IL42446A - Electro-photographic film and method of making same - Google Patents

Description

ELECTBO-PHOTOGBAPHIC FILM AND METHOD OP MAKING SAME This invention relates to a novel electrophotographic film in which a photoconductive material such as cadmium sulfide, zinc indium sulfide or the like is sputtered in a thin film layer on a substrate with an ohmic layer interposed between the substrate and the photocondutive thin film.

In electrophotographic process as known an electrostatic latent image is formed on the surface of a photoconductive member. The photoconductive member is initially charged over its entire surface area while in the dark, the charge being retained by suitably insulating the member against leakage, but the retention of such charge being dependent upon the physical character of the material. Immediately after the surface has been charged, such surface is exposed to some form of radiant energy comprising a pattern of tones, lines, text and the like which it is desired to reproduce. Such radiant energy may take the form of a projected light pattern, an X-ray projection, etc.

The areas of the surface of the photoconductive member which are exposed to the brighter portions of the pattern become more conductive than those which are exposed to the less illuminating portions of the pattern. The result of this is that electric charge is selectively and proportionally removed from the different areas of the surface of the photoconductive material in accordance The resulting geometrical charge pattern constitutes the latent image mentioned above. The charge has the property of being able to attract fine particles electrostatically, and in the art of xerography, such fine particles in the form of a comminuted powder or liquid suspension are brought into contact with the surface. The particles selectively adhere to the surface in varying degrees according to the charge pattern represented by the latent image, following which the excess is brushed or otherwise removed from the surface and the remaining "toner" as the particles are called, ■ form a visible image. This image is usually transferred to a receiving element such as a sheet of paper and is permanently fused or "burned" to the surface of the receiving element by techniques which are well known.

Generally, the most familiar structure now used commercially requires a large drum coated with selenium as the photoconductive member. The process is performed in a complex and expensive machine and the speeds, resolution, and flexibility of such machines and the processes thereof leave much to be desired.

Such processes and apparatus are not readily adaptable to use in photography as represented by a high speed camera using fine grain silver halide emulsion photographic film. Inherent faults with the known methods, apparatus and the photoconductive materials and articles used have prevented use in such fields as high speed photography, fine resolution microphotography, and many other technical areas. Record-keeping, by means of projectable microfilm is a field wherein there is a long-felt need for a process for making the projectable photographic member quickly, with high resolution, economically, with simple apparatus and having the ability to withstand long periods of storage. For example, it would be highly desirable to add information to a microfilm record from time to time without adversely' affecting the information which is already contained thereon .

Conventional photographic microfilm is not capable of being re-exposed for adding information. The inherent construction and processing thereof destroys the emulsion when the microfilm is developed. The electrophotographic process above described could provide a suitable microfilm record if it could be used to make an electrophotographic film having high resolution and prolonged storage life. As can be seen, if the photo-conductive thin film coating could be preserved indefinitely, then each time that an addition is to be made to the record already contained on the coating, one merely charges the surface of the thin film coating, exposes the same, and fixes the new image to the surface. This presumes that toner is applied directly to the ' The known transparent electrophotographic recording elements are susceptible to deterioration on prolonged exposure to light, elevated temperatures and humidity. They must be handled carefully, stored under controlled conditions and can be re-exposed only a limited number of times.

Their use for records of a permanent nature is highly limited. Accordingly, it is impractical to utilize the same for such records .

The above discussion considers only a limited aspect of the prior art deficiencies. A consideration of some of the problems solved by the invention will emphasize that the advance in the art is not confined to a small area.

The conventional silver halide gelatine coatings of photographic film achieve greater speed and better resolution than known electrophotographic films of the so-called xerographic type. Nevertheless, such gelatin emulsions are subject to disadvantages which are obviated by the invention, in addition to the fact that the electrophotographic film of the invention can be repeatedly exposed to add information to the same without deleterious effects.

The conventional silver halide film has an emulsion which is about 140 microns thick. The thin film coating of the article of the invention is a fraction of a micron thick.

The conventional silver halide film is thus not easily flexed without damage. Its resolution is limited by the amount of undercutting of the image when the silver is precipitated during development. Air bubbles in the emulsion produce flaws in the resulting developed photographic transparency. In production, the film cannot be inspected in ordinary light, it cannot be handled or transported except in special dark packages. The emulsion is soluble in ordinary liquids and is hygroscopic.

