U.S. patent application number 10/212097 was filed with the patent office on 2002-12-26 for multilevel phase change optical recording medium.
Invention is credited to Ichihara, Katsutaro.
Application Number | 20020197560 10/212097 |
Document ID | / |
Family ID | 12630568 |
Filed Date | 2002-12-26 |
United States Patent
Application |
20020197560 |
Kind Code |
A1 |
Ichihara, Katsutaro |
December 26, 2002 |
Multilevel phase change optical recording medium
Abstract
A multilevel phase change optical recording medium comprises
first to N-th (N.gtoreq.2) phase change optical recording layers,
wherein an i-th recording layer and a j-th recording layer, which
are two recording layers arbitrarily selected from the first to
N-th recording layers, meet the conditions of T.sub.i>T.sub.mi
and .tau..sub.wi<.tau..sub.xi, and T.sub.j<T.sub.mj or
.tau..sub.wj>.tau..sub.xj, with respect to a particular
recording laser beam selected from recording laser beams having
different power levels, where T is the maximum temperature of the
recording layer in a recording operation, T.sub.m is the melting
point of the recording layer, T.sub.x is the crystallizing
temperature of the recording layer, .tau..sub.w is a time required
for the recording layer to be cooled down from T.sub.m to T.sub.x
after the laser beam irradiation, and .tau..sub.x is the
crystallizing time of the recording layer.
Inventors: |
Ichihara, Katsutaro;
(Yokohama-shi, JP) |
Correspondence
Address: |
Intellectual Property Group
Pillsbury Winthrop LLP
P.O. Box 10500
McLean
VA
22102
US
|
Family ID: |
12630568 |
Appl. No.: |
10/212097 |
Filed: |
August 6, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10212097 |
Aug 6, 2002 |
|
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09030027 |
Feb 25, 1998 |
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Current U.S.
Class: |
430/270.13 ;
369/275.2; 428/64.5; 428/913; 430/945; G9B/7.014; G9B/7.024;
G9B/7.099; G9B/7.139; G9B/7.145 |
Current CPC
Class: |
Y10S 430/146 20130101;
G11B 7/24 20130101; G11B 2007/24316 20130101; G11B 7/2575 20130101;
G11B 7/259 20130101; G11B 7/257 20130101; G11B 7/2578 20130101;
G11B 2007/24314 20130101; G11B 7/244 20130101; Y10S 428/912
20130101; G11B 7/2585 20130101; G11B 2007/0013 20130101; G11B 7/126
20130101; G11B 7/2433 20130101; G11B 7/00454 20130101; G11B
2007/24312 20130101; G11B 7/0052 20130101; Y10S 428/913
20130101 |
Class at
Publication: |
430/270.13 ;
430/945; 428/64.5; 428/913; 369/275.2 |
International
Class: |
G11B 007/24 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 1997 |
JP |
9-042242 |
Claims
1. A combination comprising: (a) a recording device adapted to
generate recording laser beams having different power levels during
a recording operations; and (b) a multilevel phase change optical
recording medium received in said recording device so as to enable
said recording laser beams having said different power levels to
record data thereon during said recording operation, said recording
laser beams each irradiating the same side of said recording
medium, said medium comprising: first to Nth rewritable phase
change optical recording layers, N being greater than or equal to
two, wherein said recording layers have different compositions and
are separated by intermediate layers and are arranged within a
depth of focus of a laser beam; said recording layers including an
i-th recording layer and a j-th recording layer each being
arbitrarily selected from said recording layers and having the
following characteristics for a recording beam selected from the
aforementioned recording laser beams having different power levels:
T.sub.i>T.sub.mi and .tau..sub.wi<.tau..sub.xi, and
T.sub.j<T.sub.mj or .tau..sub.wj>.tau..sub.xj, wherein
T.sub.i and T.sub.j are the maximum temperatures to which the
selected laser beam respectively heats said i-th and j-th recording
layers during the recording operation, T.sub.mi and T.sub.mj are
the temperatures at which said i-th and j-th recording layers will
respectively melt, .tau..sub.wi and .tau..sub.wj are the times
required for said i-th and j-th recording layers to respectively
cool from T.sub.mi and T.sub.mj to respective crystallizing
temperatures T.sub.xi and T.sub.wj for each of said i-th and j-th
layers, and .tau..sub.xi and .tau..sub.xj are the times required
for said i-th and j-th layers to respectively crystallize.
