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Claims  |
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We claim:
1. An optical recording medium having lands and grooves, comprising a
recording layer, a dielectric layer and a reflecting layer,
wherein,
(a) each of said lands and each of said grooves utilizes a phase change
between an amorphous phase and a crystalline phase in said recording layer
to record and erase data,
(b) said reflecting layer has a thickness of more than 30 nm and
(c) said optical recording medium has a mirror portion having a reflectance
of:
(1) more than 15% and no more than 35% when said recording layer is in the
crystalline state and
(2) 10% or less when said recording layer is in the amorphous state.
2. An optical recording medium according to of claim 1, wherein the phase
difference between the reflected light of the amorphous phase and the
reflected light of the crystalline phase is 2 n .pi.-.pi./3 to 2 n
.pi.+.pi./3, where n is an integer.
3. An optical recording medium according to of claim 1, wherein the phase
difference between the reflected light of the amorphous phase and the
reflected light of the crystalline phase is 2 n .pi.+2 .pi./3 to 2 n
.pi.+4 .pi./3, where n is an integer.
4. An optical recording medium according to of claim 1, wherein the depth
of the groove is 1/7 to 1/5 of the wavelength of the reproducing light.
5. An optical recording medium according to of claim 1, which is a laminate
consisting of a substrate, first dielectric layer, recording layer, second
dielectric layer, and reflection layer in this order, and the refractive
indexes and thicknesses of the first and second dielectric layers are in
the ranges shown by the following formula (1) and (2), respectively.
Formulae (1)
1.5.ltoreq.n1.ltoreq.2.4
50.ltoreq.d1.ltoreq.300
Formulae (2)
1.5.ltoreq.n2.ltoreq.2.4
1.ltoreq.d2.ltoreq.50
Where n1 and n2 are the refractive indexes of the first and second
dielectric layers, and d1 and d2 are the thicknesses (nm) of the first and
second dielectric layers, respectively.
6. An optical recording medium according to of claim 1, wherein the
recording layer contains Sb or Te.
7. An optical recording medium according to claim 6, wherein the recording
layer is comprises a ternary alloy of Ge, Te and Sb, or comprises an alloy
of the three elements of Ge, Te and Sb, plus at least one metal selected
from Pd, Nb, Pt, Au, Ag, Ni and Co, the thickness of the recording layer
being 10 nm or more and 40 nm or less.
8. An optical recording medium according to claim 6, wherein the
composition of the alloy of the recording layer is expressed by Formula
(3) given below:
Formula (3)
M.sub.z (Sb.sub.x Te.sub.1-x).sub.1-y-z (Ge.sub.0.5 Te.sub.0.5).sub.y
0.4.ltoreq.x.ltoreq.0.6
0.3.ltoreq.y.ltoreq.0.5
0.ltoreq.z.ltoreq.0.05
where x, y and z denote respective molar ratios and M denotes at least one
metal selected from Pd, Nb, Pt, Au, Ag, Ni and Co;
and the reflection layer consists of an Al alloy.
9. An optical recording medium according to of claim 1, wherein the
recording layer contains at least In or Se.
10. An optical recording medium according to of claim 1, wherein the track
pitch, width of a flat portion of the land and the width of a flat portion
of the groove bottom are expressed by the following formulae:
Formula 4
Tp=a.multidot..lambda./NA
1.ltoreq.a.ltoreq.1.5
Tg=Tp.multidot.b
0.2.ltoreq.b.ltoreq.0.6
T.sub.L =Tp.multidot.c
0.2.ltoreq.c.ltoreq.0.6
where Tp is the track pitch (.mu.m): NA is the numerical aperture of the
lens; .lambda. is the reproducing wavelength (.mu.m); Tg is the width of
the flat portion of the groove (.mu.m); and T.sub.L is the width of the
flat portion of the land (.mu.m).
11. An optical recording medium according to of claim 1, wherein the land
and the groove consist substantially of a flat portion and slope portions,
the widths of the slope portions being from 0.05 .mu.m to 0.2 .mu.m.
12. An optical recording medium according to of claim 1, wherein the widths
of the marks recorded on the land are at least half the width of the flat
portion of the land, or the widths of the marks recorded on the groove are
at least half the width of the flat portion of the groove.
13. An optical recording medium according to of claim 1, wherein the
thickness of the substrate is expressed by Formula (5) which is given
below:
0.01.ltoreq.(NA).sup.3.multidot. d.ltoreq.0.20 (5)
where NA denotes the numerical aperture of the lens and d denotes the
thickness (mm) of the substrate.