A long sought after electrophotographic film is one that is highly durable. Its thin film coating should be as hard as glass; insoluble in most liquids; should have extremely fine resolution; can be applied by sputtering processes in pressure vessels and hence is dense and free of bubbles; would not be affected by light and hence, can be handled freely and readily inspected under bright light; would be nonhygroscopic and not subject to deterioration on account of any of the factors which will deteriorate the ordinary silver emulsion type of photographic coating. Fungus or other microorganisms should have no effect on an electrophotographic film.

The ordinary photographic emulsion and such electrophotographic coatings as known have a relatively limited spectral response, thus limiting their use. The photoelectric gain of known electrophotographic coatings is substantially less than that of the article of. the invention which accounts to a large extent for the inability of prior electrophotographic films to have the extremely high resolution of the invention. The greater thickness of the prior articles has been a major factor in decreasing resolution.

Prior art is typically represented by the following 0 i) United States Patents 2,861,903; 2,995,474; 3,379,527; ·*ή~ ■ ' s) (,) ·,) 3,519,480; 3,535,112; 3,573,905; 3,574,615; 3,592,643; . 3,615,lo43 3, 003^869; 3, 54?! 969; 3, 398^021 ; 3, 393^,070; 3,095,324;' 3,104,229; 3,677,816; 2,732,313; and 2,844,493.

Accordingly, the invention provides a method for manufacturing an electrophotographic film article which comprises the. steps of: depositing a thin film layer of ohmic material upon- a flexible thin organic transparent substrate in thoroughly bonded condition and a thickness which renders said thin film layer substantially transparent and flexible, sputtering a thin film coating of a wholly inorganic photoconductor material upon said ohmic material in thoroughly bonded condition and a thickness which provides a gain of said photoconductor material greater than unity and which is substantially transparent and flexible, the total thickness of said article having a l'ight absorbtivity of not greater than 30 percent and not less than about 15 percent.

Additionally, the invention provides an articl of manufacture comprising: substrate forming means to support thin film coatings, a sputtered thin film coating of a photoconductive, wholly inorganic material on said substrate forming means and means between the substrate forming means and said thin film coating to facilitate selective removal of charges from said material on selective exposure of said material to radiation .

An embodiment of the invention is illustrated in the accompanying drawings wherein: FIG. 1 is a highly schematic sectional view of a transparent electrophotographic film constructed in accordance with the invention and illustrating diagrammatically a circuit for charging the surface of the photoconductive layer; FIG. 2 is another schematic sectional view through a similar slightly modified electrophotographic film; FIG. 3 is similar to FIG. 2 but illustrating a slightly modified form; ·. . .. ' FIG. 4 is a view similar to that of FIG. 2 but illustrating schematically the manner in which toner is applied to the surface of the photoconductive layer after the same has been exposed; and FIG. 5 is a chart showing the charging and discharging voltages for the electrophotographic film of the invention as compared with a typical xerographic plate.

Briefly, there is provided an article of manufacture for use as an electrophotographic film which comprises a substrate member of flexible, transparent polymeric plastic sheeting having a thin film coating thereon, the thin film coating comprising a photoconductive material such as for example n-type cadmium sulfide or zinc indium sulfide, there being an intervening thin film layer between the substrate member and the photoconductive layer, the said intervening layer comprising an ohmic layer of indium oxide or the like.

By way of definition, the expression "thin film" as used herein is intended to mean a layer of some substance - as for example, the semiconductor or photoconductive material mentioned above, applied to a surface. Such a thin film layer is one which has a thickness that is measured in several thousands of o Angstroms, such as for example 5000 A or .5 micron.

On the other hand the expression "electrophotographic film" or "photographic film" as used herein is intended to mean a complete article with several layers or lamina for use in some photographic process. Reference to the substate or substrate member or substrate means will not include the use of the word "film" although the substrate which is contemplated by the invention could be considered 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 plastic sheeting, commonly known as plastic film.

The electrophotographic film of the invention comprises a sputtered thin film layer of an inorganic photoconductor overlying a sputtered thin film layer of a conductive material which in turn is bonded to a thin . flexible insulative substrate such as plastic sheeting of high stability. The ends sought for the electrophotographic film of the invention are transparency, flexibili great sensitivity, high photoelectric gain, economy, ease of manufacture and use and handling, ability to be exposed over, and over again, stability under varying conditions of light,heat, humidity, and other properties which will be apparent from the description which follows The attributes of the invention give rise to a wide variety of important uses not the least important of which is its use as microfilm records.

