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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a substrate for an optical recording
medium, particularly suitable for a magneto-optical recording medium
capable of direct overwrite by modification of power level and/or pulse
width of the recording optical pulse, and also relates to such a
magneto-optical recording medium using said substrate.
2. Description of the Related Art
Optical discs have been intensively investigated, developed and
commercialized as high density and capacity information storage media. The
commercialized optical discs have, adjacent to data areas, a guide in the
form of a convex or concave portion on the surface of the substrate for
servo tracking by an optical beam during recording, reproduction, erasing,
etc., of information. The typical guide is a groove formed on the surface
of the substrate in the form of spiral or concentric circles. The control
of the tracking servo is conducted by a light reflected from the guide.
It is however known that the quality of signal is degraded by the guides
existing in the vicinity of the data areas. The convex or concave guides
reflect to the recording layer which causes a deformation of bit
configuration lowering the C/N ratio (carrier to noise ratio), etc.
Some solutions have been proposed to solve the above problem. For example,
U.S. Pat. No. 5,089,358, issued on Feb. 18, 1992 for Taki et al, discloses
a flat layer formed on a base plate to provide a flat surface on which a
recording layer is formed so that the signal reproduced from the recording
layer is not deteriorated. Taki et al form the guides by patterning a
metal reflecting layer deposited on a glass base plate, followed by making
the flat layer on the guides. This process requires complex steps such as
photolithography and etching and is not adequate for mass production due
to low yield and high cost, etc.
JP-A-57-60544, published on Apr. 12, 1982, discloses a leveling resin layer
formed on a resin substrate having convex or concave portions, as a
tracking servo guide on the surface thereof. In JP-A-57-60544, since both
the planalizing layer and the substrate are made of a resin, the light
reflection from the tracking servo guide is disadvantageously low due to a
small difference of the refractive index thereof so that the tracking
servo is not efficient.
JP-A-2-152041, published on Jun. 12, 1990, discloses a leveling layer of
silicon nitride on a substrate by sputtering or evaporating silicon
nitride onto a substrate while milling the surface of the deposited
silicon nitride layer. This sputtering or evaporation while milling is
complex and does not provide a planalizing layer having a desired planer
top surface.
JP-B2-4-47910, published on Aug. 5, 1992, discloses a thin film coating on
a substrate having convex or concave portions as tracking servo guides, in
which the thin film coating covers the sharp angle edges of the convex or
concave portions and provides a relatively flat or smooth top surface by
which a recording layer to be formed thereon would not be damaged. In
JP-B2-4-47910, if the thin film coating is an organic resin layer, a
sufficient difference in the refractive index between the thin film
coating and the substrate cannot be obtained, and if the thin film coating
is an inorganic layer, the top surface of the thin film coating cannot be
made geometrically planar, causing the S/N ratio to be lowered due to
still existing geometrical convex or concave portions.
The object of the instant invention is to solve the above problems, to
provide a substrate for an optical recording medium by which a reliably
high tracking signal is obtained from the guides for a tracking servo and
an enhanced quality of reproduced signal can be obtained due to a
recording layer being formed on a geometrically planar surface of the
substrate, and which can be produced at a high yield and a low cost, and
to provide a magneto-optical recording medium using such a substrate.
SUMMARY OF THE INVENTION
To attain the above and other objects of the invention, the present
invention provides a substrate for an optical recording medium, comprising
a base having guide means in the form of convex or concave portions for
servo tracking with an optical beam, the base being made of an organic
resin material in at least a portion thereof where the guide means is
formed; a dielectric layer on the base at least in an area where the guide
means is formed; and a planalizing layer on the dielectric layer and the
base for burying the convex or concave portion of the guide means and
making a top surface of the planalizing layer flat; wherein the dielectric
layer has a refractive index higher than those of the organic resin
material forming the guide means and the leveling layer.
The present invention also provides a magneto-optical recording medium
comprising A) the substrate as described above and B) a magneto-optical
recording layer over the substrate, the magneto-optical recording layer
being capable of being directly overwritten by modification of a power
level and/or pulse duration when recording an optical pulse.
