U.S. patent application number 09/737776 was filed with the patent office on 2001-10-18 for optical recording medium and method for making.
This patent application is currently assigned to TDK Corporation. Invention is credited to Kikukawa, Takashi, Kuribayashi, Isamu, Takahashi, Makoto, Tominaga, Junji.
Application Number | 20010031332 09/737776 |
Document ID | / |
Family ID | 26527893 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010031332 |
Kind Code |
A1 |
Tominaga, Junji ; et
al. |
October 18, 2001 |
Optical recording medium and method for making
Abstract
A phase change optical recording medium has on a substrate a
recording layer consisting essentially of a Sb base thin film and a
reactive thin film. The Sb base thin film is formed by depositing a
Sb base material containing at least 95 at % of Sb to a thickness
of 70-150 .ANG.. The reactive thin film is formed of a material
which forms a phase change recording material when mixed with Sb.
The reactive thin film is typically formed of an In--Ag--Te or
Ge--Te material. Stable write/read characteristics are accomplished
at the first overwriting, initializing operation is eliminated, and
rewriting is impossible at the same linear velocity as
recording.
Inventors: |
Tominaga, Junji;
(Tsukuba-shi, JP) ; Kuribayashi, Isamu; (Nagano,
JP) ; Takahashi, Makoto; (Nagano, JP) ;
Kikukawa, Takashi; (Nagano, JP) |
Correspondence
Address: |
R. J. Lasker
Larson & Taylor
TransPotomac Plaza
1199 N. Fairfax Street Suite 900
Alexandria
VA
22314
US
|
Assignee: |
TDK Corporation
Reel 8710
0953
|
Family ID: |
26527893 |
Appl. No.: |
09/737776 |
Filed: |
December 18, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09737776 |
Dec 18, 2000 |
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08936330 |
Sep 24, 1997 |
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6114833 |
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Current U.S.
Class: |
428/64.4 ;
G9B/7.142; G9B/7.186; G9B/7.194 |
Current CPC
Class: |
G11B 2007/24308
20130101; Y02T 90/12 20130101; Y10T 428/21 20150115; G11B
2007/24312 20130101; B60L 53/00 20190201; G11B 2007/24314 20130101;
Y02T 10/7072 20130101; G11B 7/257 20130101; G11B 7/266 20130101;
B60L 2210/20 20130101; G11B 2007/24316 20130101; Y02T 90/14
20130101; H02J 7/14 20130101; G11B 2007/2431 20130101; H02J 7/0063
20130101; Y02T 10/72 20130101; Y02T 10/70 20130101; G11B 7/26
20130101; H02J 7/00 20130101; G11B 7/243 20130101; Y10S 430/146
20130101; G11B 7/2595 20130101 |
Class at
Publication: |
428/64.4 |
International
Class: |
B32B 003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 9, 1996 |
JP |
8-227824 |
Dec 12, 1996 |
JP |
8-352298 |
Claims
1. An optical recording medium comprising on a transparent
substrate a recording layer consisting essentially of at least one
antimony base thin film and at least one reactive thin film wherein
said antimony base thin film and said reactive thin film are
disposed in close contact, said antimony base thin film is formed
by depositing an Sb base material containing at least 95 at % of
antimony to a thickness of at least 70 .ANG., and said reactive
thin film is formed of a material which forms a phase change
recording material when mixed with antimony.
2. The optical recording medium of claim 1 wherein said antimony
base thin film is crystalline.
3. The optical recording medium of claim 1 wherein said reactive
thin film is formed by depositing a In--Ag--Te base material
containing indium, silver, and tellurium as major components or
indium, silver, tellurium, and antimony as major components.
4. The optical recording medium of claim 3 wherein in the
In--Ag--Te base material, the atomic ratio of indium, silver,
tellurium, and antimony is represented by the formula:
(In.sub.xAg.sub.yTe.sub.1-x-y).sub.1-zSb.sub.- z (I-1) wherein
letters x, y, and z are in the range: 0.1.ltoreq.x.ltoreq.0.3,
0.1.ltoreq.y.ltoreq.0.3, and 0.ltoreq.z.ltoreq.0.5.
5. The optical recording medium of claim 3 wherein at least one of
the Sb base material and the In--Ag--Te base material contains an
element M selected from the group consisting of H, Si, C, V, W, Ta,
Zn, Ti, Ce, Tb, and Y, the content of element M in the recording
layer is less than 5 at %, and the content of element M in the Sb
base material is less than 5 at %.
6. The optical recording medium of claim 3 wherein in the recording
layer, the silver is partially replaced by gold.
7. The optical recording medium of claim 3 wherein in the recording
layer, the antimony is partially replaced by bismuth.
8. The optical recording medium of claim 3 wherein in the recording
layer, the tellurium is partially replaced by selenium.
9. The optical recording medium of claim 3 wherein in the recording
layer, the indium is partially replaced by aluminum or phosphorus
or both.
10. The optical recording medium of claim 1 wherein the reactive
thin film is formed by depositing a Ge--Te base material containing
germanium and tellurium as major components or germanium,
tellurium, and antimony as major components.
11. The optical recording medium of claim 1 wherein the number of
interfaces between the antimony base thin film and the reactive
thin film in the recording layer is up to 20.
12. The optical recording medium of claim 1 wherein when a
reflectance is measured from the side of the transparent substrate,
the recording layer as prepared has a reflectance Ro, and the
recording layer having undergone repetitive recording includes a
crystalline portion having a reflectance Rc and an amorphous
portion having a minimum reflectance Ra, which are in the
relationship: Ra<Ro.ltoreq.Rc.
13. The optical recording medium of claim 1 wherein the recording
layer includes a crystalline portion and an amorphous portion which
have an absorption Ac and Aa at the wavelength of a write/read
laser beam, respectively, wherein Ac/Aa.gtoreq.0.9.
14. The optical recording medium of claim 1 wherein a transmittance
of at least 1% is measured when a write/read laser beam is
irradiated to the recording layer from below the transparent
substrate.
15. The optical recording medium of claim 1 wherein the recording
layer includes a crystalline portion and an amorphous portion which
have an absorption Ac and Aa at the wavelength of a write/read
laser beam, respectively, wherein Ac/Aa.gtoreq.0.9 and a
transmittance of at least 1% is measured when a write/read laser
beam is irradiated to the recording layer from below the
transparent substrate.
16. The optical recording medium of claim 1 wherein the recording
layer is irradiated with a laser beam at a linear velocity whereby
the antimony base thin film-forming material and the reactive thin
film-forming material are mixed to form a record mark, and the
laser beam irradiation at the linear velocity is insufficient to
crystallize the record mark.
17. A method for producing an optical recording medium according to
claim 1, comprising the mixing step of continuously exposing the
recording layer to a laser beam to thereby mix the antimony base
thin film-forming material with the reactive thin film-forming
material.
18. The method of claim 17 wherein in the mixing step, the
recording layer is irradiated with a laser beam at a linear
velocity Vm which is controlled relative to the linear velocity Vw
at which the recording layer is irradiated with a laser beam during
a rewriting step, so as to satisfy 0.2 Vw.ltoreq.Vm.
19. The method of claim 18 wherein the linear velocity Vm is
controlled so as to satisfy Vw.ltoreq.Vm.
20. A method for producing an optical recording medium according to
claim 1, comprising the steps of evacuating a sputtering chamber to
lower than 0.5.times.10.sup.-2 Pa, introducing a sputtering
atmosphere gas into the chamber, and sputtering the Sb base
material in the chamber to deposit the antimony base thin film.
21. A method for producing an optical recording medium according to
claim 1, comprising the steps of forming the recording layer, and
heat treating the recording layer at a temperature of 60 to
120.degree. C.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a phase change optical recording
medium and a method for preparing the same.
[0003] 2. Prior Art
[0004] Highlight is recently focused on optical recording media
capable of recording information at a high density and erasing the
recorded information for overwriting. One typical rewritable (or
erasable) optical recording medium is of the phase change type
wherein a laser beam is directed to the recording layer to change
its crystallographic state whereupon a change of reflectance by the
crystallographic change is detected. Optical recording media of the
phase change type are of great interest since they can be
overwritten by modulating the intensity of a single light beam and
the optical system of the drive unit used for their operation is
simple as compared with magneto-optical recording media.
[0005] Most optical recording media of the phase change type used
Ge--Te systems which provide a substantial difference in
reflectance between crystalline and amorphous states and have a
relatively stable amorphous state. It was recently proposed to use
new compounds known as chalcopyrites. Chalcopyrite compounds were
investigated as compound semiconductor materials and have been
applied to solar batteries and the like. The chalcopyrite compounds
are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed
in terms of the Groups of the Periodic Table and have two stacked
diamond structures. The structure of chalcopyrite compounds can be
readily determined by X-ray structural analysis and their basic
characteristics are described, for example, in Physics, Vol. 8, No.
8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No.
4 (1988), pp. 228.
[0006] Among the chalcopyrite compounds, AgInTe.sub.2 is known to
be applicable as a recording material by diluting it with Sb or Bi.
The resulting optical recording media are generally operated at a
linear velocity of about 7 m/s. See Japanese Patent Application
Kokai (JP-A) No. 240590/1991, 99884/1991, 82593/1991, 73384/1991,
and 151286/1992.
[0007] In addition to these phase change type optical-recording
media using chalcopyrite compounds, JP-A 267192/1992, 232779/1992,
and 166268/1994 disclose phase change type optical recording media
wherein an AgSbTe.sub.2 phase forms when a recording layer
crystallizes.
[0008] For prior art phase change type optical recording media,
recording layers are formed using vacuum deposition equipment and
remain amorphous immediately after formation. The recording layers
must be crystallized by an operation generally known as
initialization before the recording media can be utilized as
rewritable media.
[0009] Initialization is carried out in various ways, for example,
after a recording layer is formed on a substrate, by heating the
substrate to the crystallization temperature of the recording layer
for crystallization as disclosed in JP-A 3131/1990; irradiating a
laser beam to the recording layer for crystallization, which method
is called solid phase initialization, as disclosed in JP-A
366424/1992, 201734/1990 and 76027/1991; irradiating flash light to
the substrate to achieve pseudo-crystallization by so-called
photo-darkening, which method takes advantage of the photo
characteristics of calcogen compounds, as disclosed in JP-A
281219/1992; and high-frequency induction heating the medium. JP-A
98847/1990 proposes to heat a substrate during formation of a
recording layer to thereby crystallize the recording layer. JP-A
5246/1990 discloses a method involving the steps of forming a first
dielectric layer, forming a recording layer thereon, heating it for
crystallization, and forming a second dielectric layer thereon.
