CA1277282C - Method for overcoating optical recording media - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
IMPROVED METHOD FOR OVERCOATING OPTICAL RECORDING MEDIA

This disclosure concerns methods of applying protective overcoatings to an information storage record which includes an information-layer adapted for optical data recording. The protective overcoatings are transparent to recording radiation and especially adapted to enhance service life and recording characteristics.
A typical coating comprises the polymerization product of a formulation including at least "bulk resin"
(e.g., an acrylamide or an acrylate monomer or pre-polymer), plus an associated non-yellowing photo-initiator, and a non-yellowing adhesion-promoter.
This coating has been obtained by curing the monomer or prepolymer formulation by exposure to W
radiation so as to cure it without heating it significantly so that said radiation functions as the sole or principal polymerizing agent, acting quickly, and with little or no supplemental heat, and without extended "tackiness". This formulation will readily "level" to enable the desired coating to serve as a mechanical/chemical barrier. "Leveling" is accelerated and enhanced by dispersing the coating as "beads" with a nozzle placed close enough to the substrate to "top" the beads in the fashion of a "Doctor Blade".

Description

~7~282 IMPROVED METHOD FOR OVERCOATING OPTICAL
RECORDING MEDIA

The present invention relates to a novel information storage record, including an information-layer adapted for optical data recording, and more particularly to methods of applying protective overcoatings, especially as adapted to enhance service life and recording characteristics.
INT~QDUCTION, BACKGROUND
Optical storage of digital data is a relatively volatile technology now, being concerned with the storage and retrieval of digital information utilizing optical techniques and using a special related (ODD, "optical digital data") medium, such as an ODD disk. By analogy such data is conventionally stored on magnetic media like tapes or disks commonly used with high speed digital lS computers today.

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Here described are some novel approaches to providing protective coatings over a sensitive optical recording medium - e.g., one resisting oxidation or like environmental degradation, wherein sensitivity is i~proved, extended life is feasible and fabrication parameters are simplified over what is now con~entional.
Various types of protective overcoatings for such media have been suggested by wor~ers, especially relative to "tuned media" (e.g., media using a "dar~ mirror" effect; for instance see U.S. 4,222,071 to 3ell, et al; also see "Review of Optical Storage Media'' by Zech, SPIE Vol. 177, Optical Information Storage, 1979, paqe 56, et sequ.; also see "Optical Recording Media Review" by Bartolini, page 2, et sequ. of 1977, SPIE Vol. 123, ''Optical Storage Materials and Methods"; and see "Melting Holes in .~etal Films for Real-Time ~igh Density Data Storaqe" by Cochran and ~errier, SPIE
Proceedings, August 1977, pages 17-31; and other citations below).
--Extended Archival life:
Optical data storage technology is attractive because it pr~mises increased storage capacity. An optical data disk as here contemplated will be assumed to store information thereon for an extended archiv~1 life; the goal is 3-lO years or more under typical, and extreme, service conditions for data processing (DP) apparatus. Such extended life is a goal as yet unattained in the art, though wor~ers have long striven towards it. The present invention points toward improved ODD media better adapted for such archival life;
media which are especially adapted for ''optical mass memory"
~3~ and li~e applications, with emphasis on improved overcoat means.

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Thu~, as a feature hereof, we contemplate the use of a novel overcoat structure and materials for records which preferably exhlbit extended archival life, i.e., record~ which are made extremely resistant to oxidation or liXe environmental degradation during typical DP storage and use (thus, with little or no ''l.oss" o~ recorded Lnformation occurring over extended s~orage li~e, with rerlectivity remaining stable enough t~ "read") -- something no practical storage medium or associated system can yet provide; especially where "good" sensitivity is alsb required.
The invention teaches means toward this end.
--Overcoat; generally:
The typical recorded spots ("bits") are contemplated as belng about one micrometer in diameter. But surface "dirt"
LS (e.g., oil, fingerprints) or particulate contaminants, such a~ air-borne dust, are this large, or larger, and thus can obstruct a recorded "bit". For lnstance, common smoke p~rticles can ~e about ii~ micron~ (6 um, or about 24~
~icroinches) Ln diameter. ConsequentLy, such contaminant particles will commonLy "mask", and so obliterate, recorded "bits" (data) i one or several of them sit just abo~e the overcuat.
So, it has become conventional to specify a thick overcoating layer for defocusing such contaminant particles and all imudges, spots or smears -- e.g., here, by providing a transparent overcoating on the order of 100 to 180 microm~ters thic~. Thu-~, any dust par~icles that do settle on the s~rface of such a protective layer, (and are not wiped-away) will bs "defocused", i.e., thrown out o~ the focal range of the objective used to detect recorded data and the rest of the optical train -- optically they "disappear". As a second purpose, such an overcoat should provide mechanicaL protection for the recording layer and prevent damage from handling, etc. (e.g., during fabricatio~, te3~ing or service).

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~ ow, in ~ome case~, workers have suggested relatively "hard" materials as a protective tran~parent overcoat, while in other~ they have proposed "~o~ter"
materials. For instance, some have ~uggested an ela~tomer outer-coat (c~ a ~ilicone rubber liXe "Silastic RTV" by GE
~ 3ee U.S. 4,101,907, to Bell, et al where an "abla~able"
absorber, such as certain organic dye~tuffs, was overcoated ~ith a "barrier layer" of SiO2, or of derivatives of sucrose or resin acids; and this ~uper-coated with such a silicone resin). But known overcoatings of .of! ~ resilient (rubbery) materials have characteristically exhibited a "tacky" exposed surface which readi:Ly attrac~s and retains dust; and in certain instances, such "elastomeric"
coatin~s still ~eem to "cons~rict" the underlying absorber.
Also, elastomerR may require a curing temperature that is t~o high; or, if they cure at room temperature it may take f~ too long; yet, when heated for "quick curing" they present a serious ris~ of overheating the tri-layer (-- a silicone elastomer like RTV presents all these shortcomings, along with cure-stress, and excessive moisture-uptake in service).
On the other hand, other workers have considered a "hard" outer "sealing" overcoat applied directly over the absorbing layer (e.g., ~ee "Optical Disk System~ Emerge" by Bartolini, et al IEEE Spectrum, Augu t 1~78, where, in a "tri-layer" struc~ure, SiO2 is specified above and below a titanium absorber); yet they have been forced to concede that, such a hard overcoat (pe~haps because it unyieldingly confines and constricts the absorb~r) appears to degrade recording sensitivity, to the point where it renders an o~herwise acceptable recording medium essentially "unr~cordable".
Also, hard outer coatings like SiO2 are too absorptive (e.g., of water vapor) to be long- lived.

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' --"Hard/Soft" overcoat:
A salient aspect of this approach is to provide an overcoating which avoids most or all o the foregoing shortcomings, doing so by providing a two-part overcoating made up of a "soft pad" inner layer and a "hard" outer sealing layer -- i.e., with a "Hard/Soft" overcoat. The relatively softer inner pad is intended to be placed against the absorber, to be yielding and quite compressible (as a "mushy cushion") allowing the subjacent absorber to distort and/or move during write-heating, while also providing good thermal insulation (very low thermal conductivity; relatively low specific heat). In short, this "soft pad" seems to better isolate the absorber, mechanically and thermally;
on the other hand the "hard" outer coat gives optimal mechanical protection (e.g., a seal against vapor entry).
Of course, such layers should also inter~bond well, be highly transparent to the contemplated read/write wavelengths and preferably be convenient and inexpensive to apply.
As mentioned, the mechanical properties of certain such "soft pads" (e.g., of a fluoro-polymer, see below) appear to better accommodate motion or deformation of the underlying absorber during "write-heating" (e.g., as a "top pad"; also as a "bottom pad" if the soft material is used as a "spacer" too). Such "soft pads" -- evidently because they so decouple the absorber, mechanically and thermally, from its surrounding environment -- are found able to markedly increase "sensitivity" (e.g., well over what can be expected using only a "hard" overcoating like fused silica -- i.e., the latter will require more energy to "write" a given bit or "hole"). A "soft pad" is so effective as such isolation that even where only used as a subjacent "spacer" (e.g., with SiO2 directly over absorber) it has been seen to enhance sensitivity (e.g., vs. replacing it with an SiO2 spacer).

lZ~B2 ~r -- 6 As mentioned below, such a "soft pad" coating may, in certain cases, be applied with essentially the same facility as those used to deposit the (reflector and) absorber layex (e.g., during a related, succeeding deposition step, and with common equipment). The consequent convenience and reduced cost, time, etc., will be evident.
It is often possible to use the same "soft pad" material for both sides of an absorber (i.e., as spacer and overcoat). One may choose from a class of plasma polymerized polymers in some instances, such as polyvinyl fluoride (PVF) other fluorinated polymers such as fluorinated ethylene polymer (F-P), or polyethylene (P-e).
Preferably, one evaporo-deposits such a "soft pad" layer at the same time, and with the same equipment, as that for depositing the absorber layer (and/or the spacer layer).
Alternatively, one may in certain instances deposit by other methods, for example ~y plasma (polymerization) deposition.
The thickness of this "soft pad" overcoat is preferably such as to so decouple the absorber layer (thermally and mechanically) from any supercoating (especially a "hard" layer appli~od over the "soft pad") --and also to bond favorably with the underlying absorber (e.g., so that sensitivity is not badly compromised and so the absorber is suitably "decoupled" from a hard "outer"
overcoating, while also preventing the hard overcoating, and/or any stress therefrom, from constraining the absorber and so interfering with pit-formation therein -- yet bonded well enough to the "hard" coat to prevent "delamination", moisture intrusion, etc., in service, these easily upsetting the needed optical properties -- cf. a mere 100 A shift can destroy the required "tuning").

