US2744837A - Photo-conductive targets for cathode ray devices - Google Patents

Description

May 8, 1956 s. V. FORGUE 2,744,837

FHOTO-CONDUCTIVE TARGETS' FOR CATHODE RAY DEVICES Filed June l, 1951 (Mawr/vf reu) 33 J3 #so mfr/fam (ggg/ous 4:59 r .swr/pe) Briar/p5) /N VNTE Sian/ey WForgzze United States Patent O PHOTO-CONDUCTIVE TARGETS FOR CATHODE RAY DEVICES Stanley V. Forgue, Cranbury, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application June 1, 1951, Serial No. 229,427

6 Claims. (Cl. 117-211) This invention relates to photo-conductive target electrodes for electron discharge devices, and to methods of making the same.

Television camera tubes employing photo-conductive targets and known as Vidicons are described in an article beginning on page 70 of the May 1950 issue of Electronics, in copending U. S. patent application, Ser. No. 198,130 of S. V. Forgue led November 29, 1950, and in a U. S. patent application Serial Number 229,428, of S. V. Forgue and R. R. Goodrich tiled concurrently with this one.

A Vidicon camera tube consists of an electron gun and a photo-conductive target assembly contained in an evacuated envelope. The electron gun is of the conventional type used in the image orthicon and other television pickup tubes. The target assembly comprises a film of light-transparent, electrically-conductive material on the glass face plate of the envelope, and a layer of photoconductive material deposited upon the electrically conductive film. The target and the gun are so arranged within the envelope that the electron beam from the gun scans the photo-conductive surface of the target.

The photo-conductive material used for Vidicon targets is an electrical insulator in the dark,- but becomes electrically-conductive under the inuence of light. The conductivity increases with the amount of light affecting the material, and is limited to the immediate area under the influence of the light.

Vidicons may be operated at either high or low velocity. That is, they may be operated with the target at sufiicient voltage positive with respect to the cathode so that the electron beam will strike the target with enough force to drive secondary electrons from the photo-conductive material, thereby rendering it more positive. Or, they may have cathode and target at approximately the same potential so that the scanning electron beam deposits electrons on the target with a negligible amount of secondary emission, thereby rendering it more negative.

In one type of high velocity operation, the electron beam scans the photo-conductive surface facing the electron gun at a velocity above first crossover (i. e. at a suiciently high velocity so that the number of secondary electrons leaving the surface will be greater than the number of primary electrons which arrive at the surface to cause the secondary emission therefrom). The first crossover potential differs with different target materials and surface conditions. Two hundred volts between target and cathode has given satisfactory results with red antimony tti-sulfide targets.

The target potential is applied to the electrically-conductive lm in contact with the opposite side of the photo-conductive layer from the one scanned by the electron beam. A collector potential, about ten volts positive with respect to the target is applied to a metal screen or ring, which is termed the collector electrode and is located immediately in front of the scanned surface of the photo-conductive layer. When the high velocity beam scans this surface, it releases secondary electrons which are attracted to the collector electrode, making the scanned surface more positive until it reaches approximately the potential of the collector electrode and equilibrium is established between them. When the scanning electron beam has brought the scanned surface to collector potential, it makes that surface ten volts positive with respect to the conductive lm on the other side of the photo-conductive layer.

When a light image is focused upon the target, the photo-conductive material becomes conductive in the areas Where the light affects it. The eifect of this conductivity is to cause a leakage of electrons from the conductive film, through the photo-conductor, to the scanned surface. The amount of leakage depends upon the intensity of the light incident upon each area; and, its effect is to make the area of the scanned surface affected by the light a volt or so less positive. When the scanning electron beam re-scans an area from which the electron charge has leaked, it restores the charge. This leakage and restoration causes an electron current ow in a circuit between the light-transparent, electrically-conductive film in contact with the photo-conductive target and the source of potential to which it is connected. Variations in the electron current through this circuit, as more or fewer electrons are needed to restore the difference in potential, become the signal output of the tube.

In low velocity operation, the electrically-conductive lilm is connected to a source of potential approximately ten volts positive with respect to the cathode which is at ground potential. The electron beam scans the photoconductive surface; and, by depositing electrons thereon at a velocity less than iirst crossover (i. e. where the ratio of secondary electrons leaving the surface when primaries strike it is less than unity), brings the bornbarded surface to cathode potential and produces approximately a ten volt difference of potential across the target.

