U.S. patent application number 11/030038 was filed with the patent office on 2005-06-02 for information recording medium and method for manufacturing the same.
This patent application is currently assigned to MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.. Invention is credited to Kawahara, Katsumi, Kojima, Rie, Matsunaga, Toshiyuki, Yamada, Noboru.
Application Number | 20050119123 11/030038 |
Document ID | / |
Family ID | 26409379 |
Filed Date | 2005-06-02 |
United States Patent
Application |
20050119123 |
Kind Code |
A1 |
Yamada, Noboru ; et
al. |
June 2, 2005 |
Information recording medium and method for manufacturing the
same
Abstract
An information recording medium having such a recording material
layer on a substrate where reversible phase change between
electrically or optically detectable states can be caused by
electric energy or electromagnetic energy. The recording material
forming the recording layer is either a material having a crystal
structure including lattice defects in one phase of the reversible
phase change or a material having a complex phase composed of a
crystal portion including a lattice defect in one phase of the
reversible phase change and an amorphous portion. Both portions
contain a common element. A part of the lattice defects are filled
with an element other than the element constituting the crystal
structure. The recording medium having a recording thin film
exhibits little variation of the recording and reproduction
characteristics even after repetition of recording and
reproduction, excellent weatherability, strong resistance against
composition variation, and easily controllable characteristics.
Inventors: |
Yamada, Noboru;
(Hirakata-shi, JP) ; Kojima, Rie; (Kadoma-shi,
JP) ; Matsunaga, Toshiyuki; (Kadoma-shi, JP) ;
Kawahara, Katsumi; (Kadoma-shi, JP) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Assignee: |
MATSUSHITA ELECTRIC INDUSTRIAL CO.,
LTD.
Kadoma-shi
JP
|
Family ID: |
26409379 |
Appl. No.: |
11/030038 |
Filed: |
January 4, 2005 |
Current U.S.
Class: |
503/201 ;
257/E45.002; G9B/11; G9B/11.007; G9B/11.057; G9B/13; G9B/7.143;
G9B/7.194; G9B/9; G9B/9.024 |
Current CPC
Class: |
G11B 7/2585 20130101;
G03G 5/02 20130101; G11B 7/26 20130101; H01L 45/06 20130101; G11B
2007/24316 20130101; G11B 2007/25713 20130101; G11C 13/04 20130101;
H01L 45/1683 20130101; G11B 13/00 20130101; G11B 9/08 20130101;
G11B 2007/24322 20130101; G11B 2007/24312 20130101; G11B 9/04
20130101; G11B 2007/24308 20130101; H01L 45/1625 20130101; H01L
45/1233 20130101; H01L 45/144 20130101; G11B 7/2433 20130101; G11C
2029/0403 20130101; G11B 11/00 20130101; G11B 2007/24314 20130101;
G11C 13/0004 20130101; G11B 11/08 20130101; G11B 7/266 20130101;
G11B 2007/2571 20130101; G11B 9/00 20130101; G11B 11/12 20130101;
G11B 2007/24306 20130101; G11B 2007/2431 20130101; Y10T 428/21
20150115; G11B 7/243 20130101 |
Class at
Publication: |
503/201 |
International
Class: |
B41M 005/20 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 15, 1999 |
JP |
11-068146 |
Oct 15, 1999 |
JP |
11-293292 |
Claims
1-23. (canceled)
24. A method for manufacturing an information recording medium
having a recording material layer on a substrate, where reversible
phase change between electrically or optically detectable states is
caused by electric energy or electromagnetic energy, wherein the
recording layer is formed by using a recording material in which
one phase of the reversible phase change comprises a lattice
defect, and at least a part of the defect is filled with an
additional element.
25. The method for manufacturing an information recording medium
according to claim 24, wherein after formation of the recording
layer an element comprising the crystal lattice is deposited
outside the lattice by the additional element.
26. The method for manufacturing an information recording medium
according to claim 24, wherein the recording layer is formed by
sputtering, and a sputtering target used in the sputtering
comprises an element constituting the crystal structure and the
additional element.
27. The method for manufacturing an information recording medium
according to claim 26, wherein a gas used in the sputtering
comprises at least one gas selected from N.sub.2 gas and O.sub.2
gas.
Description
TECHNICAL FIELD
[0001] The present invention relates to an information recording
medium that can record, reproduce, erase and rewrite high-density
information by means of irradiation of laser beams and application
of a high electric field. The present invention relates to also a
method for manufacturing the information recording medium.
BACKGROUND ART
[0002] It is well known to apply as a memory a change in optical
characteristics caused by reversible phase change of a substance,
and a technique using this has come into practice as phase change
optical disks such as DVD-RAM. Specifically, recording, reproducing
and rewriting of signals will be available by rotating a disk
medium comprising a substrate on which a recording thin film for
generating reversible phase change is provided, and by irradiating
the disk medium with a laser beam drawn to a sub-micron size. In
the case of a phase change optical disk, overwriting by means of a
single laser beam is carried out. That is, irradiation is performed
by modulating the laser power between a high level and a low level
depending on the information signal, so that an amorphous phase is
generated at a region irradiated with a high power laser beam while
a crystalline phase is generated at a region irradiated with a low
power laser beam. As a result, a signal array comprising the
amorphous portion and crystal portion alternately is recorded on
the disk. Since the amorphous portion and the crystal portion are
different in the light transmittance and reflectance, the change in
the state can be read as a change in the amount of the light
transmittance or reflectance by continuously irradiating a laser
beam on this signal array, in which the laser beam is attenuated
not to change the recording film.
[0003] Such a phase change optical disk has some characteristics
such as:
[0004] (1) it enables the performance of overwriting, i.e.,
recording a new signal while erasing an old signal by using only
one laser beam; and
[0005] (2) it can record and reproduce a signal by using a change
in the reflectance, based on a principle similar to that of a ROM
medium. These characteristics lead to several merits such as
simplifying a system construction and providing devices for general
purposes, so that such phase change optical disks are expected to
be applied widely.
[0006] Recording materials used for recording layers of phase
change optical disks generally include chalcogenide semiconductor
thin films based on chalcogen elements such as Te, Se and S. A
method used in the early 1970s is crosslinking a Te network
structure for stabilizing an amorphous state by adding materials
such as Ge, Si, As and Sb to a main component of Te. However, these
materials would cause a problem. That is, when the crystallization
temperature is raised, the crystallization speed is lowered
remarkably, and this would make rewriting difficult. Alternatively,
when the crystallization speed is increased, the crystallization
temperature is lowered sharply, and thus, the amorphous state will
be unstable at a room temperature. A technique suggested for
solving the problems in the latter half of the 1980s is the
application of a stoichiometric compound composition. The thus
developed compositions include Ge--Sb--Te based materials,
In--Sb--Te based materials, and GeTe based materials. Among them,
Ge--Sb--Te based materials have been studied most since the
materials allow phase change at high speed, substantially no holes
will be formed even after repeated phase changes, and substantially
no phase separation or segregation will occur (N. Yamada et al,
Jpn. J. Appl. Phys. 26, Suppl. 26-4, 61 (1987)). An example of
material compositions other than such stoichiometric compositions
is an Ag--In--Sb--Te based material. Though this material is
reported to be excellent in the erasing performance, it has been
found that the characteristics deteriorate due to the phase
separation as a result of repeated overwriting.
[0007] Similarly, characteristic deterioration caused by repetition
may be observed even if a stoichiometric composition is used. An
example of the deterioration mechanism is a phenomenon of
micro-scaled mass transfer caused by repetition of overwriting.
More specifically, overwriting causes a phenomenon that substances
composing a recording film flow little by little in a certain
direction. As a result, the film thickness will be uneven at some
parts after a big repetition. Techniques to suppress the phenomenon
include the addition of additives to recording layers. An example
of such techniques is addition of a N.sub.2 gas at a time of film
formation (JP-A-4-10979). A document clarifies a mechanism that a
nitride having a high melting point is deposited like a network in
a grain boundary composing the recording film, and this suppresses
the flow (R. Kojima et al. Jpn. J. Appl. Phys. 37 Pt. 1, No. 4B.
2098 (1998)).
[0008] JP-A-8-127176 suggests a method of including a material
having a melting point higher than that of the recording
material.
[0009] As mentioned later, the cited reference is distinguishable
from the present invention in that the material having a high
melting point will not be dissolved in the base material but
scattered in the base material layer. According to the reference,
the scattered material having a high melting point suppresses the
mass transfer phenomenon caused by repeated overwriting so as to
improve the performance. JP-A-7-214913 suggests, without clarifying
the mechanism, the addition of small amounts of Pt, Au, Cu, and Ni
in a Ge--Sb--Te film in order to improve stability of the amorphous
phase without lowering the repeatability.
[0010] However, the repetition number tends to decrease when the
recording density is increased. Due to a recent demand for keeping
compatibility among media of various generations, recording at
higher density should be performed by using optical heads of
identical performance (i.e., laser beams of an identical wavelength
and object lenses of an identical numerical aperture). The size of
a recording mark should be reduced to raise recording density. On
the other hand, the strength of reproduced signals is lowered as
the recording mark becomes small, and the signals will be
influenced easily by a noise. Namely, during a repeated recording,
even a slight variation that may have not caused a trouble in a
conventional process will lead to errors in reading, and thus, the
number of available repetitions of rewriting is decreased
substantially. This problem can be noticeable in the a case of
so-called land-groove recording, in which a concave-convex-shaped
groove track is formed on a substrate and information is recorded
on both the groove (a region closer to the light-incident side) and
the land portion (spacing between the grooves) in order to guide a
laser beam for recording and reproducing. Specifically, since the
thermal and optical conditions are different between the land and
groove, the repeatability will deteriorate easily, especially in
the land region.
[0011] Merits provided by a recording layer comprising a compound
material have been described above. On the other hand, when the
composition of the recording layer is changed from the
stoichiometric composition, the recording performance will be
changed remarkably. In a desirable recording method, the
performance of a recording film should be controlled with further
accuracy while keeping the merits of the compound composition, and
using an identical recording film or a composition having a wide
acceptability with respect to characteristics.
[0012] Electrical switching devices comprising a chalcogenide
material and memory devices are known as well as applications of
such phase change materials. The electrical phenomenon was first
reported in 1968. Specifically, when voltage is applied gradually
to a phase change material thin film in an as-depo.-state
sandwiched between electrodes, electrical resistance between the
electrodes sharply declines at a certain threshold voltage, and a
large current will start to flow (crystallization). For reversing
this state to an initial low-resistant state (OFF state), a big and
short current pulse will be passed. A portion provided with current
is melted first and then, quenched to be amorphous so that the
electrical resistance is increased. Since differences in the
electrical resistance can be detected easily by an ordinary
electrical means, the material can be used as a rewritable memory.
Though material compositions based on Te have been used for
electrical memories, any of them require a .mu.s order period of
time for crystallization.
DISCLOSURE OF INVENTION
[0013] To solve the above-mentioned problems, a first purpose of
the present invention is to provide a phase change memory material
that will increase a number of repetitions of rewriting and enables
rewriting at a high speed. The memory device can be constituted
with either an optical memory or an electric memory. The present
invention aims to provide a recording medium comprising a recording
thin film formed on a substrate. Due to the above-mentioned
excellent characteristics of stoichiometric composition, the
recording thin film provides less influence on the characteristics
regardless of some composition variation. That is, the recording
thin film comprises a composition exhibiting easy controllability
of the characteristics. The present invention provides also a
method for manufacturing a recording medium comprising such a
recording thin film.
