Method of and photomask for manufacturing optical memory element

A method of and a photomask for manufacturing an optical memory element. The manufacturing method includes the steps of: subjecting a positive type photoresist coated on a glass substrate to exposure, by using the photomask capable of irradiating light onto a portion of the photoresist other than the remaining portion, for forming pits on the glass substrate such that the portion of the photoresist is solubilized against developing solution; developing the photoresist by using the developing solution so as to remove the portion of the photoresist; and etching the glass substrate so as to directly form, as convex portions, the pits on a surface of the glass substrate.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of manufacturing an optical 
memory element enabling at least one of recording, reproduction and 
erasure of information and a photomask used directly for manufacture of 
the optical memory element. 
In recent years, optical disks for performing reproduction, etc. of 
information by using laser beams have been greatly developed. In an 
optical disk, a thin film of recording medium is deposited on a plastic 
substrate made of polycarbonate or acrylic resin and a protective member 
is formed on the thin film in close contact therewith. A spot of laser 
beams converged to a diameter of about 1.mu.m is irradiated onto the 
optical disk rotating on an optical disk player such that recording, 
reproduction and erasure of information are performed. 
Meanwhile, when the optical disk is being rotated by a rotary motor, tracks 
on the optical disk are likely to vibrate in axial and radial directions 
of the rotary motor due to distortion of the optical disk or run-out of 
the rotary motor. Therefore, unless a countermeasure for preventing this 
vibration of the optical disk is taken, the spot of laser beams cannot 
follow the tracks accurately. This is partly because the tracks on the 
optical disk are formed at an excessively minute pitch of about 1.6.mu.m. 
Thus, conventionally, an optical disk device is usually provided with a 
system for causing, through detection of vibration of the optical disk, 
the spot of laser beams to automatically follow signal pits recorded along 
the tracks, i.e. a so-called servo system. The servo system is of either a 
continuous servo type or a sampled servo type. 
In the continuous servo type, grooves are formed spirally or coaxially on 
the optical disk such that the spot of laser beams follows the grooves. On 
the other hand, in the sampled servo type, pits are formed discontinuously 
on the surface of the optical disk such that the pits cause the spot of 
laser beams to follow the tracks. 
FIG. 1 shows a known optical disk of sampled servo type. In the known 
optical disk, staggered pits (wobble pits) 2 and 3 are alternately 
provided at opposite sides of a track centerline 1, while a clock pit 4 is 
provided on the track centerline 1. As shown in FIG. 2, if the spot of 
laser beams travels along the track centerline 1 rightwards in FIG. 1, 
signal strength obtained at the time when the spot has passed through the 
left pit 2 becomes identical with signal strength obtained at the time 
when the spot has passed through the right pit 3. On the contrary, if the 
spot of laser beams travels along a line deviating from the track 
centerline 1, the above two signal strengths become different from each 
other. Hence, as shown in FIG. 3, if a difference between the two signal 
strengths is detected by a signal processing circuit 12, a signal (radial 
error signal) indicating the direction and how far the spot of laser beams 
deviates from the track centerline 1 can be obtained such that servo 
control in the radial direction is performed in accordance with this 
signal. Meanwhile, timing of sampling is determined by the clock pit 4. 
Servo control in the focusing direction is performed by sampling a signal 
obtained at the time when the spot of laser beams is located at a portion 
A in FIG. 1. Also in this case, timing of sampling is determined by the 
clock pit 4. 
Meanwhile, such optical disk (optical memory element) is manufactured by 
two methods. In one manufacturing method, plastic such as polycarbonate, 
acrylic resin, etc. is subjected to injection molding. The other 
manufacturing method is employed for manufacture of the continuous servo 
type optical disk. In the latter manufacturing method, pits or grooves are 
directly formed on a glass substrate. 
The latter manufacturing method of the optical disk is performed as shown, 
for example, in FIGS. 4a to 4f. Initially, as shown in FIGS. 4a and 4b, a 
photoresist 6 is coated on a glass substrate 5. Then, as shown in FIG. 4c, 
a photomask 7 having a desired pattern formed thereon is laid on the 
photoresist 6 in close contact therewith. Subsequently, exposure of the 
photoresist 6 is performed from above the photomask 7 so as to print the 
above mentioned pattern onto the photoresist 6. Then, as shown in FIG. 4d, 
the pattern printed on the photoresist 6 is developed so as to be 
transferred onto the photoresist 6 such that a photoresist pattern 6' is 
obtained. Subsequently, after the glass substrate 5 has been etched as 
shown in FIG. 4e, the photoresist pattern 6' is removed as shown in FIG. 
4f and thus, concave and convex portions are formed on the surface of the 
glass substrate 5 into the desired pattern. Therefore, for example, thus 
obtained grooves 11 are formed concavely relative to the remaining 
portions on the surface of the glass substrate 5. 