[0010] However, the initialization step by laser beam irradiation
takes a long time and causes low productivity. Heating of the
overall medium rejects the use of inexpensive resin substrates.
That is, resin substrates can be distorted upon heating for
initialization, causing tracking errors. The method of irradiating
flash light is also low in productivity because several shots of
irradiation are necessary to achieve full crystallization.
[0011] Under the circumstances, the use of a so-called bulk eraser
is the only technique which is regarded commercially acceptable and
currently used. The bulk eraser irradiates a beam from a high power
gas or semiconductor laser through a relatively large aperture stop
for crystallizing a multiplicity of tracks altogether. Since the
bulk eraser permits the recording layer to be locally heated, the
substrate temperature is elevated to a little extent, enabling the
use of less heat resistant resins as substrates.
[0012] The bulk eraser, however, requires a time of several minutes
for initializing optical recording discs of 12 cm in diameter. Then
the initializing step is a rate-determining step in the making of
optical recording discs. While TeGeSb base materials are currently
most widely used for phase change recording layers, it is believed
that the initializing operation cannot be removed insofar as these
materials are used.
[0013] Prior art phase change type recording media require to
repeat rewriting several times after initialization until a
constant rate of erasure is reached. In most cases, rewriting is
repeated about ten times before performance rating is carried out.
The reason why the rate of erasure remains unstable upon rewriting
immediately after initialization is that the formation of a
AgSbTe.sub.2 or In--Ge crystalline phase is incomplete.
[0014] To eliminate the initialization step which is required by
prior art phase change type recording media, U.S. Ser. No.
08/598,913, entitled "Method for Preparing Phase Change Optical
Recording Medium" and assigned to the same assignee as the present
invention, proposes a method for forming a In--Ag--Te--Sb base
recording layer by separately effecting the step of sputtering
Sb+In and the step of sputtering Ag+Te or by separately effecting
the step of sputtering Sb, the step of sputtering In, and the step
of sputtering Ag+Te. The recording layer formed by such a series of
steps has been at least partially crystallized. After recording is
repeated on the recording layer formed by this method so that the
elements in the recording layer are fully diffused and mixed with
each other, a sufficient change of reflectance is obtained as
acquired after initialization by the bulk eraser. However, in the
duration from immediately after the formation of the recording
layer to several times of rewriting, the rate of erasure remains
unstable like prior art phase change type recording media. More
particularly, since reflectance is different between the region
crystallized during formation and the region crystallized upon
rewriting, the reflectance remains unstable until the rewritten
regions are extended by increments throughout the entire surface of
the recording layer. In the case of mark edge recording utilized in
rewritable digital video discs (DVD-RAM), such reflectance
variations can be mistaken for mark edges.
[0015] JP-A 106647/1996 discloses a phase change type recording
medium comprising a recording layer in the form of AgInSbTe system
artificial superlattice film having alternately deposited
AgSbTe.sub.2 films and In--Sb films or having alternately deposited
AgSbTe.sub.2 films, In films, and Sb films. One of the alleged
advantages is that the initialization energy required for the
entire recording layer is reduced because the crystallized
AgSbTe.sub.2 films are used.
[0016] We found that when an AgSbTe.sub.2 film and an In--Sb film
were stacked, as in the case of U.S. Ser. No. 08/598,913, the
reflectance remained unstable in the duration from immediately
after the formation of the recording layer to several times of
rewriting. Also when an AgSbTe.sub.2 film, an Sb film, and an In
film were alternately deposited, the reflectance remained unstable
until rewriting was done several times. To acquire a stable
reflectance in the crystalline region upon rewriting, the In--Ge
crystalline phase must be present in the crystalline region.
However, in the embodiment of JP-A 106647/1996 wherein indium is
not present in the AgSbTe.sub.2 film, but as the In--Sb film or In
film, it becomes difficult for indium to bond with tellurium to
form an In--Ge crystalline phase. Where initialization is carried
out with low energy as described in JP-A 106647/1996, the In--Ge
crystalline phase cannot be fully formed during the initialization.
For this reason, the reflectance remained unstable until the In--Ge
crystalline phase is fully formed by repeating rewriting several
times. It is noted that specific initializing conditions such as
linear velocity and laser power are described nowhere in JP-A
106647/1996.
[0017] In examples described in JP-A 106647/1996, the Sb films and
In--Sb films have a thickness of less than 5 nm. These films cannot
be crystalline when their thickness is less than 5 nm. As a result,
the reflectance of the recording layer immediately after its
formation is very low. Since the low reflectance hinders focusing
of a laser beam and hence, uniform heating, it becomes difficult to
achieve uniform initialization.
[0018] Still further, the content of indium in the In--Sb film is
described nowhere in JP-A 106647/1996. In Example of JP-A
106647/1996, a laminate construction wherein indium and antimony
are separated into In films and Sb films is simply compared with a
single layer construction wherein indium and antimony are not
separated, but formed into an In--Sb film. It is thus believed that
the composition of the In--Sb film is the same as the combination
of In and Sb films. Since the In and Sb films have the same gauge,
it is believed that the indium content in the In--Sb film is about
10 to 15 at %. Such a high indium content makes it difficult to
form an In--Sb film as a crystalline one even if its thickness is
increased. There still arises the above-mentioned problem
associated with initialization.
SUMMARY OF THE INVENTION
[0019] Therefore, an object of the present invention is to provide
a novel and improved phase change optical recording medium wherein
the manufacturing time is reduced and stable write/read
characteristics are accomplished at the first overwriting as
opposed to prior art optical recording media wherein the
initializing step of the recording layer is a rate-determining step
in their manufacture.
[0020] Another object of the present invention is to provide a
novel and improved write-once type phase change optical recording
medium which eliminates initializing operation and which cannot be
rewritten at the same linear velocity as recording.
[0021] In a first aspect of the invention, there is provided an
optical recording medium comprising on a transparent substrate a
recording layer consisting essentially of at least one antimony
base thin film and at least one reactive thin film. The antimony
base thin film and the reactive thin film are disposed in close
contact. The antimony base thin film is formed by depositing an Sb
base material containing at least 95 at % of antimony to a
thickness of at least 70 .ANG.. The reactive thin film is formed of
a material which forms a phase change recording material when mixed
with antimony.
[0022] Preferably, the antimony base thin film is crystalline.
[0023] In a first preferred embodiment of the optical recording
medium, the reactive thin film is formed by depositing a In--Ag--Te
base material containing indium, silver, and tellurium as major
components or indium, silver, tellurium, and antimony as major
components.
[0024] In the In--Ag--Te base material, the atomic ratio of indium,
silver, tellurium, and antimony is typically represented by the
formula:
(In.sub.xAg.sub.yTe.sub.1-x-y).sub.1-zSb.sub.z (I-1)
[0025] wherein letters x, y, and z are in the range:
0.1.ltoreq.x.ltoreq.0.3, 0.1.ltoreq.y.ltoreq.0.3, and
0.ltoreq.z.ltoreq.0.5.
[0026] Preferably, at least one of the Sb base material and the
In--Ag--Te base material contains an element M selected from the
group consisting of H, Si, C, V, W, Ta, Zn, Ti, Ce, Tb, and Y, the
content of element M in the recording layer is less than 5 at %,
and the content of element M in the Sb base material is less than 5
at %.
[0027] In the recording layer, the silver may be partially replaced
by gold; the antimony may be partially replaced by bismuth; the
tellurium may be partially replaced by selenium; the indium may be
partially replaced by aluminum or phosphorus or both.
[0028] In a second preferred embodiment of the optical recording
medium, the reactive thin film is formed by depositing a Ge--Te
base material containing germanium and tellurium as major
components or germanium, tellurium, and antimony as major
components.
[0029] The number of interfaces between the antimony base thin film
and the reactive thin film in the recording layer is preferably up
to 20.
[0030] Preferably, when a reflectance is measured from the side of
the transparent substrate, the recording layer as prepared has a
reflectance Ro, and the recording layer having undergone repetitive
recording includes a crystalline portion having a reflectance Rc
and an amorphous portion having a minimum reflectance Ra, which are
in the relationship: Ra<Ro.ltoreq.Rc.
[0031] Preferably, the recording layer is irradiated with a laser
beam at a linear velocity whereby the antimony base thin
film-forming material and the reactive thin film-forming material
are mixed to form a record mark, and the laser beam irradiation at
the linear velocity is insufficient to crystallize the record mark.
That is, when a laser beam is irradiated at the same linear
velocity as used in forming a record mark, the record mark cannot
be crystallized.
[0032] In a second aspect of the invention, there is provided a
method for producing an optical recording medium as defined above,
comprising the mixing step of continuously exposing the recording
layer to a laser beam to thereby mix the antimony base thin
film-forming material with the reactive thin film-forming
material.
[0033] In the mixing step, the recording layer is irradiated with a
laser beam at a linear velocity Vm which is preferably controlled
relative to the linear velocity Vw at which the recording layer is
irradiated with a laser beam during a rewriting step, so as to
satisfy 0.2 Vw.ltoreq.Vm. More preferably, the linear velocity Vm
is controlled so as to satisfy Vw.ltoreq.Vm.
[0034] Also provided is a method for producing an optical recording
medium as defined above, comprising the steps of evacuating a
sputtering chamber to lower than 0.5.times.10.sup.-2 Pa,
introducing a sputtering atmosphere gas into the chamber, and
sputtering the Sb base material in the chamber to deposit the
antimony base thin film.
[0035] Further provided is a method for producing an optical
recording medium as defined above, comprising the steps of forming
the recording layer, and heat treating the recording layer at a
temperature of 60 to 120.degree. C.