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It is important to protect the absorber from any such deleterious effects; for instance, especially where one uses absorbers which deform and/or are displaced in the course of recording and creating a "bit-spot". It will be apparent to workers that a hard overcoating (e.g., SiO or SiO2 as known) applied directly on the absorber layer can be expected to constrict it, and restrain such deformation or translation during "bit-writing" -- thus interfering with bit formation and degrading sensitivity and recording efficiency, so that more write-energ~ is needed. Also, most silicon oxides absorb too much moisture~ We have experienced these problems using SiO2 (evaporo-deposited on a "cool" substrate) -- much less so with materials like the preferred fluoropolymers (cf these can be deposited as relatively "non-porous" films under like circumstances).
Workers will see how important and useful a proper "soft pad" of the type described can be, especially where one wants to enhance the recording efficiency of an adjacent OD
absorber layer~ Thus, it will usually be desired to so provide a "soft pad" coating over an absorber layer and, where possible, to do so using common deposition techniques (-- whether or not one also provides a like "soft pad" spacer layer beneath the absorber -- whereby one may thermally and mechanically isolate the absorber from interference generated from above and/or below).
It will be recognized that this involves so applying a (fluoropolymer) "soft pad" which is sufficiently soft and yi~lding as to mechanically decouple the adjacent absorber layer, freeing it to "move" as written, while also isolating it thermally (i.e., to so function, either as a subjacent z~

"spacer" or as an overlying "soft overcoat" or as both). One will thus want to so provide such a "soft pad" spacer using an organic layer which is made strongly adherent to an underlying reflector layer while a:Lso being relatively differently adherent to a superposed absorber layer. One will prefer to provide such a "soft pad" overcoat which bonds to a superposed hard overcoat relatively firmly (but may bond differently to the subjacent absorber).
--Novel "Hard" supercoat:
As mentioned above, another salient feature hereof is that the above-characterized "soft pad" overcoat is, in turn, preferably super-coated with a compatible "hard" outer protective layer. When one superposes a "hard" protective overcoating outward of this "soft pad" overcoat it can serve as a good vapor barrier, and as a mechanical "cover" and an anti-static surface, as well as to-complete the necessary optical thickness for "defocusing" surface contaminants --i.e., yield a "Hard/Soft" overcoat.
A more specific feature, a family of novel "UV
cured acrylic-epoxy polymers" is here taught for such a "hard" outer coating for an archival OD (optical data) disk;
also, a preferred associated novel method is t ught for coating such disks with such material. These Acrylated epoxy polymers will be relatively clear (to recording/read beams) and somewhat flexible in addition to the mentioned "overcoat"
requirements -- e.g., passing all related environmental tests without delamination, cracking, etc.
A novel pre-polymer formulation is described below (e.g., see Mix T-l); it is intended to provide such a "hard"
protective overcoating for such OD disks (extended archival li~e, etc.) and especially as a super-coat over such a "soft pAad" overcoat. More particularly, it is intended to provide -~
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a "clear" coating (transparent at the contemplated R/W
wavelengths), of a thickness to help "defocus" surface dust, etc., (e.g., up to 6-8 mils here) and to provide an environmental barrier against mechanical interference or vapor intrusion (especially water, aqueous aerosols, sulfates or Na~L or other chlorides). It is intended to so function rather like known overcoatings (of a "glass" for instance), and to provide good mechanical protection, (e.g., allowing one to lightly ~queeze the disk, though it need not resist a positive cutting action, such as scraping with fingernail.
--Known "hard" outer-coatings:
Workers in this art have considered various materials for similar protective coatings. For instance, it~has become common to suggest a "glassy" form of overcoat, such as with "fused silica" (SiO~, or SiO) but for present purposes (OD disks, etc.) these seem to be disqualified.
For example, they are typically highly porous and can take-up too much moisture; thus they are too prone to swell and crack (especially under the mentioned extreme temperature/
humidity cycling tests) -- also such moisture contaminants badly degrade optical characteristics. Also, they are not optimal for the desired vacuum-evaporation deposition (e.g., impractical to so deposit several mils or more).
Besides such inorganic overcoatings, workers have considered certain oryanic materials for providing protective overcoats in similar situations. For instance, as mentioned, some workers have considered using a silicone rubber or like elastomeric polymer for this -- e.g., some silastics which may be conveniently curable at room temperature, typically liberate harmful contaminants like acetic acid during cure, (or see "plastic sheet" of U.S. 4,334,233).
~ In a similar vein, we have considered using various fluoropolymers; but, in the thicknesses contemplated 77Z~2 (6 to 8 mils) typical fluoropolymer deposition methods are not favored -- e.g., typically require dissipating too much solvent (~ee problems below with solvent dissipation and associated shrinkage, etc.). More seriously, this could involve a CUI. e-heating which i5 entirely too intense (at about 390 C), whereas the subject OD disks and associated coatings ar.e not intended to survive more than about 66C (e.g., otherwise their coatings, such as the organic soft fluoro-polymer overcoat and the absorber layer, would be destroyed, and/or constituents could migrate, etc.). Moreover, such polymers are apt to exhibit a "~acky", dust-retaining, surface and are not believed optimally transparent at the sub'ject read/write wavelengths (cf. 600-900 N. meters).
Also considered for such a hard protective overcoat were various "solvent-based" (solvent-applied) polymers like epoxy. However, drying (curing) these involves dissipating relatively large proportions of solvent, with a great deal of problema~ical shrinkage likely. This has seemed to disqualify these materials, especially for coatings as thick as those contemplated (also, bubbles, etc., would probably form in such a thick coating of these materials).
Also contemplated were various "two-component curing" polymers such as "RTV-6" (by GE) ~- or epoxy.
However, these are somewhat difficult to apply, typically having a relatively high viscosity ~possibly requiring probl~matical heating or dilution to soften enough for quick, smooth application -- e.g., dilute certain RTV:
example, Sylgard*184 and Dow Corning*200); they also typically present "out~gas" problems; further, many cure * Trade Mark .~

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relatively 510wly and at a relatively high temperature (e.g., 15 minutes at about 66C -- and, even then, the cured material often exhibits a tacky surface and is too apt to scratch, peel-off, etc.). Moreover, such materials typically have too brief a "pot-life" (on the order of one day) -- yet another application shortcoming.
The subject preferred radiation-cured epoxy-acrylic poLymers do not seem to present the foregoing problems, e.g., they don't require solvents and are cured at room temperature in a short time.

--Preferred materials for "HARD overcoat":
An attempt was made at usin~ a "radiation-cured" acrylic-epoxy type polymer (-acrylic monomer, or pre-polymer mix plus epoxy resins with various additives, similar to the "Mixture T-l" discussed below). It was found, somewhat surprisingly, that when properly applied (e.g., see "spiral" technique, below; with appropriate "setting surfactant" and appropriate "solvent-leveling", etc.) such an overcoat could satisfy (most, i~not all of~ the mentioned requirements, whereas other materials seem less apt for doing so. Thus, it is an object of this disclosure to teach the use of such radiation-cuxed acrylated epoxy polymers as a "hard"
protective overcoat for such optical data dis~s, as well as teaching related methods of preparing and applying them.
As detailed below, a preferred family of hard coat materials -- "radiation-cured polymers" -- is made up of epoxy plus a number o~ "acrylated monomers" (or "pre-polymers", i.e., an oligomer or resin that will undergo further polymerization -- especially where the ~; .

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, ~77282 principal constituent is a suitable acrylate or acrylamide).
A preferred version (Mix T-l) includes an appropriate acrylated epoxide together with an acrylate cross-linker, an acrylate flexibilizer and associated acrylate diluent plus UV-initiator and "clarifying-adhesion promoter", and preferably including a suitable surfactant constituent.
Also, a minor portion of th~ Mix may comprise one or more additives (preferably organics which will participate in the UV polymerization, e.g., oC -methyl styrene, vinyl acetate, etc., do this).
Such acrylics are evidently eminently suitable for several reasons: they do not include (any significant portion of) problematic co~ponents like ("shrink-prone solvents") and they require no problematic cure conditions (such as extreme heat). They seem to be especially apt for providing a final "Hard" and glossy polymeric overcoat which has the required characteristics.
"Acrylic-epoxy radiation-cured polymers"
will be recognized as satisfying essentially all the other cited requisites of the desired "~ard overcoat"; i.e., they don't readily crystallize, they have no massive solvent content or assoclated shrinkage problems, they are cured quickly and conveniently and without excessive heating; and they are relatively easy to apply (e.g., as a low-viscosity solution). They appear quite superior in resisting degradation and attack by common environmental components, they are not "tacky" or dust-retentive, and, unlike the ("two-component-cured") polymers, they are compatible with a wide number and variety of additives (e.g., their curing is not affected thereby, as seen in the Examples below).

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Workers will recognize that the required cure-radiation may be something as inexpensive, quick and convenient as a few seconds exposure to a W source (of appropriate ~, intensity, etc.) arld involve as little as a few % shrinkage. or, where cost is not a major concern, one may instead cure with electron-beam or gamma radiation.
Alternatively, a W activat:ed epoxy derivative (catalyst) cure may be feasible. Whatever the primary curing mode, it will be understood that light supplemental heat may, in certain cases, be so applied to hasten complete curing.
--Application as "spirals":
According to a related feature, such acrylic-epoxy overcoat polymers are apt for application in a spiral configuration on a host substrate-disk, being evenly distributed thereon (e.g., with appropriate disk rotation and inclusion of an appropriate leveling agent), and allowed (or in some cases induced) to settle and flow-out evenly. This is seen to spread the mix across this surface with exceptional smoothness and uniform thickness. WorXers in the art will recognize the simplicity and novel advantages of such a coating technique.
Thus, it is an object hereof to provide the foregoing, and other related, features and advantages. A
more particular object is to do so, teaching the use of "soft pad" materials adjacent an "optical recording layer"