In the above described manufacturing method, portions having a high light 
transmittance and portions having a low light transmittance are formed on 
the photomask 7. The portions having a high light transmittance correspond 
to portions of the glass substrate 5, at which the pits or the grooves are 
formed, while the portions having a low light transmittance correspond to 
portions of the glass substrate 5, at which the pits of the grooves are 
not formed. One reason for such correspondence is that a resist having a 
high resolution on the order of submicrons required of the optical disk is 
restricted to a positive type. Namely, after development, the resist 
should not remain at the portions of the glass substrate 5, at which the 
pits are formed. Thus, if the pits are formed by using such positive type 
resist, portions of the photomask 7 corresponding to the pits of the glass 
substrate 5 should be the portions having a high light transmittance. 
Another reason for the above correspondence is linked with a manufacturing 
method of the photomask 7. In order to manufacture the photomask 7, a 
metallic film 9 is initially formed on a transparent substrate 8 as shown 
in FIG. 5a. Then, as shown in FIG. 5b, a photoresist 10 is coated on the 
metallic film 9. Subsequently, as shown in FIG. 5c, laser beams are 
irradiated onto the photoresist 10 so as to record a desired pattern on 
the photoresist 10. Then, after a photoresist pattern 10' has been formed 
by development as shown in FIG. 5d, the metallic film 9 is etched so as to 
obtain a pattern 9' of the metallic film 9 as shown in FIG. 5e. 
Subsequently, as shown in FIG. 5f, the photoresist pattern 10' is removed 
and thus, the above described photomask 7 is obtained. 
In this manufacturing method of the photomask 7, it is needless to say that 
the photoresist 10 is required to have a high resolution and therefore, 
should be of positive type. Therefore, the portions of the photomask 7 
corresponding to the portions of the glass substrate, at which the pits or 
the grooves 11 are formed, are necessarily the portions having a high 
light transmittance. 
By forming on the thus obtained glass substrate 5 a recording medium 
enabling recording, reproduction and erasure of information, for example, 
a recording medium made of material utilizing phase transition between 
crystalline phase and amorphous phase or magnetooptical material based on 
perpendicular magnetization, an optical memory element is obtained. Since 
the above described glass substrate 5 is employed in the optical memory 
element and glass itself has low hygroscopicity and low gaseous 
transmission, deterioration of the recording medium with time is 
restricted, so that a highly reliable optical memory element can be 
obtained. 
Meanwhile, when the photomask 7 is laid on the photoresist 6 coated on the 
glass substrate 5 so as to be brought into close contact with the 
photoresist 6, it is necessary to judge whether or not the photomask 7 is 
properly held in close contact with the photoresist 6 by visual inspection 
from above the photomask 7. If dust is present between the photomask 7 and 
the photoresist 6, interference fringes may be observed by interference of 
light when the photomask 7 is laid on the photoresist 6 in close contact 
therewith. If the glass substrate 5 having the interference fringes is 
subjected to exposure without eliminating the interference fringes, 
defective pits or grooves are formed at portions of the glass substrate 5 
corresponding to the interference fringes and thus, various servo systems 
or reproduction signals of the optical disk are adversely affected. 
Namely, it is essential for manufacture of the optical disk that inspection 
of close contact of the photomask 7 with the photoresist 6 is performed 
securely. In manufacture of the continuous servo type optical disk, since 
the portions of the photomask 7, which have a high light transmittance, 
are relatively large in number, the above inspection can be performed 
relatively easily. 
On the other hand, in the case of the sampled servo type optical disk, 
since the portions of the photomask 7, which have a high light 
transmittance, are restricted to portions corresponding to the staggered 
pits 2 and 3 and the clock pits 4, light transmission of the photomask 7 
as a whole is low and thus, it is difficult to perform the above 
inspection. Therefore, such an inconvenience is incurred that even if dust 
or the like is present, dust or the like cannot be found. Furthermore, 
such a problem arises that since inspection of close contact of the 
photomask 7 with the photoresist 6 is insufficient, defective optical 
disks may be manufactured. 
SUMMARY OF THE INVENTION 
In order to solve the above described disadvantages inherent in 
conventional manufacturing methods of the optical memory element, a method 
of manufacturing an optical memory element, according to one aspect of the 
present invention, is characterized in that when a positive type 
photoresist coated on a glass substrate is subjected to exposure by using 
a photomask capable of irradiating light onto portions of the photoresist 
other than the remaining portions for forming pits hereinafter considered 
as physical discontinuities of the glass substrate, the portions of the 
photoresist are solubilized against developing solution and then are 
removed. Subsequently, the glass substrate is etched such that the pits 
are directly formed, as convex portions, on a surface of the glass 
substrate. 