The three important elements of the electrophotographic film 10 comprise the sputtered thin film layer 12 of photoconductive material, the sputtered ohmic or conductive thin film layer 14 and the substrate 16.

Attention is invited to Figure 5 which is a chart showing the characteristics, of a typical xerographic plate of the prior art (A) and the characteristics of an electrophotographic film element taught herein (B) .

The horizontal axis of Figure 5 is time increasing to the right, not necessarily scaled and the vertical axis is potential increasing upwards and not necessarily scaled. In the xerographic process, the photoconductive surface, typically amorphous selenium or zinc oxide - resin mixtures, is charged by subjecting the same to a corona in darkness. For the typical thickness of such surface, namely of the order of 20 to 160 microns, the potential represented by charge will follow the solid line 30 rising to about 500 volts or so This occurs in a period of the order of about 60 seconds While still in darkness, the charge commences to leak off slowly, along the solid line 32 and then along the dashed line 34. The saturation level 35 is considered the point where corona current is about the same as leakage current.

At the point in time represented by the vertical dashed line 36, shown as typically 90 seconds, the plate is exposed to a pattern of light and dark areas which, of course, represents the intelligence projected onto the plate. Thereafter the charge is selectively leaked off in amounts proportional to the amount of light which impinges against the surface. Each light photon will move an electron from the surface to combine with a "hole." The total a"bsence of electrons is represented by the condition of areas of the surface at the residual potential at 38 at the bottom of the chart - typically of the order of 30 to 50 volts potential. This latter condition represents pure white. The dead black areas, namely those which have absolutely no electrons knocked out because they have not been subjected to light, remain at the saturation level 34. All other areas are somewhere between these two extremes.

The decay of the charge continues as shown, but somewhere conveniently along the process, the surface of the xerographic plate has the toner particles applied thereto and these are transferred by offset type of contact to a sheet of paper and permanently adhered by fusing to the sheet of paper. Thereafter, the toner is brushed from the plate and the plate is ready for use. It may in the meantime be totally discharged by a brilliant light, if necessary .

The invention herein teaches utilizing what would be considered the plate to make a photographic transparency, henc there is no transfer of toner. Instead, as seen, the toner is fused directly into the electrophotographic film itself after it is exposed and toner applied, the extreme economy of the process and materials making this possible. So far as known, there is no commercially available xerographic process which uses the xerographic plate or element itself as the ultimate product. In every case there is a transfer. Zinc oxide paper are the one exception, but no transparency results. Indeed, to use one of these known elements as the ultimate product would serve no useful purpose unless it were transparent and could be used as a negative or transparency.

The thickness of the . electrophotographic element of the invention is at most a fraction of a micron exclusive of the substrate itself which is a fraction of a millimeter in thickness. Thus, the voltages which are involved are substantially less than those for the thick xerographic plates of the prior art. In order to make the comparison which is desired in the chart of FIG. 5, the characteristics are extrapolated for a structure in which the photoconductive layer is substantially the same thickness as the photoconductive layer of the xerographic plate.

Thus, for the extrapolated structure, the rate of decay of charge is so rapid that a totally different situation obtains. In the first place, the corona effect to which the photoconductive surface is subjected raises the potential thereof to something of the order of 2000 or more volts, which is many times higher than its saturation potential. This potential of course should not be above that which would break down the photoconductive surface. In so applying the corona, the potential of the photoconductive surface of the invention rises along the line 40 to the desired peak within a fraction of a second, here shown as one-half a second. As soon as the corona is discontinued, with the element still in darkness, it commences to discharge along the steep line 42, which is the. equivalent of the small solid portion of the dark discharge line 32 of A. At '· any convenient point in time, say one hundredth of a second thereafter, the surface of the photoconductive layer is exposed tp a projected scene or some other form of radiated energy such as X-rays. The speed of exposure can be quite fast due to the steepness of the decay curve which signifies the ease of removing electrons from the surface. The decay curve for black continues along the dashed line 44 which reaches saturation level much below the charging potential, while the light discharge line follows as shown at 46 down to the residual potential. It can be seen how quickly everything can be done in order to utilize the electrophotographic film of the invention in high speed cameras and the like.

