Optical disk having data stored on a land portion and a groove portion with the land portion having a greater width than the groove portion

An optical disk and its recording and playback method. The optical disk has a groove portion and a land portion, in which the land portion is at a distance of greater than or equal to 100 nm from the groove portion and the land portion has a land width greater than a groove width of the groove portion. The method includes recording the data on a groove portion and a land portion of the optical disk; and playing back the recorded data with a land reflectivity equal to groove reflectivity when the land portion is at a distance of greater than or equal to 100 nm from the groove portion and a land width of the land portion is greater than a groove width of the groove portion.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application is based upon and claims priority of Japanese Patent 
Application No. 08-042996 filed Feb. 29, 1996, the contents being 
incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to optical disks. More specifically, the 
present invention relates to an optical disk and its recording and 
playback method that reduces thermal crosstalk and improves playback 
signal levels even when the distance between the levels of the land 
portions and the groove portions of the disk is large. 
2. Description of the Related Art 
Optical disks, storing data at high density and capable of rapid data 
processing, are gaining more attention as storage mediums for computers. 
Optical disks, including for instance 5.25 inch, 3.5 inch, or the like 
diameter, of the magneto-optical type or phase change type, in read-only 
or rewritable formats, have been standardized by ISO standards and are 
expected to increase in use. Moreover, standards have been recently 
determined for digital video disks ("DVD"). The use of these and other 
types of optical disks (and their applications) are expected to increase 
in the multimedia field. 
In optical disks, a guide for tracking (i.e., a guide groove) is formed 
with a concave or convex shape, in a spiral form. An outgoing laser beam 
generated by an optical pickup portion of a recording and playback device 
uses the guide groove to pass along a data sequence. According to ISO 
standards, a concave portion of the guide groove, as seen from the pickup 
device, is termed a "land". The convex portion of the guide groove, as 
seen from the pickup device, is termed a "groove" and is closer to the 
pickup device than the land portion. Data is written on the land and/or 
the groove portions of the guide grooves. The distance from a center of a 
land (or a groove) to a center of an adjacent land (or groove) is termed 
the "track pitch". A series of convex marks, formed in advance as a series 
of pits, serve as a preformatted signal indicating a track number or 
sector number. 
A groove width W is defined as W=(W.sub.top +W.sub.bottom)/2, where 
W.sub.top is the width of the groove top and W.sub.bottom is the width of 
the groove bottom. The height from the groove bottom up to the groove top, 
namely, the difference (or distance) between the levels of the land 
portions and the groove portions, is termed the "groove depth". For the 
dimensions of a groove depth, take for example a groove width of 0.3-0.6 
.mu.m using a land recording method. With .lambda. as the wavelength of 
the laser light used for recording and playback, and "n" as the refractive 
index of the substrate, the groove depth is between 
.lambda./(10.multidot.n) and .lambda./(6.multidot.n). 
A track pitch of 1.6 .mu.m was a standard, but recently, in order to record 
data at a higher density, a narrowed track pitch of 1.4 .mu.m, 1.2 .mu.m, 
and 1.0 .mu.m has been used for recording. However, with an optical pickup 
having an objective lens with a numerical aperture (NA) of 0.5-0.6, 
problems arise when the track pitch is narrowed to less than 1.4 .mu.m. 
For instance, the effect due to data written on adjacent tracks being 
erroneously read out (i.e., "optical crosstalk") becomes extremely large. 
Moreover, because the tracking error signal necessary for tracking becomes 
extremely small, reliable tracking becomes difficult. 
As a different approach to recording data at a high density, a land-groove 
recording method had been proposed. In contrast to the above method which 
recorded on only one of the lands or grooves, the land-groove method 
records data on both the lands and the grooves. Recording density is 
increased by halving the track pitch. For example, in the case that the 
distance from the center of a land (or groove) to the center of an 
adjacent land (or groove) is 1.4 .mu.m, by recording on both lands and 
grooves, the track pitch becomes 0.7 .mu.m, and the recording density can 
be increased. In this land-groove recording method, if a suitable value 
for the groove depth is taken, the problem of optical crosstalk, in which 
data of the adjacent groove (land) track is simultaneously read out while 
reading out a land (groove) track, can be prevented. Moreover, because the 
distance between land (or groove) centers is 1.4 .mu.m, the tracking error 
signal can be kept sufficiently large. 
