Information recording medium having a glass substrate

An information recording disk for recording an information signal along a track defined by a depression on the disk as a change of physical property of a recording material deposited on the track comprises a disk-shaped glass substrate made of a glass and carries a groove corresponding to the track. The groove has a surface roughness substantially smaller as compared to the surface roughness caused at a bottom surface of a groove on a silica glass substrate when both the disk-shaped glass substrate and silica glass substrate are dry-etched under same conditions. The disk-shaped glass substrate comprises SiO.sub.2 component and one or both of Al.sub.2 O.sub.3 and BaO components with substantially no alkali components. Further, a recording layer comprised of the recording material which changes in physical property responsive to a projected energy beam is deposited on the substrate such that the depression is formed in correspondence with the track.

BACKGROUND OF THE INVENTION 
The present invention generally relates to information signal recording 
media and in particular to a disk-shaped information recording medium on 
which an information signal is recorded by means of an energy beam such as 
a laser beam (optical beam) or an electron beam. 
Optical information recording medium such as an optical disk or a 
magneto-optical disk hereinafter referred to as a disk is recorded with an 
information signal such as a video signal or an audio signal modulating an 
optical beam which moves along a spiral or concentric guide groove 
provided on the surface of the disk. At the time of reproduction, the 
guide groove is irradiated by an optical beam and the information signal 
is reproduced by processing the optical beam reflected back from the guide 
groove. For this purpose, tracking of the the optical beam must be 
controlled such that the optical beam traces the guide groove properly. 
Such a tracking of the optical beam is achieved by means of a known servo 
control system which controls the optical beam on the basis of the optical 
beam reflected back from the guide groove. The same tracking principle is 
known to be applicable to recording the information along a preformed 
guide track. Thus, the guide groove is used not only for storage of 
information signals but also for maintaining a proper tracking of the 
optical beam at the time of recording and reproducing of the information 
signal on and from the disk. The guide groove is usually a spiral-shaped 
or concentric continuous groove but may be a series of intermittent pits 
as in the case of a reproducing only type optical disk such as a so-called 
Compact Disk. 
The information recording medium having the guide groove or pits as 
aforementioned is manufactured by impressing a pattern on a metal stamper 
which is an inversion of the pattern of the groove or pits to be formed on 
the surface of the disk. The disk may be formed by injection molding or 
compression molding of a thermoplastic resin using the metal stamper as 
the mold. The manufacture of the disk by the injection molding or 
compression molding has a high productivity and is suited for automatic 
production. On the other hand, the disk thus produced has a problem in 
that the impression of the guide groove or pits by the stamper is not 
satisfactorily precise. Further, the disk tends to show birefringence, and 
the disk is deformed by the moisture in the air. Thus, these problems of 
the conventional plastic disk will create difficulties at the time of 
recording and reproducing. 
In order to eliminate these problems, use of silica glass for the substrate 
of the disk is proposed. The use of silica glass as the substrate of the 
disk is advantageous in that the disk thus produced has a small thermal 
expansion and shows virtually no absorption of moisture. Further such a 
disk has negligible birefringence. In order to provide the guide groove or 
pits on the surface of such silica glass disk, a layer of UV-cure resin 
which is a resin cured by ultraviolet radiation is deposited on the 
surface of the glass. In detail, a layer of UV-cure resin is first applied 
to the surface of the stamper in an uncured state and the resin is covered 
by the silica glass disk so as to be sandwitched between them. Next, an 
ultraviolet light is irradiated on to the resin through the glass disk and 
the resin is cured. The stamper is then removed and a two layered disk 
comprising a glass substrate and a layer of the cured resin carrying the 
pattern of groove or pits thereon is obtained. The disk thus produced is 
superior as compared with the plastic disk of thermoplastic resin in that 
a resin having a low viscosity at room temperature can be used and the 
groove or pits on the stamper is transferred to the plastic layer more 
accurately as compared to the case of the conventional disk molded from 
the usual thermoplastic resin. However, this procedure involves delicate 
steps of sandwitching uncured resin layer as well as of the separation of 
the stamper from the cured resin layer which pauses difficulties in 
automatization of its manufacture. 
