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Description  |
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TECHNICAL FIELD
The present invention relates to an improved reflective optical data
storage medium which permits instant laser recording without processing,
derived from photosensitive materials.
BACKGROUND ART
Since the early 1970s, when optical data storage and recording first became
practical, a large number of patents has issued describing such media.
Generally, optical data storage and recording media may be typified as
either reflective or absorptive in nature. Reflective media have a
reflective surface whose reflectivity is altered to encode data.
Absorptive media has an opaque absorptive surface whose opacity is altered
to encode data. There is one particular type of media that permits instant
laser recording without processing. It is sometimes referred to as
direct-read-after-write, or DRAW, media.
U.S. Pat. No. 3,889,272, issued June 10, 1975, to Lou and Willens of Bell
Telephone Laboratories describes the use of a thin metal film and an
anti-reflection layer to make a reflective optical data recording medium.
The thin continuous conductive metal layer is partially melted by laser
pulses, thereby encoding data. The anti-reflection layer is disposed atop
the metal film and is composed of metal crystallites.
U.S. Pat. No. 3,911,444, issued Oct. 7, 1975, to Lou, Watson and Willens of
Bell Telephone Laboratories describes the use of a relective metal film
disposed atop a plastic film. Again, the metal film used in this patent,
as with all of the other metal films mentioned in this prior art section,
is a continuous and electrically conductive film, usually deposited by
vacuum sputtering or evaporation.
U.S. Pat. No. 3,971,874, issued July 27, 1976, to Ohta and Takenaga of
Matsushita Electric Industrial Company describes the use of a thin
reflective film of tellurium oxide. Tellurium oxide is vacuum deposited
onto a transparent base. Initially, the tellurium oxide is brown in color,
and laser recording is possible by either heating the tellurium oxide so
that it changes color or by melting the tellurium oxide and thereby
forming a hole.
U.S. Pat. No. 3,990,084, issued Nov. 2, 1976, to Hamisch and Kaiser of
Robert Bosch Company describes the use of a thin reflective metal film of
bismuth and selenium to form a thin metal film for laser recording.
U.S. Pat. No. 4,000,492, issued Dec. 28, 1976, to Willens of Bell Telephone
Laboratories describes a recording medium comprising a reflective
radiation absorbing metal film disposed on a transparent substrate. The
novel aspect of this patent is the reduction in laser energy required to
create holes in the metal film by the introduction of an anti-reflection
layer between the thin metal film and the incident radiation. The purpose
of the anti-reflection layer is to substantially increase the amount of
energy absorbed from incident laser radiation. The anti-reflection layer
consists of BiS.sub.3, SbS.sub.3, or Se.
RCA has announced a trilayer system which consists of a dark, absorptive
surface layer; a transparent layer; and a thin, continuous metal layer. A
laser burns through the absorptive layer to record data which will be read
as a reflective spot in a dark field.
Eastman Kodak Company has recently disclosed a solvent coatable reflective
optical data storage medium which consists of a dye filled layer disposed
atop a thin, reflective, continuous metal layer. A recording laser beam is
absorbed by the dyed layer, melting it. The dyed layer is transparent to
the reading laser, allowing the data to be read as differential phase
shifts.
All of the above mentioned prior art discloses the use of a continuous
electrically conducting thin metal film to form a reflective optical data
storage and recording medium.
U.S. Pat. No. 4,176,277, issued Nov. 27, 1979, to Bricot, et al. of
Thomson-Brandt describes the use of a thin metal film disposed atop a
thermally deformable plastic. When a small localized area of the thin
metal film is heated to a high temperature, for example by a laser, the
thermally deformable plastic immediately below the heated area of the
metal film deforms thereby encoding data.
U.S. Pat. No. 4,188,214, issued Feb. 12, 1980, to Kido, et al. of Fuji
Photo Film Company describes the use of a thin metal film combined with
one or more metal sulfides. The addition of metal sulfides increases the
absorptivity of the thin metal film to incident recording radiation.
U.S. patent application Ser. No. 55,270, filed July 6, 1979, now U.S. Pat.
