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Claims  |
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We claim:
1. An optical information recording medium comprising a substrate provided with periodically wobbling guide grooves with a track pitch of 1.6.+-.0.1 .mu.m, and a lower protective layer,
a phase-change type recording layer, an upper protective layer and a reflective layer formed in this sequence on the substrate, for recording, retrieving and erasing amorphous marks in the guide grooves by modulation of light intensity of at least two
levels by means of a focused light having a wavelength of 780.+-.30 nm applied from the side of the substrate opposite to the recording layer side so that a crystalline state with a reflectance of from 15 to 25% is an
unrecorded state, and an amorphous state with a reflectance of less than 10% is a recorded state, wherein the recording layer is a thin film of an alloy of My.sub.y (Sb.sub.x Te.sub.1-x).sub.1-y where 0.ltoreq.y<0.3, 0.5<x<0.9, and My
is at least one member selected from the group consisting of In, Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, V, Nb, Ta, Pb, Cr, Co, O, S and Se, and its thickness is from 15 to 30 nm, the thickness of the lower protective layer is at least 70 nm and less
than 150 nm and thicker by more than 0 nm and not more than 30 nm than the thickness where the reflectance in the crystalline state becomes minimum, and the grooves have a depth of from 25 to 45 nm and a width of from 0.4 to 0.6 .mu.m.
2. The optical information recording medium according to claim 1, wherein the protective layers are made of (ZnS).sub.1-a (SiO.sub.2).sub.a, where 0.13.ltoreq.a.ltoreq.0.17 for the lower protective layer, and 0.18.ltoreq.a.ltoreq.0.22 for the
upper protective layer.
3. The optical information recording medium according to claim 1, wherein the guide grooves have a depth of from 30 to 40 nm and a width of from 0.45 to 0.55 .mu.m.
4. The optical information recording medium according to claim 1, wherein the lower protective layer has a refractive index of from 2.0 to 2.2 and a thickness of from 70 to 90 nm, the thickness of the upper protective layer is from 10 to 30 nm,
and the reflective layer is Al.sub.1-b M.sub.b, where 0.005.ltoreq.b.ltoreq.0.1, and M is Ta or Ti having a thickness of from 50 to 200 nm.
5. The optical information recording medium according to claim 1, wherein the phase-change type recording layer is made of Ag.sub..gamma. In.sub..delta. Sb.sub..epsilon. Te.sub..eta., where 3<.gamma.<10, 3<.delta.<8,
55<.epsilon.65, 25<.eta.<35, 6<.gamma.+.delta.<13, and .gamma.+.delta.+.epsilon.+.eta.=100.
6. The optical information recording medium according to claim 1, wherein the phase-change type recording layer has a composition of Mw.sub.w Ge.sub.z (Sb.sub.x Te.sub.1-x).sub.1-z-w, where Mw is at least one member of Ag and zn,
0.60.ltoreq.x.ltoreq.0.85, 0.01.ltoreq.z.ltoreq.0.20, 0.01.ltoreq.w.ltoreq.0.15, and 0.02.ltoreq.z+w<0.30.
7. The optical information recording medium according to claim 1, wherein to carry out an initialization operation by irradiating with an energy beam for crystallization, after forming the phase-change type recording layer, the recording layer
is locally melted and crystallized during resolidification.
8. The optical information recording medium according to claim 1, which is an optical information recording medium whereby mark length modulation recording and erasing are carried out by modulating a laser power among at least 3 power levels at
a linear velocity of from 1 to 7 m/s, wherein to form inter-mark portions, erasing power Pe capable of recrystallizing amorphous mark portions is applied, and to form mark portions having a length nT where T is a clock period and n is an integer of at
least 2, writing power Pw and bias power Pb are applied in such a manner that when the time for applying writing power Pw is represented by .alpha..sub.1 T, .alpha..sub.2 T, . . . , .alpha..sub.m T, and the time for applying bias power Pb is represented
by .beta..sub.1 T, .beta..sub.2 T, . . . , .beta..beta..sub.m T, the laser application period is divided into m pulses in a sequence of .alpha..sub.1 T, .beta..sub.1 T, .alpha..sub.2 T, .beta..sub.2 T, . . . , .alpha..sub.m T, .beta..sub.m T, to
satisfy the following formulae:
m=n-k, where k is an integer of 0.ltoreq.k.ltoreq.2, provided that n.sub.min -k.gtoreq.1, where n.sub.min is the minimum value of n; and
.alpha..sub.1 +.beta..sub.1 +. . . +.alpha..sub.m +.beta..sub.m =n-j, where j is a real number of 0.ltoreq.j.ltoreq.2;
and under such conditions that Pw>Pe, and 0<Pb.ltoreq.0.5 Pe, provided that when i=m, 0<Pb.ltoreq.Pe.