One great advantage of such high speed electrophotographic film is that the corona may be applied with the photoconductive surface already exposed, that is, in light and the toner applied whenever the desired potential is reached. This practically could be higher up on the curve 42 than shown, and even at the peak 48.

Now, with respect to the actual potentials used, since the photoconductive layer of the invention is so thin, the actual potential at the maximum point of charge designated 48 is no more than 50 or 60 volts or a voltage in that vicinity. The other voltages are proportionally lower as well/making the apparatus with which the invention can be '—' used simple to construct. The residual potential would, for example, be a few volts in the case of the invention where it is.30 to 50 volts in the case of the conventional xerographic plates. It is noted that the operating range of potential for the electrophotographic film of the invention is all below the voltages which comprise the very lowest used in conventional xerographic processes.

Reference may now be made to FIGS. 1 to 3 which illustrate the electrophotographic film of the invention in section, the dimensions being exaggerated and not proportional in order to enable explanation of the several parts of the article. In each case there is a substrate member 16, a photoconductive layer 12 and an intervening conductive or ohmic layer 14. In FIG. 1 contact is made at 18 with the ohmic layer by reason of the photoconductive layer being noncoextensive with said ohmic layer, leaving 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 subjecting the photoconductive thin film layer 12 to a surface charge.

In the structure of FIG. 2 a portion of the conductive layer 14 or an independent strip of some conductor such as aluminum is applied along the edge as shown at 22 to facilitate contact with the conductive layer 14. In FIG. 3 this strip takes the form of a member 24 that engages around the edge and a portion of the bottom surface of the substrate 16. This contact strip 22 or 24 provides a good contact with the ohmic or conductive layer 14 and is easily applied in substantial thickness to be wear-resistant. V The three elements of the basic structure are assembled by sputtering techniques carried out . in a suitable pressure chamber. The substrate member is preferably cut to proper width before coating and passed through a first pressure chamber in which the ohmic thin film layer 14 is coated on one surface. In an alternative process, the source of supply and take-up for the strip of substrate member is contained fully within the chamber. In like manner the second or photoconductive layer 12 is then sputtered over the ohmic layer. The contact area 18 or the edges 22 or the lateral members 24 may be applied by vacuum or sputtering techniques and/or by masking, usually before the two layers 14 and 12 are applied.

PHOTOCO DUCTIVE LAYER 12 The photoconductive layer 12 is the most important element of the electrophotographic film because it represents the functional and physical characteristics which make the invention advantageous over the prior art.

The material from which the layer 12 is made is a photoconductive compound or alloy which exhibits the properties described. The two examples which have been set forth below use n-type cadmium sulfide (CdS) and zinc indium sulphide (Znln2 S^) . Other materials which are capable°of being used as the layer 12 are Si N , ZnS, Sb^S.., As2S3, GaAs, CdSe, ZnSe and perhaps others. These may be variously doped. The characteristics of the material are discussed as follows: 1. The material is in very case wholly inorganic macrocrystalline, and several thousands of Angstroms thick. Prior art photoconductive layers or plates are thicker to a substantial degree and hence are not highly flexible and transparent. The prior art materials are to a great extent mixed with resinous binders and other materials which increase their opacity and as a general rule vacuum deposited in order to avoid crystalline formation because large crystals make the layers frangible. The thickness of the thin film layer 12 is preferably less than 3500 Angstroms, but could be as much as 5000 Angstroms. The conduction of electrons and holes through the layer is not in any way inhibited by such thin layers. The crystalline structure for at least one of these materials is vertically oriented, that is normal with respect to the surface upon which the layer is deposited, this resulting from the sputtering process of deposition which is used.

As an example of the flexibility which is achieved, when deposited upon a sheet of flexible polyester .005 inch in thickness, the electrophotographic film of the invention can be wrapped around a cylinder 0.25 inch in diameter without cracking or crazing. 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 fact that the layer 12 is inorganic, thin and crystalline in character is its hardness. The surface is mentioned above as being hard as glass. Abrasion resistance is important in handling the film since it obviates scratches, scores and the like which can cause loss of detail and data, especially in fine subject matter. In the manufacture of the electrophotographic film no difficulties are met where it is The material is electrically anisotropic due to its thinness and semiconductor properties. This means that the material will, at least for a substantial period of time, retain a nonuniform pattern of electrons and holes applied thereto or produced therein as required in the process of using the electrophotographic film as described. 2. The material has a high photoelectric gain characteristic. The two specific materials mentioned above are n-type cadmium sulfide and zinc indium sulfide.

