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Claims  |
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What is claimed is:
1. An optical recording medium for recording, erasing and reading-out
information by irradiation of a laser beam, comprising:
a lower dielectric protective layer, a phase-change-type recording layer,
an upper dielectric protective layer and a metal reflective layer
successively deposited on a transparent substrate formed with grooves,
in which
both of grooves and lands are used as a recorded region,
a groove depth (d) satisfies the following relation (1):
.lambda./7n<d<.lambda./5n (1)
wherein .lambda. represents a wavelength of an irradiation light and n
represents a refractive index of the substrate, and
a groove width (GW) and a land width (LW) satisfy the following relation
(2):
0.1 .mu.m<GW<LW (2).
2. An optical recording medium as defined in claim 1, wherein the land
width (LW) satisfies the following relation (3):
0.62(.lambda./NA)<LW<0.80(.lambda./NA) (3)
wherein NA represents a numerical aperture of a lens.
3. An optical recording medium as defined in claim 2, wherein a phase
difference (.alpha.) between a reflected light from an unrecorded region
and a reflected light from a recorded region of an optical recording
medium satisfies the following relation (4):
-.pi.<.alpha.<0 (4)
and
a reflectance (R.sub.1) of an unrecorded region and a reflectance (R.sub.2)
of a recorded region satisfy the following relation (5):
R.sub.2 <R.sub.1 ( 5).
4. An optical recording medium as defined in claim 2, wherein a phase
difference (.alpha.) between a reflected light from an unrecorded region
and a reflected light from a recorded region of an optical recording
medium satisfies the following relation (6):
0<.alpha.<.pi. (6)
and
a reflectance (R.sub.1) of an unrecorded region and a reflectance (R.sub.2)
of a recorded region satisfy the following relation (7):
R.sub.2 >R.sub.1 ( 7).
5. An optical recording medium as defined in claim 2, wherein a land width
(LW), a groove width (GW) and a distance between adjacent grooves (groove
pitch (PG)=LW+GW) satisfy the following relation (8):
0.02.ltoreq.(LW-GW)/PG.ltoreq.0.3 (8).
6. An optical recording medium as defined in claim 2, wherein the groove
depth is from 40 to 80 nm and the groove width (GW) satisfies the
following relation (9):
0.15(.lambda./NA)<GW<0.5(.lambda./NA) (9).
7. An optical recording medium as defined in claim 1, wherein the phase
difference (.alpha.) between a reflected light from an unrecorded region
and a reflected light from a recorded region of the optical recording
medium satisfies the following relation (4):
-.pi.<.alpha.<0 (4)
and
a reflectance (R.sub.1) of the unrecorded region and reflectance (R.sub.2)
of the recorded region satisfy the following relation (5):
R.sub.2 <R.sub.1 ( 5).
8. An optical recording medium as defined in claim 1, wherein the phase
difference (.alpha.) between a reflected light from an unrecorded region
and a reflected light from a recorded region of the optical recording
medium satisfies the following relation (6):
0<.alpha.<.pi. (6)
and
a reflectance (R.sub.1) of an unrecorded region and a reflectance (R.sub.2)
of a recorded region satisfy the following relation (7):
R.sub.2 >R.sub.1 ( 7).
9. An optical recording medium as defined in claim 1, wherein the land
width (LW), the groove width (GW) and a groove pitch (PG) satisfy the
following relation (8):
0.02.ltoreq.(LW-GW)/PG.ltoreq.0.3 (8).
10. An optical recording medium as defined in claim 1, wherein the melting
point of the recording layer is less than 700.degree. C. and the
crystallizing temperature of the recording layer is not less than
150.degree. C.
11. An optical recording medium as defined in claim 10, wherein the
recording layer comprises an alloy mainly composed of Ge, Sb and Te as a
main ingredient and has a thickness from 15 to 25 nm.
12. An optical recording medium as defined in claim 10, wherein the
reflective layer comprises an alloy of Al and Ti or Ta, and the Ti or Ta
content is from 0.5 to 3.5 at %.
13. An optical recording medium as defined in claim 10, wherein at least
one layer of the lower dielectric protective layer and the upper
dielectric protective layer comprises ZnS and SiO.sub.2 or Y.sub.2
O.sub.3, and the SiO.sub.2 or Y.sub.2 O.sub.3 content is from 5 to 40 mol
%.