[0014] For achieving the purposes, an information recording medium
according to the present invention comprises a recording material
layer formed on a substrate, and the recording material layer
enables the generation of reversible phase change by means of
electric energy or electromagnetic wave energy in an electrically
or optically detectable state. The information recording medium is
characterized in that the recording material layer is composed of
either a material having a crystal structure including lattice
defects in one phase of the reversible phase change (material `A`)
or a material in a complex phase comprising lattice defects in one
phase of the reversible phase change comprising a crystal portion
and an amorphous portion, and both the portions comprise a common
element (material `B`), and that at least one part of the
above-mentioned lattice defects is filled with an element other
than the elements composing the above-mentioned crystal
structure.
[0015] Next, a method for manufacturing an information recording
medium according to the present invention relates to an information
recording medium comprising a recording material layer formed on a
substrate, and the recording material layer generates reversible
phase change by means of electric energy or electromagnetic wave
energy in an electrically or optically detectable state. It is
characterized in that the recording layer is constituted with a
recording material having a crystal structure in which one phase of
the reversible phase change includes lattice defects, and that at
least a part of the defects is filled with additional elements.
[0016] The present invention employs the following material
compositions for generating reversible phase change between an
amorphous phase and a crystalline phase by irradiating the material
layer with a laser beam or energizing the same layer. The material
composition forms a single phase during crystallization and the
crystal lattice necessarily includes some defects. At least a part
of the lattice defects is filled with an element other than the
element composing the base material in order to exhibit a new
compound phase that has never been observed. Filling additional
elements in the lattice of the base material can change the
characteristics of the base material fundamentally.
[0017] For solving the above-mentioned problems, the present
invention employs an amorphous material layer to be crystallized by
irradiating a laser beam or by energizing. The material phase forms
a complex phase (crystalline phase) comprising a compound phase
portion having lattice defects within the crystal and an amorphous
phase portion. Here, it is important and preferred that the
compound phase portion is filled with additional elements, and the
amorphous phase is a single phase. It is preferable that a molar
ratio of the amorphous phase to the crystalline phase in the
complex phase is 2.0 at most, and further preferably, the ratio is
1.0 at most.
[0018] Regardless whether the crystalline phase is a single phase
or a complex phase, it is preferable that the compound comprises a
base material of rock-salt type structure (NaCl) having a crystal
structure with a lattice defect (vacancy). As mentioned above, at
least one part of the lattice defects included in the base material
is filled with an atom other than elements composing basic
substances of the rock-salt type structure. It is preferable for
the element to fill the lattice defects that Rim is closer to Rnc,
e.g., 0.7<Rim.ltoreq.1.05Rnc, where Rim denotes an ionic radius
of an element to fill the lattice defects, and Rnc denotes an ionic
radius of a smallest ion among elements composing the rock-salt
type crystal. When Tim denotes a melting point of an element to
fill the lattice defects and Tnc denotes a melting point of the
rock-salt type crystal, it is preferable that the Tim is closer to
Tnc, i.e., the relationship satisfies
.vertline.Tim-Tnc.vertline..ltoreq.- 100.degree. C. When Dim
denotes a concentration of an element added to fill the lattice
defects and Ddf denotes a concentration of the lattice defects in
the rock-salt type crystal, it is preferable that
Dim.ltoreq.Ddf.times.1.5. It is further preferable that
0.2.ltoreq.Dim.ltoreq.Ddf.
[0019] Specifically, the material is preferred to contain Te. A
substance to form the amorphous phase in the complex phase
comprises at least one of Sb, Bi, In, Ge and Si. At least a part of
the elements can comprise an oxide, a nitride, a fluoride, and a
nitride-oxide. It should be noted here that the compound phase and
the amorphous phase preferably contain a common element. For
example, when an element composing the crystalline phase is based
on three elements of Ge, Sb and Te, the amorphous phase is
preferred to contain Sb or Ge as a main component. Alternatively,
it is further preferable that the compound phase contains Ge, Sb
and/or Bi and Te while the amorphous phase contains Sb and/or Bi or
Ge. It is preferable that at least one element selected from Sn,
Cr, Mn, Pb, Ag, Al, In, Se and Mo is included in the crystalline
phase.
[0020] The element composing the rock-salt type crystal preferably
contains Ge and Te as its base materials, and further preferably,
it contains at least one element selected from Sb and Bi. It is
particularly preferable that the base material composition of the
rock-salt type crystal substantially corresponds to a
GeTe--Sb.sub.2Te.sub.3 quasibinary system composition, a
GeTe--Bi.sub.2Te.sub.3 quasibinary system composition or a mixture
thereof. When an element composing the rock-salt type crystal
contains Ge, Te, and Sb, or it contains Ge, Te, and Bi, the element
to fill the lattice defects is at least one selected from Al, Ag,
Pb, Sn, Cr, Mn and Mo. It is also preferable that the base material
composition of the rock-salt type crystal substantially corresponds
with (GeTe).sub.1-x(M.sub.2Te.sub.3).sub.x, in which
0.2.ltoreq.x.ltoreq.0.9 (M denotes at least one element selected
from Sb, Bi and Al, or an arbitrary mixture of these elements). It
is further preferable that 0.5.ltoreq.x.ltoreq.0.9. For improving
recording sensitivity, it is further preferable that the recording
film contains nitrogen (N) or oxygen (O). Preferably, the
concentration of the N atom (Dn) is 0.5 atom %.ltoreq.Dn.ltoreq.5
atom % since the range provides higher effects.
[0021] Filling Al, Cr or Mn in lattices is preferable to improve
repeatability, and addition of Ag is preferable to increase changes
in optical characteristics (signal amplitude change) between the
crystalline phase and the amorphous phase. Filling Sn or Pb is
effective in improving crystallization speed.
[0022] It is further effective to fill plural elements at the same
time in lattice defects for improving the characteristics. When the
material is based on Ge--Sb--Te or Ge--Bi--Te, both the
crystallization speed and the repeatability can be improved
preferably at the same time by, for example, using simultaneously
at least one of Sn and Pb together with Al, Cr or Mn. Otherwise,
simultaneous use of either Sn or Pb together with Ag is preferable
to improve the crystallization speed and the signal amplitude at
the same time. Using at least one of Al, Cr and Mn together with Ag
is preferable to improve repeatability and signal amplitude at the
same time. Furthermore, addition of at least one of Al, Cr and Mn,
at least either Sn or Pn together with Ag is preferable in
improving crystallization speed, signal amplitude and repeatability
at the same time.
[0023] Preferably, such a material layer is manufactured by
lamination such as vapor deposition and sputtering. Specifically,
it is further preferable that sputtering is carried out by using a
target including a component composing the rock-salt type crystal
and an element to fill the lattice defects. Preferably, the target
contains at least Ge and Te as elements for forming the rock-salt
type crystal, and further preferably, contains an element selected
from Al, Sb and Bi. Especially preferable elements to fill the
lattice defects include Ag, Sn, Pb, Al, Cr, In, Mn and Mo. It is
further preferable that sputtering is carried out in a gaseous
atmosphere containing Ar and N.sub.2. It is also preferable that
the sputtering gas contains at least one gas selected from N.sub.2
gas and O.sub.2 gas.
[0024] An optical information recording medium according to the
present invention can comprise a single layer medium prepared by
forming the above-mentioned recording material thin film on a
substrate. However, it is desirable to use a multilayer including
the recording layer. For example, it is preferable that a
protective layer is provided between the substrate and the
recording layer in order to reduce thermal damage in the substrate
or to utilize its optical interference effect. It is also
preferable to provide a protective layer to the opposing surface of
the recording layer as well in order to prevent deformation of the
recording layer and to utilize its optical interference effect. The
protective layer is made of a material that is stable thermally and
chemically, and transparent optically, such as an oxide, a sulfide,
a nitride, a nitride-oxide, a carbide, and fluoride. Examples of
the materials include ZnS, SiO.sub.2, ZnS--SiO.sub.2, SiNO, SiN,
SiC, GeN, Cr.sub.2O.sub.3, and A.sub.2O.sub.3. It is preferable to
provide a reflecting layer over the protective layer in order to
increase efficiency for laser beams or the like used for recording.
The reflecting layer can be a metallic material film or a
multilayer film combined with a dielectric material. The metallic
material can be Au, Al, Ag or an alloy based on these metals.
[0025] An electric information recording medium according to the
present invention can be constituted by laminating sequentially on
a substrate an electrode material, the above-mentioned material
thin film, and a further electrode material. Otherwise, such a
medium can be constituted by laminating the material thin film and
an electrode material on a metallic substrate that functions also
as an electrode.
[0026] Materials of the respective layers are formed by lamination
such as sputtering and vapor deposition similar to the case of an
optical information recording medium. Since an electric memory
system in the present invention causes variation in electrical
resistance, it can be used as a component for a variable
programmable circuit.
BRIEF DESCRIPTION OF DRAWINGS
[0027] FIG. 1 is a schematic view to show a structure (atom
position at a time of crystallization) of a representative
recording film used for an information recording medium of the
present invention, in which the crystalline phase is a single
phase. In this example, the crystalline phase is constituted with a
single compound phase (moreover, it is a rock-salt type structure).
In the lattice site position forming the rock-salt type structure,
all 4a sites are occupied by Te atoms 1, while 4b sites are
occupied by Ge atoms 2, Sb atoms 3, and occupied by also lattice
defects 4. In the present invention, atoms other than the atoms
occupying the 4b sites are filled in the lattice defects.
[0028] FIG. 2 is a schematic view to show a structure (atom
position at a time of crystallization) of another representative
recording film used for an information recording medium of the
present invention, in which the recording layer is a complex phase
(a crystalline phase). In FIG. 2, (a) denotes a crystalline phase
100. The crystalline phase is a complex phase (mixture phase) 100
comprising a component 110 having a compound structure basically
equal to that shown in FIG. 1 and also an amorphous component 120.
In FIG. 2, (b) denotes an amorphous phase 200. In (b), a single
phase is formed.
[0029] FIGS. 3A-3D are further specific examples of the structure
shown in FIG. 2.
[0030] FIGS. 4A-4J are cross-sectional views of an example of a
layer constitution of an optical information recording medium
according to the present invention. In FIGS. 4A-4J, 7 denotes a
substrate, 8 denotes a recording layer (phase change material
layer), and 9 and 10 denote protective layers. Numeral 11 denotes a
reflective layer, 12 denotes an overcoat layer, 13 denotes an
adhesive layer, and 14 denotes a protective plate. Numeral 15
denotes a surface layer, 16 and 17 denote interface layers, 18
denotes an optical absorption layer, 19 denotes a reflective layer
(light incident side), and 20 and 21 respectively denote multilayer
films of the above-mentioned thin films.
[0031] FIG. 5 is a schematic view of a crystal structure to show
positions of additional elements in the crystalline phase of a
recording film used for an information recording medium according
to the present invention. Numeral 22 denotes a position of an atom
filling a lattice defect in a rock-salt type crystal lattice.