The former has a dark resistivity of 10 12 ohm-cm., a light resistivity of 10 ohm-cm. and an energy gap of about 2.45 The latter has a dark resistivity of 10^ ohm-cm., a light resistivity of 10^ ohm-cm. to 10"1"0 ohm-cm. Their optical transmissivity should be 70% and not more than 85%. A dark-to-light resistivity ratio of 10^ or more is particularly advantageous for extremely fast electrophotographic film.

Zinc indium sulfide, because of its high dark resistivity and low light resistivity is useful in many electrophotographic applications of the invention. Cadmium sulfide, on the other hand, because of its rapid dark decay finds preferred application in such processes where images must be created very swiftly.

The high gain characteristic is of importance because it increases the sensitivity of the electrophotograhi film of the invention 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. High gain in a photoconductive material means that instead of a single electron being released when a photon strikes the material, a plurality of electrons is released. The more electrons released, the higher the gain.

The gain increase of the photoelectric material of the invention is believed to be the result of the release of free electrons from energy levels in the forbidden band of 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.

The photoconductive layer is necessarily deposited by sputtering to achieve the characteristics described.

No other method of deposition known at this time will produce these characteristics.

One may consider the nature of the gain of the photoconductive layer. If one electron is conducted away from the layer when a photon is absorbed it could be said that the gain is unity. If a plurality of electrons is "knocked out" the gain is greater than unity. It becomes clear that the thickness of the layer 12 should be such that there is sufficient 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 maximum of gain with the minimum of practical thickness. This is easily ascertained experimentally for any given material by measuring the light absorption and gauging the abrasion resistance and strength by suitable 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. With the invention it is possible to do so with a photoconductive layer that has substantially more than unity gain and excellent abrasion resistance.

It must be understood that the proportions of the elements which make up the photoconductiye layer must be stoichiometrically correct, this being achieved by control of the conditions of deposit. The dopant proportion must also be controlled, but since the entire layer is inorganic, conventional control methods make this feasible and relatively easy. 3. The material has a wide spectral response .

Peak response in the 5000 Angstrom area is particularly desirable for a broad variety of electrophotographic appli- * cations. The materials mentioned above for the thin film layer 12 exhibit broad spectral response. It is desired to have most radiation of a type that it is desired to record be capable of producing images on the electrophotographic film of the invention. All of the ordinary visible light is included along with X-rays and radiation encountered in the physical laboratories which are being recorded on other mediums at this time.

Cadmium sulfide doped with an appropriate donor material will exhibit panchromatic' response.' Zinc indium sulfide has a more universal response but must be doped selectively to improve its response when desirable. Such response normally peaks at about 4800. Angstroms. The cadmium sulfide dopant which is used in the examples described hereinafter is copper. 4. The material is easily deposited. This characteristic is important because it enables uniform, controlled high speed production. The required j form of deposition is sputtering in a suitable pressure chamber using an r.f. field. There are no pastes or resins to be handled. All the materials are introduced into the chamber, either by way of targets which are consumed, gases or sublimed compounds introduced into the atmosphere of the vessel after the process is started. Stoichiometri-cally correct proportions are easily controlled to result in a substantially perfect and uniform product.

The characteristics which are described above are not exclusive. Many other advantages accrue in connection with the photoconductive layer of the invention. In addition, the order of listing should not be inferred as the order of importance.

OHMIC LAYER 14 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 to promote the conducting of electrons from the surface of the photoconductive layer when the latter absorbs photons. It also may serve to assist in bonding the photoconductive layer to the substrate member.

This ohmic layer is very much thinner than the photoconductive 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 photoconductive layer 12 and the substrate member 16. It functions as one element of the capacitive circuit during charging of the surface of the photoconductor in addition to functioning to conduct electrons knocked out from the photoconductive layer.