[0036] In prior art phase change recording media, the single-ply
amorphous recording layer formed by sputtering is initialized or
crystallized by heating and slow cooling. When a rewriting or
overwriting laser beam is irradiated after initialization, the
recording layer is melted in the region where the recording power
is applied and thereafter, quenched into an amorphous or
microcrystalline state with a lower reflectance, forming a record
mark. In the region where the erasing power is applied, no change
occurs and the reflectance is maintained unchanged from that after
initialization. Upon subsequent rewriting, the recording power is
applied at sites where new record marks are to be formed and the
erasing power is applied at the remaining sites. Whether the state
before irradiation is crystalline or amorphous or microcrystalline,
the sites where the recording power is applied are all converted
into amorphous or microcrystalline record marks and the sites where
the erasing power is applied all assume a crystalline state.
Overwrite recording is enabled in this way.
[0037] In contrast, the optical recording medium of the present
invention is produced by depositing an antimony base thin film and
a reactive thin film and effecting a mixing treatment. The mixing
treatment is to irradiate a laser beam to the recording layer to
heat it in order to mix the elements of the antimony base thin
films with the elements Of the reactive thin films. As a result of
the mixing treatment, the recording layer assumes a state wherein
amorphous phases such as Ag--Sb--Te are dispersed in the Sb
crystalline phase. Although the reflectance of the recording layer
before the mixing treatment is relatively high due to the
crystallized antimony base thin films, the mixing treatment reduces
the reflectance. It is understood that the reflectance of the
recording layer after the mixing treatment is still higher than the
reflectance of amorphous areas or record marks.
[0038] The mixing treatment achieves the same effect as the
initialization treatment used in prior art phase change recording
media in the sense that the recording layer as formed is converted
into a recordable state. Although the prior art initialization
treatment crystallizes the recording layer to increase its
reflectance, the mixing treatment according to the invention
converts the recording layer into a state wherein amorphous phases
are dispersed in the antimony crystalline phase, inviting a drop of
reflectance.
[0039] After the mixing treatment, recording and rewriting (or
overwriting) may be carried out in the same manner as in the prior
art phase change recording media. More particularly, the recording
layer is melted in the region where the recording power is applied
and thereafter, quenched into an amorphous or microcrystalline
state, forming a record mark. In the region where the erasing power
is applied, crystallization of AgSbTe.sub.2 or the like occurs to
increase the reflectance. Subsequent rewriting is carried out in
the same manner as in the prior art phase change recording
media.
[0040] According to the invention, the reflectances of the record
mark and crystalline regions obtained upon first irradiation of
rewriting laser beam after the mixing treatment are equal to those
of the record mark and crystalline regions upon second or later
irradiation of rewriting laser beam, respectively. That is, unlike
the prior art phase change recording media wherein the single-ply
amorphous recording layer is initialized, the optical recording
medium of U.S. Ser. No. 08/598,913, and the optical recording
medium of JP-A 106647/1996, the recording layer of the invention
has a fully stable reflectance already at the first recording and
rewriting.
[0041] In the optical recording medium of the invention, provided
that the recording layer as prepared (that is, prior to the mixing
treatment) has a reflectance Ro, and the recording layer having
undergone repetitive recording includes a crystalline portion
having a reflectance Rc and an amorphous portion (or record mark)
having a minimum reflectance Ra, these reflectances are in the
relationship:
Ra<Ro.ltoreq.Rc.
[0042] Note that the reflectance is measured from the side of the
transparent substrate. The minimum reflectance of the amorphous
portion is obtained when the reflectance becomes lowest as
amorphous conversion proceeds to the most extent. The reflectance
Ro of the recording layer immediately after its formation is
generally lower than Rc, but relatively high due to the
crystallized antimony thin film as previously mentioned, for
example, at least about 60% of Rc. This enables precise control of
the focusing of a laser beam irradiated for the mixing treatment
while the mixing treatment becomes uniform. In the event where the
reactive thin film is also crystallized, Ro can be equalized to Rc
by optimizing the composition and thickness of both the thin films
and optimizing the material and thickness of a dielectric layer and
a reflective layer which are formed on the medium surface along
with the recording layer. In this event, the mixing treatment can
be removed.
[0043] According to the invention, the linear velocity at which the
medium is rotated during the mixing treatment can be set
significantly higher than the linear velocity at which the medium
is rotated during conventional initialization treatment. This
achieves an improvement in productivity.
[0044] In the conventional initialization treatment, the single-ply
amorphous recording layer formed by sputtering is heated and slowly
cooled for crystallization. In the case of phase change recording
media, when the amorphous record mark is erased or crystallized by
overwriting, the record mark is heated and then slowly cooled.
Since the recording layer as formed and the record mark are common
in that they are amorphous, but different in energy, the
initialization requires higher energy and a lower linear velocity
in order to lower the cooling rate. Herein, the linear velocity at
which the rate of erasure during overwriting is less than -25 dB is
designated a "rewritable linear velocity," and the linear velocity
at which an optimum rate of erasure is available is designated a
"rewritable optimum linear velocity." The linear velocity required
for initialization is about 1/3 to about 1/2 of the rewritable
optimum linear velocity. As a consequence, the initialization by
irradiation of a laser beam requires a longer time.
[0045] In contrast, the present invention enables the
relationship:
Vw.ltoreq.Vm
[0046] wherein Vm is the linear velocity at which the recording
layer is rotated relative to a laser beam during the mixing
treatment, and Vw is the rewritable optimum linear velocity after
the mixing treatment. As a consequence, the time required for the
mixing treatment is significantly shorter than the time required
for the conventional initialization. The linear velocity Vm can be
increased by increasing the power of a laser beam during the mixing
treatment. Although the upper limit of Vm is not particularly
specified, it is usually
Vm.ltoreq.5 Vw
[0047] where an ordinary bulk eraser and recording device are
used,
[0048] If the linear velocity Vm during the mixing treatment is
made lower, the mixing treatment is enabled by a lower power laser
beam. Then the necessary laser power can be very low when the
mixing treatment is carried out at an equal linear velocity to the
conventional initialization. However, in order to carry out the
mixing treatment at a practical rate, the linear velocity Vm is
preferably controlled to be:
0.2 Vw.ltoreq.Vm.
[0049] The optical recording medium of the present invention is
well durably rewritable. A prior art In--Ag--Te--Sb system medium
fabricated by initializing a single-ply amorphous recording layer
becomes substantially non-rewritable after about 10,000 cycles of
repetitive rewriting because the rate of erasure drops. In
contrast, the first embodiment of the present invention which is
also a In--Ag--Te--Sb system medium can maintain the rate of
erasure over about 100,000 cycles of repetitive rewriting. Also the
second embodiment of the present invention which is a Te--Ge--Sb
system medium is more durably rewritable than prior art Te--Ge--Sb
system media.
[0050] It is believed that such an improvement in rewriting
durability is accomplished by stacking a reactive thin film and a
crystallized antimony base thin film and subjecting them to mixing
treatment as will be described later.
[0051] In the first embodiment of the invention, in order to
provide an increased reflectance and an increased rate of erasure,
a microcrystalline phase of Sb must be present in addition to the
AgSbTe.sub.2 crystalline phase upon application of erasing power to
the In--Ag--Te--Sb system recording layer to effect
crystallization. In the prior art method of initializing a
single-ply amorphous recording layer, however, microcrystals of Sb
are first formed upon initialization, silver is likely to diffuse
into the antimony microcrystals, and the repetition of rewriting
further promotes the silver diffusion, whereupon the function of
antimony microcrystals is exacerbated. Accordingly, the rate of
erasure lowers as rewriting is repeated. In contrast, according to
the present invention, when a reactive thin film is mixed with a
crystallized antimony base thin film, the silver (Ag) which has
formed stable compounds of Ag--Te and AgSbTe.sub.2 is mixed with
the antimony (Sb) phase. Since silver is thus unlikely to diffuse
into antimony microcrystals, the repetition of rewriting causes
only a slight drop of the erasure rate.
[0052] Further, in the second embodiment of the invention,
tellurium (Te) behaves as does silver (Ag) in the first embodiment.
Since tellurium forms stable compounds of GeTe.sub.2 and
Sb.sub.2Te.sub.3 before mixing with the antimony (Sb) phase, the
diffusion of tellurium alone is suppressed, resulting in an
improvement in rewriting durability.
[0053] The optical recording medium of the invention is described
as the rewritable (or erasable) one although it may be used as a
write-once type. In the write-once type application of the optical
recording medium according to the invention, the treatment of
mixing both the thin films should be removed. The write-once type
optical recording medium according to the invention can be
recorded, but cannot be erased when the above-mentioned rewriting
method is used, that is, when a drive unit for rewritable recording
media is used. More particularly, mixing of the antimony base thin
film and the reactive thin film is possible when the recording
power is applied, but crystallization is impossible in the region
where mixing has occurred when the erasing power is applied to that
region at the same linear velocity as in the recording mode. Since
the optical recording medium according to the invention can have a
relatively high reflectance immediately after preparation and
achieve a substantial drop of reflectance as a result of mixing
treatment, the invention accomplishes a write-once type optical
recording medium eliminating a need for initialization and having
excellent characteristics. The recording layer capable of
write-once type recording can be established by properly selecting
the composition and thickness of both the thin films.
[0054] While the first and second embodiments of the invention have
the above-mentioned advantages, the first embodiment is superior in
rewriting durability. Since the recording layer can be made
thinner, the first embodiment has the additional advantages of
reduced light absorption and a higher reflectance. This allows the
degree of modulation to be markedly increased by the provision of a
dielectric layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0055] These and further features of the present invention will be
apparent with reference to the following description and drawings,
wherein:
[0056] FIG. 1 is a schematic cross-sectional view of a portion of
an optical recording medium according to the present invention.
[0057] FIG. 2 is a schematic cross-sectional view of a portion of
another optical recording medium according to the present
invention.
[0058] FIG. 3 shows an electron beam diffraction image of a
recording layer having a In--Ag--Te reactive thin film formed on an
antimony thin film.
[0059] FIG. 4 is a TEM electronmicrograph of the recording layer
having a In--Ag--Te reactive thin film formed on an antimony thin
film.
[0060] FIG. 5 is a schematic cross-sectional view of a portion of a
further optical recording medium according to the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0061] Briefly stated, an optical recording medium according to the
invention has on a transparent substrate a recording layer
consisting essentially of at least one antimony base thin film and
at least one reactive thin film. In the recording layer, the
antimony base thin film is disposed in close contact with the
reactive thin film.