2. The combination according to claim 1, wherien the j-th layer is
lower in the maximum temperature and shorter in the cooling time
compared with the i-th layer.
3. The combination according to claim 1, wherein said medium
further comprises an intermediate layer having lower thermal
conductivity than that of the recording layer and provided on the
j-th layers.
4. The combination according to claim 1, wherein said medium
further comprises an upper protective layer and a reflective layer
on the j-th layer, wherein the upper protective layer has a higher
thermal conductivity than that of the recording layer.
5. The combination according to claim 1, wherein the i-th layer
consists of Ge.sub.2Sb.sub.2Te.sub.5 and the j-th layer consists of
Ge.sub.2Sb.sub.2Te.sub.5 doped with 5 at % of Sb.
6. The combination according to claim 1, wherein the i-th layer is
closer to the substrate than the j-th layer.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a phase change optical
recording medium capable of multilevel recording.
[0002] Optical disk memories capable of reproducing or recording
and reproducing information by laser beam irradiation are widely
used as mass capacity, high-speed accessible and portable storage
media for data files such as audio, video and computer data and
expected for further development. In order to improve storage
density of such optical disks, many techniques are proposed: for
example, use of a short-wavelength gas laser for master disk
cutting, use of a short-wavelength semiconductor laser as an
operating light source, increasing the numerical aperture of an
objective lens, reducing the thickness of the disk, and so on. For
recordable optical disks, mark length recording and land-groove
recording, in addition to the foregoing techniques, are
proposed.
[0003] Also, as a high-density oriented technique, a method of
recording and reproducing multilevel data using a multilayered
recording medium has been proposed. A simplest multilayered
recording medium comprises two or more recording layers, which are
allocated to different focal points and accessed separately for
recording or reproducing. The method may provide higher reliability
in both recording and reproducing operations. However, because only
one of the recording layers can be accessed at once, it is
difficult to perform high-speed recording and reproducing
operations.
[0004] There is proposed a magnetooptical recording medium
comprising two or more recording layers arranged within the depth
of focus of a laser beam with an attempt at multilevel recording in
respective recording layers by using various levels of laser power
or recording field intensity and multilevel reproducing by analog
processing of read-out signals. The read-out signals from such a
magnetooptical recording medium are, however, based on slight Kerr
rotation angle from which a desirable level of carrier-to-noise
ratio (CNR) can only be obtained by binary digital processing.
Therefore, it will hardly be feasible to subject the read-out
signals to successful multilevel analog processing.
[0005] On the other hand, in a phase change optical recording
medium, particularly rewritable medium, intense read-out signals
can be obtained, so that it is expected to realize multilevel
processing. However, such a multilayered phase change optical
recording medium capable of recording and reproducing multilevel
data simultaneously at a high speed has not yet been in practical
use.
BRIEF SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide a
multilayered phase change optical recording medium that is capable
of recording and reproducing multilevel data at a high speed.
[0007] A multilevel phase change optical recording medium according
to the present invention comprises first to N-th phase change
optical recording layers (N.gtoreq.2), wherein an i-th recording
layer and a j-th recording layer, which are two recording layers
arbitrarily selected from the first to N-th recording layers, meet
the conditions of:
[0008] T.sub.i>T.sub.mi and .tau..sub.wi<.tau..sub.xi,
and
[0009] T.sub.j<T.sub.mj or .tau..sub.wj>.tau..sub.xj,
[0010] with respect to a particular recording laser beam selected
from recording laser beams having different power levels, where T
is the maximum temperature of the recording layer in a recording
operation, T.sub.m is the melting point of the recording layer,
T.sub.x is the crystallizing temperature of the recording layer,
.tau..sub.w is a time required for the recording layer to be cooled
down from T.sub.m to T.sub.x after the laser beam irradiation, and
.tau..sub.x is the crystallizing time of the recording layer.