No. 4,278,756, by Bouldin and Drexler of Drexler Technology Corporation,
describes a method of making reflective, electrically non-conductive,
silver optical data storage and recording materials. A fine grained
silver-halide emulsion photosensitive material has latent images created
in it, and subsequently the silver halide is subjected to solution
physical development. This solution physical development creates
reflective non-filamentary silver particles which form the reflective
recording and data storage surface. This is a direct-read-after-write
media.
Accordingly, it is an object of the present invention to achieve a more
sensitive and versatile reflective optical data storage and recording
medium. Another object of the present invention is to create a reflective
optical data storage and recording medium using reflective read methods
over a wavelength range of 440 nanometers to 830 nanometers, those
wavelengths being the most common wavelengths for laser sources in the
visible and near-infrared.
DISCLOSURE OF INVENTION
The present invention is a reflective optical data storage and recording
medium wherein reflective metal particles are dispersed in a low melting
temperature suspensive colloid. These metal particles are very small, on
the order of 50 to 350 angstroms in diameter. The metal particles may all
be of one metal, or a combination of two or more metals. The metal
particles are uniformly dispersed in such a fashion that the resulting
medium is reflective either before or after recording and electrically
non-conductive.
A major advantage of the present invention over the above cited prior art
is that recording is accomplished by directly melting the suspensive
colloid at a low melting temperature, not by melting the metal. A number
of arrangements is possible for the disposition of metal particles. For
example, the optical data storage and recording medium may have a
recording and storage layer consisting of one or more metals dispersed
uniformly throughout a suspensive matrix. It may be useful in this
configuration to have more of one metal than another in the recording and
storage layer, resulting in a disproportionate distribution. The recording
and storage layer of the present invention consists of two or more layers
of the same or different particles. In this case, the layers may be in
intimate contact or separated by a transparent layer. When it is necessary
to reduce surface reflectivity and increase absorptivity and optical
density, it is possible to incorporate metal particles having an
alternative absorptive crystal structure, or to incorporate metal sulfide
particles.
These structures allow for a wide range of surface reflectivity, absorption
and recording sensitivity. For example, it is desirable in some
applications to be able to record both through the substrate and from
above the reflective surface. This may be accomplished by making an
article having a transparent supporting substrate, atop which is disposed
a transparent layer of suspensive colloid, atop which is disposed a layer
of suspensive colloid containing very small reflective metal particles of
one or more elements, which in turn is covered by another layer of
transparent suspensive colloid. Alternatively, the transparent suspensive
colloid layers may contain a dye for absorbing coherent recording. After
melting holes with a laser in the metal filled or dye filled colloid
layer, the data may be read by changes, for example, in reflectivity of a
reflective surface or the phase shift through a spot of melted or unmelted
colloid.
The starting material which forms the basis of the present invention is
that described in U.S. patent application Ser. No. 55,270, now U.S. Pat.
No. 4,278,756. Specifically a silver-halide photosensitive medium is
exposed to solution physical development whereby reflective silver
particles are made. Atop this reflective silver layer is disposed at least
one layer of low melting temperature suspensive colloid which contains
either reflective metal particles, metal sulfides or a dye. The
photographic and photoplate industries have amassed substantial knowledge
in the application of colloid layers to other colloid layers. A typical
example is the color tone correcting stripping film which has three or
more gelatin layers disposed atop one another. The dispersion of metal
particles or metal sulfides is also known. One exemplary method is
dispersing an even layer of finely ground (on the order of 0.05 microns)
particles in the colloid layer prior to curing. As the layer cures, the
metal particles are incorporated therein.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a side-sectional view of the optical data storage and recording
medium of the present invention.
FIG. 2 is a side-sectional view of an alternative optical data storage and
recording medium of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is a reflective optical data storage and recording
medium having extreme versatility, comprising very small reflective metal
particles dispersed in one or more layers of a low melting temperature
suspensive colloid. These very small reflective metal particles are on the
order of 50 to 350 angstroms in diameter, although for some purposes
smaller or larger particle sizes may be useful. The reflectivity of a
layer of the reflective metal particles could fall within the range of 20%
to 80%. In some applications, it may be useful to utilize metal particles
having different reflectivities so that it is possible to vary the overall
reflectivity of the resulting optical data storage and recording medium.