[0062] The antimony base thin film is formed by depositing an
antimony base material containing at least 95 at %, preferably at
least 97 at % of antimony (Sb), for example, by a sputtering
technique. The antimony base thin film has a thickness of at least
70 .ANG.. If the Sb content or thickness is below these limits,
such antimony base thin films are difficult to crystallize, failing
to achieve the objects of the invention.
[0063] The reactive thin film is formed by depositing a material
which forms a phase change recording material when it is mixed with
antimony.
[0064] First Embodiment: In--Ag--Te--Sb System
[0065] In the first embodiment of the invention, the reactive thin
film is formed by depositing a In--Ag--Te base material, for
example, by a sputtering technique. The In--Ag--Te base material
contains indium (In), silver (Ag), and tellurium (Te) as major
components or indium, silver, tellurium, and antimony (Sb) as major
components.
[0066] In the In--Ag--Te base material, the atomic ratio of indium,
silver, tellurium, and antimony is preferably represented by the
formula:
(In.sub.xAg.sub.yTe.sub.1-x-y).sub.1-zSb.sub.z (I-1)
[0067] wherein letters x, y, and z are in the range:
0.1.ltoreq.x.ltoreq.0.3, 0.1.ltoreq.y.ltoreq.0.3, and
0.ltoreq.z.ltoreq.0.5,
[0068] more preferably in the range: 0.15.ltoreq.x.ltoreq.0.28,
0.15.ltoreq.y.ltoreq.0.28, and 0.1.ltoreq.z.ltoreq.0.5,
[0069] further preferably 0.2.ltoreq.z.ltoreq.0.4.
[0070] If the value of x is too smaller, the indium content of the
recording layer is relatively too low so that record marks become
less amorphous, resulting in a lower degree of modulation and lower
reliability. If the value of x is too larger, the indium content of
the recording layer is relatively too high so that the reflectance
of regions other than record marks becomes low, resulting in a
lower degree of modulation.
[0071] If the value of y is too smaller, the silver content of the
recording layer is relatively too low so that the recrystallization
of record marks and hence, repetitive overwriting becomes
difficult. If the value of y is too larger, the silver content of
the recording layer is relatively too high so that excess silver
solely diffuses into the antimony phase during the mixing
treatment. This results in lower rewriting durability, less
stability of both the record marks and the crystalline regions, and
a loss of reliability. Specifically, when the medium is stored at
elevated temperature, record marks crystallize more to invite drops
of C/N and modulation. Additionally, the deterioration of C/N and
modulation caused by repetitive recording is promoted.
[0072] If the value of x+y is too smaller, tellurium becomes
excessive to form a Te phase, which lowers the rate of crystal
transition to hinder erasure. If the value of x+y is too larger, it
would become difficult to make the recording layer amorphous and it
would become impossible to record signals.
[0073] If the value of z is too larger, it would become difficult
to make the recording layer amorphous and it would become
impossible to record signals. Although z may be equal to 0, it is
preferred to introduce antimony so that z may fall in the more
preferred range because the inclusion of Sb in the reactive thin
film improves rewriting durability. When z is in the optimum range
in which the crystallization temperature of the recording layer
becomes lowest, crystallization proceeds during formation of the
recording layer and the energy required for the mixing treatment is
very low or even fully stable rewriting becomes possible without a
need for mixing treatment. Even if crystallization proceeds during
formation of the recording layer, no intermixing occurs between
both the thin films. That is, both the thin films are present as
independent crystallized films.
[0074] The antimony base thin film and/or the reactive thin film
preferably contains an element M. The element M is selected from
the group consisting of H, Si, C, V, W, Ta, Zn, Ti, Ce, Tb, and Y.
The element M is effective for improving rewriting durability, more
specifically restraining the rate of erasure from lowering as a
result of repetitive rewriting. It is also effective for improving
reliability under severe conditions such as hot humid conditions.
At least one of V, Ta, Ce and Y is preferred among the elements M
because their effects are more outstanding. V and/or Ta is more
preferred, with V being most preferred.
[0075] The content of element M is 5 at % or less, preferably 3 at
% or less of the entire recording layer. If the M content of the
recording layer is too high, the change of reflectance associated
with a phase change becomes too small to provide a degree of
modulation. In order that the element M exert the above-mentioned
effects, the M content in the recording layer should preferably be
at least 0.5 at %.
[0076] Where the element M is contained in the antimony base
material, the content of element M in the Sb base material is 5 at
% or less, more preferably 3 at % or less. If the M content of the
antimony base material is too high, the antimony base thin film
cannot be converted into a crystalline film, failing to achieve the
objects of the invention. It is preferred that the antimony base
thin film consist essentially of antimony or of antimony and
element M.
[0077] The thickness of the antimony base thin film and the
thickness of the reactive thin film may be properly determined in
accordance with the compositions of both the thin films so that the
desired composition of the recording layer (which is obtained after
intermixing of both the thin films) may be eventually obtained.
[0078] The thickness of the antimony base thin film is at least 70
.ANG., preferably at least 80 .ANG.. If the antimony base thin film
is too thin, it does not become a uniform crystalline film, but an
amorphous film, failing to achieve the objects of the invention. If
the antimony base thin film is too thick, the reactive thin film
must accordingly be thick and consequently, the recording layer as
a while becomes too thick and increases light absorption. This
results in a lower reflectance and lower modulation. For this
reason, the thickness of the antimony base thin film is preferably
less than 150 .ANG., more preferably less than 110 .ANG..
[0079] The thickness of the reactive thin film corresponding to the
thickness of the antimony base thin film is preferably 25 to 100
.ANG., more preferably 30 to 90 .ANG.. If the reactive thin film is
thin, it may be island-like rather than continuous. In the first
embodiment of the invention, the reactive thin film need not be a
continuous film.
[0080] It is noted that the thickness of each thin film is
expressed by a value calculated by multiplying a deposition rate by
a deposition time.
[0081] The recording layer may consist solely of one antimony base
thin film and one reactive thin film. By constructing the recording
layer from three or more films, the rewriting durability is
significantly improved and the energy necessary for intermixing is
reduced. Where both the thin films are present in the recording
layer in a total number of at least three films, the recording
layer may be an even number ply structure consisting of alternating
antimony base thin films and reactive thin films or an odd number
ply structure having thin films of the same type at opposed
surfaces, most preferably an odd number ply structure having
reactive thin films at opposed surfaces. The recording layer of the
structure having reactive thin films at opposed surfaces, that is,
the structure wherein every antimony base thin film is interleaved
between reactive thin films is advantageous in that intermixing of
the materials of both the thin films rapidly and uniformly occurs
and the energy necessary for intermixing is further reduced.
[0082] If the number of both thin films is too large, the recording
layer becomes too thick. Therefore, the number of interfaces
between antimony base thin films and reactive thin films in the
recording layer is preferably 20 or less, more preferably 10 or
less.
[0083] The atomic ratio of elements in the entire recording layer
consisting of antimony base thin films combined with reactive thin
films is preferably represented by the following formula:
{(In.sub.aAg.sub.bTe.sub.1-a-b).sub.1-cSb.sub.C}.sub.1-dM.sub.d
(I-2)
[0084] wherein letters a, b, c, and d are in the range:
0.1.ltoreq.a.ltoreq.0.3, 0.1.ltoreq.b.ltoreq.0.3,
0.5.ltoreq.c.ltoreq.0.8- , and 0.ltoreq.d.ltoreq.0.05, more
preferably 0.15.ltoreq.a.ltoreq.0.28, 0.15.ltoreq.b.ltoreq.0.28,
0.55.ltoreq.c.ltoreq.0.65, and 0.005.ltoreq.d.ltoreq.0.05.
[0085] The reason why a and b are limited to the above range is the
same as the reason of limitation of x and y in formula (I-1). The
reason of limitation of d representative of the M content is as
previously described. If c is too small in formula (I-2), the
difference in reflectance associated with a phase change increases,
but the rate of crystal transition is drastically decelerated to
restrain erasure. If c is too large, the difference in reflectance
associated with a phase change decreases and the degree of
modulation is reduced.
[0086] Although it is preferred that the recording layer consists
essentially of silver (Ag), antimony (Sb), tellurium (Te) and
indium (In), it is acceptable that the silver is partially replaced
by gold (Au); the antimony is partially replaced by bismuth (Bi);
the tellurium is partially replaced by selenium (Se); and the
indium is partially replaced by aluminum (Al) and/or phosphorus
(P).
[0087] The percent replacement of Ag by Au is preferably less than
50 at %, more preferably less than 20 at %. With a higher percent
replacement, record marks are likely to crystallize, leading to a
loss of reliability at elevated temperature.
[0088] The percent replacement of Sb by Bi is preferably less than
50 at %, more preferably less than 20 at %. With a higher percent
replacement, the recording layer would have an increased
coefficient of absorption. As a result, the optical interference
effect and the difference in reflectance between crystalline and
amorphous regions are reduced, leading to a lower degree of
modulation and a lower C/N.
[0089] The percent replacement of Te by Se is preferably less than
50 at %, more preferably less than 20 at %. With a higher percent
replacement, the crystal transition would be retarded and the rate
of erasure be reduced.
[0090] The percent replacement of In by Al and/or P is preferably
less than 40 at %, more preferably less than 20 at %. With a higher
percent replacement, record marks would become less stable with a
resultant loss of reliability. The proportion of Al and P is
arbitrary.
[0091] It is noted that the recording layer after repetitive
rewriting has a coefficient of absorption k of about 3.3 in the
crystalline state and about 2.2 in the microcrystalline or
amorphous state.
[0092] In the recording layer, there may be present other elements
such as Cu, Ni, Zn, Fe, O, N, and C as trace impurities although
the total content of such impurity elements should preferably be
less than 0.05 at %.
[0093] The composition of the recording layer is identifiable by
electron probe microanalysis (EPMA), X-ray microanalysis, etc.
[0094] The recording layer preferably has a thickness of about 95
to 500 .ANG., more preferably about 130 to 300 .ANG.. A too thin
recording layer would restrain the growth of a crystalline phase
and provide an insufficient change of reflectance associated with a
phase change. If the recording layer is too thick, there would
occur a phenomenon during formation of record marks that silver
would diffuse in the recording layer more in a thickness direction
thereof and less in an in-plane direction. As a result, the
recording layer becomes less reliable. A too thick recording layer
would provide a lower reflectance and a lower degree of modulation
as previously mentioned.
