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Description  |
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This invention relates to novel light beam recording methods for use in the
recording of signal information on storage media and more particularly to
novel optical methods of recording video and audio signal information onto
master disc records having thin coatings of photoresist.
In U.S. Pat. No. 3,842,194, issued on Oct. 15, 1974 to Jon K. Clemens,
video disc playback systems of a variable capacitance form are disclosed.
In an arrangement therein described, an information track incorporates
geometric variations in the bottom of a spiral groove in a disc, the
surface of which comprises conductive material covered with a thin coating
of dielectric material. Variations in the capacitance presented between a
conductive electrode on a tracking stylus and the conductive material of
the disc occur as the disc is rotated by a supporting turntable; the
capacitance variations are sensed to recover the recorded information.
In one format employed for the groove bottom information track in practice
of the Clemens invention, depressed areas extending across the groove
bottom alternate with nondepressed areas, with the frequency of
alternation varying with the amplitude of video signals subject to
recording. The form of the recorded signal is thus a carrier frequency
modulated in accordance with video signals.
Optical techniques may be utilized for recording the aforementioned signal
on a disc master. Specifically, a composite color video signal, including
audio accompaniment and synchronizing signals is used to modulate a
carrier signal. The modulated carrier signal is then used to intensity
modulate a scanning laser beam. As successive groove bottom regions of a
grooved disc master, coated with a layer of light sensitive material
(forming a smoothly curved surface for the groove), pass through the laser
beam path, the regions are exposed in a pattern determined by the
intensity controlling signal. Upon subsequent development, a relief
pattern of geometric variations corresponding to the exposure pattern is
established in the groove bottom of the disc master.
A stamper disc may be derived from the recording master, by techniques
described in the Clemens patent, and utilized, through conventional record
stamping machinery, to produce a plurality of replica discs of
thermoplastic material. Each replica disc has a surface groove with a
relief pattern of geometric variations, corresponding to the pattern of
the video disc master, in the bottom thereof.
A significant element of an optical recording system as described above is
the ability of the recording lens to form the laser beam into a tightly
focused spot. The finite size of the focused spot effects an exposure on
the master disc surface of areas which should not be exposed (i.e., the
"on" recording beam overlaps in the "off" regions). Thus, the overall
response of the system is affected such that the response tends to roll
off at high spatial frequencies where the spatial frequency is a function
of the temporal frequency of the recording signal, the frequency of
rotation of the disc and the radial position of the recording beam on the
disc.
In an ideal optical recording system using a trapezoidally grooved master
disc having a thick (e.g., 2 .mu.m.) photoresist coated thereon where the
recording beam is turned on and off in response to the input signal, the
information track geometry would consist of an undisturbed groove bottom
interleaved with geometric variations or "pits" of substantially uniform
depth. In an optical recorder where the recording beam width is not
negligible compared with the signal wavelength, however, the pits tend to
become shallower at high spatial frequencies due to the nonuniform
frequency dependent response. The effect of this nonuniform response is to
introduce "signal track drop" into the record master during recording.
In the playback of a video disc with a stylus as disclosed in the Clemens
patent, the signal track drop results in a variation in the spacing
between the stylus electrode bottom and the groove bottom. Since the
dielectric support portion of the stylus is desirably of considerably
greater length (along the groove) than the conductive electrode portion,
the stylus will ride on the upper surface of the groove. As a consequence,
the maximum distance between the stylus electrode bottom (in a direction
perpendicular to the plane of the record) and the groove bottom (i.e.,
signal tops) is primarily determined by the height of the groove bottom at
low modulation frequencies (at low modulation frequencies there is little
or no track drop). When signal track drop is encountered undesirable
variations in the spacing between the stylus electrode bottom and the
groove bottom result.
These spacing variations affect the recovered signal by introducing
amplitude and phase interference therein. Although the amplitude
modulation is tolerable to a considerable degree (i.e., the influence of
these components can be reduced by limiter circuitry preceding the FM
demodulator in the playback apparatus), the phase modulation which cannot
be removed effectively introduces an interfering component in the output
signal.
In U.S. Patent Application Ser. No. 938,234, filed Aug. 30, 1978, in the
name of I. Gorog et al., now U.S. Pat. No. 4,206,477, apparatus is
described for effectively removing signal track drop. In an arrangement
described therein, a predistorted signal is recorded simultaneously with
the grooves.
