Note: Descriptions are shown in the official language in which they were submitted.
~:~915~'7
METHOD FOR FORMING VIDEO DISCS
This invention relates generally to methods for
forming video discs, and, more particularly, to methods
for producing opti~ally-readable video disc masters and
stampers for use in forming video disc replicas.
S Op~ically~readable video disc replicas are useful
in storing vast quantities of information, usually in
the form of a fr~quency-modulated (f.m.) carrier signal,
with a high recording density. The f.m. signal is
typically recorded as a sequence of spaced pits or
bump6 arranged in a succession of substantially circular
and concentric recording tracks. Each pit and adjacent
space b~tween pits represents one cycle o ~he .m.
signal.
Disc replicas are typically formed in injection-
molding apparatus using disc-shaped s~ampers derived
from recordins masters. A recording master typically
includes a glass substrate having a disc shaped,
planar surface, with a thin recording layer .quch as a
metal film overlaying it. Xnformation i~ normally
recorded in the recording layer by focusing an
intensity-modulated writing beam of light onto the
layer using a radially-movable objective lens~ as the
master is rotated at a prescribed rate. The întensity
of the beam is modulated in accordance with th f.m.
-signal to be al~ernately greater than and less than a
predetermined threshold at which the metal ilm is
melted, whereby the successlon of spaced pits is formed
in the film. The succession of pits and spaces
preferably has a nominal duty cycle of 50/50, whereby
the signal is recorded with m; n;mllm second harmonic
distortion.
The present invention resides in methods employed
in the manufacture of video disc masters of a type
having a photoresist recording layer~ and in methods
for producing stampers from such masters. The mas-ter
includes a glass substrate with a smooth, planar surface
on which a thin, unifoxm recording layer of photoresist
is deposited. An f.mO information signal is recorded
in the photoresist re~ording layer using an intensity-
modulated writing beam of light, producing a succession
of spaced exposed regions arranged in a succession of
substantially circular and concentric recording tracks.
In one aspect of the invention, the glass substrate
is initially prepared by dispensing an adhesion promoter
such as stannous chloride onto the surface of the
substrate while the substrate is rotated at a relatively
low velocity, e.gO, about 75 to 100 r~p.m. The surface
is then rinsed with water while still being rotated at
the relatively slow velocity, thereby removing residual
adhesion promoter, after which the rinsed surface is
dried by being rotated at a relatively high velocity,
e.g., about 750 to 1000 r.p.m.
The met~od ~or preparing the substrate can further
include pxeliminary steps of polishing the surface
using a polishing compound having a submicron particle
size, and then cleaning the polished surface. The
cleaning step can include steps of flushing the surface
~first with a detergent solution and then with water,
drying the surface by rotating the substrate at the
relatively high velocity, and wiping the surface with
acetone to remove traces of dust and oil.
In another aspect of the invention, the photo
resist recording layer is applied to the prepared
surface of the glass substrate by dispensing a photo-
resis~ solution onto the surface as the substrate i5
being rotated at the relatively low velocity, by then
rotating the substrate at the relatively high velocity
to partially dry the photoresist solution and form a
layer having a substantially uniform thickness, and by
finally bakiny the photoresist-coated substrate in a
prescribed fashion to completely dry the photoresist
layer. The photore~ist solution is preferably Shipley
AZ 1350 photoresist having a viscosity of about 1.3
centipoise, and in the step of baking, the mas~er is
preferably baked at about 80 degrees centigrade, for
about 20 minutes~
In another aspect o~ the invention, a thin metal
layer i5 formed on the glass substrate prior to the
application of the photosensitive recording layer~
Still anoth~r aspect of the i~vention resides in a
method for ~electing an zptimum peak intensity for ~he
intensity-modulated writing beam of light used in
recording the f.m. siynal on the photoresist recording
layer. In ~hi~ aspect of the inven~ion,-a prc5cribed
test signal is initally recorded on thC disc in a
succession o~ narrow sets of reo~r~lng track~, each set
being recorded using a writin~ ~e~m havin~ a differen~
3~
peak intenslty. Since the photoresist layer is exposed
whenever the intensity of the be~m exceeds a prede-
termined threshold, a higher peak intensity re~,ults in
exposed regions of greater length. Each separate set
5 of recording tracks thus has a different duty cycle.