14. An optical recording medium according to of claim 1, wherein the
dielectric layer contains at least ZnS, SiO.sub.2 and carbon.
15. An optical recording medium having lands and grooves, comprising a
recording layer having a photo-absorbance and a reflecting layer having a
thickness of more than 30 nm,
wherein,
(a) each of said lands and each of said grooves utilizes a phase change
between an amorphous phase and a crystalline phase in said recording layer
to record and erase data,
(b) each of said grooves has a depth corresponding to an optical path
length of 1/7 to 1/5 of the wavelength of the reproducing light, and
(c) the photo-absorbance of said recording layer in the amorphous phase and
the photo-absorbance of said recording layer in the crystalline phase
satisfy the following formula:
Aa-Ac.ltoreq.10
where Aa is the photo-absorbance (%) of said recording layer in the
amorphous phase, and Ac is the photo-absorbance (%) of said recording
layer in the crystalline phase.
16. An optical recording medium according to claim 15, wherein a mirror
portion thereof has a reflectance, when in the crystalline state, of more
than 15% and no more than 35%.
17. An optical recording medium according to claim 15, wherein a mirror
portion thereof has a reflectance, when in the amorphous state, of 10% or
less.
18. An optical recording medium according to any one of claims 16 or 17,
wherein the phase difference between the reflected light of the amorphous
phase and the reflected light of the crystalline phase is 2 n .pi.-.pi./3
to 2 n .pi.+.pi./3, where n is an integer.
19. An optical recording medium according to any one of claims 16 or 17,
wherein the phase difference between the reflected light of the amorphous
phase and the reflected light of the crystalline phase is 2 n .pi.+2
.pi./3 to 2 n .pi.+4 .pi./3, where n is an integer.
20. An optical recording medium according to any one of claims 16 or 17,
wherein the recording layer contains at least Sb or Te.
21. An optical recording medium according to any one of claims 16 or 17,
wherein the recording layer is comprises a ternary alloy of Ge, Te and Sb,
or comprises an alloy of the three elements of Ge, Te and Sb, plus at
least one metal selected from Pd, Nb, Pt, Au, Ag, Ni and Co, the thickness
of the recording layer being 10 nm or more and 40 nm or less.
22. An optical recording medium according to any one of claims 16 or 17,
wherein the composition of the alloy of the recording layer is expressed
by Formula (7) given below.
Formula (7)
M.sub.z (Sb.sub.x Te.sub.1-x).sub.1-y-z (Ge.sub.0.5 Te.sub.0.5).sub.y
0.4.ltoreq.x.ltoreq.0.6
0.3.ltoreq.y.ltoreq.0.5
0.ltoreq.z.ltoreq.0.05
where x, y and z denote respective molar ratios, and M denotes at least one
metal selected from Pd, Nb, Pt, Au, Ag, Ni and Co.
23. An optical recording medium according to any one of claims 16 or 17,
wherein the recording layer contains at least In or Se.
24. An optical recording medium according to any one of claims 16 or 17,
wherein the optical recording medium has at least a first dielectric
layer, recording layer and photo-absorbable layer.
25. An optical recording medium according to claim 24, wherein the
photo-absorbable layer is made of at least one metal selected from Ti, Zr,
Hf, Cr, Ta, Mo, Mn, W, Nb, Rh, Ni, Fe, Y, V, Co, Cu, Zn, Ru, Pd,
Lanthanides and Te.
26. An optical recording medium according to claim 24, wherein the
photo-absorbable layer is made of an alloy consisting of at least one
metal and Si and/or Ge.
27. An optical recording medium according to claims 16 or 17, wherein the
optical recording medium is a laminate of at least a substrate, first
dielectric layer, recording layer, second dielectric layer,
photo-absorbable layer and reflection layer in this order, and the
thickness of the reflection layer is 10 nm or more.
28. An optical recording medium according to claims 16 or 17, wherein the
optical recording medium is a laminate of at least a substrate, first
dielectric layer, recording layer, second dielectric layer and a
reflection layer in this order, or a laminate of at least a substrate,
first dielectric layer, recording layer, second dielectric layer,
photo-absorbable layer and reflection layer in this order and the
refractive indexes and thicknesses of the first and second dielectric
layers are in the ranges shown by the following formulae (1) and (2),
respectively.
Formulae (1)
1.5.ltoreq.n1.ltoreq.2.4
50.ltoreq.d1.ltoreq.300
Formulae (2)
1.5.ltoreq.n2.ltoreq.2.4
1.ltoreq.d2.ltoreq.50
where n1 and n2 are the refractive indexes of the first and second
dielectric layers, and d1 and d2 are the thicknesses (nm) of the first and
second dielectric layers, respectively.
