Optical recording disc

An optical recording disc which comprises a disc-shaped substrate plate, for example, a plate manufactured from synthetic resin, for example, polymethylmethacrylate, which has on at least one side an ablative recording layer, for example, an amorphorous layer of a tellurium mixture, in which the substrate on the side of the recording layer has extrinsic instabilities, in particular surface discontinuities. For example, the surface of the substrate is roughened or provided with scratches. The substrate may also be provided with a separate coating layer comprising the discontinuities, for example, an inhomogeneous vapor-deposited layer, a coarse-granular layer of a dye, or a light-cured lacquer layer which has been roughened or provided with scratches.

The invention relates to an optical recording disc in which information can 
be written and read optically, comprising a disc-shaped substrate which 
has an ablative recording layer on at least one side which upon exposure 
to energy-rich light modulated according to the information melts in the 
exposed places and forms holes. 
Such a disc is disclosed inter alia for example in Netherlands patent 
application No. 7,607,997. The disc described in said specification 
comprises a substrate plate on which a recording layer is present 
comprising at least 30 at .% Se and/or Te. Upon exposure to modulated 
laser light, liquid areas are formed in which the liquid in the exposed 
area then retracts while forming a hole which is surrounded by an upright 
edge of material originating from the liquid area. 
It is the object of the present invention to increase the sensitivity of 
materials for recording of information so that materials which previously 
were insensitive and in which an information bit (hole) could be provided 
only by the use of a comparatively large amount of laser light energy. For 
example to provide a means whereby amorphous materials on the basis of 
tellurium compounds, can also be used readily. This means that information 
bits (holes) can also be formed in such materials with a favourable amount 
of laser light. 
Another object of the present invention is to provide an optical recording 
disc in which very small holes can be formed with a small amount of laser 
light thereby increasing the information density of the disc. 
A further object of the present invention is to provide an optical 
information disc in which small holes of different diameters can be formed 
with a small amount of laser light energy. This permits an analog 
recording of information which also results in a much larger information 
density. 
The present invention is based firstly on applicant's recognition that a 
comparatively high energy must be overcome to initiate a hole in an 
ablative recording layer. This comparatively high energy barrier cannot 
exclusively be overcome by thermal activation of the liquid area. 
In non-prepublished Netherlands patent application No. 7,902,542 
corresponding to U.S. application Ser. No. 58,213 filed July 16, 1979, 
calculations are disclosed with respect to the required activation energy 
and associated minimum hole diameter. It is also stated in the Netherlands 
patent application that the Marangoni effect, that is to say a 
destabilizing process in a liquid film in which as a result of a gradient 
in the surface energy a mass transport occurs to those parts of the film 
having the highest surface energy, provides an important contribution to 
the required hole inititation energy. According to the Netherlands patent 
application, means are disclosed for optimizing the Marangoni effect. 
Applications have now gained the further recognition that extrinsic 
instabilities, i.e. instabilities which lie outside the recording layer in 
which the holes are formed, give a strong promotion of the hole 
initiation. 
It has been found experimentally that when employing the known recording 
discs, the liquid areas provided in the ablative recording layer by means 
of laser light have comparatively large diametrical dimensions before the 
liquid of said areas retracts to form holes having ridges. By providing 
extrinsic instabilities holes are formed in liquid areas (much smaller 
diametrical dimensions) obtained in the recording layer by exposure to 
laser light. This means that less laser light energy is required and a 
smaller hole will be obtained. It also means that ablative recording 
materials which so far have been too insensitive become useful. 
More particularly the invention relates more in particular to an optical 
recording disc of the type mentioned in the opening paragraph which is 
characterized in that the extrinsic instabilities are provided at the 
surface of the substrate adjacent to the recording layer. 
By way of example it can be stated that holes having a diameter of 
approximately 0.3 .mu.m can be provided by means of laser light in an 
amorphous recording layer of a tellurium alloy, for example a Ge/Te alloy, 
which has a thickness of 30-40 nm. The amount of laser light energy per 
hole (information bit) then is roughly 0.4 nJ and originates, for example, 
from a laser having an emission wavelength of 647 nm, a power of 5 mW on 
the recording layer and a pulse time of 75 ns. When the same laser is used 
but with a pulse time of 100 ns, liquid areas are obtained in the 
recording layer of larger diametrical dimensions which result in a hole 
having a diameter of approximately 1.0 .mu.m. This means that it is 
possible according to the invention to obtain holes of different 
diametrical dimensions in accordance with the amount of laser light energy 
used, of which holes the largest also still have a small diameter in the 
range up to, for example, 1.5 .mu.m. 