The disk produced by molding of the thermoplastic resin or by application 
of the UV-cure resin on the silica glass substrate is further deposited 
with a reflection layer by vacuum vapor deposition or by sputtering. 
During such procedure, there is a problem that water is released from the 
cured resin or molded plastic due to the heating and the structure and 
property of the reflection layer become deteriorated. 
In order to eliminate this problem, provision of the guide groove or pits 
directly etched on the surface of the silica glass substrate is proposed 
in the U.S. Pat. No. 4,655,876 as well as in the Japanese Laid-open Patent 
Application No. 26951/1986 in which the respective assignee and the 
applicant are same as the assignee of the present application. According 
to the procedure proposed by the aforementioned patent and patent 
application, a layer of photoresist is applied on a polished surface of a 
silica glass substrate. Then, the pattern of guide groove or pit is 
written on this photoresist by means of a focused laser beam. Then, after 
a development of the laser exposed photoresist, the substrate is subjected 
to a dry etching such as a plasma etching using a plasma gas such as 
CF.sub.4. The plasma gas selectively reacts with the silica of the glass 
and the silica material at the portion of the disk not covered with the 
photoresist is removed by the reaction. On the other hand, the portion of 
the silica masked by the photoresist is not subjected to the reaction. The 
reaction is continued until an intended groove depth is reached. After the 
groove reaches the intended depth, the plasma gas is changed to a gas 
containing oxygen (O.sub.2) and the remaining photoresist is removed by 
reaction with the oxygen. 
However, the disk thus produced shows an unsatisfactory signal-to-noise 
ratio when the recording and reproduction is made after deposition of 
recording layer and protection layer on the disk. More specifically, error 
in the reproduced signal as well as the tracking error of the optical beam 
were found to be excessive for a satisfactory recording and reproducing 
operation of the disk. The reason for this was studied by electron 
microscopic observation of the disk and it was discovered that 
deterioration in the S/N ratio is caused by the irregularity or roughness 
at bottom of the groove. Such irregularity produces an unstable reflection 
of the optical beam. When the surface roughness exceeds about 100 .ANG., 
the reflected optical beam becomes too unstable for satisfactory operation 
of the recording and reproducing system and the proper reproduction of the 
information signal or proper tracking of the optical beam is lost. At 
present, it is impossible to etch silica glass without causing 
irregularity at the bottom of the groove. Thus, it is not possible to 
obtain disk to provide satisfactory recording and reproducing results. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to provide an 
information recording medium and a manufacturing method thereof wherein 
the aforementioned problems are eliminated. 
Another object of the present invention is to provide an information 
recording medium having a depression on its surface for recording an 
information signal wherein the roughness at the bottom of the depression 
is minimized so that the signal-to-noise ratio of a reproduced signal 
reproduced from the recording medium is improved. 
Another object of the present invention is to provide a method of forming a 
depression on a surface of a recording medium comprised of a glass 
material by etching, wherein the roughness at the surface of the 
depression is substantially minimized so that the signal-to-noise ratio of 
a reproduced signal reproduced from the recording medium is improved. 
Another object of the present invention is to provide an information 
recording medium having a depression on its surface for recording an 
information signal wherein the recording medium comprises a glass 
substrate of a barium borosilicate glass containing alumina and barium 
oxide but free from alkalis and alkali earth elements and having an 
etching rate which is less than sixty percent as compared to the etching 
rate of the silica glass. According to the present invention, the 
roughness at the bottom of the depression is successfully reduced as a 
result of slow etching rate and the signal-to-noise ratio of the 
reproduced signal is substantially improved. Further, the recording medium 
of the present invention has a substantially negligible birefringence, 
virtually absorbs no water, and can be easily manufactured.