[0032] FIGS. 6A-6C are graphs to show laser modulation waveforms to
evaluate the recording performance of an optical information
recording medium according to the present invention. FIG. 6A shows
the recording performance regarding a 3T pulse, FIG. 6B shows the
recording performance regarding a 4T pulse, and FIG. 6C shows the
recording performance regarding 5T-11T pulses.
[0033] FIG. 7 is a graph to show a relationship between a proper
additive concentration and a lattice defect concentration in an
information recording medium according to the present
invention.
[0034] FIGS. 8A-8F and 9A-9E show examples of crystal structures of
recording films used for information recording media according to
the present invention. The respective structures will cope with any
compound phases shown in FIGS. 1 and 2.
[0035] FIG. 10 is a schematic view to show a basic structure of an
electric memory device (a reversible change memory of a resistor)
according to the present invention. In FIG. 10, 23 denotes a
substrate, 24 and 27 denote electrodes, 25 denotes an insulator, 26
denotes a phase change material film, 28 and 29 denote switches, 30
denotes a pulse power source, and 31 denotes an electrical
resistance meter.
BEST MODE FOR CARRYING OUT THE INVENTION
[0036] FIG. 2 is a cross sectional view to show an example (layer
constitution) of an optical information recording medium according
to the present invention. A typical information recording medium is
constituted by forming a recording layer 8 having the
above-mentioned constitution on a substrate 7 selected from
transparent polycarbonate resin, an acrylic resin, a
polyolefin-based resin, a glass sheet or the like. Protective
layers 9 and 10 can be formed on at least one surface of the
recording layer. Reflective layers 11 can be formed on the
respective protective layers. Overcoats 12 can be formed on the top
layers, or the overcoats can be replaced by protective plates 14
that are adhered by adhesive layers 13. For guiding laser beams
used in recording/reproducing, a spiral or concentric circular
concave-convex groove track, a pit array, a track address can be
formed on the substrate surface. Such a recording medium is
irradiated with a laser beam in order to cause reversible phase
change in the recording layer between a crystalline phase and an
amorphous phase, so that information can be rewritten. In the case
of crystallization, the recording medium is irradiated with a laser
beam like a pulse in order to keep the irradiated part at or above
an interim crystallization change temperature. In changing the
recording layer to be amorphous, the layer is irradiated with a
more intensive laser beam for a period equal to or shorter when
compared to a case of crystallization, so that the irradiated part
is melted instantaneously and then quenched. This reversible phase
change can be detected as a change in the reflectance or
transmittance. This reproduction is carried out by irradiating the
recording medium with a laser beam weakened not to provide any
additional influence so as to detect changes in the strength of
light reflected from the irradiated portion or transmitted.
[0037] An optical information recording medium according to the
present invention, as shown in FIGS. 4A-4J, will be characterized
by a composition of a material composing the recording layer 8 and
by the internal structure. A representative example will be
explained below with reference to a Ge--Sb--Te based material. As
reported in N. Yamada et al., J. Appl. Phy. 69(5), 2849 (1991), a
Ge--Sb--Te material is crystallized to have a face-centered cubic
structure meta-stably by irradiating a laser beam. In addition to
that, a recent research presentation by the same author
(MRS-Buttetin, 21(9), 48(1996) and a research presentation by
Nonaka et al. (papers for the tenth symposium on phase change
recording, p. 63) suggest that the metastable phase necessarily
contains many lattice defects (vacancy). The following description
is about a representative composition of a stoichiometric compound
composition of Ge.sub.2Sb.sub.2Te.sub.5. The material has a
metastable phase of rock-salt type (NaCl type). As shown in FIG. 1,
all lattice positions (4a sites) corresponding to Cl atoms are
occupied by Te atoms 1, and all lattice site positions (4b sites)
corresponding to Na atoms are occupied by Ge atoms 2 and Sb atoms 3
at random depending on the composition ratio. However, since the
total number of the Ge atoms and the Sb atoms is greater than the
number of the Te atoms, the 4a sites necessarily has lattice
defects 4 of about 20% (about 10% of the entire sites). The lattice
defects also are located at random (An example of atom positions in
4a sites is shown).
[0038] The inventors reported that such a Ge--Sb--Te system makes a
crystal having a substantially identical face-centered-cubic
crystal structure even if the composition is changed. Recent
studies show that a Sb atom does not enter a crystal lattice but an
added Sb atom exists in a separate structure on an interface of a
crystal particle even if Sb is included in a form of, e.g.,
Ge.sub.2Sb.sub.2+xTe.sub.5 (0<x.ltoreq.1) to fill the defects.
Particularly, the Sb atom will exist in an amorphous phase
especially for a case of laser crystallization. Specifically, the
result of observation by a detailed X-ray diffraction demonstrates
that even if Sb is added to a stoichiometric composition
Ge.sub.2Sb.sub.2Te.sub.5 thin film, the Sb atom does not enter the
crystal lattice to fill the lattice defect completely. As a result,
Ge.sub.2Sb.sub.2Te.sub.5 crystal and Sb will coexist in a structure
of a recording film in a crystalline state. In a typical case of
two-phase coexistent composition, repetition of a
melting-solidification process will cause a phase separation, and
this will lead to local variation in the composition. An advantage
of this case is that such a phase separation will not proceed since
the melting point of Sb is considerably close to that of Ge--Sb--Te
and since the Ge--Sb--Te also includes Sb.
[0039] Besides Sb, some additives can prevent crystal growth though
the conditions vary in many cases. For example, JP-A-7-214913
discloses the addition of Pd. This reference discloses that
crystallization becomes difficult when the amount of the additives
exceeds 2 atom %. From the fact that a very small amount of
additive causes an abrupt change in the characteristics, Pd is
considered to exist without entering the lattice defects. In other
words, even a small amount of Pd is considered to be separated
completely from Ge--Sb--Te but not to enter a crystal lattice based
on Ge--Sb--Te. However, when the Pd concentration reaches about 2
atom %, characteristics of Pd as a material having a high-melting
point become remarkable, and the Pd will restrict the movement of
atoms so as to substantially prevent crystallization. Moreover,
repetition of recording and erasing accelerates phase separation of
the Ge--Sb--Te and Pd. In other words, an additive that does not
enter a lattice cannot be suitable for controlling the
characteristics.
[0040] On the other hand, a relatively easy relationship between Sb
concentration and change in the crystallization characteristics
facilitates control of the characteristics and serves to maintain
high repeatability. This fact may suggest that the melting point of
an additional element cannot be too much higher than that of the
base material in order to change the characteristics widely and
continuously by adding the element. It is also desirable that the
additional element can enter the crystal lattice and especially,
the element does not create a separate crystalline phase. A further
merit is that entering of excessive and harmful atoms can be
prevented by previously filling the lattice defects with useful
atoms.
[0041] The inventors evaluated recording materials from the
above-mentioned aspects and found that additional elements enter
crystal lattices and thus characteristics can be controlled
continuously with high accuracy under a certain condition. The
inventors found also that some additives will take place of
elements of the base material. Moreover, the additives may change
the purged elements. In addition, the temperature and speed of
crystallization can be controlled by controlling the condition and
concentration of the purged elements, and this will lead to
desirable recording/erasing performance. It is reasonable that in
this case, a part of elements forming a compound in a crystal is
common to elements that have been purged outside the compound and
exist in an amorphous phase in the grain boundary or the like. This
means that positional uniformity of the composition will be
maintained easily all the time that phase changes between a
crystalline phase and an amorphous phase occur. Specifically, the
additives prevent the progress of phase separation even when the
crystalline phase becomes a complex phase, and thus, good
repeatability can be maintained. It can be concluded from the above
facts that a material being a single phase and necessarily
including lattice defects can provide unexpected characteristics by
filling the lattice defects appropriately with any other atoms.
Also, it is suggested that addition of a certain element can help
formation of a material having a new structure.
[0042] The following explanation is about a specific material
composition to constitute a recording layer 8. A primary condition
for a material in the present invention is to obtain a material
comprising many lattice defects. A crystalline phase comprising
lattice defects will appear as a metastable phase in materials that
can be represented by GeTe-M.sub.2Te.sub.3 (M is, for example, Sb,
Bi or Al). The examples are a Ge--Sb--Te based material comprising
a GeTe--Sb.sub.2Te.sub.3 composition, a Ge--Bi--Te material
comprising a GeTe--Bi.sub.2Te.sub.3 based composition, or a
Ge--Te--Al based material comprising a GeTe--Al.sub.2Te.sub.3 based
composition. Similarly, a crystalline phase including lattice
defects will appear as a metastable phase in compositions of the
mixtures such as Ge--Sb--Bi--Te, Ge--Sb--Al--Te, Ge--Bi--Al--Te,
and Ge--Sb--Bi--Al--Te. Similar constitutions are obtained for
Ge(Te,Se)--M.sub.2(Te,Se).sub.3 in which a part of Te is replaced
by Se. The examples are Ge--Te--Se--Sb, Ge--Te--Se--Bi,
Ge--Te--Se--Sb--Bi, Ge--Te--Se--Al, Ge--Te--Se--Sb--Al,
Ge--Te--Se--Bi--Al, and Ge--Te--Se--Sb--Bi--Al. Similar effects
were obtained by applying, for example, Ge--Sn--Te--Sb,
Ge--Sn--Te--Sb--Al, Ge--Pb--Te--Sb, and Ge--Pb--Te--Sb--Al, which
are obtained by substituting a part of the Ge with Sn or with Pb.
Similar constitutions were obtained when N was added to the
compositions. These are crystallized meta-stably to have a
face-centered-cubic crystal structure (rock-salt structure). When
the 4b sites of the rock-salt type structure are occupied by Te (or
Se) and the 4a sites are occupied by other element M as mentioned
above, Te (or Se) atoms outnumber M atoms, which will create
lattice defects at the 4a sites inevitably. The lattice defects
cannot be filled completely with the above-mentioned elements such
as Sb. The reason has not been clarified yet, but it can be deduced
that a metastable phase of a rock-salt type cannot be formed
without a certain number of lattice defects inside thereof. Namely,
filling the defects may raise the entire energy so that the
rock-salt type structure cannot be kept.
[0043] As a result of various analyses and experiments, the
inventors have found that not all elements can fill lattice defects
and that an ionic radius is an important factor to determine the
conditions. When the 4a sites have lattice defects, the defected
lattices of the base materials will be filled easily if Rim is
sufficiently close to Rnc, where Rnc denotes an ionic radius of an
element having a minimum ionic radius among elements occupying the
4a sites and Rim denotes an ionic radius of an additional element.
According to Third Revision of Manual of Basic Chemistry
(Kagaku-binran Kiso-hen) II issued by Maruzen Co., Ltd., the radius
of a Ge.sup.4+ ion is 0.67, the radius of a Sb.sup.5+ ion is 0.74
.mu.m, and the radius of a Te.sup.2- ion is 2.07 .mu.m when the
coordination number is 6. For Ge--Sb--Te, an element can enter a
lattice easily when it has an ionic radius substantially the same
or slightly smaller than the radius of a Ge ion located at a 4a
site. Each Ge ion has a smaller ionic radius than that of a Sb
ion.
