U.S. patent application number 09/832137 was filed with the patent office on 2001-09-20 for information recording medium.
Invention is credited to Andoo, Keikichi, Anzai, Yumiko, Hirotsune, Akemi, Miyamoto, Makoto, Nishida, Tetsuya, Terao, Motoyasu, Ushiyama, Junko.
Application Number | 20010023006 09/832137 |
Document ID | / |
Family ID | 17107253 |
Filed Date | 2001-09-20 |
United States Patent
Application |
20010023006 |
Kind Code |
A1 |
Miyamoto, Makoto ; et
al. |
September 20, 2001 |
Information recording medium
Abstract
An information recording medium having a substrate, a lower
superficial layer formed on the substrate and having a thickness of
not more than 25 nm, and a recording layer formed on the lower
superficial layer. Information is recorded on the recording layer
in accordance with a change of atomic arrangement caused by
irradiation of light. The information recording medium includes and
an absorption control layer formed on the recording layer.
Inventors: |
Miyamoto, Makoto; (Ome-shi,
JP) ; Hirotsune, Akemi; (Higashimurayama-shi, JP)
; Andoo, Keikichi; (Musashino-shi, JP) ; Nishida,
Tetsuya; (Odawara-shi, JP) ; Terao, Motoyasu;
(Tokyo, JP) ; Ushiyama, Junko; (Kodaira-shi,
JP) ; Anzai, Yumiko; (Ome-shi, JP) |
Correspondence
Address: |
ANTONELLI TERRY STOUT AND KRAUS
SUITE 1800
1300 NORTH SEVENTEENTH STREET
ARLINGTON
VA
22209
|
Family ID: |
17107253 |
Appl. No.: |
09/832137 |
Filed: |
April 11, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09832137 |
Apr 11, 2001 |
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09380302 |
Aug 30, 1999 |
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6231945 |
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Current U.S.
Class: |
428/64.1 ;
G9B/7.139; G9B/7.171; G9B/7.189 |
Current CPC
Class: |
G11B 7/2595 20130101;
G11B 7/24 20130101; G11B 7/2578 20130101; G11B 2007/24308 20130101;
G11B 2007/2431 20130101; Y10S 428/913 20130101; G11B 2007/24312
20130101; G11B 2007/25715 20130101; G11B 2007/2571 20130101; G11B
7/2534 20130101; G11B 7/259 20130101; Y10S 430/146 20130101; G11B
7/2585 20130101; G11B 2007/24306 20130101; G11B 2007/25713
20130101; G11B 7/252 20130101; G11B 2007/25706 20130101; G11B
7/2533 20130101; G11B 7/2531 20130101; G11B 2007/25711 20130101;
G11B 2007/24314 20130101; G11B 7/256 20130101; Y10T 428/31678
20150401; G11B 2007/24304 20130101; G11B 2007/24316 20130101; G11B
2007/25708 20130101; Y10T 428/21 20150115 |
Class at
Publication: |
428/64.1 |
International
Class: |
B32B 003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 9, 1997 |
JP |
09-243670 |
Claims
What is claimed is:
1. An information recording medium comprising: a substrate; a lower
superficial layer formed on said substrate and having a thickness
of not more than 25 nm; a recording layer formed on said lower
superficial layer and on which information is recorded in
accordance with a change of atomic arrangement caused by
irradiation of light; and an absorption control layer formed on
said recording layer.
2. An information recording medium according to claim 1, wherein
said absorption control layer has a refraction index of not less
than 1.2 and not more than 6 and an extinction of not less than 0.5
and not more than 3.3.
3. An information recording medium according to claim 1, wherein a
thickness of said absorption control layer is in a range of not
less than 10 nm and not more than 50 nm.
4. An information recording medium according to claim 1, wherein a
heat diffusion layer is formed between said substrate and said
lower superficial layer.
5. An information recording medium according to claim 1, wherein
said lower superficial layer has a thickness of not less than 2
nm.
6. An information recording medium, comprising: a substrate; a
recording layer formed on said substrate and on which information
is recorded in accordance with a change of atomic arrangement
caused by a radiation of light; an upper superficial layer formed
on said recoding layer and having a thickness of not more than 12
nm; and an absorption control layer formed on said upper
superficial layer.
7. An information recording medium according to claim 6, wherein
said absorption control layer has a refraction index of not less
than 1.2 and not more than 6 and an extinction of not less than 0.5
and not more than 3.3.
8. An information recording medium according to claim 6, wherein a
lower superficial layer is formed between said recording layer and
said substrate.
9. An information recording medium according to claim 6, wherein a
thickness of said lower superficial layer is in a range of not less
than 2 nm and not more than 25 nm.
10. An information recording medium according to claim 6, wherein
said upper superficial layer has a thickness of not less than 2
nm.
11. An information recording medium comprising a lower superficial
layer, a recording layer and an absorption control layer formed on
a substrate in order from a light incident side to the substrate,
said recording layer enabling recording of information thereon by
change of atomic arrangement caused by irradiation of light;
wherein said lower superficial layer includes any one of materials
including SiO.sub.2, Al.sub.2O.sub.3 or a mixture of
Al.sub.2O.sub.3 and SiO.sub.2, Ta.sub.2O.sub.5, a mixture between
Ta.sub.2O.sub.5 and SiO.sub.2 or Al.sub.2O.sub.3,
ZrO.sub.2--Y.sub.2O.sub.3, a mixture of ZrO.sub.2--Y.sub.2O.sub.3
with SiO.sub.2, Al.sub.2O.sub.3 or Ta.sub.2O.sub.51 CoO,
Cr.sub.2O.sub.3 and NiO.
12. An information recording medium according to claim 11, wherein
said material has a purity of not less than 90%.
13. An information recording medium according to claim 11, wherein
said absorption control layer is made of a material having a
refraction index of not less than 0.2 and not more than 6 and an
extinction coefficient of not less than 0.5 and not more than
3.3.
14. An information recording medium comprising a recording layer,
an upper superficial layer and an absorption control layer formed
on a substrate in order from a light incident side to the
substrate, said recording layer enabling recording of information
thereon by change of atomic arrangement caused by irradiation of
light; wherein said upper superficial layer includes any one of
materials including SiO.sub.2, Al.sub.2O.sub.3 or a mixture of
Al.sub.2O.sub.3 and SiO.sub.2, Ta.sub.2O,, a mixture between
Ta.sub.2O.sub.5 and SiO.sub.2 or Al.sub.2O.sub.3,
ZrO.sub.2--Y.sub.2O.sub.3, a mixture of ZrO.sub.2--Y.sub.2O.sub.3
with SiO.sub.2, Al.sub.2O.sub.3 or Ta.sub.2O.sub.51 CoO,
Cr.sub.2O.sub.3 and NiO.
15. An information recording medium according to claim 14, wherein
said material has a purity of not less than 90%.
16. An information recording medium according to claim 14, wherein
said absorption control layer is made of a material having a
refraction index of not less than 0.2 and not more than 6 and an
extinction coefficient of not less 0.5 and not more than 3.3.
17. An information recording medium according to claim 14, wherein
a lower superficial layer is formed between said recording layer
and said substrate.
18. An information recording medium according to claim 14, wherein
a lower superficial layer includes any one of materials including
SiO.sub.2, Al.sub.2O.sub.3 or a mixture of Al.sub.2O.sub.3 and
SiO.sub.2, Ta.sub.2O.sub.5, a mixture between Ta.sub.2O.sub.5 and
SiO.sub.2 or Al.sub.2O.sub.3, ZrO.sub.2--Y.sub.2O.sub.3, a mixture
of ZrO.sub.2--Y.sub.2O.sub.3 with SiO.sub.2, Al.sub.2O.sub.3 or
Ta.sub.2O.sub.5, CoO, Cr.sub.2O.sub.3 and NiO.
19. An information recording medium comprising a substrate, a
recording film formed on said substrate as a recording layer for
recording information by change of atomic arrangement caused by
irradiation of light, and an absorption control layer; wherein said
medium comprises at least one of a lower superficial layer formed
between said substrate and said lower recording film and an upper
superficial layer formed between said recording film and said
absorption control layer, said superficial layer or said upper
superficial layer includes any one of materials including AlN, BN,
CrN, Cr.sub.2N, GeN, HfN, Si.sub.3N.sub.4, Al--Sn--N group
material, Si--N group material, Si--O--N group material, TaN, TiN
and ZrN, BeO, Bi.sub.2O.sub.3, CeO.sub.2, Cu.sub.2O, CuO, CdO,
Dy.sub.2O.sub.3, FeO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, GeO,
GeO.sub.2, HfO.sub.2, In.sub.2O.sub.3, La.sub.2O.sub.3, MgO, MnO,
MoO.sub.2, MoO.sub.3, NbO, NbO.sub.2, PbO, PdO, SnO, SnO.sub.2,
Sc.sub.2O.sub.3, SrO, ThO.sub.2, TiO.sub.2, Ti.sub.2O.sub.3, TiO,
TeO.sub.2, VO, V.sub.2O.sub.3, VO.sub.2, WO.sub.2, WO.sub.3, and
carbides including C, Cr.sub.3C.sub.2, Cr.sub.23C.sub.6,
Cr.sub.7C.sub.3, Fe.sub.3C, MO.sub.2C, WC, W.sub.2C, HfC, TaC and
CaC.sub.2.
20. An information recording medium according to claim 19, wherein
said material has a purity of not less than 90%.
Description
TECHNICAL FIELD
[0001] The present invention relates to an information recording
medium used for an optical disk.
BACKGROUND ART
[0002] Various principles are known for recording information in a
thin film (recording film) by radiating a laser beam on it. Among
them, a method utilizing the change of the atomic arrangement by
the radiation of the laser beam such as the phase transition (also
called the phase change) of the film material or the photo
darkening is not substantially accompanied by the deformation of
the thin film, and therefore has the advantage that an information
recording medium of a two-side disk structure can be obtained by
attaching two disk members directly to each other.
[0003] Generally, these information recording media are configured
of a substrate, and a protective layer, a recording film of GeSbTe
group, etc., a protective layer and a reflective layer formed on
the substrate, and the reflectance is higher in crystal state than
in amorphous state. Therefore, the absorption of a recording film
is larger for amorphus. The recording mark portion in amorphous
state is liable to increase in temperature more easily than the
crystal. In the case where an overwrite operation is performed in
this state, therefore, a newly recorded mark is increased
excessively, thereby causing the reproduction signal to be
distorted.
[0004] In order to prevent this inconvenience, an effort has been
made to increase the absorption coefficient of the recording film
in crystal state as compared with that of the recording film in
amorphous state. For example, reference 1 "Yamada and three others,
Shingaku Giho MR92-71, CPM92-148, December 1992, p.37" describes a
structure formed with an Au reflective layer as thin as 10 nm to
reverse the absorption coefficient.
[0005] Also, reference 2 "Okada and six others, Shingaku Giho
MR93-53, CPM93-105, December 1993, p. 1" describes a structure in
which Si of 65 nm is used for the reflective layer thereby to
reverse the absorption coefficient.
[0006] In this specification, the term "phase change" is defined to
include not only the phase change between crystal and amorphus but
also between melting (change to liquid phase) and recrystallization
and between crystal states.
DISCLOSURE OF INVENTION
[0007] In all the conventional information recording media used as
a high-density rewritable information recording media of phase
transition type using a mark edge recording, the erase
characteristic is improved by reversing the absorption coefficient
(the absorption coefficient of amorphus is lower than that of
crystal). These media use a material with a thin reflective layer
or a material of a reflective layer through which light is
transmitted or has a structure allowing light to transmit
therethrough. This method poses the problem that each absorption
coefficient is smaller than that of the normal disk with the
absorption coefficient not reversed, resulting in a small recording
sensitivity. The laser used for recording having a wavelength
shorter than about 680 nm is still low in output, and if the linear
speed is increased for increasing the transfer rate, the recording
sensitivity tends to deteriorate. Therefore a medium of high
recording sensitivity is required.
[0008] Further, the use of a thin material of a low heat
conductivity for the reflective layer poses the problem that heat
generated at the time of recording is not easily diffused often
causing an increased jitter after a multiplicity of overwrite
cycles.
[0009] Accordingly, an object of the present invention is to solve
these problems and to provide an information recording medium which
has a superior recording/reproduction characteristic without
increasing the jitter as compared with the prior art even after the
overwrite cycle for high-density recording and reproduction.
[0010] Means for solving the problems will be described below.
[0011] (1) There is provided an information recording medium
comprising an information recording thin film as a recording layer
formed on a substrate for recording and/or reproducing information
by the change in atomic arrangement caused by the radiation of
light, and at least one protective layer, wherein the protective
layer and the recording layer are formed in that order from the
light incidence side, followed by being formed with at least one
absorption control layer.
[0012] (2) There is provided an information recording medium as
described in (1), characterized in that the thickness of the
absorption control layer is in the range of not less than 10 nm but
not more than 50 nm. The thickness of not less than 10 nm but not
more than 40 nm is more preferable.
[0013] (3) There is provided an information recording medium as
described in (1), wherein at least 95% of the total number of atoms
of the absorption control layer is composed of a mixture or a
compound of a dielectric material and a metal element.
[0014] (4) There is provided an information recording medium as
described in (1), wherein the absorption control layer is made of a
material having n (refractive index) of not less than 1.2 but not
more than 6, and k (extinction coefficient) not less than 0.5 but
not more than 3.3. If n is not less than 1.8 but not more than 5.5,
and k is not less than 0.8 but not more than 3, it is more
preferable.
[0015] (5) There is provided an information recording medium as
described in (1), wherein the absorption control layer is made of a
material having a melting point of not less than 600.degree. C.
[0016] (6) There is provided an information recording medium as
described in (1), characterized in that in the case where
information is recorded on the recording film, the reflectance in
amorphous state is lower than that in crystal state, the
reflectance in amorphous state is lower than the reflectance in
crystal state, and the mark size with the shortest mark recorded on
a material in amorphous state is equal to or smaller than the mark
size with the shortest mark recorded on a material in crystal state
under the same conditions.
[0017] (7) There is provided an information recording medium as
described in (1), wherein at least a heat diffusion layer is formed
between the substrate and the protective layer.
