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Claims  |
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What is claimed is:
1. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein said interference
layer has a refractive index of at least 3.0.
2. An information recording medium according to claim 1, wherein said
interference layer comprises Si.
3. An information recording medium according to claim 1, wherein said
recording layer comprises at least one of a Te alloy, TeTiAgSe, and
amorphous Te-C.
4. An information recording medium according to claim 1, wherein said
reflective layer comprises at least one of Al, Ag, Au, Cu, In, Ti, V, Nb,
Cr, Mo, W, Mn, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt.
5. An information recording medium according to claim 1, wherein said
interference layer includes a first interference layer formed on said
transparent substrate and a second interference layer formed on said first
interference layer.
6. An information recording medium according to claim 5, wherein said first
and second interference layers comprise Si and Si.sub.3 N.sub.4,
respectively.
7. An information recording medium according to claim 5, wherein said first
and second interference layers comprise TiO.sub.2 and Si, respectively.
8. An information recording medium according to claim 5, wherein said first
and second interference layers comprise CaF and Si, respectively.
9. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein a reflectivity at
said mirror region is at least 10% greater than a reflectivity at said pit
region.
10. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein a reflectivity at
said pit region is at least 10% greater than a reflectivity at said mirror
region.
11. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein said interference
layer includes a first interference layer formed on said transparent
substrate and a second interference layer formed on said first
interference layer, wherein refractive indices of said transparent
substrate (n.sub.0), said first interference layer (nx.sub.1), said second
interference layer (nx.sub.2), said recording layer (ny), and said
reflective layer (nz) satisfy:
n.sub.0 <nx.sub.1, nz<ny<nx.sub.2 <nx.sub.1 (in said mirror region); and
n.sub.0 <nx.sub.1, nz<nx.sub.2 <nx.sub.1 (in said pit region).
12. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein said interference
layer includes a first interference layer formed on said transparent
substrate and a second interference layer formed on said first
interference layer, wherein refractive indices of said transparent
substrate (n.sub.0), said first interference layer (nx.sub.1), said second
interference layer (nx.sub.2), said recording layer (ny), and said
reflective layer (nz) satisfy:
n.sub.0 <nx.sub.1 <nx2, nz<ny<nx.sub.2 (in said mirror region); and
n.sub.0 <nx.sub.1 <nx.sub.2, nz<nx.sub.2 (in said pit region).
13. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein said interference
layer includes a first interference layer formed on said transparent
substrate and a second interference layer formed on said first
interference layer, wherein refractive indices of said transparent
substrate (n.sub.0), said first interference layer (nx.sub.1), said second
interference layer (nx.sub.2), said recording layer (ny), and said
reflective layer (nz) satisfy:
nx.sub.1 <n.sub.0, nx.sub.1 <nx.sub.2, nz<ny<nx.sub.2 (in said mirror
region); and
nx.sub.1 <n.sub.0, nx.sub.1 <nx.sub.2, nz<nx.sub.2 (in said pit region).
14. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein said interference
layer includes a first interference layer formed on said transparent
substrate and a second interference layer formed on said first
interference layer, wherein said first and second interference layers
comprise Si and CaF, respectively.
15. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein said interference
layer includes a first interference layer formed on said transparent
substrate and a second interference layer formed on said first
interference layer, wherein refractive indices of said transparent
substrate (n.sub.0), said first interference layer (nx.sub.1), said second
interference layer (nx.sub.2), said recording layer (ny), and said
reflective layer (nz) satisfy:
n.sub.0 <nx.sub.1, nz<ny<nx.sub.2 <nx.sub.1 (in said mirror region); and
n.sub.0 <nx.sub.1, nz<nx.sub.2 <nx.sub.1 (in said pit region).
16. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein the reflectivity
(Rm), the absorptivity (Am) and phase (.phi.m) of the mirror region, and
the reflectivity (Rp), the absorptivity (Ap) and phase (.phi.p) of the pit
region satisfy:
Rp<Rm, Am.ltoreq.Ap, .vertline.cos (.phi.m-.phi.p).vertline.<1.
17. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein the reflectivity
(Rm), the absorptivity (Am) and phase (.phi.m) of the mirror region, and
the reflectivity (Rp), the absorptivity (Ap) and phase (.phi.p) of the pit
region satisfy:
Rm<Rp, Ap.ltoreq.Am, .vertline.cos (.phi.m-.phi.p).vertline.<1.
18. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein said mirror region
has a reflectivity characteristic different from that of said pit region.
19. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein a reflectivity of
said pit region changes as a thickness of said recording layer changes.
20. An information recording medium, comprising:
a transparent substrate;
an interference layer formed on said transparent substrate;
a recording layer formed on said interference layer, said recording layer
having a pit region and a mirror region;
a reflective layer formed on said recording layer; and
a coating resin formed on said reflective layer, wherein a reflectivity
characteristic of said mirror region is selectively adjustable from that
of said pit region within a range of thicknesses of said interference
layer.
21. A method for manufacturing an information recording medium, comprising
steps of:
(a) preparing a transparent substrate;
(b) forming an interference layer having a refractive index of at least 3.0
on said transparent substrate;
(c) forming a recording layer on said interference layer;
(d) forming a pit region in said recording layer;
(e) forming a reflective layer on said recording layer; and
(f) providing a coating resin on said reflective layer.
22. An information recording medium produced by a process comprising steps
of:
(a) preparing a transparent substrate;
(b) forming an interference layer having a refractive index of at least 3.0
on said transparent substrate;
(c) forming a recording layer on said interference layer;
(d) forming a pit region in said recording layer;
(e) forming a reflective layer on said recording layer; and
(f) providing a coating resin on said reflective layer. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to an information recording medium, and more
particularly to an optical information recording medium.
A super-resolution optical system is used to increase the recording density
of an optical information recording medium. The general structure of the
super-resolution optical system is disclosed in the following references:
Yamanaka et al., "High Density Optical Recording by Super-resolution
system", International Symposium on Optical Memory 1989, 27D--17, pp.
99-100, Sept. 1989, Technical Digest;
Fukuhisa et al., "Reproducing C/N Characteristic of High Density
Magneto-Optic Disk Using Super Resolution Head", Extended Abstracts of the
39th Meeting of the Japanese Society of Applied Physics and Related
Societies, 31p-L-6, p. 1002, March, 1992; and
Ichiura et al., "High Density Video Disc Using Super-resolution and Green
Laser", International Symposium on Optical Memory and Optical Data Storage
'93, Tul. 4, pp. 10-11, July 1993.
In a super-resolution optical system, diffraction means (e.g., a shield
plate or a double rhomb prism) is placed between a light source and a
medium. The diffraction means forms a diffraction pattern of the light on
the medium. The diameter of the center spot (main lobe) of the diffraction
pattern is approximately 20% smaller than that of the smallest spot formed
without the diffraction means. The reduction of the spot diameter
increases the recording density of the medium by about 1.5 times.
The super-resolution optical system is used with a conventional optical
information recording medium based on the phase-contrast technique.
Examples of the conventional optical recording medium are disclosed in the
following references.
A conventional medium including a Pb-Te-Se film is disclosed in M. Terao,
et al. "Oxidation Resistance of Pb-Te-Se Optical Recording Film", Journal
of Applied Physics 62 (3), p.1029, 1987.
A conventional medium including a Te-C film is disclosed in Katsutaro
Ichihara, Hideki Okawa, "Optical Disk Medium", Electronic Ceramics Vol. 18
(90), p.5, November 1987, and M. Mashira, N. Yasuda, "Amorphous Te-C Films
for an Optical Disk", Proceedings SPIE Optical Disk Technology Vol. 329,
pp.190-194, 1982.
A conventional medium including CS.sub.2 -Te film is disclosed in H.
Yamazaki, et al., "Plasma Polymerized CS.sub.2 -Te Film for Laser Beam
Memory", Review of Electrical Communication Laboratories Vol. 32 No. 2,
pp.260-266, 1984.
A conventional medium including an organic recording film is disclosed in a
Japanese Unexamined Patent Publication sho-62-119755.
