DESCRIPTION
TITLE OF INVENTION
OPTICAL INFORMATION RECORDING MEDIUM
TECHNICAL FIELD
The present invention relates to an optical information recording medium that
records/reproduces information such as audio/video and the like as a digital signal by
irradiating a thin film formed on a substrate with a high-energy optical beam such as a
laser beam, and particularly relates to an optical information recording medium capable
of recording/reproducing large amounts of information through multilayering
of the information layers.
BACKGROUND ART
Research in optical information recording methods has been advancing in recent
years, and such methods have come to be used widely in industrial and consumer
applications. In particular, optical information recording media capable of recording
information at high densities, such as CDs and DVDs, have become widespread. Such
optical information recording media are constructed by layering thin metal films or
thermally-recordable thin film materials upon a transparent substrate in which is formed
pits expressing an information signal, concavo-convex channels such as guidance
grooves for tracking of recording/reproducing light, and so on, and furthermore layering
thereupon a protective layer, such as a resin layer or a transparent substrate, that
protects the thin metal film or thin film material from atmospheric moisture. The
reproducing of information is carried out by irradiating the thin metal film or thin film
material with laser light and detecting changes in the amount of the resulting reflected
light.
The method for manufacturing such an optical information recording medium is
generally performed as follows.
For example, with a CD, a resin substrate having a concavo-convex form
expressing information signals on one surface is formed through injection molding or the

like using a mold, called a "stamper", that has a concavo-convex channel pattern on its
surface. A thin metal film or thin film material is then formed upon the concavo-convex
channels through deposition, sputtering, or the like, after which a protective layer is
formed by coating the film with an ultraviolet light-curable resin, thereby completing the
manufacture.
With a DVD, a resin substrate approximately 0.6 mm thick is formed through
injection molding or the like using a stamper, after which a thin metal film or thin film
material is formed upon the concavo-convex form on the resin substrate, This is then
laminated onto a separately-prepared resin substrate, approximately 0.6 mm thick,
using ultraviolet light-curable resin or the like, thereby completing the manufacture.
Such optical information recording media are seeing increased demand for
higher capacities, and due to such demand, higher densities in optical information
recording media are being sought. Dual-layer optical information recording medium
constructions, in which two signal layers, each formed of concavo-convex channels and
a thin metal film or thin film material, are constructed so as to sandwich an intermediate
layer tens of urn thick, are offered for the aforementioned DVDs as well, with the goal of
increasing the capacity thereof.
Furthermore, next generation optical information recording media, having higher
densities and higher capacities than DVDs, are in demand due to the recent spread of
digital high-definition broadcasting, and thus high-capacity media such as Blu-ray disks
are being offered. Compared to DVDs, the track pitch in the information layers formed
in concavo-convex form is narrower in Blu-ray disks, and the pits are smaller as well. It
is therefore necessary to concentrate the laser spot used to record/reproducing
information into a smaller area on the information layer. With Blu-ray disks, a violet
laser whose laser light wavelength is a short 405 nm is used, and the laser light spot is
concentrated into a small area on the information layer by using an optical head
equipped with an objective lens for laser light concentration whose numerical aperture
(NA) is 0.85. However, a smaller spot increases the influence of disk tilt. Aberration will

occur in the beam spot with even a slight tilt in the disk, causing distortion in the
concentrated beam; this results in a problem in that recording/reproducing cannot be
performed. This drawback is circumvented in Blu-ray disks by reducing the protective
layer on the laser light-entry side of the disk to a thickness of approximately 0.1 mm.
Meanwhile, in recording/reproducing systems that use optical heads having this
sort of objective lens with a high NA, aberration, such as spherical aberration arising
due to variations in the thickness from the outer surface of the disk to the information
layer, exerts a great influence on the quality of the laser light concentrated onto the
information layer. A means for correcting aberration arising due to thickness variations
is therefore provided. For example, a configuration that provides a spherical aberration
correction means using a combination lens in the optical head, a configuration that
provides a spherical aberration correction means using liquid-crystals in the optical
head, and so on have been proposed.
Meanwhile, still higher capacities are being demanded even in such high-
capacity next-generation optical information recording media such as Blu-ray disks, and
thus, as with DVDs, increasing capacity through the multilayering of information layers
is being proposed as one such method. In order to reduce the influence of disk tilt when
multilayering information layers in a Blu-ray disk, it is necessary for distance to the
information layer furthest from the laser light-entry side to be approximately 0.1 mm
from the surface of the disk, as with single-layer media. For this reason, the information
layers are layered so as to sandwich a transparent layer called an intermediate layer,
whose thickness is several urn to several tens of µm, all within a space approximately
0.1 mm thick.
Accordingly, the method of manufacture for a multilayer Blu-ray disk is generally
performed as follows. A method of manufacturing a dual-layer optical information
recording medium, having two information layers, shall be described as an example.
This method includes a step of forming a thin metal film, a thermally-recordable thin film
material, or the like upon a molded resin substrate, approximately 1.1 mm thick, having

pits, guidance grooves, and so on in a concavo-convex form on one side, thereby
forming a first information layer; a step of forming an intermediate layer several urn to
several tens of urn thick upon the information layer on the substrate, in order to
separate the information layers; a step of transferring pits, guidance grooves, or the like
onto the upper side of the intermediate layer by pressing the intermediate layer with a
stamper having a concavo-convex form corresponding to the pits, guidance grooves,
and so on; a step of forming a thin metal film or thermally-recordable thin film material,
the film being semitransparent.with respect to the wavelength of the laser light used for
recording/reproducing, upon the pits, guidance grooves, or the like transferred onto the
intermediate layer, thereby forming a second information layer; and a step of forming a
protective layer upon the second information layer in order to protect the second
information layer. When multilayering more than two layers, those multiple information
layers can be layered sequentially by repeating the steps from the formation of the
intermediate layer to the formation of the second information layer several times.
As mentioned earlier, with multilayer Blu-ray disk media constructed in this
manner, all information layers are required to be provided within a space approximately
0.1 mm thick in order to reduce the influence of disk tilt. Thus, as shown in Fig. 2, the
distance to a first information layer 202, which is furthest from the outermost surface on
the recording/reproducing light-entry side, is restricted to approximately 0.1 mm, and the
other information layers are layered thereupon moving outward toward the
recording/reproducing light-entry side.
While dual-layer media are well-known as such muItiIayered media, structures
having three or more layers are being proposed as of late. In particular, four-layer
media, which have four information layers, have been introduced.
When recording/reproducing light is focused onto the information layer to be
recorded to/reproduced in art optical information recording medium having multiple
information layers, part of the light that has been reflected by another information layer
and that is not involved in the recording/reproducing of information (this light is called

"stray light" here) is reflected In multiple by one of the information layers. When the
stray light returns to the optical head via the same optical path as the reflected light from
the information layer being recorded to/reproduced (this reflected light is called
"information light" here), the stray light interferes with the information light to be read
out, causing major fluctuations in the light amount. Problems caused by such
interference are particularly apparent in multilayer media composed of three or more
information layers. Light amount fluctuations caused by stray light reflected in multiple
returning to the optical head along the same optical path as the information light to be
read out shall be referred to here as a "back-focus issue". Various investigations are
being made with respect to the elimination of such back-focus issues.
For example, Patent Document 1 proposes a structure in which the thickness of
each intermediate layer is designed so that when light is focused on an information
layer to be read, the light does not converge on other information layers. This
document particularly discloses a structure in which the thicknesses from one of the
information layers to another one of the information layers on the inner side and the
thicknesses from that one information layer to one of the information layers on the
protective layer side are all different. Making the intermediate layers thicker (or thinner)
the further away they are from the recording/reproducing light-entry side is proposed as
a way to realize such a structure; this prevents light from converging on other
information layers when focusing on the information layer to be read.
In addition, Patent Document 2, for example, discloses a structure for a
multilayer medium having three or more information layers in which the intermediate
layers are composed having different thicknesses in order to eliminate the influence of
crosstalk between information layers (interlayer crosstalk). This document particularly
discloses a structure for a four-layer medium having four information layers, in which,
when the structure has three intermediate layers, or first, second, and third intermediate
layers, that are layered starting with the first intermediate layer, which is furthest from
the recording/reproducing light-entry side, and moving out toward the
recording/reproducing light-entry side, the second information layer has the highest

thickness, thereby preventing stray light from being focused upon other information
layers.
[Patent Document 1] JP 2001-155380A
[Patent Document 2] JP 2004-213720A
DISCLOSURE OF INVENTION
However, it has come to be understood that the patterns with which back-focus
issues arise are not limited to the case where the stray light occurring when light is
focused upon one of the information layers converges on another information layer, as
discussed in Patent Document. 1 or Patent Document 2. For example, in addition to the
pattern shown in Fig. 3(a), in which stray light 302 converges upon one of the
information layers, it is known that major fluctuations in the light amount also occur in a
pattern, shown in Fig. 3(b), where stray light 304 converges upon the surface of the
protective layer, as well as a pattern, shown in Fig. 3(c), where stray light 306 and 307
converge upon neither an information layer nor the protective layer but do return along
almost the same optical path as information light 305.
Furthermore, when manufacturing a dual-layer optical information recording
medium, a four-layer optical information recording medium, or the like, the intermediate
layers separating the information layers, the protective layer, and so on are generally
formed using a spin coat method or the like on ultraviolet light-curable resin, and thus
when forming an intermediate layer or a protective layer thereby, it is necessary for the
thickness distribution across the entire surface of the medium to. be at least
approximately ±2 µm, including lot-to-lot variability.
It is furthermore absolutely necessary to ensure that four-layer Blu-ray disks are
compatible with the single-layer and dual-layer Blu-ray disks currently being sold, and
thus the thickness from the furthest information layer to the protective layer surface
when viewed from the recording/reproducing light-entry side is restricted to
approximately 0.1 mm.