[0018] (8) There is provided an information recording medium
comprising an information recording thin film as a recording layer
formed on a substrate for recording and/or reproducing information
by the change in atomic arrangement caused by the radiation of
light, at least a protective layer, at least a heat diffusion layer
and at least a heat diffusion layer, characterized in that the heat
diffusion layer, the protective layer and the recording layer are
formed in that order from the light incidence side, followed by
being formed with at least one reflective layer.
[0019] (9) There is provided an information recording medium as
described in any one of (7) and (8), characterized in that at least
90% of the total number of atoms of the heat diffusion layer is
composed of Al--O.
[0020] (10) There is provided an information recording medium as
described in any one of (7) and (8), characterized in that the heat
diffusion layer has a layer in which at least 90% of the total
number of atoms has a composition similar to any one of
(SiO.sub.2), (Al.sub.2O.sub.3), (Ta.sub.2O.sub.5),
(Al.sub.2O.sub.3)--(SiO.sub.2), (Ta.sub.2O.sub.5)--(SiO.sub.2),
(Al.sub.2O.sub.3)--(Ta.sub.2O.sub.5) and
(Al.sub.2O.sub.3)--(SiO.sub.2)--(Ta.sub.2O.sub.5) or a mixture
composition thereof.
[0021] (11) There is provided an information recording medium as
described in any one of (7) to (8), wherein the heat diffusion
layer has a layer in which at least 90% of the total number of
atoms has a composition similar to any one of Be--O, B--N, Si--C
and Mg--O or a mixture composition thereof.
[0022] (12) There is provided an information recording medium as
described in (1), characterized by having a structure wherein a
reflection layer composed of at least one layer of a Cu alloy, an
Al alloy and an Au alloy is formed on the absorption control
layer.
[0023] (13) There is provided an information recording medium as
described in (1) or (8), characterized in that at least one surface
protect layer is formed in the boundary of the recording film.
[0024] (14) There is provided an information recording medium as
described in (1) or (8), characterized in that the recording film
satisfies the relation
Ge.sub.x-wSb.sub.yTe.sub.zM.sub.w
[0025] where 0.10.ltoreq.x.ltoreq.0.26, 0.18.ltoreq.y.ltoreq.0.33,
0.52.ltoreq.z.ltoreq.0.60, w.ltoreq.0.06 and x+y+z=1, and M is any
one of Na, Mg, Al, P, S, Cl, L, Ca, Sc, Zn, Ga, As, Se, Br, Rb, Sr,
Y, Zr, Nb, Ru, Rh, Cd, In, Sn, I, Cs, Ba, La, Hf, Ta, Re, Os, Ir,
Hg, Tl, Pb, Th, U, Ag, Cr, W, Mo, Pt, Co, Ni, Pd, Si, Au, Cu, V,
Mn, Fe, Ti and Bi.
[0026] (15) There is provided an information recording medium as
described in (1) or (8), characterized in that the protective layer
is made of a layer containing at least 80 mol % of ZnS.
[0027] (16) In the case where the absorption control layer is made
of Mo--(SiO.sub.2), the MO amount represents preferably not less
than 42 mol % of all the components. The figure of not less than 61
mol % but not more than 90 mol % is more desirable.
[0028] The use of Cr, W, Fe, Sb, C, Zn, Mn, Ti, Co, Ge, Pt, Ni, Nb,
Pd, Be or Ta as a material replacing Mo in the Mo--(SiO.sub.2) film
of the absorption control layer has produced a similar result.
Among these elements, Mo, Cr and W are more preferable as they have
a high melting point. Also, Pd and Pt are not very reactive with
other layers and the resulting increased possible number of
overwrite cycles makes these elements more preferable. When Ni, Co
or Ti is used, on the other hand, an inexpensive target can be used
as compared with other materials and the total production cost can
be reduced.
[0029] Materials which may be used in place of SiO.sub.2 in the
Mo--(SiO.sub.2) film used for the absorption control layer include
oxides including SiO, Al.sub.2O.sub.3, BeO, Bi.sub.2O.sub.3, CoO,
CaO, Cr.sub.2O.sub.3, CeO.sub.2, Cu.sub.2.degree., CuO, CdO,
Dy.sub.2O.sub.3, FeO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, GeO,
GeO.sub.2, HfO.sub.2, In.sub.2O.sub.3, La.sub.2O.sub.3, MgO, MnO,
MoC.sub.2, MoO.sub.3, NbO, NbO.sub.2, NiO, PbO, PdO, SnO,
SnO.sub.2, Sc.sub.2O.sub.3, SrO, ThO.sub.2, TiO.sub.2,
Ti.sub.2O.sub.3F TiO, Ta.sub.2O.sub.5, TeO.sub.2, VO,
V.sub.2O.sub.3, VO.sub.2, WO.sub.2, WO.sub.3, Y.sub.2O.sub.3 and
ZrO.sub.2, nitrides including AlN, BN, CrN, Cr.sub.2N, GeN, HfN,
Si.sub.3N.sub.4, Al--Si--N group material (such as AlSiN.sub.2),
Si--N group material, Si--O--N group material, TaN, TiN and ZrN,
sulfides including ZnS, Sb.sub.2S.sub.3, CdS, In.sub.2S.sub.3,
Ga.sub.2S.sub.3, GeS, SnS.sub.2, PbS, Bi.sub.2S.sub.3, SrS, MgS,
CrS, CeS and TaS.sub.4, selenides including SnSe.sub.2,
Sb.sub.2Se.sub.3, CdSe, ZnSe, Tn.sub.2Se.sub.3, Ga.sub.2Se.sub.3,
GeSe, GeSe.sub.2, SnSe, PbSe and Bi.sub.2Se.sub.3, fluorides
including CeF.sub.3, MgF.sub.2, CaF.sub.2, TiF.sub.3, NiF.sub.3,
FeF.sub.2 and FeF.sub.3, Si, Ge, borides including TiB.sub.2,
B.sub.4C, B, CrB, HfB.sub.2, TiB.sub.2 and WB, carbides including
C, Cr.sub.3C.sub.2, Cr.sub.23C.sub.6, Cr.sub.7C.sub.3, Fe.sub.3C,
MO.sub.2C, WC, W.sub.2C, HfC, TaC and CaC.sub.2, or a material
having a composition similar to any of the materials described
above or a mixture thereof.
[0030] Among these materials, the use of SiO.sub.2, Ta.sub.2O.sub.5
or Y.sub.2O.sub.3--ZrO.sub.2 makes it possible to use a target less
expensive than when using other materials, and therefore can reduce
the whole cost of production.
[0031] Al.sub.2O.sub.3 is high in heat conductivity. Therefore, a
disk having a structure lacking the first reflective layer and/or
the second reflective layer deteriorates the rewrite characteristic
to lesser degree than when using other materials.
[0032] Also, in the case where the absorption control layer
contains impurities elements not more than 5 atomic % of the
components thereof, it can desirably reduce the deterioration of
the rewrite characteristic. The content of not more than 2 atomic %
is more preferable.
[0033] (17) A preferable material of the upper surface protect
layer and the lower surface protect layer is SiO.sub.2,
Al.sub.2O.sub.3 or a mixture of Al.sub.2O.sub.3 and SiO.sub.2. In
the case where 70 mol % or more of SiO.sub.2 or Al.sub.2O.sub.3 is
contained, the crystallization rate is increased and at 18 m/s that
is the rate about twice as high as in the absence of the surface
protect layer, the erasure ratio reaches 25 dB or more.
[0034] The next preferable choice is Ta.sub.2O.sub.5 or a mixture
between Ta.sub.2O.sub.5 and SiO.sub.2 or Al.sub.2O.sub.3. The
second next preferable choice is ZrO.sub.2--Y.sub.2O.sub.3,
SiO.sub.2 or a mixture of ZrO.sub.2--Y.sub.2O.sub.3 or SiO.sub.2
with Al.sub.2O.sub.3 or Ta.sub.2O.sub.5. Among these materials,
Al.sub.2O.sub.3 is more preferable as it can suppress the
variations of the reflectance level to 5% or less and can reduce
the jitter after a multiplicity of overwrite cycles. The materials
CoO, Cr.sub.2O.sub.3 and NiO are also more preferable as a uniform
crystal grain size is obtained at the time of initial
crystallization and the jitter is increased to a lesser degree in
the initial stage of overwrite cycle.
[0035] Also, nitrides such as AlN, BN, CrN, Cr.sub.2N, GeN, HfN,
Si.sub.3N.sub.4, Al--Sn--N group material (such as AlSiN.sub.2),
Si--N group material, Si--O--N group material, TaN, TiN and ZrN are
more preferable as they increase the adhesion and deteriorate the
information recording medium to a lesser degree under external
shocks. A material of the recording film containing nitrogen or a
material having a similar composition can also improve the
adhesion.
[0036] In addition, oxides such as BeO, Bi.sub.2O.sub.3, CeO.sub.2,
Cu.sub.2O, CuO, CdO, Dy.sub.2O.sub.3, FeO, Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4, GeO, GeO.sub.2, HfO.sub.2, In.sub.2O.sub.3,
La.sub.2O.sub.3, MgO, MnO, MoO.sub.2, MoO.sub.3, NbO, NbO.sub.2,
PbO, PdO, SnO, SnO.sub.2, Sc.sub.2O.sub.3, SrO, ThO.sub.2,
TiO.sub.2, Ti.sub.2O.sub.3, TiO, TeO.sub.2, VO, V.sub.2O.sub.3,
VO.sub.2, WO.sub.2 and WO.sub.3 or carbides such as C,
Cr.sub.3C.sub.2, Cr.sub.23C.sub.6, Cr.sub.7C.sub.3. Fe.sub.3C,
Mo.sub.2C, WC, W.sub.2C, HfC, TaC and CaC.sub.2 or materials having
a similar composition can also be used.
[0037] As another alternative, any mixture of these materials is
usable.
[0038] The upper surface protect layer, the lower surface protect
layer, and the replacement materials of the upper surface protect
layer and the lower surface protect layer preferably represent 90%
or more of the total number of atoms of the respective surface
protect layer. In the case where impurities other than the
materials described above reach ten atomic % or more, the possible
number of overwrite cycles is reduced by 50% or more, or otherwise
the rewrite characteristic is deteriorated.
[0039] In the absence of the upper surface protect layer, the
reflective layer material diffuses into the recording film and the
remanence increases, the reduction in the reflectance level after
100 thousand overwrite cycles can be suppressed to as small as 5%
or less. A change in reflectance level causes the offset of the
reproduction signal level, and adds the offset jitter for an
increased jitter. Thus, the variation of the reflectance level is
preferably as small as possible.
[0040] Further, for the modulation degree to be maintained at 43%
or more, the figure of not more than 12 nm is preferable. For the
figure of 5 nm or less, the modulation degree of 47% or more can be
secured. A film of uniform thickness can be formed when the
thickness is not less than about 2 nm. In the case where the
thickness of the upper surface protect layer is 2 to 12 nm,
therefore, the recording/reproduction characteristic is desirably
improved.
[0041] In the absence of the lower surface protect layer, the
protective layer material diffuses into the recording film for an
increased remanence, so that the jitter increases beyond 6% after
100 thousand overwrite cycles. Further, for maintaining the
modulation degree at 43% or more, the thickness is desirably
maintained at 25 nm or less. The thickness of not less than 5 nm
but not more than to 10 nm can secure the modulation degree of 47%
or more. In view of the fact that a uniform film is formed for the
thickness of about 2 nm or more, the recording/reproduction
characteristic is desirably improved when the thickness of the
lower surface protect layer is 2 to 25 nm. (18) Materials of the
protective layer include any one of ZnS, Si--N group material,
Si--O--N group material, oxides such as SiO.sub.2, SiO, TiO.sub.2,
Al.sub.2O.sub.3, Y.sub.2O.sub.3, CeO.sub.2, La.sub.2O.sub.3,
In.sub.2O.sub.3, GeO, GeO.sub.2, PbO, SnO, SnO.sub.2, BeO,
Bi.sub.2O.sub.3, TeO.sub.2, WO.sub.2, WO.sub.3, Sc.sub.2O.sub.3,
Ta.sub.2O.sub.5, ZrO.sub.2, Cu.sub.2O and MgO, nitrides such as
TaN, AlN, BN, Si.sub.3N.sub.4, GeN, Al--Sn--N group material (such
as AlSiN.sub.2), sulfides such as ZnS, Sb.sub.2S.sub.3, CdS,
In.sub.2S.sub.3, Ga.sub.2S.sub.3, GeS, SnS.sub.2, PbS and
Bi.sub.2S.sub.3, selenides such as SnSe.sub.2, Sb.sub.2Se.sub.3,
CdSe, ZnSe, In.sub.2Se.sub.3, Ga.sub.2Se.sub.3, GeSe, GeSe.sub.2,
SnSe, PbSe and Bi.sub.2Se.sub.3, fluorides such as CeF.sub.3,
MgF.sub.2 and CaF.sub.2, or Si, Ge, TiB.sub.2, B.sub.4C, B, C or
materials having a similar composition to the materials described
above. Also, a layer of a mixtures a multi-layer of these materials
including ZnS--SiO.sub.2 and ZnS--Al.sub.2O.sub.3 may be used.
Among these materials, ZnS has a large n and can maintain a large
modulation degree. In the case of a mixture containing 60 mol % or
more of this material, the large n of ZnS and the superior chemical
stability of the oxide have a combined effect. Further, ZnS has a
large sputter rate, so that when ZnS represents 80 mol % or more,
the film-producing time can be shortened. Other sulfides and
selenides can also produce similar characteristics.