Despite solving the optical crosstalk problem and maintaining a suitable 
tracking error signal, there is still a problem in which the temperature 
of an adjacent track increases due to the heat of a laser beam when data 
is recorded and erased on a track. With the increased temperature on the 
adjacent track, data on the adjacent track becomes erased (i.e., "cross 
erasure" or "thermal crosstalk"). In either the magneto-optical type or 
the phase change type of optical storage mediums, because both types of 
optical disks are recorded by means of heat, if the distance between 
adjacent tracks becomes small, the transfer of heat to the adjacent tracks 
becomes large and thermal crosstalk occurs. 
To what degree the track pitch can be narrowed is decided by this 
crosstalk. In the prior art optical disks, the difference in level between 
the land portions and the groove portions (i.e., groove depth) is about 
70-80 nm. In an optical disk having this kind of guide groove form, about 
0.8 .mu.m in the magneto-optical type or the phase change type, the light 
intensity modulated overwrite magneto-optical type is limited to about 
0.9-1.0 .mu.m, and a track narrowing greater than this was found to be 
difficult. 
Consequently, the present inventors, primarily in order to reduce thermal 
crosstalk, developed an optical disk with a large groove depth. That is, 
the thermal propagation distance to adjacent tracks is made long by making 
the difference in the level between land portions and groove portions 
large. Thus, the effect of heat from adjacent tracks is reduced. For 
example, in optical disks in which recording is performed in the land 
portions only, or the groove portions only, the groove depth is 40-90 nm. 
However, the present invention is directed to deeper groove depths (e.g., 
100 nm or greater), whereupon, the track pitch can be narrowed to 0.7 
.mu.m or less. 
Nevertheless, if the groove depth is to be 100 nm or more, there is no 
guidance in the prior art to determine suitable groove depths. First, it 
is desirable for the land reflectivity and the groove reflectivity to be 
0.5 or more in order to maintain a desirable playback signal level. But, 
the land reflectivity and the groove reflectivity change due to changes in 
the distance between the level of the land portions and the groove 
portions. A fall in the playback signal level may result from certain 
groove depths, causing data readout errors and the like. Moreover, it is 
desirable for a push-pull signal modulation to be 0.2 or more in order to 
maintain tracking accuracy. But, tracking may be dislocated if the 
push-pull signal modulation becomes low due to certain groove depths. 
Consequently, high speed access becomes impossible and erroneous data 
erasure and the like may result. 
With regard to the playback signal level, if the wavelength of the light 
spot is .lambda., and the refractivity of the optical disk substrate is n, 
the playback signal level becomes greatest at a groove depth of 
approximately m.lambda./(2n). Here m is a natural number (m=1, 2, 3, 4, 5, 
6 . . . ). For example, when .lambda.=680 nm, n=1.5 and m=1, the playback 
signal is greatest at a groove depth of 226.7 nm; when m=2, the playback 
signal is greatest at a groove depth of 453.3 nm; and when m=3, it is 680 
nm. In practice, due to the effect of the direction of polarization and 
the like, the land/groove level difference at which the playback signal 
level is the greatest is not exactly m.lambda./(2n). For instance, in the 
case of linearly polarized light having a plane of polarization in a 
direction parallel to the guide groove, the groove depth at which the 
playback signal level is the greatest may be m.lambda./(1.8 n) or 
m.lambda./(1.95 n) and the like. Moreover, the groove depth at which the 
push-pull signal modulation becomes greatest is at approximately 
(2m+1).lambda./(8 n). 
When the best and most suitable groove depths are found from the above 
viewpoints, groove depth ranges of 110-220 nm, or 230-330 nm, or 350-580 
nm are considered to be desirable. Moreover, comparing the cases of 
playback by means of H polarized light and playback by means of E 
polarized light, the value of the groove reflectivity Ig/Io by means of H 
polarized light is considered to be large overall, and a good playback 
signal level is easily obtained. In the case of playback by means of H 
polarized light, if the groove depth ranges are 110-210 nm, or 230-320 nm, 
or 350-440 nm, or 450-570 nm, particularly good results are obtained. 
Moreover, at a groove depth of 350 nm or more, the thermal crosstalk 
reduction is effective for a track pitch narrowed down to about 0.3 .mu.m. 
Nevertheless, the following problems arise when the groove depth is large. 
