Note: Descriptions are shown in the official language in which they were submitted.
Background of the Invention
Photosensitive films comprising silver halides have
been a primary object of photographic research. Although
the photolytic reduction of halides to provide the latent
silver photographic image is of major interest, the reverse
reaction through which metallic silver is reconverted to a
r ~ silver halide by the action of light or heat has also been
the subject of study.
An early discussion of the changes in absorption behavior
. ~ produced in a darkened photographic plate by exposure to red
light is provided by Cameron and Taylor in Photophysical
Changes in Silver-Silver Chloride Systems, Journal of the
Optical Society of Amerlca, Volume 24, pages 316-330 (1934).
These authors verified that optically or chemically darkened
silver halide-containing emulsions can be selectively bleached,
particularly with red light, such that they become more
transparent to light of the bleaching wavelength. This
behavior is referred to as color adaptation. It was further
noted that polarized bleaching light produced a dichroic,
birefringent image in the darkened film.
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Optically-induced dichroism has also been observed in
polycrystalline silver halide layers produced by evaporation
techniques. Enhanced dichroism in silver halide films
containing additions of vacuum-evaporated silver was reported
by V. P. Cherkashin in Soviet Physics - Solid State, Volume
13, Number 1, pages 264-265 (1971).
It has been postulated that the anisotropic absorption
behavior of silver halide films is due to elongated metallic
silver colloids. This hypothesis is not inconsistent with
certain absorption characteristics which have been observed
for granular metal films. The optical properties of some
sputtered gold and silver island films are described by R.
H. Doremus in J. Chem. Phys., Volume 42, pages 414-417
(1964), by R. W. Cohen et al., in Physical Review B, Volume
8, Number 8, pages 3689-3701 (1973), and in other papers.
Summary of the Invention
, ~, We have now discovered configurations for photosensi-
.- J tive thin films, and methods for producing them, by means of
-; ; which full color images may be photographically recorded in
a single exposure utilizing ordinary white light. These
films also permit the recording of optically anisotropic
images, provided that polarized light is used to project the
image to be recorded onto the film. Images thus produced
are both dichroic (light polarizing) and birefringent.
Photosensitive thin films provided in accordance with
the invention are of the additively-colored type, which
means that, as initially formed, they contain metallic `
color centers and thus rather broadly absorb visible light.
,.~
The films include not only a polycrystalline dielectric
film, but also an underlying metallic sub-film, preferably
-2-
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-... . deposited by a sputtering or vacuum evapora~ion technique,
-- which is responsible for the additive coloration observed.
These films are photosensitive as formed, and record
optical information by means of a bleaching process wherein
the array of metallic color centers is modified by exposure
to light of a selected wavelength. The modification is such
that the film becomes substantially more transparent to
light of the wavelength used for exposure, but not signifi-
cantly more transparent with respect to light of other
wavelengths. Moreover, if pclarized bleaching light is
used, the increased transmittance induced by bleaching is
limited largely to light polarized in the same direction as
the bleaching light. In both cases, the degree of bleaching
depends on the intensity of and duration of exposure to the
bleaching light.
~ Photosensitive films provided in accordance with the
- ~ ~ invention accurately record the intensity and direction of
j polarization of light incident thereon, and, in addition,
j are capable of retaining most of the color information
~r~ 20 present in the incident light. Although somewhat less
sensitive to blue and green light than to red light, the
films are nevertheless quite satisfactory for recording
full-color optical-images. The reason for the rather broad
color sensitivity of these films is not fully understood,
~ut is believed to depend on the microstructure of the
films, which in turn depends on the process by which the
films are made. A further advantage of these films is the
extremely high image resolution obtainable therewith, which
is on the order of 1-10 microns. This value compares favor-
ably with similar values for high resolution photographicfilm.