A purity grade of semiconductor indium 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 applied by sputtering, techniques in the same apparatus as used to apply the photoconductive layer. Other methods of deposit known do not provide the density, bonding, A metallic layer of the order of 100 Angstroms thick may he deposited directly upon the substrate between the ohmic layer 14 and the substrate 16 to improve the adhesive affinity of substrate 16 and the overlying inorganic ohmic and photoconductive layers 14 and 12, but this generally is not required. Such a layer could be titanium metal and is indicated at 15 in FIG. 4. It is easily deposited by the same techniques used to deposit the other layers, that is, preferably by sputtering but possibly by other depositing techniques.

SUBSTRATE MEMBER 16 The substrate member 16 is the carrier or mechanical support for the photoconductive layer 12 and the ohmic layer 14. Its properties have been alluded to above, but have not been specifically detailed. The mechanical properties are flexibility, strength, transparency, 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 electrical 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.

Polyester sheeting of .005" thickness has been mentioned above as one example of substrate that has been satisfactory. This material is an organic polymer. Of excellent characteristics is such material made by E.I. duPont de Nemours Company of Wilmington, Delaware, U.S.A. and sold under the trademark "Mylar" . This material is sold with conditions of internal stress that are inherent due to its method of manufacture. Such stresses 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 80% relative humidity at a temperature of about 100° Celsius for a period of about 30 minutes. Such steps are known.

The substrate 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 and devoid of any static charge Radioactive brushing is accomplished prior to use.

Now we refer to a discussion of the manner in which the film 10 is manufactured.

Commencing with the fully conditioned substrate member 16 the first step of manufacture consists of deposit ing the ohmic layer 14 (which may comprise more than one lam of conductive material including a layer 15, for example) .

Considering the required method of deposit, a pressure chamber is used and the deposits are carried out by sputtering with a plasma vapor in a radio frequency electric field. The substrate is placed upon an anode or may be led over an anode in the case of production methods, the anode being of stainless steel and suitably cooled to 0 about 80 Celsius with water or other coolant. In a pre¬ ferred structure the substrate is in the form of a long strip and is led over the anode which could be in the form of a roller or drum. Small substrate members, say of the order of 5 centimeters (about 2 inches) square may be placed upon the anode of known sputtering chambers for laboratory work or low quantity production.

The cathode 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. The cathode is spaced .from the anode in accordance with the physical characteristics of the particular chamber, considering the geometry, the voltages to be used, etc. The chamber in- the example was pumped down to near the 10 torr pressure range. This, of course, is a substantial vacuum. Then ultrapure argon, that is, containing less than 10 ppm H 0 and N_ was ad- 2 2 mitted to the sputtering chamber through a servo-leak valve until a pressure of about 50 millitorr is achieved.

At a suitable point, the radio frequency field is established and the ionization of the argon produces 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 substrate member.' This sputtering is carried out at a rate which is determined by the conditions within the chamber, typically somewhat less than about 75 Angstroms per second. Thickness is monitored by optical means known in the art until a thick ness of about 500 Angstroms is reached.

The substrate member is now removed from the chamber and passed into or placed within another chamber in production. If 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.

In any event, the substrate member 16 with its first coating of the ohmic layer 14, in the case of the example being described being indium oxide, is again mounted on an anode carrier or led over a rotating anode or the like. In this case, the cooling of the anode may be in-creased to about 40 Celsius because more energy will be involved in depositing the photoconductive layer 12.

Cold water or liquid nitrogen may be used.

For a photoconductive layer of cadmium sulphide of the n-type the cathode or target will be made out of cadmium sulfide or even cadmium alone. The pressure is first dropped to 10"*8 torr before being adjusted to 60 millitor with later admitted argon gas and hydrogen sulphide The hydrogen sulfide provides the correct amount of sulfur to the vapor plasma so that a stoichiometrically correct proportion of cadmium and sulfur is deposited on top of the ohmic layer. It will be appreciated that in both depositing procedures the rear surface of the substrate member 16 is blocked or masked to prevent any deposit thereon in normal processes. In the case that a cadmium sulfide cathode is used, the amount of hydrogen sulfide admitted is about 500 ppm of argon. In other cases when a cadmium cathode is used, this proportion may be increased.

A small amount of copper in the form of sublimated copper chloride is admitted into the sputtering chamber, this being effected by keeping the copper salt in an evacuated vessel which communicates with the sputtering chamber through a control valve. Copper is the dopant in this case, making the cadmium sulfide n-type.