14. An optical recording medium as defined in claim 1, wherein the groove
depth is from 40 to 80 nm and the groove width (GW) satisfies the
following relation (9):
0.15(.lambda./NA)<GW<0.5(.lambda./NA) (9).
15. An optical recording medium as defined in claim 1, wherein a track
pitch of a region to which a file management or allocation information is
recorded is greater by 1.05 to 1.5 times a track pitch of data recorded
regions.
16. An optical recording medium as defined in claim 15, wherein the groove
width (GW) and the land width (LW) in the file management or allocation
region satisfy the following relation (10):
0.6(.lambda./NA)<(GW+LW)/2<1.0 .mu.m (10).
17. An optical recording medium as defined in claim 15, wherein the melting
point of the recording layer is less than 700.degree. C. and the
crystallizing temperature of the recording layer is not less than
150.degree. C.
18. An optical recording medium as defined in claim 15, wherein the
recording layer comprises an alloy mainly composed of Ge, Sb and Te as a
main ingredient and has a thickness from 15 to 25 nm.
19. An optical recording medium as defined in claim 15, wherein the
reflective layer comprises an alloy of Al and Ti or Ta, and the Ti or Ta
content is from 0.5 to 3.5 at %.
20. An optical recording medium as defined in claim 15, wherein at least
one layer of the lower dielectric protective layer and the upper
dielectric protective layer comprises ZnS and SiO.sub.2 or Y.sub.2
O.sub.3, and the SiO.sub.2 or Y.sub.2 O.sub.3 content is from 5 to 40 mol
%.
21. A optical recording medium as defined in claim 15, wherein a vertical
birefringence of the substrate is less than 400.times.10.sup.-6 and an
in-plane birefringence of said substrate is less than 40.times.10.sup.-6.
22. An optical recording medium as defined in claim 15, wherein a thickness
of the substrate is not less than 0.4 mm and less than 1.0 mm.
23. An optical recording medium as defined in claim 1, wherein the groove
width (GW) and the land width (LW) satisfy the following relation (10):
0.6(.lambda./NA)<(GW+LW)/2<1.0 .mu.m (10).
24. An optical recording medium as defined in claim 23, wherein the melting
point of the recording layer is less than 700.degree. C. and the
crystallizing temperature of the recording layer is not less than
150.degree. C.
25. An optical recording medium as defined in claim 24, wherein the
recording layer comprises an alloy mainly composed of Ge, Sb and Te as a
main ingredient and has a thickness from 15 to 25 nm.
26. An optical recording medium as defined in claim 24, wherein the
reflective layer comprises an alloy of Al and Ti or Ta, and the Ti or Ta
content is from 0.5 to 3.5 at %.
27. An optical recording medium as defined in claim 24, wherein at least
one layer of the lower dielectric protective layer and the upper
dielectric protective layer comprises ZnS and SiO.sub.2 or Y.sub.2
O.sub.3, and the SiO.sub.2 or Y.sub.2 O.sub.3 content is from 5 to 40 mol
%.
28. A optical recording medium as defined in claim 23, wherein a vertical
birefringence of the substrate is less than 400.times.10.sup.-6 and an
in-plane birefringence of said substrate is less than 40.times.10.sup.-6.
29. An optical recording medium as defined in claim 23, wherein a thickness
of the substrate is not less than 0.4 mm and less than 1.0 mm.
30. An optical recording medium comprising a lower dielectric protective
layer, a phase-change-type recording layer, an upper dielectric protective
layer and a metal reflective layer deposited successively on a transparent
substrate formed with grooves for reversibly recording, erasing and
reading-out information by utilizing an optically distinguishable
crystallized or amorphous state,
in which a track pitch of a region to which a file management or allocation
information is recorded is greater by 1.05 to 1.5 times a track pitch of
data recorded regions.
31. An optical recording medium as defined in claim 30, wherein a groove
width (GW) and a land width (LW) in the file management or allocation
region satisfy the following relation (10):
0.6(.lambda./NA)<(GW+LW)/2<1.0 .mu.m (10).
32. An optical recording medium as defined in claim 31, wherein a vertical
birefringence of the substrate is less than 400.times.10.sup.-6 and an
in-plane birefringence of said substrate is less than 40.times.10.sup.-6.