BRIEF DESCRIPTION OF THE INVENTION
FIG. 1 schematically illustrates a substrate for an optical disc of the
present invention.
FIG. 2 shows the dependency of the reflectivity on the AlSiN layer
thickness 2 in the structure as shown in FIG. 1.
FIGS. 3A and 3B show the pulses used to write and overwrite in an
embodiment of the present invention.
FIG. 4 is a cross-sectional view of a magneto-optical recording medium of
Example 1.
FIGS. 5 and 6 are cross sectional views of magneto-optical recording media
of Comparative Examples 1 and 2, respectively.
FIG. 7 is a cross-sectional view of a magneto-optical recording medium of
Example 2.
FIGS. 8 and 9 are cross-sectional views of magneto-optical recording media
of Comparative Examples 3 and 4.
FIGS. 10 and 11 are cross-sectional views of magneto-optical recording
media of Examples 3 and 4.
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, a light reflected from guides for servo tracking
is utilized to control the tracking servo. It is necessary to obtain a
sufficient intensity of the reflected light from the guides to attain a
stable tracking servo capability. Specifically, at least 10% of the
reflection of a tracking servo light beam from the guides is preferred. To
attain this condition, it is sufficient that a dielectric layer of a
material having a refractive index higher than that of the base in which
the guides are formed and that of the leveling layer and having a high
transparency is disposed at least on the guides. Since the typical resin
materials of the base including the guide portions has a refractive index
of 1.4 to 1.6, it is preferred that the dielectric layer has a refractive
index of not less than 1.6 for the wavelength of the light beam for servo
tracking, to obtain a sufficient intensity of reflected light.
Materials satisfying the above conditions include AlN, ZnS, Si.sub.3
N.sub.4, AlSiN, SiO, Zr.sub.2 O.sub.3, In.sub.2 O.sub.3, SnO.sub.2,
Ta.sub.2 O.sub.5, AlON, SiON, ZrON, InON, SnON and TaON, and mixtures
thereof.
The percent reflection of light for servo tracking is more preferably not
less than 15% to obtain a stabler servo tracking capability and a higher
reproduction signal intensity. To attain this, the refractive index of the
dielectric material is preferably not less than 1.8 for the wavelength of
light for servo tracking. In this respect, inorganic oxides and/or
nitrides such as AlSiN, Si.sub.3 N.sub.4, Zr.sub.2 O.sub.3, Ta.sub.2
O.sub.5, ZrON and TaON are preferable and from the viewpoint of durability
AlSiN is particularly preferable.
When n layers are stacked, a layer I.sub.m having a refractive index of
N.sub.m.sup..+-. and a thickness of h.sub.m is sandwiched by a layer
I.sub.m-1 having a refractive index of N.sub.m-1.sup..+-. and a layer
I.sub.m+1 having a refractive index of N.sub.m+1.sup..+-., and a light
having a wavelength of .lambda. is incident from the layer I.sub.m+1 side,
the amplitude reflection R.sub.m,m+1.sup..+-. is expressed by the
following formula:
##EQU1##
and j denotes the complex number. The total reflection can be obtained by
adding all of the reflections from the interfaces between adjacent layers.
For example, in the three-layer construction as shown in FIG. 1, when the
base 1 is polycarbonate, having a refractive index of 1.58, the dielectric
layer 2 is AlSiN having a refractive index of 2.05, the leveling layer 3
is a resin having a refractive index of 1.50, and the thickness of the
dielectric layer is varied, the total reflection from the three-layer
construction is shown in FIG. 2.
A higher reflection is periodically obtained when the thickness of the
dielectric layer is varied. Any thickness of the dielectric layer
providing a higher reflection may be advantageously used, but a thin
thickness is preferable from the viewpoint of productivity.
