Optical data recording medium

An optical data recording medium of the present invention includes: a substrate; a reflective material layer formed of light-reflective material, the reflective material layer being provided over the substrate; and an optical data recording layer for optically recording data therein and for optically reproducing the data therefrom, the optical data recording layer being formed over the reflective material layer. The reflective material layer is patterned so that the light-reflective material may be partly removed therefrom for selectively allowing a light beam irradiated on the substrate to pass therethrough to reach the optical data recording layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical data recording medium for 
optically recording data therein and for optically reproducing the data 
recorded therein. More specifically, the present invention relates to an 
optical data recording medium capable of recording information therein 
with high recording density and capable of reproducing the information 
recorded therein with high reproducibility. 
2. Description of Related Art 
One type of a conventional optical data recording medium is shown in FIG. 
1. The optical data recording medium 80 includes a transparent substrate 
82 in which a tracking guide groove 81 is formed in concentrical or spiral 
fashion, a first protective layer 83 formed over the substrate 82, an 
optical data recording layer 84 formed over the first protective layer, 
and a second protective layer 85 formed over the data recording layer 84. 
The substrate 82 is made of acrylic resin, polycarbonate resin, glass, or 
the like. The first protective layer 83 is formed of oxide material such 
as SiO.sub.2, nitride material such as AlN, or the like. The data 
recording layer 84 is made of magnetooptic material such as TbFeCo, GdTbFe 
and TbCo, phase-change material such as GeSbTe, pit formable material such 
as metal and dye, or the like. The second protective layer 84 is formed of 
oxide material such as SiO.sub.2, nitride material such as AlN, resin, or 
the like. 
In the data recording medium 80 having the above-described structure, as 
illustrated in FIG. 2, the tracking guide groove 81 is formed so that a 
groove width W.sub.g, a groove-to-groove distance W.sub.L, and a track 
pitch P may satisfy the following equation P=W.sub.G +W.sub.L and so that 
the track pitch P may be set to be approximately equal to a spot diameter 
dS of a laser beam spot to be irradiated on the data recording medium 80. 
In addition, as shown in FIGS. 4 and 5, preformat pits 87 are formed on 
land portions 86 of the substrate 82. 
The substrate 82 thus formed with the tracking guide groove 81 and the 
preformat pits 87 is produced through the following manner. 
First, a photoresist with its thickness having a value equal to or lower 
than 0.1 .mu.m, for example, is formed over a glass plate, through a spin 
coating process. The photoresist is irradiated with laser beam in such a 
manner that a laser beam spot formed on the photoresist is moved relative 
to the photoresist concentrically or spirally with a fixed pitch. The 
laser beam irradiation is performed continuously and intermittently. The 
continuously irradiated laser beam will form a groove extending 
concentrically or spirally and the intermittently irradiated laser beam 
will form pits which are arranged concentrically or spirally. Then, the 
photoresist is subjected to a development process, so that a mask member 
consisting of the glass substrate and the photoresist formed with the 
groove and the pits is obtained. 
Then, an electro-conductive film is provided over the mask member before 
being subjected to an electro-forming process. Thus, a nickel stamper is 
produced. The nickel stamper is then subjected to resin molding operation 
such as a photopolymerization process or an injection molding process. 
Accordingly, resin is molded into the substrate 82 onto which are 
transferred the groove and the pits. Thus, the substrate 82 formed with 
the tracking guide groove 81 and the preformat pits 87 is obtained. 
In order to record data into the data recording medium 80, on the other 
hand, as shown in FIG. 1, laser beam is irradiated on the data recording 
layer 84 through the substrate 82 to locally heat the data recording 
layer. As a result, the heated portion of the data recording layer 
undergoes inversion of magnetization, phase change, or formation of pit. 
Thus, a data recording bit is formed in the data recording layer. In order 
to reproduce the data thus recorded in the data recording layer 84, a 
laser beam is also irradiated on the data recording layer 84, and the 
laser beam reflected from the data recording layer is detected. More 
specifically, change in rotation angle of a polarization plane of the 
reflected laser beam or change in intensity of the reflected laser beam is 
detected. 
In order to increase a data recording density of the above-described data 
recording medium 80, it is necessary to narrow the track pitch P. In the 
case where the track pitch P becomes smaller than the spot diameter dS of 
the laser beam spot 91, however, the following problem will occur. As 
shown in FIG. 3, in the case where the laser beam spot 91 traces a 
particular track 92 in order to reproduce data recorded in the track, the 
laser beam spot 91 fails to be irradiated only on the track 92 but is 
irradiated not only on the track 92 but also a neighboring track 93. The 
laser beam spot is therefore erroneously irradiated onto a part of a data 
recording bit 94 which is formed in the neighboring track 93. Accordingly, 
the laser beam spot tracing the particular track 92 picks up not only 
desired information recorded on the particular track 92 but also unwanted 
information on the neighboring track 93. Thus, cross-talk is occurred. 
In addition, the conventional data recording medium 80 has a problem that 
the tracking guide groove 81 and the preformat pits 87 formed on the 
substrate 82 through the injection molding process are liable to have 
undesired shapes. In other words, a yield rate of the data recording 
medium 80 has a small value. This tendency would be particularly 
acknowledged if the groove width W.sub.G is decreased to enhance the data 
recording density of the data recording medium. 
FIG. 6 shows one example of the above-described conventional optical data 
recording medium 80 which applies magnetooptic material as the data 
recording layer 84. The conventional magnetooptic data recording medium 
170 includes a transparent substrate 172 made of polycarbonate resin or 
the like which is formed with a tracking guide groove 171, an interference 
layer 174 made of SiAlON, AlN or the like which is formed over the 
substrate 172, a magnetooptic data recording layer 176 made of GdTbFe, 
TbFeCo or the like which is formed over the interference layer 174, a 
protective layer 178 made of SiAlON, AlN or the like which is formed over 
the data recording layer 176, and a reflective layer 180 made of Al or the 
like which is formed over the protective layer 178. In order to record 
data in the data recording medium 170, laser beam is irradiated on the 
data recording layer 176 to locally heat it to a temperature equal to or 
higher than its Curie temperature or its Compensation temperature. 
Simultaneously, magnetic field is applied to the heated portion of the 
data recording layer 176, to thereby invert a direction of magnetization 
exhibited in the heated portion. 
In order to reproduce the data thus recorded in the data recording layer 
176, a linearly polarized laser beam is irradiated on the data recording 
layer 176 through the substrate 172. A polarization plane of the linearly 
polarized laser beam is rotated by a Kerr rotation angle at the time when 
the laser beam is reflected at the data recording layer 176. A direction 
in which the polarization plane is rotated at the Kerr rotation angle 
depends on the direction of the magnetization occurred in the data 
recording layer 176. Accordingly, in order to reproduce the data recorded 
in the data recording layer, the rotating direction of the polarization 
plane of the laser beam is detected. 
In the data reproducing operation, the laser beam irradiated on the data 
recording medium from its substrate side is partly reflected at an 
interface defined between the substrate 172 and the interference layer 
174, at an interface defined between the interference layer 174 and the 
data recording layer 176, at an interface defined between the data 
recording layer 176 and the protective layer 178, and at an interface 
between the protective layer 178 and the reflective layer 180. In other 
words, the laser beam undergoes multiple reflection both in the 
interference layer 174 and the protective layer 178. As a result, a 
plurality of reflective laser beams are generated at the respective 
interfaces and travel back to the substrate 172. The plural reflective 
laser beams meet one another and interfere with one another, to thereby 
constitute a single reflective laser beam to be outputted from the data 
recording medium 170. In the conventional data recording medium 170, 
thickness of the interference layer 174 is selected to approximately 
.lambda./(4n.sub.2) where n.sub.2 represents an index of refraction of the 
interference layer and .lambda. represents a wavelength of the laser beam 
in vacuum space so that the reflectivity of the interference layer 174 may 
be minimized. In addition, the thickness of the data recording layer 176 
is selected to such a value as minimizes the reflectivity of the data 
recording layer. Since the data recording medium 170 has such a structure, 
the interference occurred among the reflective laser beams generated at 
the respective interfaces undergoes destructive interference, to thereby 
largely increase an apparent Kerr rotation angle of the laser beam 
outputted from the data recording medium. (This phenomenon is called as a 
"Kerr effect enhancement", hereinafter.) As a result, carrier-to-noise 
ratio (C/N ratio) of the data recording medium is largely enhanced. 
The conventional data recording medium 170, however, has the following 
disadvantage. The thickness of the interference layer and the data 
recording layer are selected to such values as minimize the reflectivities 
of the interference layer and the data recording layer, as described 
above. Thus, the interference layer and the data recording layer have 
reflectivities of low values, at their entire areas. Thus, the 
conventional data recording medium 170 has a low value of reflectivity, 
even at its tracking guide groove 171. It is noted, however, that 
intensity of reflective laser beam reflected from the tracking guide 
groove 171 is detected and a tracking error signal is produced for 
tracking servo operation. Thus, the conventional data recording medium 170 
having the low value of reflectivity at its tracking guide groove may not 
obtain a tracking error signal having an amount proper to attain a stable 
tracking servo operation. The data recording medium therefore may not 
attain a stable recording and reproducing operations. 
SUMMARY OF THE INVENTION 
The present invention is achieved to solve the above-described problems of 
the conventional optical data recording medium. 
An object of the present invention is therefore to provide an optical data 
recording medium which has a small track pitch to attain a high data 
recording density while preventing cross-talk from being increased. 
Another object of the present invention is to provide an optical data 
recording medium in which tracking guide grooves and preformat pits having 
undesired shapes are not liable to be produced. 
A further object of the present invention is to provide a magnetooptic data 
recording medium which is capable of largely enhancing its Kerr effect to 
thereby attain an increased C/N ratio without reducing intensity of a 
tracking error signal required for the tracking servo operation to thereby 
attain a stable tracking servo operation. 