33. An optical recording medium as defined in claim 31, wherein a thickness
of the substrate is not less than 0.4 mm and less than 1.0 mm.
34. An optical recording medium as defined in claim 31, wherein the melting
point of the recording layer is less than 700.degree. C. an the
crystallizing temperature of said layer is not less than 150.degree. C.
35. An optical recording medium as defined in claim 34, wherein the
recording layer comprises an alloy mainly composed of Ge, Sb and Te as a
main ingredient and has a thickness from 15 to 25 nm.
36. An optical recording medium as defined in claim 34, wherein the
reflective layer comprises an alloy of Al and Ti or Ta, and the Ti or Ta
content is from 0.5 to 3.5 at %.
37. An optical recording medium as defined in claim 34, wherein at least
one layer of the lower dielectric protective layer and the upper
dielectric protective layer comprises ZnS and SiO.sub.2 or Y.sub.2
O.sub.3, and the SiO.sub.2 or Y.sub.2 O.sub.3 content is from 5 to 40 mol
%. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to an optical recording medium, and more in
detail, it relates to an optical information recording medium for
performing recording/reading-out and erasure of information in both of
grooves and an inter-groove portions (lands) of a substrate by irradiation
of a laser beam.
Along with increasing amount of information in recent years, a recording
medium capable of recording and reading-out a great amount of data at a
high density and at a high speed has been demanded, and an optical disk is
expected as a medium just suitable to such an application use.
Optical discs include a write-once-type disk capable of recording only for
once and a rewritable-type disk capable of recording and erasure over and
over.
As the rewritable-type optical disk, there can be mentioned a
magneto-optical recording medium utilizing an magneto-optical effect and a
phase-change medium utilizing the change of reflectance along with
reversible change between amorphous and crystal states.
The phase-change medium has a merit capable of recording/erasure by only
modulating the power of a laser beam without requiring an external
magnetic field, and capable of miniaturizing of a recording/reading-out
device.
Further it has also a merit capable of obtaining a high density recording
medium by a shorter wavelength with no particular alteration of materials
from existent medium predominant at present capable of recording/erasure
at a wavelength of about 800 nm.
As the material for the recording layer of such a phase-change medium, a
thin film of chalcogenic alloy is often used. There can be mentioned, for
example, GeTe-based materials, GeTeSb-based materials, InSbTe-based
materials and GeSnTe-based materials.
Generally, in a rewritable phase-change recording medium, recording is
performed by forming an amorphous bit from a crystallized state in a
unrecorded/erased state. The amorphous bit is formed by heating the
recording layer to a temperature higher than the melting point followed by
quenching. In this case, a dielectric layer disposed in contact with the
recording layer serves as a heatsink layer for obtaining a sufficiently
overcooled state, and a protective layer for suppressing an ablation.
On the other hand, erasure (crystallization) is performed by heating the
recording layer to a temperature higher than the crystallizing temperature
and lower than the melting point of the recording layer.
In this case, the dielectric layer serves as a heat accumulating layer for
keeping the temperature of the recording layer at a high temperature till
crystallization is completed.
Generally in a writable phase-change recording medium, a laser beam of two
different power levels is used for attaining different crystal states.
The recording film is selected from view points that the film can easily
take a crystallized state or an amorphous state moderately, has a large
difference of reflectance between the crystallized state and the amorphous
state, and shows small volume change due to phase-change, or the like.
The material for the protective layer is selected from view points, for
example, having optical transparency to a laser beam, appropriate
refractive index, high melting point, softening point and decomposition
point, ease of preparation and appropriate heat conductivity.
In a phase-change-type medium capable of 1-beam overwriting, the erasing
and rewriting steps can be performed only by the intensity modulation of
one focused beam (Jpn. J. Appl. Phis., 26 (1987), suppl. 26-4, pp. 61-66).
In the 1-beam overwritable phase-change recording medium, the time required
for writing information can be shortened. It has a further merit that a
drive can be constituted simply and inexpensively since the medium
requires no magnetic fields.
Further a write-once-type phase-change medium can also be obtained by
substantially the same material and layer constitution as those of the
rewritable-type medium by changing the composition of the recording layer
from that of the reversible phase-change type recording layer.