The conventional phase-contrast medium has a mirror surface and a bump
protruding from the mirror surface (hereinafter referred to as a mirror
region and a pit region). Typically, the height of the bump is one-quarter
the wavelength of the light.
In the conventional phase-contrast medium, contrast, which represents
"bits" of information, is produced by the destructive interference of
light waves. Therefore, the mirror region and the pit region have nearly
the same reflectivity. When the light spot is on a planar part of the
mirror region, the light is simply reflected from the mirror region. When
the light spot is on the pit region, the light is reflected from the pit
region as well as from the mirror region. The light wave reflected from
the pit region and the light wave reflected from the mirror region are in
phase opposition. Thus, the destructive interference of the two light
waves occurs to suppress light reflection in the pit region.
However, the combination of the super-resolution optical system and the
conventional media causes the following problem.
In the super-resolution optical system, the diffraction pattern formed on
the medium includes a main lobe (the center spot) and side lobes on the
both sides of the main lobe. The medium reflects the side lobes as well as
the main lobe. The reflected beams of the main lobe and the side lobes
interfere with each other to cause an edge shift in the reproduced signal.
The edge shift distorts the reproduced signal to increase the jitter in
the reproduced signal.
A filter (e.g., a slit or a pin-hole) can be incorporated in the
super-resolution optical system to remove the reflected beam of the side
lobes. However, the filter cannot remove the reflected beam of the side
lobes sufficiently to prevent distortion and jitter in the reproduced
signal.
SUMMARY OF THE INVENTION
In view of the aforementioned problems of the conventional optical
information recording medium, one object of the present invention is to
provide an information recording medium in which the phase-contrast is
produced by a principle other than the destructive interference of light
waves.
Another object of the present invention is to provide an information
recording medium having a sufficient reflective difference between a
mirror region and a pit region.
Yet another object of the present invention is to provide an information
recording medium which has an increased recording density.
Yet another object of the present invention is to provide an information
recording medium in which the jitter of the reproduced signal is reduced.
Yet another object of the present invention is to provide an information
recording medium in which the distortion of the reproduced signal is
reduced.
Yet another object of the present invention is to provide an information
recording medium in which the adverse effect of the side lobe is reduced.
According to the present invention, an information recording medium
includes a transparent substrate, an interference layer formed on the
transparent substrate, a recording layer formed on the interference layer,
a reflective layer formed on the recording layer, and a coating resin
formed on the reflective layer. The recording layer has a pit region and a
mirror region.
According to the present invention, the mirror region has a reflectivity
characteristic different from that of the pit region.
According to the present invention, a reflectivity of the pit portion
changes as a thickness of the recording layer changes.
According to the present invention, a reflectivity characteristic of the
mirror region is selectively adjustable from that of the pit region within
a range of thicknesses of the interference layer.
The interference layer may comprise Si.
The interference layer may include a first interference layer formed on the
transparent substrate and a second interference layer formed on the first
interference layer.
The first and second interference layers may comprise Si and Si.sub.3
N.sub.4, respectively.
The first and second interference layers may comprise TiO.sub.2 and Si,
respectively.
The first and second interference layers may comprise CaF and Si,
respectively.
The first and second interference layers may comprise Si and CaF,
respectively.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the present invention will become
more apparent when the following description is read in conjunction with
the accompanying drawings, wherein:
FIG. 1 shows the structure of a medium according to a first embodiment of
the present invention.
FIG. 2 shows the result of a first simulation for estimating the
relationship between the thickness of the recording layer 3, and the
reflectivity and the phase difference of the mirror region 4b of the
medium according to the first embodiment.
FIG. 3 shows the result of a second simulation for estimating the
relationship between the thickness of the recording layer 3, and the
reflectivity and the phase difference of the mirror region 4b of the
conventional medium.
FIG. 4 shows the result of a third simulation for estimating the
relationship between the distance from the center of the pit region 4a and
the reflectivity at that position.
FIG. 5 shows the result of a fourth simulation for estimating the
relationship between the distance from the center of the pit region 4a and
the phase difference at that position.
FIG. 6 shows the result of a fifth simulation for determining desired
ranges of the thickness of the interference layer 2.