[0042] The element ratio in these compounds, i.e. the ratio between
a metal element and oxygen element for oxides and the ratio between
a metal element and a sulfide element for sulfides, for example, is
preferably 2 to 3 or thereabouts for Al.sub.2O.sub.3,
Y.sub.2O.sub.3 and La.sub.2O.sub.3, 1 to 2 or thereabouts for
SiO.sub.2, ZrO.sub.2 and GeO.sub.2, 2 to 5 or thereabouts for
Ta.sub.2O.sub.5 and 1 to 1 or thereabouts for ZnS. Even a ratio
departing from the ratios specified above can product a similar
effect. In the case where the ratio is not an integral one
described above, for example, the deviation of the ratio between Al
and 0 in Al--O is preferably not more than .+-.10 atomic % in terms
of Al amount from Al.sub.2O.sub.3, the deviation of the ratio
between Si and I in Si--O is preferably not more than .+-.10 atomic
% in terms of Si amount from SiO.sub.2. In this way, the deviation
of not more than 10 atomic % is desirable. A deviation of not less
than 10 atomic % would change the optical characteristic and the
modulation degree is reduced by 10% or more.
[0043] The protective layer and the replacement material of the
protective material preferably represents at least 90% of the total
number of atoms of the respective protective layer. In the case
where impurities other than these materials increase to 10 atomic %
or more, the possible number of overwrite cycles is reduced to one
half or less or otherwise the rewrite characteristic is
deteriorated.
[0044] The thickness of the protective layer is desirably 20 to 70
nm, which can increase the modulation degree for recording to as
high as 43% or more, and more preferably, the thickness of the
protective layer is 35 to 60 nm.
[0045] (19) The preferable materials of the heat diffusion layer
are Al.sub.2O.sub.3, MgO, BeO, SiC, BN, B.sub.4C large in heat
conductivity. Also, Ta.sub.2O.sub.3, SiO.sub.2, Al.sub.2O.sub.3 and
mixtures thereof have an inexpensive target and the production cost
thereof is desirably low. On the other hand, ThO.sub.2, TiO.sub.2,
AlN and TiN are desirable for their ease to form into a film.
[0046] Other preferable materials than those described above have a
heat conductivity larger than the substrate material and an
absorption coefficient k smaller than 0.5.
[0047] A large heat conductivity can suppress the damage to the
substrate surface by heat at the time of recording, and therefore
the jitter can be suppressed to a low level after 100 thousand
overwrite cycles. Also, a small k can suppress the reduction of
modulation degree to a small level.
[0048] The heat diffusion layer and the replacement materials of
the heat diffusion layer are desirably not less than 90% of the
total number of atoms of each protective layer. In the case where
the impurities other than the materials described above reaches 10
atomic % or more, the possible number of overwrite cycles is
reduced to one half or less or otherwise the rewrite characteristic
is deteriorated.
[0049] The thickness of the heat diffusion layer is preferably 10
to 50 nm or more preferably 20 to 40 nm.
[0050] (20) The preferable material of the first reflective layer
is Al--Cr, Al--Ti, Al--Ag or the like containing an Al alloy which
can reduce the jitter to a low level at the time of overwrite
operation.
[0051] The characteristic for a multiplicity of overwrite cycles
has been found to be improved when the contents of the element
other than Al in the Al alloy reaches the range of not less than 5
atomic % but not more than 30 atomic %. A similar characteristic is
obtained also from the Al alloy other than those described
above.
[0052] A layer may be used which is composed of any one of the
element unit Au, Ag, Cu, Ni, Fe, Co, Cr, Ti, Pd, Pt, W, Ta, Mo, Sb,
Bi, Dy, Cd, Mn, Mg or V or an alloy containing any one of these
materials as a main component such as an Au alloy, Ag alloy, Cu
alloy, Pd alloy, Pt alloy, Sb--Bi, SUS, Ni--Cr or alloys between
these alloys. In this way, the first reflective layer is composed
of a metal element, a metalloid element, an alloy or a mixture
thereof.
[0053] Among these materials, such materials as Cu alloy, Al alloy
or Au alloy having a large reflectance increases the modulation
degree leading to a superior reproduction characteristic. A similar
characteristic is exhibited by the Ag alloy. In this case, if the
content of elements other than the main components is in the range
of not less than 5 atomic % but not more than 30 atomic % like the
Al alloy, the rewrite characteristic is improved further.
[0054] The preferable material of the second reflective layer is
Al--Ti, Al--Ag, Al--Cu, Al--Cr or the like material containing an
Al alloy as a main component. Al can also be used.
[0055] From this, it has been found that when the content of
elements other than Al in the Al alloy is in the range of not less
than 0.5 atomic % but not more than to 4 atomic %, the
characteristic of a multiplicity of overwrite cycles and the bit
error rate are improved, and the improvement is further enhanced in
the case where the content is in the range of not less than one
atomic % but not more than two atomic %. A similar characteristic
is obtained for other Al alloys than described above.
[0056] Also, a layer may be used which is composed of the element
unit such as Au, Ag, Cu, Ni, Fe, Co, Cr, Ti, Pd, Pt, W, Ta, Mo, Sb,
Bi, Dy, Cd, Mn, Mg or V or an alloy containing any one of these
elements as a main component such as an Au alloy, Ag alloy, Cu
alloy, Pd alloy or Pt alloy or an alloy between these alloys. In
this way, the second reflective layer is composed of a metal
element, a metalloid element, an alloy or a mixture thereof.
[0057] Among these materials, those having a large heat
conductivity such as Cu, Al, Au, Cu alloy, Al alloy and Au alloy
have a superior rewrite characteristic as the disk can be cooled
rapidly with ease. A similar characteristic is observed also for Ag
and Ag alloy. In the case where the content of the elements other
than Cu, Au and Ag making up the main components, like the Al
alloy, is in the range of not less than 0.5 atomic % but not more
than 4 atomic %, the characteristic of a multiplicity of overwrite
cycles and the bit error rate are improved. This trend is further
enhanced when the content is in the range of not less than one
atomic % but not more than 2 atomic %.
[0058] Also, a study of the refractive index (n) and the extinction
coefficient (k) of the materials of the first reflective layer and
the second reflective layer described above shows that the jitter
increase after 100 thousand overwrite cycles can be suppressed
within 3% in the case where n of the first reflective layer is
larger than n of the second reflective layer and k of the first
reflective layer is smaller than k of the second reflective
layer.
[0059] The materials of the first reflective layer and the second
reflective layer desirably represent at least 95% of the total
number of atoms of the respective reflective layer. In the case
where impurities other than the materials described above reach 5
atomic % or more, the possible number of overwrite cycles is
reduced to one half or otherwise the rewrite characteristic is
deteriorated.
[0060] The thickness of the first reflective layer is desirably not
less than 5 nm but not more than 100 nm. The thickness of the
second reflective layer, on the other hand, is desirably not less
than 30 nm but not more than 200 nm.
[0061] Examples of desirable combinations of the materials of the
first reflective layer and the second reflective layer are an
Al.sub.94Cr.sub.6 for the first reflective layer with
Al.sub.99Ti.sub.1 for the second reflective layer,
Al.sub.90Ti.sub.10 for the first reflective layer with
Al.sub.98Ti.sub.2 for the second reflective layer,
Al.sub.75Ti.sub.25 for the first reflective layer with
Al.sub.99Ti.sub.1 for the second reflective layer, etc. in which
case the first reflective layer and the second reflective layer
contain the same main component element, and elements other than
the main component element of Al are contained more in the second
reflective layer than in the first reflective layer. A similar
characteristic is obtained from the combinations of Al--Ti with
Al--Ti, Al--Cr with Al--Cr or other combinations such as Al--Ag
with Al--Cu in which the Al alloy is a main component. The Au
alloy, Ag alloy, Cu alloy or a similar composition can improve the
rewrite characteristic of a multiplicity of overwrite cycles.
[0062] (21) The substrate material may be a polycarbonate substrate
with a tracking groove formed directly in the surface thereof,
polyolefin, epoxy, acrylic resin, or a chemically reinforced glass
having the surface thereof formed with an ultraviolet setting resin
layer.
[0063] The substrate having a tracking groove is the one with the
whole or part of the substrate surface having a groove at least
.lambda./10n' (n': refractive index of the substrate material) deep
where .lambda. is the recording/reproduction wavelength. The groove
may be formed either continuously over the whole periphery or
segmented midway. It has been found that crosstalks are desirably
reduced when the groove depth is about .lambda./6n'. Further, it
has been found that although the yield for substrate production is
deteriorated but the cross erase is reduced desirably when the
groove is deeper than about .lambda./3n'.
[0064] Also, the groove may have different widths at different
places. A substrate of sample servo format lacking a groove or of
other tracking types or formats will do. A substrate having a
format capable or recording and reproduction in both grooves and
lands or a substrate having a format capable of recording and
reproduction only in grooves or lands can also be used. The disk
size is not limited to 12 cm, but other sizes including 13 cm,
3.5', 2.5', etc. are applicable with equal effect. The disk
thickness is neither limited to 0.6 mm but other thickness such as
1.2 mm or 0.8 mm can be employed.
[0065] Two disk members including a first disk member and a second
disk member are fabricated by exactly the same method, and are
attached to each other by an adhesive with the second reflective
layers thereof face to face. As an alternative, the second disk
member may be replaced by a disk member of another configuration or
a protective substrate. In the case where the disk member used for
attachment or the protective substrate has a large transmittance in
the ultraviolet wavelength area, the ultraviolet setting resin may
be used for attaching the disk members. Other methods of attaching
may also be used. A disk member of a structure having no second
reflective layer may be attached with an adhesive layer formed on
the topmost layer.
[0066] The first and second disk members described above are
attached to each other with the second reflective layers thereof
face to face through the adhesive layer. The error rate is reduced
further by coating the ultraviolet setting resin about 10 .mu.m
thick on the second reflective layers of the first and second disk
members beforehand and attaching the disk members to each other
after the resin is set.
[0067] Instead of attaching the first and second disk members to
each other, the ultraviolet setting resin may be coated to the
thickness of about 10 .mu.m on the second reflective layer of the
first disk member. In the case of a disk member of a structure
lacking the second reflective layer, the ultraviolet setting resin
may be applied on the topmost layer.
[0068] (22) In addition to the structures described above, the
structures of the disks 1 to 39 described below have a smaller
remanence due to the absorption control layer and have the effect
of reducing the jitter.
[0069] Disk 1: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5, upper
surface protect layer 6, absorption control layer 7, first
reflective layer 8, adhesive layer 10
[0070] Disk 2: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5, upper
surface protect layer 6, absorption control layer 7, second
reflective layer 9, adhesive layer 10
[0071] Disk 3: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5, upper
surface protect layer 6, absorption control layer 7, adhesive layer
10
[0072] Disk 4: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5,
absorption control layer 7, first reflective layer 8, second
reflective layer 9, adhesive layer 10
[0073] Disk 5: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5,
absorption control layer 7, first reflective layer s, adhesive
layer 10
[0074] Disk 6: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0075] Disk 7: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5,
absorption control layer 7, adhesive layer 10
[0076] Disk 8: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, second
reflective layer 9, adhesive layer 10
[0077] Disk 9: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, adhesive
layer 10
[0078] Disk 10: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, upper surface protect layer 6,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0079] Disk 11: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, upper surface protect layer 6,
absorption control layer 7, adhesive layer 10
[0080] Disk 12: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, absorption control layer 7, first
reflective layer 8, second reflective layer 9, adhesive layer
10
[0081] Disk 13: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, absorption control layer 7, first
reflective layer 8, adhesive layer 10
[0082] Disk 14: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, absorption control layer 7, first
reflective layer 8, second reflective layer 9, adhesive layer
10
[0083] Disk 15: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, absorption control layer 7, adhesive
layer 10
[0084] Disk 16: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, second
reflective layer 9, adhesive layer 10
[0085] Disk 17: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, adhesive
layer 10
[0086] Disk 18: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0087] Disk 19: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, adhesive layer 10
[0088] Disk 20: Substrate 1, heat diffusion layer 2, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, second
reflective layer 9, adhesive layer 10
[0089] Disk 21: Substrate 1, heat diffusion layer 2, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, adhesive
layer 10
[0090] Disk 22: Substrate 1, heat diffusion layer 2, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0091] Disk 23: Substrate 1, heat diffusion layer 2, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, adhesive layer 10
[0092] Disk 24: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, second
reflective layer 9, adhesive layer 10
[0093] Disk 25: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, adhesive
layer 10
[0094] Disk 26: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0095] Disk 27: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, adhesive layer 10
[0096] Disk 28: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, absorption control layer 7,
first reflective layer 8, second reflective layer 9, adhesive layer
10
[0097] Disk 29: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, absorption control layer 7,
first reflective layer 8, second reflective layer 9, adhesive layer
10
[0098] Disk 30: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, absorption control layer 7,
first reflective layer 8, adhesive layer 10
[0099] Disk 31: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, absorption control layer 7,
adhesive layer 10
[0100] Disk 32: Substrate 1, protective layer 3, recording film 5,
upper surface protect layer 6, absorption control layer 7, first
reflective layer 8, second reflective layer 9, adhesive layer
10
[0101] Disk 33: Substrate 1, protective layer 3, recording film 5,
upper surface protect layer 6, absorption control layer 7, second
reflective layer 9, adhesive layer 10
[0102] Disk 34: Substrate 1, protective layer 3, recording film 5,
upper surface protect layer 6, absorption control layer 7, first
reflective layer 8, adhesive layer 10
[0103] Disk 35: Substrate 1, protective layer 3, recording film 5,
upper surface protect layer 6, absorption control layer 7, adhesive
layer 10
[0104] Disk 36: Substrate 1, protective layer 3, recording film 5,
upper surface protect layer 6, absorption control layer 7, first
reflective layer 8, second reflective layer 9, adhesive layer
10
[0105] Disk 37: Substrate 1, protective layer 3, recording film 5,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0106] Disk 38: Substrate 1, protective layer 3, recording film 5,
absorption control layer 7, first reflective layer 8, adhesive
layer 10
[0107] Disk 39: Substrate 1, protective layer 3, recording film 5,
absorption control layer 7, adhesive layer 10
[0108] (23) The recording/reproduction characteristic is improved
simply by securing a desired range of thickness or material of each
layer independently. A higher effect can be achieved, however, by
combining the desired ranges of the respective factors.
[0109] (24) A better characteristic is obtained in the case where
the recording film has a composition defined as
0.12.ltoreq.x.ltoreq.0.24, 0.20.ltoreq.y.ltoreq.0.31,
0.54.ltoreq.z.ltoreq.58, 0.ltoreq.w.ltoreq.0 04.