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with a relatively "Harder" supercoat on the soft pad.
Another object is to teach such for improved recording sensitivity, adequate for low-power lasers; as well as for extended service life. A further object is to teach preparation of such a "Hard" supercoating using acrylated epoxy materials. Another object is to provide such "hard"
overcoatings and associated preferred materials.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages of the 10 present invention will be appreciated by workers as they become better understood by reference to the following detailed description of the present preferred embodiments, these being considered in conjunction with the accompanying drawings, wherein like reference symbols denote like 15 elements:
FIG. 1 provides a cross-sectional view of a recording medium embodiment exhibiting a construction in accordance with ~eatures of the present invention; and FIG. 2 very schematically indicates a preferred ` 20 method of applying overcoat material of the kind taught herein.
FIG. 3 is a view after the manner of FIG. 1 indicating a modified embodiment-` ~ :
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Exemplary OD recording --Example I (FIG. l;
tri-layer with "overcoat"):
FIG. 1 will be understood to (schematically and in idealized fashion) depict a fragmentary section of an optical data disk RD ~ including a substrate disk A supporting a recording tri-layer T-L and overlying protective overcoat O-C. Disk RD will be understood as intended and adapted for recording by a known radiation source (Laser L) directing a beam (LB in phantom) at a tri-layer T-L so as to record certain bits therein -- these to be "read" using prescribed associated detect (D), as known in the art.
The wavelength of the reading laser beam (LB of FIG. 1) is chosen so that unrecorded regions of the disk RD exhibit the desired anti-reflection condition; read-beam intensity will be kept low enough so as to not disturb the integrity of data recorded on the disk. Substrate A
preferably comprises a relatively conventional magnetic recording disk with a smoothing layer B, applied thereon as necessary. Tri-layer T-L preferably comprises a transparent spacer layer d atop a reflector ~ilm c, with a suitable absorber or recording film e superposed on spacer d.
It will thus be understood that the reflected read-beam will be intensity-modulated by optically detectable ~5 changes at bit sites where data ~as recorded. Thus, the read beam will experience relatively high reflection when incident on a "bit" and relatively low reflection when incident on unwritten regions. Protective means O-C is a Hard/Soft composite overcoat chosen and arranged so that dust particles on its upper surface will be displaced far .. :

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~ 16 -from the focal plane of the optical system (i.e., placed out of focus); and so have a negligible effect on the recording and reading operations.
It is conventionally assumed that, for the laser beam to "write" (i.e. "record" and produce an optically detectable disturbance in the reflPctivity of the thin film absorber layer c) absorber film e, at any given bit-site, must be heated to a prescrib2d (minimum) write-temperature (Tw). The level of minimum temperature Tw is believed to depend on the properties of absorber c (e.g., on its thickness, metallurgy, microscopic structure, etc.) and also on the properties of subjacent spacer d, as well as upon "interface characteristics" between the spacer d and absorber e, and possibly between overcoat 0-C and absorber e.
It will be found that a finite time is required for writing at a "bit site" (on which the writing laser beam is here assumed to be focused) to reach this requisite minimum "recording temperature" Tw. But while a "bit site" is being so heated, some of the applied heat is typically assumed to be escaping through underlyi~g - dielectric spacer d (also throu~h O-C, possibly) and thus "wasted". To the extent such heat is lost, more time/energy are required to "write" of course, i.e., recording sensitivity is commensurately degraded. It is also believed that such heat-loss can reduce the quality of the recording and thereby reduce "recording density" for a given medium.
-"Soft pad"as "Tri-layer-Spacer"; preferred materials:
As an optimizing feature, it is preferred to use certain "soft pad" (e.g., fluorinated hydrocarbon polymer) as such a dielectric spacer layer d (FIG. 1). Such a .

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"soft pad spacer" is believed to help reduce the 109s of write-energy (i.e., less writing-snergy escapes from the bit site).
Preferred materials are fluoro-ethylene polymer, or other fluorinated polymers or copolymers,e.g., those commercially available under the name "Teflon", a trademark of DuPont. Such a fluorinated polymer can be deposited over the reflective layer c in a thin uniform layer as workers will understand.
--Preparation of Tri-layer T; (FIG. 1):
Abou~ 600-900 A of a good archival reflector like gold (prefer about 600 A, vapor-deposited) is applied as the reflector c atop an aluminum disk A, preferably smoothed properly with a subbing layer B as known in the art. Aluminum may replace gold where reduced cost is required and archivability can be compromised.
The reflector c may be so evaporated under high vacuum, in a large, batch-coating chamber with corresponding large coating dis~ances and "double-rotation" of substrate etc., to better ensure uniformity. All dust and stains on parts should be reduced to a strict minimum, using rigorous "Clean Room" techniques.
The spacer d is similarly deposited atop reflector c. Under present practice spacer d serves as a dielectric material which is relatively transparent to the "working portion" of the laser spectrum. A one-quarter wave (of - laser L) thickness of "soft pad" fluoropolymer is preferred, for the subject purposes (e.g., assume write/read at ~ = 6328 A; ~ote: from an optical standpoint, a spacer of thickness t = 1~ n ~ will "disappear").
Absorber layer e may be understood to comprise an u~tra-thin layer of gold which is vapor-deposited (thermally evaporated) in island form onto spacer layer d (on a relatively flat -- - '1/20 ~ record surface . - ' ' '' ~' . .
., . ' ' '' 7~8~, thereof -- as contrasted with a more conventional absorber of tellurium -- e.g., see "Optical Properties of Tellurium Films Used for Data Recording" by Ash and Allen, SPIE
Proceedings, #222, 1980; and see "Design and Production of Tellurium Optical Data Disks" by Rancourt, SPIE
Proceedings, #299, 1981; or see U.S. 4,222,071 or U.S.
4,334,299).
Here, test recording will be assumed as performed with a gas (He-~e) laser beam operating at 6328 A, with recording exposure from 30-470 n.sec Esually 10 mW, 40 n.sec or about 400 p.J. -- this intended to yield minimum adequate read-out, or about 40 dB S/N, when read at lower power e.g., 150-500 pJ/cm2, where pJ = 10 watt-sec. or Joulss), with the same or similar laser equipment. Note: for this contemplated setup, assume the laser beam is focused on bit site of ~ to 1 micron diameter, (i.e., 5000-10,000 A ), with a write-pulse about 40 n.sec.
long -- this also accommodating disk rpm of 1800 and associated galvo-mirror focus characteristic~s~.
Thus, the spacer layer d (e.g., in such a "dark mirror" arrangement) will preferably comprise a "soft pad"
wnich is vapor-deposited on a reflector layer, and upon which t~e absorber (recording) layer may in turn be d~posited. This spacer layer will preferably comprise a so-deposited fluoropolymer (e.g., about 1100 A thick) which is highly transparent to the contemplated read-write wavelengths and which also provides good thermal and mechanical insulation, isolating the absorber layer from the reflector layer, (note the reflector is typically a highly conductive metal which could otherwise function as a heat sink, draining recording energy away from the absorber layer and reducing its effectiveness).

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Thus, as further described below, for one example we prefer a (vacuum-evaporated) fluoropolyrner, like polytetrafluorocarbon (Teflon) prepared from tetrafluoroethylene by plasma polymerization. Alternatively, PVF may be substituted.
~ote: this will be distinguished from hard silicate coating (silicon oxide or silicon dioxide -- cf.
"fused silica") more conventionally for such a spacer (s.g., see U.S. 4,195,312 or 4,195,313 or 4,216,501 to Bell, et al) or compare "Design ancl Production of Tellurium Optical Data Disk" by J. Rancourt, SPIE
Procsedings; Advances in Laser Scan Technology, page 57, Vol. 299, 1~81~.
Such a "soft pad" spacer material, including associated deposition methods is especially apt for such OD disks -- and even more especially such which are typically convenient for low-energy recording with present laser equipment (e.g., writing with a He-~7e laser in a 5-20 mW/40 n.sec. pulse -- cf. 25 MHz rate).
The subject record RD (FIG. 1) is so-recorded upon. It is found (relating to comparable situations in the literature, etc.) that relatively "moderate-power"
l~er pulses can heat and agglomerate the gold-island film sufficient to yield good read-out (e.g., bit reflectance of ~50% vs. background of 1-396 at = 6328 A) -- and with relatively no "noise".
Workers will be familiar with present prPferred methods for high-vacuum evaporation, and reconstitution on the Al film, of such thin layers of organic materials like the fluoropolymer (cf. cited Rancourt article also re similar deposition).
Fluoropolymers like those preferred are of a generally paraffinic structure, with some or all of the hydrogen replaced by fluorine. Both are sold by DuPont Co.

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under the trademark "TEFLON". They are highly inert (unaffected by reactive chemicals) and are quite stable chemically and mechanically, under the contemplated extremes of temperature and humidity; they have low dielectric constants and appear to bond satisfactorily.
For present purposes, "Sensitivity" will be understood as characterizing the write-energy Ew, i.e., the laser beam necessary to change reflectivity (or a like read-out characteristic) sufficient to give the specified minimum read-out.
The intensity and time exposure of the focused Write-beam here will be understood as sufficient to so elevate the temperature of absorber layer e as to cause the indicated change in reflectivity, giving desired read-out quality, etc. (e.g., so that adequate contrast, S/~ ratio) may be realized, as understood by workers in the art, --cf. an exemplary S/~ ratio of 40-50 dB (peak-to-peak signal vs. R~S noise) for a bandwidth of about 15 MHz.
Laser recordings are made on the resulting optical medium at 2400 revolutions per minute using apparatus of the general type referred-to in connection with FIG. 1 (above). A Helium-~eon laser is again used for recordiny (wavelength of 0.633 um). The focused laser beam "spot"
on the medium film 98 is approximately 0~5 um. Resulting sensitivity of such recordings will be found to be quite good -- better than conventional approaches have led one to expect.
Moreover, the fluoropolymer gives a nice optically clear layer with a relatively low refractive index (about 1-3 vs. about 1.5 for fused silica, a valu~ somewhat higher than optimum).
Alternative deposition by plasma polymerization or other techniques will be feasible in certain instances, as workers will appreciate.

.

.;

' ~, ~2 Also, workers will contemplate that other like "soft pad" polymers may be similarly deposited by vacuum evaporation, although the choice will be somewhat limited in view of the subject, rather stringent requirements. The preferred materials and thickness have been ~ound to be quite versatile; for instance, in many cases one may use a different absorber metal without changing the materials or thickness of this spacer (or of the "soft pad" overcoat, as described below).
--"Soft pad" as supercoating on absorber:
As mentioned, it is preferred to use such a "soft pad" layer as a "buffer" supercoat directly over the absorber layer, e.g., helping to further isolate it thermally and mechanically -- especially where a like "soft pad" is present underneath the absorber. For instance, it is believed that this further helps to conserve write-energy, while giving the gold-isle mass more freedom to move or deform while being write-heated (e.g., vs. a conventional silica supercoat which is believed to seriously constrict hole-formation). On both counts sensitivity should be enhanced.
Such was found to be the case as noted below.
In the course of usin~ such a "soft pad" layer (e.g., 9500 A ) as the in-contact buffer supercoat over such an absorber, a salient feature of this teaching is to, in turn, overcoat the soft overcoat with a "Hard"
barrier layer of acrylic-epoxy, etc., as specified below.
It is believed we determined that such a "soft pad"
supercoat should preferably exhibit the following characteristics (Table I):
.