Such a preferable thickness of the dielectric layer depends on the
refractive index of the dielectric layer. When the dielectric layer is an
inorganic nitride, oxide or the like, the refractive index thereof is in a
range of 1.6 to 2.3 and the thickness of the dielectric layer is
preferably in a range of 20 nm to 160 nm.
When the degree of the nitrization and/or oxidation of the nitride and/or
oxide dielectric layer is lowered, the recording and reproduction
characteristics are affected but the refractive index is increased so that
the thickness of the dielectric layer can be made thinner and the
productivity is improved.
The dielectric layer may be a single layer or a multi-layer of different
materials.
The dielectric layer can be formed by any process including PVD such as
evaporation, sputtering and CVD or the like. It is preferred for optical
discs that the dielectric layer is firmly adhered to the substrate to
prevent peeling of the dielectric layer during high temperature and high
humidity durability tests and the sputtering process is preferred in this
respect.
The dielectric layer is formed on at least the guides but is preferably
formed on the entire surface of the base including the guides since it is
easy to make.
When such a dielectric layer having a thickness of around 100 nm is
deposited on the guides or base, the top surface of the dielectric layer
retains the configuration of the guides and cannot be geometrically
planar. Since the light reflection depends on the thickness of the
dielectric layer, the thickness of the dielectric layer is generally
selected so that the light reflection from the data areas, i.e., areas
without guides of convex or concave portions, becomes maximum. It is
preferred that the dielectric layer has a uniform thickness along the top
surface of the base including the guide portions so that the light
reflection from the guides is also almost maximum. It is preferred that
the difference of the thickness of the dielectric layer along the layer is
less than 10%.
In the instant invention, the convex or concave portions of the top surface
of the dielectric layer due to the guides are filled in, for example, by
coating a material having a low viscosity to make the top surface
geometrically planar on which a recording layer is to be formed so that
the configuration of the recording bits is not affected due to the
presence of the guides and a high quality reproduction signal can be
obtained due to perfect bit configuration. The difference of the level of
the top surface of the leveling layer is preferably less than a few tens
nanometers.
The thickness of the leveling layer is preferably in a range of 50 nm to
500 nm on the guides. Since the guides have a depth or height of more than
40 nm, the leveling layer should preferably have a thickness of 50 nm or
more to cover the guides. The thickness of the leveling layer is
preferably 500 nm or less when productivity is considered.
The material to be used for the leveling layer is not particularly limited
as long as it can bury the convex or concave portions of the guides and
provide a planar top surface, but is preferably a resin due to easy
formation of the layer.
Such resins include any resins which can be used for optical discs, for
example, ultra-violet ray curable resins, electron beam curable resins,
epoxy resins, silicate resins, urethane resins, polyester resins,
thermoplastic resins, etc.
It is preferred, however, that the resins can be dissolved in general
organic solvents since the resins are to be coated at a lowered viscosity
prepared by diluting with a solvent.
In the production of optical discs, UV-curable resins and electron beam
curable resins are often used because of their high productivity and
excellent protection effect or the like. The UV-curable resins are most
often used because equipment therefor is simpler than that of the electron
beam curable resin. Such UV-curable resins are commercially available as
compositions comprising a compound called acrylate of oligomer, having a
relatively high molecular weight and obtained by acrylating or
metacrylating a compound or oligomer having a functional group such as
hydroxyl or epoxy, for example, acrylate of a bisphenol A-based epoxy
compound or oligomer, a compound having one functional group, e.g.,
(meth)acryloyloxy, or two to six functional groups, e.g.,
(meth)acryloyloxy, and an initiator. For example, SD-17, SD-301, etc.,
produced by Dainippon Ink and Chemicals, Inc. and UR-4502 produced by
Mitsubishi Rayon are commercially available. Also, MH-71 produced by
Mitsubishi Rayon, for example, is commercially available as an electron
beam curable resin.
Epoxy resins can be any ones that can be used for optical discs, typically
ones derived from bisphenol. Nevertheless, since transparency is required
for optical discs, the curing agent should be carefully selected. From the
viewpoints of curing rate and transparency, pentaerythritol-derived
diamine and the like are preferably used and, for example, Epomate N-002
produced by Yuka Shell Epoxy can be mentioned as a preferred curing agent.