FIG. 7 shows the result of an estimation of the medium according to the
first embodiment of the present invention.
FIG. 8 shows the structure of the medium according to the second embodiment
of the present invention.
FIG. 9 shows the result of a simulation for estimating the relationship
between the thickness of Si layer 12A, and the reflectivity and the
absorptivity of the mirror region 4b of the medium according to the second
embodiment.
FIG. 10 shows the result of a simulation for estimating the relationship
between the thickness of Si layer 12A, and the reflectivity and the
absorptivity of the pit region 4a of the medium according to the second
embodiment.
FIG. 11 shows the structure of the medium according to the third embodiment
of the present invention.
FIG. 12 shows the result of a simulation for estimating the relationship
between the thickness of Si layer 22B, and the reflectivity and the
absorptivity of the mirror region 4b of the medium according to the third
embodiment.
FIG. 13 shows the result of a simulation for estimating the relationship
between the thickness of Si layer 22B, and the reflectivity and the
absorptivity of the pit region 4a of the medium according to the third
embodiment.
FIG. 14 shows the structure of the medium according to the fourth
embodiment of the present invention.
FIG. 15 shows the result of a simulation for estimating the relationship
between the thickness of Si layer 32B, and the reflectivity and the
absorptivity of the mirror region 4b of the medium according to the fourth
embodiment.
FIG. 16 shows the result of a simulation for estimating the relationship
between the thickness of Si layer 32B, and the reflectivity and the
absorptivity of the pit region 4a of the medium according to the fourth
embodiment.
FIG. 17 shows the structure of the medium according to the fifth embodiment
of the present invention.
FIG. 18 shows the result of a simulation for estimating the relationship
between the thickness of Si layer 42A, and the reflectivity and the
absorptivity of the mirror region 4b of the medium according to the fifth
embodiment.
FIG. 19 shows the result of a simulation for estimating the relationship
between the thickness of Si layer 42A, and the reflectivity and the
absorptivity of the pit region 4a of the medium according to the fifth
embodiment.
In these drawings, the same reference numerals depict the same parts,
respectively.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Next is described a first embodiment of the present invention.
Referring to FIG. 1, an information recording medium according to the first
embodiment comprises a transparent substrate 1, an interference layer 2, a
recording layer 3, a reflective layer 4 and a coating resin 5. The laser
beam incident is from the transparent substrate 1.
In the first embodiment, the transparent substrate 1 is a glass (SiO.sub.2)
substrate having a thickness of about 1.2 mm. The transparent layer 1 can
be formed from a thermoplastic resin such as polymethylmethacrylate
(PMMA), polycarbonate (PC), amorphous polyolefin (APO), and/or epoxy.
The interference layer 2, which serves as an interference filter, is formed
on the glass substrate 1. In the first embodiment, the interference layer
2 is an Si film having a thickness of 30 nm. Si is a desirable material
for the interference Layer 2 because of the following reasons. First, Si
has a relatively high refractive index of 3.5. Second, Si has a relatively
low absorptivity of less than 0.1. Third, Si has a sufficient transparency
characteristics for allowing the laser beam therethrough. Other materials
preferably used as the interference layer 2 are described below.
A recording layer 3 is formed on the interference layer 2. Concave pit
portions 4a are formed in the recording layer 3. In the first embodiment,
the thickness of the recording layer 3 is about 100 nm. The recording
layer 3 is preferably formed from Te alloy, TeTiAgSe, or Te-C (amorphous).
The recording layer 3 may be formed from a photo-resist.
A reflection layer 4 is formed on the recording layer 3. In this
embodiment, the reflection layer 4 is an Al film having a thickness of
about 60 nm. The reflection layer 4 can also be formed from at least one
material selected from a group comprising Ag, Au, Cu, In, Ti, V, Nb, Cr,
Mo, W, Mn, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt. The material of the
reflection layer is selected according to the wavelength of the light
source. The depth of the reflection layer 4 is placed within the focal
depth of the laser beam.
The reflection layer 4 may further include a film of a transparent
dielectric or a semiconducting material such as Si and/or Ge. This
configuration increases the reflectivity for light having a specific
wavelength.