, - '., ~ . .

~27~;Z8;~

~"Soft Pad" Supercoat desiderata) 1. Optically compatible: good transparency at (R/W)~

2. Good uniform thickness and surface flatness-3. "Moderate-to-weak" adhesion to absorber:
~ ~ .
Little or no resistance to "hole writing"
and associated deformation ana/or movement of absorber -- yet no orange peel, lifting, delamination, etc.

4. Strong bond to ("Hard") overcoat:

5. Stable under contemplated environment: (i.e., despite varying temperature and humi~ity, contaminants, etc.):
e.g., surviving service temperature without degrading, even adjacent the hole-formation site; chemically stable too; e.g., no release of solvent or other contaminants during cure or under extended extreme temperature and humidity cycling.

6. Relative "softness": allowing movement/deformation as in #3; (considerably more than "Hard"
overcoat) ~nd thick enough to accommodate bit-writing with minimal degradation of sensitivity from overcoating(s).

7. Good thermal insulator: e.g., low thermal diffusivity, low specific heat; survives temperature of fabrication, and of "writing".

~--~772BZ

Now, others have suggested some kind of polymeric supercoating for such absorbers. For instance, U.S.
4,101,907 mentions "silicone resins" for such (e.g., General Electric's RTV 615 or RTV 602, these curing at room temperature with certain curing agents; or Dow Corning's Sylgard 184 -- e.g., suggesting these for use over titanium) -- preferably with an intervening "barrier layer" of SiO2 or certain comple~ organic materials.
--Alternative "soft pad" embodiments:
Workers will recognize that such a "soft pad spacer" may be otherwise implement:ed in appropriate instances (e.g., with another relatively "soft", relatively non-reactive, stable, durable polymer such as a like "modified fluoropolymer" or polyethylene, polypropylene or polystyrene -- these will typically decompose and polymerize in similar fashion). Likewise for such a "soft pad supercoat".
The deposition techniques which will, in appropriate cases, be feasible include a plasma deposition technique like glow-discharge (especially for fluoro-carbons) or sputtering, especially where chemical breakdown is not complete. Workers may well change the optical absorber; e.g., to another more compatible, high-sensitivity, thin-film, low thermal conductivity material which also couples properly to the "soft pad".
Further, workers will contemplate other like applications and uses of such a soft pad.
--Preferred overcoat embodiment; ("Hard/So~t"
overcoat O-C~:
Disk RD in FIG. 1 (only small schematic section shown) illustrates a preferred e~ample of the features mentioned above, and especially the (general) teaching of a "Hard" overcoat applied over a "soft pad" layer covering an absorber (optical recording film) -- i.e., a , novel "Hard/Soft" overcoating structure O~C (cf. FIG. 1, Hard coat g and soft pad layer f over absorber e, which is part of the ODD "tri-layer" T-I. applied on substrate A).
It will now be described with reference to this schematic showing.
Except as otherwise specified, workers will understand that (here and for all embodiments) all materials, methods and devices ancl apparatus herein will be understood as implemented as above or by other known expedien~s according to present good practice. In the course of this description some variations which could prove useful in certain circumstances will also be pointed out.
--Substrate:
The substrate is preferably the surface of disk A, as treated, when necessary, with a smoothing or subbing layer B to make its surface su~ficiently smooth. Thus, substrate A is preferably a common "Winchester" disk, such as used in commercial magnetic recording disks for computer media. It comprises an aluminum alloy, prepared as is typical for fabricating disks for high speed magnetic recording of digital data (e.g., as used in computer memory systems). The surface of such disks i9 commonly polished, diamond-turned or otherwise smoothed, as workers well know.
Alternativ~ly, a suitable glass or plastic disk may be substituted in certain instances.
"Subbing" layer B will be understood as applied to the bare, well-cleaned disk surface. The "subbing"
preferably comprises an organic material to so smooth the microscopic irregularities on the surface of substrate A
to well under "hole size" (e.g., about 0.5 um or less in diameter). If the surface is already smooth enough (e.g., if a highly polished glass disk is used), a subbing layer may not be ..acessary, as workers know.

' ~7~82 This substrate is thus understood as pra~arably comprising a 14" disk to be operated at about 1800 (to several thousand) rpm, with good surface smoothness.
A radiation (laser) beam of prescribed energy and wavelength will be understood as applied to medium RD
from a laser source L (see FIG. 1), being activated and focused at "write time" YO as to render a "pit", "hole" or like optical "anomaly" apt for the contempla~ed read-out on recording layer e in the course of "writing". More particularly, one may, for example, contemplate using a 10 mW gaussian beam with diameter of 0.8 um (i.e., 8000 A ) and scanning at 45 n.sec. to form an optical transition with a certain minimum length and width, e.g., 0.8 um , though not necessarily square, circular or other prescribed shape.
~ow, this requirement is too stringent for conventional means, as workers realize (e.g., or archival records).
So, where each "pit" (bit) is recorded, the "anti-reflective" background will be disrupted such as to yield "bit marks" adapted for high-contrast read-back. And, where the recording wavelength is shifted, the spacer thickness is readily altered to give like results. In this "tuned" ("tri-layer" or "Dark Mirror") configuration, sur~ace reflectance (on absorber e) can be made "zero", or other selected value, by adjusting absorber thickness and spacer thickness. (A "tri-layer" being here understood as comprising a transparent spacer with absorber on one face and reflector on the other, thicknesses being adjusted for "opt cal tuning" as workers will know).
Thus, the coating parameters here will be understood as selected to preferably provide an "anti-reflective"
condition for the so-coated disk at the contemplated recording frequency when the write beam is focused on this absorber layer. (Regarding such see above, and also:

~77~

"Anti-Reflection Structures for Optlcal_Recordin~" by Bell and Spong, Journal of Quantum Electronics, Vol.
QE 14, ~o. 7, July 1978, and, for general prior art, see exemplary articles: "Optical Disk Systems Emerge", IEEE Spectrum by Bartolini, et al, August 1978, page 20;
and "Optical Storage Materials and Methods", SPIE
Proceedings, Vol. 177, Optical Information Storage, 1979, page 56).
--Recording portion ("DarX Mirror" type):
The recording face of disk RD may be visualized as an "absorber layer" (e) together with an appropriate subjacent "spacer layer" (d) and a "reflector layer" (c), below spacer d, as well known in the art. As another aspect of this disclosure, such layers (c, d and e) are preferably applied by successive evaporative coating sequences with appropriate materials in a single high-vacuum chamber, and preferably together with "soft pad"
overcoating (f) also as described above.
Alternatively, these applications might be applied by a suitable plasma polymerization technique or other appropriate methods for producing films of the mentioned type. Workers will recognize, as a feature of advantage here, the teaching of materials and techniques which may accommodate such a series of like deposition steps using a common deposition apparatus, (e.g., especially where spacer layer d and a soft overcoating f both comprise like "soft pads").
Reflector layer c comprises, preferably, a layer of high reflectivity metal such as vapor-deposited gold or aluminum as above discussed; e.g., deposited untll layer c is "just opaque" under the contemplated illumination, as viewed through layer c (as workers knowledgable about making .

. ~ .
, ' .

77~8~

evaporated reflectors well know, too thick a reflector will degrade reflectivity). And as workers know, other metals can, at times, be used so long as they provide sufficient high reflectance at the contemplated R/W wavelengths.
Another option is to use dielectric films of alternating high and low index and with a quarter-wave reflector.
Spacer layer d, is intended to function, in combination with the reflector layer c and absorber layer e, to reduce the reflectance of the "tri-layer" assembly to zero, or to some other predetermined reflectance value.
The materials used will preferably be relatively "non-absorbing" and highly transparent to the contemplated R/W
wavelengths. The thickness of spacer d will depend on its optical properties and those of the other layers in this tri-layer. Preferably a thickness of 0.5 to 1.5 quarter waves will be used. Alternatively, multiple half-wave thicknesses can be added as workers will see. (~ote:
from an optical standpoint, a spacer of thickness t = ~ n ~ will "disappear").
Layer e ¦FIG. 1, still) is the absorbing ~ilm in which the working incident "write energy" is to be concentrated.
-~Overcoat portion:
"Soft pad" coating f preferably ronsists of a convenient thickness (e.g., a few thousand A ) of a fluoropolymer (e~g., preferably and conveniently be the same material and deposition method as for spacer layer d).
It is preferably formulated and deposited (on absorber e) as described above, most preferably being laid-down in the same overall deposition sequence; cf. with tri-layer T-L
for convenience.
Where using the "tri-layer", it will be convenient to detect and control thickness with layer f being deposited .t. :

' :

~Z'~7Z~3Z

as one or more half-waves. As workers will realize, a number of half-wave thicknesses will make the soft overcoating "disappear" optically, and thus not reflect read/write energy meant for the absorber layer (--this would reduce system efficiency).
"Soft pad" supercoating f will be sufficiently "soft" and yielding to maximize sensitivity, will be relatively non-porous, thermally insulative, with a relatively low specific heat, as well as being highly transparent to the contemplated R/W wavelengths ( ~r) as mentioned above. Also, it will bond firmly to the superposed "Hard" barrier layer, but couple rather laosely to the underlying absorber (e.g., ~hich preferably will be relatlvely non-reactive with the "pad") -- also a flash inter-coating can, of course, be used. It should also be chemically stable, compatible (not project contaminants in record ~) and able to be matched thermally and mechanically to adjust layers (i.e., to absorber e and hard coat g). Ideally it will also be cost-effective and convenient to apply te.g., with same deposition methods and equipment as layers c,d,e).
The above-described fluoropolymer material will be found to meet most, if not all, these stringent requirements (as summarized in Table I above), though other like materials (e.g., like plasma polymerized fluoropolymers) will be suitable in appropriate instances. And, when such "soft pads" sandwich an absorber on both sides, the "thermal-mechanical isolation" thus afforded will be recognized as exceptional.
Further treatment of "soft pad" overcoating f may be necessary to optimize its compatibility and bonding to co~tiguous coatings (e.g.,to enhance adhesion of its exposed surface to the "hard" overcoating and/or to weaken .-., .

