US3391297A - Photoconductive target having arsenicselenium layers of different densities on cryolite layer - Google Patents

Description

y 2, 1968 v. .1. SANTILLI 3,391,297

PHOTOCONDUCTIVE TARGET HAVING ARSENIC-SELENIUM LAYERS OF DIFFERENT DENSITIES ON CRYOLITE LAYER Filed March 29, 1965 2 Sheets-Sheet 1 OUTPUT INVENTOR Vincent J. Suntilli ATTORNE WITNESSES v. J. SANTILLI 3,391,297 PHOTOCONDUCTIVE TARGET HAVING ARSENIC-SELENIUM LAYERS OF July 2, 1968 DIFFERENT DENSITIES ON CRYOLITE LAYER Filed March 29. 1965 2 Sheets-Sheet z L|..I|||| w 5 22% TV FIELDS FIG.4.

TV FIELDS FIG-6- llllllllll 3 2 I m Ewmmau H 2205 TARGET VOLTAGE VOLTS TIME-MINUTES United States Patent 3,391,297 PHOTOCONDUCTIVE TARGET HAVING ARSENIC- SELENIUM LAYERS OF DIFFERENT DENSITIES 0N CRYOLITE LAYER Vincent J. Santilli, Corning, N.Y., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., :1 corporation of Pennsylvania Filed Mar. 29, 1965, Ser. No. 443,566 2 Claims. (Cl. 313-96) ABSTRACT OF THE DISCLOSURE This invention is directed to a radiation pickup tube. The tube includes a radiation sensitive target which comprises an electrical conductive coating, a layer of cryolite, and a first and second coating of photoconductive material of different densities.

This invention is directed to an electron discharge device and more particularly to an improved radiation sensitive target electrode.

One particular application of this invention is in a photoconductivity type of pickup tube. The most common type of photoconductivity pickup tube is a vidicon. The vidicon is comprised of an evacuated envelope in which there is provided an input radiation transmissive faceplate portion. An electrically conductive coating of input radiation transmissive material is provided on the inner surface of the faceplate. The conductive layer may be referred to as the backplate or the signal plate of the vidicon. A layer of photoconductivity material sensitive to the input radiation is deposited on the backplate and the two layers may be referred to as the target electrode. An electron gun is provided at the opposite end of the envelope with respect to the faceplate for providing an electron beam for scanning the tar-get electrode. Suitable scanning means are provided for scanning the electron beam over the target member. The beam of electrons for scanning may be of high energy type, that is, of an energy between the first and second crossover potential of the target surface or it may be of the more common type of low energy in which operation is below the first crossover potential of the target surface.

In the latter type of operation, the electrons are substantially slowed down as they approach the target surface and are deposited upon the exposed photoconductive surface to drive the surface down to substantially the potential of the cathode of the electron gun. The conductive signal plate of the target electrode is normally held at a potential of several volts to 100 volts) with respect to the cathode of the electron gun. The exposed surface of the target electrode is normally maintained at the cathode potential prior to excitation by the input radiation. In this manner, an electric field is provided across the photoconductive layer. When the input radiation image is directed onto the photoconductive layer, the film of photoconductive material is excited. The excitation of the photoconductive material by the photons causes a generation of charge carriers, electrons and/ or holes. As a result of the field impressed across the photoconductive layer, a current flow will take place through the layer in the illuminated areas and will cause these areas on the exposed surface of the photoconductive layer to tend to charge toward the potential of the conductive backplate. Those areas of the photoconductive layer which are not illuminated will remain at substantially cathode potential. The electron beam upon scanning over the exposed surface of the photoconductive layer will return the illuminated target areas to cathode potential. Since the backplate or signal plate 3,391,297 Patented July 2, 1968 is capacitively coupled with the exposed surface of the target, the instantaneous charging of the target by the beam to cathode potential will be evidenced by a voltage change in an output circuit electrically connected to the conductive backplate. This voltage change is the output signal of the vidicon. In those areas that are not illuminated, there will not be a change in potential and there will not be an output signal derived from these unilluminated areas. The term radiation means not only electromagnetic radiation such as light but also particle bombardment such as electrons.