1TABLE 1 Ionic radii and element's melting points for respective
ion species Ion species with a Element's coordination Ionic radius
melting point No. number of 6 (nm) (.degree. C.) 1 N.sup.5+ 2.7
-209.86 2 V.sup.5+ 5.0 1890 3 S.sup.4+ 5.1 112.8 4 Si.sup.4+ 5.4
1410 5 P.sup.3+ 5.8 44.1 6 Be.sup.2+ 5.9 1280 7 As.sup.5+ 6.0 817 8
Se.sup.4+ 6.4 217 9 Ge.sup.4+ 6.7 937.4 10 Mn.sup.4+ 6.7 1240 11
Re.sup.7+ 6.7 3180 12 Al.sup.3+ 6.8 660.37 13 Co.sup.3+l 6.9 1490
14 Fe.sup.3+l 6.9 1540 15 Cr.sup.4+ 6.9 1860 16 Re.sup.6+ 6.9 3180
17 Te.sup.6+ 7.0 449.5 18 Ni.sup.3+l 7.0 1450 19 As.sup.3+ 7.2 817
20 Mn.sup.3+l 7.2 1240 21 V.sup.4+ 7.2 1890 22 Mo.sup.6+ 7.3 2620
23 Sb.sup.5+ 7.4 630.74 24 Ni.sup.3+h 7.4 1450 25 Rh.sup.4+ 7.4
1970 26 W.sup.6+ 7.4 3400 27 Co.sup.3+h 7.5 1490 28 Fe.sup.2+l 7.5
1540 29 Ti.sup.4+ 7.5 1660 30 Mo.sup.5+ 7.5 2620 31 Ga.sup.3+ 7.6
29.78 32 Pd.sup.4+ 7.6 1550 33 Cr.sup.3+ 7.6 1860 34 Ru.sup.4+ 7.6
2310 35 W.sup.5+ 7.6 3400 36 Pt.sup.4+ 7.7 1770 37 Ir.sup.4+ 7.7
2410 38 Os.sup.4+ 7.7 3045 39 V.sup.3+ 7.8 1890 40 Nb.sup.5+ 7.8
2470 41 Ta.sup.5+ 7.8 2990 42 Mn.sup.3+h 7.9 1240 43 Co.sup.2+l 7.9
1490 44 Fe.sup.3+h 7.9 1540 45 Tc.sup.4+ 7.9 2170 46 Mo.sup.4+ 7.9
2620 47 W.sup.4+ 8.0 3400 48 Mn.sup.2+l 8.1 1240 49 Ti.sup.3+ 8.1
1660 50 Rh.sup.3+ 8.1 1970 51 Ru.sup.3+ 8.2 2310 52 Ir.sup.3+ 8.2
2410 53 Nb.sup.4+ 8.2 2470 54 Ta.sup.4+ 8.2 2990 55 Sn.sup.4+ 8.3
231.96 56 Ni.sup.2+ 8.3 1450 57 Mo.sup.3+ 8.3 2620 58 Hf.sup.4+ 8.5
2230 59 Mg.sup.2+ 8.6 648.8 60 Zr.sup.4+ 8.6 1850 61 Nb.sup.3+ 8.6
2470 62 Ta.sup.3+ 8.6 2990 63 Ge.sup.2+ 8.7 937.4 64 Cu.sup.2+ 8.7
1083.4 65 U.sup.5+ 8.7 1132.3 66 Cr.sup.2+l 8.7 1860 67 Zn.sup.2+
8.8 419.58 68 Sc.sup.3+ 8.8 1540 69 Co.sup.2+h 8.9 1490 70 Li.sup.+
9.0 180.54 71 Bi.sup.6+ 9.0 271.3 72 Sb.sup.3+ 9.0 630.74 73
Pd.sup.3+ 9.0 1550 74 Cu.sup.+ 9.1 1083.4 75 Pb.sup.4+ 9.2 327.502
76 Fe.sup.2+h 9.2 1540 77 V.sup.2+ 9.3 1890 78 In.sup.3+ 9.4 156.61
79 Pt.sup.2+ 9.4 1770 80 Cr.sup.2+h 9.4 1860
[0044] Atoms in a rock-salt structure are considered to have a
coordination number of 6. Table 1 is a list of ion species each
having a coordination number of 6 and ionic radius of about 0.67 in
an order of the ionic radius. Since a Ge.sup.4+ ion has ionic
radius of 0.67, ions ranging from a vanadium ion V.sup.5+ that is
about 70% of a Ge.sup.4+ ion to a Ni.sup.3+ ion that is about 105%
may enter a lattice. That is, effective elements are V, S, Si, P,
Be, As, Se, Ge, Mn, Re, Al, Co, Te, Cr, and Ni. Among them, V, S,
Si, Mn, Al, Co, Cr, and Ni etc. are suitable. The remaining
elements are not suitable, since, for example, Be, As and P may
cause problems due to the toxicity, while Ge and Te compose the
base material, and Re is a radioactive element.
[0045] Elements for filling lattices are not limited to the
above-mentioned ones. The above-mentioned condition is just one
factor to determine easy access to a lattice. An element that
composes a compound of a rock-salt type structure is observed to
enter a lattice easily. Specifically, Ag, Sn and Pb were observed
entering lattices, since Ag, Sn and Pb compose AgSbTe.sub.2, SnTe,
and PbTe respectively.
[0046] In addition to the suitability to fill a lattice, another
important factor for additional elements is the melting point.
Formation of an amorphous mark with a phase change optical disk
requires a process of melting a recording film before quenching.
For such a case, a melting point of the additive is preferred to be
close to the melting point of an entire recording film (more
preferably, a melting point of the additive is close to melting
points of all elements composing the recording film). If the
additive has a melting point much higher than the entire melting
point, phase separation will proceed easily during repetition of
melting and solidification. In such a case, it is difficult to keep
the additives stably in lattices even when the ionic radii are
closer to each other. In other words, phase separation occurs, and
the phase separation creates a region comprising more additives and
a region comprising fewer additives. It is preferable to decrease
the difference between the melting points, however, when the
difference is about 100.degree. C., lattice defects can be filled
while creating substantially no phase separation. Otherwise an
extremely uniform mixed phase can be formed even without forming a
single phase. For a case of Ge.sub.2Sb.sub.2Te.sub.5, the melting
point is about 630.degree. C. Therefore, an additive is preferred
to have a melting point in a range from about 530.degree. C. to
730.degree. C. Table 2 is a list of elements to form ions having
coordination number of 6 as mentioned above, and the elements are
described sequentially from the one with a lower melting point.
This table shows that elements ranging from No. 25 (Sb) to No. 31
(Ba) are within the range. That is, corresponding elements are Sb,
Pu, Mg, Al and Ba, from which Pu as a radioactive element and Sb as
a base material are excluded. The remaining Mg, Al, Ba or the like
are used suitably for the purpose.
2TABLE 2 Melting points of respective elements and ionic radii of
ion species Ion species with a Element's coordination Ionic radius
melting point No. number of 6 (nm) (.degree. C.) 1 Cs.sup.+ 18.1
28.4 2 Ga.sup.3+ 7.6 29.78 3 Rb.sup.+ 16.6 38.89 4 P.sup.3+ 5.8
44.1 5 K.sup.+ 15.2 63.65 6 Na.sup.+ 11.6 97.81 7 S.sup.2- 17.0
112.8 8 S.sup.4+ 5.1 112.8 9 I.sup.- 20.6 113.5 10 In.sup.3+ 9.4
156.61 11 Li.sup.+ 9.0 180.54 12 Se.sup.2- 18.4 217 13 Se.sup.4+
6.4 217 14 Sn.sup.4+ 8.3 231.96 15 Bi.sup.3+ 11.7 271.3 16
Bi.sup.6+ 9.0 271.3 17 Tl.sup.+ 16.4 303.5 18 Tl.sup.3+ 10.3 303.5
19 Cd.sup.2+ 10.9 320.9 20 Pb.sup.2+ 13.3 327.502 21 Pb.sup.4+ 9.2
327.502 22 Zn.sup.2+ 8.8 419.58 23 Te.sup.2- 20.7 449.5 24
Te.sup.6+ 7.0 449.5 25 Sb.sup.3+ 9.0 630.74 26 Sb.sup.5+ 7.4 630.74
27 Pu.sup.3+ 11.4 639.5 28 Pu.sup.4+ 10.0 639.5 29 Mg.sup.2+ 8.6
648.8 30 Al.sup.3+ 6.8 660.37 31 Ba.sup.2+ 14.9 725 32 Sr.sup.2+
13.2 769 33 Ce.sup.3+ 11.5 799 34 Ce.sup.4+ 10.9 799 35 As.sup.3+
7.2 817 36 As.sup.5+ 6.0 817 37 Eu.sup.2+ 13.1 822 38 Eu.sup.3+
10.9 822 39 Ca.sup.2+ 11.4 839 40 La.sup.3+ 11.7 921 41 Ge.sup.2+
8.7 937.4 42 Ge.sup.4+ 6.7 937.4 43 Ag.sup.+ 12.9 961.93 44
Ag.sup.2+ 10.8 961.93 45 Nd.sup.3+ 11.2 1020 46 Ac.sup.3+ 12.6 1050
47 Au.sup.+ 15.1 1064.43 48 Cu.sup.+ 9.1 1083.4 49 Cu.sup.2+ 8.7
1083.4 50 U.sup.3+ 11.7 1132.3 51 U.sup.4+ 10.3 1132.3 52 U.sup.5+
8.7 1132.3 53 Mn.sup.2+l 8.1 1240 54 Mn.sup.2+h 9.7 1240 55
Mn.sup.3+l 7.2 1240 56 Mn.sup.3+h 7.9 1240 57 Mn.sup.4+ 6.7 1240 58
Be.sup.2+ 5.9 1280 59 Gd.sup.3+ 10.8 1310 60 Dy.sup.3+ 10.5 1410 61
Si.sup.4+ 5.4 1410 62 Ni.sup.2+ 8.3 1450 63 Ni.sup.3+l 7.0 1450 64
Ni.sup.3+h 7.4 1450 65 Co.sup.2+l 7.9 1490 66 Co.sup.2+h 8.9 1490
67 Co.sup.3+l 6.9 1490 68 Co.sup.3+h 7.5 1490 69 Y.sup.3+ 10.4 1520
70 Sc.sup.3+ 8.8 1540 71 Fe.sup.2+l 7.5 1540 72 Fe.sup.2+h 9.2 1540
73 Fe.sup.3+l 6.9 1540 74 Fe.sup.3+h 7.9 1540 75 Pd.sup.2+ 10.0
1550 76 Pd.sup.3+ 9.0 1550 77 Pd.sup.4+ 7.6 1550 78 Lu.sup.3+ 10.0
1660 79 Ti.sup.2+ 10.0 1660 80 Ti.sup.3+ 8.1 1660
[0047] For example, when the base material comprises a
Ge.sub.2Sb.sub.2Te.sub.5 composition, Al is a suitable element that
can satisfy the two conditions concerning ion radius and melting
point simultaneously, while it is free of toxicity or
radioactivity. A GeTe--Sb.sub.2Te.sub.3-based composition can be
treated in the same manner as Ge.sub.2Sb.sub.2Te.sub.5. While the
melting point of the GeTe--Sb.sub.2Te.sub.3-based composition
changes continuously in a range from 593.degree. C. to 725.degree.