In o n e method, three or four sets of
tracks are recorded, having peak intensities that vary
in ~teps of about five percent. After development, in
~ which the exposed regions forming each track are trans-
formed into a succession o spaced pits, the developed
disc is ex~m;ned to determine the particular set of
tracks having spaced pits with a duty cycle closest to
an optimum value. The peak in~ensity of the beam is
then adjusted in accordance with this determination,
whereby the f.m. information signal can thereafter be
recorded with the optimum duty cycle on the rem~in;ng,
unexposed portions of the photoresist layer,
The test signal pxeferably has a prescribed,
constant frequency~ and the sets of xecording tracks
are preferably located near the periphery of the photo-
resist layer. Also, each set preferably includes
several hundred tracks, and is separated from an
adjacent set by a narrow unexposed band of the photo=
resist layer~ The developed photGresist layer is
preferably ex~mlned by scanning each set of tracks with
a reading beam of light, to produce a reflected beam
that is modulated in inten~ity in a~cordance with the
recorded pattern of spaced pits, and by then detecting
the modulated intensity and monitoring it using a
spectrum analyzer.
In yet ano~her aspect of the invention, the expo~ed
photoresist layer is developed ~y dispensiny onto the
layer, while it is rotating at the r~latively low
velocity te.g-. 75 to 100 rup.m,), first waterJ to
~ ~3~ 5~t7
pre~wet the layer, then both water and a developer
solution of a prescribed normality, to partially develop
the layer, and finally developer solution, alone, to
fully develop the layer. The developed photoresist
layer is then rinsed with water, ko eliminate residual
developer solution, and finally rotated at the rela~
tively high velocity of preferably 750 to 1000 r.p.m.,
to dry the developad layer. The photoresist layer is
preferably derived from Shipley AZ 1350 photoresist,
and the developer solution is preferably either
potassium hydroxide or sodium hydroxide, with a
normality of about q 230 to ~ 240 . In another more
detailed aspect of the invention, the step of dispensing
~oth water and developer solution has a time duration
of ~bout S to 10 seconds, the step of dispensing
developer solution, alone, has a time duration of about
20 seconds, and the step of rinsing has a time duration
of ~bout 30 to 60 seconds~
Still ano~her aspect of the invention resides in a
technique for producing a stamper from the developed
recording mas~er, for use in molding video disc replicasO
In this aspect of the invention, a first hinJ uniform
metallic film is vapor-deposited onto the developed
recording layer, after which a second ~hin, uniform,
metallic film is electroplated onto the first film, the
~wo films together forming an integral me~allic layer.
The integral metallic layer is then separated from the
underlying master recording, and residual photor~sist
material is removed from the undersurface of the sepa
rated metallic layer using a suitable solvent, thereby
foxming the stamper The first metallic film preferably
has a th.ickness of about 500 to 600 A, and the second
metallic film preferably has a thickness of about 15
mils. Both films are preferably formed of nickel.
5~t7
Many other aspects and advantages o~ the invention
will become apparent from the ~ollowi.ng detailed
description, taken in conjunction with the accompanying
drawings, which disclose, by way of example, the
principles of the invention. The accompanying
drawings illustrate the invention. In such drawings:
FIG. 1 is a simplified schematic diagra~ of
appara~us for xecording an f~m. information signal on a
recording master produced in accordance with ~he methods
of the pre~ent invention;
FIG. 2 is an enlarged plan view of a segment of
the recording master of FIG~ 1, howing a succession of
spaced exposed regions arranged in a plurality of
substantially circular and concentric recording tracks;
FIG~ 3 is a graph showing the modulated intensity
of the writing beam in the recording apparatus of
FIG. l;
FIGo 4 is a sec~ional, elevational view of a
portion of the recording mas~er, taken along a recording
track, and showing -the photoresist recording layer to
be exposed whenever the intensity of the writing beam
of FIG. 3 exceeds a predetermined threshold;
FIG. 5 is a sectional, elevational view of ~he
recording master of FIG. 4, after development to remove
the 5paced, exposed regions; and
FIG. 6 is a perspective view of a turntable
apparatus used in forming th~ photoresist recording
layer on the recording master of FIG. 1.