'Ihe vidicon type of pickup tube is a simple, reliable and rugged device. This type of tube is very desirable for use in critical environments such as space vehicles. The vidicon tubes of the prior art have several limitations at least in obtaining a tube having all of the desirable properties. One of these is lack of good sensitivity. Another is a suflicient dark resistivity to give adequate storage Within the operational environment. Another disadvantage of the prior art type of device is the long response times of the target material.

It is accordingly an object of this invention to provide an improved radiation sensitive device.

It is a further object to provide an improved radiation sensitive target having short integration time.

It is another object to provide an improved target having a short response time.

It is still another object to provide an improved radiation sensitive target for high temperature environment.

In accordance with my invention, I provide an improved radiation sensitive target electrode comprised of an electrically conductive layer having a layer of insulating material provided on one surface thereof and a radiation sensitive body consisting of two layers, one of a porous material and one of a non-porous material.

Further objects and advantages of the invention will become apparent as the following description proceeds. The features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of the description.

For a better understanding of the invention reference may be had to the accompanying drawings, in which:

FIGURE 1 is a view in section of a pickup tube embodying the teachings of this invention;

FIG. 2 is an enlarged sectional view of the target shown in FIG. 1;

FIG. 3 is a graphical representation of the short decay of the signal versus time of the target shown in FIG. 1;

FIG. 4 is a graphical representation of short build up time of the signal versus time of the target shown in FIG. 1;

FIG. 5 is a graphical representation of the sensitivity of the target in FIG. 1; and

FIG. 6 is a graphical representation of the low dark current of the target in FIG. 1 in comparison with a prior art device.

Referring now to FIGS. 1 and 2, a pickup tube is illustrated including an evacuated envelope 12 containing an electron gun assembly 20 and a target assembly 30. The electron gun 20 consists of at least a cathode 22, a control grid 24 and at least one or more accelerating anodes 26 and 28 connected by suitable lead-ins to appropriate sources of potential for generating and forming an electron beam. The specific design of the electron gun 20 is conventional and may be of any suitable type of electron gun for generating a pencil-like electron beam. The envelope 12 includes a faceplate portion 14 of the material such as glass transmissive to the input radiations from a scene. An input radiation transmissive electrical- 1y conductive coating or film 32 is provided on the inner surface of the faceplate 14. An insulating coating 34 is provided on the conductive layer 32 and two photoconductive layers 36 and 37 are provided on the insulating coating 34. The conductive film or coating 32 is a signal electrode or backplate of the target 30. An electrical lead-in 38 is provided to the exterior of the envelope 12. The lead-in 38 is connected through a resistor 41 to a voltage source 39. A signal output from the tube is derived across the output resistor 41. The target 30 will be described in more detail as to structure and manufacture in connection with FIG. 2.

Means are provided for focusing the electron beam generated by the electron gun 20 and scanning the beam over the target 30 to form a raster in a conventional well-known means. This may include a focus coil 40, deflection yoke 43 and an alignment coil 42. It is also obvious that electrostatic deflection and focusing could be utilized in place of the above-mentioned electromagnetic assembly. An electrically conductive screen electrode or mesh 33 is positioned adjacent the target 30 and during operation together with the focus coil 40, functions to insure that the electron beam from the gun 20 is directed onto the target 30 normal to the surface thereof. The electron discharge device described above is substantially of conventional design and any suitable type of structure may be utilized with the exception of the target assembly which is a new and improved target electrode.

Referring now to FIG. 2, for a more detailed description of the target 30. The target 30 is supported on the light-transmission faceplate 14 of a material such as glass. The target 30 consists of electrical conductive coating 32 of a thickness of about 500 angstroms of tin oxide and transmissive to the input radiations. The resistance of material in the layer 32 should be less than 200 ohms per square. The electrical conductive coating may be formed by spraying a solution of tin salt over the heated support faceplate 14. It is also possible to provide a conductive coating of a material such as gold according to well known and established techniques by evaporation onto the faceplate.