In other applications, a low reflectivity may be useful. This is
accomplished by incorporating metal sulfides or absorptive crystalline
metal particles in the colloid layer.
These very small reflective metal particles are dispersed in a low melting
temperature suspensive colloid. Such dispersal separates the individual
particles so that the recording and storage layer is electrically
non-conductive. As used herein, electrically non-conductive means that
there is a minimum resistance of 1 million ohms per one linear inch
created by the array of metal particles. Clearly, if the distance between
metal particles is sufficiently small, the recording and storage layer
will become electrically conductive. Virtually any metal solid at ambient
operating temperatures, may be used to form the reflective metal
particles. Liquid metals, for example mercury, are unuseable. The major
drawback of electrical conductivity is that it also allows for thermal
conductivity, thereby decreasing sensitivity for recording purposes by
conducting heat away from the point of recording. Indeed, the thin metal
films of the above cited prior art other than patent application Ser. No.
55,270, now U.S. Pat. No. 4,278,756 are all electrically conductive; and
where the thin metal film is created by vacuum sputtering, the metal
particles which are sputtered on are in intimate contact with each other
and not dispersed.
The low melting temperature suspensive colloid supports the dispersed small
reflective metal particles. In the present invention, the melting point of
the low melting temperature colloid is generally between 40.degree. C. and
300.degree. C., although any colloid having a melting temperature below
that of the metal it contains may be used. When the optical data storage
and recording medium of the present invention is used for laser recording,
the suspensive colloid is melted, thereby either reducing reflectivity in
the melted area or opening an aperture in an opaque layer to expose a
reflective underlayer. Unlike the above-cited prior art, with the
exception of patent application Ser. No. 55,270 and the Eastman Kodak
solvent coatable optical data storage medium, it is unnecessary for the
recording laser beam to heat a spot on a conductive metal film as part of
the recording process. The following table of compounds is exemplary of
low melting temperature suspensive colloids:
Gelatin
Polyvinyl alcohol and its esters and acetals
Partial esters of polyvinyl alcohol
Polyvinylpyrrolidene
Acrylamide-maleic acid mixtures
Polyamide-acid mixtures
Mixed polymers of acrylamide-acrylic acid
Polyacrylamide
Polyvinylpyridines and cellulose derivatives
Cellulose phthalate ethers
With reference to FIG. 1, the optical data storage and recording medium of
the present invention consists of at least two layers of low melting
temperature suspensive colloid, where each layer may have dispersed
therein very small reflective metal particles and at least one layer
contains silver. FIG. 1 shows a cross-sectional view of an optical data
storage and recording medium in accord with the present invention. Medium
11 is comprised of a supporting substrate, 15, atop which are disposed two
low melting temperature suspensive colloidal layers, 17 and 19. Dispersed
in each of these two layers are very small reflective metal particles, 21
and 23, though either layer may not contain metal particles but rather may
be transparent or dyed. The low melting temperature suspensive colloid
layers are 10 microns or less in thickness, although for very high density
recording it is more useful to have a total thickness on the order of
one-tenth micron. The very small reflective particles, 21 and 23, are
evenly distributed throughout their respective layers. This uniform
distribution creates a uniform reflective surface, which is advantageous
in laser recording and data reading. Supporting substrate, 15, is
generally transparent, although it is possible and in some applications
more practical to use an opaque substrate. The supporting substrate should
be dimensionally stable in both the reading and recording modes for very
high density recording. For example, if the optical data storage and
recording medium is a videodisc, it will be rotated at approximately 1500
rpm. If the modulus of elasticity and orthogonal directions is not the
same, such as in the case of extruded plastics, the substrate will
stretch, causing problems in data reading and writing. However, such
materials can be used at lower rotating speeds or for larger data spots.
Also, sheets of such materials may be bonded orthogonal to one another to
balance out the anisotropies in elasticity. Generally, glass cast plastics
are preferred substrates for rotating discs.