Meanwhile, in the photomask for manufacturing the optical memory element, 
according to another aspect of the present invention, portions for forming 
the pits of the glass substrate are so set as to have a lower light 
transmittance against exposure light than the remaining portions and are 
so provided as to be partially or wholly embedded in a transparent 
substrate. 
In the method according to one aspect of the present invention, when the 
positive type photoresist coated on the glass substrate is subjected to 
exposure, the photomask capable of irradiating light onto the portions of 
the photoresist other than the remaining portions for forming the pits, 
namely, the photomask having a high light transmittance as a whole is 
used. Therefore, inspection of close contact of the photomask with the 
photoresist can be performed securely and thus, an optical memory element 
having high quality can be manufactured. Furthermore, in the method of the 
present invention, since the pits are convexly formed on the surface of 
the glass substrate relative to portions of the glass substrate other than 
the pits, track servo control or focusing servo control is performed 
securely 
Meanwhile, in the photomask according to another aspect of the present 
invention, since portions having a low light transmittance are the 
portions for forming the pits and the remaining portions have a high light 
transmittance, light transmission of the photomask as a whole is increased 
remarkably. Therefore, when such photomask has been laid on the 
photoresist on the glass substrate so as to be brought into close contact 
with the photoresist, the interference fringes produced by dust, etc. 
present between the photomask and the photoresist on the glass substrate 
will be clearly observed and thus, it becomes possible to prevent 
occurrence of defective optical memory elements preliminarily. In 
addition, the portions having a low light transmittance are partially or 
wholly embedded in the transparent substrate. Hence, in the photomask of 
the present invention, the portions having a low light transmittance are 
less likely to sustain damage or failure in comparison with an arrangement 
in which the portions having a low light transmittance are so formed as to 
project from the surface of the transparent substrate, so that the 
photomask can be handled with much ease and has remarkably excellent 
reliability.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, there is shown in FIGS. 6 to 9, a method of 
manufacturing an optical memory element and a photomask 15 used in 
manufacture of the optical memory element, according to one embodiment of 
the present invention. As shown in FIG. 6, the photomask 15 for 
manufacturing the optical memory element is constituted by a transparent 
substrate 16 acting as portions having a high light transmittance against 
exposure light and a metallic film pattern 17' acting as portions having a 
low light transmittance against exposure light. In the photomask 15, the 
metallic film pattern 17' corresponds to portions for forming pits, 
namely, a pattern of the pits 21 formed on a glass substrate 19 as will be 
described later. The metallic film pattern 17' is so provided as to be 
wholly embedded in the transparent substrate 16. Needless to say, the 
metallic film pattern 17' may also be so provided as to be partially 
embedded in the transparent substrate 16. 
In the photomask 15 of the above described arrangement, the portions having 
a low light transmittance are limited to the portions for forming the pits 
21 and the remaining portions are the portions having a high light 
transmittance. Therefore, light transmission of the photomask 15 as a 
whole is increased remarkably. Furthermore, more, since the metallic film 
pattern 17', i.e. the portions having a low light transmittance are so 
provided as to be partially or wholly embedded in the transparent 
substrate 16, the metallic film pattern 17' is less likely to suffer 
damage or failure as compared with an arrangement in which the metallic 
film pattern 17' projects from the surface the transparent substrate 16. 
Therefore, the photomask 15 can be handled with much ease and is highly 
reliable in operation. 
In order to manufacture the photomask 15 referred to above, a positive type 
photoresist 18 is initially coated on the transparent substrate 16 made of 
glass or quartz as shown in FIG. 7a. Then, as shown in FIG. 7b, laser 
beams are irradiated onto the photoresist 18 so as to cut the photoresist 
18 into a desired pattern, for example, a pattern of sampled format. 
Subsequently, as shown in FIG. 7c, the pattern is developed into a 
photoresist patern 18'. Then, as shown in FIG. 7d, the glass substrate 16 
is etched such that recesses are formed in the glass substrate 16. This 
etching may be an arbitrary one of so-called wet etching employing 
hydrofluoric acid solution or its buffer solution and so-called dry 
etching in which gas such as CF.sub.4 or CHF.sub.3 is electrically 
discharged into plasma such that etching is performed by using the plasma. 
Thereafter, as shown in FIG. 7e, a metallic film 17 made of, for example, 
tantalum (Ta), titanium (Ti), chromium (Cr), etc. is formed by deposition 
or sputtering. The metallic film 17 is formed not only on the photoresist 
pattern 18', so as to cover the photoresist pattern l8', but in the 
recesses produced by etching so as to be embedded in the recesses. 
