CA1165613A - Method for developing photoresist layer on a video disc master - Google Patents

Abstract

METHOD FOR FORMING VIDEO DISCS
ABSTRACT OF THE DISCLOSURE

Methods for use in producing a disc-shaped recording master having a thin photoresist recording layer for storing an f.m. information signal with high density. During recording, an intensity-modulated writing beam is focused onto the photoresist layer, to form a succession of spaced exposed regions arranged in a plurality of substantially cir-cular and concentric recording tracks. The master further includes a glass substrate having a specially prepared surface, and the photoresist layer is deposited on the surface using a technique that ensures a uniform thickness and a uniform sensitivity to the writing beam. The peak intensity of the writing beam is selectively adjusted such that the succession of spaced exposed regions is formed with an optimum 50/50 duty cycle. After development to remove the spaced exposed regions, the recording master is used to produce a metallic stamper that, in turn, is used to produce disc replicas. Developing is effected by first dispensing water on the photoresist layer to pre-wet it, then both water and developer solution are dispensed and finally developer solution alone.

Description

METHOD FOR FORMING VIDEO DISCS

This invention relates generally to methods for forming video discs, and, more particularly, to methods for producing optically~readable video disc masters and stampers for use in forming video disc replicas.

Optically-readable video disc replicas are useful in storing va~t quantities of information, usually in the form of a frequency-modulated ~f.m.) carrier signal, with a hi~h recording de~sity. The f.m. signal is typically xecorded aæ a sequence of spaced pits or bumps arranged in a succession of substantially circular and concentric recording tracks. Each pit and adjacent ~pace between pits represents one cycle of t~he f.m~-signal~

Disc replicas are typically formed in inj~ction-moldiny apparatus using disc-shaped stampers derived from recording masters. A recording master typically includes a glass substxate haviny a disc shaped, planar surface, with a thin recording layer 3uch as a metal film o~erlaying it. Information is normally .r ;

5 ~
z recorded in the recording layer by focusing an intensity-modulated writing beam oE light onto the layer using a radially-movable objective lens, as the master is rotated at a prescribed rate. The intensity of the beam is modulated in accordance with the f.m.
signal to be alternately greater than and less than a predetermined threshold at which the metal film is melted, whereby the succession 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 minimum 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 mastersO The master includes a glass substrate with a ~mooth, planar surface on which a thin, uniform recording layer of photoresist is deposited. An f.m. information signal is recorded in the photoresist recording layer using an intensity-modulated writing beam of light, producing a su~cession of spaced exposed regions arranged i~ a succession of substantially circular and concentric recordiny tracks.

In one aspect of ~he 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 ~elocity, e.g., about 75 to 100 r.p~m. The surf~ce 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.

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The method or preparing the substrate can further include preliminary steps of polishing the surface using a p~lishing compound haviny 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 hi.gh 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 appli~d to the prepared surfàce of the glass substrate by dispensing a photo-resist solution onto the surface as the substrate is being rotated at the reiatively 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 baking the photoresist-coated substrate in a prescribed fashion to completely dry the photoresist layer. The photoresist solution is preferably Shipley AZ 1350 photoresist having a viscosity of about 1.3 centipoise, and in the step of baking, ~he master i5 preferably baked at about 80 deyrees centigrade, for about 20 minutes.

In another aspect of the invention, a thin metal layer is formed on the glass substrate prior to the application of the photo~ensitive recording layer.
Still another aspect of ~he in~ention re~ides in a method for selecting an optimum peak intensity for the intensity-modulated writing beam of light used in recording the f.m. ~ignal on the photoresist recording layer. In ~h.is aspect of the invention~-a prcscribed test signal is initally recorded on th~ disc in a succession of narrow sets of re~rding tracks, each set being recorded using a w~itin~ beam having a dif2rent 3 $ 6 1 3 peak intensity. Since the photoresist layer is exposed whenever the intensity o~ the beam exceeds a prede-termined threshold, a higher peak intensity re~ults in exposed regions of greater length. Each separate set 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 steps of about five percent. After development in which the exposed regions forming each track are trans-formed into a succession of spaced pits, the developeddisc is examined to determine the particular set of tracks having spaced pits with a duty cycle closest to an optimum value. The peak intensity of the beam is then adjusted in accoxdance with this determination, whereby the f.m. information signal can thereafter be recorded with the optimum duty cycle on the remaining, unexposed portions of the photoresist layer.

The test signal preferably has a prescribed, constant frequency, and the sets of recording tracks are preferably located near the periphery of the photo-resist layer. Also, each set prefera~ly includes several hundred tracks, and is ~eparated ~rom an adjacent set by a narrow unexposed band of the photo-resist layer. The developed photoresist layer i5 preferably examined by scanning each set o tracks with a reading beam of light, to produce a refle~ted beam that is modulated in intensity in accordance with the recorded pattern of spaced pits, and by then detecting the modulated intensity and monitoring it using a spectrum analyzer.