The very small reflective metal particles dispersed in each of the two
colloidal layers are composed of different elements, or different crystal
structures. Since different elements and different particle sizes have
different absorptivities, it is possible to create an optical data storage
and recording medium which would meet virtually any desirable
reflectivity, absorptivity, or opacity specifications.
In accord with the present invention, at least one layer of suspensive
colloid must contain silver. To obtain this layer of silver, it is
preferable to start with the finished product of the prior invention
disclosed in co-pending U.S. application Ser. No. 55,270 now U.S. Pat. No.
4,278,756. In that application a reflective optical data storage and
recording medium is made by silver diffusion transfer. This process starts
out with commercially available silver-halide photosensitive media.
A very thin, highly reflective surface may be formed by the diffusion
transfer of complexed silver ions to a layer of silver precipitating
nuclei. This reflective layer is electrically non-conducting and has low
thermal conductivity and may be patterned photographically, these latter
two properties being highly desirable for laser recording media. The
complexed silver ions are created by reaction of an appropriate chemical
and the silver halide used in conventional silver-halide emulsions. A
developing or reducing agent must be included in this solution to permit
the complexed silver ions to be precipitated in the nuclei layer. The
combination of developing agent and silver complexing solvent in one
solution is called a monobath solution. Preferred monobath formulations
for highly reflective surfaces include a developing agent which may be
characterized as having low activity. The specific type of developing
agent selected appears to be less critical than the activity level as
determined by developer concentration and pH.
The developing agent should have a redox potential sufficient for causing
silver ion reduction and adsorption or agglomeration on silver nuclei. The
concentration of the developing agent and the pH of the monobath should
not cause filamentary silver growth which gives a black low reflectivity
appearance. The developed silver particles should have a geometric shape,
such as a spherical or hexagonal shape which when concentrated form a good
reflectivity surface.
Developing agents having the preferred characteristics are well known in
the art and almost any photographic developing agent can be made to work
by selection of concentration, pH and silver complexing agent, such that
there is no chemical reaction between the developing agent and complexing
agent. It is well known that photographic developing agents require an
antioxidant to preserve them. The following developing agent/antioxidant
combinations produced the typical indicated reflectivities for exposed and
monobath developed Agfa-Gevaert "HD Millimask" photoplates.
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For Monobaths Using Na(SCN) As a Solvent
And Silver Complexing Agent
Approximate
Developing Agent
Antioxidant Maximum Reflectivity
______________________________________
p-methylaminophenol
Ascorbic Acid
46%
p-methylaminophenyl
Sulfite 37%
Ascorbic Acid
-- 10%
p-Phenylenediamine
Ascorbic Acid
24%
Hydroquinone Sulfite 10%
Catechol Sulfite 60%
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For Monobaths Using NH.sub.4 OH As a Solvent
And Silver Complexing Agent
Developing Agent
Antioxidant
Typical Reflectivity
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Hydroquinone Sulfite 25%
Catechol Sulfite 30%
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The preferred solvents/silver complexing agents, which must be compatible
with the developing agent, are mixed therewith, in proportions promoting
the complete diffusion transfer process within reasonably short times,
such as a few minutes. Such silver complexing agents in practical volume
concentrations should be able to dissolve essentially all of the silver
halide of a fine grain emulsion in just a few minutes. The solvent should
not react with the developing silver grains to dissolve them or to form
silver sulfide since this tends to create non-reflective silver. The
solvent should be such that the specific rate of reduction of its silver
complex at the silver nuclei layer is adequately fast even in the presence
of developers of low activity, which are preferred to avoid the creation
of low-reflectivity black filamentary silver in the initial development of
the surface latent image.
The following chemicals act as silver-halide solvents and silver complexing
agents in solution physical development. They are grouped approximately
according to their rate of solution physical development; that is, the
amount of silver deposited per unit time on precipitating nuclei, when
used with a p-methylaminophenol-ascorbic acid developing agent.
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Most Active
Thiocyanates (ammonium, potassium, sodium, etc.)
