Note: Descriptions are shown in the official language in which they were submitted.
~L~S~3~
In earlier applications th~Le was proviclecl an electro-
photographic film member which i5 not only capable oE being
u-tilize(l Eor -the same purposes as conventlonal xerographic
and elec-troEax memb~rs but also is capable of being used
in the same manner as silver halide ernulsion photographic
films.
The electrophotographic film comprises a thin film
coating of an inorganic, photoconductive, electronically
anisotropic material such as r.f. sputtered cadmium sulfide
bonded to a thin film layer of ohmic ma-terial, such as
indium oxide which in turn is honded to a thin, stable
substrate memher, preferably of flexible, plastic shee-ting.
- The thickness of the photoconductive coating is about
3000 Angstroms, of the ohmic layer, about 500 angstroms
; 15 and of the substrate member, about à fraction of a milli-
meter. The resulting electrohotographic member has a hard,
abrasion resistant surface, is highly transparent and is
flexible notwithstanding the fact that the coating is
microcrystalline. It has high photoelectric gain with
speed and sensitivity to enable its use in high speed
photography. As such it can accept a charge at a rapid
rate and will retain the same selectively after exposure
to enable practical toning with an almost infinite gradation
of pigment values.
It is desirous nevertheless to improve upon this
already superior electrophotographic member.
-2-
` lalS53(~
Accorclingly, the invention herein provides an
electrophotographic member o~ the type inc]ucling substrate
means, a thin ~ilm coatincJ of a wholly inorganic, r.f.
sputtered, photoconductive material on said substra-te
S means, sald coating being very dense, microcrystalline,
substantially transparent, having a dark resistlvity of at
least 10 ohm-centimeters and a ratio between dark and
light resistivity oE at least 10 , having the capability
of accepting a rapid charge and retaining same to enable
toning and being electrically anisotropic,and a thin film
layer of ohmic material sandwiched between the coating and
substrate means for facilitating charging of said coating
before exposure; bond enhancing means comprising an
ultrathin film layer of a transparent wholly inorganic
material between the ohmic layer and the substrate means.
The preferred embodiments of this invention will now
be described, by way of example, with reference-to the
drawings accompanying this specification in which the
figure is a diagrammatic cross-section of the electro-
photographic film according to the invention.
' Before reference is made to the drawings for a de-
tailed description of the invention herein, certain
expressions that are used hereinafter and in the art to
define observed phenomina should be reviewed.
The expression "thin film" is used herein, both in
the specification and in the claims. As a general rule
t~he scientific literature in some way attempts to def:ine
10553C~)
~hin fi1m in terms oE ~he properties oE the substance beiny
discussed, calling attelltion to those properties in contrast
to the properties of the same substance in bulk. This
la-tter is called "bu1k properties" herein. Speaking in
relatively simple terms, some ma-terials act diEferently
when constituted as a "skin" than they clo as a "body".
Reference may be had, for example, to a publication
entitled "Thin Films" by Leaver & Chapman, Wy]ceham
Publications (London)Ltd.) 1971 for a general discussion
of the differences between thin film and bu1k properties
of the same type of material. In that publication, the
thickness of a "thin film" is given as "usually less than
one micron". This general definition is required in view
of the breadth of the subject covered in that publication.
When one considers the purposes and requirements of ; -
the structures in which a certain category of material is
to be used, the boundary or boundaries between the thin
film and the bulk properties must take these purposes and
requirements into consideration. Properties which are of
no importance or interest to the solution of a problem do
not enter into the matter and hence should not establish
the physical criteria. For example, if a radical change in
the sound reflecting property of a certain material occurs
when the material is made about 2 microns thick and less
2S because of skin effect, then if that ma-terial is going to
be used in an environment which uses the sound reflecting
property, it is e~hibiting a thin film effect. On the
.. .
553~t)
other h~nd, :L:E that identi~al mate.rial changes i.ts
.resistivi.ty radically only when iks thickness i5 decreclsed
to .5 micxons or less then, for the condi-t.ions of use in
which its resist:ivity is o.E importance, the material is
still a bulk material at thicknesses grea-ter than about
.5 micron.