9. The optical information recording medium according to claim 8, wherein 0<Pb.ltoreq.0.2 Pe, provided that when i is m, 0<Pb.ltoreq.Pe, and when 2.ltoreq.i.ltoreq.m-1, .alpha..sub.i +.beta..sub.i =1.0, and 0.05<.alpha..sub.i
.ltoreq.0.5.
10. An optical information recording medium comprising a substrate provided with periodically wobbling guide grooves with a track pitch of 1.6.+-.0.1 .mu.m, and a first lower protective layer, a second lower protective layer, a phase-change type
recording layer, an upper protective layer and a reflective layer formed in this sequence on the substrate, for recording, retrieving and erasing amorphous marks in the guide grooves by modulation of light intensity of at least two levels by means of a
focused light having a wavelength of 780.+-.30 nm applied from the side of the substrate opposite to the recording layer side so that a crystalline state with a reflectance of from 15 to 25% is an unrecorded state, and an amorphous state with a
reflectance of less than 10% is a recorded state, wherein the recording layer is a thin film of an alloy of My.sub.y /(Sb.sub.x Te.sub.1-x).sub.1-y where 0.ltoreq.y<0.3, 0.5<x<0.9, and My is at least one member selected from the group consisting
of In, Ga, Zn, Ge, Sn, Si, Cu. Au, Ag, Pd, Pt, V, Nb, Ta, Pb, Cr, Co, O, S and Se, and its thickness is from 15 to 30 nm, the difference between the refractive index of the first lower protective layer and the refractive index of the substrate is less
than 0.1, the thickness of the second lower protective layer is thinner by more than 0 nm and not more than 30 nm than the minimum thickness where the reflectance in the crystalline state becomes minimum, the total thickness of the first and second lower
protective layers is at least 70 nm and less than 150 nm, and the grooves have a depth of from 25 to 45 nm and a width of from 0.4 to 0.6 .mu.m.
11. The optical information recording medium according to claim 10, wherein the grooves have a depth of from 30 to 40 nm and a width of from 0.45 to 0.55 .mu.m.
12. The optical information recording medium according to claim 10, wherein the thickness of the upper protective layer is from 10 to 30 nm, and the reflective layer is Al.sub.1-b M.sub.b, where 0.005.ltoreq.b.ltoreq.0.l, and M is Ta or Ti,
having a thickness of from 50 to 200 nm.
13. The optical information recording medium according to claim 10, wherein the phase-change type recording layer is made of Ag.sub..gamma. In.sub..delta. Sb.sub..epsilon. Te.sub..eta., where 3<.gamma.<10, 3<.delta.<8,
55<.epsilon.<65, 25<.eta.<35, 6<.gamma.+.delta.<13, and .gamma.+.delta.+.epsilon.+.eta.=100.
14. The optical information recording medium according to claim 10, wherein the phase-change type recording layer has a composition of Mw.sub.w Ge.sub.z (Sb.sub.x Te.sub.1-x).sub.1-z-w, where Mw is at least one member of Ag and Zn,
0.60.ltoreq.x.ltoreq.0.85, 0.01.ltoreq.z.ltoreq.0.20, 0.01.ltoreq.w.ltoreq.0.15, and 0.02.ltoreq.z+w<0.30.
15. The optical information recording medium according to claim 10, wherein to carry out an initialization operation by irradiating an energy beam for crystallization, after forming the phase-change type recording layer, the recording layer is
locally melted and crystallized during resolidification.
16. The optical information recording medium according to claim 10, which is an optical information recording medium whereby mark length modulation recording and erasing are carried out by modulating a laser power among at least 3 power levels
at a linear velocity of from 1 to 7 m/s, wherein to form inter-mark portions, erasing power Pe capable of recrystallizing amorphous mark portions is applied, and to form mark portions having a length nT where T is a clock period and n is an integer of at
least 2, writing power Pw and bias power Pb are applied in such a manner that when the time for applying writing power Pw is represented by .alpha..sub.1 T, .alpha..sub.2 T, . . . , .alpha..sub.m T, and the time for applying bias power Pb is represented
by .beta..sub.1 T, .beta..sub.2 T, . . . , .beta..sub.m T, the laser application period is divided into m pulses in a sequence of .alpha..sub.1 T, .beta..sub.1 T, .alpha..sub.2 T, .beta..sub.2 T, . . . , .alpha..sub.m T, .beta..sub.m T, to satisfy the
following formulae:
m=n-k, where k is an integer of 0.ltoreq.k.ltoreq.2, provided that n.sub.min -k>1, where n.sub.min is the minimum value of n; and
.alpha..sub.1 +.beta..sub.1 +. . . +.alpha..sub.m +.beta..sub.m =n-j, where j is a real number of 0.ltoreq.j.ltoreq.2;
and under such conditions that Pw>Pe, and 0<Pb.ltoreq.0.5 Pe, provided that when i=m, 0<Pb.ltoreq.Pe.