[0011] A method of recording and reproducing for a multilevel phase
change optical recording medium comprising two or more recording
layers having different melting points and/or crystallizing
temperatures according to the present invention comprises the steps
of: irradiating the multilevel phase change optical recording
medium with recording beams having different power levels, thereby
performing recording; and irradiating the recorded multilevel phase
change optical recording medium with a reproducing beam to detect
reproducing signals, followed by digitizing the reproducing
signals, thereby performing reproducing.
[0012] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0013] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
[0014] FIG. 1 is a cross sectional view of the multilayered
multilevel phase change optical recording medium according to the
present invention;
[0015] FIGS. 2A and 2B are diagrams showing the thermal response of
two recording layers (i-th and j-th layers) in the multilayered
multilevel phase change optical recording medium of the present
invention;
[0016] FIG. 3 is a cross sectional view showing an example of the
multilayered multilevel phase change optical recording medium of
the present invention;
[0017] FIG. 4 is a schematic view of an optical disk drive used in
the present invention; and
[0018] FIGS. 5A and 5B are diagrams showing recording signals and
reproducing signals in the multilayered multilevel phase change
optical recording medium of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present invention will be described in more detail.
[0020] FIG. 1 is a cross sectional view of a multilevel phase
change optical recording medium according to the present invention.
As shown in FIG. 1, a lower protective layer 2 is formed on a
substrate 1. On the lower protective layer 2, there are alternately
provided, from the lower side where a laser beam is incident on,
first to N-th (N.gtoreq.2) recording layers R.sub.1, R.sub.2, . . .
, R.sub.N-1 and R.sub.N, and intermediate layers S.sub.1, . . . ,
and S.sub.N-1. On the uppermost recording layer R.sub.N, an upper
protective layer 3 and a reflective layer 4 are formed. For
recording data on the phase change optical recording medium, a
plurality of recording laser beams having different power levels is
used. The phase change optical recording layers are located within
the depth of focus of each of the laser beams.
[0021] Now, let us study two recording layers, referred to as the
i-th layer and the j-th layer (1.ltoreq.i, 2.ltoreq.j, and
i.noteq.j), which are arbitrarily selected from the first to N-th
recording layers R.sub.1 to R.sub.N constituting the phase change
optical recording medium of the present invention. FIGS. 2A and 2B
show the thermal responses of the i-th and j-th layers,
respectively, when these layers are irradiated with the recording
laser beams. In these diagrams, two profiles denoted by the letters
"A" and "B" represent thermal responses corresponding to two
recording laser beams different in power level, respectively. The
power levels of the two laser beams are in a relationship of
A<B.
[0022] The important factors in phase change optical recording are
the maximum temperature and the time response during cooling of the
recording layer. Now, assume that T is a maximum temperature of the
recording layer in a recording operation, T.sub.m is the melting
point of the recording layer, T.sub.x is the crystallizing
temperature of the recording layer, .tau..sub.w is a time required
for the recording layer to be cooled from T.sub.m to T.sub.x after
the laser beam irradiation, and .tau..sub.x is the crystallizing
time of the recording layer. The above values for the i-th and j-th
layers are represented by subscripts "i" and "j", respectively.