DETAILED DESCRIPTION 
The present invention is based on a series of experiments exploring the 
relation between the etching rate and surface roughness at the bottom of a 
depression or groove formed as a result of etching. The experiments were 
conducted for various types of glasses as is summarized in Table 1 and a 
depression or groove having width of about 2 .mu.m and a depth of about 
0.1-0.5 .mu.m in correspondence with the actual depth of the groove of the 
disk is formed in a spiral formation on the surface of the glass by the 
plasma etching technique. For a convenience of observation, the width of 
the groove (about 2 .mu.m) is chosen to be slightly larger than the usual 
width (0.5-0.8 .mu.m) of the groove formed on the actual disk so that one 
can measure the depth of the groove precisely by a probe. Each of the 
glasses were polished so that the surface roughness becomes less than 30 
.ANG. before the start of the experiments. The plasma etching was achieved 
under a total pressure of 2.0.times.10.sup.-2 Torr using CF.sub.4 as the 
plasma gas and a high frequency power (RF power) of 200 watts is supplied 
in order to establish the plasma. 
Referring to Table I, the type II silica glass is a silica glass containing 
OH radical amounting to about 150-400 ppm and the type III silica glass is 
a silica glass containing OH radical amounting to up to 1000 ppm. The type 
II and type III silica glasses are commercially available glasses under 
the trade name of Heralux and Suprasil, respectively. The chemical 
composition of the glasses used in the experiments is listed in Table I. 
As usual, the chemical composition is represented in percent by weight of 
the respective components in the form of oxide. 
TABLE I 
______________________________________ 
sample Heralux* Suprasil* 
#7740 plate 
#0317 #7059 
i.d. glass 
Type Type II Type III boro- soda- 
soda- barium 
of silica silica silicate 
lime alumi- 
boro- 
glass glass glass glass glass 
no sili- 
silicate 
cate 
etching 470 360 380 83 93 107 
rate 
(A/min) 
surface*** 
X X X O O O 
roughness 
SiO.sub.2 
100 100 81 71 61 49 
Na.sub.2 O 4 15 13 
K.sub.2 O 3 
MgO 4 4 
CaO 7 
Al.sub.2 O.sub.3 2 2 17 10 
B.sub.2 O.sub.3 13 15 
BaO 25 
______________________________________ 
*trade name 
***surface roughness 
X: unacceptable (&gt;100 .ANG.) - 
O: substantially less than 100 .ANG.- 
From Table I, it can be seen that the type II silica glass, type III silica 
glass and the borosilicate glass (#7740) having large etching rates show 
rough surfaces having a surface roughness exceeding 100 .ANG. while the 
soda lime glass (plate glass commonly used as a panel), soda 
aluminosilicate glass (#0317) and the barium borosilicate glass (#7059) 
having small etching rates show smooth surfaces having a surface roughness 
substantially less than 100 .ANG.. Thus, the etching rate and the surface 
roughness are proportional to each other and as the etching rate 
increases, the surface roughness increases. From Table I, it can be seen 
that the glass samples which showed acceptable surface roughness were 
etched with an etching rate of 83 .ANG./min (plate glass), 93 .ANG./min 
(#0317) and 107 .ANG./min (#7059). These etching rates are substantially 
smaller than the etching rate for the silica glass samples and it can be 
safely concluded that an acceptable surface roughness is obtained when the 
etching rate is substantially less than 60% of the etching rate of the 
silica glass. 
From the Table I, it can be seen also that the etching rate is increased 
with increased content of SiO.sub.2 and B.sub.2 O.sub.3 component and is 
decreased with increased content of Na.sub.2 O, K.sub.2 O, MgO, CaO, 
Al.sub.2 O.sub.3 and BaO components. Generally, a satisfactory surface 
roughness is obtained when the content of the Na.sub.2 O, K.sub.2 O, MgO, 
CaO, Al.sub.2 O.sub.3 and BaO components exceed about 10% in weight 
individually or in combination. 