17. The optical information recording medium according to claim 16, wherein 0<Pb.ltoreq.0.2 Pe, provided that when i is m, 0<Pb.ltoreq.Pe, and when 2.ltoreq.i.ltoreq.m-1, .alpha..sub.i +.beta..sub.i =1.0, and 0.05<.alpha..sub.i
.ltoreq.0.5.
18. The optical information recording medium according to claim 1, wherein a readily crystallizable crystallization accelerating layer is formed between the substrate and the recording layer in contact with the recording layer in a thickness of
from 0.2 to 5 nm, and the recording layer is treated for initial crystallization by irradiation of light energy.
19. The optical information recording medium according to claim 18, wherein the crystallization accelerating layer has a composition close to Sb.sub.2 Te.sub.3.
20. The optical information recording medium according to claim 18, wherein a composition-adjusting layer is formed adjacent to the crystallization accelerating layer, so that the composition averaging the compositions of the
composition-adjusting layer and the crystallization accelerating layer is close to the composition of the recording layer.
21. An optical information recording medium comprising a substrate provided with periodically wobbling guide grooves with a track pitch of 1.6.+-.0.1 .mu.m, and a lower protective layer, a phase-change type recording layer, an upper protective
layer and a reflective layer formed in this sequence on the substrate, for recording, retrieving and erasing amorphous marks in the guide grooves by modulation of light intensity of at least two levels by means of a focused light having a wavelength of
780.+-.30 nm applied from the side of the substrate opposite to the recording layer side so that a crystalline state with a reflectance of from 15 to 25% is an unrecorded state, and an amorphous state with a reflectance of less than 10% is a recorded
state, wherein the recording layer is a thin film of an alloy of Ge(Sb.sub.x Te.sub.1-x).sub.1-y where 0.60.ltoreq.x.ltoreq.0.85 and 0.01.ltoreq.y.ltoreq.0.20, and its thickness is from 15 to 30 nm, the thickness of the lower protective layer is at least
70 nm and less than 150 nm and thicker by more than 0 nm and not more than 30 nm than the thickness where the reflectance in the crystalline state becomes minimum, and the grooves have a depth of from 25 to 45 nm and a width of from 0.4 to 0.6 .mu.m.
22. The optical information recording medium according to claim 21, wherein a readily crystallizable crystallization accelerating layer is formed between the substrate and the recording layer in contact with the recording layer in a thickness of
from 0.2 to 5 nm, and the recording layer is treated for initial crystallization by irradiation of light energy.
23. The optical information recording medium according to claim 20, wherein the crystallization accelerating layer has a composition close to Sb.sub.2 Te.sub.3.
24. The optical information recording medium according to claim 20, wherein a composition-adjusting layer is formed adjacent to the crystallization accelerating layer, so that the composition averaging the compositions of the
composition-adjusting layer and the crystallization accelerating layer is close to the composition of the recording layer. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical disk capable of high density recording utilizing a rewritable phase-change medium. Particularly, it relates to a phase-change medium whereby groove signals compatible with CD (compact disk) standards,
can be obtained in spite of low reflectance, and deterioration during repeated overwriting of data is little, while maintaining high contrast.
2. Discussion of the Background
Along with an increasing amount of information in recent years, a recording medium capable of recording and retrieving a large amount of data at a high density and at a high speed has been demanded, and an optical disk is expected to be just
suitable for such an application.
Optical disks include a write-once type disk capable of recording only for once and a rewritable-type disk capable of recording and erasing many times.
As the rewritable-type optical disk, a magneto-optical medium utilizing a magneto-optical effect, or a phase-change medium utilizing the change in reflectance due to the reversible change in the crystal state, may be mentioned.
The phase-change medium has a merit that it is capable of recording/erasing simply by modulating the power of a laser beam without requiring an external magnetic field, and the size of a recording and retrieving device can be made small.
Further, it has a merit that a high density recording can be attained by a shorter wavelength light source without any particular alteration of the material of e.g. the recording layer from the currently predominantly employed medium capable of
recording and erasing at a wavelength of about 800 nm.
As the material for the recording layer of such a phase-change medium, a thin film of a chalcogenic alloy is often used. For example, an alloy of GeSbTe type, InSbTe type, GeSnTe type or AgInSbTe type may be mentioned.
In a rewritable phase-change type recording medium which is practically employed at present, an unrecorded or erased state is a crystalline state, and recording is carried out by forming an amorphous bit. The amorphous bit is formed by heating
the recording layer to a temperature higher than the melting point, followed by quenching. To prevent evaporation or deformation of the recording layer by such heat treatment, it is common to sandwich the recording layer with heat resistant and
chemically stable dielectric protective layers. In the recording step, these protective layers facilitate heat dissipation from the recording layer to realize overcooled state, and thus contribute to formation of the amorphous bit.
Further, it is common that a metal reflective layer is formed on the above described sandwich structure to obtain a quadri-layer structure, whereby the heat dissipation is further facilitated so that the amorphous bit will be formed under a
stabilized condition.