In order to attain the above objects, an optical data recording medium of 
the present invention for optically recording data therein and for 
optically reproducing the data therefrom includes: a substrate; a 
reflective material layer formed of light-reflective material, the 
reflective material layer being provided over said substrate; and an 
optical data recording layer for optically recording data therein and for 
optically reproducing the data thus recorded therein, the optical data 
recording layer being formed over the reflective material layer. The 
reflective material layer is patterned so that the light-reflective 
material may be partly removed from the reflective material layer for 
selectively allowing light beam irradiated on the substrate to pass 
therethrough to reach the optical data recording layer. 
The optical data recording medium of the present invention may further 
include an interference layer provided between the reflective material 
layer and the optical data recording layer, the interference layer being 
formed of dielectric material having an index of refraction which has a 
value larger than that of the substrate. 
Or otherwise, the optical data recording medium may further include a first 
interference layer formed of dielectric material and a second interference 
layer formed of dielectric material, the first interference layer being 
provided between the substrate and the reflective material layer and the 
second interference layer being provided between the reflective material 
layer and the optical data recording layer. 
With such a structure, the reflective material layer, the optical data 
recording layer and the interference layer (the first and second 
interference layers) may be produced through a well-known film forming 
process and photolithography process. Accordingly, it becomes unnecessary 
to produce the optical data recording medium through injection molding 
process. Thus, it becomes easy to produce the optical data recording 
medium in the case where a track pitch of the data recording medium is 
narrowed to enhance a data recording density thereof. 
In the optical data recording medium of the invention, the reflective 
material layer is patterned to partly remove the light-reflective material 
from the reflective material layer so that areas from which the 
light-reflective material is thus removed are continuously arranged to 
define a data recording and reproducing area which is adapted for allowing 
a light beam irradiated on the substrate to pass therethrough to reach the 
optical data recording layer to thereby record data in the optical data 
recording layer and reproduce the data recorded in the optical data 
recording layer. The data recording and reproducing area is formed to 
continuously extend, defining a track. More specifically, the reflective 
material layer is patterned so as to form a plurality of data recording 
tracks in such a manner that the light-reflective material remains on the 
reflective material layer at a tracking guide area defined between each 
two neighboring (adjacent) tracks. 
Since the reflective material is thus remained on the reflective material 
layer at both sides of each data recording and reproducing area, even in 
the case where the light beam having the spot diameter larger than the 
track pitch is irradiated on the data recording medium at the data 
recording and reproducing area of a particular track, the light beam 
cannot reach the optical data recording layer at the data recording and 
reproducing area of a neighboring track positioned adjacent to the 
particular track. Accordingly, in the case where a laser beam is 
irradiated on the data recording medium for reproducing data recorded in a 
data recording and reproducing area of a particular track, the laser beam 
cannot reproduce data recorded in another data recording and reproducing 
area of a neighboring track, positioned adjacent to the particular track. 
Accordingly, cross-talk is not increased. 
More specifically, the data recording and reproducing area and the tracking 
guide area have their widths extending perpendicularly to the track 
extending direction. A total value of the width of the data recording and 
reproducing area and the widths of the tracking guide areas positioned on 
each side to sandwich the data recording and reproducing area is equal to 
or greater than a spot diameter of a light beam irradiated on the optical 
data recording layer. Accordingly, in the case where the light beam is 
irradiated on the optical data recording medium at a particular data 
recording and reproducing area, no part of the light beam is erroneously 
irradiated on the data recording layer at a neighboring data recording and 
reproducing area positioned adjacent to the particular data recording and 
reproducing area. Thus, in the case where the track pitch is decreased to 
enhance the data recording density of the data recording medium, 
cross-talk is not increased. 
In the data recording medium of the present invention, the reflective 
material layer may be further patterned to partly remove the 
light-reflective material from the reflective material layer in accordance 
with a preformat information so that areas from which the light-reflective 
material is thus removed are discontinuously arranged in accordance with 
the preformat information along the track so as to define a preformat area 
which is adapted for receiving a light beam irradiated on the substrate 
and for optically modulating the light beam in accordance with the 
preformat information. 
Since the preformat area is thus formed through simply removing the 
light-reflective material from the reflective material layer, the 
preformat area is not liable to be produced to have an undesired shape, 
and therefore yield rate of the data recording medium is greatly enhanced. 
The data recording and reproducing area and the preformat area have their 
widths extending perpendicularly to the data track extending direction, 
and the width of the data recording and reproducing area preferably has a 
value equal to or larger than that of the preformat area. Accordingly, the 
C/N characteristics of the data recording medium is further greatly 
enhanced. 
In the case where the substrate is of a disk shape, the width of the data 
recording and reproducing area and a track pitch defined as a distance 
between each two adjacent data tracks may preferably decrease away from 
the center toward the outer periphery of the disk-shaped substrate, so 
that the data recording density of the data recording medium may be 
further enhanced. 
The reflective material layer may be further patterned to partly remove the 
light-reflective material from the reflective material layer so that the 
light-reflective material provided at the area defined between each two 
adjacent tracks may be completely removed from said substrate to define a 
mirror part. The mirror part will provide an offset signal representative 
of a shift amount of an optical axis of the laser beam irradiated on the 
data recording medium and an inclination amount of the data recording 
medium. The offset signal will be utilized for neglecting a DC offset 
component of a tracking error signal. 
Particularly in the case where the data recording medium of the present 
invention is provided with the interference layer, since the interference 
layer is formed of the dielectric material with its index of refraction 
being greater than that of the substrate, multiple reflection of light 
beam occurs in the interference layer. Accordingly, a reflectivity of the 
data recording layer is lowered and the Kerr effect enhancement is 
attained, so that an apparent Kerr rotation angle is greatly increased and 
the C/N ratio of the data recording medium is enhanced. Furthermore, since 
the reflectivity of the tracking guide area where the light-reflective 
material remains is not lowered, it is possible to attain a stable 
tracking characteristic. 
Furthermore, particularly in the case where the data recording medium of 
the present invention is provided with the first and second interference 
layers, since the first and second interference layers are formed of the 
dielectric material with its index of refraction being greater than that 
of the substrate, multiple reflection of the light beam occurs in the 
first and second interference layers. 
Thickness of the first interference layer and thickness of the second 
interference layer preferably have values allowing a phase change amount 
of approximately .pi.+m(2 .pi.), where m is an integer, occursin a light 
beam as the light beam travels through the first and second interference 
layers from a first interface defined between the substrate and the first 
interference layer toward a second interface defined between the second 
interference layer and the optical data recording layer, returns at the 
second interface to travel again through the second and first interference 
layers in this order and reaches the first interface. Accordingly, the 
Kerr effect enhancement is increased due to the multiple reflection 
occurring in the first and second interference layers, so that the C/N 
ratio of the data recording medium is greatly enhanced. 
The thickness of the first and second interference layers may preferably 
have values allowing an amount of phase difference between a reflection 
beam reflected from the optical data recording medium at an area from 
which the light-reflective material is removed and a reflection beam 
reflected from the optical data recording medium at an area where the 
light-reflective material remains to have approximately a value of 
.pi./2+m.pi. where m is an integer. With such a structure, a push-pull 
type tracking error signal is obtained to have its maximum value. 
Accordingly, the reflectivity of the data recording medium at the tracking 
guide area where the light-reflective material remains is not decreased 
relative to that at the data recording and reproducing area from which the 
light-reflective material is removed. Accordingly, the value of the 
tracking error signal can be prevented from being lowered, and a stable 
tracking servo operation can be certainly achieved. 
More specifically, the optical data recording medium of the present 
invention should preferably be so designed that an amount of phase 
difference between a reflection beam reflected from the optical data 
recording medium at its area from which the light-reflective material is 
removed and a reflection beam reflected from the optical data recording 
medium at its area where the light-reflective material remains may have a 
value other than .pi.+m.pi. where m is an integer. For example, the 
optical data recording medium of the present invention should be so 
designed that a difference amount between a phase change amount occurring 
in a light beam as the light beam travels through the first and second 
interference layers from the first interface toward the second interface, 
reflects at the second interface to travel again through the second and 
first interference layers and returns to the first interface and a phase 
change amount occurring in a light beam as the light beam travels through 
the first interference layer from the first interface toward a third 
interface defined between the first interference layer and the 
light-reflective material remaining on the first interference layer, 
reflects at the third interface to travel again through the first 
interference layer and returns to the first interface may have 
approximately a value other than .pi.+m.pi. where m is an integer. For 
example, the thickness of the first and second interference layers may 
preferably have values allowing a phase difference amount to have the 
value other than .pi.+m.pi.. 
Furthermore, the optical data recording medium of the present invention 
should preferably be designed so that an amount of phase difference 
between a reflection beam reflected from the optical data recording medium 
at its area from which the light-reflective material is removed and a 
reflection beam reflected from the optical data recording medium at its 
area where the light-reflective material remains may have approximately a 
value of .pi./2+m.pi. where m is an integer. For example, the optical data 
recording medium should preferably be designed so that a difference amount 
between the phase change amount occurring in a light beam as the light 
beam travels through the first and second interference layers from the 
first interface toward the second interface, reflects at the second 
interface to travel again through the second and first interference layers 
and returns to the first interface and a phase change amount occurring in 
a light beam as the light beam travels through the first interference 
layer from the first interface toward a third interface defined between 
the first interference layer and the light-reflective material remaining 
on the first interference layer, reflects at the third interface to travel 
again through the first interference layer and returns to the first 
interface may have approximately a value of .pi./2+m.pi. where m is an 
integer. For example, the thickness of the first and second interference 
layers may preferably have such values allowing the phase difference 
amount to have approximately the value of .pi./2+m.pi.. 