its bond with the underlying absorber layer). For instance, it has been found that certain "promoters" applied to the exposed surface of such a "soft pad" are often preferable for enhancing the wetting, etc., of a hard overcoating g like the radiation-cured acrylics described below. Such a "promoter" can evidently reduce moisture absorption and raise the "surface energy" ~s f the soft pad, and lower the "free energy" of the substrate/coating system. One may prefer to promote wetting and hydroxyl affinity providing related "polar groups" on a TFE or FEP soft pad surface (these increasing surface energy E , e.g., vs. other coatings which raise E ). A methyl methacrylate, or MMA
provides such a (compatible) polar group. One may deposit such a "polar strike" by plasma (branching) polymerization (e.~., for 10~ minutes in the case of MMA) or by plasma etching or the like. Alternatively, one may lower E and favor coating of such a soft pad via a light transparent "striXe" of metal or metal oxide (these raising E and improving wetting). As a feature hereof, such "soft pad"
supercoatings will be seen to give strong adhesion to a hard supercoat thereon, but be coupled relatively loosely to the underlying absorber layer.
The rest of the overcoating O-C on absorber e (i.e., the outer portion) is made up -- according to a .

.
.

.. . . .
. , - ~ . .
' ' ~

~7;~82 ~ 30 -related feature hereof -- of "Hard" overcoating layer g, comprised of the below-specified acrylic-epoxy. This serves not only to provide outer mechanical protection and the needed defocusing thickness (with pad f), but also serves as a good vapor barrier and anti-static surface. The preferred formulations for hard overcoat g and related preferred methods for preparing and applying such are detailed below.
The thickness of layer g will, to some extent, depend on the optical system used (e . g., correcting spherical aberration in the focusiny objective may be involved); it has been found that thicknesses on the order of 200 micrometers are quite suitable for this embodiment.
--Results: (Ex. I, FIG. 1):
The "hard/soft" overcoat embodiment suggested above (with the acrylated epoxy as in Mix T-l below, applied on the "soft pad" with underlying absorber, tri-layer, etc.) will be seen to give surprisingly good sensitivity (e.g., superior to analogous records where a thick sio2 overcoat overlays the absorber), as well as providing the other desired characteristics mentioned above (e.g., Table I).
Of course, workers will understand that this embodiment is rather generally described, with ~urther particulars of materials, deposition, etc., of the "Hard"
and "Soft pad" coatings given elsewhere herein (cf. "Hard"
Example II below, etc.).

~t7~Z~3~

The Hard/soft overcoating will be recognized by workers as superior to such common (non-composite) coatings as fused silica (e.g., reducing required write-energy, giving longer; better environmental stability and service -- especially in respect of moisture uptake).
The "Hard" overcoat resulting not only combines well with the ~'soft pad~ (e.g., bonding satisfactorily thereto); it also exhibits the usual properties expected of such a protective outercoat (e.g., hardness, abrasion resistance, non-tacky), be readily cleaned (e.g., of dust, oil, fingerprints), be clear and transparent to ~ r and exhibit low permeability to contaminants like water vapor, o~ygen, etc.
Such a Hard coat material is preferably applied by spin-coatin~ (according to present good practice) or by other suitable techniques known to workers ~e.g., in certain instances, spray-coating, dip-coating, flow-coating or curtain coating may be feasible alternatives).
Radiation-cured acrylic-epoxy coatings like those detailed below will be understood as apt for most such instances.
--Other materials for Hard/soft overcoating:
Workers will understand that, in appropriate instances, other "soft pad" and/or "hard overcoat"
materials may be used to effect some or all of the indicated functions of the preferred embodiments here detailed. For instance, in certain instances the hard overcoat may take the form of a transparent pre-formed sheet of acrylic-epoxy -- laminated onto the "soft pad~
or such a soft pad laminated onto such a sheet -- in some instances the "soft pad" may also serve as the adhesive for the Hard coat.

. .

.
. . .
. - . . . .
.. . . .

~ ~7728~:

~ 32 ---Preferred "Hard overcoat" materials:
Expanding on the foregoing, we will next describe a family of materials which are especially, and surprisingly, apt for use in protective "Hard" overcoatings like those above discussed (i.e., as a supercoating over a "soft pad"
on the OD disk of FIG. 1, etc.). Thereafter, we will describe a preferred novel associated technique ~or applying such "hard coating" material to an OD disk or like substrate.
Example I-A: ("Hard" coating for Ex. I;
Preparation, application, curing):
This Example is intended to describe the preparation and characteristics of a apreferred radiation-cured acrylic-epoxy hard coating mixture T-l as applied to the Example I
embodiment (on "soft-pad" supercoat overlying th~ absorber) and also to describe a gensral method of applying this to a substrate and then curing it in situ. Later, further details of a particular preferred method for applying this to a prescribed optical data disk will be described (see description below in connection with FIG. 2).
Workers will agree that the desired "Hard"
outercoat for such optic~l disks should not only function as a protective layer (to protect the media from dust contamination and environmental degradation, etc.), but also should have other properties such as high optical transmission, minimum effect on sensitivity and S/~
performance of the medium, W -curable coatings are more acceptable for industrial applications than conventional thermal-cured coatings, e.g., because they have a faster cure cycle, less en~rgy consumption and less environmental pollution (no problematic solvent emission). It should , .

~ 2~72132 not attract dust (be static-free) should be very "clear"
and highly transparent to R/W ~ , very strong, somewhat flexible, adhere well to "soft pad" and not badly degrade optical R/W performance. It should have good mechanical integrity despite humidity/temperature cycling te-g-, not be brittle or easily fracture, no delaminate or curl due to internal stress) and have good abrasion-resistance.
In general, the UV curable coating here will comprise an unsaturated resin, unsaturated monomer and photoinitiator. The formulation of ingredients is "state of the art", but requires a complete understanding of the functions of the constituent parts and their function.
Working at room temperature and otherwise standard conditions, the following "Hard overcoat" prepolymer mixture T-l is prepared, being intended for application as a "Hard" protective overcoating, about 7-10 mils thick, and having the described characteristics aa uniformly spread and cured on a prescribed optical data disk surface.
This surface may be understood as comprising a properly-treated aluminum disk substrate (e.g., with smoothing pre-coat thereon) with a tri-layer optical recording matrix superposed thereon, followed by a plasma polymerized thin, "soft pad" supercoating (of "soft pad"
fluoropolymer). Such a fluoropolymer is, thus, the sub~trate of choice here.

:~ ' ` ~ . ' ` , `

_ 34 ~ 7~8~

Mix T-l Wt ~ -Approx.
Pref.Ran~ e "Celrad*3701" (Acrylated epoxy resin) 36 30-40 TMPTA (Tri-acrylated monomer - for cross-linking) 24 ~0-30 2-EhA (mono-acrylated monomer;
to flexibilizP) 36 30-40 FC-430*(Fluoro-carbon wetting agent) 1 0.5~2 I-184*(non-yellowing UV-initiator) 2 1 4 Z-6020*(clarifying adhesion-promoter) 1 0.5-2 The Celrad 3701 (Celanese Corp.) will be understood to be an acrylated epoxide "basic" bulk-resin which is readily cured by ultraviolet light (as below) when properly initiated. This basic resin is selected to impart the desired strength and chemical stability to the cured coating over relatively extend~d service life; and because it very quickly and conveniently cures and yields fairly good clarity. Importantly, it allows relatively little moisture absorption. Also, like all the other constituents it is preferred here because it is generally low~cost, easy to formulat2 and apply, and because it yields the desired "archival" protective coating (as mentioned elsewhere).
The viscosity of Mix T~l ~hould be monitored lest it become too thick and viscous to apply readily (see preferred spiral application technique below -- e.g., mix must flow through a dispensing nozzle). Also, the cured coating should exhibit little or no water uptake lest it might later tend to swell and crack.
However, since 3701 can tend to discolor slightly over time, it should be used with additives that promote clarification and resist yellowing as noted below.
* Trade Mark `, '~
.
~ ' . ' , ~X7~2~

WorXers will recognize that other like, low-viscosity co-monomers (or pre-polymer, low-viscosity diluents) may be substituted, adjusting viscosity accordingly. For instance, certain other Celrad formuLations may be suitable irl some S instances. However, other common coating polymexs are not feasible; for instance, acrylated urethane which is prone to cause "orange peel". Other like acrylated resins are not apt for substitution. For instance, Celrad 3200, another acrylated epoxide is apt to induce coating separation, delamination, cracking or racture (is less viscous, with more flexibility and less tensile strength). Celrad 1700 (acrylated acrylate) gives similar problems. And moisture-intrusion and shrinkage can be reduced by adding a saturated resin (e.g., a derivative of polystyrene like polyvinyl acetate).
The trimethyl-ol propane triacrylate (TMPTA) is a trifunctional acrylate monomer, serving to promote cross-linking in this mixture. Other like (acrylate) cross-linking agents might be substituted, such as (trLmethyl-ol trimethacrylate). Some such cross-linker will usually be used -- to enhance coating strengtn, etc., as workers well know -- preferably (another acrylate cross-linker).
Elimination o~ TMPTA or the Celrad (without replacing by equivalents) will tend to soften the cured overcoat and reduce shrinkage.
The 2-ethylhexyl acrylate (2-EHA, Celanese Corp.) is a mono-functional acrylate monomer, supplemental to the "3701" and added, here, to improve flexibility of the final poLymer coat. WorXers will recoynize that other such diluents may be substituted such as isodecyl acrylate or styrene.

. .

. .
, -- , . . . . .