1 5~3~7
Referring now to the drawings, and particularly to
FIG. 1, there is shown apparatus for recording a
frequency-modulated (f.m.) information signal on a
recording master 11. The master includes a glass
substrate 13, with a smooth, planar upper surface on
which is deposited a photoresist layer 15 having a
prescribed r uniform thickness. The photoresist layer
is exposed whenever impinged by a beam of light having
an intensity that exceeds a predetermined recording
threshold.
The recording apparatus includes a writing laser
17 such as an argon ion laser for producing a writing
beam of light 19 having a prescribed intensity, and an
intensity modulator 21 for modulating the intensity of
the writing beam in accordance with an f.m. information
signal received on line 23. The recording apparatus
further includes a spindle motor 25 for rotating the
recording master 11 at a prescribed angular velocity,
and an objective lens 27 for focusing the intensity-
modulated beam onto the photoresist layer 15 of therotating master. The objective lens in mounted on a
carriage (not 5hown) that is radially movable with
respect to the master, so that the focused beam traces
a spiral pattern on the photoresist layer.
~s shown in FIGS. 2, 3 and 4, the intensity of the
intensity-modulated heam lg is alternately greater than
and less than the predetermined recording threshold of
~he photoresist layer 15, whereby a succession of
spaced exposed regions 29, arranged in a plurality o~
substantially circular and concentric recording tracks
31, is formed in the layer. Each exposed region and
adjacent space correspond to one cycle of the f.m.
r-~7
signal. FrG. 5 depicts the recording ~aster 11 after
development to remove the exposed regions, the master
then being in suitable condition for use in producing a
stamper.
The recording master 11 is initially prepared for
use with the recording apparatus of FIG. 1 using a
special process in which the upper surface of the glass
substxate 13 is first ground and polished, and then
cleaned. The photoresist layer 15 is then formed by
dispensing a photoresist solution onto the surface,
after which it is dried and baked in a prescribed
fashion. After baking, the recording master is in
suitable condition for recording. In another aspect of
the invention, a thin metal layer is formed on the
substrate prior to forming the photosensitive layer.
More particularly, the planar surface of the ylass
substrate 13 is initially prepared by first grinding it
in a conventional manner~ using an aluminum oxide
compound having about a nine-micron gritO The surface
is then polished using a zirconium oxide or cerium
oxide polishing compound of sub-micron particle size.
Cerium oxide has been found to polish the surface more
quicklyr but is generally more difficult to clean.
The polished surface of the glass substrate 13 is
cleaned in a special three-step process. First, the
surface is flushed with high~purity, de-ionized water
and brushed with a fine brush to remov2 most of the
polishing compoundO The de ionized water preferably
has a resistivity of 18 mega-ohms centimeter. The
cleaned surface of the glass substrate 13 is thereafter
inspected by examining it with the naked eye under a
high-intensity light. Under this light, defects such
as scratches ancl microscopic pits appear as point
3 ~
sources of scattered light. ~hen a defect is detected,
a microscope is used to measur~ its size~ If any
defects larger than 25 microns are detected, or if the
number of de~ects under 10 microns exceed one per
square millimeter, the ~ubstrate is rejected and the
polishing and cleaning sequences are repeated. The
surface is then wiped with acetone to remove any traces
of dust and oil introduced during handling.
Second, the surface is flushed wi~h a detergent
solution, and third, the surface is again flushed with
de ioniæed water for a period of about ten to twenty
minutes.
After cleanlng, the substrate 13 is placed on a
turntable, as shown in FIG. 6, and rotated at an angular
velocity of about 750-1000 r. p.m., to dry the surfac~.
FIG~ 6 depicts apparatus for use in forming the photo
resist layer 15 on ~he cleaned substrate 13. The
apparatus includes a variable speed motor 33 for
rotating the substrate in a prescribed fashion, and a
pivot arm 35 on which are mounted ~hree dispensing
tubes 37, 39, and 41 for dispensing de-ionized water, a
stannous chloride solution, and a photoresist solution~
respectively, in a prescribed sequence.