29. An optical recording medium according to any one of claims 1 and 15,
wherein the recording layer contains nitrogen.
30. An optical recording medium according to claim 29, wherein the nitrogen
atom concentration of the recording layer is 0.1 atom % to 10 atom %.
31. An optical recording medium according to claim 30, wherein the nitrogen
atom concentration of the recording layer is 1 atom % to 3.5 atom %. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates to an optical information recording medium,
which allows information to be recorded, erased and reproduced by
irradiation with light and also to a method for recording on an optical
information recording medium. More particularly, the present invention
relates to a rewritable phase change type recording medium such as an
optical disc, optical card, or optical tape, which allows recorded
information to be erased or rewritten and allows information signals to be
recorded at a high speed and at a high density and also to a method for
recording on said recording medium.
BACKGROUND OF THE INVENTION
The conventional techniques of the rewritable phase change type optical
recording media and of the methods of recording on such recording media
are described below.
These optical recording media have a recording layer mainly composed of an
alloy of Te, Ge and Sb, etc. For recording, the recording layer is
irradiated with focused laser beam pulses for a short time, to partially
melt the recording layer. The molten portion is quickly cooled by thermal
diffusion and solidified, to form recorded marks of the amorphous state.
The light reflectance of the recorded marks is lower than that of the
crystalline state and the recorded marks can be optically reproduced as
recorded signals.
Furthermore, for erasure, the portion of the recorded marks is irradiated
with a laser beam, to be heated at a temperature lower than the melting
point of the recording layer and higher than the crystallization
temperature, to crystallize the recorded marks of the amorphous state, for
returning them into the original unrecorded state.
In the optical recording media with a Te alloy as the recording layer, the
crystallization rate is high and high speed overwriting by a circular beam
can be achieved by simply modulating the irradiation power into a
recording power for writing the marks and an erasing power for erasing the
recorded marks (T. Ohta et al, Proc. Int. Symp. on Optical Memory, 1989,
p.49-50). These optical recording media with a recording layer usually
have a heat-resistant and transparent dielectric layer each on both sides
of the recording layer, to prevent the recording layer from being deformed
or opened during recording. Furthermore, in another technique known, a
reflection layer of a light-reflecting metal such as Al is provided on the
side opposite to the beam incident side, to improve the signal contrast at
the time of reproduction by optical interference effect and to allow easy
formation of recorded marks of amorphous state by the effect of cooling
the recording layer and also to improve erasabilities and overwrite
cycles.
Especially the "rapid cooling structure", in which the recording layer and
the dielectric layer between the recording layer and the reflection layer
are kept as thin as about 20 nm, is considerably less lowered in
recordability in spite of repeated overwriting and wider in the erasing
power margin than the "slow cooling structure" in which the dielectric
layer is as thick as about 200 nm. These rewritable phase change type
optical recording media include optical discs. On the substrate of the
optical disc, a groove is formed beforehand to form also land. At present,
a general optical disc has a laser beam focused only on either the land or
the groove, for recording and reproducing signals.
To increase the recording capacity of the optical disc, it is practiced to
narrow the width of the land or groove, for shortening the track pitch.
However, if the track pitch are shortened, the groove makes the angle of
diffraction of reflected light large, to lower disadvantageously the
tracking error signal for enabling the focused spot accurately to follow
the track. Furthermore, if the width of the land or groove is narrowed,
the width of the recorded pits is also narrowed, to lower the amplitude of
reproduced signals as another problem. On the other hand, a technique of
recording signals on both the tracks of the land and groove for increasing
the recording capacity (JP-B 88-57859) is also known. However, if signals
are recorded on both the tracks of the land and the groove, there are such
problems (i) that the signal leak from the adjacent track (cross talk)
increases, so that regenerated signals deteriorate, thus increasing the
error, (ii) that since the difference in the amplitude of reproduced
signals between the land and the groove becomes large, data detection is
difficult and (iii) that at the time of recording, the marks of the
adjacent track already recorded are erased (cross erase).
For such an optical disc, pit position recording has been used. However, in
recent years, to meet the demand for higher density recording media, edge
recording to allow higher density recording by recording information at
edges of marks is going to be used instead of the pit position recording.
In edge recording, longer recorded marks must be formed than those of the
pit position recording and at the rear portion of a long mark, the
remaining heat effect of the recording layer widens the recorded mark, to
deform disadvantageously the front-rear symmetry of the recorded mark like
a tear drop. The deformation of the recorded mark deforms the reproduced
signal, to increase jitter as a result.
