Recording disc and method for fabricating same

A recording disc comprises a substrate having a tracking servo groove of spiral or concentric pattern on one surface thereof. The servo groove has a constant depth and a variable width which varies to allow generation of a position indicating signal. An eraseable recording layer is deposited on the surface of the substrate to store information signals. The recording disc is fabricated by a method involving modulating the intensity of a laser beam with a permanently stored signal and directing it to the surface of a photoresist layer on a master blank placed on a rotating turntable. The beam penetrates the full thickness of the layer. The optically excited portion of the layer spreads in lateral directions by an amount proportional to the energy. The turntable is moved relative to the laser to expose a portion of the photoresist layer in spiral or concentric pattern to the directed beam. The exposed portion is then etched to form the tracking servo groove. A replica of the groove is then form on a substrate followed by the deposition of a radiation sensitive layer on the substrate.

BACKGROUND OF THE INVENTION 
The present invention relates to a recording disc having spiral or 
concentric turns of a tracking servo groove and a method for fabricating 
such discs. 
One approach to high density disc recording involves forming a tracking 
servo groove on the disc surface to allow a recording or reading laser 
beam to precisely follow a desired path. 
A recording disc of the type shown and described in Japanese Patent 
Laid-open Publication 58-57643 is provided with a spiral pattern of 
tracking servo groove. The groove is modulated in depth so that lands are 
created at specified positions of tracks. The edges of the groove and the 
lands are sensed by diffraction of the laser beam to derive a tracking 
servo control signal and a position indicating signal. A recording layer 
is deposited on the groove to eraseably store information signals. 
However, the depth modulation results in the generation of a significantly 
high amplitude position signal which interferes with the information 
signal if the range of depth variations, or modulation index, is greater 
than a certain value, which is approximately 10%. However, this value of 
modulation index would require the disc surface to vary in depth within a 
range of 100 Angstroms which is impractical to manufacture. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a recording 
disc having a variable width groove which is easy to manufacture and a 
method for fabricating such discs. 
According to the invention, a recording disc comprises a substrate having a 
tracking servo groove of spiral or concentric pattern on one surface 
thereof. The servo groove has a constant depth and a variable width which 
allows a recording or reading laser beam to be modulated in intensity to 
derive a tracking control signal and a position indicating signal. An 
eraseable recording layer is deposited on the surface of the substrate to 
store information signals. For a satisfactory value of width modulation 
index, the groove is modulated in width in a range of 2000 Angstroms. 
According to a further aspect of the invention, the method for fabricating 
the recording disc comprises modulating the intensity of a laser beam in 
accordance with a position indicating signal which is to be permanently 
stored in the recording disc. The modulated laser beam is directed to the 
surface of a photoresist layer of a master blank placed on a rotating 
turntable. The laser beam penetrates the full thickness of the layer. The 
optically excited portion of the photoresist spreads in lateral directions 
by an amount proportional to the amount of the energy. One of the 
modulated laser beam and the turntable is moved relative to the other to 
expose a portion of the photoresist layer in spiral or concentric pattern 
to the directed beam. The exposed portion is then etched to form a 
tracking servo groove having a constant depth equal to the thickness of 
the layer and a width variable as a function of the optical energy of the 
laser beam. A replica of the groove is formed on a substrate followed by 
the deposition of a radiation sensitive layer on the substrate.