In accordance with the principles of the present invention, signal track
drop is effectively removed by recording the information signals on a flat
substrate which has been thinly coated with a radiant energy sensitive
material (e.g., photoresist such as Shipley AZ1350B). In this technique
the nonlinear exposure characteristics of the photoresist, the thickness
of the photoresist, the interference pattern established by optical
reflection at the surface of the substrate by the recording beam and the
development time are coordinated to provide a record master.
In further accordance with the present invention, a method of recording
information signals modulated within a given frequency range
(illustratively, 4.3-6.3 MHz) on a flat reflective storage medium is
provided. In this method, a coherent light beam of a given wavelength
incident on the flat substrate is intensity modulated in accordance with
the modulated information signals. The interaction of the incident and
reflected light beam form an optical standing wave at the surface of the
substrate and throughout the photoresist. The method comprises coating one
surface of a substantially reflective substrate with a uniform layer of
nonlinear radiant energy sensitive material to a thickness typically
greater than the first standing wave maximum of the optical standing wave
in the radiant energy sensitive material but less than the first standing
wave minimum of the optical standing wave. Afer coating the substrate, the
substrate is moved relative to the intensity modulated light beam to
expose the radiant energy sensitive material with the modulated coherent
light beam in a pattern representative of the modulated information.
Finally, the exposed radiant energy sensitive material is developed for a
time period, depending on both the thickness of the radiant energy
sensitive material and its exposure, sufficient to form a track having
geometric variations therein representative of the modulated information
in the radiant energy sensitive material. The aforementioned method
effects the production of an information storage medium wherein the depth
of the geometric variations at the high frequency end of the given
frequency range are substantially equal to the depth of the geometric
variations at the low frequency end of the given frequency range.
As mentioned previously, it is sometimes desirable to record information
signals on a trapezoidally grooved, photoresist coated, storage medium. It
has been observed, however, that recording in such a medium can produce
microscopic signal element nonuniformities as well as signal track drop.
In further accordance with the principles of the present invention, a
method of optically recording information signals modulated within a given
frequency range on a grooved storage medium is provided in which the
microscopic nonuniformities and signal track drop are effectively removed.
In this method, a coherent light beam of a given wavelength incident on a
surface of the storage medium interacts with the reflected light beam to
form an optical standing wave in the storage medium. The method comprises
forming a groove having a cusp-shaped cross section on one surface of a
substrate. After grooving, the grooved surface of the substrate is
conformally coated with a uniform layer of partially absorbing, nonlinear
radiant energy sensitive material to a thickness greater than the first
standing wave maximum of the optical standing wave in the radiant energy
sensitive material but less than first standing wave minimum of the
optical standing wave. The coated substrate is then moved relative to the
light beam while the coherent light beam is simultaneously intensity
modulated with the modulated information signals to expose the radiant
energy sensitive material along the groove with the intensity modulated
light beam in a pattern representative of said modulated information. The
exposing step is followed by a developing step as described above.
In accordance with yet another principle of the present invention, a method
is provided for optically recording modulated information signals on a
cusp-shaped grooved substrate where the radiant energy sensitive material
does not conformally coat the groove. In this method, the coherent light
beam radial intensity profile is shaped to compensate for radial
variations in the thickness of the radiant energy sensitive material
within the cusped groove.
Further aspects of the present invention will be apparent from the more
detailed description which is described with reference to the accompanying
drawing.
In the accompanying drawing:
FIG. 1 is a cross-sectional view of a portion of an information track
illustrating track drop;
FIG. 2 provides a representation of the intensity profile of a standing
wave at the surface of a reflective substrate;
FIG. 3 defines a two-dimensional boundary for development time and
photoresist thickness;
FIG. 4 shows signal profiles at various stages of the development process;
FIG. 5 illustrates in graphical form the relative signal element depth for
signals recorded in a thick photoresist and signals recorded in a thin
photoresist in accordance with the principles of the instant invention;
FIG. 6 is a flow diagram of one method in accordance with the present
invention; and
FIG. 7 is a cross-sectional representation of a cusp-shaped grooved disc
that has been coated with a layer of radiant energy sensitive material.