[0095] Second Embodiment: Te--Ge--Sb System
[0096] In the second embodiment of the invention, the reactive thin
film is formed by depositing a Ge--Te base material. The Ge--Te
base material contains germanium (Ge) and tellurium (Te) as major
components or germanium, tellurium, and antimony (Sb) as major
components.
[0097] In the Ge--Te base material, the atomic ratio of germanium,
tellurium, and antimony is preferably represented by the
formula:
Ge.sub.xSb.sub.yTe.sub.1-x-y (II-1)
[0098] wherein letters x and y are in the range:
0.12.ltoreq.x.ltoreq.0.35 and 0.ltoreq.y.ltoreq.0.3,
[0099] more preferably 0.10.ltoreq.y.ltoreq.0.3.
[0100] If the value of x is too small, record marks are more
unlikely to crystallize and the rate of erasure would be lower. If
the value of x is too large, much tellurium would bond with
germanium with the resultant precipitation of antimony, inhibiting
formation of record marks.
[0101] If the value of y is too large, antimony would precipitate
to inhibit formation of record marks. Although y may be equal to 0,
it is preferred to introduce antimony so that y may fall in the
more preferred range because the inclusion of Sb in the reactive
thin film improves rewriting durability.
[0102] The thickness of the antimony base thin film and the
thickness of the reactive thin film may be properly determined in
accordance with the compositions of both the thin films so that the
desired composition of the recording layer (which is obtained after
intermixing of both the thin films) may be eventually obtained.
[0103] The thickness of the antimony base thin film is at least 70
.ANG., preferably 80 to 100 .ANG., more preferably 80 to 90 .ANG..
If the antimony base thin film is too thin, it does not become a
uniform crystalline film, but an amorphous film, failing to achieve
the objects of the invention. If the antimony base thin film is too
thick, the reactive thin film must accordingly be thick and
consequently, the recording layer as a while becomes too thick and
increases light absorption. This results in a lower reflectance and
lower modulation. The thickness of the reactive thin film
corresponding to the thickness of the antimony base thin film is
preferably 70 to 100 .ANG., more preferably 80 to 90 .ANG..
[0104] As in the first embodiment, the recording layer of the
second embodiment may have a structure consisting of three or more
films. Also in this case, the recording layer of the structure
having reactive thin films at opposed surfaces is advantageous. The
preferred number of interfaces between antimony base thin films and
reactive thin films in the recording layer is the same as in the
first embodiment.
[0105] The atomic ratio of elements in the entire recording layer
consisting of an antimony base thin film combined with a reactive
thin film is preferably represented by the following formula:
Ge.sub.aSb.sub.bTe.sub.1-a-b (II-2)
[0106] wherein letters a and b are in the range:
0.08.ltoreq.a.ltoreq.0.25 and 0.20.ltoreq.b.ltoreq.0.40.
[0107] The reason why a is limited to the above range is the same
as the reason of limitation of x in formula (II-1). If b is too
small in formula (II-2), record marks are more likely to
crystallize with a loss of reliability. If the value of b is too
large, antimony would precipitate to inhibit formation of record
marks.
[0108] The recording layer preferably has a thickness of about 140
to 500 .ANG.. A too thin recording layer would restrain the growth
of a crystalline phase and provide an insufficient change of
reflectance associated with a phase change. A too thick recording
layer would provide a lower reflectance and a lower degree of
modulation as previously mentioned.
[0109] Formation of Recording Layer
[0110] In either of the first and second embodiments of the
invention, the antimony base thin film and the reactive thin film
are preferably formed by sputtering. Sputtering conditions are not
critical. Where a material containing a plurality of elements is to
be sputtered, either an alloy target or a multi-source sputtering
technique using a plurality of targets may be used. Either the
antimony base thin film or the reactive thin film may be deposited
first. For the write-once type application, the antimony base thin
film is first formed so that the antimony base thin film may be
disposed on the recording light incident side. This order of film
formation enables the so-called "high-to-low" recording that the
recording layer has a high reflectance prior to recording and a low
reflectance after recording.
[0111] The sputtering technique involves the steps of evacuating a
sputtering or vacuum chamber, introducing a sputtering atmosphere
gas such as argon therein, and sputtering the target. According to
the invention, the sputtering chamber used for the formation of an
antimony base thin film(s) should preferably be evacuated to a
pressure prior to the introduction of the sputtering atmosphere gas
(that is, ultimate gas pressure) of lower than 0.5.times.10.sup.-2
Pa, more preferably lower than 0.2.times.10.sup.-2 Pa, most
preferably lower than 5.times.10.sup.-4 Pa. After the sputtering
chamber is evacuated to such a vacuum, a sputtering atmosphere gas
is introduced into the chamber whereupon sputtering is carried out.
By depositing an antimony base thin film under such conditions, the
formation of antimony microcrystals is promoted. After the antimony
base thin film and the reactive thin film have been formed, the
crystallization required for the antimony base thin film has been
substantially completed. Therefore, the establishment of the
ultimate gas pressure in the above-defined range prior to the
formation of the antimony base thin film is effective for reducing
the time required for the mixing treatment. Better results are
obtained when the invention is applied to the write-once type
optical recording medium.
[0112] When the invention is applied to the write-once type optical
recording medium, the degree of crystallization of the recording
layer as formed is important because the mixing treatment is
removed. If the ultimate gas pressure in the above-defined range is
established, the crystallization of the antimony base thin film is
substantially completed during formation of the recording layer so
that there may result a greater difference in reflectance from
amorphous record marks and hence, a greater degree of
modulation.
[0113] The establishment of the ultimate gas pressure in the
above-defined range is also effective for stabilizing the
write/read characteristics of a write-once type optical recording
medium. When a record mark is formed by irradiating record light,
the temperature of an area surrounding the record mark is also
increased. If the antimony base thin film has been insufficiently
crystallized, the surrounding area would increase its reflectance
during formation of the record mark. As a result, the waveform of
read signals is deformed, adversely affecting the jitter and
causing more errors. In contrast, if the antimony base thin film
has been fully crystallized with a substantially saturated
reflectance. No further increase of the reflectance of the record
mark-surrounding area occurs.
[0114] It is noted that the establishment of the ultimate gas
pressure in the above-defined range is advantageous especially in
the first embodiment.
[0115] Where the antimony base thin film and the reactive thin film
are formed in a common sputtering chamber, the establishment of the
ultimate gas pressure in the above-defined range may be followed by
successive formation of both the thin films. When the antimony base
thin film is formed again subsequent to the reactive thin film, it
is unnecessary to evacuate the chamber again to the ultimate
pressure after the reactive thin film is formed.
[0116] When the antimony base thin film is insufficiently
crystalline owing to the insufficient evacuation prior to
sputtering, a heat treatment of the recording layer can promote the
crystallization of the antimony base thin film. Therefore, a heat
treatment may be carried out if necessary to promote
crystallization. The heat treatment is preferably at a temperature
of 60 to 120.degree. C. A lower heat treatment temperature requires
a longer time for crystallization whereas a higher heat treatment
temperature can damage the substrate if it is a resin substrate
such as polycarbonate. The heat treatment may be continued until
the increase of reflectance is saturated. The heat treatment time
is not limited in this sense, but is usually less than 1 hour in
view of production efficiency.
[0117] The heat treatment is usually applied to only the write-once
type optical recording medium. Where the invention is applied to a
rewritable (or erasable) optical recording medium, the insufficient
crystallization of the antimony base thin film can be solved by
controlling the conditions of the mixing treatment.
[0118] The heat treatment also becomes an index indicating the
degree of crystallization of the antimony base thin film. If the
reflectance of the recording layer is increased by the heat
treatment, it indicates the insufficient crystallization of the
antimony base thin film. If the reflectance of the recording layer
is not increased by the heat treatment, that is, if the reflectance
is saturated, it indicates that the antimony base thin film has
sufficiently crystallized. It is noted that if the reflectance
prior to the heat treatment is more than 95% of the reflectance
subsequent to the heat treatment, the recording layer is regarded
to have practically satisfactory characteristics and thus usable
without the heat treatment.
[0119] Mixing Treatment and Rewriting
[0120] On the optical recording medium of the invention, the mixing
treatment and rewriting are performed in the above-mentioned
manner. The recording power may be applied in pulses. If one signal
is recorded by at least two divided portions of irradiation, the
heat accumulation in the record mark is suppressed. Then the
dilation of the trailing edge of the record mark (known as a
teardrop phenomenon) can be prevented, leading to an improved C/N.
The pulse irradiation also improves the rate of erasure. The values
of recording power and erasing power can be determined without
undue experimentation. The reading laser beam should be of a low
power so that the crystalline state of the recording layer may not
be affected thereby.
[0121] When the optical recording medium of the invention is
recorded, the linear velocity of the recording layer relative to
the laser beam is generally 0.8 to 20 m/s, preferably 1.2 to 16
m/s. The rewritable optimum linear velocity can be controlled by
changing the antimony content of the recording layer in the first
embodiment and by changing the tellurium content of the recording
layer in the second embodiment. More particularly, the rewritable
optimum linear velocity can be increased by increasing the antimony
content of the recording layer in the first embodiment and by
increasing the tellurium content of the recording layer in the
second embodiment.
[0122] The light for use in the mixing treatment, rewriting and
reading of the optical recording medium of the invention may be
selected in a wide wavelength range of, for example, 100 to 5,000
nm.
[0123] Medium Structure
[0124] Referring to FIG. 1, there is illustrated one preferred
configuration of the optical recording medium according to the
present invention. The optical recording medium 1 has a lower
dielectric layer 3, a recording layer 4, an upper dielectric layer
5, a reflective layer 6, and a protective layer 7 on a substrate 2.
The recording layer 4 is of a two-ply structure.
[0125] Since the optical recording medium is adapted to be recorded
and read by directing a light beam to the recording layer 4 through
the substrate 2, the substrate 2 is preferably formed of a material
substantially transparent to such a light beam, for example, resins
and glass. For ease of handling and low cost, resins are preferred
substrate materials. A choice may be made among various resins such
as acrylic resins, polycarbonate, epoxy resins and polyolefins. The
shape and dimensions of the substrate are not critical although it
is generally of disc shape having a diameter of about 50 to 360 mm
and a thickness of about 0.5 to 3 mm. The substrate surface may be
provided with a predetermined pattern of grooves for tracking and
addressing purposes.