... ~. . I
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Photosensitive thin films provided in accordance with
the invention include at least one two-component structural
layer, hereinafter referred to as a metal-dielectric layer,
which consists of a metal sub-film and a transparent dielec-
tric over-film covering the metal sub-film. The complete
photosensitive film may consist of one or several of such
metal-dielectric layers, depending upon the degree of resolu-
tion or color-reproducing characteristics required in the
completed thin film structure.
The process of providing the film comprises the initial
step of depositing on a suitable support a discontinuous
metal island film. The metal selected for depositing the
` ~ , film must be one which, in island form, exhibits plasma
; ~ absorption of light when surrounded by a suitable dielectric
acceptor material. Such behavior renders the peak wavelength
of absorption and the peak half width dependent upon the
size and shape of the metal islands, as well as on the
refractive index of the dielectric. Among the metals suitable
for making up the metal island film are Ag, Pb, Cu, Al, Cr,
and Ge.
The absorption properties of the metallic phase of the
photosensitive thin film depend on the size and shape of the
i;~ metal particles therein. Although considerably affected by
: subsequent processing, the size and shape of the particles
depend in the first instance on the structure of the metal
island film first deposited. For best imaging properties,
we have found that an island film having an apparent thick-
ness in the range of about 15-150~, which is largely com~
posed of metal islands in a size range of about 100-1000~,
should be provided.
-4-
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` Following the deposition of a discontinuous metal
_ island film as above described, a transparent covering filmconsisting at least predominantly of a suitable dielectric
acceptor material is deposited over the island film. This
dielectric acceptor film must fulfill at least two functions.
First, it must be capable of accepting and conducting away
from a metallic color center photoemitted electrons ejected
from the metal by the action of light. Secondly, it must
~ permit diffusion of the positive metal ions produced by ~ 10 photoemission away from the metallic color centers and into
the dielectric film. Among the dielectric acceptor materials
which have been found suitable for this purpose arelAgCl,
r~ ~ AgBr, Agl and PbI2.
The dielectric acceptor material is intended to form a
transparent covering film over the discontinuous metal
. ,~;.~ .
. ~ island fiLm. In addition, its thickness has some effect on
the color imaging properties of the photosensitive thin film
structure. In general, continuous films of at least about
j 300~ thickness are preferred for best results. The maximum
._;
~ 20 thickness of the film is limited only by the need for trans-
; ~ parency, although resolution tends to be poorer with thicker
films. Films as thick as one micron have been employed, and
--~ thicker films may also be used, although no significant
advantages are offered thereby.
Through the use of the above procedures, a photosensi-
tive medium useful for recording optical information may
readily be provided. This medium includes support means in
the form of a substrate and a photosensitive thin film
deposited on the substrate. The film includes at least one
~ 11~ .
metal-dielectric layer consisting of a metal sub-film and a
transparent dielectric over-film coverin-g the metal sub-
film, produced as above described.
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In general, photos~nsitive thin films including only a
single metal-dielectric layer impart the highest image
resolving power to the recording medium. However, enhanced
color imaging is provided in some cases if several metal-
dielectric layers, e.g. 2-6 layers, are provided on a single
substrate. Photosensitive thin films including two or more
metal dielectrLc layers may be conveniently produced by
sequentially repeating the steps of island-film deposition
and dielectric film deposition as above described.
The photographic recording of a full-color optical
image in accordance with the invention comprises the step of
projecting a selected real full-color image onto a photosensi-
tive thin film which includes at least one metal-dielectric
layer produced as above described. The selected real image
may be projected onto the photosensitive thin film by any
suitable~method, including lenses, imaging mirrors or similar
focusing means. Pre-recorded images such as color transparencies
~ or the llke may be projected onto the film or transferred
_ ~ thereto by "contact-printing", i.e., direct exposure of the
film through a transparency in contact therewith.
The period of exposure required for recording a selected
,~ .
image in the photosensitive film depends primarily upon the
intensity of the recording light and secondarily upon the
particular composition of the film selected for treatment.