As a means for solving this problem, it has been proposed to divide one
recording pulse into a plurality of recording pulses (hereinafter called a
pulse train) (JP-A 91-35425). Furthermore, a technique, in which a pulse
corresponding to one half of the window margin and with a power smaller
than the erasing power is applied for irradiation after the last pulse of
each recording pulse train, is known (Proceedings of 5th Symposium on the
Research of Phase Change Recording, p. 86, 1993).
However, in general, in a phase change type optical recording medium, the
recorded marks of amorphous state are lower in reflectance and the
difference in reflectance between the recorded amorphous state and the
non-recorded crystalline state makes the difference in the photo-absorbed
quantity of the recording layer large. So, depending on whether the state
before overwriting is the crystalline state or the amorphous state with
marks, the temperature rise during recording is different. Even in the
edge recording using pulse trains, this phenomenon is liable to occur at
the rear end of a long recorded mark since the temperature reached during
recording is higher and it remarkably appears at a higher recording linear
velosity. Thus, depending on the state before overwriting, the maximum
temperature at the overwriting changes and the cooling rate of the
recording layer also changes. So, a new recorded mark is modulated by the
previous recorded mark, which is a factor to limit the jitter
characteristic at the rear end of the mark and furthermore to limit
erasability. That is, even if pulse trains are used, the deformation of
recorded marks at the time of overwriting cannot be avoided.
Moreover, even if a pulse with a duration corresponding one half of the
window margin and with a power smaller than the erasing power is added
after the last pulse, the jitter characteristic at the rear end of the
recorded mark at the time of overwriting cannot be sufficiently improved.
SUMMARY OF THE INVENTION
The present invention assresses the problems associated with the
conventional optical recording media and to provide an optical recording
medium which can decrease the cross talk without widening the track pitch
more than the track pitches of the conventional optical recording media
and without using any special optical system or signal processing circuit
for decreasing the cross talk, can keep the reproduced signal amplitudes
of the land and the groove almost equal and can also decrease the cross
erasure.
The present invention seeks also to provide an optical recording medium
high in recording sensitivity and excellent in such recording
characteristics as carrier-to-noise ratio and erasability.
The present invention seeks also to provide an edge recording method
excellent in jitter characteristic without deformation at the rear ends of
marks.
Furthermore, the present invention seeks also addresses the problems
associated with conventional optical recording media, for realizing an
optical disc high in density and large in capacity and seeks to provide an
optical recording medium for the edge recording on both the tracks of the
land and the groove, which is less in cross talk, can keep the regenerated
signal amplitudes of the land and the groove almost equal and can suppress
the jitter increase at the time of overwriting.
The present invention provides an optical recording medium excellent in
oxidation resistance and wet heat resistance and long in lifetime without
causing any defect even after storage for a long time.
The present invention provides, according to the first aspect, an optical
recording medium having each of a land and a groove, on each of which land
and groove recording and erasing of data is to be carried out by phase
change between amorphous and crystalline, and which optical recording
medium comprises at least a recording-layer, dielectric layer and
reflecting layer, characterized in that the mirror portion has reflectance
characteristics selected from at least one of the reflectance of 15% to
35% when in a crystalline state and reflectance of 10% or less when in an
amorphous state.
Such an optical recording medium has each of a land and a groove, on each
of which land and groove recording and erasing of data are to be carried
out by phase change between amorphous and crystalline. The reflectance of
the mirror portion, when in a crystalline state, is 15% to 35% and/or the
reflectance of the mirror portion, when in an amorphous state, is 10% or
less.
Furthermore, the present invention provides, according to the second
aspect, an optical recording medium having each of a land and a groove on
each of which land and groove recording and erasing of data is to be
carried out by phase change between amorphous and crystalline, wherein the
depth of the groove is 1/7 to 1/5 of the wavelength of the reproducing
light and the photo-absorbance of the recording layer in the amorphous
phase and the photo-absorbance of the recording layer in the crystalline
phase satisfy the following Formula (6).
Formula 6:
Aa-Ac.ltoreq.10 (6)
where Aa is the photo-absorbance (%) of the recording layer in the
amorphous phase and Ac is the photo-absorbance (%) of the recording layer
in the crystalline phase.
Still furthermore, according to the third aspect, the present invention
provides an optical recording medium having a mirror portion and each of a
land and a groove, on each of which land and groove recording and erasing
of data is to be carried out by phase change between amorphous and
crystalline, and which optical recording medium comprises at least a
recording layer, dielectric layer and reflecting layer, at least the
recording layer containing nitrogen, and the reflectance characteristics
of the mirror portion is selected from the reflectances, when in a
crystalline state, of 15% to 35% and, when in an amorphous state, of 10%
or less.