In this case, information can be recorded and stored for a longer period of
time since the medium has no reversibility and the information can be
stored, in principle, substantially permanently.
In a case of using the phase-change medium as the write-once-type medium,
different from an ablation-type since no raising called as a rim is not
caused to the periphery of a bit, it has a merit of providing excellent
signal quality, and, since no gap is required above the recording layer,
there is no requirement for an air sandwich structure.
The requirements of the high capacity and the high density in the recording
medium is inevitable of the times, imposed on recording media and
recording apparatus for handling an enormous amount of video information
or audio signals, and they have been ever-progressing keeping pace with
the progress of digital modulation technique and data compression
technique. These high capacity and density are also demanded in the
above-mentioned phase-change optical recording medium.
As a concrete means for increasing the recording density, in the optical
disk, there have been developed and utilized, for example, reduction of a
focused beam diameter of irradiated light and shortening of recording mark
length by shortening the wavelength of the optical source or making NA
(Numerical Aperture) of lens high, MCAV (Modified Constant Angular
Velocity) of increasing recording frequency toward an outer circumference
under constant rotational frequency, thereby making the recording density
constant from inner to outer circumferences, and a mark edge recording of
carrying information to beginning and rear ends of a mark, and means for
further increasing the density has been thought at present.
Further in the phase-change-type medium, since there is less deterioration
caused by the reduction of optical resolution and a signal amplitude can
be increased even in a case of recording at an identical track pitch
(track pitch density) and a shortest bit length (longitudinal recording
density), it has a merit capable of easily attaining increased density as
compared with the magneto-optical medium.
In an optical disk capable of recording, guide grooves are previous
engraved on a disk to form so-called tracks. Usually, information signals
are recorded, read out or erased by condensing a laser beam on a land or
in a guide groove.
In an optical disk, lands and grooves are formed alternately in a radial
direction coaxially or spirally and a focused light is guided by utilizing
a diffracted light from the portions. The system includes a push-pull
tracking-servo system of utilizing a radial difference of an intensity of
a reflected light from an optical disk, namely, utilizing a diffracted
light from a land or groove detecting 0th and 1st diffracted light by two
splitted detectors, thereof (I1-I2 signal), and a 3-beam system using
three splitted optical beams arranged in parallel in a radial direction
and guiding a focused light by the calculation of the intensity of the
reflected light for each of beams at three detector positions, that is, a
land and grooves on both sides thereof or a groove and lands on both sides
thereof. Further, the radial movement in such an optical disk is conducted
by a system of counting the number of tracks passed by a cross track
signal (I1+I2) and approaching an aimed track. In a usual optical disk,
since recording/reading-out is performed only to the lands or only to the
grooves, the width of the land (or groove) used for recording is made
wider usually by about twice compared with that of the groove (or land)
not used for recording. For further increasing the capacity, a system of
recording/reading-out in both of the land and the groove is also
considered. The capacity of the optical disk is doubled by recording both
in the land and the groove.
In ordinary optical disks marketed at present, usually, information signals
are recorded to either one of the land or the groove, and the other of
them serves only as a boundary for separating adjacent tracks to prevent
intrusion of leaked signals.
If information can be recorded also in the boundary portion, for example,
in the groove in the case of recording information on the land, or on the
land in a case of recording information in the groove, the recording
density is doubled and remarkable improvement can be expected for the
recording capacity.
A method of recording information to both of the land and the groove are
hereinafter simply referred to as "L&G recording".
L&G recording is proposed, for example, in Japanese Patent Publication
(KOKOKU) 63-57859 and a special care has to be taken for reducing
cross-talk in a case of using such L&G recording technique.
That is in the L&G recording described in Japanese Patent Publication
(KOKOKU) 63-57859, since the distance between a row of recording marks in
a certain track and a row of recording marks in a track adjacent therewith
is one-half of a focused beam diameter, the focused beam diameter lies
over the row of recording marks adjacent with the row of recording marks
to be read out. Therefore, cross talk upon reading-out is increased to
deteriorate the reading-out S/N.
For reducing the cross talk, there is proposed a method as described for
example, in SPIE Vol. 1316, Optical Data Storage (1990), pp 35, of
disposing a special optical system and a cross talk cancel circuit to an
optical disk reading-out device, thereby reducing the cross talk. However,
this method involves a disadvantage of further complicating the optical
system and the signal processing system of the device.