When the thickness composition of the intermediate layers and protective layer
are taken into consideration along with the manufacturing margin for such optical
information recording media, it becomes apparent that the thickness compositions of the
intermediate layers and protective layer disclosed in Patent Document 1 and Patent
Document 2 cannot completely eliminate the back-focus issues.
Having been conceived as a solution to the back-focus issues occurring in the
thickness compositions proposed above, it is an object of the present invention to
provide a thickness composition for intermediate layers and a protective layer that
eliminates the back-focus issues that affect the electric signal properties of an optical
information recording medium, while ensuring compatibility with the single-layer and
dual-layer Blu-ray disks currently being sold and taking into consideration the
manufacturing margin for such optical information recording media.
The present invention proposes a thickness composition for intermediate layers
and a protective layer in a multilayer optical information recording medium having three
or more information layers that solves back-focus issues while affording a
manufacturable margin and ensuring compatibility with single-layer and dual-layer
structures.
Specifically, the present invention is as follows.
An optical information recording medium according to the present invention has
at least three information layers, at least two intermediate layers separating the
information layers, and a protective layer layered upon a substrate, the optical
information recording medium being recorded and/or reproduced from the side of the
protective layer using an optical head. The round-trip optical path length difference
between information light returning to the optical head from one of the information layers
upon which recording/reproducing light is focused and reflected stray light that is a part
of stray light reflected by one of the information layers that returns to the optical head
having been reflected by the information layer or the surface of the protective layer no
more than three times is no less than 2 µm.

Preferably, the sum of the thicknesses of the intermediate layers differs from the
thickness of the protective layer.
Preferably, the thicknesses of each of the intermediate layers and the protective
layer differ from one another, and the difference between each thickness is no less than
1 µm.
Preferably, the thickness variability of each of the intermediate layers is within ±2 µm.
Preferably, the optical information recording medium comprises a first information
layer provided upon the substrate, a first intermediate layer provided upon the first
information layer, a second information layer provided upon the first intermediate layer,
a second intermediate layer provided upon the second information layer, a third
information layer provided upon the second intermediate layer, a third intermediate layer
provided upon the third information layer, a fourth information layer provided upon the
third intermediate layer, and the protective layer provided upon the fourth information
layer, with the second intermediate layer being the thinnest of the first through third
intermediate layers.
Preferably, the optical information recording medium comprises a first information
layer provided upon a substrate, a first intermediate layer provided upon the first
information layer, a second information layer provided upon the first intermediate layer,
a second intermediate layer provided upon the second information layer, a third
information layer provided upon the second intermediate layer, and the protective layer
provided upon the third information layer, with the second intermediate layer being
thinner than the first intermediate layer.
Preferably, the thickness of each intermediate layer is no less than 16 urn and no
more than 37µm.

Preferably, the thickness of the protective layer is no less than 43 µm and no
more than 59 µm.
Preferably, the thickness of the first intermediate layer is no less than 23 µm and
no more than 27 µm, the thickness of the second intermediate layer is no less than 16
µm and no more than 20 µm, and the thickness of the protective layer is no less than 55
µm and no more than 59 µm.
Preferably, the thickness of the first intermediate layer is no less than 23 µm and
no more than 27 µm, the thickness of the second intermediate layer is no less than 18
µm and no more than 22 µm, and the thickness of the protective layer is no less than 53
µm and no more than 57 µm.
Preferably, the thickness of the first intermediate layer is no less than 33 µm and
no more than 37 µm, the thickness of the second intermediate layer is no less than 18
µm and no more than 22 µm, and the thickness of the protective layer is no less than 43
µm and no more than 47 µm.
Preferably, the difference between the thickness of the first intermediate layer
and the thickness of the second intermediate layer is more than 1 µm.
Preferably, the difference between the thickness of the first intermediate layer
and the total thickness of the second intermediate layer and protective layer is more
than 1 µm.
Preferably, the difference between the total thickness of the first and second
intermediate layers and the total thickness of the protective layer is more than 1 µm.
Preferably, the difference between the thickness of the second intermediate layer
and the thickness of the protective layer is more than 1 µm.

Preferably, the difference between the thickness of the first intermediate layer
and the thickness of the protective layer is more than 1 µm.
Preferably, the thickness between an information layer A and an information layer
B on the light entry side of the information layer A and the thickness between the
information layer B and an information layer C on the light entry side of the information
layer B or the surface of the protective layer are different by more than 1 µm. This is
because with such a structure, when information light is focused on the information
layer A, the round-trip optical path length difference between the information light that
returns to the optical head from the information layer A and the stray light that returns to
the optical head after being reflected in order from the information layer B ? the
information layer C or the protective layer surface ? the information layer B is
sufficiently long.
Note that the information layer A may be any of the fourth through the second
information layers, counted in from the light entry side. Meanwhile, the information layer
B may be any of the third through the first information layers, counted in from the light
entry side. Finally, the information layer C may be any of the second through the first
information layers, counted in from the light entry side.
Preferably, the thickness between an information layer a and an information layer
b on the light entry side of the information layer a and the thickness between an
information layer c on the light entry side of the information layer b and an information
layer d on the light entry side of the information layer c or the surface of the protective
layer are different by more than 1 µm. This is because with such a structure, when
information light is focused on the information layer a, the round-trip optical path length
difference between the information light that returns to the optical head from the
information layer a and the two incidences of stray light discussed below is sufficiently
long. The first stray light returns to the optical head having been reflected by the
information layer b ? the information layer d or the protective layer surface ? the

information layer c, in that order. The second stray light returns to the optical head
having been reflected three times, by the information layer c ? the information layer d
or the protective layer surface ? the information layer b, in that order.
Note that the information layer a may be any of the fourth through the third
information layers, counted in from the light entry side. Meanwhile, the information
layers b and c may be any of the third through the first information layers, counted in
from the light entry side.
Preferably, recording and/or reproducing is performed using an optical head
including at least a laser light source having a wavelength of no less than 400 nm and
no more than 410 nm, an objective lens having an NA of 0.85, and a spherical
aberration correction element.
According to the present invention, a multilayer optical information recording
medium composed of three or more information layers is capable of reducing the
influence of interlayer crosstalk while maintaining compatibility with conventional single-
and dual-layer optical information recording media, and can eliminate back-focus issues
caused by interference between the information light and reflected stray light, in which
some of the stray light reflected by other information layers when light is focused onto
one of the information layers is reflected three times by other information layers or the
protective layer surface and returns to the optical head, while affording a process
margin sufficient for manufacturing intermediate layers, a protective layer, and so on.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is a diagram illustrating an exemplary structure of a multilayer optical
information recording medium according to a first embodiment of the present invention.
Fig. 2 is a diagram illustrating an example of a conventional structure of a
multilayer optical information recording medium.
Fig. 3 is a diagram illustrating patterns in which back-focus issues arise.
Fig. 4 is a diagram illustrating surface thickness distributions of a third

intermediate layer and thickness fluctuations per manufacture sample.
Fig. 5 is a diagram illustrating the relationship between the surrounding
temperature of a coating apparatus and the average value of the surface thickness of a
third intermediate layer.
Fig. 6 is a diagram illustrating the variability in the thickness from the protective
layer surface to a first information layer.
Fig. 7 is a diagram illustrating an exemplary structure of a dual-layer optical
information recording medium.
Fig. 8 is a diagram illustrating the thickness of an intermediate layer and the
reproducing signal properties of a recorded/reproduced signal.
Fig. 9 is a diagram illustrating the reproducing signal amplitude relative to the
difference in inter-layer thicknesses.
Fig. 10 illustrates the state of dust buildup on the protective layer surface as
observed through a photon microscope.
Fig. 11 is a diagram illustrating the relationship between the thickness of the
protective layer and the SER.
Fig. 12 is a diagram illustrating exemplary back-focus issues with three
reflections and five reflections.
Fig. 13 is a diagram illustrating the relationship between the ratio of the amount
of stray light to the amount of reproducing light and the fluctuation range of the
reproducing signal amplitude.
Fig. 14 is a diagram illustrating an exemplary pattern in which back-focus issues
arise.
Fig. 15 is a diagram illustrating the state of reproducing signal amplitude
fluctuation when interference between stray light and information light has occurred.
Fig. 16 is a diagram comparing the optical path length of information light with the
optical path length of stray light.
Fig. 17 is a diagram illustrating an exemplary configuration of an optical
information recording medium and an optical head.
Fig. 18 is a diagram illustrating an exemplary structure of a multilayer optical
information recording medium according to a fourth embodiment of the present
13
invention.
EXPLANATION OF REFERENCE
101 substrate
102 first information layer
103 second information layer
104 third information layer
105 fourth information layer
106 first intermediate layer
107 second intermediate layer
108 third intermediate layer
109 protective layer
110 objective Jens
111 recording/reproducing light
112 aberration correction element
201 substrate
202 first information layer
203 second information layer
204 third information layer
205 Nth information layer
206 objective lens
207 recording/reproducing light
301 optical path of information light to be read
302 optical path of stray light converging on third information layer
303 optical path of information light to be read
304 optical path of stray light converging on protective layer surface
305 optical path of information light to be read
306 optical path of stray light not converging on other information layers
307 optical path of stray light not converging on other information layers
701 substrate
702 second information layer

703 third information layer
704 first intermediate layer
705 protective layer
706 objective lens
707 recording/reproducing light
708 aberration correction element
1201 first information layer
1202 second information layer
1203 third information layer
1204 fourth information layer
1205 protective layer surface
1206 optical path of information light to be read
1207 optical path of stray light converging on third information layer
1208 optical path of information light to be read
1209 optical path of stray light not converging on other information layers
1210 optical path of information light to be read
1211 optical path of stray light reflected five times and then converging on
second information layer
1401 first information layer
1402 second information layer
1403 third information layer
1404 fourth information layer
1405 protective layer surface
1406 optical path of information light to be read
1407 optical path of stray light
1701 optical information recording medium
1702 optical head
1703 light source
1704 recording/reproducing light
1705 collimate lens
1706 polarizing beam splitter