In the optical recording medium in accordance with the invention any one or
more of the above features described with reference to the various aspects
may be present.
Moreover, according to the fourth aspect of the present invention, the
present invention provides a recording method in which a recording power
is directed at an optical recording medium so as to provides an edge type
recording system in which recording and erasing are effected by a phase
change between amorphous and crystalline states, which recording method
comprises directing at a recording medium a series of pulse trains to form
respective recording marks, each of which pulse trains comprises a number
of recording power pulses, which number is, for each pulse train,
independent of the number of the recording power pulses in each other
train and is at least one, and each of which pulse trains includes
additionally power pulse, of a power lower than an erasing power and of a
duration in a range 1.1 to 6, times the duration of the last recording
power pulse of the pulse train.
Still moreover, according to the fifth aspect of the present invention, the
present invention provides a recording method in which a recording power
is directed at an optical recording medium so as to provides an edge type
recording system in which recording and erasing are effected by a phase
change between amorphous and crystalline states, which recording method
comprises directing at a recording medium a series of pulse trains to form
respective recording marks, and each of which pulse trains includes
additionally a power pulse, after the last said recording power pulse, of
a power lower than an erasing power and of a duration in a range 0.6 to 3
times the duration of a window margin.
Thus, an optical recording medium to have information recorded and erased
by a phase change between amorphous and crystalline may be irradiated with
a recording power by pulse trains, respectively formed by a plurality of
recording pulses, to form recorded marks in edge recording. A pulse is
provided for irradiating with a power less than the erasing power after
the last pulse of each recording pulse train, the duration of said pulse
for recording, being 1.1 times to 6 times the duration of the last pulse
of the recording pulse train.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described below in detail.
Since the optical recording medium of the present invention can be high in
recording sensitivity, high in signal contrast and less in cross talk, it
is necessary to make a configuration, in which the reflectance of the
mirror portion in the crystalline state is 15% to 35%. If the reflectance
of the mirror portion in the crystalline state is larger than 35%, the
recording sensitivity is lowered and the irradiation power for recording
and erasure is insufficient, making it difficult to record at a high
rotational speed, while the cross talk from the recorded portion of the
adjacent track increases. If the reflectance in the crystalline state is
smaller than 15%, the difference in reflectance between the crystalline
portion and the amorphous portion with recorded marks is small, to make
the signal contrast during reproduction small. In view of these, it is
more preferable that the reflectance of the mirror portion in the
crystalline state is 15% to 30%.
The mirror portion refers to a portion with a mirror surface free from the
groove or pre-pits formed. If the reflectance of the mirror portion is
measured, the reflectance on the optical recording medium can be
accurately measured without being affected by the groove or pre-pits.
Furthermore, it is preferable to make a configuration, in which the
reflectance of the mirror portion in the amorphous state is 10% or less.
If the reflectance of the mirror portion in the amorphous state is 10% or
more, the signals of the adjacent track are easily read during
reproduction, to increase the cross talk, for deteriorating the reproduced
signals, resulting in the increase of error rate. Furthermore, if the
reflectance of the mirror portion in the amorphous state is more than 5%,
the servo of the track with recorded marks formed is stable. So, it is
especially preferable that the reflectance is 5% to 10%.
Furthermore, in order to keep the land and the groove equal in the
amplitude of reproduced signals, for decreasing the cross talk, it is
preferable to make a configuration, in which the phase difference between
the reflected light in the amorphous state and the reflected light in the
crystalline state is kept in a range from 2 n .pi.-.pi./3 to 2 n
.pi.+.pi./3, or 2 n .pi.+2 .pi./3 to 2 n .pi.+4 .pi./3. If the phase
difference is not in this range, the amplitude difference between the land
and the groove becomes large and the cross talk is also becomes large. If
the phase difference is 2 n .pi.-0.1 .pi. to 2 n .pi.+0.1 .pi., or 2 n
.pi.+0.9 .pi. to 2 n .pi.+1.1 .pi., the amplitude of reproduced signals is
especially large more preferably. In the above formulae, n stands for an
integer. Furthermore, because amplitude of reproduced signal can be large,
it is preferable that the phase difference between the reflected light in
the amorphous state and the reflected light in the crystalline state is
kept in a rage from 2 n .pi.+0.87 .pi. to 2 n .pi.+1.2 .pi..