[0110] Further, in the case where the Ge amount reaches not less
than 20 atomic % in this range, the read light endurance is
improved by 1.5 times. The read light endurance is obtained by
determining, and by comparison with, the power of the read light
for reducing the recording signal by 2 dB or more during a
five-minute reproduction. For the Ge amount of not more than 17
atomic %, on the other hand, the extinction ratio is large also in
the case where the linear speed is high, thus producing a superior
figure of not less than 30 dB for 12 m/s.
[0111] In the case where M is Ag, the recording sensitivity is
improved by 10% as compared with Ge--Sb--Te. In the case where M is
at least one of Cr, W and Mo, on the other hand, the possible
number of overwrite cycles at which the jitter increases at least
5% is improved three times or more in a multiplicity of overwrite
cycles, as compared with Ge--Sb--Te. In the case where M is at
least one of Pt, Co and Pd, the crystallization temperature is
increased by at least 50.degree. C. as compared with
Ge--Sb--Te.
[0112] Also, in the case where the impurities elements in the
recording film are not more than 5 atomic %, the deterioration of
the rewrite characteristic can be reduced desirably. The figure of
not more than 2 atomic % produces a more desirable result.
[0113] The thickness of the recording film is desirably not less
than 10 nm but not more than 30 nm, and the figure of not less than
13 nm but not more than 20 nm is more desirable.
[0114] Though somewhat time-consuming, mixing nitrogen with the
sputtering gas at the start or end of the fabrication process of
the recording film or using a target mixed with a small amount of
nitrogen in the composition of the recording film or otherwise
containing nitrogen in the neighborhood of the boundary between the
recording film and other layers improves the adhesion for an
improved characteristic.
BRIEF DESCRIPTION OF DRAWINGS
[0115] FIG. 1 is a sectional view showing a structure of an
information recording medium according to embodiment 1 of the
present invention.
[0116] FIG. 2 is a sectional view showing an information recording
medium having the current structure.
[0117] FIG. 3 shows recording waveforms used for evaluation of the
recording/reproduction characteristic of an information recording
medium according to the present invention.
[0118] FIG. 4 shows the rewrite characteristic of an information
recording medium according to the invention and an information
recording medium of the current structure.
[0119] FIG. 5 is a sectional view showing a structure of an
information recording medium according to embodiment 3 of the
invention.
[0120] FIG. 6 is a sectional view showing a structure of an
information recording medium according to embodiment 5 of the
invention.
[0121] FIG. 7 is a sectional view showing a structure of an
information recording medium according to embodiment 6 of the
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0122] The present invention will be explained in detail below with
reference to embodiments.
[0123] The reference numerals used in the drawings are defined
below.
[0124] 1, 1': Substrate
[0125] 2, 2': Heat diffusion layer
[0126] 3, 3': Protective layer
[0127] 4, 4': Lower surface protect layer
[0128] 5, 5': Recording film
[0129] 6, 6': Upper surface protect layer
[0130] 7, 7': Absorption control layer
[0131] 8, 8': First reflective layer
[0132] 9, 9': Second reflective layer
[0133] 10: Adhesive layer
[0134] 11, 11': Reflective layer
[0135] T: Window width (Tw)
[0136] Pr: Low power level
[0137] Pe: Intermediate power level
[0138] Ph: High power level
[0139] Pp: Preheat power level
[0140] Pl: Level of power 0
[0141] Tc: Cooling pulse width
[0142] Tp: Preheat level width
[0143] Other reference numerals are defined in the drawings.
[0144] (1) Embodiment 1
[0145] (Configuration and Fabrication Method)
[0146] FIG. 1 is a sectional view showing a structure of a
disk-type information recording medium according to embodiment 1 of
this invention. This medium was fabricated in the following
manner.
[0147] First, a heat diffusion layer 2 of Al.sub.2O.sub.3 film was
formed in the thickness of about 30 nm on a polycarbonate substrate
12 cm in diameter and 0.6 mm thick having a tracking groove in the
surface thereof. Then, a protective layer 3 of ZnS film about 45 nm
thick, a lower surface protect layer 4 of SiO.sub.2 film about 5 nm
thick, a recording film 5 of Ge.sub.14Sb.sub.29Te.sub.17 about 15
nm thick, an upper surface protect layer 6 of SiO.sub.2 film about
15 nm thick, an absorption control layer 7 of
MO.sub.80(SiO.sub.2).sub.20 film about 18 nm thick, a first
reflective layer 8 of Al.sub.89Ti.sub.11 about 20 nm thick, and a
second reflective layer 9 of Al.sub.98Ti.sub.2 film about 180 nm
thick were formed sequentially. The film lamination was formed by a
magnetron sputtering apparatus. In this way, a first disk member
was obtained.
[0148] On the other hand, a second disk member having the same
configuration as the first disk member was obtained by exactly the
same method. The second disk member was produced in such a manner
that a heat diffusion layer 2' of Al.sub.2O.sub.3 film, a
protective layer 3' of ZnS film about 45 nm thick, a lower surface
protect layer 4' of SiO.sub.2 film about 5 nm thick, a recording
film 5' of Ge.sub.14Sb.sub.29Te.sub.57 about 15 nm thick, an upper
surface protect layer 6' of SiO.sub.2 film about 15 nm thick, an
absorption control layer 7' of Mo.sub.80(SiO.sub.2)20 film about 18
nm thick, a first reflective layer 8' of Al.sub.89Ti.sub.11 film
about 20 nm thick, and a second reflective layer 9' of a
Al.sub.98Ti.sub.2 film about 180 nm thick, were sequentially formed
on a substrate 1' having a diameter of 12 cm and 0.6 mm thick.
[0149] After that, the first disk member and the second disk member
were attached to each other with the second reflective layers 9, 9'
thereof face to face through an adhesive layer 10, thereby
producing a disk-type information recording medium shown in FIG.
1.
[0150] (Initial Crystallization)
[0151] The recording films 5, 5' of the medium fabricated in the
manner described above were subjected to initial crystallization in
the following manner. Only the recording film 5 which is treated
exactly the same way as the recording film 5' is described
below.
[0152] The medium is rotated so that the linear speed of a point on
the recording track is 8 m/s. The laser light power of a
semiconductor laser (about 810 nm in wavelength) with a laser light
power of 800 mW having an elliptical spot long radially of the
medium was radiated on the recording film 5 through the substrate
1. The spot was moved by being displaced by 1/4 of the spot length
each time radially of the medium. By doing so, the initial
crystallization was effected. One session of the initial
crystallization is sufficient. Nevertheless, three repeated
sessions of the initial crystallization could somewhat reduce the
noise increase due to the initial crystallization. This initial
crystallization can be advantageously carried out at high
speed.
[0153] (Recording, Erasure and Reproduction)
[0154] Then, while conducting the tracking and automatic focusing
operation in the recording area of the recording film 5 for which
the initial crystallization was completed in the manner described
above, information was recorded by changing the power of the
recording laser light between the intermediate power level (4.5 mW)
and the high power level Ph (11 mW). The linear speed of the
recording track is 9 m/s, the semiconductor laser wavelength is 636
nm, and the numerical aperture (NA) of the lens is 0.6. The
amorphous or similar portion formed in the recording area by the
recording laser light constitutes a recording point. The
reflectance of this medium is higher in crystal state, and the area
which turned amorphous by recording has a lower reflectance.
[0155] The power ratio in the range of 1:0.3 to 1:0.6 between high
level and intermediate level of the recording laser light is
especially preferable. Also, other power levels can be employed for
each short time. The recording/reproduction are performed by a
device having means in which, as shown in FIG. 3, while one
recording mark is being formed, the power is repeatedly reduced to
a level lower than the intermediate power level by one half of the
window width (Tw/2) each time, and a waveform having a preheat
level Pp (4.6 mW) at the head of the recodring pulse is generated.
Then, a reproduced signal waveform was obtained which has an
especially low jitter value and low error rate. The preheat level
is slightly higher than the intermediate level and lower than the
high level. This waveform has the feature that the preheat level
width Tp (length for which the preheat level is held) changes by
the combination of the recording mark and the length of the space
formed immediately before the recording mark and the feature that
the cooling pulse width Tc (time width during which the level is
reduced to Pr at the end of the recording pulse) is determined by
the combination of the recording mark and the length of the space
following the particular mark. The shorter the space immediately
before the mark and the longer the mark, the longer the Tp, while
the longer the space immediately before the mark and the shorter
the mark, the longer the width Tp. Depending on the structure of
the medium, however, in the case where Tp of the recording waveform
of the 6 Tw mark is especially long, the jitter can be reduced
effectively. Also, the longer the space following the mark and the
longer the mark, the shorter the width Tc, while the shorter the
space following the mark and the shorter the mark, the long the
Tc.
[0156] In FIG. 3, only the recording waveforms of 3 Tw, 4 Tw, 6 Tw
and 11 Tw are shown. The waveform of 5 Tw is such that among a
series of the pulse train having a high power level in the
recording waveform of 6 Tw, one each of the high power level Ph of
Tw/2 and the immediately following low power level Ph of Tw/2 are
removed. In the recording waveforms 7 Tw to 10 Tw, on the other
hand, a set of the high power level Ph of Tw/2 and the low power
level Ph of Tw/2 is added immediately before the pulse of high
power level at the tail end of the recording waveform of 6 Tw. It
follows, therefore, that the recording waveform of 11 Tw is the
result of adding five such sets. The length of the shortest
recording mark corresponding to 3Tw is set to 0.42 .mu.m. Once the
portion to be recorded is passed, the laser light power is reduced
to the low power level Pr (1.5 mW) for reproduction (reading). The
recording signal contains dummy data with a repetition of the 4T
mark and the 4T space at the starting end and the tail end of the
information signal. The starting end also contains VFO.
[0157] In this recording method, the portion where information is
already recorded can be rewritten into new information by
overwriting the new information without erasure. In other words,
the overwrite operation with a substantially circular single light
spot is possible.
[0158] As an alternative, in the first one or a plurality of disk
rotations during the overwrite operation, the continuous light of
the intermediate power level (4.5 mW) or a power level proximate
thereto of the power-modulated recording laser light is radiated
thereby to erase the recorded information, and then during the next
one rotation, the recording operation is performed by radiating the
laser light power-modulated in accordance with the information
signal, between the low power level (1.5 mW) and the high power
level (11 mW) or between the intermediate power level (4.5 mW) and
the high power level (11 mW). By recording after erasing the
information in this way, the remanence of the information
previously written is reduced. Thus, the overwrite operation
becomes easy even when the linear speed is increased to twice.
[0159] These methods are effective not only with the recording film
used for the medium according to the invention but also with the
recording film of other media.
[0160] When the recording and erase operations are repeated in the
information recording medium according to this embodiment, the
jitter (.sigma./Tw) is reduced by 6% or more as compared with the
conventional information recording medium described in embodiment 2
for each overwrite cycle, as shown in FIG. 4. The jitter is an
indicator of the degree to which the reproduction signal fluctuates
with respect to the window width (Tw) when reproducing the position
of the edge portion of the recording mark. With the increase in the
jitter value, the detection position of the edge portion represents
a major proportion of the window width and therefore the recording
signal cannot be accurately reproduced. Thus, the jitter is
desirably as small as possible. The reason why the jitter has been
reduced is that the absorption can be controlled by the absorption
control layer so that the remanence can be reduced also at the time
of recording with high linear speed.
[0161] Observing the recording mark under the transmission electron
microscope, the mark size (mark area) was compared between the case
in which the long mark (amorphous state) is rewritten and the case
in which the long space (crystal state) is overwritten. In the case
of the information recording medium according to this embodiment,
the former has been found to be substantially the same as the
latter. Under the strong absorption control, however, the former is
slightly smaller than the latter. For the information recording
medium described in embodiment 2, on the other hand, the former is
larger than the latter.
[0162] When recording information on an information recording
medium, the number of times of recording at one point of an
information recording medium is generally said to be about 100
thousands. In this embodiment, therefore, the
recording/reproduction characteristic from the first recording to
the end of 100 thousands of overwrite cycles was studied.
[0163] The window width (Tw) in jitter measurement is 16 ns, the
shortest recording signal is 3 Tw and the longest recording signal
is 11 Tw, which are randomly recorded. A reproduction equalizing
circuit was used for the measurement.
[0164] The absorption control layer, which is effective also in
other recording schemes, has an especially high effect of reducing
the jitter by recording the edge portion accurately in the mark
edge recording. The mark edge recording is a recording scheme in
which the edge portion of a recording mark is regarded as "1" while
the space between marks and the interior of the marks are regarded
as "0". Further, the effect is conspicuous where the linear speed
is higher than 6 m/s.
[0165] The effect of the absorption control layer, though observed
also at the time of low density recording, becomes conspicuous at
the time of high density recording. An example is the case of
recording on a land-groove recording substrate at a track pitch of
not less than 0.53 .mu.m but not more than 0.65 .mu.m and/or with
the shortest mark length of not less than 0.39 .mu.m but not more
than 0.45 .mu.m. The modulation degree is increased and the rewrite
characteristic is desirably improved for the recording/reproduction
wavelength of not less than 600 nm but not more than 660 nm. Also
for the wavelength shorter than 600 nm, the medium according to
this embodiment is usable by correcting the film thickness in
accordance with the wavelength ratio.
[0166] (Absorption Control Layer)
[0167] In this embodiment, changing the thickness of the film used
in the absorption control layers 7, 7', the jitter (.sigma./Tw)
after 10 overwrite cycles was measured, and the following result
was obtained.
[0168] The square mean value (%) of the jitter and the modulation
degree (%) of the front edge and the trailing edge after ten
overwrite cycles are indicated with respect to the thickness (nm)
of the absorption control layer. Unless otherwise specified, the
jitter is expressed by the square mean of the jitter values at the
front edge and the trailing edge.
[0169] The modulation degree (Mod) was calculated according to the
following equation.
Mod (%)=100.times.(Ic-Ia)/Ic
[0170] where Ic is the highest level of reflectance of the crystal
(erased) state at the time of EFM signal recording and Ia the
lowest level of reflectance of the amorphous (recorded) state at
the time of EFM signal recording.