Silicate resins are also any ones that can be used for optical discs,
including thermoplastic types such as alkylsiloxane, and UV-curable types
such as acryl silicone compound. For example, glass resin GR-650 produced
by Showa Denko and KP-f5 and KNS-5300 produced by Shin-Etsu Chemical Co.
Ltd. can be used.
Thermoplastic resins that can be used are those which do not affect the
recording layer, are soluble in an adequate solvent, particularly an
organic solvent and provide a uniform transparent layer. For example,
acrylates resin such as polymethylmethacrylate and polyethylmethacrylate,
acrylonitrile resins such as polyacrylonitrile and polymethacrylonitrile,
fluorine-based resins such as vinyl fluoride-hexafluoropropylene
copolymer, vinyl resins such as vinyl chloride and vinyl acetate,
polyvinyl-alcohol resins, polyvinylbutylal resins, polyester resins,
polyurethane resins, etc., and mixtures and copolymers thereof can be
preferably used.
The planalizing layer can be formed by spin coating, screen printing, roll
coating, spray coating, dipping, sputtering, etc. When the planalizing
layer is a cured resin layer, the resin layer is preferably applied by
coating a resin or a diluted resin solution from the viewpoint of
workability. Further, spin coating is the most preferable considering the
productivity, cost, etc.
When the planalizing layer is applied by a spin coating method, the
viscosity of the resin to be applied must be low, e.g., by diluting a
resin material with a solvent. Preferred viscosity of the resin solution
to be applied by spin coating is not more than 50 cP at 20.degree. C. to
obtain a layer thickness of 200 to 500 nm and not more than 30 cP at
20.degree. C. for a layer thickness of 50 nm to 200 nm.
The solvent for dilution may be any ones that for an organic resin base
that do not cause chemical damage to the base and can be almost
volatilized during the curing of the resin. Preferred solvents include
isopropylalcohol, butylalcohol, ethylalcohol, etc., from the viewpoint of
handling poisonous materials.
The guides formed on the top surface of the base are typically grooves, but
are not limited thereto. The grooves preferably have a depth of not less
than 40 nm to obtain a stable servo tracking capability and more
preferably not less than 70 nm to obtain a more stable servo tracking
capability. The configuration of the grooves as the guides is not
particularly limited but a V-groove is preferred to make the reflections
from the guides and the dielectric layer overlying the guides efficient.
The material of the base is preferably an organic resin at least at a
portion where the guides are formed. Both of a base entirely made of a
single organic resin and a base with a cured resin layer prepared by the
2P(photo-polymer) method to provide guides on the surface of the base can
be used as the base of the present invention.
The organic resins used for the base include polycarbonate resins, acryl
resins, epoxy resins, 2-methyl pentene resin, polyolefin resins, or
copolymers thereof. Among others, polycarbonate resins are preferred from
the viewpoints of mechanical strength, durability, thermal resistance,
transparency and cost. It is preferred to prepare the entire base by a
polycarbonate resin from the viewpoint of productivity.
The guides used at the present for servo tracking are V-grooves in the form
of a concentric circle or spiral at a pitch of about 1.6 .mu.m with a
groove width of about 0.6 .mu.m and a group depth of typically 70 nm.
Nevertheless, the configuration of the guides and track pitch are not
limited to the above in the present invention.
The recording layer to be formed on the substrate prepared as described
above is not particularly limited. Magneto-optical recording layers of
amorphous rare earth element-transition metal alloys, inorganic or organic
phase-transition type recording layers, write once-recording type
recording layers, or any other optical recording layers can be used.
Specifically, the optical recording layer can be sandwiched by transparent
dielectric layers, a reflecting metal layer can be inserted on a side of
the recording layer opposite to the light incident side, and/or an
inorganic and/or organic protecting layer can be provided over the
recording layer.