Particularly in the case where the reflective material layer is relatively 
thick, the thickness of the first and second interference layers may 
preferably have values allowing a difference amount between the phase 
change amount occurring in a light beam as the light beam travels through 
both the first and second interference layers from the first interface 
toward the second interface, reflects at the second interface to travel 
again through both the second and first interference layers in this order 
and returns to the first interface and a phase change amount occurring in 
a light beam as the light beam travels through only the first interference 
layer from the first interface toward a third interface defined between 
the first interference layer and the light-reflective material remaining 
on the first interference layer, reflects at the third interface to travel 
again through only the first interference layer and returns to the first 
interface to have approximately a value of .pi./2+m.pi. where m is an 
integer. 
Indices of refraction of the first and second interference layers are 
preferably selected to be equal to each other. In this case, undesired 
reflection of light beam does not occur at an interface between the first 
and second interference layers so that the C/N ratio and the tracking 
characteristics of the data recording medium are greatly enhanced. 
The thickness of the second interference layer may preferably have 
approximately a value represented by [.lambda./4n.sub.2 
.+-..lambda./12n.sub.2 ]+m .lambda./2n.sub.2 where n.sub.2 represents the 
index of refraction of the second interference layer, .lambda. represents 
a wavelength, in a vacuum space, of a light beam to be irradiated on the 
optical data recording medium and m represents an integer. In this case, 
the push-pull type tracking error signal is obtained having a value 
slightly smaller than its maximum value, but a preformat pit reproducing 
signal for the preformat area is obtained having a large value. 
Accordingly, the data recording medium of the present invention can be 
used also as a preformat disk. 
Other objects, features and advantages of the present invention will become 
apparent in the following specification and accompanying drawings.

PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
A first preferred embodiment of the present invention will be described 
with reference to FIGS. 7 through 18. 
FIG. 7(a) is a cross-sectional side view of a disk-shaped optical data 
recording medium 20 of the present embodiment taken along a line extending 
in a radius direction thereof. FIG. 7(b) is a sectional view of the data 
recording medium 20 taken along a line extending in a circumferential 
direction thereof. As shown in FIGS. 7(a) and 7(b), the data recording 
medium 20 includes a transparent substrate 1, a film-shaped 
light-reflective material (which will be referred to as a "reflective 
material layer") 2 formed over the substrate, an interference layer 3 
formed over the reflective material layer, an optical data recording layer 
4 formed over the interference layer, and a protective layer 5 formed over 
the data recording layer. 
FIG. 8 is a sectional view of the reflective material layer 2 taken along a 
line extending parallel to a surface of the substrate 1. As apparent from 
FIG. 8, the reflective material layer 2 is partly removed from the 
substrate 1 in concentrical or in spiral fashion. More specifically the 
reflective material layer 2 is partly removed discontinuously so that 
there are formed a plurality of areas 6 from which the light-reflective 
material is thus removed, arranged concentrically or spirally. The parts 6 
serve as preformat pits 6 for providing sector mark signals, address mark 
signals, synchronizing signals, etc. The reflective material layer 2 is 
also partly removed from the substrate 1 continuously so that there is 
formed an area 7 from which the light-reflective material is removed and 
extends concentrically or spirally. The area 7 serves as data recording 
and reproducing area 7 into which data is to be recorded and from which 
data is to be reproduced. More specifically to say, the reflective 
material layer 2 is partly removed from the substrate 1 so that a 
plurality of data recording and reproducing areas 7 are formed to be 
arranged concentrically or spirally. Reflective material 2 remaining on 
the substrate 1 at an area defined between each two neighboring areas 7 
serves as a tracking guide area 8. 
It should be noted that a width of the data recording and reproducing area 
7 is selected to be equal to or larger than that of the preformat pit 6. 
The reflective material layer 2 is further removed partially from the 
substrate 1 so that the reflective material does not remain at an area 
between each two neighboring tracks. An area 9 from which the reflective 
material is thus completely removed serves as a mirror part. 
The transparent substrate 1 is formed of glass, resin (such as acrylic 
resin, polycarbonate resin, amorphous polyolefin resin), etc. 
Representative examples of the reflective material 2 include metal such as 
aluminum, gold, titanium, tantalum, etc., nitride such as titanium 
nitride, tantalum nitride, etc., semiconductor such as carbon, silicon, 
etc., and organic material such as dye, etc. 
The interference layer 3 is formed of a transparent dielectric film 
material with its index of refraction having a value larger than that of 
the substrate 1. Representative examples of the interference layer include 
SiO, TiO.sub.2, ZrO.sub.2, SiAlON, ZnS, ZnO, SiN, AlN, etc. 
Representative examples of the optical data recording layer 4 include: 
magnetooptic data recording material such as amorphous alloy mainly 
composing rare earth and transitional metal such as TbFeCo, GdTbFe, etc.; 
phase-change material such as GeSbTe, TeOx, etc.; and pit formable 
material such as metal of Te, Bi, etc., dye, etc. 
The protective layer 5 serves to protect the data recording layer 4 from 
being chemically changed and is formed of SiO.sub.2, AlN, SiAlON, or the 
like. 
Method of producing the data recording medium 20 having the above-described 
structure will be described below with reference to FIGS. 9(a) through 
9(e). 
First, as shown in FIG. 9(a), the reflective material 2 is formed over the 
substrate 1. A photoresist 10 is provided over the reflective material 2. 
A laser beam is irradiated onto the photoresist 10 so that a laser beam 
spot formed on a surface of the photoresist may be moved concentrically or 
spirally. The laser beam irradiation is controlled so that the laser beam 
may be irradiated on the photoresist continuously and intermittently. (It 
is noted that in order to control the laser beam to be irradiated 
intermittently on the photoresist, a laser source is preferably modulated 
in accordance with sector mark signals, etc.) As a result, the photoresist 
10 is formed with latent pattern images of the preformat pits 6, the data 
recording and reproducing areas 7 and the mirror areas 9 shown in FIG. 8. 
Thus formed latent pattern images are then developed, so that the 
photoresist 10 formed with the preformat pits, the data recording and 
reproducing areas and the mirror parts is obtained, as shown in FIG. 9(b). 
Then, the reflective material layer 2 is subjected to chemical etching 
treatment with the use of acid and alkali solution or is subjected to 
plasma etching treatment. As a result, parts of the reflective material 2 
which are not covered with the photoresist 10 (and therefore which 
correspond to the data recording and reproducing areas, the preformat pits 
and the mirror areas) are removed from the substrate 1. Then, as shown in 
FIG. 9(c), the photoresist 10 is entirely removed from the substrate 1, 
with the use of organic solvent, or the like. Accordingly, there is 
obtained the substrate 1 covered with the reflective material layer 2 
which is formed with the preformat pits 6, the data recording and 
reproducing areas 7, the tracking guide areas 8 and the mirror areas 9. 
Then, as shown in FIG. 9(d), the interference layer 3 is formed over the 
reflective material layer 2 through a well-known sputtering process or a 
still well-known vacuum deposition process. As shown in the FIG. 9(e), the 
data recording layer 4 is then formed over the interference layer 3 
through the sputtering process, the vacuum deposition process, a spin 
coating process, or the like so that the data recording layer 4 may have a 
substantially uniform thickness. As also shown in FIG. 9(e), the 
protective layer 5 is formed over the thus formed data recording layer 4 
through the sputtering process, the vacuum deposition process, the spin 
coating method, or the like. 
According to the method of producing the data recording medium of the 
present embodiment, as described above, a tracking guide groove is not 
formed on the substrate through the injection molding operation, contrary 
to the conventional method of producing the data recording medium. 
According to the present embodiment, the data recording and reproducing 
area 7 can be formed through simply removing the reflective material from 
the reflective material layer 2. Thus, the data recording and reproducing 
area 7 can be easily produced, and therefore yield rate of the data 
recording medium can be considerably enhanced. It therefore becomes 
possible to easily produce the data recording medium having a small track 
pitch. 
It is noted that in the step of FIG. 9(b) for forming the latent pattern 
images on the photoresist 10, in order to form a latent image 11 for the 
mirror area 9, as shown in FIG. 10, power of the laser beam irradiated on 
the photoresist 10 for forming the latent image 11 should be selected to 
be larger than those of the laser beam irradiated for forming a latent 
image 12 for the preformat pit 6 and a latent image 13 for the data 
recording and reproducing area 7. This is because the latent images 11 
formed on neighboring tracks through the high power irradiation of the 
laser beam can be overlapped with one another so as to form a single 
mirror area 9. 
It is further noted that in the step of FIG. 9(b), a photomask (not shown 
in the drawing) having patterns of the preformat pits 6, the data 
recording and reproducing areas 7 and the mirror areas 9 may be overlaid 
on the photoresist 10, and uniform light may be irradiated on the 
photoresist 10 through the photomask. The photomask thus formed with the 
patterns of the preformat pits 6, the data recording and reproducing areas 
7 and the mirror areas 9 may be produced through processes as described 
below. Chromium is first deposited on a glass substrate. Then, a 
photoresist is coated on the chromium thus provided on the substrate. 
Then, laser beam is irradiated on the photoresist so that a laser beam 
spot formed on the photoresist may be moved concentrically or spirally. 
The irradiation of the laser beam is controlled so that the laser beam may 
be irradiated on the photoresist continuously and intermittently. As a 
result, the latent pattern images of the preformat pits 6, the data 
recording and reproducing areas 7 and the mirror parts 9 as shown in FIG. 
8 may be formed on the photoresist. The photoresist is then subjected to 
the development process and the etching treatment. As a result, a 
photomask consisting of the glass substrate and the chromium layer formed 
with the preformat pits 6, the data recording and reproducing areas 7 and 
the mirror areas 9 is obtained. 
The optical data recording medium 20 of the present embodiment produced as 
described above is subjected to data recording and data reproducing 
operation, as described below. 