However, the proper exposure interval can be readily deter-
.~
,~*~ mined for any particular film and exposure condition by
observing the development of the positive image during the
exposure period and terminating exposure when the optimum `
- image is obtained. ~ i
The capability of photosensitive thin films provided in
accordance with the invention to record information relating
-6-
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to the polarization state of light traversing the film
permits the recording of dichroic images therein. Dichroic
recorded images are produced by projecting onto the film a
selected real image, which may be a full-color image, utilizing
light which is linearly polarized along a selected axis.
This axis is hereinafter referred to as the recording axis.
Images formed using linearly polarized light are dichroic in
that they exhibit good transmittance and are readily viewable
in transmitted light polarized in a direction parallel to
the recording axis, but decreased transmittance and low
contrast and coloration when viewed in light polarized
perpendicularly to the recording axis.
Dichroic images produced by recording in polarized
light offer two distinct advantages over conventionally-
recorded images. First, the color quality of the recorded
., . . . 1 , .
dichroic image is enhanced, by comparison with a conven-
tionally-recorded image, when viewed in white light linearly .'' .-,
polarized in the direction of the recording axis. Secondly,
~ the contrast of the dichroic image can readily be enhanced' 1 ~ 20 by viewing the recorded image in transmitted light while the
film is positioned between crossed polarizers, each polarizer
being aligned with its optic axis (axis of maximum light
,~ transmittance) at an angle of 45 degrees with the recording
axis of the dichroic image. Both advantages involve some
sacrifice in the intensity of the transmitted viewing light,
but this may not be important for some applications.
Conventional and dichroic images produced as described
may be modified by further exposure to bleaching light as
nece-ssary. Of course, when it is desired to preserve the
recorded image for long time periods, care should be taken
to prevent exposure of the recording to stray light. In the
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.~ ~ ~ absence of such light, however, the recording is quite
i stable and may be preserved indefinitely.
Films provided in accordance with the invention have
I utility not only for recording full color photographic
images, but also for other information storage applications
where information relating to the color and polarization
state of incident light is desired to be retained.
Description of the Drawings
_~
The invention may be further understood by reference to
the appended drawings, wherein
FIGURE 1 is a schematic elevational view in cross-
section of a photosensitive optical recording medium, includ-
ing a glass substrate and a photosensitive film, provided in
accordance with the invention, wherein the photosensitive
I ~ film consists of a single metal-dielectric layer which
- ~ includes a discontinuous metal sub-film comprising metal
! islands (Me) covered by a transparent over-film composed of
~ a dielectric acceptor material.
_ FIGURE 2 is a schematic elevational view in cross-
section of another embodiment of a photosensitive optical
recording medium provided in accordance with the invention,
wherein the photosensitive film consists of five metal-
dielectric layers.
:~
FIGURE 3 is an electron photomicrograph of a discon-
tinuous metal island film composed of silver deposited on a i ¦
- support consisting of glass, wherein the white bar repre-
sents a length of 0.5 microns.
FIGURE 4 is a graph plotting the light transmittance
through four selected optically-bleached regions of a multi-
layer Pb metal-AgCl dielectric photosensitive film as a
-8-
92877
;'~
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function of light wavelength. Each of the four selected
regions of the film had been previously bleached with a
different wavelength of visible light, each wavelength
producing a different coloring effect in the photosensitive
film. The unbleached transmittance of the film is also
shown.
FIGURE 5 is a graph plotting the transmittance of
polarized light through three selected optically-bleached
regions of a multi-layer Pb metal-AgCl dielectric film as a
function of wavelength. Transmittance is shown for each of
two orthogonally-polarized light beams. Each of the three
selected regions of the film had been previously bleached
with polarized light having a direction of polarization
corresponding to the light beam producing the parallel
~ transmittance curves shown in the drawing. The dichroism
.~
induced by bleaching with polarized rather than unpolarized
light is evidenced by the fact that, at each of the three
bleaching wavelengths, the transmittance of the glass is
higher with respect to light polarized parallel to the
. ,,
_ 20 direction of polarization of the bleaching light (solid
curve) than with respect to light polarized perpendicular
thereto (broken curve).