C., Al was effective as well in filling lattice defects. Similarly,
in any material compositions based on Ge and Te, Al was effective
in filling lattice defects. Needless to say, elements other than Al
were confirmed to enter lattices. It was confirmed that Ag, Cr, Mn,
Sn, Pb, Mo In and Se enter lattices.
[0048] Elements to fill lattice defects are not limited to one
kind, but plural kinds of elements can be filled simultaneously. In
an experiment performed by the inventors, the crystallization speed
was improved remarkably by, for example, filling Sn (or Pb) in
lattices when the material is Ge--Sb--Te based material or
Ge--Bi--Te based material. The repeatability was improved by
filling Cr in lattices. Therefore, the crystallization speed and
repeatability were improved at the same time by filling Sn (or Pb)
together with Cr. Similar effects were obtained by filling Mn in
place of Cr in the crystal lattices. Filling Ag was helpful in
improving optical reflectance variation between a crystalline phase
and an amorphous phase (improvement in recording signal amplitude).
Therefore, improvement in the recording signal amplitude and the
crystallization speed was achieved simultaneously by adding Ag and
Sn (or Pb) together. Signal amplitude and repeatability were
improved simultaneously by filling Ag and Cr (or Mn) at the same
time. The addition of Sn (or Pb), Ag and Cr (or Mn) together served
to improve crystallization speed, signal amplitude and
repeatability simultaneously.
[0049] FIG. 2 indicates a preferred embodiment for a recording
layer used for another optical information recording medium
according to the present invention. FIG. 2 expresses schematically
a partial microscopic structure of a recording layer 8 at a laser
irradiation part in any of FIGS. 4A-4I. In FIG. 2, (a) denotes a
crystalline phase (complex phase) 100 comprising a mixture of a
compound component 110 and an amorphous component 120, while (b)
denotes a single-amorphous phase 200. The recording material layer
is composed of the four elements of Ge, Sb, Te and Sn. The crystal
component 110 in the complex phase 100 has a NaCl type structure
comprising the four elements of Ge--Sb--Te--Sn. The 4a sites of the
NaCl type structure (sites corresponding to Cl) are occupied by Te,
while the 4b sites (sites corresponding to Na) are occupied
randomly by Ge, Sb and Sn. At the 4b sites there are lattice
defects to accept no atoms, which tends to decrease entire density.
As a result, volume variation between the crystalline phase and
amorphous phase is decreased, and inconvenience such as deformation
or perforation caused by the phase change is prevented. In the
grain boundary, components that cannot enter the lattices exist in
an amorphous state. Here, Sb exists in an amorphous state. It is
preferable that an amount of the amorphous component is twice or
less than the crystal component by number of molecules. It is
preferable A/C.ltoreq.2, or more preferably, A/C.ltoreq.1, where C
denotes a number of molecules of the crystal component and A
denotes a number of molecules of the amorphous component. When the
ratio of the amorphous component exceeds twice, the crystallization
speed will be lowered remarkably. On the other hand, when the ratio
is close to 0, the crystallization speed is increased excessively.
It is preferable that A/C.gtoreq.0.01. The element that is found as
an amorphous component in the crystalline phase is not limited to
Sb but it can be Ge. Ge is effective in raising crystallization
temperature or improving repeatability. The great viscosity of the
amorphous Ge is considered to provide such effects. It has been
confirmed that elements such as Mn and Cr can be added for
depositing Ge.
[0050] From a macroscopic viewpoint, all elements are arranged in a
substantially uniform state in the single-amorphous phase 200. It
is important for the recording film to change reversibly between
the two states during recording or rewriting information. At this
time, it is preferable that a part of the elements for forming the
amorphous phase 120 and elements for forming the compound component
110 in the complex phase 100 is common, so that the distance of
atomic diffusion is decreased at the time of phase change so as to
complete the change rapidly. It is effective also in preventing
generation of great positional compositional segregation when
rewriting is repeated many times.
[0051] A material layer composing the recording layer comprises a
material for forming a crystalline phase in a complex phase, and
the material is represented by a format of Ma-Mb-Mc-.alpha., in
which Ma comprises Ge and at least one of Sn and Pb, Mb comprises
at least one of Sb and Bi, and Mc comprises at least one of Te and
Se. Any other elements can be added if required. For example, Mn,
Cr, Ag, Al, In or the like can be added. For a material for forming
an amorphous phase in the complex phase, Sb or Ge is suitable for a
Ge--Sb--Te based material, while Ge or Bi is suitable for a
Ge--Bi--Te based material. For a AgInSbTe based material, In can be
used.
[0052] In general, protective layers 9 and 10 in FIGS. 4B-4I are
made of a dielectric material. Protective layers suggested as
optical disk media in conventional techniques can be used as well.
The examples include a material layer of an oxide alone or a
complex oxide of an element selected from Al, Mg, Si, Nb, Ta, Ti,
Zr, Y, and Ge; a material layer of a nitride or a nitride-oxide of
an element selected from Al, B, Nb, Si, Ge, Ta, Ti, and Zr; a
sulfide such as ZnS and PbS; a selenide such as ZnSe; a carbide
such as SiC; a fluoride such as CaF.sub.2 and LaF; and a mixture
thereof such as ZnS--SiO.sub.2 and ZnSe--SiO.sub.2.
[0053] A reflecting layer 11 is based on a metal such as Au, Al,
Ag, Cu, Ni, Cr, Pd, Pt, Si, and Ge, or an alloy such as Au--Cr,
Ni--Cr, Al--Cr, Al--Ta, Al--Ti, Ag--Pd, Ag--Pd--Cu, Si--W, and
Si--Ta.
[0054] An overcoat layer 12 can be made of, for example, a
photo-curable resin. An adhesive 13 can be made of, for example, a
hot-melt adhesive or a photo-curable resin such as an ultraviolet
curable resin. A protective plate 14 can be made of the same
material as the substrate. The substrate is not transparent
necessarily for a constitution to record and reproduce by
irradiating a laser beam from the side having a recording layer.
The above-mentioned substrate can be replaced by, for example, a
plate of a light metal such as Al and Cu, or a plate of alloy based
on the light metal, and a plate of ceramics such as Al.sub.2O.sub.3
and MgO.sub.2. In this case, the respective layers are formed on
the substrate in a reversed order.
[0055] Though it is not indispensable, a surface layer 15 can be
provided on the outermost in order to prevent damage caused by a
contact with an optical head. The surface layer can be made of a
lubricant material comprising e.g., a diamond-like-carbon and a
polymer material.
[0056] Interface layers 16 and 17 can be formed in an interface
between the recording layer and at least one of the protective
layers for several purposes, such as preventing atomic diffusion in
spacing between the recording layer and the protective layer.
Especially, nitrides, nitride-oxides and carbides are suitable for
the interface layer. The examples include materials of Ge--N--(O),
Al--N--(O), Si--C--N, Si--C or the like, and materials further
including Cr, Al or the like, such as Ge--C--N and Si--Al--.
Optical absorption Aa of a recording layer in an amorphous state
can be decreased relatively with respect to optical absorption Ac
of the recording layer in a crystalline state by applying an
optical absorption layer 18 over an upper protective layer of the
recording layer, or by applying a semitransparent reflecting layer
19 at the light incident side of the recording layer.
[0057] The optical absorption layer can be made of alloy materials
based on Si and Ge, or alloy materials based on Te. The reflecting
layer can be made of the same material, or it can be formed by
laminating dielectric films having different refractive indices,
such as SiO.sub.2/ZnS--SiO.sub.2/SiO.sub.2. An alternative medium
can have both surfaces made by adhering a recording medium having
these multilayer films 20 and 21 through adhesive layers 13.
[0058] A multilayer film used for an optical information recording
medium according to the present invention can be formed by an
ordinary method for forming a thin film. The method is selected,
for example, from magnetron sputtering, DC sputtering, electron
beam deposition, resistance heating deposition, CVD, and ion
plating. Especially, magnetron sputtering using an alloy target,
and also DC sputtering are excellent in obtaining uniform films
that will be used as recording films in the present invention. A
target used for sputtering contains a main component of a material
for forming the above-mentioned rock-salt structure, to which an
element for filling the lattice defects is added. Such a target can
be prepared by solidifying powders composed of respective elements
at a proper ratio, and the elements are, for example, Ge, Te, Sb
and Al; Ge, Sb, Sn, Cr and Te; Ge, Sb, Te, Sn and Ag. Though the
component ratio in the target substantially corresponds to
compositions of the recording film, minor adjustment for every
apparatus is required since the components will be influenced by
the apparatus. For example, Dad is equal substantially to
Dim.ltoreq.Ddf.times.1.5, where Dim denotes a concentration of an
additive in a film of the crystalline phase, Ddf denotes a
concentration of lattice defects, and Dad denotes a concentration
of an additive in a target. In general, an amorphous single phase
is formed just after film formation, which will be transformed into
a crystalline phase (initialization). It is possible to form a
phase as a mixture of the crystalline phase and the amorphous phase
by irradiating with a high density energy flux. In irradiation of
the high density energy flux, it is desirable to penetrate the flux
at a high temperature for a short period. Therefore, laser
irradiation and flash irradiation are used suitably.
[0059] FIG. 10 is a schematic view to show a basic structure of an
electric memory device according to the present invention (a
reversible change memory of a resistor). In FIG. 10, 23 is a
substrate selected from a glass sheet, a ceramic sheet such as
Al.sub.2O.sub.3, and sheets of various metals such as Si and Cu.
The following explanation is about a case for using an alumina
substrate. In FIG. 10, an Au layer is sputtered to provide an
electrode 24 on a substrate. Subsequently, a layer 25 of an
insulator such as SiO.sub.2 or SiN is formed thereon through a
metal mask, and further, a recording layer 26 comprising a phase
change material similar to the above-mentioned recording layer for
the optical information recording medium, and also an electrode
(Au) 27 are laminated. Between the electrodes 24 and 27, a pulse
power source 30 is connected through a switch 28. For crystallizing
the recording film that is in highly resistant under
as-depo.-condition in order to change into a low resistant state,
the switch 28 closes (switch 29 open) so as to apply voltage
between the electrodes. The resistance value can be detected with a
resistance meter 31 while opening the switch 28 and closing the
switch 29. For reversely transforming from the low resistant state
to a high resistant state, voltage higher than the voltage at the
time of crystallization is applied for the same or shorter period
of time. The resistance value can be detected with a resistance
meter 31 while opening the switch 28 and closing the switch 29. A
large capacity memory can be constituted by arranging a large
number of the memory devices in a matrix.
[0060] The present invention will be described further by referring
to specific examples.