A coating resin 5 (e.g., an ultraviolet light curing resin) is formed on
the reflection layer 4. The thickness of the coating region 5 is about 50
.mu.m.
The concave portion of the recording layer 3 forms the pit region 4a. Other
planar portions of the recording layer 3 form a mirror region 4b.
Next is described the manufacturing process of the aforementioned
information recording medium.
In a first step, the transparent substrate 1 is prepared.
In a second step, the interference layer 2 is sputtered on the transparent
substrate 1.
In a third step, the recording layer 3 is formed on the interference layer
2. Thereafter, the pit region 4a is formed by a laser cutting method. The
details of the laser cutting method is described in John Watkinson, "The
Art of Data Recording", p.478, Focal Press, Oxford, England, 1994.
In the third step, the recording layer 3 is cut sharply because, due to the
high heat conductivity thereof, the Si interference layer 3 enhances the
diffusion of heat to avoid a "half-tone" of the recording layer 3.
In a fourth step, the reflective layer 4 is sputtered on the recording
layer 3.
In a fifth step, the coating resin 5 is applied to the reflective layer 4.
Next is described the result of first and second simulations for showing
the functional difference between the medium according to the first
embodiment and the conventional medium.
Referring to FIG. 2, the first simulation shows the relationship between
the thickness of the recording layer 3 and the reflectivity of the pit
portion 4a of the medium according to the first embodiment of the present
invention.
In the first simulation, the thickness of the interference layer 2, the
recording layer 3 in the mirror portion 4b, and the reflective layer 4 are
set to 35 nm, 100 nm, and 60 nm, respectively.
The solid line shows the reflectivity of the pit portion 4a. The marks "+"
show the phase difference between the pit portion 4a and the mirror
portion 4b, referred to as the reflectivity of the mirror portion 4b
(.phi.=0).
The reflectivity of the pit portion 4a decreases as the thickness of the
recording portion 3 decreases. When the thickness of the recording portion
3 is 0, the reflectivity of the pit portion 4a becomes about 10% and is
enough to provide a sufficient reflective difference between the pit
portion 4a and the mirror portion 4b.
Referring to FIG. 3, the second simulation shows the relationship between
the thickness of the recording layer 3 and the reflectivity of the pit
portion 4a of the conventional medium.
In the second simulation, the thickness of the interference layer 2, the
recording layer 3 in the mirror portion 4b, and the reflective layer 4 are
set to 0 nm, 100 nm, and 60 nm, respectively. This configuration means
that this medium does not have the interference layer 3.
The reflectivity of the pit portion 4a is constant irrespective of the
thickness of the recording layer 3, which is completely different from the
case of medium according to the first embodiment shown in FIG. 2.
Referring to FIGS. 2 and 3, the phase difference is suppressed in the
medium according to the first embodiment compared with that of the
conventional medium.
Next is described the result of a third simulation. The third simulation
shows the relationship between the position on the medium and the
reflectivity at that position.
Referring to FIG. 4, the solid line, the long-dashed line, and the
short-dashed line show the depth of the pit portion 4a, the reflectivity
of the conventional medium, and the reflectivity of the medium according
to the first embodiment. Reflected light on the oblique surface of the pit
portion 4a is assumed to be reflected in an incident light direction.
The reflectivity of the conventional medium is a substantially constant
value of 84% irrespective of the position on the medium. The reflectivity
of the medium according to the first embodiment suddenly decreases in
spite of the predetermined gradient of the surface of the pit portion 4a.
Next is described a fourth simulation. The fourth simulation shows the
relationship between the position on the medium and the phase difference
at that position.
Referring to FIG. 5, the solid line, the long-dashed line, and the
short-dashed line show the depth of the pit portion 4a the phase
difference of the conventional medium, and the phase difference of the
medium according to the first embodiment. Reflected light on the oblique
surface of the pit portion 4a is assumed to be reflected in an incident
light direction.
The medium of the first embodiment has a phase difference less than that of
the conventional medium.
Next is described the result of a fifth simulation for determining the
preferred thickness of the interference layer 2.