1707 quarter wave plate
1708 objective lens
1709 aperture
1710 detection lens
1711 cylindrical lens
1712 photodetector
1801 substrate
1802 first information layer
1803 second information layer
1804 third information layer
1805 first intermediate layer
1806 second intermediate layer
1807 protective layer
1808 objective lens
1809 recording/reproducing light
1810 aberration correction element
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments of the present invention shall now be described with reference to
the drawings.
(First Embodiment)
Fig. 1 illustrates an exemplary configuration of a four-layer optical information
recording medium according to a first embodiment of the present invention. The optical
information recording medium examined here is a disk-shaped optical information
recording medium with an outer diameter of approximately 120 mm and a thickness of
approximately 1.2 mm, and Fig. 1 is a diagram illustrating a part of a cross section
thereof.
The following descriptions discuss examination results for a write-once, four-layer
optical information recording medium having information layers composed of a write-
once phase change material. "Write-once phase change material" refers to a material

that can take on two or more states having different optical properties by being heated
through the irradiation of recording/reproducing light. Preferably, this is a material in
which the stated reaction can result in an irreversible change. For example, a material
containing O and M is preferable (where M is a single element or plural elements
selected from Te, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ru, Rh,
Pd, Ag, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Pt, Au, and Bi). A structure in which a dielectric
material is also layered in addition to those materials is also preferable. However, the
materials contained in the information layers are not limited to these materials. In
addition, the effects of the present invention are the same even if a metal reflective film,
such as Ag or Al alloys used in read-only media, is used rather than a write-once phase
change material. Furthermore, the effects of the present invention are the same even if
a phase change material capable of repeated recording is used.
A resin substrate 101 is a resin substrate, approximately 1.1 mm thick, made up
of polycarbonate resin, and guidance grooves of a concavo-convex form are formed in
one side thereof. The structure is such that a first information layer 102 containing a
phase change recording material, a first intermediate layer 106 (thickness t1) composed
of ultraviolet light-curable resin, a second information layer 103, a second intermediate
layer 107 (thickness t2), a third information layer 104, a third intermediate layer 108
(thickness t3), a fourth information layer 105, and a protective layer 109 (thickness tc)
are layered, in that order, upon the resin substrate 101. The external surface of the
protective layer 109 is referred to as a protective layer surface 109a. Because it is
necessary for the second information layer 103 to the fourth information layer 105 to
both reflect recording/reproducing light and allow recording/reproducing light to pass
through to the information layer furthest from the recording/reproducing light-entry side,
those layers are composed of a thin film material that is semitransparent with respect to
recording/reproducing light. Furthermore, the transmissibility and reflectance of each
information layer is designed so that the amount of light reflected from each information
layer to the optical head is approximately the same. For this reason, the materials are
designed so that the transmissibility increases from the first information layer 102 to the
fourth information layer 105.

The first intermediate layer 106 to the third intermediate layer 108 are formed
through coating of an ultraviolet light-curable resin, which is cured after being pressed
on one side with a stamper having guidance grooves of a concavo-convex form, and
then transferring the concavo-convex form onto their surfaces following the removal of
the stamper. The protective layer 109 is also formed through coating with an ultraviolet
light-curable resin. It is preferable for the resin material used in the intermediate layers
and protective layer to be approximately transparent with respect to the wavelength of
the recording/reproducing light. "Approximately transparent" refers to a resin that
preferably has a transmissibility of 90% or more for the wavelength of the
recording/reproducing light. For example, a resin having a transmissibility of 90% or
more for a wavelength of 405 nm is preferable.
Meanwhile, the optical head that records to/reproduces this optical information
recording medium is configured with a 405 nm wavelength semiconductor laser as its
light source, an objective lens 110 with an NA of 0.85, and an aberration correction
element 112 configured of a combination lens. •
Fig. 17 illustrates exemplary configurations of an optical information recording
medium 1701 and an optical head 1702 according to the first embodiment of the
present invention.
A light source 1703 emits a divergent beam 1704 of linearly-polarized light with a
wavelength of 405 nm. The beam 1704 emitted from the light source 1703 is
transformed into parallel light by a collimate lens 1705 having a focal distance f1 of 18
mm, and then passes through a polarizing beam splitter 1706; after then passing
through a quarter wave plate 1707 and being transformed into circular polarized light,
the beam 1704 is transformed into a convergent beam by an objective lens 1708 having
a focal distance of f2 of 2 mm, and is then concentrated upon the optical information
recording medium 1701. The aperture of the objective lens 1708 is restricted by an
aperture 1709, which has a numerical aperture NA of 0. 85. The collimate lens 1705 is

adjusted in the direction of the optical axis using an aberration correction element
configured of a stepping motor and the like, so that the spherical aberration on the
information layer is approximately 0 m?. The beam reflected by the information layer
passes through the objective lens 1708 and the quarter wave plate 1707, is transformed
into linearly-polarized light 90 degrees different from that in the round-trip path, and is
then reflected by the polarizing beam splitter 1706. The beam reflected by the
polarizing beam splitter 1706 is then divided by a diffraction grating, which is a beam
dividing element, into a beam of zero-order light and first-order light, passes through a
detection lens 1710 having a focal distance f3 of 30 mm and a cylindrical lens 1711, and
enters into a photodetector 1712. The beam that enters the photodetector 1712 is given
astigmatism upon passing through the cylindrical lens 1711.
The aberration correction element plays the part of adding aberration that
counteracts aberration components arising in each information layer in order to correct
aberration components, such as spherical aberration, arising due to differences in the
thickness from the protective layer surface of the optical information recording medium
to the information layer to/from which information is recorded/reproduced. Originally,
this optical head has an optical design aimed at reducing the aberration for the
information layer in a single-layer medium, and considering the recording/reproducing of
up to a dual-layer medium, is set so that the position of minimum aberration, design-
wise, is approximately 80 to 90 urn from the protective layer surface. For this reason,
when concentrating recording/reproducing light onto information layers of different
thicknesses from the position of minimum aberration, it is necessary for aberration
correction values to be set and correction performed for each information layer by the
aberration correction element.
Note that the wavelength of 405 nm for the semiconductor laser used as the light
source is set with a permissible wavelength range of 400 nm to 410 nm, due to slight
changes in the wavelength caused by the design or due to changes in
temperature/driving current. The effects of the present invention do not change
throughout the 400-to-410 nm wavelength range, and thus the same effects can be

obtained.
The optimal design values with respect to the thicknesses of the intermediate
layers and the protective layer was then considered, taking the thickness of the first
intermediate layer 106 as t1, the thickness of the second intermediate layer 107 as t2,
the thickness of the third intermediate layer 108 as t3, and the thickness of the
protective layer 109 as tc.
Note that values measured using a thickness gauge having a confocal optical
system are used as the thickness values mentioned here. This gauge is configured so
as to concentrate a beam using an optical head having a 405 nm-wavelength light
source, an objective lens, and an actuator onto an optical information recording medium
and receive light reflected by that optical information recording medium using a
photodetector having a pinhole in the previous stage. This creates an optical design
whereby when the beam is focused on the boundary surface of the optical information
recording medium, the beam is also focused on the surface of the photodetector; light
passes through the pinhole provided in the stage previous to the photodetector only
when the beam is focused on the boundary surface of the optical information recording
medium, whereas a major portion of the light is blocked by the pinhole when the beam
is focused on a position aside from the boundary surface of the optical information
recording medium. Whether or not the beam is focused on the boundary surface of the
optical information recording medium can be determined by measuring the optical
intensity detected by the photodetector in this manner. The beam is focused on each
information layer while using the actuator to move the optical head in the direction of the
optical axis at which light enters the optical information recording medium, and each
position that is in focus is calculated based on the distance the actuator has moved;
each of these is taken as a thickness result. Note that this gauge is calibrated to
measure an accurate thickness when the refraction index n with respect to the
wavelength of 405 nrh for the intermediate layers or protective layer is 1.6, and thus the
optical thickness will vary depending on the value of the refraction index n of the
material from which the intermediate layers and protective layer are formed. The

thickness values discussed in the first embodiment of the present invention refer to
thicknesses found when the refraction index n has been converted to 1.6.
"Thicknesses found when the refraction index n has been converted to 1.6" refers
to the data measured by the stated thickness gauge when the refraction index n of each
resin layer has been set to 1.6. Taking the refraction index n of the resin at a
wavelength of 405 nm and the actual thickness as d(um) when measuring the
thicknesses of the resin layers using this thickness gauge, 1.6 x d/n is obtained as the
measured data when the refraction index is set to 1.6. In this specification, the
"thickness value" refers to a value obtained by this thickness gauge (under these
thickness measurement conditions). Discussions regarding thickness are therefore not
concerned with the actual thickness d in this specification.
Next, variability in the thickness of the intermediate layers or protective layer
when manufacturing a four-layer optical information recording medium were
investigated. The desired thicknesses of the intermediate layers and protective layer
were 24 µm for t1, 13 µm for t2, 18 µm for t3, and 45 µm for the thickness tc of the
protective layer, with the thickness from the protective layer surface to the first
information layer at 100 µm. Here, the intermediate layers and protective layer were
manufactured using a process for coating an ultraviolet light-curable resin through a
spin coat method.
Fig. 4 illustrates surface thickness distributions of the thickness t3 of the third
intermediate layer and thickness fluctuations per manufacture sample, for the
manufactured optical information recording medium. 150 samples were manufactured,
and a single sample was removed every ten samples and the thickness of that sample's
intermediate layer measured. Fig. 4 shows the average surface thickness value of the
optical information recording medium and the maximum and minimum values in the
surface using an error bar. As can be seen in Fig. 4, there are variabilities in the surface
thickness from medium to medium. This is a thickness distribution that depends on
thickness variations resulting from the spin coat method, such as thickness variations