Namely, in the prior art optical disks in which the land width and the 
groove width were equal and when the difference in level of the land 
portions and the groove portions was made large, there was a tendency for 
the reflectivity in the land portions to become small in comparison with 
the reflectivity in the groove portions. This discrepancy between land and 
groove reflectivity is something that arises due to the properties of 
light as waves. Because the land surfaces seen from the pickup are remote 
(namely, concave portions), the land surfaces can be considered as a kind 
of slit having a width equal to a width of the lands. Because it is 
difficult for light waves to enter the interior of a slit, the reflected 
light at a land surface is small in comparison with the reflected light at 
a groove surface. This phenomenon is particularly noticeable in the case 
where the plane of polarization of the light is parallel with respect to 
the groove (termed "E polarized light"). In this manner, the land 
reflectivity is small in comparison to the groove reflectivity, and the 
playback signal level (carrier level) becomes small when playing back data 
recorded in the land portions, causing readout errors and the like. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to solve the 
above-identified problems and improve playback signal levels from land 
portions of an optical storage medium when the groove depth is large. 
It is a further object of the present invention to prevent the playback 
signal levels from land portions of an optical storage medium from 
becoming correspondingly small when the groove depth is made large. 
It is another object of the present invention to maintain land reflectivity 
and groove reflectivity to be approximately the same when the groove depth 
is made large in an optical storage medium. 
It is yet another object of the present invention to prevent readout errors 
when the groove depth is large in an optical storage medium. 
Objects of the present invention are achieved by providing an optical disk 
having a groove portion and a land portion, in which the land portion is 
at a distance of greater than or equal to 100 nm from the groove portion 
and the land portion has a land width greater than a groove width of the 
groove portion. 
Objects of the present invention are also achieved by providing an optical 
disk having a groove portion and a land portion, in which the land portion 
is at a distance of greater than or equal to 100 nm from the groove 
portion, the land portion has a land width greater than a groove width of 
the groove portion with a land width to groove width ratio of greater than 
or equal to 1.03, and a distance from a center of said land portion to a 
center of said groove portion is less than or equal to 0.7 .mu.m. The land 
portion may be at the distance from the groove portion in the range of at 
least one of 110-220 nm, 230-330 nm, and 350-580 nm. 
Objects of the present invention are also achieved by providing an optical 
disk having a groove portion and a land portion, in which the land portion 
is at a distance of greater than or equal to 100 nm from the groove 
portion and the land portion has a land width greater than a groove width 
of the groove portion to maintain an equal land reflectivity and groove 
reflectivity when incident light is directed onto the optical disk for 
recording and playback of data. 
Objects of the present invention are further achieved by providing a method 
to record and playback data on an optical disk. The method includes 
recording the data on a groove portion and a land portion of the optical 
disk; and playing back the recorded data with a land reflectivity equal to 
groove reflectivity when the land portion is at a distance of greater than 
or equal to 100 nm from the groove portion and a land width of the land 
portion is greater than a groove width of the groove portion. 
Moreover, objects of the present invention are achieved by an optical disk, 
including: a groove portion having a groove width; and a land portion 
having a land width greater than the groove width, wherein the land 
portion is at a distance of greater than or equal to 
.lambda./(4.multidot.n)nm from the groove. 
Still further objects of the invention are achieved by an optical disk, 
including: a groove portion having a groove width; and a land portion 
having a land width greater than the groove width with a land width to 
groove width ratio of greater than or equal to 1.03, wherein the land 
portion is at a distance of greater than or equal to 
.lambda./(4.multidot.n) nm from the groove portion and a distance form a 
center of the land portion to a center of the groove portion adjacent to 
the land portion is less than or equal to 0.7 .mu.m. 
Further objects of the invention are achieved by an optical disk on which 
an incident light is directed, including a groove portion having a groove 
width; and a land portion having a land width, wherein the land portion is 
at a distance of greater than or equal to .lambda./(4.multidot.n)nm from 
the groove portion and the land width is greater than the groove width to 
maintain an equal land reflectivity and groove reflectivity when the 
incident light is directed onto the optical disk.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made in detail to the present preferred embodiments 
of the present invention, examples of which are illustrated in the 
accompanying drawings. In order to solve the aforementioned problems, a 
preferred embodiment of the present invention includes an optical disk 
having a groove depth of 100 nm or more, and a land width that is wider 
than a groove width. 