Other methods of doping are ion implantation, diffusion migration and the like.

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 of, the photoconductive layer 12. The rate of deposit in the test conducted was about 50 Angstroms per second. The copper is admitted in small controlled quantities sufficient to dope the cadmium sulfide on the ohmic layer in an amount of 5 x 10"^ percent by weight. The sputtering is continued until the thickness of the layer 12 reaches 3000 Angstroms. In the test made the layer has a microcrystalline structure with the mean diameter of the crystals being about 0.1 micron or about one-third the thickness of the layer itself.

The resulting electrophotographic film has the properties described above for the article 10. The experimental article had a yellowish cast which is characteristic of the thin films deposited thereon.

Electrophotographic film using zinc indium sulfide would not require copper doping. In this case, the color, of the film would be bluish. . ..

In use, the electrophotographic film is charged .to a high potential by means of corona as explained in connection with FIG. 5, this potential being very high compared to what would be considered the- normal saturation level of the electrophotographic film. Exposure is effected high on the dark decay curve. Thus, the article is charged to the point 48 on the curve 40 and then exposed a fraction of a second later after only the portion 42 of the curve has been traversed in the dark-decay mode.

The proper timing is accomplished by monitoring with an exposure meter so that the charge builds up to an optimum value for the particular light condition under which the electrophotographic film is to be exposed, this being accomplished automatically.

It will be appreciated that the method of using the electrophotographic film 10 which is preferred is one in which the film is shocked, in effect. In con- ' ventional xerography the medium or plate is charged to saturation, that is, to the point where the charge leaking off the medium is approximately equal to the charge building up. This is shown on the curve 30 at about the point 32.

' In the case of the electrophotographic film 10 of the invention, the medium is charged very quickly far in excess of saturation, then exposed to lower the voltage quickly.

After the electrophotographic film of the invention has been exposed, toner is applied to the surface of th photoconductive layer very rapidly and uniformly. The toner is preferably applied in the presence of a bias voltage in close proximity to the film surface to accelerate the toner particles toward the surface and to provide an even particle distribution. Conventional fine particle carbon toners can be used for black and white transparencies Colored resins may also be used.

I Finally, excess toner is immediately swept from the surface and the remaining toner fused to the surface of the film by a uniform flash of infrared radiation or the like so that the entire process is completed before the surface voltage on the electrophotographic film dissipates to its lowest or background level. This time is of the order of a small fraction of the second and preferably of the order of about ten milliseconds. The toner thereafter permanently adheres to the surface 28 as indicated at 26 in FIG. 4.

In the process of using the electrophotographic film.10", - once the toner has been applied, and before it is fused, the distribution thereof is fixed.

Accordingly, the charge decay thereafter will not materially affect the now visible image represented by the positions of the toner particles. No high speed apparatus is essential for fusing the toner immediately after it has been distributed and the excess removed. This can be done in a reasonable length of time. There is another advantage which is achieved however, especially in the case of use of the electrophotographic film in experimental work. Prior to fusing the toner, the operator can examine the same carefully to see whether the desired image has been achieved. If not satisfied, he may change lighting conditions, time of exposure, adjust focus, etc., in order to get a better "picture." As for the toner image achieved from the previous exposure, he merely rubs the same off the film by any simple means such as a swab or cloth, leaving the surface clean. Accordingly, there is no waste of electrophotographic film, and no waiting or time lost in order to achieve the desired results.

The resulting article is a transparency, suitable for projection or for use in making prints. The image is characterized by a high degree of resolution, making the article and process highly advantageous in producing microimages. It has excellent contrast and exceptionally clean background. When projected on a greatly enlarged scale for viewing or copying purposes, the resultant enlarged image still has good detail and is relatively free of imperfections in the white or light areas.

The compounds mentioned including cadmium sulfide and zinc indium sulfide have characteristics that differ from compound to compound. Requirements of spectral response, dark and light decay resistivity, gain, ease of sputtering, etc. must all be considered, and perhaps experimentally determined for the circumstances in which the compound is to be used. Generally, all compounds which are known photoconductors may be used with varying degrees of effectiveness.