In order to record desired information in the data recording medium 20, a 
laser beam is irradiated on the data recording layer 4 through the 
substrate 1, to thereby locally heat the data recording layer. As a 
result, inversion of magnetization, phase change or a pit formation is 
occurs locally in the data recording layer 4, so that the desired 
information is recorded therein. In order to reproduce the information 
thus recorded in the data recording layer, on the other hand, the laser 
beam is irradiated on the data recording layer 4, and the laser beam 
reflected from the data recording layer is detected. More specifically to 
say, change in an amount of a rotation angle of a polarization plane of 
the reflected laser beam or change in intensity of the reflected laser 
beam is detected, to thereby reproduce the information recorded in the 
data recording layer. In order to perform a tracking operation, a 
well-known push-pull type tracking operation can be achieved with the use 
of the laser beam reflected to be diffracted at the tracking guide area 8 
and the data recording and reproducing area 7. A well-known three beam 
type tracking operation can be also performed. 
In the data recording medium 20 of the present embodiment, as apparent from 
FIG. 11, a width W.sub.r of the data recording and reproducing area 7 is 
set to be equal to or larger than a width B .sub.s of the preformat pit 6. 
In other words, the widths W.sub.r and B.sub.s have such values as 
satisfying the following equation (1), 
EQU W.sub.r .gtoreq.B.sub.s (1) 
Thus, the width of the data recording and reproducing area 7 is 
sufficiently large, and therefore it becomes possible to maintain a high 
C/N ratio, even in the case where the track pitch is narrowed to perform a 
high density data recording operation. Accordingly, it is certainly 
possible to perform the high density data recording operation without 
deteriorating the C/N ratio. 
According to the data recording medium 20 of the present embodiment, 
furthermore, as illustrated in FIG. 12(a), in the case where a laser beam 
spot 14 is irradiated on the data recording medium for recording data in 
the data recording layer 4 at a particular data recording and reproducing 
area 7, the laser beam spot 14 traces the particular data recording and 
reproducing area. As a result, most part of the laser beam spot is 
irradiated on the particular data recording and reproducing area, but a 
small part of the laser beam spot is irradiated on a pair of tracking 
guide areas 8 which are positioned to sandwich therebetween the particular 
data recording and reproducing area. The reflective material remaining at 
the pair of tracking guide areas 8 can prevent the part of the laser beam 
spot thus irradiated thereon from reaching the data recording layer 4. 
Accordingly, a width of a data bit or a magnetic domain to be formed in 
the data recording layer 4 at the particular data recording and 
reproducing area 7 is limited to a width of the data recording and 
reproducing area. 
As apparent from FIG. 12(a), a total width W' of the width W.sub.r of the 
data recording and reproducing area 7 and the widths of the tracking guide 
areas 8 provided on both sides of the data recording and reproducing area 
7 to sandwich therebetween the area 7 satisfies the following equation (2) 
, 
EQU W'=P+(P-W.sub.r)=2P-W.sub.r (2) 
where P represents the track pitch. In such a construction, therefore, in 
the case where the total width W' is equal to or larger than the spot 
diameter d of the laser beam spot irradiated on the data recording layer 
4, no part of the laser beam spot can be irradiated on neighboring data 
recording and reproducing areas 7' which are positioned adjacent to the 
particular data recording and reproducing area 7, and therefore cross-talk 
does not be occur. More specifically to say, in the case where the track 
pitch P and the width W.sub.r satisfy the following equation (3), even if 
the track pitch P is narrowed to enhance its data recording density, 
cross-talk will not be increased. 
EQU d=2P-W.sub.r (3) 
Accordingly, the data recording medium 20 of the present embodiment is 
designed so as to satisfy the equation (3). Thus, it becomes possible to 
select the value of the track pitch P to be smaller than the laser beam 
spot diameter d while preventing the cross-talk from being increased. 
Accordingly, it is possible to narrow the track pitch of the data 
recording medium for enhancing the data recording density. 
In addition, as illustrated in FIG. 12(b), in the case where the laser beam 
spot 14 traces the particular data recording and reproducing area 7 for 
reproducing data recorded in the area 7, the pair of tracking guide areas 
8 provided to sandwich the area 7 therebetween can also prevent the part 
of the laser beam spot 14 irradiated thereon from reaching the data 
recording layer 4. No part of the laser beam spot 14 can therefore reach 
the data recording layer 4 at the neighboring data recording and 
reproducing areas 7' which are positioned next to the particular data 
recording and reproducing area 7. Accordingly, the laser beam spot 14 
cannot reproduce data bits 16 which are recorded in the neighboring data 
recording and reproducing areas 7'. In the data recording medium 20 of the 
present embodiment having the above-described structure, therefore, it is 
possible to narrow the track pitch to attain a high density data recording 
operation while preventing the cross-talk from being increased. 
Additionally, as illustrated in FIG. 12(c), during when the data bit 16 is 
recorded in the data recording layer 4 at the neighboring data recording 
and reproducing area 7', there is a possibility that the area of the data 
bit 16 may expand its area in the data recording layer 4 toward such an 
area as positioned under the tracking guide film area 8, as indicated by 
slanted lines in FIG. 12(c), due to heat diffusion occurred in the data 
recording layer. In such a case, when the laser beam spot 14 traces the 
particular data recording and reproducing area 7 for reproducing data 
recorded in the area 7, the reflective material 2 remained at the tracking 
guide area 8 prevents the part of the laser beam spot irradiated thereon 
from reaching the data recording layer 4. Accordingly, the data bit 16 
recorded in the data recording layer 4 under the tracking guide area 8 is 
never reproduced, and therefore the cross-talk is not increased. 
The mirror area 9 provides an offset signal representative of a shift 
amount of an optical axis of the laser beam irradiated on the data 
recording medium 20 and an inclination angle of the data recording disk 
20. With the use of the offset signal, it is possible to neglect a DC 
offset component of a tracking error signal. 
It is further noted that values of the widths of the data recording and 
reproducing area 7 and the preformat pit 6 should not be particularly 
limited. However, the width of the data recording and reproducing area 7 
should preferably have a value equal to or close to that of the diameter 
of the laser beam spot irradiated on the date recording medium in the data 
reproduction operation. In addition, the width of the preformat pit 6 
should preferably have a value equal to or close to that of a half of the 
spot diameter of the data reproducing laser beam. 
In addition, it may be preferable that values of the width of the data 
recording and reproducing area 7 and the track pitch should decrease away 
from the center of the disk-shaped data recording medium 20 so that an 
outer peripheral part of the data recording medium has the smallest width 
of the data recording and reproducing area and the smallest track pitch 
for the following reasons. When information is recorded in the data 
recording medium 20, the disk-shaped data recording medium is rotated with 
a fixed rotational speed. Accordingly, the length of the data bit recorded 
in the data recording medium increases away from the center of the data 
recording medium. Therefore, even though the width of the data recording 
and reproducing area 7 at the outer peripheral side of the data recording 
medium 20 has a small value, it is possible to prevent the data recording 
and reproducing characteristics such as the C/N ratio at the outer 
peripheral side from being deteriorated. Accordingly, it is possible to 
narrow the track pitch at the outer peripheral side of the data recording 
medium to thereby enhance the data recording density, while preventing the 
data recording and reproducing characteristics from being deteriorated. 
In the data recording medium of the present embodiment, thickness of the 
interference layer 3 is selected to such a value as can enhance the C/N 
ratio without deteriorating the tracking servo performance, as will be 
described below in great detail with reference to an example of a 
magnetooptic data recording medium to which applied is the present 
embodiment. 
FIG. 13 shows a magnetooptic data recording medium 21 to which applied is 
the present embodiment. The data recording medium 21 includes the 
substrate 1, the reflective material layer 2, the interference layer 3, 
the data recording layer 4 formed of magnetooptic data recording material, 
and the protective layer 5. Representative examples of the magnetooptic 
data recording material include rare earth and transitional metal alloy 
such as TbFeCo, GdTbFe, etc., a multi-layered film of PtCo and PdCo, 
magnetic oxide such as rare earth ferrum garnet, etc., and combination 
material formed of the above materials. 
As shown in FIG. 14, in the present example, the reflective material layer 
2 is partly removed from the substrate 1 so as to form the data recording 
and reproducing areas 7 and the preformat pits 6. The remaining part of 
the reflective material layer constitutes the tracking guide area 8. The 
mirror area 9 is not formed on the magnetooptic data recording medium of 
the present example. 
In order to record desired information in the data recording layer of the 
magnetooptic data recording medium 21, laser beam is irradiated on the 
data recording layer 4 to thereby locally heat it and increase temperature 
of the heated portion to its Curie temperature or its Compensation 
temperature. Simultaneously, direction of magnetic field applied to the 
data recording layer is controlled in response to the information desired 
to be recorded in the data recording layer. Accordingly, when the heated 
portion is cooled, the direction of the magnetization occurring in the 
heated portion is changed in accordance with the applied magnetic field 
direction. Thus, the desired information is recorded in the data recording 
layer. 
In order to reproduce the data thus recorded in the data recording layer 4, 
a linearly polarized laser beam is irradiated on the data recording layer 
through the substrate 1. When the laser beam is reflected at the data 
recording layer, a polarization plane of the laser beam is rotated by a 
Kerr rotation angle determined dependently on the direction of the 
magnetization which is locally presented in the data recording layer. The 
Kerr rotation angle is detected, to thereby reproduce the data recorded in 
the data recording layer. 
More specifically, the reflected laser beam is first separated, by a 
polarization beam splitter, into its s-polarized component beam and its 
p-polarized component beam. Intensities of the s- and p- polarized 
component beams are detected by a pair of photodetectors, and a 
differential amplifier connected to the photodetectors outputs a 
differential signal representing an amount of a difference between the 
intensities. The differential signal therefore corresponds to a value of 
the Kerr rotation angle, i.e., the data recorded in the data recording 
layer. 