caused by differences in the radial centrifugal force exerted on the resin drawn out by
the rotation of the spin table during spinning and thickness variations caused by the
resin at the edges bulging outward due to the influence of surface tension in the resin
coat edges after the spinning has been stopped, or due to the influence of resin flow
occurring during pressing with a stamper following the resin coating; the distribution is
approximately 3 µm depending on the state of differences between the maximum and
minimum thicknesses throughout the entire surface of the medium. Various methods
aside from the spin coat method can be considered as methods for forming the
intermediate layers or protective layer, such as, for example, screen printing, gravure
printing, and so on; however, although the shape of the thickness distribution is
different, a thickness distribution of approximately 3 µm appears no matter what
technique is used. Also, because this method includes a process of coating a liquid
ultraviolet light-curable resin, the influence of the surrounding environment of the
coating apparatus, and the influence of changes in the temperature and humidity in
particular, is great. For example, the temperature of the ultraviolet light-curable resin
increases with increase in the surrounding temperature, causing a drop in viscosity.
When resin is coated using the spin coat method, for example, in such a state, the
intermediate layer or protective layer that is formed will be thinner by the amount at
which the viscosity dropped. Although adding a temperature adjustment function to the
coating apparatus itself can reduce the degree of thickness fluctuations due to changes
in temperature, the influence thereof cannot be completely eliminated; thus thickness
variability appears among the optical information recording media as multiple optical
information recording media are manufactured. Fig. 5 illustrates the relationship
between the surrounding temperature of the coating apparatus and the average value of
the surface thickness of the third intermediate layer. The data in Fig. 5 shows that a
change in thickness of approximately 0.5 µm occurs for a temperature change of
approximately 1° C. A change in thickness of approximately 1 µm arises between
media even when the surrounding temperature is restricted to a change of
approximately 1° C by a temperature adjustment function. Including this thickness
variability in the medium surface and thickness fluctuations between media results in
the occurrence of a maximum variation of approximately 4 µm with respect to the

desired thickness. For this reason, the thickness of each intermediate layer or the
thickness of the protective layer have a process-related fluctuation factor of
approximately ±2 µm with respect to the desired thickness. Although these descriptions
focus only on the thickness t3 of the third intermediate layer, the same effects are
obtained for the thickness t1 of the first intermediate layer, the thickness t2 of the
second intermediate layer, and the thickness tc of the protective layer, and thus a
thickness fluctuation range in the intermediate layers and protective layer of
approximately ±2 µm with respect to the desired thickness can be expected. In other
words, when mass-producing the optical information recording medium, it is possible
that the thicknesses of the intermediate layers and protective layer are each off from the
desired thickness by approximately ±2 µm, and thus it is necessary to take this
fluctuation range into consideration when designing the thicknesses of the intermediate
layers and protective layer of a four-layer medium.
Next, the degree of variability that appears in the thickness from the protective
layer surface to the first information layer in a four-layer optical information recording
medium manufactured by layering intermediate layers, a protective layer, and so on
having this ±2 µm thickness fluctuation range was examined. The thickness from the
protective layer surface to the first information layer in an optical information recording
medium in which a first intermediate layer, a second intermediate layer, a third
intermediate layer, and a protective layer are layered was examined in 150 samples.
The root-sum-square of the surface thickness variability in each intermediate layer were
also examined for the same samples at this time. Fig. 6 illustrates those results. It can
be seen that although the thickness of each intermediate layer has a range of ±2 µm,
the thickness variability in a structure where four such layers are layered is not simply
the sum of ±2 µm, or ±8 µm; rather, the variation is within ±4 µm, or the root-sum-
square of each thickness variation. For this reason, if the desired thickness from the
protective layer surface to the first information layer is 100 µm, that thickness ultimately
has a fluctuation range of ±4 µm, and thus fluctuates within a range from 96 µm to 104 µm.

Next, the relationship of the intermediate layer thickness and the quality of the
recording/reproducing signal was examined for two information layers that sandwich
that intermediate layer when the thickness of that intermediate layer is changed. Here,
in order to create a simplified model for the thickness of the intermediate layer and the
influence of interlayer crosstalk between the information layers sandwiching that
intermediate layer, a dual-layer optical information recording medium, such as that
shown in Fig. 7, was used for evaluation. However, the same recording film as used for
the second information layer and third information layer of the four-layer optical
information recording medium was used for the two information layers here. "Interlayer
crosstalk" as mentioned here refers to noise leaking into the signal to be read when
focusing recording/reproducing light onto the information layer that is to be recorded
to/reproduced. This is caused by more concentrated recording/reproducing light being
irradiated onto other layers due to the diameter of the recording/reproducing light spot
on other information layers dropping as a result of the thickness of the intermediate
layer being reduced. Although the dual-layer optical information recording media were
manufactured with several varying types of intermediate layer thicknesses, the
thickness of the protective layer in all the media was 63 µm.
As an evaluation method, a signal was recorded to both information layers at the
same radial position, and jitter values containing signal leakage from the other layer
were examined. "Jitter value" refers to the amount of fluctuation or deviation from the
desired temporal position of the recorded signal, and the lower the jitter value, the
higher the reproducing quality of the signal. Fig. 8 illustrates the thickness of the
intermediate layer and the reproducing signal properties of a signal recorded
to/reproduced from each information layer. Note that the recording/reproducing of the
signal was performed at a linear speed of 4.9 m/s, and the jitter was evaluated in a state
boosted by a limit equalizer. A jitter value of 8.5% or less was used as a benchmark for
determining the quality of the medium. If this jitter value can be obtained, error
correction can be performed with almost no problems, and is thus a level that enables
reproducing. As can be seen in Fig. 8, the thinner the intermediate layer is, the worse
the jitter value becomes in both layers, due to the influence of interlayer crosstalk. The

degradation of the jitter value becomes particularly apparent at thicknesses of less than
10 µm, and it is thus necessary for the intermediate layer to be at least 10 µm in order
to meet the criteria for jitter value. Furthermore, based on this graph, the influence of
interlayer crosstalk from an adjacent information layer are extremely small when the
intermediate layer is 18 µm or thicker, and the influence on the jitter value is minor.
Next, the influence of interference caused by multisurface reflected light shall be
evaluated. As shown in Figs. 3(a) to 3(c), when the recording/reproducing light is
focused upon the information layer to be read out, if some of the stray light reflected by
another layer is reflected in multiple via one of the information layers, the protective
layer surface, or the like, and then enters the photodetector in the optical head with the
same optical path length and with the same beam diameter as the information light to
be read out, those stray light components enter the photodetector having been reflected
by multiple information layers. The stray light components therefore have a much
smaller light amount relative to the information light to be read out, but also enter the
photodetector with the same optical path length and with the same beam diameter,
resulting in major influence exerted by interference. Thus a minute change in the
thicknesses of the intermediate layer or protective layer causes a major fluctuation in
the light amount, making stable signal detection difficult.
Fig. 9 illustrates the reproducing signal amplitude relative to the difference in
inter-layer thicknesses when the light amount ratio of the information light to be read out
to the stray light returning to the photodetector in a pattern as shown in Fig. 3 is 100:1.
Note that "difference in inter-layer thicknesses" refers to the difference in the
thicknesses of the first intermediate layer, the second intermediate layer, and the third
intermediate layer. In Fig. 9, the horizontal axis expresses the difference in inter-layer
thicknesses, and the vertical axis expresses the reproducing signal amplitude; the value
has been normalized to a DC light amount found when only the information light to be
read out is received by the photodetector. It can be seen in Fig. 9 that when the
difference in inter-layer thicknesses drops below 1 µm, the reproducing signal amplitude
fluctuates dramatically. Based on this, it is considered preferable to provide a difference

of 1 µm or more for the thicknesses of the first intermediate layer, the second
intermediate layer, and the third intermediate layer.
Next, the relationship between the thickness of the protective layer and the
information signal recorded to or reproduced from the information layer shall be
evaluated. There is a high likelihood that the surface of the protective layer will become
soiled by dirt, dust, or fingerprints, or scratched. When such blemishes are present on
the surface of the protective layer, the recording/reproducing light for recording to or
reproducing from the information layers is blocked, the angle at which the
recording/reproducing light enters changes, and so on, which significantly affects the
quality of the signal for recording to or reproducing from the information layer. In
addition, the thinner the protective layer is, the smaller the diameter of the beam of
recording/reproducing light becomes on the protective layer surface when the
recording/reproducing light is focused onto the information layer; therefore, if particles of
dirt that are smaller than the diameter of the beam of recording/reproducing light on the
protective layer surface are present, the size of the dust particles becomes larger
relative to the beam diameter on the protective layer surface, even if the dust particles
are the same size. This expands the area that blocks the recording/reproducing light,
which is assumed to have major influence on the quality of the signal recorded
to/reproduced from the information layer. Accordingly, a single-layer medium having
recording film of the same structure as the fourth information layer of a four-layer optical
information recording medium was evaluated; samples in which the thickness of the
protective layer was changed from 100 µm to 30 µm were manufactured, blemishes
were added to the protective layer surfaces of those media, and the error rate was
examined to discover how much influence the blemishes had on the
recording/reproducing signal in the information layer. Note that the recorded signal was
a random-pattern signal modulated according to the 1-7PP modulation technique, with a
reference clock frequency of 66 MHz and a minimum mark length of 149 nm, and the
recording/reproducing linear speed was set to 4.9 m/s.
The evaluation method shall be described next. Most dust particles in a general