In yet another aspect of the invention, the exposed photoresist layer is developed by dispensing onto the layer, while it is rotating at the relatively low velocity (eOg., 75 to 100 r.p.m.), first water, to l 3 pre-wet the layer, then both water and a developer solution of a prescribed normality, to partially develop the layerl and finally developer solution, alone, to fully develop the layer. The developed photoresist layer is ~hen rinsed with water, to sliminate residual developer solution, and finally rotated at the rela~
tively high velocity of preferably 750 to 1000 r.p.m., to dry the developed 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 .230 to .240. In another more detailed aspect of the invention, the step of dispensing both water and developer solution has a time duration of ~bout 5 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 another aspect of the invention resides in a technique for producing a stamper from the developed recording master, for use in molding video disc replicas.
In this aspect of the invention, a fixst thin~ uniform metallic film is vapor-deposited onto the d~veloped recording layer, after which a second thin, uniform, metallic film is electroplated onto the first film, the two films together forming an integral metallic layer.
The integral metallic layer is then separated from the underlying master recording, and residual photoresist material is removed from the undersurface of the sepa-rated metallic layer using a suitable solvent, therebyforming the stamper. The first metallic film preferably has a thickness of about 500 to 600 A, and the second metallic film preferably has a t~ickness of about 15 mils. Both films are preferably formed of nickel.

Many other aspects and advantages of the invention will become appaxent from the following detailed description, taken in conjunction wi~h the accompanying drawings, which disclose, by way o~ example, the 5 - principles of the invention. The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a simplified schematic diagram of apparatus for recording an f.m. information signal on a recording master produced in accordance with the methods of the present invention;

FIG. 2 is an enlarged plan view of a segment of ~he recording master of FIG. 1, showing 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;

FIG. 4 is a sectional, elevational view of a portion of the recording master, taken along a recording track, and showing the photoresist recording layer to : be expo~ed whenever the intensit,y of the writing beam of FIG. 3 exceeds a predetermined threshold~

FIG. 5 is a sectional, elevational view of the recording master of FIG. 4, after development to remove the spaced, exposed regions; and FIG. 6 is a perspective view of a turntable apparatus used in forming the photoresist recording layer on the recording master of FIG. 1.

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- 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, uniform thickness. The photore~ist 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 ar~on 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 layex 15 of therotating master. The objective lens in mounted on a carriage (not shown) that i~ radially movable with respect to the master, so that the focused beam trac0s a spiral pattern on the photoresist layer.

As shown in FIGS. 2, 3 and 4, the intensity of the intensity-modulated beam 19 is alternately greater ~han and less than the predetermined recording ~hreshold o the photoresist layer 15/ whereby a succession of spaced exposed regions 29~ arranged in a plurality of substan~ially 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.

9 I t~ . 3 : signal. FIG. 5 depicts the recording master 11 a~ter deYelopment to remoYe the exposed xegions, 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 substrate 13 is first ground and polished, and then '3 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 o the glass substrate 13 is initially prepared by first grinding it in a conventional manner, using an aluminum oxide compound having about a nine~micron grit. The surace 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 surfac~ more quickly, but is generally more difficult to clean~

The polished sur~ace of the glass substrate 13 is cleaned in a special three-step pro~ess. First, the surface is flushed with high-purity, de-ionized water and brushed with a fine brush to remove most of the polishing compound. 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 lightr defects such as scratches and microscopic pits appear as point ~ lBS~;13 souxces of scattered light. When a defect is detected, a micxoscope is used tv m~asure its size. If any defects larger than 25 microns are detected, or if the number of defects under 10 microns exceed one per square millimeter, the substrate is rejected and the polishing and cleaning sequences are repeated. The surface is then wiped wi~h acetone to remove any traces of dust and oil introduced during handling.

Second, the surface i5 flushed with a detergent solution, and third, the surface is again flushed with de-ionized water for a period of about ten to twenty minutesO

After cleaning, the substrate 13 is placed on a turntable, as shown in FI5. 6, and rotated at an angular velocity of about 750-1000 r.p.m., to dry the surface.
FIG. 6 depicts apparatus for use in forming the photo-resist layer 15 on the ~leaned substrate 13. The apparatus includes a variable speed motor 33 ~or rotating the substrate in a prescribed fashion, and a pivot arm 35 on which are mounted three dispensing tubes 37, 39, and 41 for dispensing de-ionized water, a stannous chloxide solution, and a photoresist solution, respectively, in a prescribed sequence.