As a method of reducing the cross talk with no additional provision of
special optical system or signal processing circuit for reducing the cross
talk upon reading-out, it has been proposed to make the width of the
groove (guide groove) equal with that of the land and define the groove
depth within a certain range corresponding to a wavelength of a
reading-out light (Jpn. J. Appl. Phys. Vol. 32 (1993), pp 5324 5328).
This proposal shows, by calculation and based on experiment that cross talk
is reduced under the condition that the lane width is equal with the
groove width and the groove depth is from .lambda./7n to .lambda./5n
(.lambda.: wavelength of reading-out light, n: refractive index of
substrate).
This is disclosed also in Japanese Patent Application Laid-Open (KOKAI)
5-282705.
In the above-mentioned proposals, based on the premise that the groove
width is equal with the land width, an effect of reducing the cross talk
is shown by computer simulation, examples of actually manufacturing,
evaluating disks are given, and their effectiveness is mentioned.
It has been reported that a high density 3 to 4 times the current density
can be attained by L&G recording method using the phase-change medium in
CD size (diameter: 120 nm) by combination with an optical head at 680 nm
and 0.6 NA (numerical aperture of a condensing lens) available at present
(Jpn. J. Appl. Phys., 32 (1993), pp 5324 5328). It is also said that high
quality moving picture for not less than one hour can be recorded in
combined use with image compression technique.
However as a result of the further present inventions' earnest study, it
has been found that as the recording density is increased by restriction
of a track pitch by narrowing the groove width while keeping the groove
width-to-land width ratio at 1:1, characteristics are remarkably
deteriorated on the lands in view of residue after erasure of a preceding
mark or worsening jitter of the recording mark after repetitive
overwriting, and on the other hand, that erasing characteristic or jitter
is less worsened after repetitive recording overwriting in the groove even
if the track pitch is narrowed.
Further according to the dependence of the CN ratio (carrier-to-noise
ratio) and the cross talk on the groove depth described in the
afore-mentioned report, although an effect of reducing the cross talk can
be obtained by optimizing the groove depth, balance of the CN ratio is
lost between the lands and the grooves.
In the L&G recording, it is not preferable in view of the signal quality of
a disk that a difference is caused between the carrier level on the lands
and the carrier level in the grooves, and as a result, the CN ratio for
one of them is remarkably deteriorated. The difference thereof should be
fallen within a specified range.
Since atoms can migrate in the phase-change optical disk upon recording and
erasure, there is a problem of characteristic deterioration caused by
repetitive recording and erasure.
Although the repetitive recording characteristic can be improved to some
extent by optimizing, for example, the material for the recording layer
and the protective layer, the layer constitution and conditions for
preparing each of the layers, it is not yet sufficient. As the cause for
the deterioration due to repetitive recording and erasure, there may be
considered, for example, film deformation, material transfer in the
recording film and segregation. It has not seen a reason why such
phenomenon became conspicuous.
Also, there has been proposed for example, a sample servo method for
guiding an optical beam by the arrangement of pits formed with unevenness
without disposing the guide grooves, other than L & G recording.
Although a narrow track pitch can be achieved by the method, in a case of
recording at a track pitch of not more than 1.0 .mu.m, an extremely small
spot diameter of the focused beam should be obtained by using an optical
system at a short wavelength and with a great NA, but it has been known
that the focal depth is also reduced in such an optical system.
Specifically, for the wavelength .lambda. and the lens numerical aperture
NA, there are the following relationship.
Beam spot diameter.varies..lambda./NA
Focal depth.varies..lambda./(NA)2
The focal depth is abruptly reduced as the spot diameter at a focal point
is restricted. Accordingly, if the focal point is automatically adjusted
to the surface of the recording layer, a margin of a focus servo system is
extremely narrowed. At the same time, the coma aberration caused by the
tilt of the substrate increases.
One of the solutions is to reduce the thickness of the substrate to not
more than 1.2 mm of the prior art (T. Sugaya, et al., Jpn. J. Appl. Phys.
32, 5402 (1993)).