Further, although a magneto-optical recording layer is sandwiched by
transparent dielectric layers to enhance the Kerr effect and the thus
sandwiched structure is to be formed on the planalizing layer of the
substrate, the dielectric layer between the recording layer and the
planalizing layer may be eliminated by utilizing the planalizing layer as
the enhancement layer. In this case, the thickness of the planalizing
layer is preferably 200 nm to 300 nm to obtain a maximum enhancement
effect. The material of the planalizing layer is not particularly limited
and may be, for example, UV-curable resin, electron beam curable resin,
epoxy resin, silicate resin, urethane resin, polyester resin,
thermoplastic resins, or the like.
The substrate for an optical recording medium of the present invention is
particularly advantageously applicable to a magneto-optical recording
medium which can be overwritten only by modification of power level and/or
pulse width of an optical pulse.
The drawback of the magneto-optical recording media in comparison with
floppy discs, hard discs, etc., is the fact that direct overwrite is
difficult. Here, the direct overwrite means writing information while
erasing already written information.
Various direct overwrite methods have been proposed for magneto-optical
recording media. Among others, a method disclosed in U.S. Pat. No.
4,888,750; J. Appl. Phys. Vol. 63 No. 8 (1988) 3844; IEEE TRANS. Magn.
Vol. 23 No. 1 (1987) 171; Appl. Phys. Lett. Vol. 49 No. 8 (1986) 473; IEEE
TRANS. Magn. Vol. 25 No. 5 (1989) 3530; J. Appl. Phys. Vol. 69 No. 8
(1991) 4967; and others has attracted attention because it does not
require modification of the conventional magneto-optical recording
apparatus in their optical system, magneto and so on. The proposed method
uses a magneto-optical recording layer in which the direction of the net
remnant magnetization can be self-inverted at a portion of magnetic domain
wall region when heated by a laser beam, and carries out the direct
overwrite by modifying power level and/or pulse width of an optical pulse
without changing the direction and intensity of the bias magnetic field.
The descriptions of the above publications are incorporated by reference.
The present inventors carried out experiments to confirm the direct
overwrite as proposed above. The medium used comprises a polycarbonate
substrate having a diameter of 130 mm and a thickness of 1.2 mm and having
1.6 .mu.m pitch spinal grooves, a magneto-optical layer of a rare
earth-transition metal amorphous alloy (Gd.sub.25 Tb.sub.75).sub.28
(Fe.sub.80 CO.sub.20).sub.72, 150 nm thick, as the above self-invertible
magneto-optical recording layer on the substrate, and transparent
dielectric layers of AlSiN 80 nm thick sandwiching the magneto-optical
recording layer.
The overwrite operation was conducted on the above medium. The rotation
speed of the medium was a linear speed of 11.5 m/sec at a point of a
radius of 30 mm. The writing and erasing were conducted by 4 MHz pulse
signals as shown in FIG. 3A under an external bias magnetic field of 350
Oe in the direction of the bit recording. The power level of the laser
having a wavelength of 830 nm was 15.0 mW for writing and 9.0 mW for
erasing. The reading was conducted by a continuous light, DC laser of 1.0
mW. Thus, the C/N of the reproduced signal was evaluated to be about 37
dB.
Next, the direct overwrite was conducted on the above medium on the same
track as evaluated above under an external bias magnetic field of 350 Oe
using 3 MHz pulse-signals as shown in FIG. 3B. The reproduced signals were
measured by a continuous light, DC laser, of 1.0 mW to find that the
initially recorded 4 MHz signals were completely erased and only 3 MHz
signals were recorded. The C/N ratio of the reproduction signal was then
about 37 dB. Thus, the overwrite operation by the above laser pulse
modification was confirmed. Nevertheless, the characteristics of the
reproduced signal, i.e., the C/N ratio, was so low, i.e., about 37 dB,
that it necessitated a remarkable improvement for practical use.