household environment are 20 µm or less in diameter. Therefore, in order to reproduce
the dust particle buildup conditions of a general household environment, it is preferable
to cause dust particles 20 µm or less in size to build up using a dust environment tester
or the like. The amount of dust actually building up changes, of course, depending on
environmental differences, the storage environment (whether the medium is stored in a
case or not), how long the medium is stored, and so on, but when an optical information
recording medium is actually inserted into a drive and rotated, almost all dust aside from
that building up due to static electricity is shaken off. Taking this into consideration, a
concentration of the occupancy of dust particles 20 µm or less in diameter on the
protective layer surface (called "dust occupancy" here) of approximately 1% is said to
be acceptable. Therefore, in the present first embodiment, dust of a dust occupancy of
approximately 1% is caused to build up, using a dust environment tester, on the
protective layer surfaces of samples in which the thicknesses of the protective layers
have been changed, and the degree to which the error rate worsened when that
medium was recorded to/reproduced was evaluated. The JIS test powder I, class 10
(fly ash, ultrafine) was used as the powder caused to build up. This powder has a
particle diameter distribution in which 2 µm-sized dust is 82 ±5%, 4 µm-sized dust is 60
±3%, 8 µm-sized dust is 22 ±2%, and 16 µm-sized dust is 3 ±3%, and is considered an
extremely favorable powder for evaluating the influence of an environment with dust
particles of 20 µm or less. An environment with a constant dust concentration was
created using a dust environment tester (a Shibata AP-355), and a test optical
information recording medium was left within the tester for a set amount of time,
allowing dust to build up thereon. Fig. 10 illustrates the state of dust buildup on the
protective layer surface as observed through a photon microscope. The buildup of dust
resulted in a dust ocoupancy of approximately 1.3%.
The error rate (SER: Symbol Error Rate) was then evaluated using this sample.
As a benchmark for an acceptable SER, an error rate of less than 4.2x10-3 was
considered to be non-problematic. This error rate value is a level at which there is the
possibility that information cannot be read from one out of 1,000,000 media, and thus it
can be said that there are no problems with the recording/reproducing properties of the

optical information recording medium if the error rate is below this value. Furthermore,
aside from the recording conditions at which the recording/reproducing signal quality is
optimal when recording a signal, the recording stress state or reproducing stress state
that actually occurs was given the same SER evaluation, and the acceptability thereof
was determined. The recording stress state used here was determined by estimating
the defocus amount of the recording/reproducing light, the influence of tilt that can occur
in the disk, and the amount of spherical aberration that can occur, as well as setting
errors in the recording power, errors in recording power learning for optimizing the
recording power, recording power errors caused by temperature changes, and so on.
Here, as a recording stress state, the power was set to 8.8% lower than the optimal
recording power. Meanwhile, the reproducing stress state was determined by
estimating the manufacturing variability in the optical head that performs readout, a
defocus amount, track skew caused by disk tilt, and the like. Here, the power was set to
29% lower than the normal reproducing power.
Fig. 11 illustrates SER value evaluations for an optical information recording
media whose protective layers vary from 100 µm to 30 µm. Here, the SER value when
recording under the optimal recording conditions, the SER value when recording under
recording stress conditions, and furthermore the SER value when the signal recorded
under the recording stress conditions was reproduced under reproducing stress
conditions, were evaluated using a sample in which dust was allowed to build up on the
protective layer surface in the manner shown in Fig. 10. An SER obtained when a
sample in which no dust builds up on the protective layer surface has been recorded to
under optimal recording conditions and reproduced at a normal reproducing power was
used as a reference value. Because the results show that the SER value does not
exceed 4.2x10-3 under the dust buildup state, recording stress state, and reproducing
stress state, it can be said that a protective layer thickness of 40 µm or more is
acceptable.
Next, the degree of influence of back-focus issues was evaluated. With a four-
layer optical information recording medium, a total of five reflective surfaces, or the
information layers from the first information layer to the fourth information layer and the
surface of the protective layer, are present. When the recording/reproducing light is
focused onto one of the information layers, some of the stray light reflected from the
other reflective layers undergoes repeated multisurface reflection, and returns to the
photodetector provided in the optical head. The stray light that returns to the
photodetector always returns to the photodetector having been reflected by one of the
boundary surfaces an odd number of times. Accordingly, a pattern in which the stray
light returns to the photodetector after three reflections and a pattern in which the stray
light returns to the photodetector after five reflections were considered, and the degree
of influence thereof was evaluated.
The reflectances and transmissibilities of the information layers in the four-layer
optical information recording medium according to the present first embodiment are
shown in Table 1.
The reflectances and transmissibilities of the information layers are set so that
the reflectances of the information layers are approximately identical when those
information layers are reproduced. For this reason, the nearer to the first information
layer from the protective layer, the higher and lower, respectively, the reflectances and
transmissibilities of the information layers are. When this information layer composition
is used, the reflectances of the information layers during recording/reproducing
balances out at around 4 - 5%.
Fig. 12 illustrates an example of back-focus issues that may arise, including a

back-focus issue with three reflections and a back-focus issue with five reflections.
Because the reflectances of the information layers are higher the closer to the first
information layer, the amount of stray light that returns to the photodetector is greater
when multisurface reflection occurs in one of the information layers closer to the first
information layer than to the protective layer surface. For example, the pattern shown in
Fig. 12(a) illustrates stray light that returns after being reflected three times, by the
second information layer, the third information layer, and the second information layer
again, when the recording/reproducing light is focused on the first information layer. In
this case, the stray light is undergoes multisurface reflection between the second
information layer and the third information layer, which have high reflectances, and thus
among the three reflections that can occur, this pattern has the highest amount of stray
light relative to the amount of light reflected by the first information layer onto which the
recording/reproducing light is focused. In this pattern, the amount of stray light is
approximately 1.4% of the amount of reproducing light on the first information layer.
Fig. 13 illustrates the relationship between the ratio of the amount of stray light to the
amount of reproducing light and the fluctuation range of the reproducing signal
amplitude. In the pattern shown in Fig. 12(a), the amount of stray light is approximately
1.4% of the amount of reproducing light, and thus the reproducing signal amplitude
fluctuates by approximately 45% as a result.
Meanwhile, the pattern shown in Fig. 12(b) illustrates stray light that returns after
passing through the second information layer, the fourth information layer, and the third
information layer, when the recording/reproducing light is focused on the first
information layer. At the same time, stray light that returns to the photodetector after
being reflected by the third information layer, the fourth information layer, and the
second information layer also occurs in this pattern; because two incidences of stray
light return to the photodetector, the influence thereof is great. In the present first
embodiment, the amount of stray light relative to the amount of reproducing signal light
is approximately 0.87%. In addition to a case where a single beam of stray light occurs,
as mentioned earlier with reference to Fig. 12(a), Fig. 13 also shows correlation data for
a case where two beams of stray light occur, as shown in Fig. 12(b). In a pattern where

two beams of light return to the photodetector, as shown in Fig. 12(b), the reproducing
signal amplitude fluctuates by approximately 50% when the amount of stray light is
taken as approximately 0.87% of the amount of reproducing signal light.
Next, the influence of stray light reflected five times shall be evaluated. Because
multilayer reflection occurring at an information layer toward the first information layer
side is multilayer reflection by an information layer with a higher reflectance, a greater
amount of stray light returns to the photodetector. For this reason, the pattern of five
reflections shown in Fig. 12(c), in which the light returns to the photodetector having
been reflected by the second information layer, the third information layer, the second
information layer, the third information layer, and the second information layer when the
recording/reproducing light is focused on the first information layer, is considered to be
the pattern with the greatest amount of stray light, compared to the other patterns in
which the stray light returns after five reflections. In this case, the amount of stray light
is approximately 0.02% of the amount of reproducing light, and thus estimating the
fluctuation of the reproducing signal amplitude based on Fig. 13 shows a fluctuation
range of approximately 2 - 3%. Fluctuation of this degree does not exert significant
influence on the quality of the recording/reproducing signal. Therefore, stray light that
returns to the photodetector after being reflected five times can be ignored. Based on
this, it is acceptable to consider only stray light that returns to the photodetector having
been reflected three or fewer times by one of the information layers or the protective
layer surface to be detrimental to the quality of the recording/reproducing signal due to
the influence of back-focus issues.
Fig. 15 illustrates the fluctuation of the reproducing signal amplitude in the case
where interference occurs between stray light that returns to the photodetector having
been reflected three times and the information light. Fig. 15(b) illustrates the fluctuation
of the reproducing signal amplitude caused by the influence of stray light entering the
photodetector with the same optical path length and the same beam diameter as the
information light from the first information layer that is to be read out, with that stray light
having been reflected three times, by the fourth information layer, the protective layer