Erasing (crystallization) is carried out by heating the recording layer to a temperature higher than the crystallization temperature and lower than the melting point of the recording layer. In this case, the above-mentioned dielectric protective
layers serve as heat accumulating layers for keeping the recording layer at a temperature sufficiently high for solid phase crystallization.
A phase-change medium which is capable of carrying out the erasing and rewriting steps solely by intensity modulation of one focused light beam, is called a 1-beam overwritable phase-change medium (Jpn. J. Appl. Phys., 26 (1987), suppl. 27-4,
pp. 61-66).
With a recording system employing the 1-beam overwritable phase-change medium, the multilayer structure of the recording medium and the circuit structure of the drive can be simplified. Therefore, an attention has been drawn to this system as an
inexpensive high density and large capacity recording system.
In recent years, rewritable compact disks (CD-Rewritable, CD-RW) have been proposed ("CD-ROM professional", USA, September, 1996, p. 29-44, or preparatory papers for Phase-change Optical Recording Symposium, 1995, p. 41-45).
With CD, rows of pits with length modulated by data sequence, formed on a substrate with a pitch of 1.6.+-.0.1 .mu.m are scanned by a focused laser beam having a wavelength of 780.+-.30 nm from the rear side of the substrate to read out the
recorded information. Here, the reflectance at a non-pitted portion is stipulated to be at least 70%.
With CD-RW, it is difficult to accomplish compatibility with CD if such a high reflectance as at least 70% is included. However, by bringing the reflectance at an unrecorded portion to a level of from 15 to 25% and the reflectance at a recorded
portion to a level of less than 10%, compatibility with CD can be secured with respect to the record signals and groove signals, and by adding an amplifying system to cover the low reflectance to the retrieving system, compatibility can be secured within
the range of the existing CD drive technology.
In CD-RW, grooves are used as recording tracks, and recording is carried out in the grooves, and wobbling is used for these grooves to include address information (JP-A-5-210849).
FIG. 1 shows a schematic view illustrating wobbling grooves 2 formed on the surface of a substrate 1. However, the wobble amplitude is exaggerated. The wobbling is called "wobble" and frequency-modulated (FM) by a carrier frequency of 22.05
kHz, and its amplitude (Wobble Amplitude) is very small at a level of 30 nm as compared with the pitch of grooves 2 (i.e. the distance between the imaginary center lines of grooves 2: usually about 1.6.+-.0.1 .mu.m).
Such a wobble frequency-modulated by absolute time information or address information, is called ATIP (Absolute Time In Pre-groove) or ADIP (Address In Pre-groove), which has already been used in a recordable compact disk (CD-Recordable, CD-R) or
in a mini disk ("CD family", coauthored by Heitaro Nakajima, Takao Inohashi and Hiroshi Ogawa, Ohm-sha (1996) chapter 4 and Proceedings of the IEEE, vol. 82 (1994) p. 1490).
The recording process of the above phase-change medium involves a drastic heat cycle such that the recording layer is melted and then quenched to a temperature lower than the melting point within a few tens nano seconds. Therefore, even if the
recording layer is sandwiched by dielectric protective layers, microscopic deformations and segregations will be accumulated by repetitive overwriting for a few thousands to a few tens thousands times, and will eventually lead to an increase of optically
recognizable noises or formation of local defects of micron order (J. Appl. Phys., 78 (1995), pp 6980-6988).
Substantial improvements have been made by modifying the materials for the recording layer and the protective layers, or the multilayer structure. However, there is essentially an upper limit in the number of rewritable times, and it is usually
smaller by at least one figure than a usual magnetic recording medium or magneto-optical recording medium.
With the above-described CD-RW, recording is carried out at a low linear velocity at a level of at most 6 times of the CD linear velocity and under such a severe condition as mark length modulation recording, and a higher level of repetitive
overwriting durability is required.
Further, in the mark length modulation recording employing a phase-change medium for forming amorphous marks while an unrecorded state is a crystalline state, it is desired that the outlines of the amorphous marks are smooth and distinct. For
this reason, in place of a conventional GeTe-Sb.sub.2 Te.sub.3 pseudo binary alloy, a material for a recording layer having a smaller grain size, is desired.
Further, from the study by the present inventors, it has been found that the groove geometry is required to secure the compatibility of groove signals with the CD standards, rather lowers the repetitive overwriting durability of the phase-change
medium. Namely, within a range of the groove geometry (depth: 20 to 100 nm, width: 0.2 to 0.8 .mu.m) where there will be no trouble in tracking servo (a push-pull method or a 3 beam method) with a focused light having a wavelength of 780 nm.+-.30 nm,
the groove depth is required to be less than 60 nm, and the groove width is required to be within a range of from 0.3 to 0.6 .mu.m in order to bring push-pull signals after recording to the same level as ROM standards (about 0.04 to 0.09) to secure the
compatibility with CD-ROM (JP-A-8-21550, but this patent concerns nothing about the repetitive overwriting durability). This relation is a parameter determined substantially solely by the groove geometry, which does not substantially depend on the
multilayer structure of the phase-change medium.