. . .
Detailed Description
While the precise mechanism for broad-spectrum photo-
sensitivity in films provided in accordance with the inven-
tion has not been definitely established, it is believed
that the discontinuous metal-dielectric film structure is
_ sensitive to light in the following way. When light is
incident on the structure, the light excites electrons of
_9_
1 '
. . .
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, the metal. This can either be due to interband transition
.,,~ ~ or due to transfer of energy of plasma modes to single
electron states. These plasma modes correspond to collec-
I tive excitations of many electrons and are characteristic of
free or nearly free electrons. If the excited electron
1 level exceeds the energy corresponding to the difference in
.~ energy of the,bottom of the conduction band of the dielectric
q~
'- ~ and the Fermi energy of the metal, then the electron can
escape the metal into the conduction state of the dielectric.
, 10 This process corresponds to photoemission where the effective
, ~ work function of the metal (energy required to remove electron
from metal) is lo,wered due to the proximity of a dielectric
having a suitable conduction band energy.
' ~ Another aspect of the proposed mechanism deals with the
nature of the dielectric film. In addition to providing a
~ way to allow the electron to escape via a conduction band,
,' ~ it also must have the ability to incorporate the metal ions
- '~ into the lattice. The diffusion of the metal ions should be
'b - ~ relatively rapid to allow the metal dissolution process to
proceed rapidly. It has been found that heating the struc-
ture can improve the sensitivity to light in certain cases
where the metal ion has a slow diffusion coefficient at room
,, temperature.
By the combined process of electron emission from the
metal and diffusion of a positive metal ion'into the dielec-
tric lattice, the metal island loses an atom. The continua-
tion of the process of photoemitted electron and diffusion
' of the positive metal ion ultimately dissolves the metal
_ island. One can write then, where (Mn) represents a n-atom
metal island,
-10-
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10~877
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(M)n light (M)n_lM~ + e~
(M)n_lM~ (M)n_l + M
In terms of the transmission of light through the film,
as the light exposure continues, the metal island dissolves,
thus decreasing the absorption, making the film more trans-
mitting in the region of exposure.
The color aspect of the process described above comes
,~ from the fact that if the excited electron state arises
through the damping of the plasma oscillation, then the peak
absorption of the plasma oscillation is dictated critically
~ by the size and shape of the metal island colloid as well as
; ~ the optical constants of the metal and the refractive index
of the dielectric medium surrounding the metal island. If
i ~ the discontinuous metal island film is put down in such a
, ~ ,.............................. .
way as to produce a distribution of sizes and shapes of
metal islands, then exposure of the thin film structure to a
:~ particular visible wavelength of light will preferentially
j remove those metal island colloids whose size and shapeproduce a peak absorption corresponding to the illumination
light wavelength. This renders that portion o~ the structure
more transmitting to that wavelength then to the other
visible wavelengths. It is in this way that full colored
images are produced.
; ~ The explanation of the dichroic effect ~ollows closely
from the postulated coloration mechanism. I~ the metal i q
island colloid is not spherical in shape, then its optical
absorption is also anisotropic. For example, if the shape `
of the metal particle is represented by a prolate ellipsoid
of revolution, then the absorption in the long direction
(major axis) would be greater for light polarized along that
direction then that perpendicular to the long direction. We
-11 -
~ 109~8~
. ~; ` 7
~ , presume that the original orientation of non-spherical metal
- particles is random in space. However upon exposure to
- polarized light, those metal island colloids whose orienta-
tion in space is such that they are more nearly parallel to
the polarization direction of the exposing light will be
dissolved at a more rapid rate than those whose orientation
__
space is more nearly perpendicular to the polarization
~ direction of the incident light. Thus the exposure to
- ~ polarized light renders the thin film structure dichoric.