In accordance with the preferred embodiments of the present invention, 
multiple magneto-optical disks are prepared in which the sum of the land 
width and the groove width is 1.4 .mu.m and 1.2 .mu.m. Values of land 
width to groove width ratio range from 1 to 1.42 in steps of 0.01, and six 
different values of the groove depth are used, i.e., 100, 150, 180, 210, 
245 and 280 nm. A silicon nitride layer, a TbFeCo magneto-optical 
recording layer, and a silicon nitride layer are formed in succession by 
sputtering onto these magneto-optical disks. Moreover, the refractive 
index of the substrate is 1.5. 
Setting these magneto-optical disks in succession into a magneto-optical 
recording and playback device, standardized at a reflectivity "Io" in a 
region having no guide grooves, the land reflectivity "II/Io" and the 
groove reflectivity "Ig/Io" are measured. The light source wavelength 
.lambda. is 680 nm, the numerical aperture NA of the objective lens is 
0.55, and the wave front aberration is 0.04 .lambda. (rms value). 
Moreover, the direction of polarization of the incident light is in a 
parallel direction with respect to the guide groove (E polarized light). 
FIG. 1 is a graph of the above measurement, showing the relationship of the 
values of land width to groove width ratios (land width/groove width 
values) at which the land reflectivity and the groove reflectivity in E 
polarized light become equal for various values of the difference in level 
of the land portions and the groove portions. It can be seen from FIG. 1 
that according to the groove depth, by making the land width wide in 
comparison with the groove width, it is possible to do away with the 
difference between the land reflectivity and the groove reflectivity. For 
instance, if the value of the land width/groove width ratio is 1.05 or 
more, the land reflectivity and the groove reflectivity can be 
approximately the same at a groove depth of about 100 nm. Equal land and 
groove reflectivity is similarly achieved if the land width/groove width 
value is about 1.08 and the groove depth is about 150 nm; if the land 
width/groove width value is about 1.16 and the groove depth is about 200 
nm; if the land width/groove width value is about 1.3 and the groove depth 
is about 230 nm; and if the land width/groove width value is about 1.4 and 
the groove depth is about 270 nm. 
Next, a setup similar to the above is disclosed according to a preferred 
embodiment of the present invention. However, in this case, the direction 
of polarization of the incident light is now in a direction at right 
angles to the guide groove (H polarization). FIG. 2 is a graph showing the 
relationship of the values of land width to groove width ratios (land 
width/groove width values) at which the land reflectivity and the groove 
reflectivity in H polarized light became equal for various values of the 
groove depth. It can be seen from FIG. 2 that for H polarization, in 
results similar to those in E polarization, making the land width wide in 
comparison with the groove width removes the difference between the land 
reflectivity and the groove reflectivity at the various groove depths. For 
instance, with H polarization incident light, equal land and groove 
reflectivity is achieved if the value of the land/groove width ratio is 
1.03 or more at a groove depth of about 100-170 nm. Equal land and groove 
reflectivity is similarly achieved if the land width/groove width value is 
about 1.1 and the groove depth is about 230 nm; and if the land 
width/groove width value is about 1.15 and the groove depth is about 280 
nm. 
The above results are substantially unaltered whether the sum of the land 
width and groove width is 1.4 .mu.m or 1.2 .mu.m. The distance between a 
land center and a groove center adjacent to the land center is 0.7 .mu.m 
or less. 
According to the above preferred embodiments of the present invention, even 
when the difference in level of the land portions and the groove portions 
is large, because the land reflectivity and the groove reflectivity can be 
made approximately the same, the playback signal level (carrier level) 
does not become correspondingly small when playing back data which was 
recorded on the land portions. Thus, an optical disk can be provided in 
which there are no readout errors of data. 
When the land width is greater than the groove width, manufacture of the 
substrate of the optical disk by injection molding is simplified. 
Moreover, a higher recording density may be accomplished by shortening the 
track pitch, especially if the track pitch is less than or equal to 0.6 
.mu.m. 
Furthermore, the present invention is more effective when a short wave 
length light source, for example a blue semiconductor laser beam 
(.lambda.:approximate to 420 nm, and the track pitch have to be in the 
range less than 0.5 .mu.m) is used. In such occasions, the heat influence 
from an adjacent track becomes larger because the light spot can be more 
concentrated. 
FIG. 3 is an elevated perspective view of an optical disk 100 having data 
stored on land portion 102 and groove portion 103 according to a preferred 
embodiment of the present invention. 
Although a few preferred embodiments of the present invention have been 
shown and described, it would be appreciated by those skilled in the art 
that changes may be made in these embodiments without departing from the 
principles and spirit of the invention, the scope of which is defined in 
the claims and their equivalents.