The substrate 13 is first xotated at an angular
velocity o about 75 to 100 r.pOm. r while stannous
chloride is dispensed onto the cleaned upper surface
through the dispensing tube 39. The pivo arm 35 is
pivoted manually so that stannous chloride is applied
to the entire surface. It is believed that the stannous
30- chloride molecules adh re to the cleaned surface of the
substrate, and thereby promote a subsequent adhesion of
the photoresist solution.
Water is then dispensed onto the surface through
the dispensing tube 37, to rinse off residual stannous
chloride solution, and the angular velocity of the
motor 33 is then increased to about 750-1000 r.p.m., to
dry the rinsed surface. The surface is now in proper
condition for dispensing of the photoresist solution.
The photoresist solution is prepared by diluting
Shipley AZ 1350 photoresist with Shipley AZ thinner,
which is believed to include cellosolve acetate, in a
ratio of about 3 to 1. This provides a solution having
a viscosity of about 1.3 centipoise, which is then
filtered to remove particles larger than about one half
micron. Viscosity can be measured by standard
techniques, such as, for example, using a Canon-Finske
viscometer.
The diluted photoresist solution is then dispensed
through the tube 41 onto the prepared surface of the
substrate 13, as the substrate is being rotated by the
variable speed motor 33 at an angular velocity of about
75-100 r.p.m. Again, the pivot arm 35 is pivoted
manually so that the solution is dispensed across the
entire radius of the substrate. At the speeds below
about 75 r.p.m., a film of substantially uniform
thickness can be achieved only if a relatively lengthy
spread time is allowed but this increases the likelihood
of contamination of the layer. At speeds above about
100 r.p.m., on the other hand, radial streaks and flow
marks can result, affecting the quality of the
subsequent recording of information. About 35 ml of
photoresist solution are required to fully coat a
substrate having a diameter of about 35.56 cm.
11
After the photoresist solution has been coated
onto the surface of the substrate 13, the angular
velocity of the motor 33 is increased to about 750-1000
r.p.m., until dry. This provides a prescribed, uniform
thickness for the photoresist layer 15.
Recognizing the fact that the thickness of the
photoresist layer 15 is inversely proportional to both
r.p.m. and temperature, the specific angular velocity
at which the substrate is rotated to partially dry the
photoresist solution can be conveniently adjusted to
provide the prescribed thickness. The appropriate
angular velocity can be determined in a conventional
manner using, for example, a Tolansky interferometer.
This technique provides an indication of the relative
thickness of the layer and, using an iterative process,
in which the angular velocity is successively adjusted,
the optimum velocity can be determined. If the
viscosity and temperature of the photoresist solution
can be maintained substantially uniform, this angular
velocity calibration need be performed only infre-
quently. It is presently preferred that the layer have
a thickness of about 1150 A° to 1350 A°, and replica
discs subsequently produced will have information-
bearing bumps or pits of corresponding height.
If it is determined that the dried photoresist
layer 15 is defective in any way (e.g., containing
radial streaks or foreign particles), the layer can be
removed using a suitable solvent such as Shipley AZ
thinner. A new layer can then be applied in the manner
described above.
12
After removal from the turntable apparatus of
FIG. 6, the recording master 11 is baked, to fully dry
the photoresist layer 15 and thereby maximize its
exposure tolerance. The master is preferably baked at
about 80° centigrade for about 20 minutes. These
parameters must be maintained to a tight tolerance, to
minimize variations in exposure tolerances when succes-
sively recording information on a number of recording
masters.
Since the exposure sensitivities of each of a
number of recording masters is likely to be slightly
different from the others, it is desirable to optimize
the peak intensity for the intensity modulated beam of
light 19 (FIG. 1) for each master, so that the f.m.
information signal can be recorded with an optimum
50/50 duty cycle. In accordance with another aspect of
the invention, a prescribed test signal is recorded on
each master 11 in a plurality of sets of adjacent
recording tracks, each set recorded with a different
peak intensity. After the test signal tracks have been
developed, using a developing technique described in
detail below, the recording master is examined to
determine the particular set of tracks having a duty
cycle closest to the desired 50/50 value. The optimum
peak intensity thereby can be determined and the f.m.
information signal thereafter can be recorded with the
optimum duty cycle on the remaining, unexposed portions
of the recording master.