772B%

The "Irgacure*184" t"I-184"; Ciba Geigy) is a photoinitiator apt for such (UV) curing of such a mixture.
This UV-initiator is found surprisingly (possibly uniquely) apt for such purposes, especially because it is surprisingly resistant to discoloration (yellowing) of the cured overcoat (e.g., when used in such a mixture as T-l, including Z-6020 as discussed below)0 This is especially surprising because such discoloration (yellowing) does result when a closely-similar companion UV-initiator Irgacure #651 (by Ciba-Geigy also) replaces the Irgacure #184 (possibly because #651 has more unsaturated bonds and/or might include quinonidal end-groups; cf. #184:oC hydroxy-cyclo hexyl phenone; ~651:
2 J 2-dimethoxy-2-ph~nyl acetophenone).
The Z-6020 (by Dow Corning) is a diamino primer added to T 1 to promote coating adhesion (to "soft pad"
substrate) and also to clarify the coating (reduce "yellowing" or amber color otherwise resulting). This clarification is somewhat unexpected. The "yellowing"
mechanism is not fully understood; hydroxyl groups may play a role.
For instance, replacement of Z-6020 with another conventional adhesion-promoter, (Z-6040 or Z-6030 are good), leaves the T-1 coating subject to yellowing.
Thus, it will be understood as critical to the desired results to ~mploy an initiator like I-184 and an adhesion promoter like Z-6020.

=:
By contrast, elimination of Z-6020 in T-l and replacement of I-184 with the mentioned I-651 yielded a coating (T-2) that exhibited decided "yellowing" under (ambient) conditions; also toughness was inferior. Viscosity was about 110 CE~ at 25 C, density 1.07 gm/cc.
* Trade Mark ,~
- ' .

7~

~ 37 -~ ow, replicating Mix T-2, but replacing I-651 with I-184, reduces the yellowing, but still leaves the coating with a light amber tone. This coating is tougher than that of T-2. A thickness of about 10 mils gave transmission of as high as 92.4% at 6328 A.
MIX T-l Now, adding Z-6020 to Mix T-3 to produce T-l, essentially eliminates all discoloration leaving a very clear, transparent coating).
This "promoter" (Z-6020) is believed to react with the moisture and hydroxyl groups in the mix solution. I
believe it removes the amber colorant of the hydroxyl group. Tests indicate this T-l film has much stronger adhesion to the substrate and maintains good flexibility.
(after being in the environmental chamber for 50 hours at 70 C and 80% R.H., this film did not show any crack or delamination on the tri-layered disk).
That lS, a disk with an overcoat film made with T-l mix will pass severe environmental testing conditions (MIL-STD-810C). It can be placed in a chamber with conditions of 70 C and 80% R.~. for 50 hours and will show no visible delamination or cl~cks at all.
The "FC-430" is a fluoropolymer "surfactant"
additive (by 3M Co.) characterized as a "non-ionic surfactant" for organic polymeric coating systems. It is added to promote good wetting, leveling and spreading functions and as a rlow control agent, being adapted for reducing surface tension of certain coatings on certain substrates. It is promoted as being very non-reactive and as compatible with water-based or solvent-based systems .

, :

, , ~ ~ ' . ' ' ..

~;277Z8~

(and with most polymers). "FC-430" might, with certain adjustments, be replaced by a like surfactant such as zonyl FSN*by DuPont.
The Mixture T-l should b~ "viscosity adjusted" to optimize spreading and disk application; here, final viscosity should be about 41 cp (25C, density: 1.07 gm/cc), given the subjsct ambient conditions (room temperature, fluoropolymer substrate surface, etc.).
The T-l formulation (and s;imilar mixtures~ is quite tolerant of any number of other additives of widely varying chemistry; so, where appropriate, these may be added (e.g., an anti-static agent.
--Curing:
With the material spread evenly across the subject disk 5fluoropolymer) surface and essentially all oxygen driven-off (e.g., by N2 or like inert pre-flush, etc., as detailed below), the coating is photo-cured by exposure to ultraviolet light for a few minutes while the disk is slowly rotated. This renders a good fully-cured "hard"
overcoating (no supplemental heat needed, no aging time necessary for complete polymerization).
More particularly, and preferably, a nitrogen pre-flush is invoked (e.g., for about l minute to drive off all oxygen); then exposure, under nitrogen, to UV
for about 3-5 minutes, or sufficient to cure the coating as desired. Preferably, this is done while slowly rotating the disk (e.y., 20 rpm; note: the preferred UV beam falls mostly in ~ 0.3 to 0.4 um. range, with intensity varying with ~ -- e.g., 50 mW/cm2 for 3.5 minutes at .366 um. --longer if less initiator is used).

* Trade Mark .

..
, .
. . ',:
-Workers will recognize that other related techniques and/or materials and associated adjustments may be substituted in appropriate cases, taking care to assure adequate stability (over extended archival life) and to avoid inducing stress cracks or decomposition of materials.
Radiation-curing is preferred over other (superficially-related) methods. For instance, thermal curing is unduly complex and hard to control; also it uses more energy and introduces so]vent pollution risks.
--Results:
Mixture T-l, when so applied on a disk, (fluoropolymer surface) and so cured, will be seen to provide a hard clear protective coating, essentially satisfying all of the mentioned subject requirements; e.g., resisting moisture intrusion (and associated swell-cracking, shrinkage), with fine optical clarity and exhibiting good scratch resistance, while being easily surface cleaned.
Moisture resistance was particularly surprising and impressive -- e.g., though not 100% impermeable, this hard coat will exhibit no swell-cracking even after extended immersion in water. Similarly, the hard overcoat has been observed to withstand extended extreme temperature/humidity cycli~g (e.g., from room temperature to 140 C and from about 40% humidity up to 80~ humidity, for many weeks).
Further, this Hard outer-coating will be observed to exhibit extended stability -- e.g., withstanding extended exposure to a rather extreme temperature/humidity cycling.
This "stability" and associated toughness, etc., is believed to derive from the relatively cross-linked, long-chain polymer (epoxy) groups produced.

, - - :
- ~ . .
- -:
' ' " ' 7~

AlSO, this hard coat adheres (satisfactorily) to the fluoropolymer "soft pad", as is desired. Such adhesion might not result where the hard coat and/or the "soft pad"
were changed - in such a case, a separate intermediate compatible (e.g., fully transparent) "adhesive inter-layer"
might be called-for; however it is disfavored (e.g., it complicates thickness control).
EXAMPLE II
Example I is repeated, except that proportions are 10 modified as below (Mix T-4), otherwise it is similarly fonmulated, applied and cured.
Mix T-4 Parts by wt.
Celrad 1700 17 Celrad 3200 17 2-EHA (ethylhexyl acrylate)31 Darocur*1173 (vs. I-184) 2 --Results: 100 The results were essentially like those in Example I, except that the overcoat was more brittle and more prone to moisture lntrusion and "swell-cracking". Compared with T-l, this mix ~ave a coating with much less ultimate tensile strength (e.g., _ 2000 psi; vs. about 4000 psi with T-1).
EXAMPLE III
Another alternative Mix, T-5, is formulated, applied and cured as with T-l.
Mix T-5 Parts by wt.

TRPGDA (Celanese) 39 ~-VP (GAF) 14 Methyl diethanolamine 3 Irgacure 651 5 * Trade Mark ffJ`~

`-: - -, : ' , ' ':: ' ' ~ ' :
'- :

-. ~ , .

~ ~77;ZB~

--Results:
Essentially as with T-l, except for improved surface hardness, but with orange peel on the surace.
--Disfavored formulations:
Somewhat surprisingly, cert'ain similar "radiation-cured acrylic" mixtures do not seem practical and are disfavored for the instant purposes. For instance, a formulation like Mix T-6 below will not be ufficiently clear and transparent (at the contemplated 0.4 - 0.8 um.
wavelengths).
Mix T-6 Mix T-l is replicated, except that Z 6030 replaces the Z-6020 "adhesion promoter".
--Results:
Clarity badly impaired; Z-6020 evidently incompatible with the other ingredients.
--Coating methods:
Following are examples of novel techniques for depositing "hard coating" mixtures like those in the foregoing Examples onto OD disk substrates (like fluoropolymer) to yield an outer protective overcoat thereof -- especially one that is several mils thicX, yet highly-uniform, is radiation-cured in situ, giving the mentioned environmental and other protection for such a disk over a prescribed extended life. Workers will recognize that these techniques emphasize convenient, cost-effective methods of coating and curing, with very close control of thickness, and thickness uniformity.
While the subject coating is applied to give a highly uniform thickness of about 7 mils, workers will appreciate that thicknesses of up to about 20 mils can ba satisfactorily rendered.

.

~2~7~8~

Workers will recognize that "hard coat formulations"
like those described are quite apt for a "spiral" method of application (e.g., to an OD disk, as below) according to another feature hereof, such material lending itself to such surprising simplicity and ease of dispensing, yet under close control and yielding the described surprisi~gly precise control of thickness uniformity.
Formulation T-1 will now be understood as to be applied to the OD disk surface f in FIG. 1 in a certain preferred spiral fashion. This will be understood as an aluminum disk on which the described tri-layer optical recording structure has been applied and, over this, a layer of fluoropolymer (or of a like "soft pad" polymeric surface).
In general, the method will be seen as involving the deposition of the coating material on the prescribed (fluoropolymer) disk surace in a prescribed number of spiral rows, or "beads" so the beads are ~pread out, or "leveled" into a very smooth, very uniform coating; and thereafter curing and hardening this coating to render the desired "Hard" protective overcoat. Some particular and preferred forms of this application method will now be described.
Example M-l: Application of T-l to fluoropolymer substrate:
Step #l Mix preparation:
A preferred form of the novel coating method will now be described wherein a preferred Hard coating mix (preferably T 1 described above), will be understood as selected, prepared and disposed for application to the disk in a spiral row of uniform symmetrical "beads", being , .