Finally, as shown in FIG. 7f, the photoresist pattern 18' is removed by 
using a proper solvent or stripping agent and thus, the photomask 15 is 
obtained. Meanwhile, when the photoresist pattern 18' is removed, the 
metallic film 17 disposed on the photoresist pattern 18' is also removed 
at the same time. Therefore, these portions of the photomask 15, which 
have the metallic film 17 and the photoresist pattern 18' removed 
therefrom, occupy the portions having a high light transmittance. On the 
other hand, since the metallic film 17 embedded in the recesses of the 
transparent substrate 16 remain rn the recesses as it is, the metallic 
film pattern 17' is formed by the metallic film 17 so as to form the 
portions having a low light transmittance. 
Hereinbelow, the method of manufacturing the optical memory element by 
using the above described photomask 15 is described with reference to 
FIGS. 8a to 8e. Initially, as shown in FIG. 8a, a photoresist 20 is coated 
on a glass substrate 19. Then, as shown in FIG. 8b, the photomask 15 is 
laid on the photoresist 20 in close contact therewith. Then, light is 
irradiated from above the photomask 15. Hence, light is irradiated onto 
only portions of the photoresist 20 other than the remaining portions for 
forming the pits 21 and thus, the portions of the photoresist 20 are 
solubilized against developing solution. As described above, light 
transmission of the photomask 15 as a whole is increased greatly. 
Therefore, when the photomask 15 has been laid on the photoresist 20 on 
the glass substrate 19 so as to be brought into close contact with the 
photoresist 20, interference fringes produced by dust, etc. present 
between the photomask 15 and the photoresist 20 can be observed clearly 
and thus, it becomes possible to distinguish defective optical memory 
elements from nondefective optical memory elements easily. 
Then, as shown in FIG. 8c, development of the photoresist 20 is performed 
so as to remove the solubilized portions of the photoresist 20 such that a 
photoresist pattern 20' is obtained. Subsequently, as shown in FIG. 8d, 
etching of the glass substrate 19 is performed so as to directly form, as 
convex portions, the pits 21 on the surface of the glass substrate 19. 
Meanwhile, this etching may be an arbitrary one of wet etching and dry 
etching in the same manner as in etching of the photomask 15 performed 
during its manufacture referred to earlier. Finally, as shown in FIG. 8e, 
the photoresist pattern 20' is removed by washing employing oxygen plasma 
or by using a stripping agent. Thus, as shown in FIG. 9, the pits 21 are 
convexly formed on the surface of the glass substrate 19. Thereafter, by 
forming on the surface of the glass substrate 19 a film of recording 
medium made of magnetooptical material containing Gd-Tb-Fe series 
amorphous substance, the optical memory element is obtained in which 
recording, reproduction and erasure of information can be performed and 
track servo control add focusing servo control can be performed securely 
by the convex pits 21. 
As will be seen from the foregoing description, in the manufacturing method 
of the optical memory element, according to one aspect of the present 
invention, when the positive type photoresist coated on the glass 
substrate is subjected to exposure, the portions of the photoresist other 
than the remaining portions for forming the pits are solubilized against 
developing solution by using the photomask capable of irradiating light 
onto the portions of the photoresist other than the remaining portions for 
forming the pits and then, are removed. Subsequently, the glass substrate 
is etched such that the pits are directly formed, as convex portions, on 
the surface of the glass substrate. Therefore, in accordance with the 
manufacturing method of the present invention, since inspection of close 
contact of the photomask with the photoresist can be performed easily, the 
high-quality optical memory element can be manufactured. Furthermore, 
since the pits are convexly formed on the surface of the glass substrate, 
track servo control and focusing servo control can be performed securely 
by using the convex pits. 
Meanwhile, in the photomask for manufacturing the optical memory element, 
according to another aspect of the present invention, the portions for 
forming the pits are so set as to have a lower light transmittance against 
exposure light than the remaining portions and these portions having a low 
light transmittance are partially or wholly embedded in the transparent 
substrate. 
In accordance with the photomask of the present invention, light 
transmission of the photomask as a whole is increased greatly. Thus, when 
the photomask has been laid on the photoresist and the glass substrate so 
as to be brought into close contact with the photoresist, interference 
fringes produced by dust, etc. present between the photomask and the 
photoresist can be clearly observed, so that manufacture of defective 
optical memory elements can be prevented in advance. In addition, since 
the portions having a low light transmittance are so provided as to be 
partially or wholly embedded in the transparent substrate, the portions 
having a low light transmittance are less likely to sustain damage or 
failure as compared with an arrangement in which the portions having a low 
light transmittance project from the surface of the transparent substrate. 
Therefore, the photomask of the present invention can be handled easily 
and has remarkably excellent reliability. 
Although the present invention has been fully described by way of example 
with reference to the accompanying drawings, it is to be noted here that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as being 
included therein.