In addition to binary information recording the invention also provides an 
analog information recording, for example, a ternary recording (large 
hole, small hole, no hole) which has a significantly higher information 
density. As a matter of fact, for example, 8 bits are necessary for the 
binary (digital) recording of 200 characters (2.sup.7 &lt;200&lt;2.sup.8) and 
only 5 bits are necessary for ternary recording (3.sup.4 &lt;200&lt;3.sup.5). 
The extrinsic instability may be, for example, a chemical impurity in the 
surface of the substrate adjoining the recording layer, for example, atoms 
diffused in the substrate surface. 
In a favourable embodiment of an optical recording disc of the invention 
the surface of the substrate adjoining the recording layer contains a 
surface discontinuity at least in those locations where holes are formed 
in the overlying recording layer upon exposure. 
In this favourable embodiment the extrinsic instability has a mechanical 
character. At the area of the discontinuity the surface shows a difference 
in levels as a result of a local recess of the surface. The strong effect 
the surface discontinuity has on the hole initiation as a result of which, 
as already said, the liquid areas in the recording layer burst open with 
much smaller diameter and hence much sooner, are still not fully 
explained. However it is believed that this phenomenon can partly be 
explained as a result of an increased internal liquid pressure in the 
liquid area caused by laser light. At the area of the discontinuity the 
surface shows a ridge which, compared with a flat surface, has a reduced 
radius of curvature. The inwardly directed liquid pressure is inversely 
proportional to the radius of curvature and is significantly increased at 
the area of the discontinuity. 
It is of importance for the radius of curvature of the substrate surface to 
be as small as possible at the location of the discontinuity and hence for 
the above-mentioned ridge to make an angle with the substrate surface 
which is as acute as possible. 
For a better understanding of the phenomenon it may be noted that in 
applicant's opinion there exists an analogy with the action of a boiling 
stone which as is generally known promotes the initiation of a boiling 
process. 
In a further favourable embodiment of the recording disc with which 
excellent results are obtained, the surface discontinuity has diametrical 
dimensions of 30 to 300 nm and a level difference with the substrate 
surface of 8 to 80 nm, the recording layer having a thickness in the order 
of magnitude of 30-40 nm. 
The term diametrical dimensions is to be understood to also include the 
width dimensions of, for example, scratches provided in the substrate 
surface. 
In a favourable practical embodiment of the disc in accordance with the 
invention a substrate of glass or synthetic resin is used whose surface 
adjoining the recording layer has been roughened or provided with 
scratches chemically or mechanically, or of which the surface adjoining 
the recording layer is a copy of the surface of a matrix comprising the 
surface discontinuity. 
The chemical roughening can be achieved by treating the substrate surface 
with an etchant. As an example the roughening of a substrate manufactured 
from polymethyl methacrylate may be mentioned in which the substrate 
surface is treated for 5 to 10 seconds with a suitable etchant, in 
particular perchloric acid. 
The mechanical roughening may be obtained according to known methods in 
which, for example, the substrate surface is subjected to a bombardment of 
hard particles, as is known, for example, by the name of sand-blasting. 
Preferably a substrate is employed the surface of which is a copy of a 
matrix surface comprising the discontinuity. The manufacturing process of 
the recording disc is considerably simplified by this procedure because it 
is not necessary for each substrate disc to be roughened individually. 
Quite a large number of suitable substrate discs can be manufactured 
according to usual techniques by means of a matrix which comprises in its 
surface a negative of the discontinuity desired in the substrate surface. 
A suitable technique is, for example, a compression technique in which a 
quantity of a plastic synthetic resin, for example PVC, placed centrally 
on a lower matrix, is deformed at elevated temperature and pressure to a 
substrate disc by means of a die having the desired discontinuity. Instead 
of the dye, the lower matrix may be provided with the surface 
discontinuity. Alternatively, both matrices may have a surface 
discontinuity. Another technique is an injection moulding technique in 
which a liquid composition of, for example, a plastic synthetic resin, for 
example in particular polymethyl methacrylate, is compressed in a mould, 
the moulding cavity of the mould having one or two matrices comprising the 
surface discontinuity. 
The matrix is usually manufactured from metal, for example, nickel, and can 
be provided with a negative of the desired surface discontinuity, as 
already said chemically or mechanically. It is also possible to take the 
desired surface roughness into account during the manufacture of the 
matrix in which a master plate, which is a glass plate having a 
photolacquer in the desired configuration, is first covered in an 
electroless manner and is then electroplated with metal, after which the 
grown metal peel is removed from the master plate. In this manner it may 
be ensured that the surface of the photo-lacquer has the desired roughness 
due to the choice of photolacquer and photolacquer developer. 