The format employed for a groove information track using a capacitive
stylus as practiced in the aforementioned Clemens patent is remarkably
tolerant of variations in signal element geometry and stylus
characteristics. Capacitive stylii are, however, nonlinear and read the
recorded information with an amplitude and phase response that depends on
the height of the stylus over the surface. To insure an acceptable
signal-to-noise ratio in playback, the signal geometry must be maintained
within certain limits. FIG. 1 illustrates a cross-sectional view of an
information track. A recording system where a constant amplitude recording
signal is employed produces pit regions of relatively shallow depth
(illustratively P.sub.3, P.sub.4 in FIG. 1) at a high spatial frequency
such as 1/W.sub.2 in FIG. 1 where the spatial frequency 1/W is a function
of the instantaneous modulation frequency (f) of a high frequency FM
modulator, the disc rotation rate, and the radial position of the
recording beam on the surface of the disc record. At lower spatial
frequencies, such as 1/W.sub.1 in FIG. 1, pit regions of deeper depth are
produced (illustratively, P.sub.1, P.sub.2 in FIG. 1). Since the average
light beam intensity for an optical recorder is constant for the range of
frequencies to be recorded, a track drop TD1 is effected at the low
spatial frequency 1/W.sub.1, a track drop TD2 is effected at the high
spatial frequency 1/W.sub.2 and a relative track drop TD3 is effected
between signal elements at the low spatial frequency and signal elements
at the high spatial frequency.
It has been discovered that track drop, and therefore the signal geometry,
can be maintained within acceptable limits by taking advantage of the
nonlinear characteristics of the photoresist coated on the storage medium
and the interference effect of the incident recording light beam (e.g.,
laser) during the optical recording process.
FIG. 2 illustrates the interference pattern established at the surface of a
reflective substrate by a coherent light beam incident on said surface.
E(x) denotes the optical energy density as a function of the coordinate x.
The solid-line portion of the E(x) curve defines a standing-wave pattern
at the surface of a substrate and the dotted portion defines an analytical
continuation of this pattern inside the substrate. The energy density is
normalized so that in the absence of standing waves the total exposure is
unity at the point of signal maximum. The optical standing wave
established at the surface of the substrate also describes a standing wave
established in a weakly-absorbing photoresist coated on the surface
thereof. For simplicity, the standing wave shown in FIG. 2 disregards
reflections at the air-resist boundary and absorption in the photoresist.
The resist thickness x.sub.s is chosen to be less than x.sub.1 which is
the first minimum of the standing wave and greater than x.sub.m which is
the first maximum of the standing wave. For a 442 nm wavelength, HeCd
recording laser having a wavelength of 268 nm in a typical photoresist
(e.g., Shipley AZ1350B positive photoresist) x.sub.1 is chosen to be less
than 134 nm which is .lambda./2. The reflectivity of typical substrates
(e.g., copper) is less than unity and thus as shown in FIG. 2 a standing
wave minimum is not located at the substrate-resist interface.
The rate of photoresist development is, in general, a nonlinear function of
exposure. The nonlinear development characteristic and the standing wave
pattern established in a photoresist provide an opportunity to make a
significant improvement in the frequency response of the recording system
by matching the photoresist thickness and development time. The shaded
space "a" shown in FIG. 3 defines a two-dimensional boundary for
photoresist depth D.sub.0 (in angstrom units) and development time in
minutes within which, for a given set of signal standards, the signal
geometry can be maintained within acceptable limits. Of course, it should
be noted that the development times as disclosed in FIG. 3 are not
absolute and can be adjusted by appropriate dilution of the developer.
FIG. 4 illustrates the evolution of the signal pattern during development.
For a resist thickness of approximately 90 nm, the signal profiles are
plotted in 0.1 of a minute intervals between 0.1 and 0.9 of a minute
development time.
FIG. 5 demonstrates the calculated improvement of the high frequency
response of an optical recorder using a 300 nm
full-width-at-half-intensity Gaussian recording light beam (FWHI) as a
result of matching the nonlinearity and interference effects. The ordinate
of FIG. 5 defines the relative signal depth, peak-to-peak, while the
abscissa defines the modulation frequency for a disc record at the 3.25"
radius turning at 450 RPM. The spatial wavelengths (W) which correspond to
the modulation frequencies are also indicated on the abscissa of FIG. 5.