Referring to FIG. 6, the first simulation shows the relationship between
reflectivity of the medium and the thickness of the interference layer 2.
The solid and the broken lines show the reflectivity of the mirror portion
4b and that of the pit portion 4a of the medium shown in FIG. 1,
respectively. The marks "+" show the phase difference between the pit
portion 4a and the mirror portion 4b. In this simulation, the wavelength
of the laser beam is set to 680 nm.
When the thickness of the interference layer 2 is within the range of 30 nm
to 45 nm or within the range of 125 to 130 nm, a sufficient reflective
difference is provided between the mirror portion 4a and the pit portion
4b. When the thickness of the interference layer 2 is within the range of
125 to 130 nm, the reflectivity of the pit portion 4b and the phase
difference between the mirror portion 4a and the pit portion 4b are
minimized simultaneously.
When the thickness of the interference layer 2 is within the range of 80 nm
to 95 nm, the pit portion 4a has a higher reflectivity than that of the
mirror portion 4b. In this configuration, the reflectivity of the mirror
section 4b is decreased thereby also lowering noise among adjacent pits.
Next is described the evaluation of the medium according to the first
embodiment with respect to the reflectivity thereof.
The reflectivities of the mirror portion 4b and the pit portion 4a of the
medium having the interference layer 2 of 30 nm were 77.91% and 35.71% of
the reflectivity of the medium having no interference layer 2,
respectively.
The reflectivities of the mirror portion 4b and the pit portion 4a of the
medium having the interference layer 2 of 90 nm were 52.67% and 8.73% of
the reflectivity of the medium having no interference layer 2,
respectively.
Next is described the evaluation of the medium according to the first
embodiment with respect to 3T jitter which indicates the distortion of the
reproduction signal.
The evaluation was performed on a medium in which the thickness of the
transparent substrate, the interference layer, the recording layer, the
reflective layer and the coating resin were 1.2 mm, 110 nm, 90 nm, 60 nm
and 0.9 .mu.m, respectively.
Referring to FIG. 7, the open and closed triangles indicate the 3T jitter
of the conventional medium evaluated by a normal optical system and by the
super-resolution optical system, respectively.
The open and closed circles indicate the 3T jitter of the aforementioned
medium evaluated by a normal optical system and by the super-resolution
optical system, respectively.
In the conventional medium, jitter of the super-resolution optical system
was worse than that of normal optical system. On the other hand, in the
aforementioned medium according to the present invention, jitter of the
super-resolution optical system was much improved as compared to that of
the normal optical system.
Next is described the causes of the aforementioned improvement. Presently,
it is believed that the following reasons are the basis for the
improvement.
First, the effect of the side lobes is reduced because the reflectivity of
the pit region 4a, which has a relatively large phase difference, is lower
than that of the mirror region 4b as shown in FIG. 4. When the side lobes
are reflected from the pit region 4a, the amount of the reflected beam of
the side lobes is reduced because of the low reflectivity of the pit
region 4a. When the side lobes are reflected from the mirror region 4b,
the reflected beam of the side lobes have little effect on the
reproduction signals because the phase difference in the mirror region 4b
is relatively small.
Second, the effect of the side lobes is reduced because the phase
difference between the pit region 4a and the mirror region 4b is reduced
as shown in FIGS. 3 and 5.
Next is described the technical advantages of the first embodiment.
First, the distortion (e.g., jitter ) in the reproduction signal is
reduced. More specifically, the distortion caused by the side lobes of the
diffraction patterns is reduced. The reduction of the signal distortion
increases the recording density of the medium.
Second, the recording layer 3 is cut sharply because the "half-tone" of the
recording layer 3 is avoided due to the high heat conductivity of the
interference layer 2.
Next is described a second embodiment of the present invention. The feature
of the second embodiment is the structure of the interference layer 2 and
the other structures are the same as those of the first embodiment.
Referring to FIG. 5, the interference layer 2 of a medium according to the
second embodiment includes Si layer 12A and Si.sub.3 N.sub.4 layer 12B.
The thickness of the layers 12A and 12B are 30 nm and 90 nm, respectively.