[0126] The lower dielectric layer 3 plays the role of preventing
oxidation of the recording layer 4 and protecting the substrate by
shutting off the heat which can otherwise conduct from the
recording layer to the substrate upon recording. The upper
dielectric layer 5 plays the role of protecting the recording layer
and helps the heat remaining in the recording layer after
completion of recording release through heat transfer. Further the
provision of both the dielectric layers is effective for improving
a degree of modulation. The lower and upper dielectric layers 3 and
5 are made of any desired dielectric material, for example, silicon
oxide such as SiO.sub.2, silicon nitride such as Si.sub.3N.sub.4,
zinc sulfide such as ZnS, mixtures thereof, various transparent
ceramics and various species of glass. Also useful are so-called
LaSiON materials containing La, Si, O, and N, so-called SiAlON
materials containing Si, Al, O, and N, SiAlON containing yttrium,
etc. Preferred among these are those materials having a refractive
index of at least 1.4, especially at least 1.8 in the wavelength
range of 400 to 850 nm. This wavelength range covers 780 nm which
is the wavelength used in current CD players and 630-680 nm which
is a candidate wavelength of the next generation recording
technology and represents the range over which the optical
recording medium of the invention is advantageously operated.
Preferred examples of the dielectric material are Si.sub.3N.sub.4,
a mixture of ZnS and SiO.sub.2, a mixture of ZnS and
Si.sub.3N.sub.4, and a mixture of ZnS and Ta.sub.2O.sub.5.
[0127] The lower dielectric layer 3 is preferably about 500 to
3,000 .ANG. thick, more preferably 1,000 to 2,500 .ANG. thick.
Within this thickness range, the lower dielectric layer is
effective for preventing any damage to the substrate upon recording
and higher modulation is available. The upper dielectric layer 5 is
preferably about 100 to 300 .ANG., more preferably about 150 to 200
.ANG. thick. This thickness range ensures a fast cooling rate and
thus permits to define a record mark with a clear edge, resulting
in reduced jitter. Also higher modulation is available. Each of the
upper and lower dielectric layers 3 and 5 may be formed of two or
more dielectric laminae of different compositions as will be
described later. The dielectric layers are preferably formed by gas
phase growth methods such as sputtering and evaporation.
[0128] The reflective layer 6 may be formed of any desired
material, typically high reflectance metals, for example, Al, Au,
Ag, Pt, and Cu alone or alloys containing at least one of these
metals. The reflecting layer is preferably about 300 to 2,000 .ANG.
thick. Reflectance would be short with a thickness below this
range. A thickness beyond this range would provide no further
improvement in reflectance and add to the cost. The reflecting
layer is preferably formed by gas phase growth methods such as
sputtering and evaporation.
[0129] The protective layer 7 is provided for improving scratch
resistance and corrosion resistance. Preferably the protective
layer is formed of organic materials, typically radiation curable
compounds or compositions thereof which are cured with radiation
such as electron and UV radiation. The protective layer is
generally about 0.1 to 100 .mu.m thick and may be formed by
conventional techniques such as spin coating, gravure coating,
spray coating, and dipping.
[0130] In another preferred embodiment of the invention wherein the
optical recording medium has a sufficiently high reflectance, the
lower dielectric layer includes at least one laminate consisting of
two dielectric laminae having different refractive indexes. The
dielectric laminae having a higher refractive index in the laminate
is disposed adjacent to the substrate.
[0131] FIG. 2 shows one preferred configuration for this
embodiment. The optical recording medium 1 is shown as having a
high refractive index dielectric layer 31, a low refractive index
dielectric layer 32, a recording layer 4, an upper dielectric layer
5, a reflective layer 6, and a protective layer 7 on a substrate 2.
In this configuration, increase by overwriting. The reason why the
construction of FIG. 5 is selected is described below.
[0132] Since a phase change type optical recording medium generally
utilizes a difference of reflectance between crystalline and
amorphous states, regions (in crystalline state) other than record
marks of the recording layer and record marks (in amorphous state)
have an absorption Ac and Aa, respectively, which are often
different. In most cases, Ac<Aa. Note that the absorption Ac or
Aa is as measured at the wavelength of a write/read laser beam.
Then, the recording sensitivity and the rate of erasure become
different depending on whether an overwritten spot has been
crystalline or amorphous. As a consequence, the record mark formed
by overwriting varies in length and breadth, and such variations
lead to an increase of jitter which may cause an error. Where mark
edge recording of carrying information at either edge of a record
mark is employed for high density recording purposes, the influence
of the variation in length of a record mark becomes significant,
resulting in more errors. This problem is solved by approaching Ac
to Aa, preferably Ac/Aa.gtoreq.0.9, more preferably Ac=Aa. Ac>Aa
is further desirable when the influence of latent heat is taken
into account. Such a relationship may be accomplished between Ac
and Aa by controlling the thickness of the recording layer and the
thickness of dielectric layers sandwiching the recording layer. In
a medium of the ordinary structure, however, if Ac/Aa.gtoreq.0.9,
then the difference between the reflectance Rc from the medium at
regions other than record marks and the reflectance Ra from the
medium at record marks becomes undesirably smaller, resulting in a
lower C/N ratio.
[0133] Under such circumstances, JP-A 124218/1996, for example,
proposes an optical information recording medium comprising a first
dielectric layer, a recording layer, a second dielectric layer, a
reflective layer, a third dielectric layer, and a UV-cured resin
layer stacked on a substrate in the described order wherein
Ac>Aa, the reflective layer is a transmissive ultrathin metal
film of Si or Ge, and the third dielectric layer is of a dielectric
material having an index of refraction of at least 1.5. By
providing the light transmissive reflective layer and the third
dielectric layer having a high index of refraction, it is possible
to control Ac/Aa to fall in the above-mentioned range while
maintaining a satisfactory reflectance difference (Rc-Ra).
[0134] It is noted that Ac and Aa are calculated from the optical
constant of each layer and the wavelength of a write/read laser
beam.
[0135] The optical recording medium of FIG. 5 is a one-side
recording medium comprising a reflective layer 6 which is of the
same construction as the reflective layer described in JP-A
124218/1996 and an uppermost dielectric layer 8 between the
reflective layer 6 and a protective layer 7. Note that the
substrate 2, lower dielectric layer 3, recording layer 4, upper
dielectric layer 5, and protective layer 7 are the same as in the
optical recording medium of FIG. 1. Although one-side recording
media are illustrated in FIGS. 1, 2, and 5, it is acceptable that
two such media be joined to form a double-side recording medium and
that a protective support be joined to a one-side recording
medium.
[0136] In FIG. 5, the reflective layer 6 is preferably constructed
as a ultrathin metal layer having a high light transmittance or a
layer of Si or Ge having high transmittance in the near-infrared to
infrared spectrum encompassing the write/read wavelength. The
thickness of the reflective layer may be properly determined such
that the difference in absorption between regions other than record
marks of the recording layer and record marks may be corrected.
Since the preferred thickness range of the reflective layer largely
varies with the composition thereof, the thickness may be properly
determined in accordance with a particular composition. For
example, when metals such as Au are used, the reflective layer
preferably has a thickness of less than 400 .ANG., more preferably
100 to 300 .ANG.. When Si or Ge is used, the reflective layer
preferably has a thickness of less than 800 .ANG., more preferably
400 to 700 .ANG.. If the reflective layer is too thin, C/N would
lower. If the reflective layer is too thick, the absorption
correcting effect would become insufficient.
[0137] Where the reflective layer is constructed of a metal, gold
(Au) and gold alloys are preferred. The gold alloys are gold base
alloys containing at least one of Al, Cr, Cu, Ge, Co, Ni, Mo, Ag,
Pt, Pd, Ta, Ti, Bi, and Sb. The reflective layer is preferably
formed by a vapor phase growth method such as sputtering and
evaporation.
[0138] The uppermost dielectric layer 8 which is optionally
disposed on the reflective layer 6 is preferably constructed of a
material having a higher index of refraction than the protective
layer 7. By providing such an uppermost dielectric layer, it is
possible as in JP-A 124218/1996 to increase Ac/Aa while maintaining
a satisfactory reflectance difference between record marks and
other regions. The material of which the uppermost dielectric layer
is formed may be selected from the dielectric materials previously
mentioned for the lower and upper dielectric layers.
[0139] The uppermost dielectric layer preferably has a thickness of
300 to 1,200 .ANG., more preferably 400 to 900 .ANG.. If the
uppermost dielectric layer is too thin, signal outputs would become
lower. If the uppermost dielectric layer is too thick, there would
occur a cross-erasure phenomenon that signals in adjacent tracks
are erased.
[0140] In the above-mentioned construction having controlled Ac and
Aa, the transmittance of the medium measured when a write/read
laser beam is irradiated from below the transparent substrate, that
is, the ratio of transmitted light to incident light is preferably
at least 1%, more preferably at least 3%. The term "transmittance"
is measured when only inorganic layers are present on a transparent
substrate. That is, the protective layer 7 is removed from the
structure shown in FIG. 5. By the term is meant a transmittance
resulting from multiple reflection among inorganic layers including
the recording layer, dielectric layers and reflective layer. When a
transmittance of at least 1% is available, the ratio of Ac to Aa
increases so that it may become easy to control the Ac/Aa ratio to
fall within the above-defined range.
[0141] It is noted that the transmittance is measured by a
spectrophotometer. The region where measurement is made is not
particularly limited. Measurement may be made in either a
crystalline region or an amorphous region. Typically, measurement
is made in a crystalline region (mirror portion) where no grooves
are present.
EXAMPLE
[0142] Examples of the present invention are given below by way of
illustration and not by way of limitation.
Example 1
[0143] In--Ag--Te--Sb System (Sb Thin Film.fwdarw.Reactive Thin
Film)
[0144] An optical recording disc as shown in FIG. 1 was prepared by
injection molding polycarbonate into a disc shaped substrate 2
having a diameter of 120 mm and a thickness of 0.6 mm. A groove was
formed in one major surface of the substrate simultaneous with
injection molding. The groove had a width of 0.74 .mu.m, a depth of
650 .ANG., and a pitch of 1.48 .mu.m. On the grooved surface of the
substrate, there were formed a lower dielectric layer 3, a
recording layer 4, an upper dielectric layer 5, a reflective layer
6, and a protective layer 7.