On the other hand, there is a tendency that the repetitive overwriting durability is better when the groove is deep and narrow in width. From the study by the present inventors, it has been found that the repetitive overwriting durability
abruptly deteriorates when the groove depth becomes shallower than 50 nm.
Thus, to secure the compatibility with the groove signals of conventional CD, the overwriting durability has to be sacrificed to some extent, but it is desired to minimize such a sacrifice.
On the other hand, in addition to the above described restriction derived from the groove signals, a new phenomenon for deterioration has been found with a CD-RW medium employing a phase-change medium, such that wobble signals are likely to leak
into recorded signals by repetitive overwriting. The wobble is essential also to CD-RW to impart address information essential to detect an unrecorded region where information is to be recorded. If it is attempted to reduce the groove width to overcome
the deterioration of the overwriting durability due to reduction of the groove depth, the groove walls tend to be damaged by the heat by a recording light beam edge, whereby deterioration of the signals attributable to the wobble signals is believed to
be accelerated.
Further, the groove bottom also undergoes deformation by the heat generation of the recording layer. The lower protective layer has a function of not only suppressing the temperature rise of the substrate surface by the heat insulating effect
but also mechanically suppressing the deformation of the substrate. Accordingly, a ZnS-SiO.sub.2 mixture film or the like is widely used from the viewpoint of the thermal conductivity and mechanical properties.
From the study by the present inventors, it has been found very difficult to satisfy both the productivity and the repetitive overwriting durability because of the restriction of the thickness of the lower protective layer due to the optical
requirements for the compatibility with the CD standards. Due to such an additional condition required to secure the compatibility with the CD standards, the number of repetitive overwriting further decreases by at least one figure to a level of a few
thousands times.
The method of rewriting information per a sector unit as in a magneto-optical disk, has not yet been established with CD-RW. However, if such a method will be practically used, it is likely that the number of rewriting to a specific sector will
exceed 1,000 times, whereby the problem of deterioration due to repetitive overwriting will be more serious.
Accordingly, it has been an acute demand to improve the repetitive overwriting durability while securing the compatibility with the current CD standards as far as possible.
SUMMARY OF THE INVENTION
The present inventors have found the following solutions to the above-mentioned problems encountered in the course of developing CD-RW employing a phase-change medium, and the present invention has been accomplished on the basis of such
solutions.
Namely, they have found specific groove width and groove depth whereby the compatibility of groove signals with CD can readily be obtained, and they have found a combination of a groove width, a recording layer composition and a multilayer
structure, whereby adequate repetitive overwriting durability can be obtained.
Firstly, by the specific groove width, acceleration of the deterioration by repetitive overwriting due to a wobble has been reduced.
Further, they have found a recording layer composition whereby jitter can be suppressed to a low level in mark length modulation recording and which is excellent also in repetitive overwriting durability. They have also found that some recording
layer composition is applicable also to media other than CD-RW.
Still further, they have found a multilayer structure whereby repetitive overwriting durability is good, uniformity of reflectance can readily be obtained, and the productivity is good.
Namely, in a first aspect, the present invention provides an optical information recording medium comprising a substrate provided with periodically wobbling guide grooves with a track pitch of 1.6.+-.0.1 .mu.m, and a lower protective layer, a
phase-change type recording layer, an upper protective layer and a reflective layer formed in this sequence on the substrate, for recording, retrieving and erasing amorphous marks in the guide grooves by modulation of light intensity of at least two
levels by means of a focused light having a wavelength of 780.+-.30 nm applied from the side of the substrate opposite to the recording layer side so that a crystalline state with a reflectance of from 15 to 25% is an unrecorded state, and an amorphous
state with a reflectance of less than 10% is a recorded state, wherein the recording layer is a thin film of an alloy of My.sub.y (Sb.sub.x Te.sub.1-x).sub.1-y where 0.ltoreq.y<0.3, 0.5<x<0.9, and My is at least one member selected from the
group consisting of In, Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, V, Nb, Ta, Pb, Cr, Co, O, S and Se, and its thickness is from 15 to 30 nm, the thickness of the lower protective layer is at least 70 nm and less than 150 nm and thicker by more than 0 nm
and not more than 30 nm than the thickness where the reflectance in the crystalline state becomes minimum, and the grooves have a depth of from 25 to 45 nm and a width of from 0.4 to 0.6 .mu.m.