Claims (22)

Appln. 246/2 C L AI MS i ,

1. A method for manufacturing an eleotrophotographic film artiole which comprises the steps ofί A. depositing thin film layer of ohmio material Upon a flexible thin organic transparent substrate in thoroughly bonded condition and a thickness which renders said thin film layer substantially transparent and flexible, BY directly in an r.f* field sputtering thin film coating of a wholly inorganio microcrys- talline phtoconduotor material upon said ohmlc material in thoroughly bonded condition and a thickness which provide a gain of said pfccfco- oonductor material greater than unity and which is substantially transparent and flexible* the total thickness of said artiole' having a light tranemis* slvity of greate than 70$ and not more than 85$.

2. The method according to Claim 1 in which tfcte photo-conduotor material is doped during deposit by introducing a dopant into said r.f* field*

3. The method according to claims 1 or 2 in which the sputtering step is gradual in order to build up the thickness of said photoconduotive material* and the sputtering process is maintained until the thioknese of the photoconduotive material provides an optimum combination of gain* transparency flexibility and abrasion resistance.

4. * An artiole of inanufaoture comprisingt A* substrate forming(means to support thin Appln 1*0*42446/2, B. a thin film coating of photoconduotive wholly inorganic macrocrystalline material on the substrate forming means, the, coating having a microcrystaliine structure the crystals of which are oriented vertically so as to be normal with respect to the surface oh which the coatin is supported, C, a thin film layer between the substrate forming means and said coating, Di said coating being electrically anisotropic and said layer and coating exhibitin greater, than 70$ light transmission but no more than 85 light transmission*

5. An article according to Cl im 4 in which said substrate forming means is a flexible, transparent} organic, polymer sheeting member; in which said thin film coating material is a semiconductor characterized by a better than 705» light transmission? and in which said layer to facilitate selective removal of electrons comprises a thin film layer of ohmic material between ' said substrate forming means and said thin film coating.

6. &ς An article according to claim £ 4,5 *ΚΧΧ&Β&ΠΚ&. in which the thin film coating has a dark resistivity of at least 10 12 ohm-centimeters and a ratio, between dark and light resistivity of at least about 10^ ohm-centimeters and is substantially transparent to light. 5

7. _¾s An article according to claim -6 in which said thin film coating and said thin film layer are less than around 5000 Angstroms thick, are flexible, and together with said substrate forming means are generally transparent, absorbing less than 30 percent of light incident thereon but not more than about 15 percent. 7 .

8. ¾ An article according to any one of claims** to & in which said thin film coating is about 3000 Angstroms thick and said thin film layer is less than 1000 Angstroms thick. 4 8

9.$Qx An article according to any one of claims ¾ to # in which said coating is an n-type semiconductor. 4 9

10. &fc$ An article according to any one of claims S to Sftx in which said thin film coating is primarily cadmium sulfide. 4 9

11.ii. An article according to any one of claims $ to 2R£ in which said thin film coating is primarily zinc indium sulfide. 4 11

12. 32. An article accordin to an one of claims & to 10c in

13. #^. An article according to any one of claims 5 to Ψ5 in which said thin film layer is primarily indium oxide. 4 13

14. An article according to any one of claims ^ to Ϊ¾Ρ in which means are provided for increasing the physical bond between said thin film layer and thin film coating on the one hand and the substrate forming means on the other hand. 14

15. An article according to claim £5 in which said physical bond increasing means comprise a thin film metallic interface. 4 15

16.l¾. An article according to any one of claims 9- to ¾¾ in which means are provided to establish electrical contact from said ohmic layer to circuitry exterior of the article. 4 16

17. ¾8c An article according to any one of claims S to Vt in which the photoelectric gain of said coating is substantially more than unity.

18.¾9>ς An article according to any one of claims¾ to -E8 in which said thin film layer is crystalline in nature. 9. x¾Gt. A method of using the article according to 4 18 any one of claims & to comprising the steps of: A. charging the surface of the coating by corona at a potential substantially greater than the saturation level thereof B. exposing said surface to a radiation pattern after the said surface has been charged but before any substantial decay has occurred, C. applying toner to said surface as soon after exposure as feasible to achieve a distribution of said toner in accordance with said pattern.

19

20. it. The method according to claim 28 the toner is fused after application.

21.-2Φς A method for manufacturing an electrophotographic film article substantially as described in the foregoing specification.

22.2^. An article of manufacture substantially as described in the foregoing specification with reference to the accompanying drawings.