[0023] First, the thermal responses when the recording layers are
irradiated with the recording laser beam "A" are described. The
i-th layer is heated up to higher than its melting point T.sub.mi
and takes a time .tau..sub.wiA to be cooled down from the melting
point T.sub.mi to the crystallizing temperature T.sub.xi. Amorphous
recording marks are formed in the i-th layer if the crystallizing
time t.sub.xi of the i-th layer is longer than the time
.tau..sub.wiA, whereas no recording marks are formed in the
contrary case. Here, assume that the recording marks are formed in
the i-th layer. The j-th layer is restrained its temperature raise
by making use of a material having higher melting point T.sub.mj or
by the effect of the intermediate layer. In the j-th layer, the
maximum temperature T.sub.j is lower than the melting point
T.sub.mj. Accordingly, no recording marks are formed in the j-th
layer regardless of the cooling speed. Therefore, with respect to
the recording laser beam "A", the conditions of T.sub.i>T.sub.mi
and .tau..sub.wi>.tau..sub.- xi for i-th layer, and
T.sub.j<T.sub.mj for j-th layer are established simultaneously.
This permits the recording laser beam "A" to form recording marks
in the i-th layer selectively.
[0024] Next, the thermal responses when the recording layers are
irradiated with the recording laser beam "B" are described. The
laser beam "B" is higher in intensity compared with the laser beam
"A" and causes the i-th layer to rise up far over the melting point
T.sub.mi. The cooling time .tau..sub.wiB of the i-th layer is
longer than the cooling time .tau..sub.wiA. Amorphous recording
marks are formed in the i-th layer if the crystallizing time
.tau..sub.xi (>.tau..sub.wiA) of the i-th layer is longer than
the time .tau..sub.wiB, whereas no recording marks are formed in
the contrary case. Any one of the two cases may be selected
depending on involved conditions. Here, assume that the recording
marks are also formed in the i-th layer under the power level "B".
When the laser beam "B" is used, the j-th layer is heated up to
higher temperature than the melting point T.sub.mj with the cooling
time being .tau..sub.wjB. Recording marks are formed in the j-th
layer if the crystallizing time .tau..sub.xj of the j-th layer is
greater than .tau..sub.wjB, whereas no recording marks are formed
in the contrary case. Here, assume that the recording marks are
also formed in the j-th layer.
[0025] Although not shown in FIGS. 2A and 2B in order to avoid
complexity, when a recording laser beam "C" having a higher power
level than that of the laser beam "B" is used, thermal responses
are as follows. In this case, both the i-th and j-th layers are
heated up to higher temperatures than their melting points. When
the cooling time .tau..sub.wiC of the i-th layer is longer than
.tau..sub.xi (>.tau..sub.wiB), no recording marks are formed in
the i-th layer. If the cooling time .tau..sub.wjC of the j-th layer
is shorter than .tau..sub.xj (>.tau..sub.wjB), recording marks
are formed in the j-th layer.
[0026] Table 1 shows the relationship between recording power
levels and whether recording marks are formed or not. In the Table
1, the case where the recording marks are formed is expressed by
"1", and the case where no recording marks are formed is expressed
by "0". Note that "O" means a power level lower than the threshold
for forming recording marks in the i-th layer.
1 TABLE 1 Power level O A B C i-th 0 1 1 0 layer j-th 0 0 1 1
layer
[0027] As apparent from Table 1, the phase change optical recording
medium of the present invention can attain four different recording
states by irradiating two recording layers, which are different in
thermal response, with recording laser beams having different power
levels. On the other hand, conventional phase change recording
medium permits only two recording states of "0" and "1".
Accordingly, the present invention can improve recording density to
two times greater than that of the conventional recording medium.
Even if the selection of power level corresponding to the recording
laser beam "C" shown in Table 1 is not allowed, the recording
density can be improved by 1.5 times greater than that of the
conventional recording medium. Moreover, the two recording layers
can be accessed simultaneously, so that the high-speed recording
operation can be possible.
[0028] Furthermore, more than two recording layers may be used for
multilevel recording, although design of the recording medium
becomes difficult. In principle, with respect to a recording medium
having N recording layers, recording density can be improved to
2.sup.N times at maximum, to 2N times even if a large design margin
is taken into account, and to (N+1)/2 times at least.
[0029] The phase change optical recording medium of the present
invention is reproduced by continuously irradiating tracks on which
recording patterns are formed with a laser beam having a read-out
level. In this operation, a plurality of output levels is obtained
depending on recording states. Intensity of output signals from the
phase change optical recording medium is high enough, and a
difference between output levels is also high enough. Therefore, a
multilevel processing can be easily performed.