The following Table II indicates the temperatures at which the vapor 
pressure equals 10 Torr for various fluoride species. These fluorides are 
formed as a result of reaction between the plasma gas and the oxide 
components in the glass. Thus, the SiO.sub.2 component in the glass reacts 
with the CF.sub.4 plasma gas to form the SiF.sub.4 component. Similarly, 
the Na.sub.2 O, K.sub.2 O, Al.sub.2 O.sub.3 and B.sub.2 O.sub.3 components 
in the glass form the NaF, KF, AlF.sub.3 and BF.sub.3 components in the 
plasma gas as the product of reaction. The temperatures listed in Table II 
characterize the thermodynamic properties of the components in the glass 
and can be regarded as a characteristic temperature characterizing the 
thermodynamic properties. It is noted that the SiO.sub.2 and B.sub.2 
O.sub.3 components have relatively low characteristic temperatures while 
the Na.sub.2 O, K.sub.2 O and Al.sub.2 O.sub.3 have relatively high 
characteristic temperatures. 
TABLE II 
______________________________________ 
Temperature at which the vapor pressure of the 
fluoride component reaches 10 Torr 
Fluoride Temperature 
______________________________________ 
SiF.sub.4 -130.4 
NaF 1240 
KF 1039 
AlF.sub.3 1324 
BF.sub.3 -141.3 
______________________________________ 
The low characteristic temperature indicates that the corresponding oxide 
component in the glass reacts fast with the plasma gas and the etching 
rate is high. On the other hand, the high characteristic temperature 
indicates that the corresponding oxide component in the glass reacts slow 
with the plasma gas and the etching rate is decreased. Thus, the 
observation in Table I that the increase in the SiO.sub.2 and B.sub.2 
O.sub.3 components in the glass increases the etching rate and the 
increase in the Na.sub.2 O, K.sub.2 O, MgO, CaO, Al.sub.2 O.sub.3 and BaO 
components decreases the etching rate is supported by thermodynamic 
consideration. In the sample #7059, the B.sub.2 O.sub.3 component is 
comparable to that of the sample #7740, however, the effect of the B.sub.2 
O.sub.3 component in the sample #7059 is considered to be masked by the 
existence of the Al.sub.2 O.sub.3 and BaO components and further by the 
decrease of the SiO.sub.2 component. 
In the experiments described heretofore, the soda lime glass and the soda 
aluminosilicate glass which are the glasses commonly used for general 
purpose also falls in the preferable range as far as the etching rate and 
surface roughness are concerned. However, such glasses contain alkalis 
such as Na.sub.2 O and K.sub.2 O as well as the alkali earth elements such 
as MgO and CaO which tend to cause corrosion or pinholes in the recording 
layer of the optical and magneto-optical disk. Thus, these glasses are not 
eligible for the substrate of the disk. In other words, the glass material 
to be used for the substrate of the optical and magneto-optical disks 
should not only show the etching rate less than 60% of the etching rate of 
the silica glass but should also be free from alkali which include one or 
more elements selected from the group consisting of sodium, magnesium, 
potassium, and calcium. In the optical disk of the present invention to be 
described, the barium borosilicate glass containing alumina and barium 
oxide (#7059) is used for the substrate of the disk. This glass contains 
the Al.sub.2 O.sub.3 and BaO components in more than 10 and 25 percent by 
weight respectively. Note that the total content of Na.sub.2 O, K.sub.2 O, 
MgO and CaO in sample No. 7059 is less than about 1 wt. % as shown in 
Table I. 
As the Na.sub.2 O, K.sub.2 O, MgO and CaO components are undesirable in the 
constituent of the glass because of the corrosion and pinhole formation as 
previously described, the glass to be used for the substrate of the disk 
should be the one containing Al.sub.2 O.sub.3 and BaO components amounting 
to more than 10 percent by weight individually or in combination. At the 
same time, such glass should show an etching rate which is less than 60% 
of the etching rate of the silica glass. 