Mod (%)=100.times.(Ic-Ia)/Ic
[0171]
1 Absorption control Jitter after 10 Modulation layer thickness
(nm) overwrite cycles (%) degree (%) 2 25 -- 5 20 -- 10 15 53 20 13
51 40 13 47 50 -- 43 60 -- 40
[0172] This indicates that in the case where the thickness of the
absorption control layer is reduced, the jitter after 10 overwrite
cycles increases while in the case where the thickness of the
absorption control layer is increased, the modulation degree is
increased. The jitter is increased with the decrease in the
absorption control layer thickness, probably because the reduction
in the absorption ratio (Ac/Aa), i.e. the ratio between the
absorption coefficient Ac of the recording film in crystal state
and the absorption coefficient Aa of the recording film in
amorphous state fails to sufficiently control the absorption, and
the remanence is caused. The absorption ratio (Ac/Aa), which cannot
be actually measured, was determined by optical calculations. The
result shows that the thickness of the absorption control layer is
desirably not less than 5 nm but not more than 50 nm, and more
desirably not less than 10 nm but not more than 40 nm.
[0173] In this embodiment, optical calculations were carried out by
changing the input values n and k and the optical constants of the
film used for the absorption control layers 7, 7'. First,
maintaining k at 1.7 while changing n, the absorption ratio (Ac/Aa)
was determined as follows.
2 n Ac/Aa 0.5 0.9 1.2 1.0 1.8 1.05 2.0 1.11 3.0 1.13 4.5 1.12 5.5
1.07 6.0 1.00 6.5 0.9
[0174] This indicates that with the change in n of the absorption
control layer, the absorption ratio (Ac/Aa) changes. Therefore, n
of the absorption control layer is desirably not less than 1.2 but
not more than 6 and more desirably not less than 1.8 but not more
than 5.5.
[0175] Then, maintaining n at 3.3 and changing k, the absorption
ratio (Ac/Aa) was determined as follows.
3 k Ac/Aa 0 0.97 0.5 1.02 0.8 1.1 1.5 1.11 1.8 1.13 2.5 1.13 3.0
1.08 3.3 1.01 4.5 0.95
[0176] This indicates that with the change in k of the absorption
control layer, the absorption ratio (Ac/Aa) changes. Therefore, k
of the absorption control layer is desirably not less than 0.5 but
not more than 3.3 and more desirably not less than 0.8 but not more
than 3.
[0177] According to this embodiment, the jitter (.sigma./Tw) and
the recording sensitivity after 10 overwrite cycles were measured
by changing the composition ratio of Mo--(SiO.sub.2) used for the
absorption control layers 7, 7', and the following result was
obtained. The recording sensitivity, which was based on
Mo.sub.80(SiO.sub.2).sub.20, is indicated as + when improved, as -
when deteriorated and as 0 when unchanged.
4 Absorption control Recording layer composition Jitter (%)
sensitivity (%) Mo.sub.20(SiO.sub.2).sub.80 25 Not measured
Mo.sub.35(SiO.sub.2).sub.65 22 Not measured
Mo.sub.42(SiO.sub.2).sub.58 20 +10 Mo.sub.50(SiO.sub.2).sub.50 18
+10 Mo.sub.61(SiO.sub.2).sub.39 15 +5 Mo.sub.67(SiO.sub.2).sub.33
14 +3 Mo.sub.72(SiO.sub.2).sub.28 13 0 Mo.sub.80(SiO.sub.2).sub.20
13 0 Mo.sub.90(SiO.sub.2).sub.10 -- 0 Mo -- -5
[0178] This indicates that with the increase in the amount of Mo
with respect to the composition of the absorption control layer,
the jitter after 10 overwrite cycles can be reduced. The jitter is
reduced probably by reason of the fact that the absorption ratio
(Ac/Aa) is so large that the remanence is hard to occur. The Mo
amount that represents of the total composition of the absorption
control layer, therefore, is preferably not less than 42 mol %.
Also, Mo itself has a larger heat conductivity than Mo--(SiO.sub.2)
and therefore, the use of Mo alone, somewhat reduces the recording
sensitivity. The desirable amount of Mo is not less than 61 mol %
but not more than 90 mol %.
[0179] In this embodiment, the Mo--(SiO.sub.2) film used for the
absorption control layers 7, 7' was analyzed by the X-ray
photoelectron spectrography, and it was found that the
Mo--(SiO.sub.2) film is composed mainly of a metal (Mo) and a
dielectric material (SiO.sub.2). Depending on the composition
ratio, the peak of the metal Mo sightly lowers with the peak
corresponding to the Mo oxide appearing. However, the main peak
appears at the position of the metal Mo, and if an oxide, oxygen is
not saturated. This is because oxygen is exchanged between the
metal (Mo) and the dielectric material (SiO.sub.2) in the
absorption control layer, and the composition ratio for the
compound as a whole is Mo--(SiO.sub.2). In the case where the
dielectric material is a compound other than the oxide, the same
effect is obtained as if oxygen has directly replaced other
elements. A sulfide such as Sb.sub.2S.sub.3, for example, includes
Mo and Sb.sub.2S.sub.3, and depending on the composition ratio, a
peak corresponding to the Mo sulfide appears other than the peak of
the metal Mo.
[0180] As described above, it was found that the absorption control
layer is composed of a metal element or an unsaturated metal oxide
and a dielectric material.
[0181] A similar result was obtained by use of Cr, W, Fe, Sb, Mn,
Ti, Co, Ge, Pt, Ni, Nb, Pd, Be or Ta as a replacement material of
Mo in the Mo--(SiO.sub.2) film used for the absorption control
layers 7, 7'. Among these elements, Re and W are high in melting
point and more desirable. Pd and Pt, on the other hand, are less
reactive with other layers and therefore have desirably a further
increased possible number of overwrite cycles. The use of Ni or Co
makes an inexpensive target usable as compared with other elements,
and can reduce the total production cost. Cr and Ti have a high
anticorrosiveness and the result of the life test on them was
superior to that of others. Also, Tb, Gd, Sm, Cu, Au, Ag, Ca, Al,
Zr, Ir, Hf, etc. are also usable.
[0182] In this embodiment, the materials usable instead of
SiO.sub.2 in the Mo--(SiO.sub.2) film used with the absorption
control layers 7, 7' are oxides including SiO, Al.sub.2O.sub.3,
BeO, Bi.sub.2O.sub.3, CoO, CaO, Cr.sub.2O.sub.3, CeO.sub.2,
Cu.sub.2O, CuO, CdO, Dy.sub.2O.sub.3, FeO, Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4, GeO, GeO.sub.2, HfO.sub.2, In.sub.2O.sub.3,
La.sub.2O.sub.3, MgO, MnO, MoO.sub.2, MoO.sub.3, NbO, NbO.sub.2,
NiO, PbO, PdO, SnO, SnO.sub.2, Sc.sub.2O.sub.3, SrO, ThO.sub.2,
TiO.sub.2, Ti.sub.2O.sub.3, TiO, Ta.sub.2O.sub.5, TeO.sub.2, VO,
V.sub.2O.sub.3, VO.sub.2, WO.sub.2, WO.sub.3, Y.sub.2O.sub.3 and
ZrO.sub.2, nitrides including AlN, Bn, CrN, Cr.sub.2N, GeN, HfN,
Si.sub.3N.sub.4, Al--Sn--N group material (such as AlSiN.sub.2),
Si--N group material, Si--O--N group material, TaN, TiN and ZrN,
sulfides including ZnS, Sb.sub.2S.sub.3, CdS, In.sub.2S.sub.3,
Ga.sub.2S.sub.3, GeS, SnS.sub.2, PbS, Bi.sub.2S.sub.3, SrS, MgS,
CrS, CeS and TaS, selenides including SnSe.sub.2, Sb.sub.2Se.sub.3,
CdSe, ZnSe, In.sub.2Se.sub.3, Ga.sub.2Se.sub.3, GeSe, GeSe.sub.2,
SnSe, PbSe and Bi.sub.2Se.sub.3, fluorides including CeF.sub.3,
MgF.sub.2, CaF.sub.2, TiF.sub.3, NiF.sub.3, FeF.sub.2 and
FeF.sub.3, Si, Ge, borides including TiB.sub.2, B.sub.4C, B, CrB,
HfB.sub.2, TiB.sub.2 and WB, carbides including C, Cr.sub.3C.sub.2,
Cr.sub.23C.sub.6, Cr.sub.7C.sub.3, Fe.sub.3C, MO.sub.2C, WC,
W.sub.2C, HfC, TaC and CaC.sub.2, or a material having a
composition similar to the materials described above or a mixture
thereof. In addition, In--Sb, Ga--As, In--P, Ga--Sb, In-As, etc.
could also be used.
[0183] Among these materials, the use of SiO.sub.2,
Ta.sub.2O.sub.3, Y.sub.2O.sub.3'ZrO.sub.3 or the like oxides makes
it possible to use a target less expensive than when using other
materials, and therefore can reduce the total cost of production.
Among the oxides, SiO.sub.2, Ta.sub.2O.sub.5,
Y.sub.2O.sub.3--ZrO.sub.2 are less reactive and desirably have an
increased number of overwrite cycles. BeO and Cr.sub.2O.sub.3 are
also desirable as they have a high melting point. Al.sub.2O.sub.3
is high in heat conductivity, and therefore, in the case where it
is made into a disk having a structure lacking the first reflective
layer and/or the second reflective layer, is less deteriorated than
other materials in the rewrite characteristic.
[0184] The use of a nitride, on the other hand, increases the
adhesion with the layers adjoining the absorption control layer and
becomes resistant to an external shock. When a sulfide or selenide
is used, on the other hand, the sputter rate can be increased for a
shortened film-making time. In the case where a carbide is used,
the hardness of the absorption control layer is increased thereby
to suppress the flow of the recording film for a multiplicity of
overwrite cycles.
[0185] A metal element and/or a dielectric material, if having a
melting point higher than the melting point (about 60.degree. C.)
of the recording film, can suppress the jitter increase at the time
of ten thousand overwrite cycles. In the case where the two
materials have a melting point of not lower than 600.degree. C.,
the jitter increase can be desirably suppressed to not more than
3%.
[0186] Also, in the case where the impurities elements in the
absorption control layer exceeds 2 atomic % of the components of
thereof, the jitter at the front or trailing edge after 10
overwrite cycles is found to exceed 15%. Further, when the
impurities elements exceed 5 atomic %, the jitter is found to
increase to 18% or more. Thus, the deterioration of the rewrite
characteristic can be desirably reduced when the impurities
elements contained in the absorption control layer is not more than
5 atomic %. The figure of not more than 2 atomic % is more
desirable.
[0187] (Measurement of Optical Constants of Absorption Control
Layer)
[0188] Separating the disk member between the upper surface protect
layer and the absorption control layer, the reflectance was studied
for the recording/reproduction wavelength. Specifically, the second
reflective layer 9 about 180 nm thick, the first reflective layer 8
of Al.sub.89Ti.sub.11 film about 20 nm thick and the absorption
control layer 7' of MO.sub.80(SiO.sub.2).sub.20 film about 18 nm
thick are deposited on the adhesive layer 10.
[0189] Then, by the counter sputtering (Ar gas etching) with the
sputtering device, the reflectance was measured with the
MO.sub.80(SiO.sub.2).sub.20 film reduced in thickness. The
thickness of the film etched was masked partly at the time of
etching, and after etching, the mask was removed and the
reflectance was measured with step meter. After repeating this
operation twice, the following values of the absorption control
layer thickness and the reflectance were obtained.
5 Absorption control layer thickness Reflectance (%) 18 27 10 49 0
75
[0190] As to n and k of the reflective layer, on the other hand,
the surface of the reflective layer was exposed by separation and
the values n and k were determined by the ellipsometry with
variable wavelengths.
[0191] From the reflectance and n and k of the reflective layer
thus obtained, the values n and k which indicate the reflectance of
structures having different thicknesses of the absorption control
layer were determined by calculation. As a result, it has been
found that n is 3.3 and k is 1.3.
[0192] (Surface Protect Layers)
[0193] A desirable material replacing SiO.sub.2 of the upper
surface protect layer 6 and the lower surface protect layer 4 is
Al.sub.2O.sub.3 or a mixture of Al.sub.2O.sub.3 and SiO.sub.2. In
the case where the SiO.sub.2 or Al.sub.2O.sub.3 content is not less
than 70 mol %, the crystallization rate is increased, so that the
extinction ratio increases to not less than 25 dB even for the
speed of 18 m/s which is about twice as high as the speed in the
absence of the surface protect layer.
[0194] The next desirable materials replacing SiO.sub.2 of the
upper surface protect layer 6 and the lower surface protect layer 4
are Ta.sub.2O.sub.5, a mixture between Ta.sub.2O.sub.5 and
SiO.sub.2 or Al.sub.2O.sub.31 followed by ZrO.sub.2-Y.sub.2O.sub.3,
SiO.sub.2 or a mixture thereof with Al.sub.2O.sub.3 or
Ta.sub.2O.sub.5. Among them, Al.sub.2O.sub.3 can suppress the
fluctuations of the reflectance level during a multiplicity of
overwrite cycles to not more than 5%, thus reducing the jitter
desirably. CoO, Cr.sub.2O.sub.3 and NiO are more desirable as a
uniform crystal grain size is obtained by initial crystallization
so that the jitter increases to lesser degree in the initial stage
of overwrite operation.
[0195] Also, nitrides such as AlN, BN, CrN, Cr.sub.2N, GeN, HfN,
Si.sub.3N.sub.4, Al--Sn--N group material (such as AlSiN.sub.2),
Si--N group material, Si--O--N group material, TaN, TiN and ZrN
increases the adhesion, and the information recording medium is
desirably less deteriorated under external shocks. A material
having the same composition as the recording film containing
nitrogen or a material of a similar composition also improves the
adhesion.
