit is trapezoidal in the
molded substrate and triangular on the surface of the reflecting
film. Namely, when manufacturing such an optical disk, as to the
shortest pit, the pit cross-sectional shape before forming the
reflecting film is trapezoidal, and the pit cross-sectional shape
after forming the reflecting film is triangular.
[0055] By stipulating the shortest pit to this size, the pit is not
filled by formation of the film, and the signal is not
deteriorated. Additionally, since the conical pit can be obtained
after forming the film, the intensity of the signal from the
shortest pit can be appropriately suppressed. As a result, it is
possible to prevent deterioration in the asymmetry or erroneous
reading of signals from pits with any other sizes, especially
erroneous reading of the 2 T and 3 T pits.
[0056] In the optical disk manufactured by the current original
board manufacturing process technology, the wall angle .theta. of
the pit is generally approximately 40 degrees. Further, if a blue
laser beam is used as a reproduction light ray, when trying to
obtain the reflectivity of, e.g., approximately 70%, the reflecting
film thickness of approximately 25 nm is required in case of Al
(aluminium), and approximately 50 nm is required in case of Ag
(silver). Applying these parameters to the expression of this
embodiment, the bottom width of the shortest pit is 32.+-.6 nm in
case of Al, and it is 64.+-.13 nm in case of Ag.
[0057] It is to be noted that only the bottom width x of the
shortest pit is not in proportion to the bottom width of another
pit. That is, in cases where the shortest pit is the 2 T code pit,
the bottom width x is not 2/3 of, e.g., the 3 T code pit, but it is
set in accordance with a thickness of the reflecting film.
Furthermore, the bottom width x is a value which is not directly
proportionate to the recording density.
[0058] Advantages obtained when stipulating the shortest pit to
this size will now be described. First, when manufacturing the
disk, the pit shape and the reproduction signal are stabilized
since the pit cross section of the molded substrate has a
trapezoidal shape. That is because, when the cross section of the
molded substrate has a triangular shape, irregularities in original
board exposure condition or irregularities in film thickness when
forming the film scatter the pit size, and the reproduction signal
from that pit is sensitively affected by the pit size and becomes
unstable. As another advantage, this size is a limit size that the
pit is not filled when forming the film. As a result, the pit can
become dense to the limit density which does not fill the pit and
does not deteriorate the reproduction signal. Moreover, since the
bottom is filled and the conical pit shape can be obtained after
forming the film, the reproduction signal from the shortest bit
does not become too large, and the asymmetry approximates 0, which
is also advantageous in discrimination of the signal.
[0059] As described above, in the optical disk to which the light
enters from the surface opposite to the molded substrate in order
to reproduce information, the shortest pit has a conical shape
after forming the film by using the pit shape according to the
present invention, and the signal waveform from the shortest pit
can be appropriately obtained while increasing the recording
density.
[0060] Description will now be given as to an embodiment of an
optical disk apparatus which records/reproduces information by
using the optical disk having the pit with the above-described
shape formed thereto. FIG. 9 is a block diagram showing a structure
of the optical disk apparatus according to this embodiment.
[0061] An optical disk 61 is an optical disk dedicated to reading
or an optical disk which can record user data. The disk 61 is
rotated and driven by a spindle motor 63. Recording and
reproduction of information with respect to the optical disk 61 are
carried out by an optical pickup head (which will be referred to as
a PUH hereinafter) 65. The PUH 65 is connected to a thread motor 66
through a gear, and this thread motor 66 is controlled by a thread
motor control circuit 68.
[0062] A seek destination address of the PUH 65 is inputted to the
thread motor control circuit 68 from a CPU 90, and the thread motor
control circuit 68 controls the thread motor 66 based on this
address. A permanent magnet is fixed inside the thread motor 66,
and the PUH 65 moves in a radial direction of the optical disk 61
when a drive coil 67 is excited by the thread motor control circuit
68.
[0063] To the PUH 65 is provided an object lens 70 which is
supported by a wire or a flat spring which is not illustrated. The
object lens 70 can move in a focusing direction (direction of an
optical axis of the lens) by drive of a drive coil 72, and it can
move in a tracking direction (direction orthogonal to the optical
axis of the lens) by drive of a drive coil 71.
[0064] A semiconductor laser 79 emits a laser beam by a laser drive
circuit 75 in a laser control circuit 73. The optical disk 61 is
irradiated with the laser beam emitted from the semiconductor laser
79 through a collimator lens 80, a half prism 81 and an object lens
70. The reflected light from the optical disk 61 is led to a
photodetector 84 through the object lens 70, the half prism 81, a
condensing lens 82 and a cylindrical lens 83.
[0065] The photodetector 84 consists of, e.g., four divided
photodetector cells, and a detection signal from each divided
photodetector cell is outputted to an RF amplifier 85. The RF
amplifier 85 combines signals from the photodetector cells, and
generates a focus error signal FE indicative of an error from just
focusing, a tracking error signal TE indicative of an error between
a beam spot center of the laser beam and a track center and an RF
signal which is a full addition signal of the photodetector cell
signals.
[0066] The focus error signal FE is supplied to the focusing
control circuit 87. The focusing control circuit 87 generates a
focus control signal FC in accordance with the focus error signal
FE. The focus control signal FC is supplied to the drive coil 72 in
the focusing direction, and focus servo is carried out so that the
laser beam is constantly just focused on the recording film of the
optical disk 61.
[0067] The tracking error signal TE is supplied to the tracking
control circuit 88. The tracking control circuit 88 generates a
tracking control signal TC in accordance with the tracking error
signal TE. The tracking control signal TC is supplied to the drive
coil 72 in the tracking direction, and tracking servo is carried
out so that the laser beam constantly traces on the track formed on
the optical disk 61.
[0068] When the focus servo and the tracking servo are effected, a
change in reflected light from, e.g., the pit formed on the track
of the optical disk 61 is reflected to the full addition signal RF
of output signals from the respective photodetector cells of the
photodetector 84. This signal is supplied to a data reproduction
circuit 78. The data reproduction circuit 78 reproduces recorded
data based on a reproduction clock signal from a PLL circuit
76.
[0069] When the object lens 70 is controlled by the tracking
control circuit 88, the thread motor 66, i.e., the PUH 65 is
controlled by the thread motor control circuit 68 in such a manner
that the object lens 70 is positioned in the vicinity of a
predetermined position in the PUH 65.
[0070] The motor control circuit 64, the thread motor control
circuit 68, the laser control circuit 73, the PLL circuit 76, the
data reproduction circuit 78, the focusing control circuit 87, the
tracking control circuit 88, an error correction circuit 62 and
others are controlled by a CPU 90 through a bus 89. The CPU 90
comprehensively controls this recording/reproducing apparatus in
accordance with an operation command provided from a host device 94
through an interface circuit 94. Furthermore, the CPU 90 uses an
RAM 91 as a working area and performs a predetermined operation in
accordance with a program recorded in an ROM 92.
[0071] The data reproduction circuit 78 reproduces information by a
binarization method which binarizes an information reproduction
signal waveform by slicing using a threshold voltage or a PRML
method which converts an amplitude of the information reproduction
signal waveform into multiple values. It is designed to reproduce
information of the optical disk that the asymmetry of a
reproduction signal is not more than +0.10 and a ratio of a signal
amplitude of the shortest pit relative to a signal amplitude of the
longest pit, i.e., the resolution (A2T/AMT) is not more than 15%
when reproducing information by the PRML method.
[0072] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general invention concept as defined by the
appended claims and their equivalents.
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