EXAMPLE 1
[0061] Example 1 is directed to a method for manufacturing an
optical information recording medium according to the present
invention. A substrate used in this example was a disc-shape
polycarbonate resin substrate that was 0.6 mm in thickness, 120 mm
in diameter and 15 mm in inner diameter. A spiral groove was formed
substantially on the whole surface of the substrate. The track was
a concave-convex groove having a depth of 70 nm. Both the groove
portion and the land portion of the track had a width of 0.74
.mu.m. A multilayer film would be formed on the surface later. A
laser beam for recording/reproducing an information signal can move
to an arbitrary position on the disk by a servo signal provided
from the concave-convex shape. On the substrate, the following
layers were formed in this order: a ZnS:20 mol % SiO.sub.2
protective layer 150 nm in thickness; a
Ge.sub.2Sb.sub.2Te.sub.5Al.sub.0.5 thin film 20 nm in thickness; a
GeN interface layer 5 nm in thickness; a ZnS:20 mol % SiO.sub.2
protective layer 40 nm in thickness; and an Al.sub.97Cr.sub.3 alloy
reflecting plate 60 nm in thickness. The protective layers were
prepared by magnetron sputtering using a ZnS--SiO.sub.2 sintered
target and Ar sputtering gas. The recording layer and the
reflecting layer were prepared by DC sputtering in which respective
alloy targets and Ar sputtering gas were used. The interface layer
was formed by a reactive magnetron sputtering using a Ge target and
a sputtering gas as a mixture of Ar gas and N.sub.2 gas. In any
cases, N.sub.2 gas can be added to a sputtering gas. After
completing the film formation, an ultraviolet curable resin was
spin-coated, and a polycarbonate plate the same as a substrate was
adhered to serve as a protective plate, and this was irradiated by
a ultraviolet beam lamp subsequently for curing, before subjecting
the disk to an initial crystallization by irradiating a laser beam.
The thus obtained optical information recording medium can record
and reproduce by means of laser irradiation. In an inspection with
an X-ray diffraction, the part that was subjected to the initial
crystallization was a NaCl type single-crystalline phase having Al
in the crystal lattices, though a slight halo peak was observed.
The same inspection was carried out for the other additive
elements, and similar results were observed for Mn, Ag, Cr, Sn, Bi,
and Pb.
EXAMPLE 2
[0062] On a quartz substrate, eight kinds of thin film material
were formed by DC sputtering. The materials were represented by
Ge.sub.2Sb.sub.2Te.sub.5Al.sub.x, in which A1:x=0.0, A2:x=0.2,
A3:x=0.5, A4:x=1.0, A5:x=1.5, A6:x=2.0, A7:x=2.5, and A8:x=3.0. The
base vacuum degree was 1.33.times.10.sup.-4 Pa, and Ar was
introduced to make the vacuum degree to be 1.33.times.10.sup.-1 Pa.
Under this condition, 100 W power was applied between a cathode and
an alloy target of 100 mm .PHI. in diameter so as to form a thin
film having a thickness of 20 nm. These samples were monitored by
using a He--Ne laser beam in the varying strength of the
transmitted light while being heated at a programming rate of
50.degree. C./minute in order to measure a temperature at which
transmittance was decreased remarkably as a result of
crystallization. The results are shown in Table 3.
3TABLE 3 Relationship between Al concentration in a
Ge.sub.2Sb.sub.2Te.sub.5 thin film and crystallization temperature
.multidot. crystallization speed Sample A1 A2 A3 A4 A5 A6 A7 A8 Al
con..sup.1) 0% 2.2% 5.3% 10% 14.3% 18.2% 21.7% 25% T.sub.x
180.degree. C. 183.degree. C. 189.degree. C. 200.degree. C.
227.degree. C. 255.degree. C. 305.degree. C. 350.degree. C.
T.sub.cry .circleincircle. .circleincircle. .circleincircle.
.circleincircle. .largecircle. .DELTA. X X
[0063] The increase of the crystallization temperature becomes
sharp when the Al concentration is at a level of the sample E. For
this composition, Ddf (concentration of lattice defects) occupies
10% of the whole sites (20% of the 4b sites). For the respective
samples, ratios that Al atoms fill lattice defects to Ddf are as
follows: A1:0, A2:0.2.times.Ddf, A3:0.5.times.Ddf,
A4:1.0.times.Ddf, A5:1.5.times.Ddf, A6:2.0.times.Ddf,
A7:2.5.times.Ddf, and A8:3.0.times.Ddf. For the samples A5-A8,
there are more Al atoms than the lattice defects to be filled.
Percentage of the Al atoms to the whole compositions in the
respective samples are as follows. A1: 0%, A2: 2.2%, A3: 5.3%, A4:
10%, A5: 14.3%, A6: 18.2%, A7: 21.7%, and A8: 25%.
[0064] Regarding the samples A3 and A4, a Rietveld method was
performed to identify the structures in detail by using an X-ray
diffractometry so as to confirm that Al entered the crystal sites
in any of the samples. FIG. 5 is a schematic view to show such a
sample. The probability that the lattice defects are filled with
the additives is determined randomly as well. For the samples A5,
A6, A7 and A8, excessive atoms that cannot enter the crystal
lattices will exist among the crystal particles. Such excessive
atoms are not always Al, but other elements such as Sb or Ge may
deposit as a result of substitution with Al. Laser irradiation
period for causing crystallization would be extended when the Al
concentration is increased. In the Table, .circleincircle.
indicates that crystallization occurred within 70 ns, .largecircle.
indicates that crystallization occurred within 100 ns, .DELTA.
indicates that crystallization occurred within 200 ns, and x
indicates that crystallization required more than 200 ns. When an
effective optical spot length is represented by 1/e.sup.2, an ideal
value would be about 0.95 .mu.m since an optical system used for
the current DVD-RAM has a wavelength of 660 nm, and NA of an
objective lens is 0.6. It takes about 160 ns for the laser spot to
traverse a disk rotating at a linear velocity of 6 m/s, which
corresponds to a velocity for DVD-RAM. Therefore, a disk with a
.largecircle. mark can be applied to a current DVD-RAM system. It
can be applied to a system having a linear velocity of at least 9
m/s as well. A disk with .circleincircle. mark can cope with an
even higher linear velocity of at least 12 m/s.
EXAMPLE 3
[0065] Eight optical disks from a1 to a8 were prepared by using the
compositions of Example 2 in the method of Example 1. These disk
media were rotated at a linear velocity of 9 m/s, and light beams
having a wavelength of 660 nm emitted from a laser diode were
focused on the disks by using an optical system comprising an
object lens having NA of 0.6. At this time, as shown in FIGS.
6A-6C, overwriting recording was carried out in a 8-16 modulation
(bit length: 0.3 .mu.m) by applying a multi-pulse waveform
corresponding to waveforms of signals ranging from a 3T signal to a
11T signal. The peak power and bias power were determined as
follows. First, a power to provide an amplitude of -3 dB to a
saturation value of the amplitude was obtained and the power was
multiplied by 1.3 to provide a peak power. Next, the peak power was
fixed while the bias power was determined to be variable for
conducting 3T recording. 11T recording was conducted with the same
power for measuring a damping ratio of the 3T signal, which was
established as an erasing rate. Since the erasing rate was
increased gradually, experienced a substantially flat region and
turned into decrease, the bias power was determined to be a central
value of the upper limit power and a lower limit power with an
erasing rate of more than 20 dB.
[0066] Table 4 shows recording power (peak power/bias power) at a
time of land recording for each disk, C/N, a maximum value for
elimination rate, and a number of times that a jitter value is 13%
or less when random signals are overwrite-recorded repeatedly.
4TABLE 4 Relationship between Al concentration in
Ge.sub.2Sb.sub.2Te.sub.5 thin film and disk performance Disk a1 a2
a3 A4 a5 a6 a7 a8 Al con. 0% 2.2% 5.3% 10% 14.3% 18.2% 21.7% 25%
Power 10.5/4.5 mW 10.5/4.5 mW 10.5/4.5 mW 10.5/4.5 mW 10.1/4.6 mW
10.0/4.9 mW -- -- mW C/N 50 dB 51.5 dB 52 dB 52.5 dB 52.5 dB 52.5
dB 52.0 dB -- Erasing 25 dB 30 dB 34 dB 35 dB 29 dB 21 dB 10 dB --
rate NT 3 .times. 10.sup.4 1 .times. 10.sup.5 >1 .times.
10.sup.5 >1 .times. 10.sup.5 1 .times. 10.sup.5 2 .times.
10.sup.4 -- -- .sup.1): Al concentration .sup.2): Number of
times
[0067] The results show that addition of Al improves erasing rate
and increases a number of repetitions. When the Al concentration
was not higher than a concentration (10%) of the lattice defects,
erasing rates exceeded 30 dB and the repetition numbers exceeded
100,000 for any of the disks a2, a3, and a4. It was found that
optimum values were obtained for C/N, erasing rate and repetition
number when the Al concentration matches the concentration Ddf of
the lattice defects. High-speed crystallization performance was
maintained up to the time that the Al concentration became 1.5
times of the lattice defect concentration. For the disk a5, the
repetition number was increased when compared to a disk including
no additives. When the additive concentration is increased
excessively, the crystallization velocity is lowered and thus, the
erasing rate is decreased and the jitter becomes large. For the
disks a7 and a8, the jitter was over 13% from the initial stage. It
was observed for these disks having improved repeatability that
mass transfer was restrained.
EXAMPLE 4
[0068] Various disks were manufactured by determining the
composition of the recording film in Example 1 to be
(GeTe).sub.x(Sb.sub.2Te.sub.3).sub.- 1-x, where the x value was
varied in a range from 0 to 1. For every disk, D.sub.1 and D.sub.2
were measured. D.sub.1 denotes a proper range of Al concentration,
and D.sub.2 denotes an optimum range among D.sub.1. The
concentration was determined first to be 0.2% and 0.5%, and
subsequently, it was increased by 0.5%, i.e., 1%, 1.5%, 2%, 2.5% .
. . The proper range was determined to be a concentration range to
provide a repetition number larger than that of a disk including no
additives, and the determination was based on the methods described
in Examples 2 and 3. The optimum range was a concentration range in
which the repetition number was doubled at least when compared to a
disk including no additives and a range that a high crystallization
velocity was obtainable. Namely, it is a range to allow
crystallization by irradiating a laser beam for 150 ns at most.
5TABLE 5 Optimum Al addition concentration for
(GeTe).sub.x(Sb.sub.2Te.sub.3).sub.1-x Al concentration within Al
concentration within X value Ddf for NaCl structure proper range:
D1 optimum range: D1 Notes 0 16.7% -- -- Sb.sub.2Te.sub.3 itself
0.1 16.1% 0.2% .ltoreq. D1 .ltoreq. 24.0% 3.0% .ltoreq. D2 .ltoreq.
16.0% 0.2 15.4% 0.2% .ltoreq. D1 .ltoreq. 23.0% 3.0% .ltoreq. D2
.ltoreq. 15.0% 0.33 14.3% 0.2% .ltoreq. D1 .ltoreq. 22.0% 3.0%
.ltoreq. D2 .ltoreq. 14.0% GeSb.sub.4Te.sub.7 0.5 12.5% 0.2%
.ltoreq. D1 .ltoreq. 19.5% 2.0% .ltoreq. D2 .ltoreq. 12.5%
GeSb.sub.2Te.sub.4 0.67 10.0% 0.2% .ltoreq. D1 .ltoreq. 16.0% 1.5%
.ltoreq. D2 .ltoreq. 11.0% Ge.sub.2Sb.sub.2Te.sub.5 0.8 7.1% 0.2%
.ltoreq. D1 .ltoreq. 11.5% 0.5% .ltoreq. D2 .ltoreq. 8.5% 0.9 4.2%
0.2% .ltoreq. D1 .ltoreq. 6.5% 0.2% .ltoreq. D2 .ltoreq. 4.5% 0.91
3.8% 0.2% .ltoreq. D1 .ltoreq. 6.0% 0.2% .ltoreq. D2 .ltoreq. 4.0%
1 0% -- -- GeTe itself
[0069] Table 5 shows the test results. The table includes also
calculation results of the concentration Ddf of lattice defects.