The refractive indices of the transparent substrate 1 (n.sub.0), the Si
layer 12A (nx.sub.1), the Si.sub.3 N.sub.4 layer 12B (nx.sub.2), recording
layer 13 (ny), and the reflective layer 14 (nz) satisfy the following
relationship:
n.sub.0 <nx.sub.1, nz<ny<nx.sub.2 <nx.sub.1 (in the mirror region 4b)
n0<nx.sub.1, nz<nx.sub.2 <nx.sub.1 (in the pit region 4a).
Next is described the result of a simulation for determining the preferable
thickness of the Si layer 12A. In this simulation, the thickness of the
Si.sub.3 N.sub.4 layer 12B and the wavelength of the laser beam are 90 nm
and 680 nm, respectively.
Referring to FIG. 9, the open squares and the closed squares respectively
show the reflectivity and the absorptivity of the medium in the mirror
region 4b.
Referring to FIG. 10, the open squares and the closed squares respectively
show the reflectivity and the absorptivity of the medium in the pit region
4a.
According to FIGS. 9 and 10, when the thickness of the Si layer 12A is
within the range of 35 nm to 45 nm or within the range of 115 nm to 135
nm, the reflectivity of the pit region 4a becomes greater than that of the
mirror region 4b and is enough to provide a sufficient reflective
difference.
Next is described the third embodiment of the present invention. The
feature of the third embodiment is the structure of the interference layer
2 and the other structures are the same as those of the first embodiment.
Referring to FIG. 11 the interference layer 2 of a medium according to the
third embodiment includes TiO.sub.2 layer 22A and Si layer 22B. The
thickness of the layers 22A and 22B are 50 nm and 85 nm, respectively.
The refractive indices of the transparent substrate 1 (n.sub.0), the
TiO.sub.2 layer 22A (nx.sub.1), the Si layer 22B (nx.sub.2), recording
layer 13 (ny), and the reflective layer 14 (nz) satisfy the following
relationship:
n.sub.0 <nx.sub.1 <nx2, nz<ny<nx.sub.2 (in the mirror region 4b)
n.sub.0 <nx.sub.1 <nx.sub.2, nz<nx.sub.2 (in the pit region 4a).
Next is described the result of a simulation for determining the preferable
thickness of the Si layer 22B. In this simulation, the thickness of the
TiO.sub.2 layer 22A and the wavelength of the laser beam are 50 nm and 680
nm, respectively.
Referring to FIG. 12, the open and closed squares respectively show the
reflectivity and the absorptivity of the mirror region 4b.
Referring to FIG. 13, the open and closed squares respectively show the
reflectivity and the absorptivity of the pit region 4a.
According to FIGS. 12 and 13, when the thickness of the Si layer 22B is
within the range of 80 nm to 90 nm, the reflectivity of the mirror region
4b becomes sufficiently greater than that of the pit region 4a and the
absorptivity of the pit region 4a becomes nearly equal to that of the
mirror region 4b.
Next is described the fourth embodiment of the present invention. The
feature of the fourth embodiment is the structure of the interference
layer 2 and the other structures are the same as those of the first
embodiment.
Referring to FIG. 14, the interference layer 2 of a medium according to the
fourth embodiment includes CaF layer 32A and Si layer 32B. The thickness
of the layers 32A and 32B are 100 nm and 85 nm, respectively.
The refractive indices of the transparent substrate 1 (n.sub.0), the CaF
layer 32A (nx.sub.1), the Si layer 32B (nx.sub.2), recording layer 13
(ny), and the reflective layer 14 (nz) satisfy the following relationship:
nx.sub.1 <n.sub.0, nx.sub.1 <n.sub.2, nz<ny<nx.sub.2 (in the mirror region
4b)
nx.sub.1 <n.sub.0 nx.sub.1 <nx.sub.2, nz<nx.sub.2 (in the pit region 4a).
Next is described the result of a simulation for determining the preferable
thickness of the Si layer 32B. In this simulation, the thickness of the
CaF layer 32A and the wavelength of the laser beam are 100 nm and 680 nm,
respectively.