[0145] The lower dielectric layer 3 was formed by sputtering a
target of ZnS and SiO.sub.2. The value of SiO.sub.2/(ZnS+SiO.sub.2)
was 15 mol %. The lower dielectric layer had a refractive index of
2.33 at wavelength 780 nm and a thickness of 2,000 .ANG..
[0146] Next, a vacuum chamber of a sputtering apparatus was
evacuated to a vacuum of 0.04.times.10.sup.-2 Pa and then charged
with argon gas to a pressure to 5.times.10.sup.-1 Pa. By a
sputtering technique, an antimony base thin film (Sb 100%) of 90
.ANG. thick was deposited and sequentially, a reactive thin film
((Ag.sub.0.25In.sub.0.25Te.sub.0.5).su- b.0.7Sb.sub.0.3) of 80
.ANG. deposited, completing the recording layer 4. The composition
of the reactive thin film was analyzed by ICP. It is noted that in
Examples set forth herein, the composition of all recording layers
is represented by an atomic ratio.
[0147] The recording layer was examined for crystallinity by
electron beam diffratometry to find that the antimony thin film had
crystallized while the reactive thin film was amorphous. An
electron beam diffraction image of this recording layer is similar
to the view of FIG. 3 which is an electron beam diffraction image
of a recording layer consisting of an antimony thin film (Sb 100%)
of 120 .ANG. thick and a reactive thin film
(Ag.sub.0.25In.sub.0.2Te.sub.0.55) of 50 .ANG. thick. The presence
of Sb crystals is evident from FIG. 3. FIG. 4 is a photograph under
a transmission electron microscope of the surface of the same
recording layer as analyzed in FIG. 3. In the photograph of FIG. 4,
a high brightness region indicates the antimony thin film and a low
brightness region indicates the reactive thin film. It is seen that
the reactive thin film is not a continuous film, but island-like.
The antimony thin film is a continuous film.
[0148] The upper dielectric layer 5 was formed by the same
procedure as the lower dielectric layer 3. The upper dielectric
layer had a thickness of 200 .ANG.. The reflecting layer 6 was
formed by sputtering a target of Au to a thickness of 1,500 .ANG..
The protective layer 7 was formed by applying a UV curable resin by
spin coating and exposing it to UV for curing. The protective layer
as cured had a thickness of 5 .mu.m.
[0149] The thus prepared phase change type optical recording disc
was designated sample 1A.
[0150] For comparison purposes, a sample 1B was prepared as was
sample 1A except that a recording layer of a single-ply structure
was formed using an alloy target. The recording layer of sample 1B
had the same thickness as the combined thickness of the antimony
thin film and the reactive thin film of sample 1A. The composition
of the recording layer of sample 1B was a combination of the
antimony thin film and the reactive thin film of sample 1A.
[0151] Sample 1A had a reflectance of 15% and sample 1B had a
reflectance of 6%. In all Examples, the reflectance was determined
by directing light of 680 nm in wavelength from the substrate side
and converting an RF signal output of a disc tester.
[0152] While sample 1B was being rotated at a linear velocity of 3
m/s, the sample was irradiated with a laser beam at a power of 8 mW
for initializing or crystallizing the recording layer. The laser
beam used in Examples had a wavelength of 680 nm. After the
initialization, the reflectance increased to 20%. Also after the
initialization, the rewritable optimum linear velocity was measured
to be 6 m/s.
[0153] Next, while sample 1A was being rotated at a higher linear
velocity of 8 m/s (about 2.7 times the linear velocity for
initialization) than the rewritable optimum linear velocity, the
sample was irradiated with a laser beam at a power of 8 mW. After
the laser beam irradiation, the reflectance decreased to 10%,
indicating that the antimony thin film and the reactive thin film
were mixed with each other.
[0154] Next, the samples were compared for record/erase
characteristics.
[0155] While each sample was being rotated at the rewritable
optimum linear velocity of sample 1B, a 1-7 modulated signal was
recorded with a recording power of 10 mW and an erasing power of 5
mW. With respect to the reflectance after the recording, both the
samples had a reflectance of 20% in the crystalline region where
the erasing power was applied and 8% in the amorphous region which
was the record mark. The C/N of a 7T (249.48 ns) signal was 55 dB
for both the samples, indicating a satisfactory signal intensity.
However, the rate of erasure of sample 1B remained unstable until
the 5th overwriting. Specifically, the rate of erasure of sample 1B
improved from -22 dB at the 1st overwriting to -28 dB at the 6th
overwriting. After the 6th overwriting, the rate of erasure
stabilized approximately at -28 dB. After about 10,000 overwriting
operations, the rate of erasure exceeded -25 dB and rewriting
became difficult. In contrast, sample 1A showed a stable rate of
erasure of -30 dB from the 1st overwriting to about the 100,000th
overwriting.
[0156] It is evident from the results of this Example that the time
required for the mixing treatment of the antimony thin film and the
reactive thin film in the present invention is extremely shorter
than the time required for initialization in the prior art. The
invention provides at least equivalent record/retrieval
characteristics to the prior art and especially, is successful in
improving the number of overwritable operations by a factor of
about 10.
Example 2
[0157] In--Ag--Te--Sb System (Reactive Thin Film.fwdarw.Sb Thin
Film)
[0158] A sample 2 was prepared by the same procedure as sample 1A
in Example 1 except that formation of the reactive thin film was
followed by formation of the antimony thin film. The ultimate gas
pressure of the vacuum chamber in the recording layer forming step
was the same as in Example 1. Sample 2 had a reflectance of 13%.
Electron beam diffractometry analysis showed that the antimony thin
film was crystalline while the reactive thin film was
amorphous.
[0159] Sample 2 was subjected to mixing of the antimony thin film
and the reactive thin film as in sample 1A. After the laser beam
irradiation, the reflectance decreased to 10% as in sample 1A,
indicating full intermixing.
[0160] Sample 2 was examined for record/erase characteristics as in
sample 1A. The reflectance after recording and the C/N of a 7T
(249.48 ns) signal were the same as in sample 1A. Like sample 1A,
sample 2 showed a stable rate of erasure of -30 dB from the 1st
overwriting to about the 100,000th overwriting.
Example 3
[0161] In--Ag--Te--Sb System (V Added)
[0162] A sample 3 was prepared by the same procedure as sample 1A
in Example 1 except that the composition of the antimony thin film
was changed to Sb.sub.99V.sub.1. The ultimate gas pressure of the
vacuum chamber in the recording layer forming step was the same as
in Example 1. Sample 3 had a reflectance of 15%. Electron beam
diffractometry analysis showed that the antimony base thin film was
crystalline.
[0163] Sample 3 was subjected to mixing of the antimony base thin
film and the reactive thin film as in sample 1A. After the laser
beam irradiation, the reflectance decreased to 13%.
[0164] Next, sample 3 was examined for record/erase characteristics
as in sample 1A except that the recording power was slightly
increased to 12 mW. Sample 3 had a reflectance of 20% in the
crystalline region where the erasing power was applied, which was
equal to sample 1A, and 6% in the amorphous region or the record
mark, which was lower than sample 1A. The C/N of a 7T (249.48 ns)
signal was 58 dB, which was higher than sample 1A, indicating an
improvement in signal intensity. Sample 3 showed a stable rate of
erasure of -30 dB from the 1st overwriting to about the 150,000th
overwriting. The number of overwritable operations was improved
over sample 1A by a factor of about 1.5 and over sample 1B by a
factor of about 15.
Example 4
[0165] In--Ag--Te--Sb System (Ta Added)
[0166] A sample 4 was prepared by the same procedure as sample 1A
in Example 1 except that the composition of the antimony thin film
was changed to Sb.sub.99Ta.sub.1. The ultimate gas pressure of the
vacuum chamber in the recording layer forming step was the same as
in Example 1. Sample 4 had a reflectance of 15%. Electron beam
diffractometry analysis showed that the antimony base thin film was
crystalline.
[0167] Sample 4 was subjected to mixing of the antimony base thin
film and the reactive thin film as in sample 2. After the laser
beam irradiation, the reflectance decreased to 9%.
[0168] Next, sample 4 was examined for record/erase characteristics
as in sample 2 except that the recording power was slightly
increased to 12 mW. Sample 4 had a reflectance of 22% in the
crystalline region where the erasing power was applied and 10% in
the amorphous region or the record mark, which were both slightly
higher than sample 2. The C/N of a 7T (249.48 ns) signal was 58 dB,
which was higher than sample 2, indicating an improvement in signal
intensity. Sample 4 showed a stable rate of erasure of -30 dB from
the 1st overwriting to about the 130,000th overwriting. The number
of overwritable operations was improved over sample 2 by a factor
of about 1.3 and over sample 1B by a factor of about 13.
Example 5
[0169] In--Ag--Te--Sb System (Three-Ply Structure)
[0170] A sample 5 was prepared by the same procedure as sample 1A
in Example 1 except that a recording layer of a three-ply structure
was formed. The ultimate gas pressure of the vacuum chamber in the
recording layer forming step was the same as in Example 1. The
recording layer was formed in the order of a reactive thin film, a
Sb thin film, and a reactive thin film from the substrate side. The
antimony thin film was of the same composition as in sample 1A and
had a thickness of 90 .ANG.. The reactive thin films were of the
same composition as in sample 1A and had a thickness of 40 .ANG..
The total thickness of the recording layer was then 170 .ANG.,
equal to that in sample 1A. Sample 5 had a reflectance of 18%.
Electron beam diffractometry analysis showed that the antimony thin
film was crystalline while the reactive thin films were
amorphous.
[0171] Sample 5 was subjected to mixing of the antimony thin film
and the reactive thin film as in sample 1A. After the laser beam
irradiation, the reflectance was 15%.