[0196] Other materials usable include oxides including BeO,
Bi.sub.2O.sub.3, CeO.sub.2, Cu.sub.2O, CuO, CdO, Dy.sub.2O.sub.3,
FeO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, GeO, GeO.sub.2, HfO.sub.2,
In.sub.2O.sub.3, La.sub.2O.sub.3, MgO, MnO, MoO.sub.2, MoO.sub.3,
NbO, NbO.sub.2, PbO, PdO, SnO, SnO.sub.2, Sc.sub.2O.sub.3, SrO,
ThO.sub.2, TiO.sub.2, Ti.sub.2O.sub.3, TiO, TeO.sub.2, VO,
V.sub.2O.sub.3, VO.sub.2, WO.sub.2 and WO.sub.3, and carbides
including C, Cr.sub.3C.sub.2, Cr.sub.23C.sub.6, Cr.sub.7C.sub.3,
Fe.sub.3C, Mo.sub.2C, WC, W.sub.2C, HfC, TaC and CaC.sub.2, or a
material having a composition similar to the materials described
above.
[0197] Also, a mixture of these materials may be used.
[0198] The upper surface protect layer 6, the lower surface protect
layer 6, or the materials used in place of the upper surface
protect layer 6 and the lower surface protect layer 4 desirably
represent not less than 90% of all the atoms of the respective
surface protect layers. In the case where the impurities other than
these materials increases to 10 atomic % or more, the possible
number of overwrite cycles is reduced by 50% or more or otherwise
the write characteristic is deteriorated.
[0199] In the absence of the upper surface protect layer, diffusion
occurs into the recording film of the reflective layer material,
and the remanence increases, so that the reduction in the
reflectance level due to 100 thousand overwrite cycles is small and
can be suppressed to 5% or less. With the change in the reflectance
level, the reproduction signal level develops an offset, and the
jitter increases by the amount equal to the offset resulting in an
increased jitter. For this reason, the variations of the
reflectance level are better smaller.
[0200] Further, in order to maintain the modulation degree at 43%
or more, the figure of no more than 12 nm is desirable. For the
figure of not more than 5 nm, the modulation degree can be
increased to 47% or more. A uniform film is formed with the figure
of about 2 nm or more, and therefore, when the thickness of the
upper surface protect layer is maintained between 2 and 12 nm, the
recording/reproduction characteristic is desirably improved.
[0201] In the absence of the lower surface protect layer, diffusion
occurs into the recording film of a protective layer material, and
the remanence increases, so that the jitter increases by more than
6% at the time of 100 thousand overwrite cycles. Further, in order
to maintain the modulation degree at 43% or more, the thickness is
desirably not more than 25 nm. For the thickness of 5 nm or more
but not more than 10 nm, the modulation degree can be maintained at
47% or more. A uniform film is formed when the thickness is not
less than about 2 nm, and therefore, by maintaining the thickness
of the lower surface protect layer between 2 and 25 nm, the
recording/reproduction characteristic can be improved
desirably.
[0202] (Protective Layer)
[0203] According to this embodiment, the protective layer 2 is
formed of ZnS.
[0204] The materials that can replace ZnS of the protective layer 2
include a Si--N group material, a Si--O--N group material, oxides
such as SiO.sub.2, SiO, TiO.sub.2, Al.sub.2O.sub.3, Y.sub.2O.sub.3,
CeO.sub.2, La.sub.2O.sub.3, In.sub.2O.sub.3, GeO, GeO.sub.2, PbO,
SnO, SnO.sub.2, BeO, Bi.sub.2O.sub.3, TeO.sub.2, WO.sub.2,
WO.sub.3, Sc.sub.2O.sub.3, Ta.sub.2O.sub.5, ZrO.sub.2, Cu.sub.2O
and MgO, nitrides such as TaN, AlN, BN, Si.sub.3N.sub.4, GeN and
Al--Sn--N group material (such as AlSiN.sub.2), sulfides such as
ZnS, Sb.sub.2S.sub.37 CdS, In.sub.2S.sub.3, Ga.sub.2S.sub.3, GeS,
SnS.sub.2, PbS and Bi.sub.2S.sub.3, selenides such as SnSe.sub.2,
Sb.sub.2Se.sub.3, CdSe, ZnSe, In.sub.2Se.sub.3, Ga.sub.2Se.sub.3,
GeSe, GeSe.sub.2, SnSe, PbSe and Bi.sub.2Se.sub.3, fluorides such
as CeF.sub.3, MgF.sub.2 and CaF.sub.2, or Si, Ge, TiB.sub.2,
B.sub.4C, B, C, or materials having a similar composition to the
materials described above. Also, ZnS--SiO.sub.2,
ZnS--Al.sub.2O.sub.3, etc. or a mixture layer or a multi-layer of
these materials may be used. Among them, ZnS has a large n and can
maintain a large modulation degree. In the case of a mixture
containing 60 mol % or more of this material, the advantageous
points of the large n of ZnS and the chemical stability of the
oxide are combined with each other. Further, ZnS has a large
sputter rate, so that when ZnS represents at least 80 mol %, the
film-fabrication time can be shortened. Other sulfides and
selenides can produce a similar characteristic.
[0205] The desirable element ratio of these compounds, as expressed
by the ratio between metal element and oxygen for oxides and the
ratio between metal element and sulfide element for sulfides, is
desirably approximately 2 to 3 for Al.sub.2O.sub.3, Y.sub.2O.sub.3
and La.sub.2O.sub.3, 1 to 2 for SiO.sub.2, ZrO.sub.2 and GeO.sub.2,
2 to 5 for Ta.sub.2O, and 1 to 1 for ZnS. Nevertheless, a similar
effect can be produced even when the ratio deviates from these
figures. The deviation of the ratio from the integral ratio
described above, if any, is desirably not more than .+-.10 atomic %
in Al amount from Al.sub.2O.sub.3 in terms of the Al-to-O ratio in
Al--O, not more than .+-.10 atomic % in Si amount from SiO.sub.2 in
terms of the Si-to-O ratio in Si--O. In this way, the deviation of
the metal element is desirably not more than 10 atomic %. Once the
deviation increases to 10 atomic % or more, the resulting change in
the optical characteristics reduces the modulation degree by 10% or
more.
[0206] The material of the protective layer 2 and the replacement
material of the protective layer 2 desirably represent at least 90%
of all the atoms of each protective layer. In the case where the
impurities other than these materials reach 10 atomic % or more,
the possible number of overwrite cycles is reduced to one half or
otherwise the rewrite characteristic is deteriorated.
[0207] By changing the thickness of the protective layer used in
this embodiment, the modulation degree and the jitter (.sigma./Tw)
after ten overwrite cycles were measured, and the following result
was obtained. The calculation formula for the modulation degree
(Mod) is as follows.
Mod (%)=100.times.(Ic-Ia)/Ic
[0208] where Ic is the reflectance level of crystal (erased) state
at the time of EFM signal recording, and Ia is the reflectance
level of the amorphous (recorded) state at the time of EFM signal
recording.
6 Protective layer Modulation thickness (nm) degree (%) Jitter (%)
15 41 -- 20 43 -- 35 48 15 45 51 14 60 50 15 70 -- 18 80 -- 22
[0209] The thickness of the protective layer is desirably 20 to 70
nm at which the modulation degree for recording can be increased to
43% or more. The thickness of 35 to 60 nm is more desirable.
[0210] (Heat Diffusion Layer)
[0211] According to this embodiment, the heat diffusion layer 1 is
formed of Al.sub.2O.sub.3.
[0212] The desirable materials replacing Al.sub.2O.sub.3 of the
heat diffusion layer 1 are MgO, BeO, SiC, BN are B.sub.4C which
have a large heat conductivity. Also, Ta.sub.2O.sub.5, SiO.sub.2,
Al.sub.2O.sub.3 and a mixture of any combinations thereof have an
inexpensive target and therefore desirably result in a low
production cost. ThO.sub.2, TiO.sub.2, AlN and TiN, on the other
hand, are desirable in view of the ease with which to fabricate the
film.
[0213] Other desirable materials than those described above
desirably have a heat conductivity larger than the substrate
material and an absorption coefficient k smaller than 0.5.
[0214] A large heat conductivity can suppress the thermal damage to
the substrate surface at the time of recording, and the jitter can
be held at a low level after 100 thousand overwrite cycles. Also, a
small k can hold the reduction in modulation degree to a small
value.
[0215] The materials and the replacement materials of the heat
diffusion layer 1 desirably represent 90% or more of all the atoms
of each protective layer. Once the impurities other than the
materials described above reach 10 atomic % or more, the possible
number of overwrite cycles is reduced to one half or less, or
otherwise the rewrite characteristic is deteriorated.
[0216] Changing the thickness of the heat diffusion layer used in
this embodiment, the jitter (.sigma./Tw) after 100 thousand
overwrite cycles was measured, and the following result was
obtained. Also, the study made of the film-fabrication time for all
the layers shows that since the sputter rate of the heat diffusion
layer is low, the film-fabrication time depends to a large measure
on the thickness of the heat diffusion layer. The film-fabrication
time for the heat diffusion layer of 30 nm is assumed to be
unity.
7 Heat diffusion Film-fabrication layer thickness (nm) Jitter (%)
time 0 21 -- 10 18 -- 20 15 one time 30 15 one time 40 15 one time
50 -- 1.2 times 60 -- 1.2 times
[0217] This indicates that the thickness of the heat diffusion
layer is desirably 10 to 50 nm, and more desirably 20 to 40 nm.
[0218] Also, the jitter increase is suppressed after a multiplicity
of overwrite cycles not only for the disk having the structure
shown in this invention but also for the disk having the current
structure and the phase change disk having other heat diffusion
layer.
[0219] (Reflective Layer)
[0220] The material Al--Cr used for the first reflective layer 6 in
this embodiment is desirably replaced by a material containing an
Al alloy as a main component such as Al--Ti, Al--Ag or Al--Cu which
can reduce the jitter at the time of overwrite operation.
[0221] This indicates that the characteristic at the time of a
multiplicity of overwrite cycles is improved in the case where the
content of the elements other than Al in the Al alloy is not less
than 5 atomic % but not more than 30 atomic %. A similar
characteristic is obtained for an Al alloy other than those
mentioned above.
[0222] As an alternative, a layer comprising any of the element
units of Au, Ag, Cu, Ni, Fe, Co, Cr, Ti, Pd, Pt, W, Ta, Mo, Sb, Bi,
Dy, Cd, Mn; Mg and V, or an alloy with any of these elements as a
main component such as an Au alloy, Ag alloy, Cu alloy, Pd alloy,
Pt alloy, Sb--Bi, SUS or Ni--Cr, or a combination of any of these
alloys. In this way, the first reflective layer is composed of a
metal element, a metalloid element, an alloy or a mixture
thereof.
[0223] Among them, a material having a high reflectance such as Cu
alloy, Al alloy or Au alloy increases the modulation degree and
exhibits a superior reproduction characteristic. The Ag alloy also
has a similar characteristic. The contents of elements other than
the main component, like the Al alloy, is set in the range of 5
atomic % to 30 atomic % inclusive, whereby the rewrite
characteristic is improved further.
[0224] According to this embodiment, the materials of the second
reflective layer replacing Al--Ti used in the second reflective
layer 9 are desirably Al--Ag, Al--Cu, Al--Cr or the like materials
containing the Al alloy as a main component. Al is also usable.
[0225] From these facts, it has been found that in the case where
the contents of the elements other than Al in the Al alloy is in
the range of 0.5 atomic % to 4 atomic % inclusive, the
characteristic after a multiplicity of overwrite cycles and the bit
error rate are improved and the improvement is further enhanced in
the case where the contents are in the range of one atomic % to 2
atomic % inclusive. An Al alloy other than those mentioned above
can also produce a similar characteristic.
[0226] Also, a layer comprising the element units of Au, Ag, Cu,
Ni, Fe, Co, Cr, Ti, Pd, Pt, W, Ta, Mo, Sb, Bi, Dy, Cd, Mn, Mg or V,
or an alloy with any of these elements as a main component such as
an Au alloy, Ag alloy, Cu alloy, Pd alloy or Pt alloy, or an alloy
of any of these alloys as a main component or an alloy of any
combinations of these alloys. In this way, the second reflective
layer is composed of a metal element, a metalloid element or an
alloy or a mixture thereof.
[0227] Among them, Cu, Al, Au, Cu alloy, Al alloy, Au alloy or the
like which has a large heat conductivity tends to cool the disk
rapidly and exhibits a superior rewrite characteristic. The Ag and
Ag alloys have also a similar characteristic. The content of
elements other than the main component such as Cu, Au or Ag, like
the Al alloy, is desirably in the range of 0.5 atomic % to 4 atomic
% inclusive, in which case the characteristic after a multiplicity
of overwrite cycles and the bit error rate are improved, and the
improvement is further enhanced when the contents are in the range
of one atomic % to 2 atomic % inclusive.
[0228] Also, a study of the refractive index (n) and the extinction
coefficient (k) of the materials of the first reflective layer and
the second reflective layer shows that n of the first reflective
layer is larger than n of the second reflective layer, and in the
case where n of the first reflective layer is larger than n of the
second reflective layer and k of the first reflective layer is
smaller than k of the second reflective layer, the increase of
jitter after one hundred thousand overwrite cycles can be
suppressed within 3%.
[0229] The materials of the first reflective layer and the second
reflective layer are desirably not less than 95% of the total
number of atoms of each reflective layer. In the case where the
impurities other than the materials described above amount to 5
atomic % or more, the possible number of overwrite cycles is
reduced to one half or otherwise the rewrite characteristic is
deteriorated.
[0230] In the case where the second reflective layer is thinner
than 30 nm, the strength is so low and the heat diffusion is so
small that the recording film is liable to flow. Thus, the jitter
after 100 thousand overwrite cycles increases beyond 15%. The
thickness of 40 nm can reduce the jitter to 15%. In the case where
the thickness of the first reflective layer is larger than 100 nm
or the thickness of the second reflective layer is larger than 200
nm, the time for fabricating the respective reflective layer is
lengthened to such an extent that the fabrication is divided into
two or more processes or two or more vacuum chambers are provided
for sputtering, thereby doubling the time required for fabrication.
In the case where the thickness of the first reflective layer is
not more than 5 nm, on the other hand, it is difficult to form a
uniform film.