In the data recording medium 21 of the present example shown in FIG. 13, 
since the interference layer 3 is formed of the transparent dielectric 
material which has an index of refraction of a high value, the laser beam 
irradiated on the data recording medium 21 for reproducing the data 
recorded therein is partly reflected at an interface F.sub.13 defined 
between the substrate 1 and the interference layer 3 and partly reflected 
at an interface F.sub.34 defined between the interference layer 3 and the 
data recording layer 4. In other words, multiple reflection has occurred 
in the interference layer 3. As indicated by an arrow A in FIG. 13, a 
phase change amount A occurs in the laser beam as the laser beam travels 
through the interference layer 3 from the interface F.sub.13 toward the 
interface F.sub.34, reflects at the interface F.sub.34 and returns to the 
interface F.sub.13 satisfies the following equation (4), 
EQU A=2 (2.pi./.lambda.)n.sub.1 d.sub.1 (4) 
where d.sub.1 represents a thickness of the interference layer 3, n.sub.1 
represents the index of refraction, and .lambda. represents wavelength of 
the laser beam in vacuum space. The phase change amount A therefore 
represents an amount of phase difference between a reflection beam I.sub.0 
' which is reflected from the interface F.sub.13 and another reflection 
beam I.sub.1 which is reflected from the interface F.sub.34. 
As well known in the art, in the case where the phase change amount A 
satisfies the following equation (5), Kerr effect enhancement obtained at 
the data recording layer 4 is maximized. In other words, an apparent Kerr 
rotation angle is largely increased, so that the C/N ratio of the data 
recording medium is considerably enhanced, in the case where the phase 
change amount A satisfies the following equation (5). 
EQU A=.pi.+2m.pi. (where m is an integer) (5) 
The manner how the apparent Kerr rotation angle is increased through the 
maximization of the Kerr effect enhancement will be described in detail, 
with reference to FIGS. 15(a) through 15(c), hereinafter. 
Now assume that a linearly polarized incident laser beam I.sub.0 irradiated 
on the data recording medium 21 at the data recording and reproducing area 
7 is partly reflected at the interface F.sub.13 and partly reflected at 
the interface F.sub.34, respectively, and reflection beams I.sub.0 ' and 
I.sub.1 are obtained. Where the laser beam I.sub.0 is reflected at the 
interface F.sub.34 to form the reflective laser beam I.sub.1, its 
polarization plane is rotated by a Kerr rotation angle .theta..sub.k. 
Accordingly, as shown in FIG. 15(b), a polarization plane of the 
reflection beam I.sub.1 forms the Kerr rotation angle .theta..sub.k with 
respect to that of the incident laser beam I.sub.0. More specifically to 
say, the polarization plane of the reflection beam I.sub.1 has a 
y-directional component I.sub.1y which extends parallel to that of the 
incident beam I.sub.0 and an x-directional component I.sub.1x which 
extends perpendicularly to that of the incident beam I.sub.0. 0n the other 
hand, where the laser beam I.sub.0 is reflected at the interface F.sub.13 
to form the reflective laser beam I.sub.0 ', its polarization plane is not 
rotated. Therefore, the polarization plane of the reflective beam I.sub.0 
' extends parallel to that of the incident beam I.sub.1. In the case where 
the phase changing amount A satisfies the equation (5), where the 
reflective beam I.sub.0 ' meets the reflective beam I.sub.1 to constitute 
a single reflective laser beam I.sub.1 ', the reflective beam I.sub.0 ' 
and the component I.sub.1y of the reflective beam I.sub.1 undergo 
destructive interference. Accordingly, as shown in FIG. 15(c), a 
y-directional component I.sub.1y ' of the composite reflective beam 
I.sub.1 ' has an intensity amount equal to a value obtained by subtracting 
an intensity amount I.sub.0 ' from an intensity amount I.sub.1y, while an 
x-directional component I.sub.1x ' has an intensity amount equal to an 
intensity amount I.sub.1x. Since the intensity amount of I.sub.1y ' of the 
composite reflective beam I.sub.1 is thus smaller than that of I.sub.1y of 
the reflective beam I.sub.1y ' an apparent Kerr rotation angle 
.theta..sub.k ' of the composite reflective beam I.sub.1 ' becomes larger 
than the value .theta..sub.K of the real Kerr rotation angle .theta..sub.k 
of the reflective beam I.sub.1. As apparent from the above description, an 
apparent Kerr rotation angle .theta..sub.k ' is largely increased, in the 
case where the phase change amount A satisfies the equation (5). 
In order to allow the phase changing amount A to satisfy the equation (5), 
the thickness d.sub.1 of the interference layer 3 should satisfy the 
following equation (6), 
EQU d.sub.1 =.lambda./(4n.sub.1)+m.lambda./(2n.sub.1) (6) 
where m is an integer. 
On the other hand, a push-pull type tracking operation is applied for the 
data recording medium 21 of the present example. In the push-pull type 
tracking operation, laser beam is irradiated on the data recording medium 
so that the laser beam may be reflected and diffracted at the data 
recording and reproducing area 7 and the tracking guide area 8. A pair of 
photodetectors PD are provided for detecting intensities of the reflection 
beams from the data recording medium as illustrated in FIG. 16(a), and a 
differential signal representative of a difference value between the 
intensities detected by the pair of photodetectors is used as a tracking 
error signal. It is noted that how the laser beam is reflected and 
diffracted at the data recording medium 21 of the present example that is 
illustrated in FIG. 16(a) is the same as how laser beam is reflected and 
diffracted at a data recording medium of a pre-grooved type in which a 
tracking guide groove is formed in a substrate 1' that is illustrated in 
FIG. 16(b). In other words, the structure of the data recording medium 21 
in which the interference layer 3 has a thickness of d.sub.1 is equivalent 
to that of the pre-grooved type data recording medium in which the 
substrate 1' is formed with the tracking guide groove having a depth of 
d.sub.1. As well known in the art, in the pre-grooved type data recording 
medium as shown in FIG. 16(b), the value of the tracking error signal 
periodically varies according to change of the groove depth d.sub.1, as 
shown in FIG. 16(c). More specifically, the value of the tracking error 
signal is maximized in the case where the groove depth d.sub.1 has a value 
of .lambda./8n+m.lambda./4n, and is minimized to have a value of zero (0) 
in the case where the groove depth d.sub.1 has a value of 
.lambda./4n+m.lambda./4n, where n is an index of refraction of the 
substrate 1', m is an integer and .lambda. is a wavelength of the laser 
beam. Accordingly, the value of the tracking error signal obtained for the 
data recording medium 21 of the present example is maximized in the case 
where the interference layer thickness d.sub.1 has a value of 
.lambda./8n.sub.1 +m.lambda./4n.sub.1, and is minimized to have a value of 
zero (0) in the case where the interference layer thickness d.sub.1 has a 
value of .lambda./4n.sub.1 +m.lambda./4n.sub.1, where n.sub.1 is an index 
of refraction of the interference layer 3, m is an integer and .lambda. is 
a wavelength of the laser beam. That is, the value of the tracking error 
signal is maximized in the case where the above-described phase changing 
amount A has a value of .pi./2+m.pi., and is minimized to have a value of 
zero (0) in the case where the amount A has a value of .pi.+m.pi.. In 
other words, the tracking error signal has the maximum value and the zero 
value, in the case where a phase difference amount between a reflective 
beam I.sub.1 reflected at the data recording and reproducing area 7 (that 
is, a reflective beam I.sub.1 reflected from the data recording layer 4) 
and a reflective beam I.sub.2 reflected at the tracking guide area 8 (that 
is, a reflective beam I.sub.8 reflected from the reflective material 2) 
has values of .pi./2+m.pi. and .pi.+m.pi., respectively, as illustrated in 
FIG. 16(a). 
As described above in great detail, therefore, in order to allow the 
tracking error signal to have the maximum value, the thickness d.sub.1 of 
the interference layer 3 has to satisfy the following equation (7). 
EQU d.sub.1 =.lambda./8n.sub.1 +m.lambda./4n.sub.1 (7) 
On the other hand, in the case where the thickness d.sub.1 of the 
interference layer 3 satisfies the following equation (8), the tracking 
error signal will have the minimum value (zero). 
EQU d.sub.1 =.lambda./4n.sub.1 +m.lambda./4n.sub.1 (8) 
As apparent from the equations (6) and (8), such a value of d.sub.1 as 
satisfying the equation (6) always satisfies the equation (8). 
Accordingly, in order to perform the push-pull type tracking operation on 
the data recording medium 21 of the present example, the thickness d.sub.1 
of the interference layer 3 should be shifted from the value as satisfying 
the equation (6), so as to enable the tracking operation and therefore 
enable the data recording and reproducing operations. 
According to the present invention, therefore, the value of the thickness 
d.sub.1 of the interference layer 3 is selected to be within any of ranges 
between values satisfying the equation (6) and values satisfying the 
equation (7), i.e., in such ranges as enabling the tracking operation. 
Preferably, the thickness d.sub.1 is set to be closer to the value 
satisfying the equation (6) than to the value satisfying the equation (7). 
In the case where the value of the thickness d.sub.1 has a value which 
falls within any range between the values satisfying the equations (6) and 
(7) and which is closer to the value satisfying the equation (6), the 
thickness becomes relatively small with respect to that satisfying the 
equation (6), but is very close thereto. According to the present 
invention, therefore, the Kerr effect enhancement is not maximized, but is 
almost maximized, so that the C/N ratio of the data recording medium is 
greatly enhanced. 