[0030] An example of the present invention will now be described
referring to the accompanying drawings.
[0031] FIG. 3 is a cross sectional view of the multilayered
multilevel phase change optical recording medium in this example.
As shown in FIG. 3, a lower protective layer 2 is formed on a
substrate 1. On the lower protective layer 2, there are provided a
first recording layer (the i-th layer) R.sub.1, an intermediate
layer S.sub.1, a second recording layer (the j-th layer) R.sub.2,
an upper protective layer 3, and a reflective layer 4. The
thicknesses of these layers are so adjusted that the first and
second recording layers are located within the depth of focus of
recording laser beams.
[0032] FIG. 4 illustrates the construction of an optical disk drive
used for recording and read-out operations. The optical disk 11
shown in FIG. 3 is mounted to a rotary shaft of a spindle motor 12.
In recording operation, a laser 22 is operated by a light source
controller 21 to emit a short-pulsed laser beam having a relatively
high power level. The laser beam is passed through an objective
lens 23, a half mirror 24 and a focusing lens 25 and then is
incident onto the optical disk 11, thereby forming recording marks.
In the read-out operation, a laser 22 is operated to emit a laser
beam having a low power level. The laser beam is incident onto the
optical disk 11 on which recording marks are formed. The laser beam
reflected from the optical disk 11 is passed through the focusing
lens 25 and reflected by the half mirror 24, and then is sent to
the reproducing unit 26 where reflectance change between recording
marks and non-recorded region is detected.
[0033] The optical disk drive has substantially similar arrangement
to a conventional type, but the laser 22 can emit recording laser
beams having different power levels by multilevel power
modulation.
[0034] The operational conditions of the optical disk drive are as
follows: linear velocity of the disk is 10 m/s, recording pulse
frequency is 10 MHz, recording pulse width is 50 ns, laser beam
wavelength is 650 nm, and numerical aperture (NA) of the objective
lens is 0.6. When NA of the objective lens is 0.6, full width at
half maximum (FWHM) of the beam spot becomes about 0.5 .mu.m. The
recording pulse width of 50 ns is substantially equal to duration
within which FWHM of the beam spot passes the recording layer. This
example uses recording laser beams having four different power
levels of 6 mW, 9 mW, 12 mW and 15 mW corresponding to the power
levels "O", "A", "B" and "C" shown in Table 1.
[0035] The thermal characteristics of the first and second
recording layers are so adjusted that they meet suitable conditions
for multilevel recording. The thermal conductivity and thickness of
other layers are also designed in accordance with the thermal
characteristics of the recording layers.
[0036] In this example, the first recording layer consists of 15
nm-thick Ge.sub.2Sb.sub.2Te.sub.5, and the second recording layer
consists of 15 nm-thick Ge.sub.2Sb.sub.2Te.sub.5+5 at % -Sb. The
first recording layer has a melting point T.sub.m1 of 630.degree.
C. and a crystallizing time .tau..sub.x1 of 50 ns. The second
recording layer has a melting point T.sub.m2 of 630.degree. C.,
equal to that of the first recording layer, and a crystallizing
time .tau..sub.x2 of 70 ns. The lower protective layer 2 consists
of about 150 nm-thick ZnS--SiO.sub.2, the intermediate layer
S.sub.1 consists of polytetrafluoroethylene (PTFE) which is a good
thermal insulator, the upper protective layer 3 consists of 20
nm-thick Si--N which is high in heat radiating effect, and the
reflective layer 4 consists of 50-nm thick Al having a high thermal
conductivity. These layers are deposited by conventional magnetron
sputtering.
[0037] Thermal responses of both recording layers calculated from
the above conditions are as follows: When the recording laser beams
having power levels of 6 mW, 9 mW, 12 mW and 15 mW are used,
respectively, the maximum temperatures will be 500.degree. C.,
700.degree. C., 900.degree. C. and 1100.degree. C. for the first
recording layer, and 450.degree. C., 600.degree. C., 750.degree. C.
and 900.degree. C. for the second recording layer.