[0231] This indicates that the desirable thickness of the first
reflective layer is not less than 5 nm but not more than 100 nm,
and that of the second reflective layer is not less than 30 nm but
not more than 200 nm.
[0232] (Combination of Materials of First Reflective Layer and
Second Reflective Layer)
[0233] The materials described in this embodiment can be used for
the first reflective layer and the second reflective layer. By
selecting a combination of the materials, however, it has been
found that the increase of jitter after 100 thousand overwrite
cycles can be suppressed to not more than 3% for an improved
rewrite characteristic. A preferable combination is the first
reflective layer of Al.sub.94Cr.sub.6 with the second reflective
layer of Al.sub.98Ti.sub.1, the first reflective layer of
Al.sub.90Ti.sub.10 with the second reflective layer of
Al.sub.95Ti.sub.2, the first reflective layer of Al.sub.75Ti.sub.25
with the second reflective layer of Al.sub.99Ti.sub.1, or the like
combinations in which the first reflective layer and the second
reflective layer contain the same main component element and the
second reflective layer contains more elements other than the main
component element Al than the first reflective layer. The
combination of Al--Ti with Al--Ti, the combination of Al--Cr with
Al--Cr or the like combination containing an Al alloy such as
Al--Ag or Al--Cu as a main component has produced a similar
Characteristic. Following these materials, an Au alloy, Ag alloy,
Cu alloy or a material of a similar composition has exhibited an
improved rewrite characteristic after a multiplicity of overwrite
cycles.
[0234] (Substrate)
[0235] According to this embodiment, a polycarbonate substrate 1
having a tracking groove directly formed in the surface thereof is
used. In place of this, polyolefin, epoxy, acrylic resin or a
chemically reinforced glass having the surface thereof formed with
an ultraviolet setting resin layer may be used with equal
effect.
[0236] The substrate having a tracking groove is defined as a
substrate having a groove at least .lambda./10n' deep (n':
refractive index of substrate material), where .lambda. is the
recording/reproduction wavelength, formed over the whole or in a
part of the substrate surface. The groove may be formed either
continuously around the whole periphery or in a form split midway.
It has been found that crosstalks are reduced desirably for the
groove depth of about .lambda./6n'. Further, in the case where the
groove is deeper than about .lambda./3n', the cross erase is
desirably reduced at the sacrifice of a lower yield of the process
for forming the substrate.
[0237] Also, the groove width may be different at different places.
Further, a substrate of sample servo format lacking a groove, or a
substrate of other tracking systems or other formats will do. The
substrate is either in a format capable of recording and
reproduction in both the groove and the land, or in a format
capable of recording only in the groove or the land. The disk size
is not limited to 12 cm but may assume other sizes such as 13 cm,
3.5' or 2.5'. The disk may be 1.2 mm, 0.8 mm thick or otherwise
thick as well as 0.6 mm thick.
[0238] According to this embodiment, two disk members are
fabricated by exactly the same method, are these disk members are
attached to each other with the second reflective layers 9, 9'
thereof face to face through an adhesive layer. In place of the
second disk member, however, a disk member of another structure or
a protective substrate may be used. In the case where the disk
members used for attachment or the protective substrate has a large
transmittance in the ultraviolet wavelength area, the ultraviolet
setting resin may be used for attachment. Other methods may also be
used for attachment. In the disk member of a structure lacking the
second reflective layer 9, an adhesive layer may be formed on the
topmost layer.
[0239] In this embodiment, two disk members are prepared, and the
first and second disk members are attached to each other with the
second reflective layers 9, 9' thereof face to face through the
adhesive layer 10. If an ultraviolet setting resin is coated to the
thickness of about 10 .mu.m on the second reflective layers 9, 9'
before attaching them and they are attached to each other after the
resin is set, then the error rate can be reduced further.
[0240] Also, according to this embodiment, two disk members are
prepared, and the first and second disk members are attached to
each other with the second reflective layers 9 thereof face to face
through the adhesive layer 10. Without so attaching, however, the
ultraviolet setting resin about 10 .mu.m thick may be coated on the
second reflective layer 9 of the first disk member.
[0241] In the case of a disk member lacking the second reflective
layer 9, on the other hand, the ultraviolet setting resin may be
coated on the topmost layer.
[0242] (Disk Structure)
[0243] The structures of the disks 1 to 39 described below other
than the structure described above in this embodiment also have the
effect of reducing the jitter by reducing the remanence due to the
presence of an absorption control layer. The materials, thickness,
etc. of each layer are described in detail in the embodiments 1, 3,
4, 5. Also, the disk 3 is described in detail in embodiment 3, the
disk 4 in embodiment 6, and the disk 24 in embodiment 5. Among
these structures, in the case where the number of layers is 5 or 6
except for the substrate and the adhesive layer, the apparatus used
for producing a film is inexpensive and the whole production cost
can be reduced.
[0244] Disk 1: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5, upper
surface protect layer 6, absorption control layer 7, first
reflective layer 8, adhesive layer 10
[0245] Disk 2: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5, upper
surface protect layer 6, absorption control layer 7, second
reflective layer 9, adhesive layer 10
[0246] Disk 3: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5, upper
surface protect layer 6, absorption control layer 7, adhesive layer
10
[0247] Disk 4: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5,
absorption control layer 7, first reflective layer 8, second
reflective layer 9, adhesive layer 10
[0248] Disk 5: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5,
absorption control layer 7, first reflective layer 8, adhesive
layer 10
[0249] Disk 6: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0250] Disk 7: Substrate 1, heat diffusion layer 2, protective
layer 3, lower surface protect layer 4, recording film 5,
absorption control layer 7, adhesive layer 10
[0251] Disk 8: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, second
reflective layer 9, adhesive layer 10
[0252] Disk 9: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, adhesive
layer 10
[0253] Disk 10: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, upper surface protect layer 6,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0254] Disk 11: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, upper surface protect layer 6,
absorption control layer 7, adhesive layer 10
[0255] Disk 12: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, absorption control layer 7, first
reflective layer 8, second reflective layer 9, adhesive layer
10
[0256] Disk 13: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, absorption control layer 7, first
reflective layer 8, adhesive layer 10
[0257] Disk 14: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, absorption control layer 7, first
reflective layer 8, second reflective layer 9, adhesive layer
10
[0258] Disk 15: Substrate 1, heat diffusion layer 2, protective
layer 3, recording film 5, absorption control layer 7, adhesive
layer 10
[0259] Disk 16: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, second
reflective layer 9, adhesive layer 10
[0260] Disk 17: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, adhesive
layer 10
[0261] Disk 18: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0262] Disk 19: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, adhesive layer 10
[0263] Disk 20: Substrate 1, heat diffusion layer 2, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, second
reflective layer 9, adhesive layer 10
[0264] Disk 21: Substrate 1, heat diffusion layer 2, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, adhesive
layer 10
[0265] Disk 22: Substrate 1, heat diffusion layer 2, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0266] Disk 23: Substrate 1, heat diffusion layer 2, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, adhesive layer 10
[0267] Disk 24: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, second
reflective layer 9, adhesive layer 10
[0268] Disk 25: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, first reflective layer 8, adhesive
layer 10
[0269] Disk 26: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0270] Disk 27: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, upper surface protect layer 6,
absorption control layer 7, adhesive layer 10
[0271] Disk 28: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, absorption control layer 7,
first reflective layer 8, second reflective layer 9, adhesive layer
10
[0272] Disk 29: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, absorption control layer 7,
first reflective layer 8, second reflective layer 9, adhesive layer
10
[0273] Disk 30: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, absorption control layer 7,
first reflective layer 8, adhesive layer 10
[0274] Disk 31: Substrate 1, protective layer 3, lower surface
protect layer 4, recording film 5, absorption control layer 7,
adhesive layer 10
[0275] Disk 32: Substrate 1, protective layer 3, recording film 5,
upper surface protect layer 6, absorption control layer 7, first
reflective layer 8, second reflective layer 9, adhesive layer
10
[0276] Disk 33: Substrate 1, protective layer 3, recording film 5,
upper surface protect layer 6, absorption control layer 7, second
reflective layer 9, adhesive layer 10
[0277] Disk 34: Substrate 1, protective layer 3, recording film 5,
upper surface protect layer 6, absorption control layer 7, first
reflective layer 8, adhesive layer 10
[0278] Disk 35: Substrate 1, protective layer 3, recording film 5,
upper surface protect layer 6, absorption control layer 7, adhesive
layer 10
[0279] Disk 36: Substrate 1, protective layer 3, recording film 5,
absorption control layer 7, first reflective layer 8, second
reflective layer 9, adhesive layer 10
[0280] Disk 37: Substrate 1, protective layer 3, recording film 5,
absorption control layer 7, second reflective layer 9, adhesive
layer 10
[0281] Disk 38: Substrate 1, protective layer 3, recording film 5,
absorption control layer 7, first reflective layer 8, adhesive
layer 10
[0282] Disk 39: Substrate 1, protective layer 3, recording film 5,
absorption control layer 7, adhesive layer 10
[0283] (Thickness and Material of Each Layer)
[0284] The recording/reproduction characteristic is improved simply
by employing a preferable range of the thickness and the material
of each layer independently. By combining preferable ranges,
however, the effect is further enhanced.
[0285] (2) Embodiment 2
[0286] (Configuration and Fabrication Method)
[0287] For clarifying the effects of the absorption control layer,
a disk-type information recording medium of a structure lacking the
absorption control layer was prepared. FIG. 2 is a sectional view
showing the structure of this medium. This medium is fabricated in
the following way.
[0288] First, a heat diffusion layer 2 of Al.sub.2O.sub.3 film
about 30 nm thick was formed on a polycarbonate substrate 12 cm in
diameter, 0.6 mm thick and having a tracking groove in the surface
thereof. Then, a protective film 3 of ZnS about 45 nm thick, a
lower surface protect layer 4 of SiO.sub.2 film about 5 nm thick, a
recording film 5 of Ge.sub.14Sb.sub.29Te.sub.57 about 15 nm thick,
an upper surface protect layer 6 of SiO.sub.2 film about 15 nm
thick and a reflective layer 11 of Al.sub.98Ti.sub.2 about 200 nm
thick were sequentially formed, and two disk members prepared in
similar manner are attached to each other thereby to produce a
disk-type information recording medium shown in FIG. 2.
[0289] (Recording/reproduction Ccharacteristic)
[0290] The initial crystallization, recording, erasure and
reproduction are performed in the same manner as in embodiment 1.
With an information recording medium having a configuration lacking
an absorption control layer according to this embodiment, the
overwrite operation, if performed when repeating the recording and
reproduction, increases the jitter considerably as compared with
the information recording medium as described in embodiment 1, as
shown in FIG. 4.
[0291] This indicates that the absence of the absorption control
layer increases the jitter after 10 overwrite cycles. This
increased jitter probably stems from the fact that the absorption
coefficient cannot be sufficiently controlled due to the absorption
ratio (Ac/Aa) as small as about 0.9, thereby causing a remanence.
The remanence is more liable to occur when the linear speed is
high.
[0292] (3) Embodiment 3
[0293] An information recording medium was prepared as follows in
the same manner as in embodiment 1 except that the first reflective
layer 8 of embodiment 1 is eliminated. Specifically, in the
information recording medium according to embodiment 3, a heat
diffusion layer 2 of Al.sub.2O.sub.3 film about 30 nm thick was
formed on a polycarbonate substrate 1 having a diameter of 12 cm
and a thickness of 0.6 mm with a tracking groove formed in the
surface thereof. Then, a protective film of ZnS film about 45 nm
thick, a lower surface protect layer 4 of SiO.sub.2 film about 5 nm
thick, a recording film 5 of Ge.sub.14Sb.sub.29Te.sub.17 about 15
nm thick, an upper surface protect layer 6 of SiO.sub.2 film about
15 nm thick, an absorption control layer 7 of
Mo.sub.80(SiO.sub.2).sub.20 film about 18 nm thick, and a second
reflective layer 9 of Al.sub.98Ti.sub.2 film about 180 nm thick,
were sequentially formed. Two disk members prepared in a similar
manner were attached to each other thereby to produce a disk-type
information recording medium as shown in FIG. 5.
[0294] With the disk according to this embodiment, the first
reflective layer is eliminated and therefore the time for preparing
the disk could be shortened by the time corresponding to one layer
as compared with the disk of embodiment 1.
[0295] (Recording/reproduction Characteristic)
[0296] The recording/reproduction characteristic was studied in the
same manner as in embodiment 1. As a result, the same effect of the
absorption control layer was obtained as in embodiment 1.
[0297] In addition, in view of the fact that the second reflective
layer having a large heat conductivity is in direct contact with
the absorption control layer, the heat is easily lost and the cross
erase could be reduced. The cross erase is a phenomenon in which
assuming that the recording is made in a track (T2) adjacent to a
track (T1) where the mark is recorded, the signal amount is reduced
due to the disappearance of the recording mark written in the track
(T2) by the heat generated at the time of recording. On the other
hand, the jitter at the front edge after 10 overwrite cycles
increased by 3%.
[0298] The matter not described in this embodiment is similar to
the corresponding matter in embodiment 1.
[0299] (4) Embodiment 4
[0300] With the exception that the composition of the recording
films 5, 5' of embodiment 1 was changed in the following manner, an
information recording medium having the following composition of
the recording film was prepared in the same manner as in embodiment
1. The initial crystallization, recording, erasure and reproduction
were carried out in the same manner as in embodiment 1.
[0301] (Composition of recording film)
[0302] The composition of the recording films 5, 5' used in this
embodiment are changed along the lines connecting GeTe and
Sb.sub.2Te.sub.3 in a triangular diagram, and the jitter
(.sigma./Tw) was measured after ten overwrite cycles with the
following result.