As apparent from the above description, according to the present invention, 
in order to largely increase the Kerr effect enhancement, the thickness 
d.sub.1 of the interference layer 3 is selected to such a value as is 
close to a value satisfying the equation (6). Accordingly, the reflective 
beams I.sub.1 and I.sub.0 ' as shown in FIG. 15(a) undergo the destructive 
interference, so that the reflectivity of the data recording and 
reproducing area 7 is decreased to be approximately minimized. In other 
words, the reflectivity of the data recording medium 21 is approximately 
minimized at its data recording and reproducing area 7, due to the 
interference phenomenon occurred in the interference layer 3. It is noted, 
however, that the reflectivity of the data recording medium is not 
decreased at all, at its tracking guide area 8 where the reflection 
material 2 remains. Accordingly, the data recording medium of the present 
invention prevents the value of the tracking error signal from being 
decreased, relative to the conventional data recording medium formed with 
the tracking guide grooves. According to the data recording medium of the 
present invention, therefore, it becomes possible to perform a stable 
tracking operation. 
In the case where the preformat pits 6 are formed in the data recording 
medium 21 as shown in FIG. 14, furthermore, the thickness d.sub.1 of the 
interference layer 3 should be selected so as to not only greatly increase 
the Kerr effect enhancement and allow the tracking error signal to have 
its approximately maximum value but also allow a reproducing signal for 
the preformat pit to have its approximately maximum value. 
It is noted that the value of the reproducing signal for the preformat pit 
6 is maximized in the case where the thickness d.sub.1 of the interference 
layer 3 satisfies the equation (6), for the following reason. In order to 
reproduce the preformat pit 6, laser beam is irradiated on the data 
recording medium 21, and intensity of laser beam reflected from the data 
recording medium is detected. At the time when the laser beam is 
irradiated on the data recording medium at the tracking guide area 8 but 
is not irradiated on any preformat pit 6, the laser beam is fully 
reflected at the reflection material layer 2. Intensity of the laser beam 
thus reflected from the reflection material layer 2 is detected as a 
reference level. In the case where the laser beam is irradiated on the 
preformat pit 6, on the other hand, as shown in FIG. 17, a part of the 
laser beam is reflected at the data recording layer 4 positioned at the 
preformat pit 6 to form a reflective beam I.sub.1, and another part of the 
laser beam is reflected at the reflective material layer 2 positioned at 
the tracking guide area 8 which is positioned to surround the preformat 
pit 6 to thereby form another reflective beam I.sub.2. In the case where 
the thickness d.sub.1 of the interference layer 3 satisfies the equation 
(6), where the reflective beam I.sub.1 meets the reflective beam I.sub.2 
to form a composite reflective beam, the reflective beams I.sub.1 and 
I.sub.2 undergo destructive interference, so that intensity of the 
composite reflection beam is minimized. Since a signal representative of a 
difference between the intensity of the composite reflection beam and the 
reference level is used as the preformat pit reproducing signal, the value 
of the preformat pit reproducing signal is maximized in the case where the 
thickness d.sub.1 satisfies the equation (6). 
However, in the case where the thickness satisfies the equation (6), as 
described already, the tracking error signal has the minimum value (zero 
value), and therefore it becomes impossible to perform the push-pull type 
tracking operation. 
According to the present invention, therefore, in the case where the data 
recording medium is formed with the preformat pits 6, the thickness 
d.sub.1 of the interference layer 3 is shifted from the value satisfying 
the equation (6) but satisfies the following equation (9), 
EQU d.sub.1 =[.lambda./4n.sub.1 .+-..lambda./12n.sub.1 ]+m.lambda./2n.sub.1(9). 
In the case where the thickness d.sub.1 thus satisfies the equation (9), 
the tracking error signal and the pit reproducing signal may have their 
proper values, and the Kerr effect enhancement is approximately maximized. 
Accordingly, excellent data recording and reproducing operations can be 
attained. 
It is noted, however, that the thickness of the data recording layer 4 and 
the reflection material layer 2 should not be particularly limited. 
For example, as shown in FIG. 18, the thickness of the data recording layer 
4 may be selected to have a small value, and a reflection layer 40 may be 
provided on the protective layer 5. In this case, since the data recording 
layer 4 is thin, the laser beam irradiated on the substrate 1 can passes 
through the data recording layer 4 to be reflected at the reflection layer 
40. The laser beam then again passes through the data recording layer 4. 
Thus, in this case, not only the Kerr effect but also Faraday effect occur 
at the data recording layer 4, so that the polarization plane of the laser 
beam is rotated by a rotation angle of a greater value. As a result, the 
C/N ratio of the data recording medium is considerably enhanced. In this 
case, since the reflectivity of the tracking guide area 8 can be prevented 
from being deteriorated similarly as described above, a stable tracking 
operation can be achieved. 
Similarly, the thickness of the reflection material layer 2 may be selected 
to have a small value. In this case, height of stepped portions formed in 
the data recording layer 4 is decreased so that the data recording layer 4 
may be made flattened. Accordingly, it is possible to prevent the data 
recording layer 4 from being deteriorated at its stepped portions. 
It is noted that in the case where the thickness of the reflection material 
layer 2 is thus small, the laser beam irradiated on the data recording 
medium at the tracking guide area 8 on which the reflective material 2 
remains is partly reflected at the reflective material, but partly passes 
through the reflective material. The part of the laser beam thus passing 
through the reflective material further travels through the interference 
layer 3 to be reflected at the data recording layer 4. Accordingly, in 
this case, the multiple reflection occurs in the interference layer 3 not 
only at its area corresponding to the data recording and reproducing area 
7 from which the reflective material is removed but also at its area 
corresponding to the tracking guide area 8 where the reflective material 
remains. As a result, reflective laser beams are generated to be 
propagated in the interference layer 3 not only at its area corresponding 
to the data recording and reproducing area 7 but also at its area 
corresponding to the tracking guide area 8. Phase change amounts H occur 
in the respective laser beams as they travel from the substrate 1 toward 
the data recording layer 4, reflect at the data recording layer and return 
to the substrate have therefore various values. In other words, a 
plurality of laser beams outputted from the optical data recording medium 
have various values of phase difference amount therebetween. Accordingly, 
when the reflective laser beams meet one another to form a single 
composite reflective beam, they undergo various types of interference. 
However, in the case where a phase difference amount D between the phase 
change amount H occurring in a reflective laser beam traveling at the data 
recording and reproducing area 7 and a phase change amount H occurring in 
a reflective laser beam traveling at the tracking guide area 8 has a value 
approximately equal to .pi./2+m.pi.(where m is an integer), the tracking 
error signal having the maximum value can be obtained. In other words, in 
the case where an amount of phase difference between a reflection beam 
reflected from the optical data recording medium at its area 7 from which 
the reflective material is removed and a reflection beam reflected from 
the optical data recording medium at its area 8 where the reflective 
material remains has approximately a value of .pi./2+m.pi. where m is an 
integer, the tracking error signal having the maximum value can be 
obtained. Accordingly, in the present invention, the thickness of the 
respective layers should be preferably selected to such values as allowing 
the phase difference D to become approximately equal to the value of 
.pi./2+m.pi.. Of course, the thickness of the layers may be shifted from 
the values allowing the phase difference D to become approximately equal 
to the value of .pi./2+m.pi.. However, the thickness should not be 
selected to such values as allowing the phase difference D to become 
approximately equal to the value of .pi.+m.pi., since the tracking error 
signal has a value of zero in the case where the phase difference D has a 
value approximately equal to the value of .pi.+m.pi.. Accordingly, the 
thickness should have such values as allowing an amount of phase 
difference between a reflection beam reflected from the optical data 
recording medium at its area 7 from which the reflective material is 
removed and a reflection beam reflected from the optical data recording 
medium at its area 8 where the reflective material remains to have a value 
other than .pi.+m.pi. where m is an integer. 
Similarly as described above, in the case where the data recording layer 4 
is thin, a plurality of laser beams outputted from the optical data 
recording medium have various values of phase difference amount 
therebetween. Accordingly, when the reflective laser beams meet one 
another to form a single composite reflective beam, they undergo various 
types of interference. However, the thickness of the respective layers 
should not be selected to values allowing the phase difference D to become 
approximately equal to the value of .pi.+m.pi., but should be preferably 
selected to such values as allowing the phase difference D to become 
approximately equal to the value of .pi./2+m.pi.. 
It should be further noted that in the case where a well-known three-beam 
tracking operation is achieved on the data recording medium of the present 
invention, the thickness d.sub.1 of the interference layer 3 should have a 
value satisfying the above-described equation (6). 
In the case where the data recording layer is formed of data recording 
material other than the magnetooptic material such as the phase-change 
material, the pit-formable material and the organic material, the 
thickness d.sub.1 of the interference layer 3 should be selected so that 
the phase changing amount A of the laser beam may have a value 
approximately equal to .pi.+m(2.pi.), where m is an integer, in order to 
enhance the data recording characteristics of the data recording medium. 
A second preferred embodiment of the present invention will be described 
with reference to FIGS. 19 through 25. 
FIG. 19 illustrates an optical data recording medium 22 according to a 
second preferred embodiment of the present invention in which another 
interference layer 30 formed of transparent dielectric material (such as 
SiO, SiAlON, etc.) with its index of refraction being larger than that of 
the substrate 1 is provided under the reflective material layer 2. 
Accordingly, the reflective material layer 2 is sandwiched between the 
interference layers 3 and 30. The interference layer 30 can further 
enhance Kerr effect, to thereby further increase the C/N characteristics. 
The interference layer 30 will be referred to as a "first interference 
layer", and the interference layer 3 will be referred to as a "second 
interference layer", hereinafter. 
The method of producing the data recording medium of the second embodiment 
is the same as that of the first embodiment, except that the first 
interference layer 30 having a predetermined thickness is first provided 
over the substrate 1 through a sputtering process or the like and then the 
reflective material layer 2 is formed over the first interference layer 30 
thus formed on the substrate. 