When light is focused upon an area of the target, it renders the photo-conductive material conductive in that particular area, and causes the corresponding portion of the scanned surface of the photo-conductor to give up electrons and come a volt or `so closer in potential to the conductive iilrn. The next time the electron beam scans this area it restores to cathode potential the area from which electrons have leaked under the light-induced conductivity. This return to cathode potential restores the ten volt difference across the target and causes an electron current to the conductive film from the source of potential to which it is electrically connected. This electron current through an output resistor provides the video signal output from the tube.

Some of the qualities of photo-conductive materials which affect their usefulness as Vidicon targets are: sensitivity, resistivity in the dark, lag, useful life, and currentto-light response.

Sensitivity has reference to the ability of the material to become conductive under the influence of light. It is measured in micro-amperes of video current output per lumen of light on the target.

Resistivity in the dark has reference to that quality of the photo-conductive material which enables it to store an electrical charge in a given spot without leakage from front to back surface as long as there is no light on the target. Dependent upon the resistivity of the target material is its dark current which is the current that ows through the material between energized electrodes when there is no light on the target.

By lag is meant rapidity of response of the target to changes in light, i. e. the ability of the target to erase a signal in a given period of time without showing a shadow or trail of light. The problems arising from lag become acute when a light-colored moving object is televised against a dark background.

Useful life has reference to the hours of operation that can be expected from a target, and its ability to stand f up under handling.

Current-to-light response indicates the range of changes Ain light intensity which can be covered within given limits of current output.

The concurrently filed application of Forgue and Goodrich, identied above, describes a method for evaporating a red antimony tri-sulfide photo-conductive Vtarget onto the face plate of a Vidicon. Targets prepared by this method have especially high sensitivity, but they would be better suited to some types of yoperation if they had better resistivity in the dark (i. e. less dark current), less lag, and a spectral response closer to that of the human eye.

Accordingly, one object of the present invention is to provide an improved photo-sensitive target for electron discharge devices.

Another object is to provide an improved red antimony tri-sulfide target for such devices, and one characterized by better resistivity in the dark, less lag, and a spectral response closer to that of the human eye.

Another object is to provide an improved television camera tube.

The above, and other related objects are accomplished in accordance with the invention by evaporating red antimony tri-sulde upon a light-transparent, electricallyconductive surface in a purposely low vacuum.

The invention is explained in more detail by reference to the accompanying single sheet of drawings wherein:

Fig. 1 is a sectional view, partly diagrammatic, of apparatus for preparing a photo-conductive target after the manner of the invention;

Fig. 2 s a fragmentary plan view of a red antimony tri-sulfide photo-conductive target prepared in a high vacuum after the manner of the prior art;

Fig. 3 is a plan view of a red antimony tri-sulfide photo-conductive target prepared in a purposely low vacuum after the manner of the invention;

. Figure 4 is a partial sectional view of the vtarget structure shown in Figure l.

Referring to Fig. 1, the apparatus shown and its method of operation in applying photo-conductive targets by the evaporation method in a high vacuum are described in the concurrently tiled application of Forgue and Goodn'ch identified above.

The apparatus comprises principally an evacuation chamber 11 consisting of a bell jar 13 mounted on a base plate 15 with an evacuating pump 17 communicating .with the interior of the chamber 11 by means of a pipe 19 and a bushing 21. The junction of the bell jar 13 with the base plate 15, and the bushing 21, are both vacuum tight.

A glass blank or envelope 23 is placed with its face plate 25 in an upright position on a supporting member 27 which rests upon the base 15. Inside the envelope 23 there is inserted a crucible 24 for holding the photoconductive material which is to be evaporated onto the face plate 25 which has been previously coated with a lm 29 of light-transparent, electrically-conductive material such as an oxide or chloride of tin.