DETAILED DESCRIPTION 
Referring now to FIG. 1, there is shown an apparatus for producing a 
variable width, constant depth tracking servo groove on a master disc 
according to the present invention. The apparatus generally comprises an 
optical system 10 and a mechanical drive system 12. The drive system 
includes a turntable 14 driven by a motor 16 mounted on a carrier 18. 
Carrier 18 is threadably mounted on an externally threaded drive shaft 20 
which is supported at one end on a support 22 and connected at the other 
end to a motor 24. 
The optical system 10 includes a light modulator 26 which modulates the 
intensity of a laser beam emitted by a laser source 30 in accordance with 
address signals supplied from a signal source 28. The modulated laser beam 
is reflected by a mirror 32 to a lens system 34 which shapes the cross 
section of the light beam into a rectangular section with the longer sides 
of the rectangle being parallel with the radial directions of the 
turntable 14. 
On the turntable 14 is placed a master blank 36 below lens system 34. As 
shown in FIG. 2A, the blank 36 comprises a glass substrate 38 and a layer 
40 of photoresist deposited on the substrate 38. 
With the motors 16 and 24 being energized, the turntable 14 is spun at a 
constant angular velocity, for example. During each revolution the 
turntable is moved in a radial direction by the motor 24 to permit the 
light beam from lens system 34 to follow a spiral pattern. The operation 
starts with the laser beam positioned on the outermost convolution and 
terminates when the beam reaches the innermost convolution. 
As shown at FIG. 2B, the light exposed portion of the photoresist layer 40 
is etched away in a known manner to create a spiral groove 41 having a 
width that is variable as a function of the intensity of the beam incident 
on the photoresist 40, and hence in response to the address signals. 
As will be described later, the thickness of photoresist layer 40 and the 
minimum intensity of the laser beam are determined so that the beam 
incident thereon penerates the full thickness of photoresist layer 40 when 
its intensity is minimum, and therefore the groove has a uniform depth 
equal to the thickness of the photoresist layer at any intensity of the 
beam. 
As shown in FIG. 2C, aluminum or nickel is sputtered on the glass substrate 
38 to form a metal film 42 that follows the surface contour of the spiral 
groove 41. Nickel is then electroplated over the metal film 42 to form a 
metal lining 43 as a reenforcement. The metal film 42 and lining 43 form a 
die 44, which is removed from the glass substrate 38 to be stamped on a 
disc 45 of thermosetting resin to create a groove pattern 46 which is a 
replica of the groove 41 (FIG. 2D). A radiation sensitive layer 47 is then 
deposited in a known manner on the resin disc 45 (FIG. 2E) where spiral 
groove 46 has been formed. The radiation sensitive material is formed of a 
phase transformable material such as TeO.sub.x GeSn which changes between 
crystalline and amorphous states in response to the application of a laser 
beam or formed of a thermomagnetic material such as MnBi, MnCuBi and GdCo. 
These materials permit signals to be stored by application of laser beam 
and erased by reapplication of the beam. 
As shown in FIG. 3, the depth of exposure to light from the surface of a 
photoresist, and hence the depth of a groove created is proportional to 
the incident beam intensity until a critical point I.sub.t is reached. 
According to the invention, the laser beam at minimum intensity produces an 
exposure which is greater than the critical point I.sub.t and the groove 
produced by etching has a constant depth t.sub.o which is equal to the 
thickness of the photoresist layer. The laser beam incident on the 
photoresist has an intensity greater than the critical point. The portion 
of the photoresist layer which is optically excited by such laser beam 
spreads in lateral directions by an amount proportional to the amount of 
applied energy, creating a groove having a variable width proportional to 
the incident beam intensity. 
The laser beam has a normal value of intensity I.sub.c in the absence of 
the modulating signal. Upon modulation with the digital address signals, 
the beam intensity increases to a maximum value I.sub.h in response to the 
presence of a digital pulse of a first address signal indicating a track 
number, for example, and returns to the normal intensity in response to 
the absence of the pulses of the first address signal and decreases to a 
minimum intensity I.sub.l in response to the presence of a digital pulse 
of a second address signal representing a sector number, for example, and 
returns to normal in response to the absence of the pulses of the second 
address signal. As shown in FIG. 4, the width of the groove in the 
photoresist varies nonlinearly as a function of exposure. The minimum 
intensity is higher than the critical point I.sub.t and the normal 
intensity I.sub.c preferably corresponds to a point where the incremental 
rate of width variation begins to decrease with increase in exposure. 
A section of a variable width tracking servo groove 46 is shown in FIG. 5. 
When the beam is at normal intensity I.sub.c, the groove has a normal 
width W. In the absence of any address signals the normal width portion of 
the groove continues and a read beam senses the edges of this portion by 
diffraction in order to derive a servo control signal. When the beam is at 
maximum intensity I.sub.h, the edges of the groove are offset outwardly 
from the center of the groove to form a portion 50 having a larger width 
Wa and when it is at minimum intensity I.sub.l the edges of the groove are 
offset inwardly toward the center of the groove to form a portion 51 
having a smaller width Wb. As the read beam scans the groove it is 
maintained along its center in response to the servo control signal and is 
modulated in intensity with the width variations. 
Information signals are recorded in the portion of recording layer 47 
deposited on the groove 46. 
In comparison with the prior art depth modulation technique, the width 
modulation technique as taught by the present invention ensures greater 
tolerances in manufacture. For 10% modulation index, it is shown that the 
invention allows the groove to vary in width within the range of 2000 
Angstroms (typically, between 7000 and 9000 Angstroms) with a constant 
depth of 650 Angstroms. The variation range of the invention is thus 20 
times greater than the variation range of the prior art.