Thiosulphates (ammonium, potassium, sodium, etc)
Ammonium hydroxide
Moderately Active
.alpha.picolinium - .beta.phenylethyl bromide
Ethylenediamine
2-Aminophenol furane
n-Butylamine
2-Aminophenol thiophene
Isopropylamine
Much Less Active
Hydroxylamine sulfate
Potassium chloride
Potassium bromide
Triethylamine
Sodium sulfite
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From the above it can be seen that the thiocyanates and ammonium hydroxide
are amongst the most active solvents/complexing agents. While almost all
developers suitable for solution physical development can be made to work
in the silver diffusion transfer process of the present invention with the
proper concentration and pH, not all solvents/complexing agents will work
within the desired short development time or in the proper manner. For
example, the thiosulfate salts, the most common silver-halide solvent used
in photography and in Polaroid-Land black and white instant photography's
diffusion transfer process, does not work in this process for two reasons.
Its complexed silver ions are so stable that it requires a strong reducing
agent to precipitate the silver on the nuclei, and this strong reducing or
developing agent would have the undesirable effect of developing low
reflective black filamentary silver. It has another undesirable effect,
also exhibited by the solvent thiourea; namely, that it forms black, low
reflecting silver sulfide with the developing silver grains. On the other
hand in the black and white Polaroid-Land process black silver is a
desirable result. Sodium cyanide is not recommended, even though it is an
excellent silverhalide solvent, because it is also an excellent solvent of
metallic silver and would thus etch away the forming image. It is also
about 50 times as toxic as sodium thiocyanate, which is a common
photographic reagent.
The process chemicals can be applied by a variety of methods, such as by
immersion, doctor blades, capillary applicators, spin-spray processors and
the like. The amount of processing chemicals and temperature thereof
should be controlled to control reflectivity. Preferably, the molar weight
of the developing agent is less than 7% of the molar weight of the solvent
since higher concentrations of developing agent can lead to low reflective
filamentary silver growth.
The concentration of the solvent in the form of a soluble thiocyanate or
ammonium hydroxide should be more than 10 grams per liter but less than 45
grams per liter. If the concentration is too low the solvent would not be
able to convert the halide to a silver complex within a short process time
and if the solvent concentration is too great the latent image will not be
adequately developed into the necessary silver precipitating nuclei
causing much of the silver complex to stay in solution rather than be
precipitated. The process by which the silver complex is reduced at the
silver precipitating nuclei and builds up the size of the nuclei is called
solution physical development.
It is important to note that in solution physical development, as used
herein, the silver particles do not grow as filamentary silver as in
direct or chemical development, but instead grow roughly equally in all
directions, resulting in a developed image composed of compact, rounded
particles. As the particles grow, a transition to a hexagonal form is
often observed. If the emulsion being developed has an extremely high
density of silver nuclei to build upon and there is sufficient
silver-halide material to be dissolved, then eventually the spheres will
grow until some contact other spheres forming aggregates of several
spheres or hexagons. As this process takes place the light transmitted
through this medium initially takes on a yellowish appearance when the
grains are very small. This changes to a red appearance as the particles
build up in size and eventually will take on a metallic-like reflectivity
as the aggregates form.
In summary, silver precipitating nuclei are formed on one of the surfaces
of a silver-halide emulsion either in the emulsion manufacturing process,
by actinic radiation, or by a fogging agent; and if this emulsion is then
developed in a monobath solution containing a weak developer and a very
fast solvent which forms complexed silver ions which are readily
precipitated by catalytic action of silver nuclei; and if the solvent does
not form silver sulfide; then a reflective coating is developed on one of
the emulsion surfaces thereby creating a medium for data storage and laser
recording. Any of the common developing agents will work whereas only a
small number of solvents/complexing agents have all of the desired
properties, the most successful of these being the soluble thiocyanates
and ammonium hydroxide.
Atop this layer of silver is disposed one or more layers of metal particles
or a dye, suspended in a low melting temperature colloid. These layers are
applied in the conventional photographic-photoplate manner, save that
metal particles are dispersed in the colloid prior to curing.