surface, and the third information layer, as shown in Fig. 14. Fig. 15(a), meanwhile,
shows a reproducing signal waveform occurring when the protective layer of the optical
information recording medium shown in Fig. 14 is manufactured with a thickness
increased by approximately 3 µm. The influence of interference has been eliminated by
shifting the optical path length of the same thrice-reflected stray light from the optical
path length of the information light from the first information layer.
Here, it was examined how far the optical path length needed to be shifted from
the optical path length of the information light to be read out in order to eliminate the
influence of interference. It can be seen, from looking at the reproducing signal
waveform in Fig. 15(b), that there are regions of large and small amplitude fluctuations.
The optical path length of the information light was compared with the optical path
length of the stray light in these respective regions. The comparison results are shown
in Fig. 16. The horizontal axis expresses the radius of the optical information recording
medium, whereas the vertical axis expresses the difference between the optical path
length of the information light (the round-trip optical path length, from when the
recording/reproducing light enters from the protective layer surface and exits from the
protective layer surface) and the optical path length of stray light resulting from three
reflections as shown in Fig. 14. In other words, the portion in the vertical axis where the
optical path length difference between the information light and the stray light is 0
represents conditions in which the information light and the stray light return to the
photodetector with the same optical path length and the same beam diameter.
However, it can be seen, in the data shown in Fig. 16, that large fluctuations occur in the
signal amplitude not only in regions where the optical path length difference is 0, but
also in regions where the optical path length difference is 0 ±2 µm. Based on this result,
it was concluded that a optical path length difference of ±2 µm or more is preferable.
Next, the conditions required to set the optical path length difference between
the information light and the stray light to ±2 µm or more shall be described.
When focusing recording/reproducing light on an information layer even further
than the fourth information layer (on the side opposite to the light entry side) in an
optical information recording medium that has four information layers, stray light
problems can occur in the following two patterns. Note that in the following
descriptions, the information layer to/from which recording/reproducing is performed
shall be called the "recording/reproducing information layer".
The first pattern of stray light problem assumes stray light reflected three times,
from an information layer B on the light entry side of the recording/reproducing
information layer A ? an information layer C further on the light entry side or the
protective layer surface ? information layer B, in which case the round-trip optical path
length difference between the information light that returns to the optical head from the
recording/reproducing information layer A and the aforementioned stray light is less than
2 µm; therefore, interference can occur between the information light and the stray light.
This first pattern of stray light problem is solved by setting the round-trip optical
path length difference between the thickness between the recording/reproducing
information layer A and the information layer B and the thickness between the
information layer B and the information layer C/protective layer surface to a value that
exceeds 1 µm. Note that the "thickness" mentioned here refers to the thickness
measured by the aforementioned thickness gauge.
To be more specific, when the information layer to be recorded to/reproduced
from is the first information layer 102, it is necessary for the following six patterns to
hold true in order to prevent interference between the information light and the stray
light.
(1) |t1-t2|>1µm
(This makes it possible to prevent interference with stray light reflected by the
second information layer 103 ? the third information layer 104 ? the second
information layer 103, in that order.)
(2) |t1-(t2 + t3)|>1µm
(This makes it possible to prevent interference with stray light reflected by the

second information layer 103 ? the fourth information layer 105 ? the second
information layer 103, in that order.)
(3) |t1-(t2 + t3 + tc)|>1µm
(This makes it possible to prevent interference with stray light reflected by the
second information layer 103 ? the protective layer surface 109a ? the second
information layer 103, in that order.)
(4) |(t1+t2)-t3|>1µm
(This makes it possible to prevent interference with stray light reflected by the
third information layer 104 ? the fourth information layer 105 ? the third information
layer 104, in that order.)
(5) |(t1+t2)-(t3 + tc)|>1µm
(This makes it possible to prevent interference with stray light reflected by the
third information layer 104 ? the protective layer surface 109a ? the third information
layer 104, in that order.)
(6) |(t1 +t2 + t3)-tc|>1µm
(This makes it possible to prevent interference with stray light reflected by the
fourth information layer 105 ? the protective layer surface 109a ? the fourth
information layer 105, in that order.)
When the information layer to be recorded to/reproduced from is the second
information layer 103, it is necessary for the following three patterns to hold true in order
to prevent interference between the information light and the stray light.
(7) |t2-t3|>1µm
(This makes it possible to prevent interference with stray light reflected by the
third information layer 104 ? the fourth information layer 105 ? the third information
layer 104, in that order.)
(8) |t2-(t3 + tc)|>1µm
(This makes it possible to prevent interference with stray light reflected by the
third information layer 104 ? the protective layer surface 109a ? the third information
layer 104, in that order.)
(9) |(t2 + t3)-tc|>1µm
(This makes it possible to prevent interference with stray light reflected by the

fourth information layer 105 ? the protective layer surface 109a ? the fourth
information layer 105, in that order.)
When the information layer to be recorded to/reproduced from is the third
information layer 104, it is necessary for the following pattern to hold true in order to
prevent interference between the information light and the stray light.
(10) |t3-tc|>1µm
(This makes it possible to prevent interference with stray light reflected by the
fourth information layer 105 ? the protective layer surface 109a ? the fourth
information layer 105, in that order.)
The second pattern of stray light problem assumes stray light reflected three
times, from an information layer b on the light entry side of the recording/reproducing
information layer a ? an information layer c further on the light entry side or the
protective layer surface ? an information layer d closer to the light entry side than the
information layer b and on the side opposite to the light entry side of the information
layer c or the protective layer surface, and is solved by reducing the round-trip optical
path length difference between the information light returning to the optical head from
the recording/reproducing information layer a and the aforementioned stray light to less
than 2 µm. Note that stray light reflected three times, from the information layer d ? the
information layer c or the protective layer surface ? information layer b, also occurs in
this second pattern, and thus interference is caused by the two light beams.
This second pattern of stray light problem is solved by setting the thickness
between the information layer a and the information b and the thickness between the
information layer c and the information layer d/protective layer surface to have a
difference that exceeds 1 µm.
To be more specific, when the information layer to be recorded to/reproduced
from is the first information layer 102, it is necessary for the following four patterns to
hold true in order to prevent interference between the information light and the stray
light.
(11) |t1-t3|>1µm

(This makes it possible to prevent interference with stray light reflected by the
second information layer 103 ? the fourth information layer 105 ? the third information
layer 104, in that order, and at the same time prevent interference with stray light
reflected by the third information layer 104 ? the fourth information layer 105 ? the
second information layer 103, in that order.)
(12) |t1 - (t3 + tc) | > 1 µm
(This makes it possible to prevent interference with stray light reflected by the
second information layer 103 ? the protective layer surface 109a. ? the third
information layer 104, in that order, and at the same time prevent interference with stray
light reflected by the third information layer 104 ? the protective layer surface 109a ?
the second information layer 103, in that order.)
(13) |t1-tc|>1µm
(This makes it possible to prevent interference with stray light reflected by the
second information layer 103 ? the protective layer surface 109a ? the fourth
information layer 105, in that order, and at the same time prevent interference with stray
light reflected by the fourth information layer 105 ? the protective layer surface 109a ?
the second information layer 103, in that order.)
(14) | (t1 +12) - tc | > 1µm
(This makes it possible to prevent interference with stray light reflected by the
third information layer 104 ? the protective layer surface 109a ? the fourth information
layer 105, in that order, and at the same time prevent interference with stray light
reflected by the fourth information layer 105 ? the protective layer surface 109a ? the
third information layer 104, in that order.)
When the information layer to be recorded to/reproduced from is the second
information layer 103, it is necessary for the following pattern to hold true in order to
prevent interference between the information light and the stray light.
(15) |t2-tc|>1µm
(This makes it possible to prevent interference with stray light reflected by the
third information layer 104 ? the protective layer surface 109a ? the fourth information
layer 105, in that order, and at the same time prevent interference with stray light

reflected by the fourth information layer 105 ? the protective layer surface 109a ? the
third information layer 104, in that order.)
Next, a specific disk composition (the thicknesses of each film) shall be
examined. It is possible for the thicknesses of the layers to exhibit a variability in the
range of ±2 µm. Therefore, in addition to taking into consideration a thickness variability
of ±2 µm for cases in which the thickness from the protective layer surface to the first
information layer was 100, 101, and 102 mm (±4 µm), respectively, the influence of
back-focus was examined as more specific case.
Thickness compositions that meet the conditions evaluated thus far and do not
experience back-focus issues of up to three reflections are indicated in Table 2. Note
that the conditions evaluated thus far are a thickness variability of ±2 µm in the
intermediate layers and protective layer, a protective layer thickness of 40 µm or more,
a minimum intermediate layer thickness of 10 µm or more, an interlayer thickness
difference of 1.0 µm or more, a thickness between the protective layer surface and the
first information layer of 100 ±4 µm, and an optical path length difference between the
information light and the stray light of ±2 µm or more.
The following experimental examples illustrate the range of the upper limit of the
thickness to the lower limit of the thickness, taking into consideration the thickness
variability of ±2 µm in each layer.
In Table 2, No. 1 - No. 6 have thickness compositions in which back-focus issues
of up to three reflections do not occur; No. 7 and No. 8, however, have thickness
compositions in which back-focus issues of up to three reflections occur. Looking at the
thickness compositions of No. 1 - No. 6 in Table 2, when the thickness from the
protective layer surface to the first information layer is 100 µm, and considering that the
second intermediate layer is the thinnest and a thickness variability of ±2 µm is present,
the minimum intermediate layer thickness is 10 µm, and the maximum intermediate
layer thickness is 26 µm. Furthermore, the minimum protective layer thickness is 42
urn, and the maximum protective layer thickness is 47 µm.
The recording/reproducing properties of optical information recording media
having these eight thickness compositions were investigated. Limit-equalized jitter was
evaluated as the indicator of these properties. Recording/reproducing was performed at
a recording/reproducing linear speed of 4.9 m/s using an optical head having a
wavelength of 405 nm and an objective lens with an NA of 0.85. There are no problems
with performance if the respective jitter values are under 8.5%. When performing the

evaluations, a signal is recorded onto all information layers within the same radius, and
thus the results shown here indicated states having signal crosstalk from other layers.
In the patterns from No. 1 to No. 6, the thicknesses of the intermediate layers and the
thickness of the protective layer all have the desired thickness variability within ±2 µm in
that surface, and reproducing signal amplitude fluctuations due to back-focus issues do
not occur in any of the regions within the surface of the medium; moreover, jitter of less
than 8.5% was confirmed in all information layers.
With the thickness composition in No. 7, back-focus issues with three reflections,
from the third information layer, the protective layer surface, and the fourth information
layer, occurred when recording to/reproducing from the first information layer, causing
signification fluctuations in the reproducing signal amplitude. Furthermore, the jitter
value in the first information layer at that time exceeded 8.5%. The reason for this is
that in the above pattern (14), the value of |(t1 + t2) - tc| is less than 1 µm, and as a
result, the optical path length difference between the information light and the stray light
is sometimes less than 2 µm.
Meanwhile, with the thickness composition in No. 8, back-focus issues with three
reflections, from the fourth information layer, the protective layer surface, and the fourth
information layer, occurred when recording to/reproducing from the second information
layer. The jitter value greatly exceeded 8.5%. The reason for this is that in the above
pattern (9), the value of |(t2 +t3) - tc| is less than 1 µm, and as a result, the optical path
length difference between the information light and the stray light is sometimes less
than 2 µm.
Although only the thickness compositions indicated in Table 2 have been
described here, the Compositions are not limited to those patterns as long as the stated
conditions, where the thickness variability of the intermediate layers and protective layer
is ±2 µm, the thickness of the protective layer is 40 µm or more, the minimum
intermediate layer thickness is 10 µm or more, the interlayer thickness difference is 1.0
µm or more, the thickness from the protective layer surface to the first information layer