:
, : 1 ~ 7728;~
- ~3 -thereafter "leveled" by a prescribed wetting (to induce a rapid, highly-uniform "leveling" of the beads on the prescribed surface) with the disk contemporaneously rotated slowly -- i.e., just fast enough to induce inter-merging of adjacent beads.
Step #20 Dispense as "Spiral Beads":
More particularly, and with illustrative reference to FIG. 2, Mix T-l will b~ supplied as known by workers to a prescribed controlled-rate dispensing means n (like a syringe-nozzle n, as workers know) affixed on a reciprocable arm A. Nozzle n is adapted and controlled (by known means) to dispense a prescribed, carefully-controlled, uniform stream st of the mix down onto~the - receiving (fluoropolymer) surface on the subject disk d at a constant rate. q'hewhile arm A will b~ understood as to be continuously shifted radially (inward) of disk d, carefully controlled so that this stream ~t moves radially of disk d while the disX rotates whereby to describe the specified spiral SR (e.g~, arm A translated by a linear motor as with magnetic recording heads -- maintaining uniform separation, and size, of the beads). DisX rpm may also be varied, as necessary, (see below). As workers will appreciate, one may vary one or several of the three variables of: disk rpm, arm velocity and dispensing rate, while keeping the other variables constant -- to deliver uniform size beads.
Thus, nozzle n is controllably swept across a prescribed radius of disk d, as the disk is rotated, deploying mix in the continuous uniform spiral SR (of "bead" segments b bsing of uniform separation, size and shape, as workers in the art will appreciate). The Mix may be supplied to nozzle n via a Xnown syringe pump (not .
- ~ :

1277~3?-~, - 4~ -detailed), arranged to dispense at a prescribed rate to form such a spiral (e.g., at 1-3 gm/min. yielding about 40 beads across a 3.5" radial band Bb).
Control of mix viscosity is ound to be very important to get good distributioIl and uniform settling.
Care should be taken to avoid "holidays" or "pinholes" (voids where little or no solvent condenses, giving a different "wetting" there or none at all --note: increased ambient temperature seems to enlarge such voids, probably because too much solvent evaporate6 too fast).
Step #2-A: bead-leveling:
The technique of applying such a precisely-uniform polymer coating (thickness of 170 + 10 um) on a fluoropolymer surface is difficult. The following procedure is a preferred method of overcoating via a spinning technique.
The spinning process includes the dispensing of coating - solutions on a spinning substrate followed by leveling and curing. To have a uniform coating we dispense the exact amount of coatiny solution at low spin speed (preferably 4-16 rpm here) on the substrate surface in a spiral fashion.
It is important that this coating solution properly "wet"
this substrate surface, this is controll d by the viscosity and the surface tension of the coating solution and by the surface tension of the substrate surface material, as well as by outward-spreading forces (the effect of the centrifugal force induced by spinning the disk-substrate).
The coating beads o~ each track will be laid down so as to "just barely" touch one another and thus wet the entire surface. Spinning rpm should be carefully ,, ' '- ' ~, :
, ' , ~7'72~
._ controlled such that the coating solution will not move radially-outward appreciably (under the influence of centrifugal force) yet so the surface tension forces and centrifugal force will overcome the retarding coating viscosity and thus spread the coating solution uniformly.
Because the surface tension of a fluoropolymer is quite low, such an applied coating solution is apt to "wet"
only very slowly. To improve and accelerate such wetting, W2 maintain a relatively low flow-rate (from the dispensing syringe), with a relatively high spinning rpm during dispensing -- leading to a relatively large number o~
relatively "thin" beads (spiral track) on the substrate, with adjacent beads kept tangent to one another and the substrate so-wetted more quickly and completely (across lS its entire surface).
The dispensing rate may be kept, for instance, at a constant 1 gr./min. to 3 gr./min. One possible problem is that the flow stream (bead spiraL) will not be continuous unless the syringe tip is kept relatively close to the (disk) substrate. Thus, to render a continuous spiral track using a tip with .033" ID, one must keep this distance between the tip and the coating surface to about 170-250 um. The tip can now help spread out the dispensed drops and level them. This was observed to work quite successfully.
The dispensing tip was translated radially (i.e., relatively to the center of the spinning substrate) so as to lay down enough beads (tracks) to cover the entire surface. In addition to so controlling radial translation spaed, disk rpm ~spinning speed) was also varied relatively continuously from 4 rpm at the OD to 11 rpm at the ID.

.

~;~77%8.~:

_ ep #3: Cure:
~ fter the entire substrate is 80 covered with coating, the disk is preferably spun-up to enhance (facilitate, accelerate) leveling (here about 4 rpm for about 7 minutes is satisfactory). The coating may then be cured; e.g., 3 minutes under ambient (UV) conditions; then another 3 minutes UV exposure under a N2 enviro~ent. Such an initial "air-cure" (first 3 min.) is preferred to avoid "wrinkling". We find, surprisingly, that if the initial UV cure takes place ln an ~2 atmosphere, the top of the coating is apt to retard penetration of the shorter wavelengths and become "wrinkled" -- evidently because its "base" then cures less (or more slowly -- e.g., it'may remain "fluid" longer).
With leveling complete and the coating thus evenly distributed across the face of disk d, it will now be cured, in situ, (and otherwise treated) to yield the desired hard protective overcoating. Thus, disk rotation may cease and the disk be subjected to curing conditions -- preferably without moving it from the "coating station", lest coating uniformity be disturbed or contaminants be introduced (e.g., dust settle on the now-tacky surface).
UV curing may be invoked at a curing station. That is, with the material evenly spread across the subject disk surface, the coating is photo-cured by exposure to ultraviolet light "under air"; then under an inert atmosphere (e,g., N2 flush to expel all oxygen) until the coating is properly cured and "hard". We find about 3 minutes total exposure to 0.3 - 0.4 um UV (e.g., 50 mW/cm2 intensity at .366 um) "in air"; then a like exposure "under N2" is quite satis~actory.

. .

' ' .

~;~772i~:~
- ~7 -Alternatively, workers will understand that other like curing m~thods (e.g., other radiation) may be used in certain in~tances, with appropriate adjustments (e.g., of the type, concentration of photoinitiator).
--Results:
As mentioned before, the thickness uniformity is quite excellent (on the order of + 168-182 um. over a 3.5" band for a "nominal 7 mil" coating is impressive, especially in view of the simplicity o~ the application apparatus and the type of coating mixture involved). As mentioned, the cure times and temperatures are quite convenient, as are the rest of the treatment conditions.
--Exampls M-2 (SiO~ flash on fluoro~olymer):
Whatever bead application technique is used, it may be advisable to pre-treat the substrate as sugyested elsewhere to enhance wetting, adhesion and related characteristics. For instance, in Example M-l above, or a modification thereof, one may wish to enhance the hydrophilicity of the substrate and the wetting thereto of the T-l beads. In such a case, we have ~ound it advantageous to apply or etch a very transparent "flash"
coating of SiO2 on the fluoropolymer prior to applying the be~ds (of T-l or the like, cf. SiO2 on layer f of FIG. 1).
--"~ncapsulated" record, FIG. 3:
FIG. 3 depicts a modified record R' in the manner of FIG. 1 and with all elements thereof identical (prime-designation) in structure, material and fabrica~ion to RD
except as otherwise stated. Here, the substrate disk A' is smoothed with a primer coat P' and subbing layer B', on which a mirror layer c' is laid, with a spacer d' atop mirror c' and absorber layer e' atop the spacer. A similar Hard/Soft overcoat OC' is applied atop the absorber e', . .
' ' ' , 7'~
~,.

except that it is made to surround and "encapsulate" the sensitive layers and so enhance archival life. Thus, soft pad coating f' extends beyond the recording tri-layer T-L' and along the exposed periphery of layexs e', d', c' (protectively sealing the outer edges and interfaces thereof) to bond with the radially extended outer poxtion of subbing B'. In liXe fashion, Hard overcoat layer g' is preferably extended radially beyond soft layer f' and subbing B', and down along their outer peripheral edges -- sealingly -- to bond with extanded outer portions of disk A', or primer P' thereon.
--Alternative uses:
Workers will recognize that one may prepare and apply such a "Hard" coating to other, somewhat different, surfaces, such as on a modified "soft pad" coating and, even where the substrate surface is radically different (e.g., a silicone elastomer), workers wlll recognize that an "otherwise-unsuitable" substrate may be pre-coated or otherwise treated, in certain instances, to accommodate application of a "Hard" overcoat as above. For instance, in the plastic coating and converting arts, ways are known for treating a wide variety of polymeric substrates to enhance their "wettability". Such may, in appropriate instances, be adopted and combined with the invention.
It will be understood that the preferred embodiments described herein are onIy exemplary, and that the invention is capable of many modifications and variations in construction, arrangement and use without departing from the spirit of the invention.
For example, "~ard" outer coatings like those here taught may, of course, be used to cover and protect other .' t ~ ~77Z15 2 substrates for like purposes, and may be applied in other than the described "spiral" coating methods and may be applied in other than the describPd "spiral" coating methods (and with other materials, with appropriate adjustments).
Such coating structures may in appropriate instances be otherwise rendered -- e.g., deposit a "soft pad" onto a "Hard coating" substrate (e.g., onto an epoxy acrylate disk), then deposi-~ the absorber onto soft pad, then deposit spacer/reflector, etc., onto absorber as required, and, finally, applying adhesive and prec;s-bonding thi'; onto associated "Winchester disX", or li.ke "carrier".
Further modifications of the invention are also possible. For example, the means and methods disclosed herein are also applicable to "soft pad" coated recording tape, floppy disks and the like. Also, the present invention is applicable for providing a like protective outer coating for media used in other forms of recording and/or reproducing systems, such as those in which data is recorded and/or reproduced using exposure with different radiation.
The above examples of possible variations of the present invention are merely illustrative. Accordingly, the present invention is to be considered as including all possible modifications and variations coming within the scope of the invention as defined by the appended claims.

Claims (40)

1. A method of applying a coating of moderate-to-high viscosity to a disk substrate surface to yield very precise thickness uniformity, comprising the steps of:
formulating a "hard" protective coating mixture;
applying this mixture in a stream, with mechanical leveling action, to the disk so as to form one or several arcuate bead segments of constant cross-section, extending arcuately about the disk surface;
the mixture being so formulated and so applied as to be capable of quickly leveling at the prevailing temperature and so providing the desired coating thickness; then curing in situ, wherein the mixture is so formulated and the bead segments are so distributed, in tangency or near-tangency to one another, as to be "self-leveling" and to "wet" the disk surface.