Curves b and c illustrate the relative signal depth for a disc master
where the recording is made on a thickly coated substrate without standing
wave enhancement and where the recording is made in accordance with the
principles of the present invention respectively. The difference between
curves b and c show an improvement from 0.35 to 0.85 relative response at
a 0.5 micron spatial wavelength by utilizing the techniques of the instant
invention. The significance of the above frequency-response improvement
can be further appreciated by reference to a specific example. In the
Clemens system, the ratio of the depth of the 6.3 MHz signal elements to
the depth of the 4.3 MHz signal elements advantageously is to be greater
than 0.7. In the absence of matching standing waves and non-linearities,
0.76 microns, which corresponds to 6.3 MHz at a 4" recording radius, is
the shortest signal wavelength recordable with a 300 nm FWHI Gaussian
beam. In accordance with the instant invention, this short wavelength
limit can be extended down to 0.40 microns corresponding to 6.3 MHz at a
radius of 2.1".
One explanation for the improvement in frequency response is as follows.
The presence of nonlinearities increases the differential rate of
development between high and low exposure regions of the resist. This
differential rate can then be utilized to flatten the frequency response
by choosing a combination of resist thickness and development time such
that in regions of maximum signal, the resist is developed down to the
substrate before significant development can occur in regions of minimum
signal. Furthermore, during development, the signal pit will reach the
substrate first in the regions of low-frequency-signal maxima. With
further development, the regions of high-frequency-signal maxima will
eventually also reach the substrate surface, and, at that time, the pit
depths at the low- and high-frequency-signal maxima are equal.
The foregoing assumes that the initial resist thickness is less than the
first interference minimum. If the resist thickness is greater than the
first interference minimum, then a standing wave minimum may occur inside
of the resist. With sufficient substrate reflectivity, the exposure at
this standing-wave minimum is sufficiently small to inhibit development
beyond the first minimum encountered by the growing signal pit during
development. Thus, the signal depth will be effectively clipped at the
plane of this minimum. Using a photoresist which is thicker than the first
interference minimum protects the substrate throughout the development
process by providing a layer of undeveloped resist. Frequently, when the
resist is fully developed to the substrate, contamination or etching of
the substrate surface may render it useless for subsequent reuse.
FIGS. 3 and 4 indicate that for a given resist thickness, the development
time must be carefully adjusted. Conversely, for a given development time,
the thickness variations must be maintained within a narrow range,
illustratively, less than 5 nm. Rate of etching of exposed photoresist is
nonlinearly related to the exposure. In general, increasing the
photoresist exposure-development nonlinearly (i.e., increasing the
exposure) and reducing the spot size will increase the tolerable thickness
variation and the allowed development time error. Outside the closed
boundary of the acceptable regions in FIG. 3, signal geometry fails for
the following reasons. For a given development time, if the thickness is
increased beyond the acceptable boundary, then the high frequency signal
becomes shallow and the signal-amplitude-ratio specification cannot be
met. As the thickness is further increased, eventually the depth of the
low frequency signal goes out of limit. If, for a constant development
time, the thickness is reduced below the acceptable boundary, the relative
track drop (TD3) increases and then, with further thickness reduction, the
track drop (TD1) also increases beyond its allowed range. Furthermore, if
the thickness is sufficiently reduced, the signal-depth requirement cannot
be met. All of these conclusions are fundamental consequences of the
combined effects of the standing waves and of the nonlinearities present
in the thin-resist exposure and development process.
A description of the theoretical considerations as described above is
provided at pages 427-457 of RCA Review, Vol. 39, No. 3, September 1978.
In the aforementioned recording technique, a grooveless record master is
produced. In some cases, for sample, where a stylus of the Clemens type is
used during playback, it is desirable to produce a record master having a
groove. As discussed above, it has been discovered that optical recording
on resist coated, trapezoidally grooved record masters may produce
unacceptable results due to the nonuniform response without using a thick
resist coating and predistorting the signals to be recorded. Additionally,
microscopic signal element non-uniformities may appear where the recording
is effected on a trapezoidally grooved disc. These non-uniformities are
caused by uncontrolled reflections of the light beam from the bottom and
side walls of the groove. It has been discovered that it is possible to
eliminate these undesirable nonuniformities by changing the groove
geometry and substrate structure in such a manner that the phase of the
interfering reflected wave is constant across the groove. A shallow
cusp-shaped groove, coated conformally with a thin layer of resist with
precisely controlled thickness, is one geometry which has provided
excellent results, i.e., the signal element nonuniformities have been
eliminated and the relative record response has been enhanced. It is
difficult, however, to effect perfect conformal coating of the groove.