As described above, in conventional optical discs, the C/N ratio of the
reproduced signals is lowered by the reflection of the configuration of
the guides to the recording layer. The magneto-optical recording medium of
the direct overwrite type as described above encounters the same problem
more severely. In the considered overwrite method, the erasing of
information occurs during the course of the temperature profile formed by
scanning of a laser beam having an erase power level approaching the
already written recording bit. Specifically, prior to when the maximum
temperature portion of the temperature profile enters into the written
recording bit, a portion of the magnetic domain wall region reaches a
certain temperature lower than said maximum temperature and the net
remnant magnetization then self-inverted to result in the erasure.
Accordingly, if the recording layer has a convex or concave portion due to
the guides, the above-mentioned temperature profile and the self-inversion
of net remnant magnetization are affected or varied and the erasing step
may be disturbed or hindered.
In accordance with the present invention, the above problem of a
magneto-optical recording medium driven by direct overwriting through only
modification of power and width of an optical pulse can be solved by
providing a planar top surface of the substrate on which the recording
layer is formed while obtaining a desired level of the reflection of an
optical beam by insertion of a dielectric layer and thus the C/N ratio of
the reproduced signals can be remarkably improved.
Thus, in accordance with the present invention, there is also provided a
magneto-optical recording medium comprising A) a substrate comprising i) a
base having guide means in the form of convex or concave portions for
servo tracking with an optical beam, the base being made of an organic
resin material in at least a portion thereof where the guide means is
formed, ii) a dielectric layer on the base at least in an area where the
guide means is formed, and iii) a leveling layer on the dielectric layer
and the base for burying the convex or concave portion of the guide means
and making a top surface of the planalizing layer flat, wherein the
dielectric layer has a refractive index higher than that of the organic
resin material forming the guide means and the planalizing layer, and B) a
magneto-optical recording layer over the substrate, the magneto-optical
recording layer being capable of being directly overwrite by modification
of power level and/or pulse duration of a recording optical pulse.
The recording layer used in the above magneto-optical recording medium of
the present invention may be any perpendicularly magnetizable layers in
which the direction of the net remnant magnetization can be self-inverted
at least a portion of the magnetic domain wall region by heating with an
optical beam without changing the direction and intensity of the bias
magnetization, if present. Such layers include, for example, amorphous
alloys of rare earth element and transition metal as main components such
as TbFe, GdFe, DyFe, TbFeCo, GdFeCo, DyFeCo, DyTbFeCo, GdTbFeCo, GdDyFeCo,
GdDyTbFeCo, NdDyFeCo, NdDyTbFeCo, NdFe, PrFe, CeFe, etc., garnet layers,
multilayers such as Co/Pt and Co/Pd, CoPt alloy layer, CoPd alloy layer,
and so on.
The above recording layer may contain an additional element up to about 10
atom % as long as the perpendicular magnetization anisotropy does not
disappear. For example, one or more of rare earth elements, Fe, Co and Ni,
and other elements such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Tc, Re, Ru,
Os, Ir, Si, Ge, Bi, Pd, Au, Ag, Cu, Pt, etc. may be contained.
Particularly, Ti, Zr, Hf, Ta, Cr and Re may be preferably added to prevent
corrosion of the recording layer by oxidation.
It is preferred that the compensation temperature T.sub.comp of the
recording layer is in a range of 50.degree. C. to 250.degree. C., more
preferably 80.degree. C. to 160.degree. C. and the Curie temperature of
the recording layer is in a range of 100.degree. C. to 350.degree. C.,
more preferably 200.degree. C. to 250.degree. C., to obtain a higher C/N
ratio of the reproduction signal.
The thickness of the recording layer is preferably in a range of 10 nm to
200 nm. When the thickness of the recording layer is less than 10 nm, the
layer may have problems in the layer structure such as continuity and
uniformity of the layer. When the thickness of the layer is higher than
200 nm, the heat capacity of the layer becomes so large that a higher
optical beam power is required for writing and erasing.
When a transparent dielectric layer is disposed between the substrate and
the recording layer to enhance to Kerr effect, the dielectric layer is
preferably made of a material having a refractive index of not less than
1.6, more preferably not less than 1.8.