2. The method as recited in claim 1 wherein the mixture is dispensed as a continual stream through nozzle means, kept close to the disk surface for said leveling; being formulated to exhibit a viscosity apt for such dispensing, as well as to exhibit a thixotropy and wetting characteristic relative to said disk surface whereby the deposited bead segments will "set-up" thereon for sufficient time to allow the leveling action.

3. The method as recited in claim 2 wherein the disk surface and nozzle means are kept rotating slowly relative to one another, in a uniform relative velocity, with the nozzle means continually shifting radially-inward.

4. The method as recited in claim 3 wherein the disk surface comprises the surface of a "soft pad"
modified fluoro-polymer; the mixture being adapted to quickly "wet" this fluoro-polymer surface and self-level the bead segments with little or no radial shift or mass, given the prevailing temperature, disk rpm and centrifugal force, and the viscosity and surface tension of the mixture as applied.

5. The method as recited in claim 4 wherein the cured coating is several mils thick.

6. The method as recited in claim 5 wherein the mixture is pumped at several gms/min through a syringe-type nozzle held about one hundred to several hundred um above the disk surface, rotating at about 4-10 rpm, to help quickly level the bead segments by "topping" them, wherein the curing is principally, or solely, carried out with UV radiation and with little or no need for supplemental heating, and wherein the UV exposure takes place first in ambient air, then is accompanied bv an inert atmosphere, so as to minimize "wrinkling".

7. The method as recited in claim 6 wherein about 4-6 beads/cm. radially are applied to yield a highly uniform coating thickness of about 7 mils.

8. The method as recited in claim 6 wherein the cured coating thickness varies from one hundred to several hundred µm over a 7-10 cm. radial band and wherein the curing is carried out principally, or solely, with UV
radiation and with little or no need for supplemental heating; and the UV exposure is applied, first for one to several minutes in ambient air, then for one to several minutes accompanied by an inert atmosphere, whereby to minimize "wrinkling".

9. The method as recited in claim 6 wherein the mixture includes UV-curable monomer or pre-polymer moieties having viscosity apt for such nozzle-dispensing, the radiation being after the coating step.

10. The method as recited in claim 2 wherein the mixture is so formulated and the bead segments are so distributed, in contiguity or near-contiguity to one another, as to be "self-leveling".

11. The method as recited in claim 2 wherein the bead-forming step is followed by application of a solvent adapted to wet said mixture and the disk surface, this being applied to wet and coat the so-applied beads and the intervening disk surface relatively continuously, this solvent also being selected and applied so as to induce and enhance said leveling.

12. The method as recited in claim 1 wherein a bead of each bead segment is applied as one continuous spiral with spacing suitable for successive adjacent segments kept spaced by a bead-spacing suitable for such leveling action.

13. The method as recited in claim 12 wherein the mixture is dispensed through nozzle means, being formulated to exhibit a viscosity apt for such dispensing, as well as to exhibit a thixotropy and wetting characteristic relative to said segments and said disk surface whereby a bead of each bead segment will "set-up"
thereon for sufficient time to allow the leveling action, with the aid of contact with the nozzle means.

14. The method as recited in claim 13 wherein the mixture includes at least one acrylate plus at least one associated viscosity-controlling diluent and a compatible cross-linking entity, along with one or more compatible surfactants adapted for imparting said "wetting" and said "set-up".

15. The method as recited in claim 2 wherein said nozzle means is adapted and controlled to so dispense the mixture at a predetermined delivery rate, while "blading"
the mixture "level"; wherein the nozzle means is mounted on arm means adapted and controlled to be translated radially of said disk.

16. The method as recited in claim 15 wherein the delivery rate, translation rate and disk rpm are inter-controlled to effect deposition of said segments of a uniform size and configuration, and as a constant segment separation.

17. The method as recited in claim 16 wherein deposition places said segments contiguous, or nearly so, whereby to induce self-leveling and wherein deposition proceeds "from outward-inward" so as to help compensate for inward-pushing by an adjacent segment.

18. The method as recited in claim 17 wherein for a constant delivery rate, disk rpm is continually increased as a function of segment position, radially, and arm velocity is also controllably varied whereby to improve the thickness uniformity of the ultimate coating.

19. In a record unit comprising a relatively flat disk recording surface with at least one thermal recording area wherein information is to be thermally recorded with prescribed laser radiation, the combination therewith of:
A hard protective outer-seal coating, so formulated and so applied to the outer surface of the recording area as to provide a mechanical/chemical barrier, while also being relative-ly transparent to said radiation, this coating being comprised of the self-leveling polymerization product of a formulation including at least one acrylate pre-polymer, wherein the formulation includes one or more acrylate pre-polymer entities, and a compatible co-functional low molecular weight flexibilizing diluent, together with sufficient low viscosity cross-Linker to give the desired coating toughness, plus sufficient compatible low molecular weight diluent to give the needed low viscosity for application to the disk surface.

20. A record unit as recited in claim 19 wherein the pre-polymer entities include a clear, high molecular weight acrylate plus sufficient related low-molecular weight compatible diluent to reduce viscosity to allow convenient application and spreading of the formulation.

21. A record unit as recited in claim 20 wherein the cross-linker comprises a multi-functional acrylate which is a triacrylate or trimethacrylate of tri-methyl-ol propane.

22. A record unit as recited in claim 21 wherein the entities comprise a high molecular weight acrylated acrylate plus sufficient low molecular weight acrylated epoxide to so reduce viscosity.

23. A record unit as recited in claim 22 wherein the formulation also includes sufficient of an organic flexibilizer-diluent to further lower viscosity enough for convenient application spreading.

24. A record unit as recited in claim 19 wherein the record unit is an optical mass memory disk for laser recording and wherein the coating is the order of several mils thick having been applied over a recording layer, or over a super-coat thereon.

25. A record unit as recited in claim 21 wherein the polymer groups have been radiation-cured.

26. A record unit as recited in claim 19 wherein a "soft pad" isolation layer is interposed between the recording area and said outer-seal coating.

27. A record unit as recited in claim 26 wherein the isolation layer comprises a fluoropolymer.

28. A record unit as recited in claim 27 wherein the fluoropolymer is the product of vacuum evaporation and condensa-tion-repolymerization.

29. An optical disk record comprising a disk substrate including at least one recording area wherein-information is to be recorded with prescribed laser radiation, and a relatively "hard" outer-seal coating has been applied over the disk substrate to a very high thickness uniformity, this coating being derived from the polymerization product of a formulation including at least one acrylate pre-polymer;
and having been applied in a stream to the outer surface of the recording area, while the disk substrate was rotating at a prescribed rpm, so as to form one or several annular bead segments, extending circularly or spirally about the disk surface;
the coating mixture having been so formulated and so applied as to be capable of leveling and so providing the desired coating; this coating serving as a mechanical/chemical barrier, while also being relatively transparent to said radiation, wherein the mixture has been so formulated and the beadsegments so dis tributed in contiguity or near-contiguity to one another, as to be "self-leveling".

30. The record as recited in claim 29 wherein the formulation includes one or more acrylate pre-polymer entities, and a compatible co-functional low molecular weight flexibilizing diluent, together with sufficient low viscosity cross-linker to give the desired coating toughness, plus sufficient compatible low molecular weight diluent to give the needed low viscosity for application to the disk surface; and also sufficient of a "setting-surfactant" to promote this bead "set-up".

31. The record as recited in claim 30 wherein the pre-polymer entities include a clear, high molecular weight acrylate plus sufficient related low molecular weight compatible diluent to reduce viscosity to allow convenient spreading of the formulation.

32. The record as recited in claim 31 wherein the cross-linker comprises a multi-functional acrylate which is a triacrylate or trimethacrylate of tri-methyl-ol propane.

33. The record as recited in claim 30 wherein the entities comprise a high molecular weight acrylated acrylate plus sufficient low molecular weight acrylated epoxide to so reduce viscosity.

34. A record as recited in claim 33 wherein the formulation also includes sufficient or an organic flexibilizer-diluent to further lower viscosity enough for convenient application spreading.

35. The record as recited in claim 30 wherein the record unit is an optical mass memory disk for laser recording and wherein the coating is the order of several mils thick having bee applied over a recording layer, or over a thin super-coat thereon.

36. The record as recited in claim 32 wherein the pre-polymer entities have been radiation-cured.

37. An optical disk record comprising a disk substrate having at least one recording area wherein information is to be recorded with radiation, plus a protective outer-seal coating of very high thickness-uniformity applied over the disk substrate, this coating comprising the product of a formulation including at least one acrylate pre-polymer applied on the record-ing area, while the optical disk record was rotated at a prescribed rpm, so as to form one or several annular bead segments, extending circularly or spirally about the disk surface;
this formulation having been so formulated and so applied as to be capable of leveling and so providing the desired coating; this coating serving as a mechanical/chemical barrier, while also being relatively transparent to said radiation; and having been cured in situ;
wherein the formulation includes one or more acrylate pre-polymer entities, and a compatible co-functional low molecular weight fexibilizing diluent, together with sufficient low viscosity cross-linker to give the desired coating necessary toughness and strength,plus sufficient compatible low molecular weight diluent to give the needed low viscosity for application to the disk surface; and wherein the pre-polymer entities include a clear, high molecular weight acrylate plus sufficient related low molecular weight compatible diluent to reduce viscosity to allow convenient spreading of the formulation.

38. The record as recited in claim 30 wherein the bead has been applied as one continuous spiral with successive adjacent segments kept spaced by a bead-spacing apt for such leveling action.

39. The record as recited in claim 32 wherein the mixture has been dispensed through a nozzle, being formulated to exhibit a viscosity apt for such dispensing, as well as to exhibit a thixotropy and wetting characteristic relative to said disk surface whereby the beads will "set-up"
thereon for sufficient time to allow the leveling action:
the mixure having been formulated to include said "setting-surfactant" specified for the indicated "wetting", while also imparting said "set-up" action or the beads.

40. The record as recited in claim 29 wherein the mixture includes at least one acrylate plus at least one associated viscosity-controlling diluent and a compatible cross-linking entity, along with one or more compatible surfactants adapted for promoting "wetting" of the beads to the disk surface and bead "set-up"
thereon; and wherein the disk surface is comprised of a polymer and a corresponding surfactant tailored to impart said wetting to the beads and this polymer surface, as well as to "set-up" the beads.