The use of the materials involved here.in relate to
several properties which are beneficial and advantayeous
for the invention, and the meaning of the expression "thin
film" as used herein will be related only to these properties,
irrespective of the properties of any other materials for
any other purposes which may have been referred to as thin
films in thicknesses other than those which will be defined
The words "thin film" ~hen used in the specification and in
the claims wi.ll be taken to mean a thickness at which the
desired properties of the material in question cease acting
as bulk properties and commence acting as a skin or thin
film. The thickness in all known examples which have
actually been made is substantially less than a micron
20 . (lO,000 Angstroms).and ver~ few of the coatings or layers
tested exceed 5,000 Angstroms. Accordingly, a "thin film"
will be considered one that is substantially less than a
micron thick.
The expression "photoelectric gain" as used herein
has a meaning requiring explanation. The speed and
efficiency of an electrophotographic member is d~rectly
rblated to the "hole-electron pairs" produced when
1~)553()V
subjec~ed to ]ight. In prior art photoconductiv~ coati.nys
usecl in xero~raphy or electrofax, it requires many photons
(extreme]y br:ight light) to produce a si.ny].e hole~elec-tron
pair. The number is usu~lly upwards o:E a thousand~ It
~ollows that if an el.ec-trop}lo-tographic ~ilm can produce a
hole-election pair upon the incidence of a single photon
or even two photons its "photoelectric gain" is very
substantially great. Aceordingly, in order to provide an
expression for the gain of the eleetrophotographic members
of the type with whieh we are concerned "high photoelectric
gain" will be intended to mean a condition in which, at
most, several photons are re~uired to produce a single
hole election pair. The term "high photoelectrie gain"
implies also the ability of the member to which the term
is applied to permit the recom~ination of the pairs result-
ing in discharge.
The expression "electrophotographic film" or "photo- ::
graphic film" as used herein is intended to mean a eomplete
article with several layers or lamina for use in some
photographic process. Reference to the substrate or
subtrate member or substrate means will not include the
use of the word "film" although the substrate whieh is
eontemplated by the invention eould be eonsidered a film
in the ordinary meaning of the word. As will be seen, it
is preferred that the substrate be a thin flexible
transparent member of plastie sheeting, eommonly known
a~s plastic film.
--6--
~.~)$5300
The improved electrophotographic Eilm or the herein
invention comprises a -thin E:ilm coating oE a wholly
inorganic, crystalLine, r.E. sputtexed photoconductive
material overly:ing thin Eilm layer oE an ohmic or conductive
material which in turn is bondecl to an ul-txathin coating
of the same ~ype of r.f. sputtered photoconductive material
as above mentioned applied to the substrate means. The
preferred form of substrate means is a thin, flexible,
insulative, plastic sheeting of high stability~ such as
polyethylene terpthalate (sold under the well known
Trademark "Mylar").
The photoconductive coating or layer 12 is the most
important element of the improved electrophotographic
film as well as the basic ~ilm since it represents the
func-tional and physical characteristics which make the
same advantageous over the prior art.
The material from which the photoconductive layer or
coating is made and which will be described in detail
below is one of several known photoconduc-tive compounds.
These compounds have been used in the past, but so far as
known, have not been successfully incorporated into an
electrophotographic member having the properties of the
member of the t~pe concerned herein. For example, the
preferred compound which will be discussed in considerable
detail below is cadmium sulfide. The compound had been
incorporated in thick photoconductive coatings comminuted
a~d embedded in organic matrices and has even been
-7-
16~5S3~
reactively sputtere~d in Eorming wholly inoryanic coatinys,
but without achievement oE the advantageous results
characteristic of the herein descîibed and the earlier
electrophotographic mem~er oE which this is an improvement.
Like the earlier electrophotographic members of the
type concerned, the best results here have been with
cadmium sulfide (CdS). Other photoconductive materials are
suitable such as zinc indium sulfide (ZnIn2S~), arsenic
trisulfide (As2S3), zinc selenide (znSe), zinc sul~:ide (ZnS),
zinc telluride (ZnTe), cadmium selenide (CdSe), cadmium
telluride (CdTe), yallium arsenide (GaAs), antimony
trisulfide (Sb2S3) and perhaps others.
The ~ollowing are characteristics of the earlier as
well as the subject improved electrophotographic member.
The photoconductive covering is wholly inorganic
microcrystalline and several thousand Angstroms thick. The
only known useful cadmium sulfide coatings have been
mixtures with organic binders and matrices, of great
thickness and no substantial transparency or flexibility.