is 100 ±4 µm, and the optical path length difference between the information light and
the stray light is ±2 µm taking into consideration stray light that returns to the optical
head after up to three reflections, are met.
(Second Embodiment)
In the present second embodiment, the thickness composition and
recording/reproducing signal properties were evaluated for a four-layer optical
information recording medium in the case where the thickness from the protective layer
surface to the first information layer is 101 ±4 µm. The necessary conditions are the
same as described in the first embodiment; namely, a thickness variability in the
intermediate layers and protective layers of ±2 µm, a protective layer thickness of 40 µm
or more, a minimum intermediate layer thickness of 10 µm, an interlayer thickness
difference of 1.0 µm or more, and an optical path length difference between the
information light and the stray light of ±2 µm or more.
Aside from the thickness from the protective layer surface to the first information
layer being 101 µm, the composition is exactly the same as in the first embodiment.
The reason the thickness from the protective layer surface to the first information
layer was increased to 101 µm in the present second embodiment is that current dual-
layer Blu-ray disks allow a variability in the thickness to the first information layer of 100
µm ±5 µm. The thickness from the protective layer surface to the first information layer
is set to 101 ±4 µm to create a range that is compliant with current dual-layer drives.
The thickness compositions and recording/reproducing properties of the four-
layer optical information recording media of the present second embodiment are shown
in Table 3.
In Table 3, the patterns in No. 1 - No. 8 have thicKness compositions in which
back-focus issues of up to three reflections do not occur, whereas No. 9 has a thickness
composition in which back-focus issues of three reflections occur.
Looking at the thickness compositions of No. 1 - No. 8 in Table 3, when the
thickness from the protective layer surface to the first information layer is 101 ±4 µm,
and considering that the second intermediate layer is the thinnest and a thickness
variability of ±2 µm is present, the minimum intermediate layer thickness is 10 µm, and
the maximum intermediate layer thickness is 27 µm. Furthermore, the minimum
protective layer thickness is 42 µm, and the maximum protective layer thickness is 48
µm.
Favorable results were obtained in No. 1 to No. 8, where the thicknesses of the
intermediate layers and the thickness of the protective layer all have the desired
thickness variability within ±2 µm in that surface, reproducing signal amplitude
fluctuations due to back-focus issues do not occur in any of the regions within the
surface of the medium, and the target jitter of less than 8.5% was met in all information
layers.
However, with No. 9, back-focus issues with three reflections, from the fourth
information layer, the protective layer surface, and the fourth information layer, occurred
when recording to/reproducing from the second information layer. As a result, the jitter
value degraded considerably, exceeding 8.5%. The reason for this is that in the above
pattern (9), the value of |(t2 +t3) - tc| is less than 1 µm, and as a result, the optical path
length difference between the information light and the stray light is sometimes less
than 2 µm.
(Third Embodiment)
In the present third embodiment, the thickness composition and
recording/reproducing signal properties were evaluated for a four-layer optical
information recording medium in the case where the thickness from the protective layer
surface to the first information layer is 102 ±4 µm. The necessary conditions are the
same as described in the first embodiment; namely, a thickness variability in the
intermediate layers and protective layers of ±2 µm, a protective layer thickness of 40 µm
or more, a minimum intermediate layer thickness of 10 µm, an interlayer thickness
difference of 1.0 µm or more, and an optical path length difference between the
information light and the stray light of ±2 µm or more.
Aside from the thickness from the protective layer surface to the first information
layer being 102 µm, the composition is exactly the same as in the first embodiment.
The reason the thickness from the protective layer surface to the first information
layer was increased to 102 µm in the present third embodiment is that doing so makes it
more possible to design the thicknesses of the intermediate layer to be wider. As
illustrated in Fig. 8, the jitter value degrades dramatically due to the influence of
interlayer crosstalk when the thickness of the intermediate layers is reduced. Designing
the thickness from the protective layer surface to the first information layer to be wider
makes it possible to increase the minimum intermediate layer thickness.
The thickness compositions and recording/reproducing properties of the four-
layer optical information recording media of the present third embodiment are shown in
Table 4.

In Table 4, the patterns in No. 1 - No. 8 have thickness compositions in which
back-focus issues of up to three reflections do not occur, whereas No. 9 has a thickness
composition in which back-focus issues of three reflections occur.
Looking at the thickness compositions of No. 1 - No. 8 in Table 3, when the
thickness from the protective layer surface to the first information layer is 102 ±4 µm,
and considering that the second intermediate layer is the thinnest and a thickness
variability of ±2 µm is present, the minimum intermediate layer thickness is 10 µm, and
the maximum intermediate layer thickness is 27 pm. Furthermore, the minimum
protective layer thickness is 43 µm, and the maximum protective layer thickness is 48
µm.
Favorable results were obtained in No. 1 to No. 8, where the thicknesses of the
intermediate layers and the thickness of the protective layer all have the desired
thickness variability within ±2 µm in that surface, reproducing signal amplitude
fluctuations due to back-focus issues do not occur in any of the regions within the
surface of the medium, and the target jitter of less than 8.5% was met in all information
layers. The conditions of No. 5 and No. 6 in particular allow the second intermediate
layer to be designed with a wider thickness of 14 ±2 µm, which enables the influence of
interlayer crosstalk on the jitter values of the second information layer and the third
information layer to be reduced; these are therefore extremely desirable compositions.
However, with No. 9, back-focus issues with three reflections, from the third
information layer, the protective layer surface, and the fourth information layer, occurred
when recording to/reproducing from the first information layer. As a result, the jitter
value degraded considerably, exceeding 8.5%. The reason for this is that in the above
pattern (14), the value of |(t1 + t2) - tc| is less than 1 µm, and as a result, the optical
path length difference between the information light and the stray light is sometimes less
than 2 µm.
(Fourth Embodiment)
In the present fourth embodiment, a three-layer optical information recording
medium, such as that illustrated in Fig. 18, shall be described. The structure is such
that a first information layer 1802 containing a phase change recording material, a first
intermediate layer 1805 (thickness t1) composed of ultraviolet light-curable resin, a
second information layer 1803, a second intermediate layer 1806 (thickness t2), a third
information layer 1804, and a protective layer 1809 (thickness tc) are layered in that
order upon a resin substrate 1801. The external surface of the protective layer 1809 is
referred to as a protective layer surface 1809a.
Thickness compositions and recording/reproducing signal properties were

evaluated for this optical information recording medium. The necessary conditions are
the same as described in the first embodiment; namely, a thickness variability in the
intermediate layers and protective layers of ±2 µm, a protective layer thickness of 40 µm
or more, a minimum intermediate layer thickness of 10 µm, an interlayer thickness
difference of 1.0 µm or more, and an optical path length difference between the
information light and the stray light of ±2 µm or more.
Next, the conditions required to set the optical path length difference between the
information light and the stray light to ±2 µm or more shall be described.
When focusing recording/reproducing light on an information layer even further
than the third information layer (on the side opposite to the light entry side) in an optical
information recording medium that has three information layers, stray light problems can
occur in the following two patterns. Note that in the following descriptions, the
information layer to/from which recording/reproducing is performed shall be called the
"recording/reproducing information layer".
The first pattern of stray light problem assumes stray light reflected three times,
from an information layer B on the light entry side of the recording/reproducing
information layer A ? an information layer C further on the light entry side, or the
protective layer surface ? information layer B, in which case the round-trip optical path
length difference between the information light that returns to the optical head from the
recording/reproducing information layer A and the aforementioned stray light is less than
2 µm; therefore, interference can occur between the information light and the stray light.
This first pattern of stray light problem is solved by setting the round-trip optical
path length difference between the thickness between the recording/reproducing
information layer A and the information layer B and the thickness between the
information layer B and the information layer C/protective layer surface to a value that
exceeds 1 µm. Note that the "thickness" mentioned here refers to the thickness
measured by the aforementioned thickness gauge.