After the electrically conductive coating 32 has been provided on the faceplate, the structure is placed in a vacuum of about 10' torr and a layer 34 of a suitable insulating material of low dielectric constant of about 1.36, such as cryolite (Na AlF is deposited on the conductive coating 32. The insulating coating 34 has a thickness of about 100 to 1500 angstroms. The substrate should be heated to a temperature of 100 to 250 C. prior to the deposition of the insulator to prevent any outgassing of the support material during subsequent depositions, and to insure good adhesion between the coatings 34 and 32. The deposition rate must be rapid to prevent any increase in the dielectric constant. A typical evaporation consists of about 80 milligrams of cryolite evaporated at a distance of about eight inches. The speed of evaporation is about ten seconds. The layer 34 must be at least 100 angstroms thick in order to yield an insulating layer which is not too high in capacitance. Thicker layers may be used but they should not exceed 1500 angstroms.

The layer 36 of asuitable photoconductive material including arsenic and selenium such as As Se is evaporated onto the layer 34. Other suitable materials are CdSe mixed with As Se CdS mixed with As Se or As Se S. This layer 36 may be produced by evaporation in a container having an atmosphere of a suitable inert gas such as argon at a pressure of about 1001b. The layer 36 should be of a thickness of about 0.2 to 0.3 micron. This provides a porous layer having a density of less than percent of its normal bulk density, The proper thickness is normally determined by monitoring transmission of light through the substrate in a well known manner to provide transmission of about 10 percent of the light. A

4 solid layer 37 is provided over the layer 36 by evaporating in a vacuum of about l0- torr, one of the above materials and to a thickness such that only about 1 percent of the light is transmitted. This corresponds to a thickness of about 2 to 3 microns for the solid layer. This layer is at approximately its normal bulk density.

It is found that a target fabricated in the above manner provides a high sensitive vidicon type tube with short response time.

In FIG. 3, the fast decay properties of the target are illustrated. FIG. 3 indicates that the signal decays to less than 25 percent of the maximum signal in three scans, each ,6 of a second. In the absence of the scanning beam, the target will retain the stored imaged for at least 10 to 15 seconds. This makes the tube suitable for slow scan operation.

FIG. 4 illustrates the fast build up of the target. FIG. 5 illustrates the sensitivity of the target and FIG. 6 compares the dark current of this invention illustrated by curve 51 with that of the prior art type of device illustrated by curve 52.

While there have been shown and described what are presently considered to be the preferred embodiments of the invention, modifications thereof will readily occur to those skilled in the art. For example, the photoconduct-ive layers may be reversed with the solid layer on the insulator and the porous layer on the solid layer. It is not desired, therefore, that the invention be limited to the specific arrangement shown and described, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

I claim as my invention:

1. A pickup tube comprising an evacuated envelope containing means to develop a scanning beam, a radiation sensitive target assembly within said envelope to be scanned by said electron beam, said target assembly comprising an electrically conductive film transmissive to input radiations, a layer of cryolite of a thickness of to 1500 angstroms, a first photoconductive layer including arsenic and selenium deposited on said cryolite layer and having a density not greater than 10% of its normal bulk density and a second coating of photoconductive material including arsenic and selenium deposited on said first photoconductive layer, said second photoconductive layer having a substantially normal bulk density.

2. A pickup tube comprising an evacuated envelope containing means to develop a scanning beam, a radiation sensitive targe assembly within said envelope to be scanned by said electron beam, said target assembly comprising an electrically conductive film transmissive to input radiations, a layer of cryolite of a thickness of 100 to 1500 angstroms, a first photoconductive layer including arsenic and selenium deposited on said cryolite layer and having substantially normal bulk density and a second coating of photoconductive material including arsenic and selenium deposited on said first photoconductive layer, said second photoconductive layer having a density not greater than 10% of its normal bulk density.

References Cited UNITED STATES PATENTS 2,887,596 5/1959 Rijssel et al. 2,967,254 l/196l Forgue. 3,069,551 12/1962 Haine 315-10 X 3,046,431 7/ 1962 Nicholson. 3,213,315 10/1965 Lempert. 3,268,764 8/1966 Simms.

ROBERT SEGAL, Primary Examiner.

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