Further if the track pitch is narrowed to 1.0 .mu.m in the L&G recording,
sample servo recording and land and groove recording, there is a problem
that a slight deviation (offset) of the focus servo system as described
above increases leak signal from adjacent tracks (cross talk).
It has become apparent recently that the focus offset is increased by
astigmatism caused by vertical birefringence of a substrate (M. R. Latta,
et al, Proceeding of SPIE, vol. 1663 (1992), pp 157).
That is since the focused beam has astigmatism, the focal position is
separated into two points to provide a focal position in which a beam is
restricted into an elongated shape along the direction of the track and a
focal position in which the beam is restricted in an elongate shape in the
direction perpendicular to the track.
Such astigmatism is particularly conspicuous in a case of using a linearly
polarized beam.
It depends on the combination of individual drives or substance, to which
of positions focusing is adjusted automatically. Further, it is not always
adjusted to one of the focusing points but focusing may be conducted at an
intermediate position.
If the beam at the focused position take such a position as it becomes
elongate in a direction perpendicular to the track the cross talk from the
adjacent track increases upon reading-out. Further, if such a beam shape
is formed during recording, it may possibly erase an amorphous bit already
recorded in the adjacent track.
This is because the temperature of the adjacent track is easily elevated by
heating to a bottom portion of the focused beam if the track pitch is not
more than 1.0 .mu.m and smaller than the spot diameter of the focused
beam.
It tends to occur easily that a portion of the amorphous bit is
crystallized and erased during repetitive recording although such
phenomenon is not caused by the recording only for once.
The problem around the birefringence of the substrate described above has
been considered, as a problem, in a magneto-optical medium for detecting a
minute Kerr rotation angle (W. A. Challener, et al., Applied Optics, Vol.
26, (1987), pp 3974, or I. Prikeryl, Applied Optics, vol. 31, (1992), pp
1853).
A phase difference caused by birefringence of the substrate presents a
problem because of the physical property of the magneto-optical medium of
detecting a minute ellipticity from a linearly polarized light and it has
been considered that this scarcely causes a problem in the phase-change
medium for detecting the intensity of the reflected light.
Therefore, only the in-plane birefringence, is noted for instance and it
has been emphasized as a merit of the phase-change medium that it suffers
from no effect of the noises or signal intensity even if the in-plane
birefringence exceeds 20.times.10.sup.-6.
Accordingly, it can be said that no appropriate counter-measure has
substantially been taken in the phase-change medium.
However, the problem of the astigmatism exists, for example, in a case of
using a linearly polarized beam of a semiconductor laser for ensuring
compatibility with a magneto-optical medium, or in a case of saving
circular polarization by using a .lambda./4 plate for simplifying the
structure of an optical head and even the phase-change medium not
detecting the polarized state tends to undergo the effect of the
astigmatism by the presence of birefringence.
By the way, although the recording density can be increased more as the
pitch of the groove or pit for tracking (track pitch) disposed on the
substrate is smaller, there is a limit for narrowing the track pitch since
there is a diffraction limit in the beam spot system.
Usually, the track pitch may be selected such that the amount of the cross
talk is reduced to less than a predetermined level, but there is another
problem to be considered in the phase-change medium.
This is a problem that the amorphous bit in the adjacent track is erased
(recrystallized) upon repetitive overwriting to a certain track.
The reason is not always apparent but it is assumed that the temperature of
the adjacent track is elevated by a weak laser beam at the bottom portion
of an intensity distribution of the focused beam upon recording the
adjacent track, so that the amorphous bit is heated to a temperature
higher than the crystallization temperature.
While the heating time per once is within several hundreds nano seconds, it
is recrystallized although gradually during repetitive heating.
For instance, the C/N ratio (carrier-to-noise ratio) of the adjacent track
is reduced from 55 dB at the initial state to not more than 50 dB after
10,000 cycles of repetitive overwriting.
The problem is hereinafter is simply referred to as "cross erasure". In the
phase-change medium, a care has to be taken to a minimum track pitch by
cross erasure rather than the optical diffraction limit, but the limit is
not always apparent.
In a case of performing L&G recording or sample servo recording, since no
unevenness effective to shielding for heat conduction is present between
the adjacent track, different from the recording only to one of the land
or the groove of the prior art, the temperature of the adjacent track
tends to be raised more easily by heat diffusion. Such cross erasure
become a serious problem.