The substrate 13 is ~irst rotated at an angular velocity of about 75 ko 100 r.p.m., while stannous chloride is dispensed onto the cleaned upper surface through the dispensing tube 39. The pivot arm 35 is pivoted manually so that stannous chloride is applied to the entire surface. It is believed that the stannous chloride molecules adhere to the cleaned surface of the substrate, and thereby promote a ~ubsequent adhesion of the photoresist solution.

Water is then dispensed onto khe surface through the dispensing tube 37, to rinse of residual stannous chloride solution, and the angular velocity o~ the motor 33 is then increased to about 750-1000 r.p.m., to 5-- dry the rinsed surface. The surface is now in proper condition for dispensing of the pAotoresist 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 ahout 75-100 r.p~m. Again, the pivot arm 35 is pivoted manually so that the solutisn is dispensed across the entire radius of the substrate. At the ~peeds below about 75 r.p.m., a fi].m of substantially uniform thickness can be achieved only if a relati~ely length~
spread time is allowed but thlæ 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 ully coat a substrate having a diameter of about 35.56 cm.

1 ~ ; 1 3 After the photoresist solution has been coated onto the sur~ace 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 la~er 15~

Recognizing the fact that the thickness of $he 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 - lO 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 Il50 A to 1350 A, and replica discs sub~equently produced will have information-- bearing b~mps or pits of corresponding height.

If it is determined khat the dried photoresist layer 15 is de~ective in any way (elg., containing radial ~treaks 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.

. . .

~ ~5~13 After removal from the turntable apparatus of FIG. 6, the recordin~ master 11 is baked, to fully dry the photoresist layer 15 and thereby maximize its exposure tolerance. The master is preferably baked at 5' 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 O

Since the exposure sensitivities of each of a number of recording masters is likely to be slightly diffexent 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 r each set recorded with a different peak intensity. After the test signal tracks have been developed, using a developing technique described in detail be}ow, the recording master i5 exami~ed to determine the particular set of txacks havi~y 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 mastex.

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 ~hat v~ries by about five percent. Each set is recorded for about 10 seconds, corresponding to several hundred recording ~ 3~5~t3 tracks. Also the sets are pxe~erably separated ~rom each other by narrow bands o~ 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 recordiny 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 i~
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 an~ 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 recoxded 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 o~
tracks at a time. The reading beam is preferabl~
produced by a helium-neon laser so that its wavelenyth will not expose the photoreslst layer 15.

Using the recording apparatus of FIG. 1, with the peak intensity of the writing be~m lg adjusted tv the prescribed, optimum value, the f.m. information signal is then recorded on the remaining, unexposed portion of the photoresist recording layer 150 The recorded master 11 is then developed to convert each recording 6 ~ 3 ~.~

track into a succession of spaced pits of uniorm depth and width and of continuously-variable ler!gth.

In yet another novel aspect of the inve~tion, 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 ~y a turntable of the type depicted in FIG. 6, at a velocity of about 75 to lO0 r.p.m. In the process, wa~er 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 r~sidual 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 lO
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 ll. In another aspect of the invention, the stamper is produced by first vapor depositing a uniform metallic film of about 500-6G0 A thickness onto the photoresis~ recording layer 15, and then el~ctroplating 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 ~ ~6~61;~

pits formed in the photoresist layer, and the two films together form an integral mekallic layer. This integral metallic layer can be separated from the underlying xecord1ng master and residual pho-toresist material 5 - 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 no~el 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 claim~.

Claims (5)

1. A method for developing an exposed photoresist layer of a prescribed, uniform thickness on a video disc master, comprising steps of: rotating the video disc master at a first prescribed velocity; selecting a photoresist developer solution having a prescribed normality; dispensing water upon the photoresist layer of the rotating video disc master, to pre-wet the layer; dispensing both water and the developer solution upon the photoresist layer; dispensing the developer solution, alone, upon the photoresist layer, to fully develop the layer; rinsing the developed photoresist layer with water, to eliminate residual developer solution from the layer; and rotating the video disc master at a second prescribed velocity, to dry the developed photoresist layer.

2. A method as defined in Claim 1, wherein: the photoresist layer is derived from Shipley AZ 1350 photoresist; and the photoresist developer solution is selected from the group including potassium hydroxide and sodium hydroxide.

3. A method as defined in Claim 2, wherein the photoresist developer solution is selected to have a normality in the range of about .230 to about .240.

4. A method as defined in Claim 1, wherein: the first prescribed velocity is in the range of about 75 r.p.m. to about 100 r.p.m.; and the second prescribed velocity is in the range of about 750 r.p.m. to about 1000 r.p.m.

5. A method as defined in Claim 1, wherein: the step of dispensing both water and the developer solution has a time duration in the range of about 5 seconds to about 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 in the range of about 30 seconds to about 60 seconds.