The photoconductive coating herein is deliberately made
O O
crystalline and thin - being 3500 A to 5000 A in thickness -
and is thus extremely flexible and transparent. The
conduction of electrons and holes through the coating is
enhanced by the manner in which the coating is produced.
25 ~ The crystals are believed to be vertically oriented, that
is normal with respect to the surface upon which same are
deposited, this resulting from a sputteri~lg process in
S53~t~
wh:ich a second dark space is establlshed between the plasma
and anode ln addition to the cathodic dark space conventionally
achieved usln~ r.f. sput~ering techniques.
It has been found that the "edge effect" characteristic
of prior art xerography, -~or example, is eliminated to a
substantial degree in toning the surEace of the photo-
conductive layer 12 of the invention. This "edge effect"
consists of the cen-ter of a reproduction of an image having
a solid pigmented area being light and the edges being dark.
The larger the area the more obvious the results of the "edge
efEect" so that large solid areas that are required to be
black throughout come out while in the center. Photographs
are impossible to reproduce with even a fraction of their
oxiginal quality without the use of relatively course screens
overlying the original. Negative originals, that is,
documents which are illustrated as fine white lines on a
black background are incapable of being reproduced with the
modern xerographic and electrofax methods because of this
"edge effect".
The earlier type as well as the subject electrophoto-
graphic film as described is capable of faith-Eully
reproducing documents and photographs without the use of
any intervening screens and without toner biasing. This
includes negatives which come out clear and sharp without
"edge effect". Toner biasing practically eliminates any
v,estiges of "edge effect" making possible the very highest
quality of photographic reproduction. As a matter of fact,
.
1~5~3(~
the qua]ity oE which tlle photoconductive coatiny 1~ is
capable is greater than that available from most ordinary
photographs today because the latter have grain oE
macroscopic size while the only limi-ting factors for the
texture of reproduced images on the coating 12 is the size
of the toner particles and the size of the crystals
comprising the coating. Both of these are typically of the
order of a small fraction of a micron, that is microscoplc.
It is believed that this benefit is achieved because
each crystal is arranged perpendicular to the substrate
and forms an individual ~ield when affected by electrons
in its vicinity. The toner particles are thus attracted
by the myriads of fields and not to the areas where the
gradients between presence and absence of charge are the
greatest. This latter condition is the reason for "edge
effect" commonly experienced.
As an example of the flexibility which is achieved, when
the photoconductive and ohmic layers are deposited upon a
sheet of flexible polyester .005 inch in thickness, the
resulting electrophotographic film can be wrapped around a
cylinder 0.25 inch in diameter without cracking or crazing
even though the photoconductive layer is crystalline. The
ability to be wrapped around cylinders a fraction of an
inch in diameter is representative of the ability of
transporting the electrophotographic film through handling `
and display machinery without problem.
Another characteristic which is related to the ~act
--10--
~)553(~0 :
that the layer 12 is inorganic, thin and crystalline in
character is lts extreme density and hardness. rrhe surEace
is mentioned above as being hard as glass. Abrasion
resistance is important in handling the film since it
obvîates scratches, scores and the like ~hich can cause loss
of detail and data, especially in fine subject matter. In
the manufacture o~ ~he electrophotographic film no difficulties
are met ~here it is necessary frictionally to move the same
by engagement of the surface by friction rollers and the like.
The abrasion resistance of the photoconductive coàting
12 is believed related to ~ e density of the compound oc-
casioned by the manner in which it is deposited. This ~-
radically improves the electrical properties as well, over ;
known coatings. ~
The material is electrically anisotropic due to its ;
thinness and semiconductor properties, among other reasons.
This means that the material will, at least for a
suhstantial period of time retain a nonuni-form charge `~
pattern applied thereto or produced therein as re~uired in ~ ,
its utilization electrophotographically and as a photo-
conductor. It also means that the finest resolution
pattern can be accurately and faithfully produced in the
latent image.
The coating 12 has a high photoelectric gain (as
defined hereinabove) characteristic. Thus, instead o
the large number of photons being needed to create a
hole-electron pair in the photoconductors of the prior
11~ '
l~S5300
art, only one or two photons are needed to drive the
charge carriers to the trapping or recombination centers,
thus producing a coating of much greater electrophoto-
graphic efficiency. This mechanism is what is intended
S to define "gain" as referred to herein. The "gain" for
doped film of the invention is many times greater than
that of undoped film.