In another favourable embodiment, the surface of the substrate facing the 
recording layer comprises a coating layer containing the surface 
discontinuity. 
A suitable coating layer is a non-continuous inhomogeneous, 
vapour-deposited layer. During the vapour deposition, a so-called island 
stage is reached in the first instance, in which the vapour-deposited 
material is deposited on the substrate to be coated as islands (insulae) 
which are distributed regularly over the substrate surface. A vapour 
deposition layer in the island stage can readily be used as a coating 
layer. An example hereof is a vapour-deposited layer of Au in the island 
stage. 
Another suitable coating layer is a coarse granular vapour-deposited layer 
of dye. A typical example hereof is a layer of vapour-deposited vanadyl 
phtalocyanine in which during the vapour deposition the temperature of the 
substrate to be coated is maintained at a minimum value of approximately 
60-80.degree. C. As a result of this tempering, a coarse granular layer of 
vanadyl phtalocyanine is obtained which can readily be used as the coating 
layer containing the surface discontinuity. 
In another preferred form of the disc according to the invention the 
coating layer is a light-cured layer of lacquer of which the surface 
adjoining the recording layer is roughened or provided with scratches 
mechanically or chemically or the surface of which adjoining the recording 
layer is a copy of the surface of a matrix comprising the surface 
discontinuity. 
The light-cured lacquer layer maybe, for example, a layer of ultraviolet 
light-cured lacquer on the basis of acrylic acid esters. Preferably the 
lacquer layer is provided on the substrate by means of a matrix which has 
the desired surface discontinuity. For this purpose a liquid, 
light-curable lacquer layer is provided on the matrix surface having the 
discontinuity and the substrate plate is then placed on the lacquer layer. 
The lacquer layer is exposed to ultraviolet light via the substrate or via 
the matrix, after which the substrate and the cured lacquer layer 
connected thereto and in which the texture of the matrix surface is 
copied, is removed from the matrix. The lacquer layer is finally provided 
with an amorphous recording layer, for example, a vapour-deposited layer 
of tellurium-germanium in a thickness of 30-40 nm. It is to be noted that 
for exposing the lacquer via the substrate or via the matrix, the 
substrate and the matrix, respectively, should be transparent to the 
ultraviolet light used. To be preferred is a transparent substrate 
manufactured from glass or a transparent synthetic resin, for example, 
polymethylmethacrylate, polysulphon, polycarbonate or PVC. 
In still another preferred embodiment actually reduced to practice, the 
substrate on the side of the recording layer has an optically readable 
servo track which partly has a relief structure of servo areas situated 
alternately at a higher and lower level. 
The servo track generally is spiral-like or constructed from concentric 
circles, the servo areas situated alternately at a higher and a lower 
level giving the track a crenelated profile. The longitudinal dimensions 
of the servo areas which show the form of blocks and pit, vary in 
agreement with the stored servo data and are roughly from 0.3 to 3.mu.. 
The difference in height between blocks and pits is roughly 0.1 .mu.m. 
The device according to the invention having a servo track has the 
significant advantage that the recording of information in the recording 
layer is accurately controlled by the servo data present in the servo 
track which comprise, for example, order with respect to the velocity of 
information recording and the location of recording. The laser light beam 
scanning the servo track via the transparent substrate, hereinafter 
referred to as servo laser light, transfers the servo data to a control 
mechanism which switches the laser light beam at a considerably higher 
energy content with which the desired information can be recorded in the 
recording layer. 
The recording of information preferably takes place in those parts of the 
recording layer which are situated on the servo track between the servo 
data. 
The servo track is provided in the substrate by means of a matrix which is 
provided with a servo track according to known and above-mentioned 
methods, for example, a moulding method or an injection moulding process. 
It is recommendable to use a matrix which in addition to the servo track 
also has the desired surface discontinuity. This is not necesssary because 
the substrate comprising a servo track can be roughened mechanically or 
chemically in an after-treatment on the side of the servo track or be 
provided with scratches or be provided differently with a coating layer 
comprising the surface discontinuity, for example, a vapour-deposited 
layer of Au in the insula stage or a coarse-granular layer of vanadyl 
phtalocyanine. The last-mentioned method, however, requires an extra 
process step. 
