CA1191597A - Method for forming video discs - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method for forming a stamper for use in molding video disc replicas in which a disc-shaped master recording is formed which includes a glass substrate and a thin photore-sist recording layer. An information signal is recorded in the recording layer in a sequence of spaced pits arranged in a plurality of substantially circular and concentric recording tracks. A first thin, uniform, metallic film is deposited on the master recording. A second thin, uniform, metallic film is electroplated on the first film so that the first and second films together form an integral, metallic layer. The metallic layer is separated from the underlying master and the residual photoresist is removed from the undersurface of the separated metallic layer to form a stamper suitable for use in molding video disc replicas.

Description

~:~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.

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.

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 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 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.

Claims (3)

1. A method for forming a stamper for use in molding video disc replicas, comprising steps of: forming a disc-shaped master recording that includes a glass substrate and a thin photoresist recording layer, an information signal being recorded in the recording layer in a sequence of spaced pits arranged in a plurality of substantially circular and concentric recording tracks; vapor depositing a first thin, uniform, metallic film on the recording layer of the master recording; electroplating a second thin, uniform, metallic film on the first film, the first and second films together forming an integral, metallic layer;
separating the metallic layer from the underlying master recording; and removing residual photoresist material from the undersurface of the separated metallic layer, to form a stamper suitable for use in molding video disc replicas.

2. A method as defined in Claim 1, wherein: the first metallic film, formed in the step of vapor depositing, has a thickness in the range of about 500 A° to about 600 A°; and the second metallic film, formed in the step of electroplating, has a thickness of about 15 mils.

3. A method as defined in Claim 1, wherein both the first and the second metallic films are formed of nickel.