Of course, the foregoing explanation is hypothetical
only, and is not intended to limit the scope of the inven-
tion as herein described.
The selection of a substrate to be utilized as support
means for photosensitive films to be provided in accordance
with the invention is not critical. Any substrate material
which does not harmfully interact with the dielectric and
~'~ ~ metallic film constituents to be applied thereto may be
:- j employed. Examples are inert ceramic materials, glass-
~. ~ ceramics, and even treated paper. Although reflective
_ 20 substrates can be used, light-transmitting substrates,
preferably transparent, will normally be preferred in order
to facilitate the retrival of stored information from the
~, ' film. The particularly preferred substrate for use in the
invention is glass.
In some cases, it may be desirable to deposit a base
.~
layer on the substrate to insure compatibility with the
photosensitive film. Thus an organic substrate such as
paper may be provided with a base layer of SiO! or a glass
substrate may be provided with a base layer, for example, of
a dielectric acceptor material.
-12-
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~- _3 The deposition of a suitable discontinuous metal island
_ film onto the substrate is preferably acoomplished utilizing
either sputtering or vacuum evaporation techniques. However,
other metal deposition methods, including certain plating
processes, may also be employed. While the apparatus utilized
7~ for the deposition process may be conventional, the deposition
conditions employed have an important effect on the imaging
properties of the resultant film.
D-C or radio-frequency sputtering may be used to deposit
any of Ag, Pb, Cu, Cr, Ge and Al. Control over the thickness
and island size of the discontinuous metal film is achieved
by varying the sputtering voltage, `the temperature of the
, ~ substrate and the biasing voltage between the substrate and
, ~ target. The condition of the target which is the source of
metal for the film is also important; for example, a silver
,1~
target which has been partially oxidized in use appears to
provide better results in certain cases than a clean,
unoxidized silver targ~t.
Vacuum evaporation processes can also be used to pro-
5. . ' . I
duce the island film, and are particula~rly useful for
depositing Ag, Ge, Pb, and Cu. Control over island film
thickness and island size distribution are exercised by
, varying substrate temperature, deposition rate, the temp-
; erature of the evaporated material, and the base pressure of
the vacuum system.
_~___ Table I below sets forth vacuum evaporation conditions
for some metals which are illustrative of the conditions
which may be employed to produce island films therefrom
exhibiting good imaging properties. The table reports the
base pressure in the system, the substrate temperature, and
the deposition rate provided.
- 13 -
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These conditions are of course merely illustrative
of the various deposition conditions which could bP employed.
TABLE I
Evaporated System Base Substrate Film Deposition
j Metal Pressure Temperature Rate
Ag 5 x 10-6 Torr 150C. l~/second
, ~ Pb 5 x 10-6 Torr 25C. lA/second
Cu 5 x 10-6 Torr 25C. lA/second
~; ~ Ge 5 x 10-6 Torr 25C. l~/second
Conditions similar to those described in Table I above
are useful not only to deposit the initial island film, but
also to deposit subsequent films when multi-layer photosen-
sitive films are to be provided.
- Following the deposition of the selected metal island
~ film, the dielectric acceptor material may be vacuum eva-
; ' porated directly onto the metal island film to provide the
.' transparent covering film. The imaging properties of the
. j resulting metal-dielectric layer are affected by the rate of
evaporation of the dielectric, the thickness of the dielec-
tric film, and the composition of the film.
The dielectric covering film should consist at least
- predominantly of one or more dielectric acceptor materials
selected from the group consisting of AgCl, AgBr, AgI and
PbI2. For the purpose of the present description, the
covering film consists predominantly of these acceptor
materials if the materials constitute at least about 50% of
the film. Although the film may consist entirely of these
materials, we have found that the color imaging properties
of the metal dieIectric layer can be improved in some cases
if minor amounts, e.g., up to about 30% by weight, of a
` -14-
~' ' ._
109~77
.~.~.`~
,~ .