Accordingly, the substantial limit for the track density is restricted
rather by the limit of thermal separation (cross erasure) than the optical
resolution power, that is, signal leakage from the adjacent track (cross
talk).
According to the study made by the present inventors, if L&G recording is
conducted at a linear velocity of 3 m/s to a medium having grooves and
lands each at 0.7 .mu.m in width, by using a semiconductor laser at a
wavelength of 680 nm and an optical head of 0.55 NA, the carrier level of
signals recorded in the adjacent land or groove was lowered by 3 to 5 dB
after 1,000 times of overwriting.
However in usual recording media, repetitive recording for more than 100
times is often conducted only in a case of rewriting the file management
or allocation information on the recording medium. That is, only a limited
region disposed to the inner circumference or the outer circumference of a
disk referred to as FAT in the DOS format or TOC in the CD format is
rewritten frequently.
The frequently rewriting region is less than 1% of the entire recordable
region.
There may be such a case as in UNIX in which file management or allocation
information is physically dispersed but it may suffice to consider an
average number of writing and there is scarcely a possibility that a
specified region is rewritten over 10,000 times.
It is considered that the situation will not change also in feature formats
that recording is conducted while distinguishing the file management or
allocation region and the contents thereof, and rewriting is concentrated
only to a physically distinguished narrow region.
That is the recording density for the entire medium is restricted a present
by less than 1% frequently rewritable region.
As a result of an earnest study of the present inventors, it has been found
that an optical recording medium for recording, erasing and reading-out
information by irradiation of a laser beam, comprising a lower dielectric
protective layer, a phase-change-type recording layer, an upper dielectric
protective layer and a metal reflective layer deposited orderly on a
transparent substrate formed with grooves in which the grooves are formed
on the transparent substrate such that the groove depth (d) can satisfy
the following relation (1) and the groove width (GW) and land width (LW)
can satisfy the following relation (2):
.lambda./7n<d<.lambda./5n (1)
0.1 .mu.m<GW<LW (2)
in which both of the grooves and the lands are used for the recording
region can reduce the cross talk from adjacent tracks and has excellent
respective overwriting characteristics in the lands. On the basis of the
findings the present invention have accomplished.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a high density optical
disk with a high reliability, particularly, an L&G recording type optical
disk using a laser beam as a light source, capable of keeping repetitive
overwriting characteristic to a high level both for the lands and the
grooves in a case if at least lands and grooves are used as the recording
region.
Another object of the present invention is to provide a high density
optical disk, particularly, an L&G recording type optical disk capable of
eliminating loss of balance of the carrier levels of the recording marks
between the lands and the grooves, and capable of obtaining equally high
signal quality upon recording to either of the lands and the grooves.
To accomplish the aims, in a first aspect of the present invention, there
is provided an optical recording medium for recording, erasing and
reading-out information by irradiation of a laser beam, comprising:
a lower dielectric protective layer, a phase-change-type recording layer,
an upper dielectric protective layer and a metal reflective layer
successively deposited on a transparent substrate formed with grooves,
in which
both of grooves and lands are used as a recorded region,
a groove depth (d) satisfies the following relation (1):
.lambda./7n<d<.lambda./5n (1)
(wherein .lambda. represents a wavelength of an irradiation light and n
represents a refractive index of the substrate), and
a groove width (GW) and a land width (LW) satisfy the following relation
(2):
0.1 .mu.m<GW<LW (2)
In a second aspect of the present invention, there is provided an optical
recording medium for recording, erasing and reading-out information by
irradiation of a laser beam, comprising:
a lower dielectric protective layer, a phase-change-type recording layer,
an upper dielectric protective layer and a metal reflective layer
successively deposited on a transparent substrate formed with grooves,
in which
both of grooves and lands are used as a recorded region,
a groove depth (d) satisfies the following relation (1):
.lambda./7n<d<.lambda./5n (1)
(wherein .lambda. represents a wavelength of an irradiation light and n
represents a refractive index of the substrate),
a groove width (GW) and a land width (LW) satisfy the following relation
(2):
0.1 .mu.m<GW<LW (2)
and
the land width (LW) satisfies the following relation (3):
0.62(.lambda./NA)<LW<0.80(.lambda./NA) (3)
(wherein NA represents a numerical aperture of a lens).