The lattice defects are formed inevitably in a crystal structure
under a hypothetical circumstance that these material thin films
form metastable phases of a rock-salt type by laser irradiation. As
indicated in the table, the concentration Ddf of the lattice
defects increases when a (GeTe).sub.x(Sb.sub.2Te.sub.3).sub.1-x
quasibinary system composition transfers from the GeTe side to the
Sb.sub.2Te.sub.3 side. On the other hand, when the proper range of
Al amount reaches a range higher than a range for the defect
concentration, the range up to about 1.5.times.Ddf is effective in
improving the characteristics.
[0070] FIG. 7 is a graph to show the relationships. The solid line
denotes Ddf, while .circle-solid. denotes the upper limit of the
proper range and .DELTA. denotes the upper limit of the optimum
range. The upper limit of the optimum range substantially coincides
with the Ddf value while the x value is small and Ddf absolute
value is big. However, the upper limit will be bigger than Ddf by
about 20% when the x value is increased and Ddf value is decreased.
The reason can be estimated as follows. Since a part of the Al
additive is modified due to oxidization, nitriding or the like, a
percentage for entering the crystal lattices is lowered, and thus,
the amount of the additive should be increased.
EXAMPLE 5
[0071] Disks of Example 4 were subjected to 10000 times of
overwrite-recording of a single frequency signal having a mark
length of 0.3 .mu.m before a measurement of the CN ratio.
Subsequently, the disks were kept in a thermostat at a temperature
of 90.degree. C. and humidity of 80% RH for 200 hours and the CN
ratio of the same track was measured. The results are shown in
Table 6. In the table, .circleincircle. indicates that the initial
CN ratio was at least 50 dB and a decrease in the CN ratio was at
most 1 dB even after a 200 hours of acceleration test.
.largecircle. indicates that the initial CN ratio was at least 50
dB and a decrease in the CN ratio was at most 3 dB after a 100
hours of acceleration test. .DELTA. indicates that the initial CN
ratio was at least 50 dB while the CN ratio was decreased by at
least 3 dB in the acceleration test. x indicates that problems
occurred during the initial overwriting of 10000 times, e.g., the
CN ratio was decreased.
6TABLE 6 Result of acceleration test of disks based on
(GeTe).sub.x(Sb.sub.2Te.sub.3).sub.(1-x) containing Al X 0 0.1 0.2
0.33 0.5 0.67 0.8 0.9 0.91 1 Result .DELTA. .DELTA. .largecircle.
.largecircle. .circleincircle. .circleincircle. .circleincircle.
.circleincircle. X X
EXAMPLE 6
[0072] A similar test was carried out by changing the composition
of the recording film of Example 4 to
(GeTe).sub.x(Bi.sub.2Te.sub.3).sub.1-x. Similar results were
obtained for the effects caused by the Al addition and the proper
concentration.
EXAMPLE 7
[0073] A similar test was carried out by changing the composition
of the recording film of Example 4 to
(GeTe).sub.x(M.sub.2Te.sub.3).sub.1-x (M: a mixture comprising Sb
and Bi at an arbitrary ratio). Similar results were obtained for
the effects caused by the Al addition and the proper
concentration.
EXAMPLE 8
[0074] Disks having films with varied N concentration were prepared
by varying partial pressures of Ar gas and N.sub.2 gas, in which
the recording layers were formed by adding 7% Al to
(GeTe).sub.0.8(Sb.sub.2Te- .sub.3).sub.0.2. The concentration of N
in the films was identified by using SIMS. The thus obtained disks
were subjected to recording of random signals having a bit length
of 0.26 .mu.m under a condition that the recording power was 11 mW
(peak power)/5 mW (bias power) and the linear velocity was 9 m/s in
order to examine the overwriting characteristics. The evaluation
results are shown in Table 7.
[0075] Table 7 indicates that addition of N improves recording
sensitivity. When excessive N was added, the optical constant was
reduced and C/N was lowered. The effects became apparent when 0.5%
of N was added, and the preferable amount of N was about 5%.
7TABLE 7 Relationship between N concentration in recording thin
film and disk performance Disks A B C D E F G H N con. 0% 0.1% 0.5%
1% 3% 5% 10% 20% C/N 51.0 dB 51.0 dB 52.0 dB 52.0 dB 52.5 dB 52.5
dB 49.5 dB 45.0 dB Power 11.5/5.0 mW 11.4/4.9 mW 11.1/4.6 mW
10.8/4.4 mW 10.5/4.1 mW 10.0/4.0 mW 10.0/4.2 mW 10/4.4 mW mW N
con.: N concentration
EXAMPLE 9
[0076] Various additives other than Al were added to
Ge.sub.2Sb.sub.2Te.sub.5 recording films for the purpose of
examining the recording performance of the films. Additives were
selected from elements having ion radii similar to an ionic radius
of Al, i.e., V, S, Si, P, Se, Ge, Mn, Re, Al, Co, Te, Cr, Ni;
elements having melting points similar to that of Al, i.e., Sb, Pu,
Mg, Al, Ba; and elements of a separate group, i.e., Ag, Pb, and Sn.
Each additive of about 5 atom % was added for examining the
effects.
[0077] Disks were manufactured in accordance with Examples 1 and 3
in order to examine the overwriting repeatability. Even if an
element had an ion radius value similar to that of Al, the element
often caused phase separation during repetition when the melting
point is far from that of Al. For an element having a melting point
similar to that of Al, degradation occurred due to mass transfer as
a result of repetition if the ion radius value was far apart from
that of Al. When Pb or Sn was added, both the repeatability and
crystallization speed were improved, while the crystallization
temperature lowered to some degree. When Ag was added, the signal
amplitude was improved, and the repetition number was increased
slightly. In conclusion, a maximum repetition number was obtained
for a disk including an additive having an ion radius and a melting
point similar to that of Al.
EXAMPLE 10
[0078] Various additives were added to Ge.sub.3Al.sub.2Te.sub.6
recording films for the purpose of examining the recording
performance of the films. For the additives, Sn, Pb and Ag were
selected, since these elements win form a rock-salt type crystal
structure with Te (SnTe, PbTe, AgSbTe.sub.2) in a thermally
equilibrium state. Concentrations of the respective elements were
5% and 8.5%. Disks were manufactured in accordance with Examples 1
and 3 for examining the laser crystal portions to find s rock-salt
type crystal of a single phase. In an examination on the
overwriting repeatability, no mass transfer occurred even after
10000 times of repetition.
[0079] FIG. 8A-8F and FIGS. 9A-9E show crystal structures for
representative examples in Examples 10 and 11. In the drawings,
only some of the structures include lattice defects, which
indicates that lattice defects are formed depending on the
compositions. Te or Se atoms occupy the 4a sites while the other
atoms and lattice defects (vacancy) occupy the 4b sites. The atoms
occupy the respective sites at random and the rate is influenced by
the composition.
EXAMPLE 11
[0080] A recording film was formed in which Sb of Example 4 was
replaced by Al. The composition of the recording film was
(GeTe).sub.x(Al.sub.2Te.- sub.3).sub.(1-x) (x=0.67, 0.8). The
recording film was irradiated with a laser beam so as to obtain a
metastable single phase. In an evaluation of the disk performance,
overwrite-recording at a linear velocity of 9 m/s was achieved.
Recording sensitivity was increased by about 10% in disks
comprising the composition together with 3 atom % of Sb or Bi.
EXAMPLE 12
[0081] In accordance with Example 1, various (100 kinds) optical
disks were manufactured in which the composition is represented by
[(Ge+Sn).sub.4Sb.sub.2Te.sub.7].sub.(100-y)Cr.sub.y. In the
composition, x indicates a percentage of Sn in the entire
composition and y indicates atom %. The values of x and y were
varied in the following range:
[0082] x=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%
[0083] y=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%.
[0084] A substrate used in this example is a disc-shape
polycarbonate resin substrate that is 0.6 mm in thickness, 120 mm
in diameter and 15 mm in inner diameter. A spiral groove was formed
on substantially the whole surface of the substrate. The track was
a concave-convex groove having a depth of 70 nm. Both the groove
portion and the land portion of the track had a width of 0.615
.mu.m. A multilayer film would be formed on the surface. A laser
beam for recording/reproducing an information signal can move to an
arbitrary position on the disk by a servo signal obtained from the
concave-convex shape. On the substrate, the following layers were
formed in this order: a ZnS:20 mol % SiO.sub.2 protective layer 100
nm in thickness; a GeN-based interface layer 5 nm in thickness; a
recording layer 9 nm in thickness having the above-identified
composition; a GeN interface layer 5 nm in thickness; a ZnS:20 mol
% SiO.sub.2 protective layer 40 nm in thickness; a Ge-based or
Si-based alloy layer 40 nm in thickness; and an Ag-based metal
reflecting layer 80 nm in thickness. The disk characteristics were
evaluated on three criteria, i.e., signal volume, repetition
number, and stability of rewriting sensitivity (after an
environmental test at 80.degree. C., 90% RH for 200H). In an
evaluation carried out by taking a disk of y=0 and z=0 as a
standard, the crystallization speed was increased with an increase
of Sn concentration, while excessive Sn decreased stability of an
amorphous state. When Cr concentration was increased, the
crystallization speed and signal amplitude were lowered and
rewriting sensitivity was lowered due to an environmental test,
while the stability of the amorphous state and repetition number
were increased. It was confirmed that equivalent or better
performance was obtainable for all the three criteria when the Sn
concentration was in a range from 3% to 15% and the Cr
concentration was in a range from 1% to 10%. It was effective
especially in improving both the repetition number and the
stability of rewiring sensitivity when the Sn concentration was in
a range from 5% to 10% and the Cr concentration was in a range from
1% to 5%.
EXAMPLE 13
[0085] In accordance with Example 12, 100 kinds of optical disks
were manufactured in which the composition is represented by
[(Ge+Sn).sub.4Sb.sub.2Te.sub.7].sub.(100-z)Ag.sub.z. In the
composition, x indicates a percentage of Sn in the entire
composition and z indicates atom %. The values of x and z were
varied in the following range:
[0086] x=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%
[0087] z=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%.
[0088] The thickness of the respective layers and evaluation
criteria are identical to those of Example 12. It was confirmed
that crystallization speed was raised with an increase of Sn
concentration, but stability of an amorphous state deteriorated
when the concentration was increased excessively. It was confirmed
also that increase of Ag concentration increased signal size,
though excessive Ag lowered the repeatability.
[0089] It was confirmed that equivalent or better performance was
obtainable for all the three criteria in a comparison with a case
where no additives were included, when the Sn concentration was in
a range from 3% to 15% and the Ag concentration was in a range from
1% to 10%. It was effective especially in improving both the signal
amplitude and the stability of rewiring sensitivity when the Sn
concentration was in a range from 5% to 10% and the Ag
concentration was in a range from 1% to 3%.