~ dopant selected from the group consisting of CuCl, CuC12 and
_ CdC12 is included in the film. T~e decision to include
;~ ~ these optional dopants will depend on the intended use of
the film. The film may of course include other constituents
which do not interfere with the imaging properties of the
system.
. _
Table II below sets forth vacuum evaporation conditions
which are typical of conditions useful in depositing a
transparent dielectric covering film over a metal island
~ 10 film to provide good imaging properties in the resulting
¦ ~ ~ metal-dielectric layer. The table reports the system pressure,
the rate of deposition of the dieIectric, and the temperature
~r
of the substrate for a number of different dielectric acceptor
materials. Again these conditions are merely illustrative
of the range of conditions which may be employed.
.~æi ,
TABLE II
7_7
~ Dielectric System Dielectric
_ ! Acceptor Base Evaporation Substrate
'i; ~ Material Pressure Rate Temperature
__~ 20 AgCl 1 x 10-4 Torr 50~/second 25C.
AgBr 1 x 10-4 Torr 50~/second 25 C.
o
AgI 1 x 10-~ Torr 50A/second 25vc.
PbI2 1 x 10-4 Torr 50A/second 25C.
Of course, alternative methods of applying both the
discontinuous metal island films and the covering dielectric
films may be employed, provided that the requisite film
thicknesses and island size and size distribution for good i
; imaging are provided.
The invention may be further understood by reference to
the following detailed examples illustrating some of the
.
-15-
.
v'~
10~;2877
' preferred procedures for producing and utilizing photosensi-
tive films in accordance therewith.
Exa~
A clean glass slide having a smooth surface suitable
for film deposition is positioned in a vacuum evaporation
coating chamber together with a quantity of AgCl. The
~ chamber is evacuated to a pressure of 5 x 10-6 Torr and,
- ~ while the slide is maintained at about 25C., the AgCl is
heated to cause evaporation and subsequent deposition onto
the slide at a rate of about lOA/second. The deposition
process is continued until an `AgCl film about 400A in thick-
ness`has been provided on the glass substrate.
Following the deposition of an AgCl base layer as
- described, a discontinuous metal island film composed of Pb
is vacuùm-deposited over the AgCl film by heating a quantity
of Pb introduced into the chamber. The chamber pressure is
j maintained at about 5 x 10-6 Torr during evaporation. The
deposition of Pb occurs at a rate of about lA/second and is
continued until a thickness equivalent to about 30A is
provided. Thereafter, a transparent covering layer of AgCl,
about 400A in thickness, is vacuum-deposited o~er the Pb
layer utilizing the procedure previously employed for the
deposition of the AgCl base layer.
The process of Pb deposition followed by AgCl deposi-
tion is repeated three times to provide a photosensitive
thin film structure consisting of a base AgCl layer and four
superimposed Pb metal-AgCl dieIectric layers on the glass ~ ~ ¦
slide. The slide is then removed from the vacuum chamber
and examined. A transparent film exhibi~ing a light purple
coloration in transmitted light is observed on the glass
surface. 7
-16- 1
. .
~ ~ .. . ~
`~it`;~ " 109~
.
. .. . `~ .
The optical properties of the thin film structure thus
provided may be demonstrated by irradiating selected regions
of the film with visible bleaching light of several differ-
! ent-wavelengths for time intervals on the order of about a
; - minute at light intensities on the order of about 10-20
milliwatts/cm2. The results of such bleaching are shown in
FIGURES 4 and 5 of the drawing.
FIGURE 4 illustrates the effect on the transmittance of
the film induced by bleaching with four different wave-
lengths of unpolarized light. Each of the different sourcesproduces bleaching in and near its own wavelength, producing
a different coloring effect in the film.