The crucible 24 is formed by twisting tungsten filament wire into the cone-like shape shown and subsequently coating it with aluminum oxide. The wires which form the crucible 24 extend within the glass envelope 23, through openings in the supporting member 27, to vacuum tight terminals 31, 31 where they make contact with a source of electrical current external to the chamber 11 and not shown in the drawing. Current from this source heats the crucible 24 and causes the red antimony trisulde contained therein to be evaporated in a layer 33 on the face plate 25 of the tube 23. A metal shield 35 is inserted inside the glass envelope 23 to prevent the evaporated material from-settling on the walls thereof. Sleeves 37 of insulating material are used to prevent the wires leading to the crucible 24 from short-circuiting against the metal shield 35 or the supporting member 27.

In the method taught by the concurrently filed application of Forgue and Goodrich referred to above, the chamber 11 is evacuated to about 10"5 mm. or less of mercury. The present inventor has discovered that when evacuation is limited to no better than about 10-1 mm. of mercury, the evaporated target 33 has different physical and electrical characteristics, such as better qualities of resistivity in the dark and lag, than when the same material is evaporated in a higher vacuum. The degree of vacuum may be controlled by an escape system 39 having a vacuum indicator 41 and an escape vent 43.

Physically, the red antimony tri-sulfide target evaporated in a poor vacuum may be said to have a mat surface. The term mat is used to represent a dull or smoky appearance as indicated in Figure 2 and which contrasts sharply with the smooth and shiny surface obtained when the same material is evaporated in a higher vacuum. This surface may also be described as spongy, or porous, to distinguish it fromthe hard surface of the high vacuum targets.

The mat surface is subject to smudging (i. e. it may be wiped from its supporting surface), but it is sufficiently adherent so that no di'iculty is encountered in its handling 0r use. Although these mat targets appear dull to the eye, they do not cause a grainy appearance in the transmitted picture.

Another desirable feature of the poor vacuum targets is that much thicker layers of this material can be built up on a surface without the peeling diiculties encountered with the shiny targets prepared in the higher vacuum. This is important when thicker targets are required to overcome capacitance lag.

Improvements of 102 and 103 in resistivity in the dark over the shiny type of red antimony tri-sulfide photoconductive targets have been observed in the mat surface targets. Also, the speed of response to sudden changes in light has been found to be appreciably better (i. e. the material lag is less).

The sensitivity of the mat surface is somewhat inferior to that of the smooth surface, but it is adequate for most television purposes.

Targets have been produced at various vacuums of less than 10-1 mm. of mercury with very satisfactory results at l mm.

In the above specification a mat surface, porous, or spongy type of red antimony tri-sulde photo-conductive target and its method of preparation have been described. Both the method of preparation, and a photo-conductive target of the physical characteristics set forth, are claimed to be within the scope of the invention.

What is claimed is:

l. A light-sensitive target for a cathode ray device comprising a layer of red antimony tri-sulde having a mat surface.

2. A light sensitive target for an electron discharge device, said target comprising, a support member and a lm of porous red antimony trisuliide 0n said support member.

3. A target electrode for an electron discharge device comprising, a transparent support member and a lm of porous red antimony trisulde on a surface of said support member.

4. A targetelectrode for an electron discharge device comprising, a transparent support member, a lm of porous red antimony trisulfide on a surface of said support member, and a conductive film between said supporting member and said antimony trisulde lm. i

5. The method of making a photoconductive target vwhich comprises the steps of, providing an atmosphere having an absolute pressure between lO-l mm. of mercury and l mm. of mercury, placing a target structure within said atmosphere, and evaporating onto a surface of said target structure red antimony tri-sulfide within said atmosphere.

6. The method of making a light-sensitive target on a supporting structure for a cathode ray tube, said method comprising, establishing an atmosphere of air having a pressure between 1(11 mm. of mercury and 1 mm. of mercury, placing said target structure within said atmosphere, and evaporating red antimony tri-sulde onto a surface of said target structure.

References Cited in the le of this patent UNITED STATES PATENTS 2,167,777 Rentschler et al. Aug. 1, 1939 6 2,236,172 Gray Mar. 25, 1941 2,236,647 Mcllvaine Apr. 1, 1941 2,404,098 Schade July 16, 1946 2,574,356 Sommer Nov. 6, 1951 2,642,367 Pfund June 16, 1953 2,654,852 Goodrich Oct. 6, 1953 2,687,484 Weimer Aug. 24, 1954 OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol 9, page 521.

De Ment: Fluorochemistry, page 358.

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