[0038] Also, calculated values of duration within which the
recording layers are retained below the melting point and above the
crystallizing temperature in the cooling process after the
irradiation of recording laser beams of various power levels
(referred to as a cooling time hereinafter) are as follows. The
cooling times for the first recording layer will be 30 ns, 45 ns
and 60 ns corresponding to the power levels of "A", "B" and "C",
respectively. The maximum temperature of the second recording layer
is maintained below the melting point in the case where the power
level "A", so that there is no need to consider its cooling time.
The cooling times for the second recording layer will be 30 ns and
50 ns corresponding to the power levels of "B" and "C",
respectively.
[0039] As described above, the second recording layer is lower in
the maximum temperature and shorter in the cooling time compared
with the first recording layer. This can be explained by the fact
that the recording laser beam is incident onto the side of the
first recording layer, temperature difference between the first and
second recording layers is maintained because the intermediate
layer having a low thermal conductivity, and heat is taken away
from the second recording layer through the upper protective layer
and the reflective layer having a high thermal conductivity.
[0040] Judging from the relationship between the maximum
temperature and the cooling time of each recording layer, recording
marks will be formed neither first nor second layers if the power
level is "O", will be formed only in the first recording layer if
the power level is "A", will be formed in both first and second
recording layers if the power level is "B", and will be formed only
in the second recording layer if the power level is "C".
[0041] The actual operations of recording and reproducing by the
optical disk drive shown in FIG. 4 are as follows. While the
spindle motor is operated to rotate the optical disk at a linear
speed of 10 m/s, a laser beam spot having a read-out power level
(for example, 1 mW) is focused on a desired track of the optical
disk and then recording is performed by using laser beams having
different recording power levels. Read-out is performed by
continuously irradiating the recorded tracks with a laser beam
having a read-out power level to detect read-out signals.
[0042] FIG. 5A shows an example of recording signal train. This
diagram represents that recording is performed by irradiating a
track with recording laser beams having three different power
levels of "A", "B" and "C" after the irradiation of laser beam
having a read-out power level R. The recording frequency is set to
10 MHz. In FIG. 5A, the power level is set to "B" during the period
of time t.sub.1, t.sub.5, and t.sub.7, to "C" during the period of
time t.sub.2 and t.sub.4, and to "A" during the period of time
t.sub.3 and t.sub.6. In a sequence of t.sub.1 to t.sub.7, a
recording pattern of "1010111" is formed in the first recording
layer, and simultaneously, another recording pattern of "110110" is
formed in the second recording layer.
[0043] The read-out signals are shown in FIG. 5B. The read-out
signals include a peak level at a position where recording marks
are formed in both first and second recording layers, a bottom
level (without including the non-recording level) at a position
where a recording mark is formed in only the second recording
layer, and an intermediate level at a position where a recording
mark is formed in only the first recording layer. This result can
be explained by the fact that reflected laser beam from the first
recording layer is directly incident upon the detecting system,
whereas reflected laser beam from the second recording layer passes
through the first recording layer and is then incident upon the
detecting system.
[0044] The read-out signals shown in FIG. 5B can be digitized by
setting windows with appropriate slice levels including the
individual output levels, even if the output level is fluctuated to
some extent. For example, the output levels of "A", "B" and "C" may
be digitized to "10", "11", and "01". Assuming that the
non-recording level is "00", the recording and read-out can be
performed at as a high density as two times that of the
conventional medium.
[0045] The material of the recording layer is not limited to
GeSbTe-based material employed in the above example and may be
selected from other phase change optical recording materials
including InSbTe-based and GeSbTe-based materials. Although the
thermal responses of the recording layers in the above example are
adjusted by varying the composition in the same material system,
they may be controlled by using two or more materials having
appropriate melting points and crystallizing times.
[0046] 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 inventive concept as defined by the
appended claims and their equivalents.
* * * * *