The high gain charac-teristic is of importance because
it increases the sensitivity of the electrophotographic --
film of type concerned to a point where it is commensurate
with the sensitivities of most high speed photographic
films, but not necessarily with the same characteristic
loss of detail due to large grain. There is no grain in
the material of the invention, the crystalline structure
being microscopic.
The gain increase of the photoelectric material of
the type concerned is believed to be the result of the
release of free electrons from energy levels in the for-
bidden band oE the photoconductor and is exponentially ;
related to the thinness of the photoconductor. In other
words, the thinner the layer, the greater the release of ~ ~;
electrons and the more sensitive the electrophotographic
film.
Since the absorption of photons of light i5 needed ;~.~
-to discharge the photoconductive coating, it is clear
that there must be a certain degree of absorptivity o~
visible light or other electromagnetic radiation by
-12-
. ^ : , . . . -- .- :
- .. . . . -
l~S5300
the photoconductive coat:ing. ~n the other hand, the gain
is higher for thinner coa-tln-;s.
It becomes clear that the thickness of the layer 12
should be such that there is suEficient material to give
the desired light absorptivity and abrasion resistance
qualities and yet thin enough to give the desired gain.
What one can do is to deposit a thickness of the layer
which gives the ma~imum of gain with the minimum of
pxactical thickness. This is easily ascertained ex-
perimentally for any given material by measuring the light
absorption and gauging the abrasion resistance and strength
by su.itable means, continuing to deposit the material until
a practical compromise is made between these qualities and
the desired photoelectric gain. -
The requirements of light absorption must be met,
in any event. ~ '
The photoconductive coating 12 has a high dark
resistivity which promotes charge acceptance and charge
retention~ The cadmium sulfide coating which is the
preferred photoconductive coating is inherently n-type and
'
in its purest form, as deposited according to the method
described has a dark resistivity of 10 2 to 1014 ohm-
cen~imeters. Its light resistivity is about 10 ohm-
centimeters. Its energy gap is about 2.45 eV. These
measures of resistivity are static and are made by known
methods of bonding electrodes to the surface or surfaces
of the photoconductive coating, applying d.c. voltage,
-13-
~553(~
measuriny current and computiny the values from the geometry.
The dark resistivity measurements are made in darkness.
It is pointed out, however, that this is done without a
charge on the photoconductive layer. Since the photo-
conductive layer of the type concerned is very thin, when ~ ~
the charge is applied to the surface it enters such surface ~ -
and drives free carriers toward the ohmic layer. Its effect
is felt to a great extent through the photoconductive layer.
Absent such carriers, during the period after charying,
discharge is inhibited, and the dark resistivity should
be increased. Dynamic measurement of dark resistivity can
be efected by considering the dark decay characteristic i;~;; ;
to be a conventional RC discharge of a condenser, and
comparlng such characteristics with the computed and graphed
characteristics for different resistivities. Using such
techniques it has been determined that the dark resistivity
of a cadmium sulfide layer of the invention that has been
charged is substantially increased at least several times ~ ~
at the beginning of the characteristic to as much as lO00 ~ -
time ~hereafter. Obviously the dynamio ratio of dark to
~light resistivity also increases.
The reference hereinafter, both in the specification
and claims to resistivities will be considered static.
As stated, the dark resistivity 15 10 to 10 ohm-
centimeters and higher. So far as known, the resistivities
of relatively thicker art photoconductive members of types
other than the electxophotographic members concerned, are
-14-
,, . . - . . . .
. .. - .. . . . . .. . . .. . . . ..
~055300 `~`
the same or little diferent whether considered statically
and dynamically.
The high dark resistivity of the coating 12 represents
an e~cellent insulating material; the h:Lgh ratio of dark
to light resistivity i5 of the order of 105 represents a
radical change in the resistance. This coating is one
which had a thickness of about 3500 A and an optical trans-
missivity of between 70% and 85%. The conductivity increase
when illuminated is related to the sensitivity of the ;;
coating.