In a second aspect, the present invention provides an optical information recording medium comprising a substrate provided with periodically wobbling guide grooves with a track pitch of 1.6.+-.0.1 .mu.m, and a first lower protective layer, a
second lower protective layer, a phase-change type recording layer, an upper protective layer and a reflective layer formed in this sequence on the substrate, for recording, retrieving and erasing amorphous marks in the guide grooves by modulation of
light intensity of at least two levels by means of a focused light having a wavelength of 780.+-.30 nm applied from the side of the substrate opposite to the recording layer side so that a crystalline state with a reflectance of from 15 to 25% is an
unrecorded state, and an amorphous state with a reflectance of less than 10% is a recorded state, wherein the recording layer is a thin film of an alloy of My.sub.y (Sb.sub.x Te.sub.1-x).sub.1-y where 0.ltoreq.y<0.3, 0.5<x<0.9, and My is at
least one member selected from the group consisting of In, Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, V, Nb, Ta, Pb, Cr, Co, O, S and Se, and its thickness is from 15 to 30 nm, the difference between the refractive index of the first lower protective layer
and the refractive index of the substrate is less than 0.1, the thickness of the second lower protective layer is thinner by more than 0 nm and not more than 30 nm than the minimum thickness where the reflectance in the crystalline state becomes minimum,
the total thickness of the first and second lower protective layers is at least 70 nm and less than 150 nm, and the grooves have a depth of from 25 to 45 nm and a width of from 0.4 to 0.6 .mu.m.
In a third aspect, the present invention provides an optical information recoding medium which is a rewritable optical information recording medium comprising a substrate and a phase-change type recording layer comprising Sb.sub.x Te.sub.1-x,
where 0.6.ltoreq.x.ltoreq.0.85, as the main component, formed on the substrate, wherein a readily crystallizable crystallization accelerating layer is formed between the substrate and the recording layer in contact with the recording layer in a thickness
of from 0.2 to 5 nm, and the recording layer is treated for initial crystallization by irradiation with light energy.
In the accompanying drawings:
FIG. 1 is a schematic view illustrating wobbling grooves.
FIG. 2 is a schematic view illustrating an embodiment of the optical recording medium of the present invention.
FIG. 3 is a schematic view illustrating another embodiment of the optical recording medium of the present invention.
FIG. 4 is a view showing an example of the irradiation pattern of a laser power during optical recording.
FIG. 5 is a graph showing the temperature change of the recording layer when optical recording was carried out on the medium of the present invention.
FIG. 6 is an explanatory view for the modulation.
FIG. 7 is an explanatory view for the phase difference.
FIG. 8 is an explanatory view for the reflectance in a quadri-layer structure and the phase difference .delta. between the crystalline state and the amorphous state.
FIG. 9 is a view illustrating the relation between the overwriting durability and the thickness of the lower protective layer.
FIG. 10 is a schematic view illustrating an embodiment of the optical recording medium of the present invention.
FIG. 11 is a view illustrating an embodiment of the EFM random signals.
FIG. 12 is a view illustrating the change of the overwriting characteristic due to the wobble amplitude.
FIG. 13 is a view illustrating the relation between the wobble and the recording laser beam.
FIG. 14 is a view illustrating the change of the overwriting characteristic due to the groove width.
FIG. 15 is an explanatory view for the distribution of reflectance of disks in Examples of the present invention.
FIG. 16 is an explanatory view for the distribution of reflectance of disks in Comparative Examples.
FIG. 17 is a view illustrating the dependency of the reflectance and the modulation on the thickness of the lower protective layer.
FIG. 18 is a view illustrating the overwriting durability against the thickness of the lower protective layer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in FIG. 2, the optical information recording medium used in the present invention, has a structure of substrate 1/dielectric lower protective layer 3/recording layer 4/dielectric upper protective layer 5/reflective layer 6. It is
preferred that the top is coated with an ultraviolet-curable or thermosetting resin (protective coating layer 7).
As the substrate for the medium, a transparent resin such as polycarbonate, acrylic resin or polyolefin, or glass, may be employed. Among them, a polycarbonate resin is most preferred, since it is inexpensive and has been most commonly
practically used in CD.
The recording layer 4, the protective layers 3 and 5 and the reflective layer 6 are formed by e.g. a sputtering method. With a view to preventing oxidation or contamination among the respective layers, it is preferred to carry out the film
formation in an in-line apparatus wherein the target for the recording layer, the targets for the protecting layers and, if necessary, the target for the reflective layer, are disposed in the same vacuum chamber.
To prevent deformation due to a high temperature during recording, the lower protective layer 3 is provided on the surface of the substrate 1, and the upper protective layer 5 is provided on the recording layer 4, usually in a thickness of from
10 to 500 nm.
If the thickness of such a protective layer made of e.g. a dielectric material, is less than 10 nm, the effect for preventing deformation of the substrate 1 or the recording layer 4 tends to be inadequate, and such a layer tends to be useless as
a protective layer. If the thickness exceeds 500 nm, the internal stress of the dielectric material itself or the difference in the elastic property from the substrate 1 tends to be distinctive, whereby cracking is likely to occur.