Such a transparent dielectric layer may be AlN, ZnS, Si.sub.3 N.sub.4,
AlSiN, SiO, Zr.sub.2 O.sub.3, In.sub.2 O.sub.3, SnO.sub.2, Ta.sub.2
O.sub.5, AlON, SiON, ZrON, InON, SnON, TaON or a mixture thereof.
Particularly, Si.sub.3 N.sub.4, AlSiN, ZnS, Zr.sub.2 O.sub.3, Ta.sub.2
O.sub.5, ZrON and TaON are preferred since these materials have a
refractive index of not less than 1.8.
The transparent dielectric layer may be not only a single layer of a single
material but also a multilayer of a plurality materials.
It is also preferred that the recording layer has a thickness of 15 nm to
100 nm, more preferably not more than 60 nm, particularly not more than 40
nm and a reflecting metal layer is disposed on a side of the recording
layer opposite to the substrate side, to increase the C/N ratio of
reproduction signal.
The reflecting metal layer preferably has a higher reflection of an optical
beam of a drive head than the reflection by the recording layer, to
increase the S/N ratio. Specifically, when the complex index of refraction
of a material is expressed as (n+ik), it is preferred to select a material
having a refractive index n and an extinction coefficient k of
n.ltoreq.3.5 and k.gtoreq.3.5, more preferably n.ltoreq.2.5 and
4.5.ltoreq.k.ltoreq.8.5 for the wavelength of the optical beam used. The
magneto-optical recording medium using a reflecting metal layer satisfying
the above conditions has a higher light reflection to enhance the Kerr
effect and thus improve the C/N ratio.
If the reflecting metal layer has a high thermal conduction coefficient
during recording with heat by an optical beam, the heat diffusion or
conduction through the reflecting metal layer is so high that a high power
of the optical beam is required. Thus, in order to make the recording
possible with a commonly used semiconductor laser having a power of not
more than 10 mW, the material of the reflecting metal layer preferably has
a thermal conduction coefficient of not more than 100 W/(m.multidot.k),
more preferably not more than 80 W/(m.multidot.k), further preferably not
more than 50 W/(m.multidot.k).
The materials satisfying the above conditions include Al or Ag alloyed with
Au, i.e., AlAu alloy or AgAu alloy. If the content of Au is less than 0.5
atom %, the reduction of the thermal conduction coefficient by the
alloying is less and if the content is more than 20 atom %, the light
reflection by the layer is lowered. Thus, the content of Au in the above
alloys is preferably in a range of 0.5 to 20 atom %.
To suppress lowering of the light reflection in comparison with the metal
Ag layer to not more than 2% and prevent lowering of the C/N ratio, the
content of Au in the AlAu or AgAu alloy is preferably in a range of 0.5 to
15 atom %, more preferably 0.5 to 10 atom %.
The above low content of Au is also advantageous in reduction of costs of
the target and medium.
To allow the minimum content of Au, one or more of certain elements such as
Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Tc, Re, Ru, Os, Ir, etc. may be
additionally added. The content of these additional elements should be not
more than 5 atom % to prevent lowering of the light reflection by the
reflecting metal layer and lowering of the C/N ratio. If the content of
the additional elements is not more than 5.0 atom %, the lowering of the
reflection of a semiconductor laser beam having a wavelength of 830 nm
used in magneto-optical recording and reading units cannot be more than
2%. If the content of the additional element is less than 0.3 atom %, the
increase of the thermal conduction efficient due to the save or reduction
of the Au content cannot be complemented. Thus, the content of the
additional elements should be in a range of 0.3 to 5.0 atom %. By addition
of an additional element in an amount of 0.3 to 5.0 atom % in combination
of the content of Au of 0.5 to 10 atom %, the lowering of the light
reflection by the reflecting layer in comparison with that by an Al or Ag
metal layer can be suppressed to less than 2%, the cost of Au can be
reduced, and the thermal conduction coefficient of the reflecting layer
can be set in a range of 20 to 100 W/(m.multidot.k).