Zinc indium sulfide, one of the other useful photo-
conductive compounds has a dark resistivity of about the
same order as that o~ the cadmium sulfide with a light
resistivity somewhat higher so that the ratio is not as
great. The energy gap of zinc indium sulfide is about
2.3. Its performance as a photoconductive coating is not
as good as that of cadmium sulfide, at least in the electro-
photographic films that were tested using the zinc indium
sulide as the photoconductive layer.
Although not required, cadmium sulfide can be doped `~
~ .
with known dopants, such as, for example, minute quantities
of copper, iodine and the like, to provide additional
carriers of electrons. This should rsnder the coatiny
even more n-type than the pure cadmium sulfide and give
a greater gain.
It must bs understood that the proportions of the
elements which make up the photoconductive layer must be
-15-
~S530~
stoichiome~rically correct, this being achieved by con-trol
of the conditions of deposit. The dopant proportions, if
dopant is used, must also be controlled, but since the
entire layer is inorganic, conventional control methods make
this feasible and relatively easy.
The photoconductive coating of the type describea which
is made from cadmium sulfide is practically panchromatic.
Tlle photoconductive coating ofthis invention as well
as the earlier electrophotographic member of the type
described is easily deposited in the special manner which
gives it its unusual properties. This guarantees uniform
deposit and high speed controlled production.
The photoconductive coating L2 in all cases is
deposited by r.f. sputtering in a vacuum chamber~ All o~
the materials which go to rnake up the coating, whether
dopants are included or not, are introduced into the
vacuum chamber. The materials are introduced either by
way of the consumable target or by gases or sublimed
compounds introduced into the atmosphere of the vessel
after the process has been started. Stoichiometrically
correct proportions are easily controlled by techniques
which are known to result in a substantially perfect and
uniform product.
The sputtering of the photoconductive layer 12 is
a critical part of the invention in that, so far as known,
the vast improvement over the prior art is achieved by
establishing a second dark space. This can be done by
-16-
~0553~0
connecting the radio frequency circuit of the sputtering
apparatus in a bias arranyement. In certain causes, the
second dark space can be self-induced.
The characteristics ~hich have been described above
are not exclusive, but are believed to be the most important.
Many other advantages accrue concurrent]y, either as a result
of the characteristics which have been men-tioned or in ad~
dition thereto.
The ohmic layer 14 is a conductive layer that is ~-
deposited on the substrate member 16 before the deposition
of the photoconductive layer 12. Its primary purpose is ta
facilitate the charging of the surface of the photoconductive
layer. It also may serve to assist in bonding the photo-
conductive layer to the substrate member. Under circum-
lS stances that a p-type coating or layer 12 is used, the
ohmic layer 14 may assist in discharge. In the use of the
coating 12 to produce an electrophotographic film, the
layer 14 is transparent.
This ohmic layer is very much thinner than the photo-
conductive layer 12, preferably being of the order of 500
Angstroms. This thickness will not interfere with the
transparency or flexibility of the final electrophotographic
film product. It forms the interface between the photo-
conductive layer 12 and the substrate member 16. It
functions as one element of the capacitive circuit during
çharging of the surface of the photoconductor.
A purity grade of semiconductor indium oxide either
-17-
lOS53~)0
alone or co~bined with a small percentage ~about 10%) of
tin oxide is a suitable material for use as the ohmic
layer 14. It is easily bonded to aluminum edges or
conducting strips. It is aLso easily and preferably applied
by sputtering techniques in the same apparatus as used to
apply the photoconductive layer. This latter is the method
used to make the practical embodiments of the inventionO
Vacuum or vapor deposition may be used but will probably
not provide as dense and smooth a layer nor one which is
so well-bonded to the substrate.
The substrate member 16 is the carrier or mechanical
support for the photoconductive layer 12, the ohmic layer
14 and as will be seen the bond enhancing layer 18. The
mechanical properties are flexibility, strength, trans-
parency, ability to adhere to the deposited layers and of
great importance - stability. The stability refers to
dimensional stability, stability in retaining thickness,
stability in resisting any changes which may occur due to
being subjected to the temperatures and elec~rical phenomena
which occur within the pressure vessel during the depositing
processes~ Resistance to abrasion is a good property to
include in choosing the substrate material.
Polyetheylene terphthalate sheeting oE .005" thickness
has been mentioned above as one example of substrate that
has been satisfactory. This material is an organic polymer.