As shown in FIG. 20, similarly as in the optical data recording medium of 
the first embodiment, the data recording medium 22 of the present 
embodiment is designed so as to satisfy the already-described equation 
(3). Thus, it becomes possible to select the value of the track pitch P to 
be smaller than the laser beam spot diameter d while preventing the 
cross-talk from being increased. Accordingly, it is possible to narrow the 
track pitch of the data recording medium for enhancing the data recording 
density. 
In the data recording medium of the second embodiment, similarly as in that 
of the first embodiment, thickness of the interference layers 3 and 30 are 
selected to such values as can enhance the C/N ratio without deteriorating 
the tracking servo performance. The thickness of the interference layers 3 
and 30 will be described below in great detail with reference to an 
example of a magnetooptic data recording medium to which applied is the 
present embodiment. 
FIG. 21 shows a magnetooptic data recording medium 23 to which the present 
embodiment is applied. The data recording medium 23 therefore includes the 
substrate 1, the first interference layer 30, the reflective material 
layer 2, the second interference layer 3, the data recording layer 4 
formed of the magnetooptic data recording material, and the protective 
layer 5. The structure of the magnetooptic data recording medium 23 is 
therefore the same as that of the already-described magnetooptic data 
recording medium 21 of the example of the first embodiment, except for the 
first interference layer 30. Also similarly as in the data recording 
medium 21, the reflective material in the reflective material layer 2 of 
the data recording medium 23 is partly removed from the first interference 
layer 30, as shown in FIG. 22, so that the data recording and reproducing 
areas 7, the preformat pits 6, and the tracking guide areas 8 are formed 
on the reflective material layer 2. 
Similarly as in the data recording medium 21 of the first embodiment, as 
shown in FIG. 21, an incident laser beam I.sub.0 irradiated on the data 
recording medium 23 at its data recording and reproducing area 7 for 
reproducing data recorded therein is partly reflected at an interface 
F.sub.130 defined between the substrate 1 and the first interference layer 
30 to form a reflection beam I.sub.0 ' and is partly reflected at an 
interface F.sub.34 defined between the second interference layer 3 and the 
data recording layer 4 to form another reflection beam I.sub.1 ', since 
the first and second interference layers 30 and 3 are formed of the 
material which has an index of refraction of a high value. In other words, 
multiple reflection occurs both in the first and second interference 
layers 30 and 3. As indicated by an arrow B in FIG. 21, a phase change 
amount B which occurs in the laser beam as it travels through the 
interference layers 30 and 3 from the interface F.sub.130 toward the 
interface F.sub.34, reflects at the interface F.sub.34 and returns to the 
interface F.sub.130 satisfies the following equation (10), 
EQU B =2(2.pi./.lambda.)(n.sub.1 'd.sub.1 '+n.sub.2 'd.sub.2 ')(10), 
where d.sub.1 ' and d.sub.2 ' represent thickness of the first and second 
interference layers 30 and 3, n.sub.1 ' and n.sub.2 ' represent the 
indices of refraction of the first and second interference layers 30 and 
3, and .lambda. represents wavelength of the laser beam in vacuum space. 
Especially in the case where the first and second interference layers 30 
and 3 are formed of the same single material, the phase change amount B 
satisfies the following equation (11), 
EQU B=2n.sub.I (2.pi./.lambda.)(d.sub.1 '+d.sub.2 ') (11) 
where n.sub.I represents the indices of refraction of the first and second 
interference layers. 
Similarly as in the data recording medium 21 of the first embodiment, in 
the case where the phase change amount B satisfies the following equation 
(12), the Kerr effect enhancement is maximized, so that the apparent Kerr 
rotation angle is obtained to be maximized to largely enhance the C/N 
ratio of the data recording medium 23. 
EQU B=.pi.+2m.pi.(m is an integer) (12). 
As apparent from the equations (11) and (12), in the data recording medium 
23 of the present example, in order to maximize the Kerr effect 
enhancement, the thickness d.sub.1 and d.sub.2 ' of the first and second 
interference layers 30 and 3 have to satisfy the following equation (13). 
EQU d.sub.1 '+d.sub.2 '=.lambda./4n.sub.I +m.pi./2n.sub.I (13) 
In the data recording medium 23 of the present invention, therefore, the 
total thickness (d.sub.1 '+d.sub.2 ') of the first and second interference 
layers 30 and 3 is selected to have a value corresponding approximately to 
a quarter of a wavelength of the laser beam travelling in the layers 30 
and 3 or a value corresponding approximately to a sum of the quarter of 
the wavelength and a value obtained by multiplying a half value of the 
wavelength by any integer. As a result, the Kerr effect enhancement is 
maximized, so that the C/N ratio of the data recording medium 22 is 
largely enhanced. 
In order to obtain a push-pull type tracking error signal for the data 
recording medium 23, the laser beam is irradiated on the data recording 
medium. Similarly as in the data recording medium 21 of the first 
embodiment, in the case where a phase difference amount between a 
reflective laser beam I.sub.1 reflected at the data recording and 
reproducing area 7 (that is, a reflective laser beam I.sub.1 reflected 
from the data recording layer 4) and another reflective laser beam I.sub.2 
reflected at the tracking guide area 8 (that is, another reflective laser 
beam I.sub.2 reflected from the reflective material 2) has a value 
approximately equal to .pi./2+m.pi.(where m is an integer), the tracking 
error signal is obtained to have its maximum value. In the case where the 
phase difference amount between the reflective laser beams I.sub.1 and 
I.sub.2 has a value approximately equal to .pi.+m.pi., on the other hand, 
the tracking error signal is obtained to have its minimum value (zero 
value). The phase difference amount between the reflective laser beams 
I.sub.1 and I.sub.2 corresponds to a difference between the phase changing 
amount B occurring in the laser beam as it travels through both the 
interference layers 30 and 3 from the interface F.sub.130 toward the 
interface F.sub.34, reflects at the interface F.sub.34, and returns to the 
interface F.sub.130 and a phase changing amount C occurring in the laser 
beam as it travels through only the interference layer 30 from the 
interface F.sub.130 toward an interface F.sub.302 which is defined between 
the interference layer 30 and the reflective material layer 2, reflects at 
the interface F.sub.302 and returns to the interface F.sub.130, as shown 
in FIG. 21. The phase changing amount C therefore satisfies the following 
equation (14). 
EQU C=2n.sub.I (2.pi./.lambda.)d.sub.1 ' (14) 
Accordingly, in order to allow the tracking error signal to have its 
maximum value, the following equation (15) has to be satisfied, 
EQU B-C=.pi./2+k.pi. (15) 
where k is an integer. 
Thus, in order to not only maximize the Kerr effect enhancement but also 
maximize the amount of the tracking error signal, the thickness d.sub.1 ' 
and d.sub.2 ' of the interference layers 30 and 3 have to satisfy both the 
equations (12) and (15). Accordingly, the thickness d.sub.1 ' and d.sub.2 
' have to satisfy the following equations (16) and (17), 
EQU d.sub.1 '=.lambda./8n.sub.I +(2m-k).lambda./4n.sub.I (16), 
EQU d.sub.2 '=.lambda./8n.sub.I +k.lambda./4n.sub.I (17). 
With the above-described structure, the reflectivity of the data recording 
and reproducing area 7 from which the reflective material is removed can 
be minimized, but the reflectivity of the tracking guide area 8 on which 
the reflective material remains is not decreased. It therefore becomes 
possible to prevent the tracking error signal from being deteriorated, 
contrary to the conventional data recording medium formed with the 
tracking guide grooves. 
It is noted furthermore that in the case where the phase difference amount 
B-C satisfies the following equation (18), the tracking error signal has 
the minimum amount of 0, and therefore data recording and reproducing 
operation may not be achieved. 
EQU B-C=.pi.+k.pi. (18) 
In the case where the equation (18) is satisfied, the thickness d.sub.2 ' 
of the interference layer 3 satisfies the following equation (19) 
regardless of the value of the thickness d.sub.1 of the interference layer 
30, 
EQU d.sub.2 '=.lambda./4n.sub.I +k.lambda./4n.sub.I (19). 
According to the present invention, therefore, the thickness d.sub.2 ' of 
the interference layer 3 should not be selected to such a value as 
satisfies the equation (19) so that the tracking error signal may have a 
value not equal to zero but the data recording and reproducing operation 
can be certainly achieved. Both in the case where the thickness d.sub.2 ' 
of the interference layer 3 is thus selected to a value which does not 
satisfy the equation (19) and in the case where the thickness d.sub.1 ' of 
the interference layer 30 is selected so that the selected values of 
d.sub.1 ' and d.sub.2 ' may satisfy the equation (13), the Kerr effect 
enhancement can be maximized to largely increase the C/N ratio while 
obtaining the tracking error signal proper to achieve a good tracking 
operation. 
In addition, similarly as in the data recording medium 21 of the first 
embodiment, the reproducing signal of the preformat pits 6 has the maximum 
value, in the case where the phase difference amount B-C satisfies the 
following equation (20). 
EQU B-C=.pi.+2k.pi. (20) 
However, the thickness d.sub.2 ' of the interference layer 3 satisfying the 
equation (20) will always satisfy the equation (19). Accordingly, if the 
thickness d.sub.2 ' satisfies the equation (20), the tracking error signal 
will have a value of zero and the tracking operation may not be achieved. 
Accordingly, the thickness d.sub.2 ' should be shifted from the value 
satisfying the equation (19), but should be selected to satisfy the 
following equation (21) or (21'). 
EQU d.sub.2 '=[.lambda./4n.sub.I +.lambda./12n.sub.I ]+k.lambda./2n.sub.I(21) 
EQU d.sub.2 '=[.lambda./4n.sub.I -.lambda./12n.sub.I ]+k.lambda./2n.sub.I(21') 
In the case where the d.sub.2 satisfies the equation (21) or (21'), if the 
thickness d.sub.1 ' of the interference layer 30 satisfies the following 
equation (22) or (22'), the thickness values d.sub.1 and d.sub.1 satisfy 
the equation (13), so that the Kerr effect enhancement can be maximized. 