To be more specific, when the information layer to be recorded to/reproduced
from is the first information layer 1802, it is necessary for the following three patterns to
hold true in order to prevent interference between the information light and the stray
light.
1) |t1-t2|>1µm
(This makes it possible to prevent interference with stray light reflected by the
second information layer 1803 ? the third information layer 1804 ? the second
information layer 1803, in that order.)
2) |t1 - (t2 + tc) | > 1 µm
(This makes it possible to prevent interference with stray light reflected by the
second information layer 1803 ? the protective layer surface 1809a ? the second
information layer 1803, in that order.)
3) |(t1 +t2)-tc|>1µm
(This makes it possible to prevent interference with stray light reflected by the
third information layer 1804 ? the protective layer surface 1809a ? the third
information layer 1804, in that order.)
When the information layer to be recorded to/reproduced from is the second
information layer 1803, it is necessary for the following pattern to hold true.
4) |t2-tc|>1µm
(This makes it possible to prevent interference with stray light reflected by the
third information layer 1804 ? the protective layer surface 1809a ? the third
information layer 1804, in that order.)
The second pattern of stray light problem assumes stray light reflected three
times, from an information layer b on the light entry side of the recording/reproducing
information layer a ? an information layer c further on the light entry side, or the
protective layer surface ? an information layer d closer to the light entry side than the
information layer b and on the side opposite to the light entry side of the information
layer c or the protective layer surface, and is solved by reducing the round-trip optical
path length difference between the information light returning to the optical head from
the recording/reproducing information layer a and the aforementioned stray light to less

than 2 µm. Note that stray light reflected three times, from the information, layer d ? the
information layer c or the protective layer surface ? information layer b, also occurs in
this second pattern, and thus interference is caused by the two light beams.
This second pattern of stray light problem is solved by setting the difference
between the thickness between the information layer a and the information b and the
thickness between the information layer c and the information layer d/protective layer
surface to a value that exceeds 1 µm.
To be more specific, when the information layer to be recorded to/reproduced
from is the first information layer 1802, it is necessary for the following pattern to hold
true in order to prevent interference between the information light and the stray light.
5) |t1-tc|>1µm
(This makes it possible to prevent interference with stray light reflected by the
second information layer 1803 ? the protective layer surface 1809a ? the third
information layer 1804, in that order, and at the same time prevent interference with
stray light reflected by the third information layer 1804 ? the protective layer surface
1809a ? the second information layer 1803, in that order.)
The thickness compositions and recording/reproducing properties of the three-
layer optical information recording media of the present fourth embodiment are shown in
Table 5.
Because the surface thickness distribution of each intermediate layer is ±2 µm,
the thicknesses fluctuates ±2 µm central to the thickness design values shown in the
table. For this reason, the reproducing signal fluctuates significantly in regions in which
the thickness within the surface does not meet the aforementioned necessary
conditions.
In Table 5, the patterns in No. 1 - No. 3 have thickness compositions in which
back-focus issues of up to three reflections do not occur, whereas No. 4 and No. 5 have
a thickness compositions in which back-focus issues of three reflections occur.
Looking at the thickness compositions of No. 1 - No. 3 in Table 5, considering
that a thickness variability of ±2 µm is present, the minimum intermediate layer
thickness is 12 µm, and the maximum intermediate layer thickness is 37 µm.
Furthermore, the minimum protective layer thickness is 43 µm, and the maximum
protective layer thickness is 47 µm.
Favorable results were obtained in No. 1 to No. 3, where the thicknesses of the
intermediate layers and the thickness of the protective layer all have the desired
thickness variability within ±2 µm in that surface, reproducing signal amplitude

fluctuations due to back-focus issues do not occur in any of the regions within the
surface of the medium, and the target jitter of less than 8.5% was met in all information
layers.
However, with No. 4, back-focus issues with three reflections, from the second
information layer, the third information layer, and the second information layer, occurred
when recording to/reproducing from the first information layer. As a result, the jitter
value degraded considerably, exceeding 8.5%. The reason for this is that in the above
pattern 4), the value of |t1 -t2| is less than 1 µm, and as a result, the optical path length
difference between the information light and the stray light is sometimes less than 2 µm.
Furthermore, with No. 5, back-focus issues with three reflections, from the
second information layer, the protective layer surface, and the third information layer,
occurred when recording to/reproducing from the first information layer. As a result, the
jitter value degraded considerably, exceeding 8.5%. The reason for this is that in the
above pattern 5), the value of |t1 - tc| is less than 1 µm, and as a result, the optical path
length difference between the information light and the stray light is sometimes less
than 2 µm.
Although these five thickness compositions were evaluated in the present fourth
embodiment, the thickness compositions are not limited thereto, and as long as the
conditions described in the first embodiment, namely, a thickness variability of ±2 µm in
the intermediate layers and protective layer, a protective layer thickness of 40 µm or
more, a minimum intermediate layer thickness of 10 µm or more, an interlayer thickness
difference of 1.0 µm or more, and an optical path length difference between the
information light and the stray light of ±2 µm or more, are met, favorable
recording/reproducing properties can be obtained without the occurrence of back-focus
issues.
INDUSTRIAL APPLICABILITY
The optical information recording medium of the present invention, implemented

as a four-layer optical information recording medium composed of four information
layers, is capable of reducing the influence of interlayer crosstalk while maintaining
compatibility with conventional single- and dual-layer optical information recording
media, and can eliminate back-focus issues caused by interference between the
information light and reflected stray light, in which some of the stray light reflected by
other information layers when light is focused onto one of the information layers is
reflected up to three times by other information layers or the protective layer surface
and returns to the optical head, while affording a process margin sufficient for
manufacturing intermediate layers, a protective layer, and so on.
The present invention can be used in high-capacity multilayer optical information
recording media capable of recording/reproducing a high-quality signal.

WE CLAIM:
1. An optical information recording medium having at least three information layers,
at least two intermediate layers separating the information layers, and a protective layer
layered upon a substrate, the optical information recording medium being recorded
and/or reproduced from the side of the protective layer using an optical head,
wherein the round-trip optical path length difference between information light
returning to the optical head from one of the information layers upon which
recording/reproducing light is focused and reflected stray light that is a part of stray light
reflected by one of the information layers that returns to the optical head having been
reflected by the information layer or the surface of the protective layer no more than
three times is no less than 2 µm.
2. The optical information recording medium according to claim 1, wherein the sum
of the thicknesses of the intermediate layers differs from the thickness of the protective
layer.
3. The optical information recording medium according to claim 1 or 2, wherein the
thicknesses of each of the intermediate layers and the protective layer differ from one
another, and the difference between each thickness is no less than 1 µm.
4. The optical information recording medium according to one of claims 1 to 3,
wherein the thickness variability of each of the intermediate layers is within ±2 µm.
5. The optical information recording medium according to one of claims 1 to 4,
comprising:
a first information layer provided upon the substrate;
a first intermediate layer provided upon the first information layer;
a second information layer provided upon the first intermediate layer;

a second intermediate layer provided upon the second information layer;
a third information layer provided upon the second intermediate layer;
a third intermediate layer provided upon the third information layer;
a fourth information layer provided upon the third intermediate layer; and
the protective layer provided upon the fourth information layer,
wherein the second intermediate layer is the thinnest of the first through third
intermediate layers.
6. The optical information recording medium according to one of claims 1 to 4,
comprising:
a first information layer provided upon the substrate;
a first intermediate layer provided upon the first information layer;
a second information layer provided upon the first intermediate layer;
a second intermediate layer provided upon the second information layer;
a third information layer provided upon the second intermediate layer; and
the protective layer provided upon the third information layer,
wherein the second intermediate layer is thinner than the first intermediate layer.
7. The optical information recording medium according to claim 36, wherein the
thickness of each intermediate layer is no less than 16 µm and no more than 37µm.
8. The optical information recording medium according to claim 36 or 37, wherein
the thickness of the protective layer is no less than 43 µm and no more than 59 µm.
9. The optical information recording medium according to one of claims 36 to 38,
wherein the thickness of the first intermediate layer is no less than 23 µm and no more
than 27 µm, the thickness of the second intermediate layer is no less than 16 µm and
no more than 20 µm, and the thickness of the protective layer is no less than 55 µm and
no more than 59 µm.
10. The optical information recording medium according to one of claims 36 to 38,

wherein the thickness of the first intermediate layer is no less than 23 µm and no more
than 27 µm, the thickness of the second intermediate layer is no less than 18 µm and
no more than 22 µm, and the thickness of the protective layer is no less than 53 µm and
no more than 57 µm.
11. The optical information recording medium according to one of claims 36 to 38,
wherein the thickness of the first intermediate layer is no less than 33 µm and no more
than 37 µm, the thickness of the second intermediate layer is no less than 18 µm and
no more than 22 µm, and the thickness of the protective layer is no less than 43 µm and
no more than 47 µm.
12. The optical information recording medium according to claim 36, wherein the
difference between the thickness of the first intermediate layer and the thickness of the
second intermediate layer is more than 1 µm.
13. The optical information recording medium according to claim 36, wherein the
difference between the thickness of the first intermediate layer and the total thickness of
the second intermediate layer and protective layer is more than 1 µm.
14. The optical information recording medium according to claim 36, wherein the
difference between the total thickness of the first and second intermediate layers and
the total thickness of the protective layer is more than 1 µm.
15. The optical information recording medium according to claim 36, wherein the
difference between the thickness of the second intermediate layer and the thickness of
the protective layer is more than 1 µm.
16. The optical information recording medium according to claim 36, wherein the
difference between the first intermediate layer and the thickness of the protective layer
is more than 1 µm.
17. The optical information recording medium according to claim 1, wherein the
thickness between an information layer A and an information layer B on the light entry
side of the information layer A and the thickness between the information layer B and an
information layer C on the light entry side of the information layer B or the surface of the
protective layer are different by more than 1 µm.
18. The optical information recording medium according to claim 1, wherein the
thickness between an information layer a and an information layer b on the light entry
side of the information layer a and the thickness between an information layer c on the
light entry side of the information layer b and an information layer d on the light entry
side of the information layer c or the surface of the protective layer are different by more
than 1 µm.
19. The optical information recording medium according to one of claims 1 to 5 and
36 to 48, wherein recording and/or reproducing is performed using a optical head
including at least a laser light source having a wavelength of no less than 400 nm and
no more than 410 nm, an objective lens having an NA of 0.85, and a spherical
aberration correction element.

The recording/reproducing quality of a multilayer optical information recording medium
deteriorate not only due to interference from other layers caused by light converging on
other information layers but also due to stray light converging on the surface of a
protective layer and stray light that does not converge on other information layers but
rather returns to an optical head through the same optical path as the reproducing
signal. The thickness composition of intermediate layers (106, 107, and 108) and the
protective layer (109) in a four-layer optical information recording medium are set so as
to eliminate the influence of interference caused by stray light from other layers
reflected up to three times.