- 18 -
105S30~
of excellellt characteristics is such material made by the
E. I. duPon-t cle Nemours company and sold under the Trademark
"M~lar." Internal stresses thereof are preferably required
to be removed prior to use, the process of doing so being
referred to as normalization. This can be done by subjecting
the film to a temperature of about 190 celsius for a period
of about 30 minutes. Such steps are known.
The su~strate material should not have any occluded
gases, and these can be removed by outgassing the same in
suitable chambers. Likewise, the sheeting should be
perfectly clean.
The above descriptions comprise the details concern- `
ing the principal elements of the electrophotographic
film 10 o the electrophotographic film of the type
lS concerned.
In particular, a major aspect of the improvement herein
relates to the provision of an electrophotographic film
member having a bonding layer 18 of ultrathln dimension,
; namely, of the order of 5~ to 300 Angstroms thick which is
deposited directly upon the substrate between the ohmic
layer 14 and the substrate 12. The adhesive affinity of
the substrate or the overlying ohmlc and photoconductive
. . .
.
layers 1~ and 12 respectively, is improved. The so-called
bonding layer 18 is formed of cadmium sulfide r~f.
sputtered directly upon the substrate under the conditions
as employea in the deposition of the photoconductive layer
12. It should be noted that the thickness of said bonding
layer are of the order not readily measurable, even by
-19- :
. , ~ .. ., ~ . . . . , :
30al
~e~nl ~les : :
interferometric ~ ~ but are estimated b~ comparison
with the measurable thickness of the photoconductive
coating deposited. The ohmic layer 14 of the order of 300
Angstroms preferably ic r.f. sputtered on the bonding layer
18, and the photoconductive layer 12 o-f cadmium sulfide
r.f. sputtered upon the ohmic layer 14. The bonding layer ~ ;
18 of cadmium ~sulfide is believed to become effectively a
part of the substrate but its thickness is such as to have
no dlscernible effect on total transparency of the film
member.
As shown in the Figure, in use, co~ can be made
at 19 with the ohmic layer by reason of the photoconductive
layer being noncoextensive with said ohmic layer, leavlng
a portion exposed. The reference numeral 20 signifies a
high voltage source and the reference numeral 21 represents
a corona generator, the circuit being symbolic of a charging
circuit for sub~ecting the photoconductive thin film layer
12 to a surface charge. ~ battery is no-t intended by
s~mbol 21.
The cathode or target of such apparatus is formed of
the material from which the layer is to be made, or several
of the elements to be used. Other elements can be added by
introduction into the chamber. In one example carried out
for testing purposes the cathode was semiconductor grade'
indium oxide. This was for the deposit of the ohmic layer
14. ~he cathode is spaced from the anode in accordance
~ith the physical characteristics of the particular chamber,
-20-
,-
~0553~0 ~ ~ ~
considering the geometry, the voltages to be used, etc.
The chamber in the example was pumped do~l to near the
lO torr pressure range. This, of course, is a substantial 1
~ vacuum. The~ ~ltrapure argon, that is, containing less
S ~ ~ than lO ppm ~ and N2 was admitted to the sputtering
chamber through a servo-leak valve until a pressure of about
20 millitorr is achieved.
At a suitable point, the radio frequency field is
established and the ionization of the argon produced
electrons which bombard the target or cathode, knocking
''".. ~, "'.
the particles of indium oxide out of the target thereby
producing the plasma vapor between the cathode and anode
and carrying the particles toward the anode there to be
deposited upon the previously deposited bond enhancing ~-
layer on the substrate member.
This sputtering is carried out at a rate which is
determined by the conditions within the chamber, typically
about 15 to 40 Angstroms per second for a commercial ;~
version using approximately 1 to 2 square feet of target
area. Thickness is monitored by optical means known in the
art until a thickness o~ about 500 Angstroms is reached.
,. . , ~
The substrate member is now removed from the chamber
., ~ .
and passed into or placed within another chamber in
production. I the process is a laboratory process or in
very small production, the same chamber may be used, but
the cathode or target must be changed. Likewise stringent
steps must be taken to remove all possible residual
-21-
~:
:
1(~55300
material to avoid contamination Careful shielding oE the `~
target or targets and the plasma can minimiæe contamination
in the chamber.