The test signal preferably has a constant frequency
of about 7 to 8 MHz, and the signal is preferably
recorded on three or four sets of tracks each set
recorded with a peak beam intensity that varies by
about five percent. Each set is recorded for about 10
seconds, corresponding to several hundred recording
13
tracks. Also the sets are preferably separated from
each other by narrow bands of unexposed portions of the
photoresist layer 15, and are located in a narrow
region adjacent the inner periphery of the layer.
The successive sets of developed recording tracks
can be conveniently examined using a reading beam of
light (not shown) for scanning each set to produce a
reflected beam having an intensity that is modulated in
accordance with the recorded test signal. The reflected
beam is modulated in intensity because the reflectance
of unaltered portions of photoresist layer 15 is about
4 percent, whereas the reflectance of altered portions
of the layer is essentially zero. The modulated beam
is suitably detected and monitored in a conventional
spectrum analyzer, to determine the presence of second
harmonic distortion. This distortion is a minimum when
the test signal is recorded with the optimum 50/50 duty
cycle.
It is not necessary that the reading beam follow
any individual recording track in each set of tracks,
since the same test signal is recorded on adjacent
tracks. Care must be taken, however, to ensure that
eccentricities in the recording master 11 do not cause
the reading beam to scan more than a single set of
tracks at a time. The reading beam is preferably
produced by a helium-neon laser so that its wavelength
will not expose the photoresist layer 15.
Using the recording apparatus of FIG. 1, with the
peak intensity of the writing beam 19 adjusted to the
prescribed, optimum value, the f.m. information signal
is then recorded on the remaining, unexposed portion of
the photoresist recording layer 15. The recorded
master 11 is then developed to convert each recording
14
track into a succession of spaced pits of uniform depth
and width and of continuously-variable length.
In yet another novel aspect of the invention, the
exposed recording master 11 is developed in a special
process in which a succession of fluids are dispensed
onto the master as it is being rotated by a turntable
of the type depicted in FIG. 6, at a velocity of about
75 to 100 r.p.m. In the process, water is first
dispensed to pre-wet the layer, then both water and a
developer solution of prescribed normality are dispensed,
to partially develop the layer, and finally developer
solution, alone is dispensed, to fully develop the
layer. The developed photoresist layer is then rinsed
with water, to remove residual developer solution,
after which the angular velocity of the rotating master
is increased to about 750-1000 r.p.m., to dry the
developed layer.
The preferred developer solution is selected from
a group including potassium hydroxide and sodium
hydroxide, and has a normality of about 0.230 to 0.240.
Preferably, the step of dispensing both water and
developer solution has a duration of about 5 to 10
seconds, the step of dispensing developer solution,
alone, about 20 seconds, and the step of rinsing about
30 to 60 seconds.
A stamper, suitable for use in molding video disc
replicas, is produced from the developed recording
master 11. In another aspect of the invention, the
stamper is produced by first vapor depositing a uniform
metallic film of about 500-600 A° thickness onto the
photoresist recording layer 15, and then electroplating
a second uniform metallic film of about 15 mils thick-
ness onto the first film. The undersurface of the
first film conforms exactly to the pattern of spaced
pits formed in the photoresist layer, and the two films
together form an integral metallic layer. This integral
metallic layer can be separated from the underlying
recording master and residual photoresist material
removed using a suitable photoresist thinner, thereby
forming the stamper. In one embodiment, both
metallic films are formed of nickel.
It will thus be appreciated from the foregoing
description that the present invention provides a
number of novel techniques for use in efficiently
producing recording masters. The masters include
photoresist recording layers in which f.m. information
signals can be recorded with high signal-to-noise
ratios and high density. In other aspects of the
invention, metallic stampers are produced from these
recording masters, for use in molding replicas of the
master.
Although the invention has been described in detail,
it will be understood by one of ordinary skill in the
art that various modifications can be made without
departing from the spirit and scope of the invention.
Accordingly, it is not intended that the invention be
limited, except as by the appended claims.