Next, manufacturing of the disk will be described with reference to FIGS. 
1(A)-(G) which show a series of manufacturing steps of a disk 10 which is 
illustrated in the completed form in FIG.1(G). In the present embodiment, 
the disk 10 is an erasable optical disk having a TeOx recording layer 
which changes the reflectivity responsive to the recording made by 
irradiation of the optical beam. However, the present invention is by no 
means limited to this particular type of the disk but may be applicable to 
other optical and magneto-optical type disks in general. 
Referring to FIG. 1(G), the optical disk 10 comprises a disk-shaped 
substrate 16 bounded by a flat bottom 16a, a TeOx recording layer 18 
deposited on a surface 16b of the substrate 16 and a protection layer 19 
formed on the recording layer 18. The protection layer 19 may be 
transparent or may be opaque and may be bounded by a flat surface 19a or 
may be bounded by a surface 19a which is not substantially flat. The 
substrate 16 is made of the #7059 glass in Table I which is transparent to 
an energy beam used for recording and reproducing the information signal 
on and from the recording layer 18 and having the etching rate less than 
60% of the etching rate of silica glass. The substrate 16 is formed with 
grooves 13 having a concentric or spiral-shaped pattern on its surface 16b 
and the surface 16b of the substrate is covered by a recording layer 18 of 
TeOx. TeOx is a non-stoichiometric compound of Te and TeO.sub.2 and causes 
a phase transition responsive to the heating by a relatively intense 
optical beam. Responsive to the phase transition, the reflectivity of the 
TeOx layer 18 is changed and the recording of information signal is 
achieved as a change in the reflectivity of the recording layer 18. The 
recorded information is erased by annealing the TeOx layer 18 by a 
relatively low energy optical beam. TeOx may be doped with Ge and Sn. The 
principle of the erasable recording system using TeOx as the recording 
medium is well known and no further description will be given. The guide 
groove has a width of less than 1 .mu.m corresponding to the beam spot of 
the optical beam used for the recording and reproducing of the information 
signal as is usual in the art. Similarly, the depth of the groove is less 
than 0.1 .mu.m as is usual. 
Referring to FIG. 1(A), the substrate 16 is first applied with a 
photoresist 12 with a uniform thickness. The photoresist 12 may be any 
photoresist commonly used in the patterning of semiconductor chips. Then, 
a focused laser beam 17 is irradiated on the surface of the photoresist 
continuously while revolving the substrate 16 and the photoresist 12 
unitarily around a central axis of the substrate as shown in FIG. 1(B). At 
the same time, the laser beam 17 is scanned on the surface of the 
photoresist 12 and a spiral-shaped pattern is drawn on the surface of the 
photoresist 12 by the laser beam. The spot of the laser beam on the 
surface of the photoresist, the seed of revolution of the substrate 16, 
and the speed of scanning of the laser beam are chosen such that a 
predetermined spiral pattern is written with a predetermined thickness. If 
the laser beam is intensity-modulated, a series of pits will be recorded 
accordingly along the predetermined spiral pattern. 
Next, the photoresist 12 is developed as shown in FIG. 1(C) wherein a 
portion of the photoresist 12 irradiated with the laser beam is removed. 
In other words, the portion of the substrate corresponding to the portion 
of the photoresist irradiated with the laser beam 17 is exposed. 
The substrate 16 having a reminder of the developed photoresist is then 
brought into a reaction chamber of a plasma etching apparatus (not shown). 
The reaction chamber is supplied with a CF.sub.4 gas and the exposed 
portion of substrate is subjected to dry etching in a plasma 14 of 
CF.sub.4 shown in FIG. 1(D). During this plasma etching, the etching rate 
is controlled so that the etching rate is less than 60% of the etching 
rate of the silica glass and a spiral groove 13' is formed. The groove 13' 
corresponds to the guide groove 13 of the disk. The groove 13' thus formed 
has a bottom surface 13a which is sufficiently smooth and the surface 
roughness of the surface 13a is limited to substantially below 100 .ANG.. 