EQU d.sub.1 '=(m-k).lambda./2n.sub.I -.lambda./12n.sub.I (22) 
EQU d.sub.1 '=(m-k).lambda./2n.sub.I +.lambda./12n.sub.I (22') 
In the case where the thickness values d.sub.1 ' and d.sub.2 ' satisfy the 
equations (21) or (21') and (22) or (22'), therefore, the tracking error 
signal and the preformat pit reproducing signal may have sufficiently 
values, respectively, so that excellent data recording and reproducing 
operations can be achieved. 
It is noted that the thickness of the data recording layer 4 and the 
reflection material layer 2 should not be particularly limited. 
For example, as shown in FIG. 23, the thickness of the data recording layer 
4 may be selected to have a small value, and the reflection layer 40 may 
be provided on the protective layer 5, similarly as in the data recording 
medium 21 of the first embodiment shown in FIG. 18. 
The thickness of the reflection material layer 2 may also be selected to 
have a small value, similarly as in the data recording medium 21 of the 
first embodiment. In the case where the reflection materiallayer 2 is thus 
thin, the laser beam irradiated on the data recording medium at the 
tracking guide area 8 on which the reflective material 2 remains is partly 
reflected at the reflective material, but partly passes through the 
reflective material. The part of the laser beam thus passing through the 
reflective material further travels through the interference layer 3 to be 
reflected at the data recording layer 4. Accordingly, in this case, the 
multiple reflection occurs in the interference layer 3 not only at its 
area corresponding to the data recording and reproducing area 7 from which 
the reflective material is removed but also at its area corresponding to 
the tracking guide area 8 where the reflective material remains. As a 
result, reflective laser beams are generated to be propagated in the 
interference layer 3 not only at its area corresponding to the data 
recording and reproducing area 7 but also at its area corresponding to the 
tracking guide area 8. Phase change amounts H occurred in the respective 
laser beams as they travel from the substrate 1 toward the data recording 
layer 4, reflect at the data recording layer and return to the substrate 
have therefore various values. In other words, a plurality of reflective 
laser beams outputted from the data recording medium have various values 
of phase difference amount therebetween. Accordingly, when the reflective 
laser beams meet with one another to form a single composite reflective 
beam, they undergo various types of interference. However, in the case 
where a phase difference amount D between the phase change amount H 
occurring in a reflective laser beam traveling at the data recording and 
reproducing area 7 and a phase change amount H occurring in a reflective 
laser beam traveling at the tracking guide area 8 has a value 
approximately equal to .pi./2+m.pi. (where m is an integer), the tracking 
error signal having the maximum value can be obtained. In other words, in 
the case where an amount of phase difference between a reflection beam 
reflected from the optical data recording medium at its area 7 from which 
the reflective material is removed and a reflection beam reflected from 
the optical data recording medium at its area 8 where the reflective 
material remains has approximately a value of .pi./2+m.pi. where m is an 
integer, the tracking error signal having the maximum value can be 
obtained. Accordingly, in the present invention, the thickness of the 
respective layers should be preferably selected to such values as allowing 
the phase difference D to become approximately equal to the value of 
.pi./2+m.pi.. Of course, the thickness of the layers may be shifted from 
the values allowing the phase difference D to become approximately equal 
to the value of .pi./2+m.pi.. However, the thickness should not be 
selected to values allowing the phase difference D to become approximately 
equal to the value of .pi.+m.pi., since the tracking error signal has a 
value of zero in the case where the phase difference D has a value 
approximately equal to the value of .pi.+m.pi.. Accordingly, the thickness 
should have values allowing an amount of phase difference between a 
reflection beam reflected from the optical data recording medium at its 
area 7 from which the reflective material is removed and a reflection beam 
reflected from the optical data recording medium at its area 8 where the 
reflective material remains to have a value other than .pi.+m.pi. where m 
is an integer. 
Similarly as described above, in the case where the data recording layer 4 
is thin, a plurality of laser beams outputted from the optical data 
recording medium have various values of phase difference amount 
therebetween. Accordingly, when the reflective laser beams meet one 
another to form a single composite reflective beam, they undergo various 
types of interference. However, the thickness of the respective layers 
should not be selected to values allowing the phase difference D to become 
approximately equal to the value of .pi.+m.pi., but should be preferably 
selected to values allowing the phase difference D to become approximately 
equal to the value of .pi./2+m.pi.. 
It should be further noted that in the case where a well-known three-beam 
tracking operation is achieved on the data recording medium of the present 
invention, the thickness d.sub.1 ' and d.sub.2 ' of the interference 
layers should have a value satisfying the above-described equation (13). 
In the case where the data recording layer is formed of data recording 
material other than the magnetooptic material such as the phase-change 
material, the pit-formable material and the organic material, the 
thickness d.sub.1 ' and d.sub.2 ' of the interference layers should be 
selected so that the phase changing amount B of the laser beam may have a 
value approximately equal to .pi.+m(2.pi.), where m is an integer, in 
order to enhance the data recording characteristics of the data recording 
medium. 
Additionally, the material of the interference layers 30 and 3 should not 
be particularly limited. The material of the interference layers 30 and 3 
may not be the same with each other. The interference layers 30 and 3 may 
be formed of material different from each other. For example, as shown in 
FIG. 24, the interference layer 30 may be formed of SiAlON or the like, 
and the interference layer 3 may be formed of TiO.sub.2, PLZT or the like 
formed through a spin coating method and a heat treatment. In this case, 
the data recording layer 4 may be made flattened. Furthermore, as shown in 
FIG. 25, the interference layer 3 may include a flattened layer 50 formed 
of TiO.sub.2, PLZT or the like produced through a spin coating method and 
a dielectric film 60 formed of SiAlON or the like which is formed over the 
flattened layer 50 through a sputtering process or the like. To summarize, 
each of the interference layers 30 and 3 is unnecessarily formed of a 
single material, but may be formed of a multi-layered film formed of 
plural kinds of material. 
According to the data recording medium of the present invention, when the 
interference layers 3 and 30 are unnecessary, they may be eliminated from 
the data recording medium. FIG. 26 shows an optical data recording medium 
of a third preferred embodiment of the present invention in which the data 
recording layer 4 is provided over the reflective material layer 2 in such 
a manner that the data recording layer is in direct contact with the 
reflective material layer. 
FIG. 27 illustrates an optical data recording medium according to a fourth 
preferred embodiment of the present invention in which an optical data 
recording layer 72 is provided over a substrate 71, and a reflective 
material layer 74 is provided over the data recording layer, and a 
protective layer 76 is provided over the reflective material layer. In 
order to perform data recording and data reproducing operations onto the 
data recording medium, a laser beam is irradiated on the data recording 
medium from its side opposite to the substrate. 
As described above, in the optical data recording medium of the present 
invention, the reflective material layer provided on the substrate is 
patterned through partially removing the reflective material from the 
substrate. The patterned reflective material layer defines therein the 
preformat areas and the data recording and reproducing area. With this 
structure, the data recording medium enables high density data recordation 
without increasing cross-talk. In addition, since the data recording and 
reproducing area and the preformat pits are thus formed through simply 
removing the reflective material from the substrate, yield rate of the 
data recording medium is considerably enhanced. In addition, the mirror 
part can be formed through simply removing the reflective material 
completely from the substrate. The mirror part serves to provide the 
offset signal representative of the shift amount of an optical axis of the 
laser beam irradiated on the data recording medium and the inclination 
amount of the disk-shaped data recording medium. The offset signal will be 
used for neglecting the DC offset component of the tracking error signal. 
In addition, since the interference layer is formed of material with its 
index of refraction being greater than that of the substrate, the Kerr 
effect enhancement is maximized, so that the apparent Kerr rotation angle 
is increased and therefore the C/N ratio of the data recording medium is 
considerably enhanced. Furthermore, since the value of the tracking error 
signal can be prevented from being deteriorated, it is possible perform a 
stable tracking operation. 
While the present invention has been described in detail and with reference 
to specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof. 
For example, the structures shown in FIGS. 18, 23 through 25 may be applied 
to not only the magnetooptic data recording medium but also to optical 
data recording medium of various types provided with the data recording 
layer formed of the phase-change material, the pit-formable material and 
the organic material, etc. 
Furthermore, as illustrated in FIG. 28, the mirror part 9 may be formed 
from an area on which the reflective material 2 completely remains. 
Similarly, as illustrated in FIG. 29, the preformat pit 6 may be formed 
from an area on which the reflective material 2 remains. Furthermore, 
though the data recording and reproducing area is formed to extend 
concentrically or spirally and the preformat pits are formed to be 
arranged concentrically or spirally in the above-described embodiments, 
the data recording and reproducing area may be formed to extend linearly 
and the preformat pits may be formed to be arranged linearly. 
Furthermore, the reflective material layer 2 provided on the substrate 1 
may be simply removed continuously in concentrical or spiral fashion so as 
to only form the data recording and reproducing areas 7 and the tracking 
guide film parts 8. In other words, the mirror parts 8 and the preformat 
pits 6 may not be formed on the reflective material layer through partly 
removing the reflective material therefrom. In such a case, the preformat 
signals may be recorded in the data recording layer 4. 
In addition, a pair of protective layers may be provided on both sides of 
the data recording layer 4 so that the data recording layer may be 
sandwiched between the pair of protective layers. 
Material of the interference layer, the protective layer and the substrate 
should not be particularly limited. Material and a reflection factor of 
the reflective material layer 2 should not be particularly limited. The 
reflective material layer may be formed of material with its light 
absorption having a large value. Material of the interference layer is 
required to have a value of an index of refraction higher than that of the 
substrate, but is not particularly limited. For example, the interference 
layer may be formed of a multi-layered film made of a plurality of 
materials.