In an attractive embodiment the servo track is provided in a light-cured 
lacquer layer which is present on the substrate and also comprises the 
surface discontinuity. A recording disc according to the last-mentioned 
embodiment can be manufactured by providing a lacquer layer of a 
light-curable lacquer on the surface of a matrix comprising both the servo 
track and the surface discontinuity, then providing a transparent 
substrate plate, exposing the lacquer layer via the substrate plate to 
ultra-violet light, removing the substrate plate and the cured lacquer 
layer connected thereto in which the servo track and the surface 
discontinuity have been coupied from the matrix surface and providing the 
cured lacquer layer with a recording layer. A matrix with servo track can 
be manufactured by providing in the photolacquer layer of the master plate 
a servo track by exposure and development, which track is then copied in 
the grown metal peel (father plate), after which successively further 
galvanic prints, for example mother plates, son plates and matrices are 
manufactured. Before providing the surface discontinuity, the matrix 
having a servo track may be roughened or be provided with scratches 
chemically or mechanically or be covered with a coating layer comprising 
the discontinuity, for example, a vapour-deposited Au layer in the insula 
stage or a coarse granular layer of vanadyl phtalocyanine in a thickness, 
for example, of 30 nm. 
The recording layer used in the recording disc according to the invention, 
for example, is a metallic recording layer or a layer of a chalcogenide 
glass which can be provided by a vapour deposition or sputtering process 
and has a thickness of approximately 30-40 nm. A very suitable recording 
layer is an amorphous layer of a mixture of tellurium with one or several 
elements selected from the group As, Sb, Bi, Ge, Sn, Si, Ga, In, Tl, Se 
and S. Readily useful tellurium-containing recording layers are described 
inter alia in the non-prepublished Netherlands patent application No. 
7,902,543 corresponding to the U.S. patent application Ser. No. 5,677, 
filed July 11, 1979 in the name of applicants. 
The substrate used in the recording disc according to the invention 
preferably is transparent and is manufactured from glass or from the 
above-mentioned transparent synthetic resin. In such a disc, information 
can be recorded or read via the substrate by means of laser light so that 
the contaminations such as dust particules, present on the surface of the 
disc fall beyond the depth of focus of the objective which focuses the 
laser light on the recording layer.

Reference numeral 1 in FIG. 1 denotes a .mu.m thick substrate plate of 
polymethylmethacrylate roughened mechanically on one side. The resulting 
scratch profile 2 is shown diagrammatically in the Figure. The scratches 
have a width of approximately 50 nm and a depth of 20 nm. The surface 
provided on such a surface discontinuity comprises a vapour-deposited 
recording layer 3 of Te.sub.1 Se.sub.1 Sb.sub.1 in a thickness of 30 nm. 
The surface of recording layer 3 remote from the substrate plate shows a 
scratch profile 4 which corresponds to the scratch profile 2. In this 
connection it is to be noted that upon vapour-depositing the recording 
layer, the resulting layer follows the profile of the substrate surface. 
For recording information, the optical recording disc shown in FIG. 1 is 
exposed, via substrate plate 1, to pulsated laser light originating from a 
laser having a power of 2.3 mW on the recording layer, an emission 
wavelength of 800 nm and a pulse time of 500 ns. As a result of the 
exposure for 500 ns, a hole 5 is formed in the recording layer 3. The 
resulting hole has a diameter of 0.2 .mu.m and comprises a ridge portion 6 
which contains recording material which in a liquid form originates from 
the hole and has then solidified to form a ridge portion 6. 
Reference numeral 7 in FIG. 2 denotes a plate of polymethyl methacrylate 
having a thickness of 1 mm which comprises on one side a vapour-deposited 
coating layer 8 of vanadyl phtalocyanide the surface of which remote from 
the plate 7 shows a coarse-granular structure 9. A structure detail of the 
coarse-granular surface has diametrical dimensions of approximately 70-100 
nm. The coarse-granular layer is made by vapour-depositing vanadyl 
phtalocyanide at normal temperature (room temperature) and then keeping 
the vapour deposited layer at a temperature of 80.degree. C. for several 
hours. It is also possible during the vapour deposition of vanadyl 
phthalocyanide, to keep the plate at a temperature of 80.degree. C. The 
vapour-deposited coating layer 8 has a thickness of 26.5 nm and supports a 
vapour-deposited recording layer 10 of Te.sub.1 Se.sub.1 Sb.sub.1 in a 
thickness of 30 nm. The recording layer also has a coarse granular 
structure 11 which corresponds to that of layer 8. 
For recording information, the recording layer 10 is exposed to pulsated 
laser light originating from a laser having a power of 3 mW on the 
recording layer, an emission wavelength of 647 nm and a pulse time of 1 
.mu.s. As a result of the exposure for 1 .mu.s, a hole 12 is formed in the 
recording layer which is surrounded by a ridge portion 13 of recording 
material originating from the hole. The hole 12 has diametrical dimensions 
of 0.75 .mu.m. In the absence of coating layer 8 in which the recording 
layer of Te.sub.1 Se.sub.1 Sb.sub.1 provided directly on the plate 7 does 
not show a coarse-granular surface structure, the minimum hole dimension 
is at least 1.3 .mu.m. 
