Note: Descriptions are shown in the official language in which they were submitted.
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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).
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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
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"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
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(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
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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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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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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
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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).
-
'
..
~2~7%~3~
-- 19 --
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.
" ' : , .
- '
~7~
- 20 -
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
- 21 -
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;~
TABLE 1
~"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
- 23 -
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
,
- 24 -
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
- 25 -
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~
- 26
"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~
- 27 -
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
-- 28 --
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
.-.,
.
- 29 -
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
- 33 -
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~
- 35 -
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%
- 36 -
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.
MIX T-2
=:
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
,~
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MIX T-3
~ 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
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(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
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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.
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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
TMPTA 32
2-EHA (ethylhexyl acrylate)31
FC-430
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.
RDX-52225 39
TRPGDA (Celanese) 39
~-VP (GAF) 14
Methyl diethanolamine 3
Irgacure 651 5
100
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--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.
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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
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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
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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
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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.
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_ 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.
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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',
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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
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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.