8 Recording film Jitter at Jitter at composition front edge (%)
trailing edge (%) Ge.sub.8Sb.sub.34Te.sub- .58 23 --
Ge.sub.10Sb.sub.32Te.sub.58 18 -- Ge.sub.12Sb.sub.27Te.sub.56 15 14
Ge.sub.14Sb.sub.29Te.sub.57 14 14 Ge.sub.24Sb.sub.21Te.sub.55 14 15
Ge.sub.26Sb.sub.19Te.sub.55 -- 18 Ge.sub.28Sb.sub.17.5Te.sub.54.5
-- 22
[0303] This indicates that with the increase in Ge amount, the
jitter at the front edge is reduced while the jitter at the
trailing edge is increased. Thus, the Ge amount exhibiting a
superior jitter characteristic is in the range of not less than 10
atomic % but not more than 26 atomic %, and a better characteristic
is exhibited in the range of not less than 12 atomic % but not more
than 24 atomic %.
[0304] Then, while maintaining a constant Te amount in the
composition of the recording film and changing the Te and Sb
amounts, the jitter (.sigma./Tw) after 10 overwrite cycles was
measured with the following result.
9 Recording film Jitter at Jitter at composition front edge (%)
trailing edge (%) Ge.sub.27Sb.sub.16Te.su- b.57 -- 22
Ge.sub.25Sb.sub.18Te.sub.57 -- 18 Ge.sub.23Sb.sub.20Te.sub.57 15 15
Ge.sub.14Sb.sub.29Te.sub.57 14 14 Ge.sub.12Sb.sub.31Te.sub.57 14 15
Ge.sub.10Sb.sub.33Te.sub.57 18 -- Ge.sub.8Sb.sub.35Te.sub.57 23
--
[0305] This indicates that with the increase in Sb amount, the
jitter at the front edge is increased while the jitter at the
trailing edge is reduced. Thus, the Sb amount exhibiting a superior
jitter characteristic is in the range of not less than 18 atomic %
but not more than 33 atomic %, and a better characteristic is
exhibited in the range of not less than 20 atomic % but not more
than 31 atomic %.
[0306] Then, while maintaining a constant Sb amount in the
composition of the recording films 5, 5' and changing the Te and Ge
amounts, the jitter (.sigma./Tw) after 10 overwrite cycles was
measured with the following result.
10 Recording film composition Jitter at trailing edge (%)
Ge.sub.9Sb.sub.29Te.sub.62 23 Ge.sub.11Sb.sub.29Te.sub.60 18
Ge.sub.13Sb.sub.29Te.sub.58 15 Ge.sub.14Sb.sub.29Te.sub.57 14
Ge.sub.17Sb.sub.29Te.sub.54 15 Ge.sub.19Sb.sub.29Te.sub.52 18
Ge.sub.20Sb.sub.29Te.sub.5- 1 22
[0307] This indicates that the jitter at the trailing edge
increases regardless of whether the Te amount is increased or
decreased. Thus, the Te amount associated with a superior jitter
characteristic is in the range of not less than 52 atomic % but not
more than 60 atomic %, and a better jitter characteristic is
exhibited in the range of not less than 54 atomic % but not more
than 58 atomic %.
[0308] In this embodiment, by adding Ag to the recording film to
form an Ag--Ge--Sb--Te recording film, it has been found that as
compared with the Ge--Sb--Te, the number of a multiplicity of
overwrite cycles at which the jitter at the front edge increases by
5% or more increases twice. In view of this, while maintaining
constant Sb and Te amounts in the composition of the recording
films 5, 5' and changing the Ge and Ag amounts, the jitter
(.sigma./Tw) after 5 overwrite cycles was measured with the
following result. Also, the number of overwrite cycles at which the
jitter increases at least 5% was studied.
11 Recording film Number of rewrite composition Jitter (%)
operations Ge.sub.14Sb.sub.29Te.sub.57 14 One time
Ag.sub.1Ge.sub.13Sb.sub.29Te.sub.57 14 1.5 times
Ag.sub.2Ge.sub.12Sb.sub.29Te.sub.57 15 Twice
Ag.sub.4Ge.sub.10Sb.sub.29Te.sub.57 15 Twice
Ag.sub.6Ge.sub.8Sb.sub.29Te.sub.57 20 -- Ag.sub.8Ge.sub.6Sb.sub.2-
9Te.sub.57 24 --
[0309] This indicates that addition of a small amount of Ag
improves the number of possible overwrite cycles. With the increase
in Ag amount, however, it has been found that the jitter also
increases. Thus, the Ag amount associated with a superior jitter
characteristic is in the range of not more than 6 atomic % and a
better jitter characteristic is exhibited in the range of not more
than 4 atomic %.
[0310] From the foregoing description, it is seen that in the case
where the composition of the recording film is expressed as
Ge.sub.x-wSb.sub.yTe.sub.zM.sub.w (x+y+z=1), a superior
characteristic is exhibited in the range of
0.10.ltoreq.x.ltoreq.0.26, 0.18.ltoreq.y.ltoreq.0.33,
0.52.ltoreq.z.ltoreq.0.60, 0.ltoreq.w.ltoreq.0.06. A better
characteristic is exhibited in the range of
0.12.ltoreq.x.ltoreq.0.24, 0.20.ltoreq.y.ltoreq.0.31,
0.54.ltoreq.z.ltoreq.0.58, 0.ltoreq.w.ltoreq.0.04.
[0311] Further, when the Ge amount reaches 20 atomic % or more in
this range, the read light endurance is improved by 1.5 times. The
read light endurance is determined by comparison with the power of
the read light at which the recording signal is reduced by at least
2 dB during a five-minute reproduction. Also, in the case where the
Ge amount is not more than 17 atomic % and the linear speed is
high, the extinction ratio is large. Even with the linear speed of
12 m/s, the extinction ratio was as superior as not less than 30
dB.
[0312] Elements added to the recording film in place of Ag include
Na, Mg, Al, P, S, Cl, L, Ca, Sc, Zn, Ga, As, Se, Br, Rb, Sr, Y, Zr,
Nb, Ru, Rh, Cd, In, Sn, I, Cs, Ba, La, Hf, Ta, Re, Os, Ir, Hg, Tl,
Pb, Th, U, Cr, W, Mo, Pt, Co, Ni, Pd, Si, Au, Cu, V, Mn, Fe, Ti and
Bi. It has been found that even when Ag is replaced by at least one
of these elements, the jitter is not easily increased after many
overwrite cycles.
[0313] Among these elements, the addition of Ag increases the
recording sensitivity by 10% as compared with Ge--Sb--Te, the
addition of at least one of Cr, W and Mo at least triples the
number of a multiplicity of overwrite cycles at which the jitter
increases 5% or more, and the addition of at least one of Pt, Co
and Pd increases the crystallization temperature by 50.degree. C.
or more as compared with Ge--Sb--Te.
[0314] Also, it has been found that when the impurities elements
contained in the recording film exceeds 2 atomic % of the recording
film components, the jitter at the front edge or the trailing edge
after 10 overwrite cycles exceeds 15%. It has further been found
that in the case where the impurities elements exceeds 5 atomic %,
the jitter increases to at least 18%. Thus, a desirable content of
the impurities elements in the recording film is not more than 5
atomic % of the recording film components at which the
deterioration of the rewrite characteristic is not reduced so much.
The content of not more than 2 atomic % is more desirable.
[0315] While changing the thickness of the recording films 5, 5'
used in this embodiment, the jitter (a/Tw) after 10 overwrite
cycles and 100 thousand overwrite cycles was measured and the
following result was obtained. For each recording film thickness
(nm), the value of jitter (%) at the front edge or the trailing
edge, whichever is worse, is shown for the characteristic after 10
overwrite cycles, and the value of jitter (%) at the front edge is
shown for the characteristic after 100 thousand overwrite
cycles.
12 Recording film Jitter after Jitter after thickness 10 rewrites
10.sup.5 rewrites 8 23 -- 10 20 -- 13 15 15 18 14 15 20 15 15 30 --
20 40 -- 25
[0316] This indicates that with the decrease in the thickness of
the recording film, the jitter after ten overwrite cycles is
increased due to the flow or segregation of the recording film,
while with the increase in the thickness of the recording film, the
jitter after 100 thousand overwrite cycles increases. It is thus
seen that the desirable thickness of the recording film is in the
range of not less than 10 nm but not more than 30 nm, or more
desirably in the range of not less than 13 nm but not more than 20
nm.
[0317] Though somewhat time consuming for fabricating the recording
film, it has been found that mixing nitrogen with the sputtering
gas in the initial or last stage of the recording film fabrication,
using a target containing a slight amount of nitrogen in the
composition of the recording film or otherwise adding nitrogen in
the neighborhood of the boundary between the recording film and
other layers, the adhesive amount is increased for an improved
characteristic.
[0318] The matters not described in this embodiment are similar to
the corresponding matters in embodiments 1 and 3.
[0319] (5) Embodiment 5
[0320] (Configuration and Fabrication Method)
[0321] The following information recording medium was fabricated in
a manner similar to embodiment 1 except that in this embodiment,
the heat diffusion layer 2 is not included.
[0322] Specifically, a protective layer 3 of ZnS film about 45 nm
thick, a lower surface protect layer 4 of SiO.sub.2 film about 5 nm
thick, a recording film 5 of Ge.sub.14Sb.sub.29Te.sub.17 about 15
nm thick, an upper surface protect layer 6 of SiO.sub.2 film about
15 nm thick, an absorption control layer 7 of
Mo.sub.80(SiO.sub.2).sub.20 film about 18 nm thick, a first
reflective layer 8 of Al.sub.89Ti.sub.11 film about 20 nm thick and
a second reflective film 9 of Al.sub.98Ti.sub.2 about 180 nm were
formed in that order sequentially. Two disk members fabricated in
the same manner are attached to each other thereby to produce a
disk-type information recording medium shown in FIG. 6.
[0323] Also, with the disk according to this embodiment, in which
the heat diffusion layer is not included, the disk fabrication time
could be shortened by a length equivalent to one layer as compared
with the disk of embodiment 1. The heat diffusion layer takes a
long time to form, and therefore the absence of the heat diffusion
layer could reduce the total fabrication time by about one
fourth.
[0324] (Recording/reproduction Characteristic)
[0325] The recording/reproduction characteristic was studied by the
same method as in embodiment 1. As a result, the effect of the
absorption control layer similar to that of embodiment 1 was
obtained.
[0326] In addition, the temperature rise of the substrate which has
been suppressed by the presence of the heat diffusion layer
increases the jitter after 100 thousand overwrite cycles to 18% as
compared with the case having the heat diffusion layer.
Substantially no difference is observed for ten thousand or less
overwrite cycles.
[0327] Those matters not described in this embodiment are similar
to the corresponding matters in embodiments 1, 3 and 4.
[0328] (6) Embodiment 6
[0329] (Configuration and Fabrication Method)
[0330] An information recording medium was fabricated as follows in
the same manner as in embodiment 1 except =that the first
reflective layer 8 and the second reflective layer 9 were not
included.
[0331] Specifically, in the information recording medium according
to embodiment 6, a heat diffusion layer 2 of Al.sub.2O.sub.3 film,
a protective layer 3 of ZnS film about 45 nm thick, a lower surface
protect layer 4 of SiO.sub.2 film about 5 nm thick, a recording
film of Ge.sub.14Sb.sub.29Te.sub.57 about 15 nm thick, an upper
surface protect layer 6 of SiO.sub.2 film about 15 nm thick and an
absorption control layer 7 of Mo.sub.80(SiO.sub.2).sub.20 film
about 18 nm thick, were formed sequentially in that order on a
polycarbonate substrate 1 having a diameter of 12 cm and a
thickness of 0.6 mm with the surface thereof formed with a tracking
groove. Two disk members prepared in the same manner are attached
to each other thereby to produce a disk-type information recording
medium shown in FIG. 7.
[0332] Also, the absence of the heat diffusion layer in the disk
according to this embodiment could reduce the disk fabrication time
by a time corresponding to two layers as compared with the time
required in embodiment 1. As a result, the total production time
could be shortened considerably to about two thirds.
(Recording/reproduction characteristic) The recording/reproduction
characteristic was studied in the same manner as in embodiment 1,
and it was found that as in embodiment 1, the jitter value can be
reduced as compared with a disk structure having no absorption
control layer.
[0333] (Thickness of Absorption Control Layer)
[0334] In the case where the reflective layer is lacking as in this
embodiment, it has been found that the thickness of the absorption
control layer not less than 50 nm is sufficient for maintaining a
high strength. Also, the thickness of not more than 200 nm is
desirable for shortening the fabrication time.
[0335] The matters not described with this embodiment are similar
to the corresponding matters in embodiments 1 and 3 to 4.
[0336] As described above, according to the present invention,
there is provided an information recording medium comprising an
information recording thin film formed on a substrate as a
recording layer for recording and/or reproducing the information by
the change of atomic arrangement caused by radiation of light, and
at least one protective layer, characterized in that the protective
layer and the recording layer are formed in that order from the
light incidence side, the medium further comprising at least one
absorption control layer, whereby the jitter (.sigma./Tw) at the
time of overwrite operation can be reduced as compared with the
information recording medium lacking the absorption control layer.
This is by reason of the fact that the remanence can be reduced due
to the presence of the absorption control layer.
[0337] The absorption control layer is composed of a metal element
or an unsaturated metal oxide and a dielectric material, and has
the function of controlling the absorption coefficient Ac of the
recording film in crystal state to not less than the absorption
coefficient Aa of the recording medium in amorphous state.
[0338] Further, in the case where the thickness of the absorption
control layer is set in the range of not less than 5 nm but not
more than 50 nm, the jitter after 10 overwrite cycles can be
reduced with an increased modulation degree.
[0339] The surface protect layer is formed in the boundary of the
recording medium and has the effect of increasing the
crystallization rate of the recording film and improving the
extinction characteristic thereof. The protective layer is formed
between the recording film and substrate, and has the effect of
protecting the recording film. The heat diffusion layer, which is
made of a material having a larger heat conductivity than the
substrate and formed directly on the substrate, has the effect of
preventing the temperature increase of the substrate at the time of
a multiplicity of overwrite cycles thereby to reduce the increase
in jitter. The first reflective layer has the effect of preventing
the increase in jitter at the front edge, and the second reflective
layer, due to its large heat conductivity, has the effect of
reducing the increase in jitter after a multiplicity of overwrite
cycles. Further, both the first reflective layer and the second
reflective layer prevent the light from being transmitted through
the medium and thus improve the recording sensitivity.
* * * * *