Reference numeral 14 in FIG. 3 denotes a 1 mm thick substrate plate of 
polymethyl methacrylate which is provided on one side with an 
ultra-violet-light-cured lacquer layer 15 in which a servotrack 16 is 
provided. Lacquer layer 15 is provided by providing a matrix surface 
comprising the servo track with a layer of liquid, light-curable lacquer 
on the basis of acrylic acid esters, laying hereon the glass plate, then 
curing the lacquer layer with ultra-violet light via the substrate and 
finally removing the assembly of glass plate and cured lacquer layer 
connected thereto in which the servo track has been couped, from the 
matrix surface. The servo track has a width of approximately 0.6 .mu.m and 
a depth of 0.2 .mu.m. Lacquer layer 15 has a coarse-granular layer of 
vanadyl phtalocyanine 17 in a thickness of 66 nm obtained by 
vapour-depositing vanadyl phtalocyanine at room temperature and then 
tempering the resulting layer at 80.degree. C. for 10 hours. Layer 17 in 
turn comprises a recording layer 18 of Ge.sub.15 Te.sub.85 in a thickness 
of 30 nm. The surface of layer 18 shows a structure 19 which corresponds 
to that of layer 17. 
For recording information, the recording layer 18 at the area of the servo 
track 16 is exposed via the substrate 14 to pulsated laser light 
originating from a laser having a power of 2 mW on the recording layer, an 
emission wavelength of 800 nm and a pulse time of 500 ns. As a result of 
the exposure for 500 ns, a hole 20 is formed in the recording layer 18 
having a diameter of 1.0 .mu.m and surrounded by a ridge portion 21 of 
recording material originating from the hole 20. If no coarse-granular 
coating layer 17 of vanadyl phtalocyanine is used and the recording layer 
18 consequently does not show a coarse-granular surface texture, a hole is 
formed in the recording layer only with a laser light power of 4.6 mW and 
a pulse time of 500 ns, which hole then has a diameter of 1.5 .mu.m. 
FIG. 4 is a photograph of a part of the surface of the recording layer 
shown in FIG. 3. The various servo tracks and the holes (information bits) 
provided in the recording layer at the area of the servo tracks and having 
a ridge portion are very clearly visible. The coarse-granular surface 
texture of the recording layer can also be readily observed. The broken 
line present in the lower part of the photograph is a measure line, each 
part of the line shown having a length of 1 .mu.m. 
FIG. 5 is further enlargement of FIG. 4 in which notably the 
coarse-granularity of the recording layer is clearly observable. 
Reference numeral 12 in FIG. 6 denotes a 1 mm thick substrate plate of 
polymethyl methacrylate which is provided on one side with an 
ultraviolet-light-cured lacquer layer 23 having a servo track 24 and a 
coarse granular (roughened) surface 25. Lacquer layer 23 with servo track 
24 and rough surface 25 has been obtained by providing on a 
coarse-granular matrix surface comprising the servo track a light-curable 
lacquer on the basis of, for example, acrylic acid esters, laying thereon 
the substrate plate, then curing the lacquer layer via the substrate with 
ultraviolet light and removing from the matrix the resulting assembly of 
substrate plate and cured lacquer layer connected thereto in which the 
servo track and the surface texture of the matrix have been copied. A 
matrix with servo track and coarse-granular surface can be manufactured, 
for example, by vapour-depositing at a temperature of 80.degree. C. a 
layer of vanadylphtalocyanine on a nickel matrix having a smooth surface 
and comprising the servo track. 
Lacquer layer 23 has a vapour-deposited recording layer 26 of In.sub.50 
Bi.sub.50 in a layer thickness of 30 nm. Recording layer 26 has a surface 
texture 27 which corresponds to that of layer 23. 
For recording information, recording layer 26 is exposed via substrate 22 
to pulsated laser light originating from a laser having a power of 2.5 mW 
on the recording layer, an emission wavelength of 676 nm, and a pulse time 
of 100 ns. As a result of the exposure for 100 ns, a hole 28 is formed in 
the recording layer 26 having a diameter of 0.2 .mu.m and comprising a 
ridge portion 29. When using a laser light power of 8.5 mW and a pulse 
time of 100 ns, a hole is obtained having a diameter of 0.75 .mu.m so that 
analog recording of information is possible.