[0172] Next, sample 5 was examined for record/erase characteristics
as in sample 1A except that the recording power was slightly
increased to 12 mW. Sample 4 had a reflectance of 25% in the
crystalline region where the erasing power was applied, which was
higher than sample 1A, and 8% in the amorphous region or the record
mark, which was equal to sample 1A. The C/N of a 7T (249.48 ns)
signal was 56 dB, which was higher than sample 1A, indicating an
improvement in signal intensity. Sample 5 showed a stable rate of
erasure of -30 dB from the 1st overwriting to about the 200,000th
overwriting. The number of overwritable operations was improved
over sample 1A by a factor of about 2 and over sample 1B by a
factor of about 20.
Example 6
[0173] In--Ag--Te--Sb System (Write-Once Type)
[0174] A sample 6 was prepared by the same procedure as sample 1A
in Example 1 except that an antimony thin film (Sb 100%) of 80
.ANG. thick was first formed by sputtering and a reactive thin film
(Ag.sub.0.25In.sub.0.25Te.sub.0.5) of 100 .ANG. thick was formed by
sputtering, completing the recording layer. The ultimate gas
pressure of the vacuum chamber in the recording layer forming step
was the same as in sample 1A. Sample 6 had a reflectance of 20%.
Electron beam diffractometry analysis showed that the antimony thin
film was crystalline while the reactive thin film was
amorphous.
[0175] While sample 6 was being rotated at a linear velocity of 6
m/s, a 1-7 modulated signal was recorded with a recording power of
10 mW and without an erasing power. Sample 6 had a reflectance of
8% in the amorphous region which was the record mark. The C/N of a
7T (249.48 ns) signal was 55 dB, indicating a satisfactory signal
intensity.
[0176] A laser beam was irradiated to the recorded sample 6 at the
same linear velocity of 6 m/s as in writing, finding that the
record mark could not be erased or crystallized whether the power
of laser beam is high or low. To facilitate erasure, the linear
velocity was slowed down to 1.2 m/s, which is equal to the compact
disc, but erasure was still impossible. Erasure became possible
when the linear velocity was further slowed down to 0.6 m/s.
[0177] It is thus evident that according to the present invention,
a phase change type optical recording medium which is useful as the
write-once type can be established without initialization.
[0178] Comparison in Terms of the Ultimate Gas Pressure in the
Recording Layer Forming Step
[0179] Samples as reported in Table 1 were prepared by the same
procedure as sample 6 except that the ultimate gas pressure of the
vacuum chamber in the recording layer forming step was changed. The
reflectance of these samples is shown in Table 1. Thereafter, the
samples were placed in a dry oven where they were heat treated at
80.degree. C. The samples were taken out of the oven, fully cooled,
and measured for reflectance again. In this way, the time of heat
treatment passed until the reflectance was saturated was
determined. The results are shown in Table 1. The results of sample
6 are also shown in Table 1.
1 TABLE 1 Ultimate Heat treatment time Sample gas pressure
Reflectance until reflectance No. (.times.10.sup.-2 Pa) (%) is
saturated 6 0.04 20 -- 601 0.2 15 1 hour 602 0.5 6 2 hours 603 1.0
6 7 hours
[0180] It is evident from Table 1 that when the ultimate gas
pressure is higher than 0.5.times.10.sup.-2 Pa, heat treatment must
be continued for a long time until reflectance is saturated. In
contrast, when the ultimate gas pressure is lower than
0.5.times.10.sup.-2 Pa, especially lower than 0.2.times.10.sup.-2
Pa, the heat treatment time required until reflectance is saturated
is short, which is advantageous in mass manufacture. No heat
treatment is necessary in the case of sample 6 because reflectance
is already saturated at the end of its preparation. The saturated
reflectance of samples 601 to 603 was equal to the reflectance of
sample 6.
Example 7
[0181] Te--Ge--Sb system (Sb thin film.fwdarw.reactive thin
film)
[0182] A sample 7A was prepared by the same procedure as sample 1A
in Example 1 except that an antimony thin film (Sb 100%) of 76
.ANG. thick was first formed by sputtering and a reactive thin film
(Ge.sub.2Te.sub.5) of 264 .ANG. thick was formed by sputtering,
completing the recording layer. The ultimate gas pressure of the
vacuum chamber in the recording layer forming step was the same as
in Example 1. Electron beam diffractometry analysis of sample 7A
showed that the antimony thin film was crystalline while the
reactive thin film was amorphous.
[0183] For comparison purposes, a sample 7B was prepared as was
sample 7A except that a recording layer of a single-ply structure
was formed using an alloy target. The recording layer of sample 7B
had the same thickness as the combined thickness of the antimony
thin film and the reactive thin film of sample 7A. The composition
of the recording layer of sample 7B was a combination of the
antimony thin film and the reactive thin film of sample 7A.
[0184] Sample 7A had a reflectance of 13% and sample 7B had a
reflectance of 4%.
[0185] While sample 7B was being rotated at a linear velocity of 3
m/s, the sample was irradiated with a laser beam at a power of 8 mW
for initializing or crystallizing the recording layer. After the
initialization, the reflectance increased to 18%. Also after the
initialization, the rewritable optimum linear velocity was measured
to be 6 m/s.
[0186] Next, while sample 7A was being rotated at a higher linear
velocity of 8 m/s (about 2.7 times the linear velocity for
initialization) than the rewritable optimum linear velocity of 6
m/s, the sample was irradiated with a laser beam at a power of 8
mW. After the laser beam irradiation, the reflectance decreased to
8%, indicating that the antimony thin film and the reactive thin
film were mixed with each other.
[0187] Next, the samples were compared for record/erase
characteristics.
[0188] While each sample was being rotated at the rewritable
optimum linear velocity of 6 m/s for sample 7B, a 1-7 modulated
signal was recorded with a recording power of 10 mW and an erasing
power of 5 mW. With respect to the reflectance after the recording,
both the samples had a reflectance of 18% in the crystalline region
where the erasing power was applied and 5% in the amorphous region
which was the record mark. The C/N of a 7T (249.48 ns) signal was
55 dB for both the samples, indicating a satisfactory signal
intensity. However, the rate of erasure of sample 7B remained
unstable until the 10th overwriting. Specifically, the rate of
erasure of sample 7B improved from -20 dB at the 1st overwriting to
-28 dB at the 11th overwriting. After the 11th overwriting, the
rate of erasure stabilized approximately at -28 dB. After about
4,000 overwriting operations, the rate of erasure exceeded -25 dB
and rewriting became difficult. In contrast, sample 7A showed a
stable rate of erasure of -28 dB from the 1st overwriting to about
the 10,000th overwriting.
[0189] It is evident from the results of this Example that the same
advantages as in Example 1 are obtained with a recording layer of a
Te--Ge--Sb system.
Example 8
[0190] Te--Ge--Sb System (Reactive Thin Film.fwdarw.Sb Thin
Film)
[0191] A sample 8 was prepared by the same procedure as sample 7A
in Example 7 except that formation of the reactive thin film was
followed by formation of the antimony thin film. Sample 8 had a
reflectance of 11%. Electron beam diffractometry analysis showed
that the antimony thin film was crystalline while the reactive thin
film was amorphous.
[0192] Sample 8 was subjected to mixing of the antimony thin film
and the reactive thin film as in sample 7A. After the laser beam
irradiation, the reflectance decreased to 8% as in sample 7A.
[0193] Sample 8 was examined for record/erase characteristics as in
sample 7A. The reflectance after recording and the C/N of a 7T
(249.48 ns) signal were the same as in sample 7A. Like sample 7A,
sample 8 showed a stable rate of erasure of -28 dB from the 1st
overwriting to about the 10,000th overwriting.
Comparative Example
[0194] A comparative sample was prepared by the same procedure as
sample 1A in Example 1 except that a recording layer of a four-ply
structure was formed. The recording layer was formed in the order
of a Sb thin film, a reactive thin film, a Sb thin film, and a
reactive thin film from the substrate side. The antimony thin films
were of the same composition as in sample 1A and had a thickness of
45 .ANG.. The reactive thin films were of the same composition as
in sample 1A and had a thickness of 40 .ANG.. The total thickness
of the recording layer was then 170 .ANG., equal to that in sample
1A. Electron beam diffractometry analysis of the comparative sample
showed that the antimony thin films were amorphous while the
reactive thin films were amorphous.
[0195] The comparative sample had a very low reflectance of 4%. For
the comparative sample, mixing of the antimony thin film and the
reactive thin film was performed as in sample 1A. After the laser
beam irradiation, the reflectance was 5%. When rewriting laser beam
was irradiated after the mixing treatment, the region where the
erasing power was applied was not crystallized. Thus the
comparative sample could not be used as a recording medium.
[0196] Instead of the mixing treatment, the comparative sample was
subjected to initialization treatment under the same conditions as
in sample 1B whereby the reflectance was improved to 20%.
[0197] It is evident from the results of Comparative Example that
the advantages of the invention are not achievable it the antimony
base thin film is too thin to crystallize.
Example 9
[0198] Structure of FIG. 5: In--Ag--Te--Sb System (Sb Thin
Film.fwdarw.Reactive Thin Film)
[0199] An optical recording disc sample 9 of the structure shown in
FIG. 5 was prepared.
[0200] The layers of sample 9 were the same as in sample 1A of
Example 1 except that the lower dielectric layer 3 was 2,300 .ANG.
thick, the antimony thin film was 85 .ANG. thick, the reactive thin
film was 75 .ANG. thick, the reflective layer 6 was 100 .ANG.
thick, and the uppermost dielectric layer 8 was of the same
composition as the lower dielectric layer and 500 .ANG. thick.
[0201] Sample 9 showed an Ac/Aa ratio of 0.94 at the wavelength 680
nm. While a laser beam of the wavelength 680 nm was irradiated from
below the substrate 2 to the structure with the protective layer 7
removed, a transmittance in a mirror portion was measured by a
spectrophotometer. The transmittance was 8.4%.
[0202] Sample 9 had a reflectance of 17%. After sample 9 was
subjected to mixing of the antimony thin film and the reactive thin
film as in sample 1A, record/erase characteristics were found to be
the same as in sample 1A.
[0203] The effectiveness of the invention is evident from the
results of Examples.
[0204] Japanese Patent Application No. 352298/1996 is incorporated
herein by reference.
[0205] Although some preferred embodiments have been described,
many modifications and variations may be made thereto in the light
of the above teachings. It is therefore to be understood that
within the scope of the appended claims, the invention may be
practiced otherwise than as specifically described.
* * * * *