Moreover, in the optical recording medium of the present invention, it is
preferable that the depth of the groove is 1/7 to 1/5 of the wavelength of
the reproducing light, since the cross talk can be decreased. If the
optical path length is less than 1/7 or more than 1/5 of the wavelength of
the reproducing light, cross talk increases, making accurate reproduction
difficult. Furthermore, to keep the jitter within the value required for
the edge recording, it is preferable to make a configuration, in which the
difference in absolute value between the photo-absorbance of the recording
layer in the amorphous phase and the photo-absorbance in the crystalline
phase is smaller than 10%.
If the difference (Aa-Ac) is more than 10%, photo-absorbance in the
amorphous phase is still larger than that in the crystalline phase. So,
even if both portions are irradiated with same quantity of light, the
crystalline portion rises much slower in temperature. Furthermore, at the
melting point, temperature increase is suspended in the crystalline
portion while the latent heat is absorbed, while temperature rises in the
amorphous portion. So the temperature difference between the crystalline
portion and the amorphous portion increases much more. After completion of
melting, since there is no difference in state between both the portions,
equal photo-absorption can be achieved. Therefore, if a proper difference
in photo-absorbance is not set, the difference between the tempratures
reached in both portions become large. As the results, the marks recorded
by overwriting are formed with the different size with being affected by
the previous state, and more deformed marks are formed. That is, the
jitter will be increased.
The refractive index of the first and second dielectric layers is
preferably 1.5 to 2.4, for the signal contrast during reproduction due to
the optical interference effect. If the refractive index is less than 1.5,
the signal contrast during reproduction cannot be sufficient and if the
refractive index is larger than 2.4, the reflectance depends more on the
thickness of the dielectric layer.
The thickness of the first dielectric layer is preferably 50 nm or more,
because it cannot be easily separated from the substrate or the recording
layer and because such defects as cracking are less liable to be caused.
Considering the production cost and quality control regarding thickness, a
thickness of less than about 200 nm is preferable.
The reflectance can be controlled by any method and for example, can be
controlled by adjusting the refractive index and thickness of the first
dielectric layer. If a transparent substrate like a policarbonate or glass
is used, the refractive index of the substrate is about 1.5 and the
refractive index of the first dielectric layer is about 1.5 to 2.4. In
order to keep the reflectance of the mirror portion in the crystalline
state in a range from 15% to 35% and the reflectance of the mirror portion
in the amorphous state in a range of 10% or less by the optical
interference effect due to the refractive indexes, it is preferable that
the optical path length n1 d1 of the first dielectric layer is in a range
expressed by the following formula:
Formula (7)
(N/4-0.1).lambda..ltoreq.n1 d1.ltoreq.(N/4+0.1).lambda.
1.5.ltoreq.n1.ltoreq.2.4
where n1 is the refractive index of the first dielectric layer; d1 is the
thickness (nm) of the first dielectric layer; .lambda. is the wavelength
(nm) of light; and N is 1 or 3.
Furthermore, the reflectance can be controlled by letting the recording
layer contain nitrogen. The reflectance is controlled by the nitrogen atom
concentration in the recording layer. It is preferable that the recording
layer is configured to contain 0.1 atom % to 10 atom % of nitrogen. If
nitrogen atoms of this range are introduced into the recording layer, the
thermal conductivity of the recording layer is lower than that of the
recording layer not containing nitrogen atoms and the heat applied to the
recording layer goes out less. In this case, recording at a lower power
can allowed.
If the nitrogen atom concentration in the recording layer is larger than
10%, the reflectance is so low as to cause an insufficient quantity of
light to be reflected, making it difficult to achieve focusing. More
preferably, the nitrogen atom concentration is 3.5% or less. If the
nitrogen atom concentration is more than 3.5%, the noise is so large as to
lower the C/N. If the nitrogen atom concentration of the recording layer
is smaller than 0.1%, the effect of changing the reflectance by
introducing nitrogen atoms into the recording layer cannot be expected.
More preferably, the nitrogen atom concentration is 1% or more.
The thickness of the second dielectric layer is about 1 to about 250 nm. A
thickness of 1 to 50 nm is preferable since the range of erase powers to
give a good erasability is wide. If the thickness of the second dielectric
layer is larger than 50 nm, the merit of the rapid cooling structure
cannot be obtained. If the thickness is smaller than 1 nm, the
recordability in repeated recording is remarkably lowered. Furthermore,
the optical path length n2.multidot.d2 of the second dielectric layer is
preferably in a range expressed by the following formula:
Formula (8)
.lambda.(1/50).ltoreq.n2.multidot.d2.ltoreq..lambda.(1/10)
1.5.ltoreq.n2.ltoreq.2.4
where n2 is the refractive index of the second dielectric layer; d2 is the
thickness (nm) of the second dielectric layer; and .lambda. is the
wavelength (nm) of light.