Alternatively, a dye may be added prior to curing, in place of or in
addition to metal particles. Since metal particles have a greater density
than the uncured colloid, the metal particles will penetrate the colloid
layer and become incorporated therein. Alternatively, layer 17 may contain
suspended metal particles and layer 19 contain reflective silver
particles. Layer 17 may be applied to substrate 15 in the conventional
photographic-photoplate manner and have metal particles distributed
therein. Layer 19 may be similarly disposed atop layer 17 and would
contain photosensitive silver halide. Layer 19 would then be treated as
described above to produce reflective silver.
FIG. 2 is a side-sectional view of an alternative method of the present
invention which consists of three low melting temperature suspensive
colloidal layers. Optical data storage and recording medium, 25, consists
of supporting substrate, 15, atop which are disposed three low melting
temperature colloidal layers, 27, 29 and 31. It is not necessary that all
three layers contain very small reflective metal particles, but that at
least one of the colloidal layers does. In one alternative medium of the
present invention, metal particles 35 are composed of a different element
than those of 33. Alternatively, particles 35 may be metal sulfide
particles or filamentary silver particles, and reflective metal particles
33 may be silver. This would allow for an optical data storage and
recording medium whose unrecorded appearance would be dark, but wherein
data would be recorded to create reflective spots. A recording laser would
burn through the absorptive sulfide or filamentary silver layer, 31, and
reveal reflective layer, 27. In this configuration, it would be
advantageous for layer 29 to consist of a relatively higher melting
temperature material so that the recording laser would not significantly
melt it.
In an alternative optical data storage and recording medium, layer 29 would
contain reflective silver particles, 37, and layers 27 and 31 would be
either transparent or dyed. When layers 27 and 31 are transparent and if
the substrate, 15, is also transparent, it would be possible to record
through the substrate. The recording laser beam would be focused on the
reflective metal particles in layer 29 either through the substrate or
from above the substrate. Recording through the substrate has two
advantages. First, it would be possible to read the data either through
the substrate or from above; and second, if reading were to be done from
above, a glass protective coverplate could be placed atop layer 31 and any
Newton rings which would result would have a reduced effect upon reading
owing to layer 31.
When layers 31 and 27 contain a dye, it is possible to read the resulting
medium by phase shift at a wavelength where the dye is not absorptive. An
optical data storage and recording medium of this sort could be prepared
as follows. A high-density photosensitive silver-halide photographic
medium, for example a photoplate using gelatin as a colloidal carrier,
could be treated with sodium thiocyanate to remove the silver halide from
the upper third of the photosensitive emulsion. A thin layer of silver
precipitating nuclei is made in the remaining silver halide such that the
greater density of precipitating nuclei is distal to the substrate. The
entire silver-halide photosensitive medium would then be treated as
described in co-pending patent application Ser. No. 55,270 now U.S. Pat.
No. 4,278,756. This creates a thin layer of reflective silver
approximately midway between the air-gelatin and the substrate-gelatin
interfaces. Subsequently, the entire emulsion layer could be dyed. If the
dye is red in color, recording could be accomplished with a blue or
perhaps a green laser. Typically the red dye would have an optical density
of 0.5 to 1.5 in the blue and less than 0.25 in the red. This laser would
melt the dyed gelatin and not affect the reflective layer. Data reading
would be accomplished with a red or infrared laser which would detect a
phase shift in those areas where the gelatin was ablated away by the
recording laser. In an alternative method, the narrow band absorptive dye
could be replaced by a uniform low density distribution of fine particles
of metal 150 to 350 angstroms in size, which would create a red
transparent layer with a typical optical density of under 0.25 in the red
which would be highly absorptive in the blue and green with a typical
optical density of 0.5 to 1.5.
In adding the metal particles the reflective character of the very thin,
highly reflective silver particle surface, previously described, is
modified. In general, the silver surface will become less reflective with
the addition of the particles. The extent of reduction of reflectivity
depends upon the metal and its concentration within the supporting
colloid. This reduction in reflectivity increases the efficiency of the
laser recording medium. A minimum reflectivity should be maintained so
that a satisfactory contrast ratio exists for the selected laser
wavelength and power.
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