The materials for the upper and lower protective layers are determined taking into consideration the refractive indices, the thermal conductivities, the chemical stability, the mechanical strength, the adhesion, etc. In general, an oxide, sulfide
or nitride of e.g. Mg, Ca, Sr, Y, La, Ce, Ho, Er, Yb, Ti, Zr, Hf, V, Nb, Ta, Zn, Al, Si, Ge or Pb, or a fluoride of Ca, Mg or Li having high transparency and high melting point, can be used. These oxides, sulfides, nitrides and fluorides may not
necessarily take stoichiometrical compositions. It is effective to control the compositions to adjust the refractive indices or the like, or to use them in admixture. From the viewpoint of the repetitive recording characteristic, a dielectric mixture
is preferred. More specifically a mixture of ZnS or a rare earth sulfide with a heat resistant compound such as an oxide, nitride or carbonate, may be mentioned.
A particularly preferred structure may, for example, be such that the
portion of the lower protective layer in a thickness of from 1 to 10 nm on the side which is in contact with the recording layer, is made of a mixture comprising a chalcogen compound and a heat resistant compound having a decomposition
temperature or melting point of at least 1,000.degree. C., which is not a chalcogen compound, and the remaining portion is made of a heat resistant compound of the type which is the same or different from the above heat resistant compound.
The chalcogen compound may, for example, be, in addition to ZnS and ZnSe, a sulfide of a Group IIa element such as MgS, CaS, SrS or BaS, a sulfide of a rare earth, such as La.sub.2 S.sub.3 or Ce.sub.2 S.sub.3, a selenium compound of a Group IIa
element such as MgSe, CaSe, SrSe or BaSe, or a selenium compound of a rare earth such as La.sub.2 Se.sub.3 or Ce.sub.2 Se.sub.3. Further, a sulfide or a selenium compound of Ta or Nb may also be used.
The above sulfides or selenium compounds contain chalcogen elements and thus have good adhesion with chalcogen elements mainly contained in the phase-change type recording layer and with the surrounding elements. Thus, a substantial improvement
in the repetitive recording characteristic is observed as compared with a case where a dielectric layer made merely of an oxide is employed.
The heat resistant compound other than the chalcogen compound, may, for example, be an oxide of Al, Si, Ge, Y, Zr, Ba, Ta, Nb, V, W, Hf, Sc or a lanthanoid, a nitride of Al, Si, Ge, Ta or B, a fluoride of Mg, Ca, Nd, Tb or La or a carbide of Si
or B.
When a fluoride is used among them, it is preferred to use an oxide in combination, so that the brittleness may be overcome.
From the viewpoint of the costs and efficiency for the production of targets, it is preferred to employ silicon dioxide, yttrium oxide, barium oxide, tantalum oxide, LaF.sub.3, NdF.sub.3, TbF.sub.3, SiC, Si.sub.3 N.sub.4 or AlN.
The total amount of the above chalcogen compound and the heat resistant compound other than a chalcogen compound in the protective layer, is preferably at least 50 mol %, more preferably at least 80 mol %. If their content is less than 50 mol %,
the effect for preventing deformation of the substrate or the recording layer tends to be inadequate, and the layer tends to be useless as a protective layer.
The content of the chalcogen compound is preferably from 10 to 95 mol % of the entire protective layer. If the content is less than 10 mol %, the desired property tends to be hardly obtainable. On the other hand, if it exceeds 95 mol %, the
optical absorption coefficient tends to be large, such being undesirable. The content is more preferably from 15 to 90 mol %.
The content of the above heat resistant compound is preferably from 5 to 90 mol % in the entire protective layer, more preferably at least 10 mol %. If the content is outside this range, the desired property may not sometimes be obtained.
The heat resistant compound is required to have a heat resistance of at least 1,000.degree. C., and at the same time, required to be optically adequately transparent to the laser beam to be used for recording and retrieving. Namely, in a
thickness of 50 nm, the imaginary part of the complex refractive index in a wavelength region of at least about 600 nm is desired to be at most 0.1.
To obtain such optical transparency, it is preferred to use a gas mixture of Ar with oxygen and/or nitrogen during the sputtering for forming the film.
S or Se in a sulfide or a selenium compound has a high vapor pressure, and a part thereof tends to evaporate or undergo decomposition during the sputtering. If such deficiency of S or Se in a protective layer becomes substantial, the optical
absorptivity tends to be defective, and the protective layer tends to be chemically unstable. Addition of oxygen or nitrogen to the sputtering gas as mentioned above, is intended to replace such deficiency with oxygen or nitrogen. Here, an oxide or
nitride of the metal element of the above chalcogen compound will be formed partially in the film, but such an oxide or nitride serves as a part of the heat resistant compound, whereby the properties of the film will not be impaired.
The film density of these protective layers is preferably at least 80% of the bulk density from the viewpoint of mechanical strength (Thin Solid Films, vol., 278, (1996), p. 74-81).