In a third aspect of the present invention, there is provided an optical
recording medium for recording, erasing and reading-out information by
irradiation of a laser beam, comprising:
a lower dielectric protective layer, a phase-change-type recording layer,
an upper dielectric protective layer and a metal reflective layer
successively deposited on a transparent substrate formed with grooves,
in which
both of grooves and lands are used as a recorded region,
a groove depth (d) satisfies the following relation (1):
.lambda./7n<d<.lambda./5n (1)
(wherein .lambda. represents a wavelength of an irradiation light and n
represents a refractive index of the substrate),
a groove width (GW) and a land width (LW) satisfy the following relation
(2):
0.1 .mu.m<GW<LW (2)
the land width (LW) satisfies the following relation (3):
0.62(.lambda./NA)<LW<0.80(.lambda./NA) (3)
(wherein NA represents a numerical aperture of a lens),
a phase difference (.alpha.) between a reflected light from an unrecorded
region and a reflected light from a recorded region of an optical
recording medium satisfies the following relation (4):
-.pi.<.alpha.<0 (4)
and
a reflectance (R.sub.1) of an unrecorded region and a reflectance (R.sub.2)
of a recorded region satisfy the following relation (5):
R.sub.2 <R.sub.1 ( 5)
In a fourth aspect of the present invention, there is provided an optical
recording medium for recording, erasing and reading-out information by
irradiation of a laser beam, comprising:
a lower dielectric protective layer, a phase-change-type recording layer,
an upper dielectric protective layer and a metal reflective layer
successively deposited on a transparent substrate formed with grooves,
in which
both of grooves and lands are used as a recorded region,
a groove depth (d) satisfies the following relation (1):
.lambda./7n<d<.lambda./5n (1)
(wherein .lambda. represents a wavelength of an irradiation light and n
represents a refractive index of the substrate),
a groove width (GW) and a land width (LW) satisfy the following relation
(2):
0.1 .mu.m<GW<LW (2)
the land width (LW) satisfies the following relation (3):
0.62(.lambda./NA)<LW<0.80(.lambda./NA) (3)
(wherein NA represents a numerical aperture of a lens),
a phase difference (.alpha.) between a reflected light from an unrecorded
region and a reflected light from a recorded region of an optical
recording medium satisfies the following relation (6):
0<.alpha.<.pi. (6)
and
a reflectance (R.sub.1) of an unrecorded region and a reflectance (R.sub.2)
of a recorded region satisfy the following relation (7):
R.sub.2 >R.sub.1 ( 7).
In a fifth aspect of the present invention, there is provided an optical
recording medium for recording, erasing and reading-out information by
irradiation of a laser beam, comprising:
a lower dielectric protective layer, a phase-change-type recording layer,
an upper dielectric protective layer and a metal reflective layer
successively deposited on a transparent substrate formed with grooves,
in which
both of grooves and lands are used as a recorded region,
a groove depth (d) satisfies the following relation (1):
.lambda./7n<d<.lambda./5n (1)
(wherein .lambda. represents a wavelength of an irradiation light and n
represents a refractive index of the substrate),
a groove width (GW) and a land width (LW) satisfy the following relation
(2):
0.1 .mu.m<GW<LW (2)
and
the land width (LW) satisfies the following relation (3):
0.62(.lambda./NA)<LW<0.80(.lambda./NA) (3)
(wherein NA represents a numerical aperture of a lens), and
the land width (LW), the groove width (GW) and a distance between adjacent
grooves (groove pitch (PG)=LW+GW) satisfy the following relation (8):
0.02<(LW-GW)/PG.ltoreq.0.3 (8)
In a sixth aspect of the present invention, there is provided an optical
recording medium for recording, erasing and reading-out information by
irradiation of a laser beam, comprising:
a lower dielectric protective layer, a phase-change-type recording layer,
an upper dielectric protective layer and a metal reflective layer
successively deposited on a transparent substrate formed with grooves,
in which
both of grooves and lands are used as a recorded region,
a groove depth (d) satisfies the following relation (1):
.lambda./7n<d<.lambda./5n (1)
(wherein .lambda. represents a wavelength of an irradiation light and n
represents a refractive index of the substrate),
a groove width (GW) and a land width (LW) satisf | | |