EXAMPLE 14
[0090] In accordance with Examples 12 and 13, 1000 kinds of optical
disks were manufactured in which the composition is represented by
[(Ge+Sn).sub.4Sb.sub.2Te.sub.7].sub.(100y-z)Cr.sub.yAg.sub.z. In
the composition, x indicates a percentage of Sn in the entire
composition and y indicates atom %. The values of x, y and z were
varied in the following range:
[0091] x=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%
[0092] y=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%
[0093] z=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%.
[0094] The thickness of the respective layers and evaluation
criteria are identical to those of Examples 12 and 13. It was
confirmed that equivalent or better performance was obtainable for
all the three criteria when the Sn concentration was in a range
from 3% to 15%, the Cr concentration was in a range from 1% to 5%,
and the Ag concentration was in a range from 1% to 10%. It was
effective especially in improving signal amplitude, stability of
rewiring sensitivity and repeatability when the Sn concentration
was in a range from 5% to 10%, the Cr concentration was in a range
from 1% to 3%, and the Ag concentration was in a range from 1% to
3%.
EXAMPLE 15
[0095] Similar results were obtained in an evaluation in accordance
with Examples 12, 13 and 14, where Cr was replaced by Mn.
EXAMPLE 16
[0096] The tests of Examples 12, 13, 14, and 15 were carried out
after replacing the base material by a
(GeTe).sub.x(Sb.sub.2Te.sub.3).sub.(1-x) quasibinary system
material (0<x<1) and a GeTe--Bi.sub.2Te.sub.3 quasibinary
system material (0<x<1), and similar effects were obtained.
Particularly, when 0.5.ltoreq.x.ltoreq.0.9, both the repeatability
and amorphous stability were obtainable. The Sn concentration was
preferably 1/2 or less of the Ge concentration in the base
material, since the amorphous phase stability deteriorates when the
Sn concentration exceeds the limitation.
EXAMPLE 17
[0097] On a 0.6 mm thick polycarbonate substrate, a
Ge.sub.19Sn.sub.2.1Sb.sub.26.3Te.sub.52.6 (atom %) thin film having
a thickness of 1 .mu.m was formed by sputtering. The whole surface
of the film was irradiated with a laser beam for crystallization,
and subsequently, an x-ray diffraction pattern was observed and the
structure was analyzed by a Rietveld method (a method to identify
by measuring several model substances and comparing the substances
with a target substance) and a WPPF (whole-powder-peak-fitting)
method. It was confirmed that the film comprised a NaCl type
crystalline phase and amorphous phase, and that there were about
20% of lattice defects at the 4b sites. The above-identified thin
film composition can be represented by
(Ge+Sn).sub.2Sb.sub.2.5Te.sub.5, in which about 0.5 mol of the 2.5
mol Sb cannot enter the lattices and the excessive Sb will be
deposited as an amorphous component. At that time, the molar ratio
(r) of the composition of the amorphous phase to that of the
crystalline phase was about 0.5/1=0.5. In a test where the Sb
concentration was varied on a basis of the composition,
crystallization characteristics were kept experimentally when `r`
was 2.0 or less. When `r` was 1.0 or less, the crystallization
speed would be increased further.
EXAMPLE 18
[0098] Similar analysis was carried out by varying the composition
of recording films in Example 17. Table 8 shows the test results.
The right column in the table indicates speed of crystallization
caused by laser irradiation. The mark .circleincircle. indicates
that the time for crystallization is 100 ns or less. .largecircle.
indicates that the time is 200 ns or less, .DELTA. denotes that the
time is 500 ns or less and x denotes the time exceeds 500 ns. A
recording film with a mark .largecircle. will be applied preferably
to recent systems, however, a recording film with a mark .DELTA.
also can be applied to the systems. As indicated in the table, all
of these compositions include lattice defects inside thereof, and
one phase forms a complex phase comprising a NaCl type crystalline
phase and an amorphous phase. When a ratio `r` of the amorphous
phase to the crystalline phase in the complex phase is 1 or less,
high speed crystallization is available. Crystallization will be
difficult when the ratio `r` exceeds 2.
8TABLE 8 Compositions and structures of materials and
crystallization performance Lattice Crystallization No. Total
composition Structure of complex phase defect r performance 1
Ge.sub.3Sb.sub.2.5Te.sub.6 NaCl type crystalline phase 1 mol + Sb
16% 0.5 .circleincircle. amorphous phase 0.5 mol 2
Ge.sub.3Bi.sub.2.8Te.sub.6 NaCl type crystalline phase 1 mol + Bi
16% 0.8 .circleincircle. amorphous phase 0.8 mol 3
GeSb.sub.2.5Bi.sub.2Te.sub.7 NaCl type crystalline phase 1 mol + Sb
+ Bi 28% 0.5 .circleincircle. amorphous phase 0.5 mol 4
Ge.sub.3SnBi.sub.2.7Te.sub.7 NaCl type crystalline phase 1 mol + Sb
16% 0.7 .circleincircle. amorphous phase 0.7 mol 5
Ge.sub.2Sb.sub.2Cr.sub.0.3Te.sub.5 NaCl type crystalline phase 1
mol + Sb 20% 0.3 .circleincircle. amorphous phase 0.3 mol 6
GeSb.sub.2In.sub.0.2Te.sub.4 NaCl type crystalline phase 1 mol + Sb
25% 0.2 .circleincircle. amorphous phase 0.1 mol 7
GePb.sub.0.1Bi.sub.2Te.sub.4 NaCl type crystalline phase 1 mol + Bi
25% 0.1 .circleincircle. amorphous phase 0.1 mol 8
GeSb.sub.2.2Se.sub.0.1Te.sub.3.9 NaCl type crystalline phase 1 mol
+ Sb 20% 0.2 .circleincircle. amorphous phase 0.2 mol 9
Ge.sub.3.5Sn.sub.0.01Sb.sub.3Te.sub.7 NaCl type crystalline phase 1
mol + Sb 16% 0.01 .circleincircle. amorphous phase 0.01 mol 10
Ge.sub.3.5Sn.sub.0.1Sb.sub.3.5Te.sub.7 NaCl type crystalline phase
1 mol + Sb 16% 0.3 .circleincircle. amorphous phase 0.3 mol 11
Ge.sub.3.5Sn.sub.0.5Sb.sub.3Te.sub.7 NaCl type crystalline phase 1
mol + Sb 16% 1.0 .circleincircle. amorphous phase 1.0 mol 12
Ge.sub.3.5Sn.sub.0.5Sb.sub.3.5Te.sub.7 NaCl type crystalline phase
1 mol + Sb 16% 1.5 .largecircle. amorphous phase 1.5 mol 13
Ge.sub.3.5Sn.sub.0.5Sb.sub.4Te.sub.7 NaCl type crystalline phase 1
mol + Sb 16% 2.0 .DELTA. amorphous phase 2.0 mol 14
Ge.sub.3.5Sn.sub.0.5Sb.sub.4.5Te.sub.7 NaCl type crystalline phase
1 mol + Sb 16% 2.5 X amorphous phase 2.5 mol
EXAMPLE 19
[0099] A polycarbonate disk substrate having a diameter of 120 mm
and thickness of 0.6 mm was prepared, and a continuous groove 60 nm
in depth and 0.6 .mu.m in width was formed on the surface. On this
disk substrate, a multilayer film comprising the recording films of
Nos. 9-18 in Example 18 was formed in a predetermined order by
sputtering, a protective plate was adhered by using an ultraviolet
curing resin, and subsequently, the recording layers were
crystallized by means of laser irradiation. Each multilayer film
structure has six layer lamination on a substrate, and the layers
are ZnS--SiO.sub.2: 20 mol % layer 90 nm in thickness, a Ge--N
layer 5 nm in thickness, a recording layer 20 nm in thickness, a
Ge--N layer 5 nm in thickness, a ZnS--SiO.sub.2: 20 mol % layer 25
nm in thickness, and an Al alloy layer 100 nm in thickness.
[0100] A deck for evaluating the disk characteristics comprises an
optical head equipped with a red semiconductor laser having a
wavelength of 650 nm and an object lens having NA of 0.6. The
rotation velocity of each disk was varied to find the linear
velocity range where recording and erasing (overwriting) were
available. Modulation frequencies (f1 and f2) were selected so that
recording marks would be 0.6 .mu.m and 2.2 .mu.m under any linear
velocity conditions, and recording was carried out alternately in
order to find repeatability based on the C/N and the erasing rate.
In Example 19, the recording portion was the groove. DC erasing was
carried out after the recording. The results are shown in Table 9.
The linear velocity demonstrated in Table 9 is the upper limit of
linear velocity allowing the C/N that has been amorphous-recorded
at f1 to exceed 48 dB and at the same time, the DC erasing rate
(crystallization) of a f1 signal to exceed 25 dB.
[0101] Table 9 shows that applicable range of linear velocity can
be selected continuously in an arbitrary manner in accordance with
change of the r value. Under each maximum linear velocity
condition, any disks provided excellent repeatability of more than
10000 times.
9TABLE 9 Material composition and limitation of applicable linear
velocity Linear No. Composition R Repetition number velocity limit
9 Ge.sub.3.5Sn.sub.0.01Sb- .sub.3Te.sub.7 0.01 >500,000 50.0 m/s
10 Ge.sub.3.5Sn.sub.0.1Sb.- sub.3.5Te.sub.7 0.3 >500,000 30.0
m/s 11 Ge.sub.3.5Sn.sub.0.5Sb.- sub.3Te.sub.7 1.0 300,000 10.0 m/s
12 Ge.sub.3.5Sn.sub.0.5Sb.sub.3.- 5Te.sub.7 1.5 100,000 3.0 m/s 13
Ge.sub.3.5Sn.sub.0.5Sb.sub.4Te.su- b.7 2.0 50,000 1.0 m/s 14
Ge.sub.3.5Sn.sub.0.5Sb.sub.4.5Te.sub.7 2.5 10,000 0.3 m/s
EXAMPLE 20
[0102] An apparatus as shown in FIG. 10 was assembled. In Example
20, a Si substrate having a nitrided surface was prepared. An
electrode of Au having a thickness of 0.1 .mu.m was provided on the
substrate by sputtering and subsequently, a SiO.sub.2 film having a
thickness of 100 nm was formed thereon through a metal mask
provided with a circular hole 0.5 mm in diameter. Next, a
(Ge.sub.3Sn.sub.1Sb.sub.2Te.sub.7).sub.95Cr.s- ub.5 film was formed
thereon to have a thickness of 0.5 .mu.m, an Au electrode was
sputtered to have a thickness of 0.5 .mu.m, and the respective
electrodes were bonded to Au leads. By applying 500 mV voltage
between these electrodes for a period of a pulse width of 100 ns,
the device transformed from a high resistant state to a low
resistant state. When this device was charged with current of 100
mA for a period of a pulse width of 80 ns in the next step, the
state of the device was reversed from the low resistant state to a
high resistant state.
Industrial Applicability
[0103] As mentioned above, the present invention provides an
optical information recording medium having a recording thin film.
The recording medium having a recording thin film exhibits little
variation of the recording and reproduction characteristics even
after repetition of recording and reproduction, excellent
weatherability. The present invention provides also a method of
manufacturing the information recording medium. The present
invention provides a recording medium having a recording thin film
that has strong resistance against composition variation and easily
controllable characteristics.
* * * * *