Among the above additional elements, Ti, Zr, Nb, Ta, Cr and Re are
preferable since they can improve the durability of the reflecting metal
layer. The reflecting metal layer generally has a thickness of 10 to 50
nm, and 30 to 200 nm is preferable and 40 to 100 nm is more preferable to
prevent the lowering of the C/N ratio due to lowering of the reflection
and allow the recording by a laser power of 10 mW.
With the Au content and/or additional element content as described above,
the thermal conduction coefficient of the reflecting layer cannot be more
than 100 W/(m.multidot.k) and the recording can be made by a laser power
of 10 mW.
The location of the reflecting metal layer is not particularly limited as
long as it is disposed on a side of the recording layer opposite to the
light beam incident side. Namely, the reflecting metal layer may be
disposed directly on the magneto-optical recording layer, or a transparent
dielectric layer may be inserted between the reflecting metal layer and
the magneto-optical recording layer, or an inorganic and/or organic
protection layer, for example, a transparent dielectric layer, may be
further provided over the reflecting metal layer formed on the recording
layer.
The inorganic layers of the transparent dielectric layer, the recording
layer and the reflecting metal layer can be formed by any known process
including PVD such as evaporation and sputtering, CVD, and others.
Nevertheless, since the magneto-optical recording layer should preferably
be firmly bonded to the underlying layer, e.g., a polymer substrate, to
prevent peeling off in a high temperature and high humidity atmosphere.
The organic protection layer may be a photo-curable and/or heat-curable
resin or a thermoplastic resin, or the like and can be applied by coating,
etc., as in the case of the planalizing layer. The protection layer on a
side of the recording layer opposite to the substrate side preferably also
covers the end sides of the recording layer.
The wave configuration of the optical beam applied for writing and erasing,
i.e., overwriting is not limited to those shown in FIGS. 3A and 3B. For
example, the recording and erasing pulses can be changed to a series of
narrower or closer pulses, or a combination of such a series of narrower
or closer pulses and a continuous pulse as shown in FIGS. 3A and 3B.
The power of an optical beam should be selected depending on the recording
sensitivity, i.e., the Curie temperature of the recording layer, and the
layer structure of the medium.
Here, the disclosures contained in the publications mentioned before as
disclosing the overwrite process by only modification of power and pulse
width of an optical beam are incorporated herewith by reference.
In the magneto-optical recording medium of the present invention, excellent
overwrite characteristics, particularly a remarkably improved C/N ratio,
can be obtained by operation of overwrite with only modification of power
and/or pulse width of an optical beam.
It should be noted that although U.S. Pat. No. 4,888,750 discloses that the
direct overwrite is observed without using external magnetic bias aiding
the recording process, the present inventors found that the C/N ratio can
be improved by providing a certain magnetic bias during the overwrite
operation by modification of power and/or width of an optical beam. In the
latter case, the direction and intensity of the magnetic bias are not
changed during the overwrite operation. It is preferred that the substrate
or the top surface thereof has a thermal conduction coefficient of not
more than 0.5 W/(m.multidot.k), the content X of the rare earth element in
atom % of the magneto-optical recording layer of a rare earth-transition
metal amorphous alloy is in a range of 20 atom % to 28 atom %, and the
applied magnetic bias Hex in Oe is not less than 17.times.(X-24).sup.2
+100 and not more than 30.times.(X-24).sup.2 +400 .
EXAMPLES
Example 1 and Comparative Examples 1 and 2
Magneto-optical recording media having the constructions as shown in FIGS.
4 to 6 were manufactured and evaluated. In these figures, 11 denotes a
base having guide grooves 12 on the surface thereof, 13 denotes a
dielectric layer on the base 11 and the guide grooves 12, 14 a leveling
layer, 15 a dielectric layer underlying a recording layer 16, 16 denotes a
recording layer, 17 a dielectric layer overlying the recording layer 16,
18 a reflecting metal layer and 19 an organic protection layer.
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