In formula (8), if n2.multidot.d2 is smaller than .lambda. (1/50) or larger
than .lambda. (1/10), it is very difficult to secure the contrast between
the crystalline portion and the amorphous portion. Furthermore, if
n2.multidot.d2 is smaller than .lambda. (1/50), durability (cyclability)
is lowered. So, n2.multidot.d2 is preferably in the range expressed by the
above formula.
To obtain sufficient reproduced signals and to minimize cross talk,
n2.multidot.d2 should be in the above mentioned range, and, the mirror
portion has reflectance characteristics selected from at least one of the
reflectance of more than 15% to 35% when in a crystalline state and
reflectance of 10% or less when in an amorphous state. And, further, it is
also preferable that the phase difference between the reflected light of
the amorphous phase and the reflected light of the crystalline phase is 2
n .pi.-.pi./3 to 2 n .pi.+.pi./3, more preferably, 2 n .pi.+2 .pi./3 to 2
n .pi.+4.lambda./3, to arrange the amplitudes of the land and the groove.
A four-layer configuration consisting of a first dielectric layer,
recording layer, second dielectric layer and reflection layer in this
order in addition to the substrate is preferable since the rapid cooling
structure is liable to be secured with the durability (cyclability)
enhanced as a result.
Moreover, a configuration consisting of at least a first dielectric layer,
recording layer and photo-absorbable layer is preferable since the
photo-absorbance of the recording layer in the amorphous state and the
photo-absorbance in the crystalline state can be controlled to further
decrease jitter at the time of overwriting. The above mentioned
photo-absorbable properties can be incorporated to at least one layer of
the optical recording medium of this invention or can be incorporated into
a layer separately prepared. Since photo-absorption may accompany heat
generation, disc structure design is easy if the photo-absorbable layer is
provided as an additional independent layer. For easier realization of the
rapid cooling structure, its resultant higher durability (cyclability) and
less jitter at the time of overwriting, it is an example to adopt a
five-layer configuration consisting of a first dielectric layer, recording
layer, second dielectric layer, photo-absorbable layer and reflection
layer in addition to the substrate. If the photo-absorbable layer is
provided, the reflection layer is not necessarily required, but it is
preferable to have the reflection layer in view of sensitivity adjustment
and contrast.
The photo-absorbable layer can also function to control the cooling of the
recording layer by its thermal conductivity, specific heat, etc. This
enables it to control diffusion of the absorbed heat of the recording
layer, and to decrease its influence (for example, cross erase) to marks
of the next track. In addition to the photo-absorbable layer, by making
the reflectance of the mirror portion 15% to 35% when in a crystalline
state and/or 10% or less when in an amorphous state, jitter at overwriting
becomes small and durability of cross erase becomes high, and cross talk
decreases.
The material of the photo-absorbable layer is not especially limited, but
can be preferably at least one metal selected from Ti, Zr, Hf, Cr, Ta, Mo,
Mn, W, Nb, Rh, Ni, Fe, Y, V, Co, Cu, Zn, Ru, Pd, lanthanides and Te, or
any of their mixtures and alloys, since they are excellent in heat
resistance, strength and corrosion resistance. Even if the
photo-absorbable layer is provided, the thickness of the second dielectric
layer is preferably 1 nm to 50 nm, more preferably 1 nm to 30 nm,
considering the thermal conductivity, etc.
It is also especially preferable to use an alloy of Si and/or Ge as the
photo-absorbable layer. In view of storage stability ot the recording
medium, it is especially more preferable to use an alloy consisting of one
or more metals higher in the absolute value of oxide production heat than
Si and/or Ge and Si and/or Ge. In this case, the metal in the alloy of Si
and/or Ge can be preferably selected from Be, Al, Se, Ti, Cr, Mn, Fe, Co,
Ni, Cu, Y, Zr, Nb, Ru, Rh, Pd, Ag, Hf, Re, Os, Ir, Pt and Au. Above all,
Zr, Ti and Hf are more preferable since they are excellent in storage
stability. Furthermore, it is preferable that the crystallographic
structure of the material of the photo-absorbable layer is substantially
amorphous. If the material has a crystalline structure, it can happen that
a phase is segregated to make the film heterogeneous, or that the
temperature rise during recording of marks causes structural phase
transition, to cause film separation, etc., thus lowering the overwrite
cyclability.