In any event, the substrate mernber 16 with its first
coating of the ohmic layer 14 and a previous underlying
bonding coating 18, in the case of the example being
described being indium oxide alone or com~ined with tin
oxide,is again mounted on an anode carrier or led over a
rotating anode or the like.
For a photoconductive layer of cadmium sulfide,the
cathode or target will be made out of cadmium sulfide or
even cadmium alone. The pressure is first dropped to 10 6
torr before being adjusted to 20 millitorr with later
admitted argon gas and hydrogen sulfide. The hydrogen
sulfide provides the correct amount of sulfur to the vapox
plasma so that a stolchiometrically correct proportion o
cadmium and sulfur is deposited on top of the ohmic layer.
Actually, the hydrogen sulfide serves as a background gas
. .
to counterbalance the vapor pressure of sulfur. Tbis
prevents decomposition of the cadmium sul~ide and thus
controls stoichiometry. It will be appreciated that in
Z ~
both depositing procedureZs the rear surface of the substrate
~; member 16 is blocked or masked to prevent any deposit thereon
in normal processes. A first dark space induced by a
.~ ....
, 25 shield around the target inhibits side and back deposits.
In the case that a cadmium sulfide cathode is used, the
àmount of hydrogen sulfide admitted is about 500 to 15,000
Z -22-
': " . ~'
... . . ~ - - . . .
l~5S30~
ppm in argon. In other cases when a caclmium cathode is used,
these proportlons may be increased. The final pressure o~
deposit was between 7 and 15 millitorr.
A small arnount of copper deposit in the form of sub-
limated copper chloride may be admitted into the sputterlng
chamber, this being effected by keeping -the copper salt in
an evacuated vessel which communic~tes with the sputtering
chamber through a control valve. Copper is the dopant in
this case, increasing the trapping levels in the inherently
n-type cadmium sulfide. Hydrogen iodide may alternatively
be used to provide iodine dopant to give additional trapping
levels in the cadmium sul~ide deposi-t.
Other methods of doping are ion implantation, diffusion
migration and the 1ike.
The application of the radio frequency high voltage
creates the necessary plasma to effect deposit of the cadmium ;
sulfide onto the ohmic layer to form o~ the photoconductive
layer 12. The rate of deposit in tests conducted was about 6
to 15 Angstroms per second. Greater rates as mentioned above
can be achieved in commercial e~uipemnt. The copper or
hydrogen iodide if used is admitted in small controlled
quantities sufficient to dope the cadmium sulfide on the
ohmi~ layer in an amount of 5 x 10 percent by weight.
Most practical examples were totally pure. The sputtering
is continued until the thickness o~ the coating 12 reaches
3000 to 3500 Angstroms.
As previously mentioned, one of the most important
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aspects of the invention revolves around the special method
of sputtering which is used. ~lile used ~or the deposit of
the bond enhanc.ing layer 18, the ohmic layer 14 and the
photoconductive coating 12, the most important application
o~ -this method is in sputtering the photoconductive material
to :Eorm both the bond enhancing layer/and the photoconductive
coating 12.
In the method o~ sputtering which is conventional, the
cathode or target is connected to the "hi~h" side o:E the .
output of the radio requency generatorJ normally through
a matching network, and the anode or substrate sùpport is
connected to ground. The radio frequency energy ionizes
the argon gas which is introduced into the chamber and there
is a plasma ~ormed between the target and anode, there being ~ ::
.
a first dark space of relatively short dimension just at the ~
surface of the target. Atoms of the.target are literally . ~ .
knocked out o~ the target by the ions OL the argon gas and
are driven across the intervening space through the plasma
and impinge against any article that overlies the anode.
This would be a substrate member and the particles themselves
or after reacting with other reagent elements which may have
been introduced into the chamber are deposited onto the
substrate.
It has been discovered that by biasing the radio
requency circuit in the manner to be described the atoms
of depos.ited material are deposited in a very dense manner
and that the unusual electrical properties descr.ibed result
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there:Erom. Thls biasing arrangement produces a second dark
space immediately above the anode.
It has also been discovered that the second dark space
can be achieved sometimes by adjusting the geometry o the
target, shields, anode, etc. within the chamber. When this
second dark space appears, the desirable qualities of the
deposit are achieved without changing the circuit configura-
tion which would indicate, o course, that the presence of
the second dark space is the desidera-tum rather than the
circuitry.
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