The etching is continued until the groove 13' reaches a predetermined 
depth as shown in FIG. 1(D). The plasma etching apparatus is well known 
apparatus commonly used in the manufacturing of the semiconductor 
integrated circuit chips and the description thereof will be omitted. 
Next, the gas supplied to the reaction chamber is changed from the CF.sub.4 
gas to an O.sub.2 and the remaining photoresist 12 is removed by reaction 
with the plasma gas now containing O.sub.2. This procedure is called 
ashing. As a result, the substrate 16 is formed with the groove 13' is 
obtained as shown in FIG. 1(E). 
The substrate 16 thus obtained is then deposited with a layer 18 of TeOx 
compound by sputtering as shown in FIG. 18(F). The thickness of the layer 
18 is usually about 0.1 .mu.m and the guide groove 13 is formed on a 
surface of the layer 18. Then the protection layer 19 is applied on the 
layer 18 by applying a resin such as the UV-cured resin or by depositing 
SiO.sub.2, Si.sub.3 N.sub.4, SiC or C on the surface of the layer 18 by 
sputtering or vacuum evaporation. Thus, the disk 10 as shown in FIG. 1(G) 
is obtained. When the protection layer 19 is formed by the resin, the 
surface 19a may be generally flat. On the other hand, when the protection 
layer 19 is provided by sputtering or vacuum evaporation, the surface 19a 
is formed with a pattern corresponding to the groove 13 on the recording 
layer. Because the reduced surface roughness at the bottom of the guide 
groove at which the incident optical beam is reflected, the S/N ratio in 
the optical beam reflected back from the guide groove of such disk is 
improved and the recording and reproduction of the information signal on 
and from the disk is performed satisfactorily. 
Further, the disk 10 thus obtained is subjected to an environmental test 
for reliability. The disk 10 is held at a temperature of 60.degree. C. and 
relative humidity of 90% for a period exceeding 1000 hours. In this test, 
no appearance of corrosion or pinholes was observed. Thus, the disk of the 
present invention can be stored without problem for a prolonged period of 
time. 
Although the manufacturing of the disk 10 used the laser beam 17 for 
writing the pattern of the guide groove 13 on the photoresist 12, the 
patterning of the guide groove 13 on the photoresist 12 may be achieved by 
irradiation of ultraviolet light using a photomask formed with a pattern 
of the guide groove, 13, or by using an electron beam projected on the 
layer of photoresist in the vacuum chamber. 
Further, the gas used for the plasma etching is not limited to the CF.sub.4 
gas but may be any gas containing fluoride vapor such as CHF.sub.3 as far 
as they react with the glass. For example, the plasma gas may contain 
C.sub.2 F.sub.6, C.sub.3 F.sub.8, NF.sub.3 and a mixture thereof. 
FIG. 2 shows a second embodiment of the disk of the present invention. In 
the drawing, those portions corresponding to the portion already 
illustrated in FIGS. 1(A)-(G) are given identical reference numerals and 
the description thereof will be omitted. In order to facilitate the 
understanding of the drawing, the protection layer 19 is not illustrated 
in FIG. 2. Referring to the drawing, the guide groove 13 is defined by a 
pair of tapered portion 13b which extends along the groove 13 at the both 
sides which also subject the reflection of the optical beam, thus the 
tracking of the optical beam is improved. Such a tapered portion may be 
formed according to a method disclosed in the aforementioned U.S. Pat. No. 
4,655,876. 
Further, the guide groove is not limited to the spiral groove but may be a 
plurality of concentric formation. 
Further, the present invention in not limited to the embodiment described 
heretofore, but various variations and modification may be made without 
departing from the scope of the invention.