The material of the recording layer of the present invention is a chalcogen
compound mainly composed of Te, which can have at least two states of
crystalline state and amorphous state. Preferably, this recording layer is
made of ternally alloy Ge, Te and Sb, or comprises an alloy of the three
elements of Ge, Te and Sb, plus at least one metal selected from Pd, Nb,
Pt, Au, Ag, Ni and Co. Additionally, as previously described, nitrogen may
also be present. Thus, the material of the recording layer of the present
invention can be selected from, though not limited to, Ge--Sb--Te alloy,
Pd--Ge--Sb--Te alloy, Nb--Ge--Sb--Te alloy, Pt--Ge--Sb--Te alloy,
Au--Ge--Sb--Te alloy, Ag--Ge--Sb--Te alloy, Ni--Ge--Sb--Te alloy,
Co--Ge--Sb--Te alloy, Pd--Nb--Ge--Sb--Te alloy, In--Sb--Te alloy,
Ag--In--Sb--Te alloy and In--Se alloy. Among them, Pd--Ge--Sb--Te alloy,
Nb--Ge--Sb--Te alloy, Pd--Nb--Ge--Sb--Te alloy, Ni--Ge--Sb--Te alloy,
Ge--Sb--Te alloy and Co--Ge--Sb--Te alloy are preferable since many times
of repeated rewriting are possible. Especially Pd--Ge--Sb--Te alloy and
Pd--Nb--Ge--Sb--Te alloy are preferable because of shorter erasing time,
higher repeatability of recording and erasure and excellent recordability
such as C/N and erasability. Above all, Pd--Nb--Ge--Sb--Te alloy is more
preferable since it is excellent in the properties stated above. It is
preferably a layer having the composition as shown by the following
formula because they allow repeated rewritings.
Formula (3)
M.sub.z (Sb.sub.x Te.sub.1-x).sub.1-y-z (Ge.sub.0.5 Te.sub.0.5).sub.y
0.4.ltoreq.x.ltoreq.0.6
0.3.ltoreq.y.ltoreq.0.5
0.ltoreq.z.ltoreq.0.05
where x, y and a are a molar ratio, respectively; and M comprises, and
preferably, elements of at least one of Pd, Nb, Pt, Au, Ag, Ni and Co,
more preferably, Pd, Nb or Pt.
The value of y is more preferably 0.3 to 0.4 because of higher durability
(cyclability) of rewriting and higher thermal stability in the amorphous
state. The value of z is more preferably 0.001 to 0.1, because of the
higher crystallization rate, higher durability (cyclability) and higher
thermal stability in the amorphous state.
The thickness of the recording layer is preferably 10 nm to 40 nm. If the
thickness is less than 10 nm, the contrast in reflectance between the
crystalline state and the amorphous state cannot be sufficient. If the
thickness is more than 40 nm, the quantity of the thermal conduction of
the recording layer is so large as to increase the cross erase.
In the optical recording medium of the present invention, the width of the
slope of the groove is preferably 0.2 .mu.m or less, since the groove
becomes closer to a rectangle, to intercept on the slope the heat
generated when marks are recorded, thereby improving the cross erase
durability. However, if the width of the slope of the groove is less than
0.05 .mu.m, it is difficult to separate the substrate from the stamper at
the time of substrate molding.
If the track pitch of the optical recording medium of the present invention
is less than .lambda./NA (.lambda. is the wavelength of the recording and
reproducing light and NA is the numerical aperture of the lens), cross
erase cannot be avoided. If the track pitch is more than 1.5 .lambda./NA,
the track pitch is so wide as to make the high density recording
meaningless. So, it is preferable that the track pitch is kept in this
range. Furthermore, the rate of the flat portions of the land and the
groove to the track pitch is preferably 0.2 to 0.6. If the rate is not in
this range, the land and the groove are greatly different in the width of
flat portion, to make the amplitude of reproduced signals on the land and
the groove greatly different, or the width of the slope of the groove is
so wide as to impair the above effect. It is especially preferable that
the rate of the flat portions of the land and the groove to the track
pitch is 0.3 to 0.5.
The widths of recorded marks of the land and the groove are preferably 1/2
or more of the respective flat portions of the land and the groove in view
of a larger amplitude of the reproduced signals. To prevent the widths of
the marks from becoming so large as to cause cross erase, it is preferable
that the widths of the recorded marks of the land and the groove are less
than the widths of the respective flat portions of the land and the
groove.
The material of the substrate of the present invention can be any of
various known transparent synthetic resins, or transparent glass, etc. To
avoid the influence of the flaws, etc. of the substrate, it is preferable
to use a transparent substrate and to use a focused beam for recording
from the substrate side. The material of the transparent substrate can be
selected from glass, polycarbonates, polymethyl methacrylate, polyolefin
resins, epoxy resi | | |