As the bulk density of a thin film of a dielectric mixture, a theoretical density of the following formula is employed, wherein mi is the molar concentration of each component i, and .sigma..sub.i is the bulk density of the component alone:
The recording layer of the medium of the present invention is a phase-change type recording layer, and its thickness is usually preferably within a range of from 10 to 100 nm. If the thickness of the recording layer is thinner than 10 nm, no
adequate contrast tends to be obtained, and the crystallization speed tends to be slow, whereby it tends to be difficult to erase the recorded information in a short time. On the other hand, if it exceeds 100 nm, an optical contrast tends to be hardly
obtainable, and cracking is likely to occur, such being undesirable. Further, in order to obtain a contrast sufficient for the compatibility with CD, the thickness of the recording layer is limited within a range of from 15 to 30 nm. If the thickness
is less than 15 nm, the reflectance tends to be too low, and if it exceeds 30 nm, the heat capacity tends to be large, whereby the recording sensitivity tends to be poor.
In a case where mark length recording is applied as in the case of CD-RW, it is preferred to use a thin film of an alloy comprising, as the main component, a SbTe alloy close to the Sb.sub.70 Te.sub.30 eutectic point, for the recording layer,
from the viewpoint of the after-mentioned optical properties and crystallizability.
Heretofore, an alloy material close to a eutectic composition has been believed to be unsuitable as a recording layer for a rewritable optical recording medium, since it undergoes phase separation during crystallization although its amorphous
forming ability is high, and it can not be crystallized by heating in such a short period of time as less than 100 nano seconds (Appl. Phys. Lett., vol., 49 (1986), p. 502).
Particularly, when an attention is drawn to a GeSbTe ternary alloy, no practical crystallization speed has been obtained in the vicinity of the Te.sub.85 Ge.sub.15 eutectic composition.
On the other hand, U.S. Pat. No. 5,015,548 discloses that the in the vicinity of the Sb.sub.70 Te.sub.30 eutectic composition, a Sb.sub.x Te.sub.1-x (0.58<x<0.75) binary alloy is useful for repeated recording and erasing between the
crystalline and amorphous states, although the method is quite primitive wherein only the change in reflectance is monitored. As the prior art disclosing a composition having a third element, particularly Ge, added to Sb.sub.70 Te.sub.30, JP-A-1-115685,
JP-A-1-251342, JP-A-1-303643 and JP-A-4-28587 may, for example, be mentioned.
However, with respect to phase-change media in the vicinity of the SbTe eutectic composition, there has been no practical progress since then. Especially, there has been a serious problem that the initialization operation to crystallize the
recording layer after film formation, is difficult, and the productivity is too low for practical application.
Accordingly, it has been considered that only a material close to the composition of a readily initializable intermetallic compound or its pseudo binary alloy exhibits practical properties (JP-A-2-243388, JP-A-2-243389, JP-A-2-243390,
JP-A-2-255378, JP-A-63-228433, JP-A-61-89889, Jpn. J. Appl. Phys., vol. 69, (1991), p. 2849).
For example, with respect to a GeSbTe ternary alloy, only the composition close to a GeTe-Sb.sub.2 Te.sub.3 pseudo binary alloy has attracted an attention and has been practically used in recent years. Such a trend is distinct, for example, from
the proceedings (as disclosed in the abstracts from Japan Society of Applied Physics) for "Symposium on Phase-Change Recording" which was held every year since 1991.
The present inventors have paid a particular attention to a binary alloy composed of SbTe for simplification and have conducted a review of the crystallization/amorphous conversion characteristics in the vicinity of the eutectic composition from
the viewpoint of applicability to mark length recording by means of an optical disk evaluating machine more suitable for high density recording, without being influenced by the conventional theories. As a result, it has been found that a recording layer
comprising, as the main component, a SbTe alloy close to the Sb.sub.70 Te.sub.30 eutectic composition is hardly susceptible to initial crystallization, but once it has been initially crystallized, the subsequent writing and erasing by the phase change
between the amorphous and crystalline states can be carried out at an extremely high speed.
Further, evaluation has been carried out with respect to materials having various elements incorporated in the vicinity of this eutectic composition, whereby it has been found that alloys close to the SbTe eutectic composition have merits such
that in repetitive overwriting employing a certain specific recording pulse pattern, deterioration is less than the well known material close to the GeTe-Sb.sub.2 Te.sub.3 pseudo binary alloy, and in mark length recording, the jitter of the mark edge is
low. Further, it has been found that the crystallization temperature is higher than the Sb.sub.7 Te.sub.30 binary eutectic alloy, and the archival stability is excellent.
Specifically, as the recording layer, a thin layer of an alloy of the formula My.sub.y (Sb.sub.x Te.sub.1-x).sub.1-y, (0.ltoreq.y<0.3, 0.5<x<0.9, and My is at least one member selected from the group consisting of In, Ga, Zn, Ge, Sn, Si,
Cu, Au, Ag, Pd, Pt, V, Nb, Ta, Pb, Cr, Co, O, S and Se), is used. Among them, In, Ga, Ge, Sn, Cu. Pb, V, Nb, Ta, O, Se and S are effective to increase the crystallization temperature of the SbTe eutectic alloy and thus to improve the stability with
time. Further, | | |