A method of optical recording on a cusp-shaped, grooved master disc which
provides satisfactory results without perfectly conformally coating the
substrate will be described with reference to FIGS. 6 and 7. A metal disc
having a shallow cusp-shaped spiral groove on its surface is formed (see
FIG. 7). The spiral groove, which has a typical pitch of 2000-9000
convolutions per inch, is cut into the metal disc with a
smooth-radius-tipped tool. This metal disc may be formed in any of a
number of ways. For example, one method of providing a grooved disc
includes the steps of forming an aluminum blank about 14" in diameter and
one-half inch thick, then, machining the disc flat to a tolerance of
0.0002". A coating of copper is thereafter formed on the flat surface to a
thickness of about 0.003". The copper-coated disc is then placed in a jig
bore lathe and the copper surface polished to form a smooth, flat disc. A
diamond cutting stylus is thereafter utilized to inscribe a spiral groove
having the desired pitch in the copper surface of the disc.
Subsequently a layer of radiant energy sensitive material or photoresist
material is deposited on the grooved copper surface. A typical photoresist
that has been successfully utilized in this application is Shipley No.
AZ1350B photoresist. The photoresist material may be applied to the disc
surface by any one of a number of techniques. In one technique the disc is
spun at a rate in the range of about 200-2000 RPM (typically at about 450
RPM) while the radiant energy sensitive material of a viscosity of about
4.5 centipoise is applied (e.g. poured) on the grooved surface. The disc
rotation throws off the excess of radiant energy sensitive material
leaving a continuous coating over the grooved region of the disc.
Referring now to FIG. 7, a cross-section of the substrate 10 is shown
having a cusp-shaped groove 12 where R.sub.s is the radius of curvature of
the grooved bottom. After the photoresist 14 has dried, it will assume a
radius of curvature as shown by R.sub.r. The groove width and signal width
in the radial direction of the disc master are denoted by g and s
respectively. The resist thickness, which is thickest at the center of the
groove, is denoted by d. Illustratively, a 5,555--grooves-per-inch disc
format has typical dimensions of R.sub.s =7 microns, R.sub.r =8 microns,
s=3.5 microns and d in the range of 80-100 nm.
The groove geometry described with reference to FIG. 7 indicates that the
resist does not coat the substrate conformally. Illustratively, the
difference in resist thickness between that at groove center and at
locations.+-.1.75 microns off center is approximately 30 nm. This
differential thickness is outside of the range of allowable tolerance
indicated in FIG. 3.
After the photoresist layer has been dried, the disc is positioned on a
turntable under a coherent light source (e.g., helium-cadmium laser) for
exposure by the coherent source. Disc rotation is set at about 450 RPM.
During the signal information recording process, the light beam is
precisely focused upon the disc providing a small spot size suitable for
high resolution recording within the previously formed spiral groove.
Illustratively, the spot size is approximately 350 nm. Intensity
modulation of the light beam is utilized for recording the signal
information on the disc master. Relative motion is established between the
intensity modulated light beam and the disc surface such that the light
beam is caused to successively traverse the grooved region of the disc
exposing a pattern of elemental slots.
In the case of a nonconformally coated master disc as illustrated in FIG.
7, the systematic thickness variation across the grooves may be
compensated for by appropriately shaping the recording-beam intensity
profile. In one format a variable density filter is employed to shape the
recording beam profile. In this format a filter which passes the coherent
beam having the intensity undisturbed in the center and having the
intensity off center reduced by an appropriate amount is interposed
between the coherent source and the disc surface. By varying the intensity
of the recording beam across the groove, the photoresist variation can be
accommodated.
The exposed disc is then developed to remove photoresist material in the
exposed regions on the disc. In the thickness range of interest for
constant exposure, the thin regions develop faster than do the thick
regions. Since the off-track-center beam intensity with respect to its
on-center value has been reduced, the development time is chosen for the
thick center region (see FIG. 3). Development may be carried out in
Shipley No. AZ1350 Developer.
After the disc has been developed and the signal representative topography
formed in the spiral groove, the disc is replicated in a material such as
nickel. This nickel replica is a negative of the photoresist-coated disc
and may be utilized to stamp or emboss vinyl records for use in a signal
information playback system.
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