FIGURE 5 illustrates the dichroism induced by bleaching
with polarized light at three different wavelengths. The
bleached règions exhibit not only coloration but also sub-
_~
I ~ stantially greater transmittance, near the bleaching wave-
__ length, of light polarized parallel to the direction of
~_ , polarization of the bleaching light than light polarized
~ perpendicularly thereto. I
.~ , . I Example 2
A clean glass slide having a smooth surface suitable
for film deposition is positioned in the substrate holder of
a triode-type radio-frequency sputtering apparatus com-
J
prising a silver target. With the substrate holder main-
tained at about 25C., the deposition chamber is evacuated `
to a pressure of 10-6 Torr. The chamber is then back-filled
with argon to a pressure of 5 x 10-3 Torr, and an r-f volt- ~ ¦
age of 400V. is applied to the target. Simultaneously, a
d.c. bias voltage of 45 V. is applied bètween the target and
the substrate. These conditions produce a sputtering rate
-17-
.
, . ,
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,
providing Ag deposition at about 9A/minute onto the surface
, , ,of the glass slide. Sputtering is continued'for a time
interval sufficient to provide an Ag film lOOA in'thickness
on the glass surface.
. , .
' Following the deposition of the silver film, the glass
slide is removed from the sputtering apparatus and posi-
tioned in a vacuum evaporation coating chamber together with
- a quantity of AgCl. The chamber is then evacuated to a
pressure of 1 x 10 6 Torr and, while the slide is maintained
at a temperature of about -70 C., the AgCl is heated to
cause evaporation and subsequent deposition onto the silvered
glass at a rate of,about 50A/second. This deposition process
, is continued until an AgCl film about 1500A in thickness is
provided.
The image-forming properties of the film thus provided
f
are tested by projecting a real image of a photographic
transparency illuminated by a 100-watt tungsten iodide lamp '
, ~, onto the surface of the film with a photographic lens. An
exposure time on the order of about 15 seconds ~mder these
conditions produces an image exhibiting excellent color
reproductlon, contrast and image resolution in the photo-
sensitive film.
.. .
, ` Example 3
... ...i
A clean glass slide having a smooth surface suitable
for film deposition is positioned in a vacuum evaporation
chamber together with a quantity of silver. The chamber is
~, evacuated to a pressure of 5 x 10 6 Torr and, while the
glass slide is maintained at a temperature of about 50C.,
the silver is heated to cause vaporization and subsequent
deposition on the surface of the glass slide at a rate of
-18-
, ,.
,j ~
~-- 10~287~
. -. .
~ about l~/second, The deposition process is continued until
o
_ i a silver film about 50A in thickness is provided.
Following the deposition of the silver film a mixture
consisting mainly of AgCl but with a minor proportion of
CdC12 is introduced into the chamber and heated to cause
- ~ deposition of an AgCl film comprising about 10% CdC12 by
weight onto the silvered glass surface, this deposition
f~?~ occurring at a rate of about lOO~lsecond. The deposition
process is continued for a time sufficient to provide a
transparent covering film about 1500~ in thickness over the
silver film. The coated slide is then removed from the
chamber.
The Ag metal-~gCl dielectric film produced by the
i ~ described process exhibits imaging properties which include
_ ~ very good color reproduction, contrast and image resolution.
In`all cases of film fabrication in accordance with the
invention as above described, there is evidence that sub-
stantial interaction between the selected metal and the
selected dielectric occurs during the deposition of one film
type onto the other. For this reason it is doubtful that
the original island structure of the metallic film is pre-
` served unchanged through the deposition process. Unfortu-
nately, resolution of the final structure of the photosen-
sitive film is presently impossible due to the volatile
nature of the component species, which interferes with
electron microscopic examination. Nevertheless, it pre-
sently appears that the use of deposition conditions leading
to the formation of an island film of the specified struc-
, ,.
ture is an essential step in the production of photosensi-
~ ~ 30 tive films exhibiting reasonably broad-band color sensitivity
; , in accordance with the invention herein described.
. -19-
. .' f.;