Referring to FIG. 15, the open and closed squares respectively show the
reflectivity and the absorptivity of the mirror region 4b, respectively.
Referring to FIG. 16, the open and closed squares respectively show the
reflectivity and the absorptivity of the pit region 4a, respectively.
According to FIGS. 15 and 16, when the thickness of the Si layer 32B is
within the range of 35 nm to 45 nm or within the range of 115 nm to 130
nm, the reflectivity of the mirror region 4b becomes sufficiently greater
than that of the pit region 4a and the reflectivity of the pit region 4a
becomes less than 10%.
Next is described the fifth embodiment of the present invention. The
feature of the fifth embodiment is the structure of the interference layer
2 and the other structures are the same as those of the first embodiment.
Referring to FIG. 17, the interference layer 2 of a medium according to the
fifth embodiment includes Si layer 42A and CaF layer 42B. The thickness of
the layers 42A and 42B are 35 nm and 120 nm, respectively.
The refractive indices of the transparent substrate 1 (n.sub.0), the Si
layer 42A (nx.sub.1), the Si layer 42B (nx.sub.2), recording layer 13
(ny), and the reflective layer 14 (nz) satisfy the following relationship:
n.sub.0 <nx.sub.1, nz<ny<nx.sub.2 <nx.sub.1 (in the mirror region 4b)
n.sub.0 <nx.sub.1, nz<nx.sub.2 <nx.sub.1 (in the pit region 4a).
Next is described the result of a simulation for determining the preferable
thickness of the Si layer 42A. In this simulation, the thickness of the
CaF layer 42B and the wavelength of the laser beam are 100 nm and 680 nm,
respectively.
Referring to FIG. 18, the open and closed squares respectively show the
reflectivity and the absorptivity of the mirror region 4b, respectively.
Referring to FIG. 19, the open and closed squares respectively show the
reflectivity and the absorptivity of the pit region 4a, respectively.
According to FIGS. 18 and 19, when the thickness of the Si layer 42A is
within the range of 40 nm to 45 nm or within the range of 120 nm to 135
nm, the reflectivity of the pit region 4a becomes sufficiently greater
than that of the mirror region 4b and the reflectivity of the mirror
region 4b becomes less than 10%.
Next is enumerated the materials preferably used as the interference layer
2. The interference layer 2 may comprise more than one layer formed from
the following materials.
Preferably the interference layer 2 has a refractive index less than 1.5.
However, materials having a higher refractive index may also be used
according to the designer's requirements.
The preferred materials having the refractive index less than 1.5 include
LiF (1.39), NaF (1.46), KF (1.32), RbF (1.40), CsF (1.48), CaF (1.32),
RbCl (1.48), MgF.sub.2 (1.38), CaF.sub.2 (1.43), SrF.sub.2 (1.44),
BaF.sub.2 (1.47), BaMgF.sub.4 (1.47), LiYF.sub.4 (1.45), Na.sub.2
SbF.sub.5 (1.47), SiO.sub.2 (1.46), RbClO.sub.3 (1.46), NH.sub.4 ClO.sub.4
(1.48), LiClO.sub.4-- 3H.sub.2 O (1.48), KB.sub.5 O.sub.8-- 4H.sub.2 O
(1.49), NH.sub.4 B.sub.5 O.sub.8-- 4H.sub.2 O (1.42), BeSO.sub.4--
4H.sub.2 O (1.47), Li.sub.2 SO.sub.4-- H.sub.2 O (1.46), Li.sub.2
SeO.sub.4-- H.sub.2 O (1.46), LiKSO.sub.4 (1.46), LiNaSO.sub.4 (1.47),
(NH.sub.4 CH.sub.3)Al(SO.sub.2).sub.-- 12HO (1.46), KNa(C.sub.4 H.sub.4
O.sub.6).sub.-- 4H.sub.2 O (1.49), and [CH.sub.2-- CF.sub.2 ]n (1.43). The
number in the parentheses following the name of the material is the
refractive index of the material.
Among the aforementioned materials, CaF, LiF, NaF, KF, MgF.sub.2, and
SiO.sub.2 are esp | | |
