US3761762A

US3761762A - Image intensifier camera tube having an improved electron bombardment induced conductivity camera tube target comprising a chromium buffer layer

Info

Publication number: US3761762A
Application number: US00225520A
Authority: US
Inventors: Henry W Nelson; Kramer W Meigs
Original assignee: RCA Corp
Current assignee: Burle Technologies Inc
Priority date: 1972-02-11
Filing date: 1972-02-11
Publication date: 1973-09-25
Anticipated expiration: 1990-09-25
Also published as: JPS4894321A; DE2304151C3; CA963064A; FR2171389B1; DE2304151B2; AU464930B2; AU5203273A; JPS5120243B2; GB1411853A; DE2304151A1; NL173578B; NL7301859A; FR2171389A1; NL173578C

Abstract

An image intensifier camera tube of the type having an image intensifier section for accelerating photoemitted electrons to average energies greater than 1000 electron volts and for focusing the accelerated electrons to one face of a charge storage target which is scanned on its other face by an electron beam. The improvement comprises a buffer layer of chromium on the one face of the target on which the accelerated electrons impinge.

Description

United States Patent Henry et a1.

IMAGE INTENSIFIER CAMERA TUBE HAVING AN IMPROVED ELECTRON BOMBARDMENT INDUCED CONDUCTIVITY CAMERA TUBE TARGET COMPRISING A CHROMIUM BUFFER LAYER Inventors: William Nelson Henry; William Meigs Kramer, both of Lancaster, Pa.

Assignee: RCA Corporation, New York, NY.

Filed: Feb. 11, 1972 Appl. No.: 225,520

U.S.Cl 3l5/l0,3l5/11,315/l2, 313/65 T Int. Cl. I-I0lj 31/26 Field of Search 315/10, 11, 12; 313/65 R, 66, 107, 65 T; 250/213 References Cited UNITED STATES PATENTS 6/1966 MacKenzie et a1. 313/65 T [451 Sept. 25, 1973 3,363,126 1/1968 Schaefer 313/65 T 3,584,251 6/1971 Banks et a1 313/65 T 2,171,213 8/1969 Janes 313/66 3,366,816 1/1968 Saum 313/65 T 3,433,994 3/1969 Gibson, Jr 313/10 3,440,476 4/1969 Crowell et a1. 315/10 3,628,080 12/1971 Lindequist 315/10 X Primary Examiner-Carl D. Quarforth Assistant ExaminerP. A. Nelson Attorney-Glenn I-I. Bruestle, Donald S. Cohen and Sanford J. Asman [57] ABSTRACT 5 Claims, 3 Drawing Figures PATENTEDSEPZSIQH Fia.

38 32 4O 42 fie. 3

FIG. 1 I

IMAGE INTENSIFIER CAMERA TUBE HAVING AN IMPROVED ELECTRON BOMBARDMENT INDUCED CONDUCIIVITY CAMERA TUBE TARGET COMPRISING A CHROMIUM BUFFER LAYER BACKGROUND OF THE INVENTION The invention relates to image intensifier camera tube targets.

Image intensifier camera tubes generally comprise an image intensifier section and a camera tube section. In the image intensifier section, a photocathode emits electrons in response to a light image. The electrons are accelerated and focused as an electron image on one face, the input face, of a charge storage target. A large number of secondary charge carriers are generated in the bulk of the target by the accelerated electrons. These secondary carriers diffuse to the other face, the output face, of the target and are stored there as an intensified charge pattern. The intensified charge pattern is read from the target with an electron beam by the camera tube section, which is essentially a vidicon camera tube.

In one type of image intensifier camera tube, the photoemitted image electrons are electrostatically focused and accelerated to average energies of several thousand electron volts before striking the input face of a monocrystalline silicon vidicon target wafer having an array of charge storage diodes on its output face. Such an image intensifier camera tube is described, for instance, in U. S. Pat. No. 3,440,476 issued to M. H. Crowell et al. on 22 Apr. 1969. An electrostatically focused image tube which may be adapted for use as the image intensifier section by substituting the camera tube target input face for the surface of the phosphor ordinarily on the output faceplate of the image tube is described, for instance, in U. S. Pat. No. 3,280,356 issued to R. G. Stoudenheimer et al. on 18 Oct. 1966.

It is generally desirable that the image intensifier section be operated with the anode cone and the input face of the target at several thousand volts positive with respect to the photocathode. At voltages lower than about a thousand volts, the accelerated electrons become poorly focused at the target, resulting in loss of resolution. However, when electrons with average en'- ergies of more than about 1,000 electron volts strike the semiconductor wafer, there are so many secondary carriers generated that the charge storage capability of the target becomes quickly saturated, even with a relatively low light level input to the photocathode. The result is a loss of gray-scale in the output signal of the tube.

One present approach to preventing the loss of grayscale is to apply a buffer layer of some material, such as aluminum metal, to the input face of the target to absorb some of the energy of the electrons before they impinge on the silicon, so that less secondary carriers are generated in the target. However, the addition of a buffer layer to the target has heretofore created local non-uniformities in the operating characteristics of the target. These non-uniformities result in a mottling, or smudging, in the displayed image signal from the target.

SUMMARY OF THE INVENTION The novel image intensifier camera tube comprises a target having a buffer layer of chromium metal on its input face.

Whereas the prior aluminum buffer layers resulted in non-uniformities such as mottling in the signal from the target, the chromium metal buffer layer of the novel tube performs the desired buffeiing function of absorbing the desired portion of the energy of the accelerated electrons, while having no noticeable effect on the operation uniformity of the target.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cut-away, sectional view of an image intensifier camera tube in accordance with one embodiment of the invention.

FIG. 2 is an exaggerated, sectional view ofa fragment of the target of the tube of FIG.

FIG. 3 is an exaggerated sectional view of a fragment of a target in accordance with another embodiment of the invention.

PREFERRED EMBODIMENT OF THE INVENTION EXAMPLE I One embodiment of the novel image intensifier camera tube is shown in FIG. 1 of the drawings. The tube 10 has an image intensifier section 12 and a camera tube section 14. The image intensifier section 12 is es- 'sentially a conventional image inverter diode image tube without its output faceplate and phosphor screen. It has a fiber optic bundle faceplate 16 having a flat exterior face 18 and a concave interior face 20. A photocathode 22 is deposited on the interior face 20. In a central portion of the image intensifier section 12 is an anode cone 24 for accelerating electrons emitted from the photocathode 22. At the opposite end of the image intensifier section 12 is a silicon diode array photoconductive charge storage target 26. The target 26 is positioned with an input face 28 in the electron image output plane of the image intensifier section 12, where the accelerated electrons are in focus.

The camera tube section 14 is a vidicon which has, instead of a glass faceplate, the image intensifier section 12 sealed to the end of its envelope. The camera tube section 14 includes inside the base end an electron gun, not shown, for generating a beam of electrons 30 which scans the target 26 on its output face 32.

FIG. 2 shows an exaggerated sectional view of a fragment of the target 26. The target 26 comprises a monocrystalline wafer 34 of silicon about 8 micrometers thick. On the input face 28 of the target 26, there is an evaporated buffer layer 36 of chromium metal about 60 nanometers thick. On the output face 32 of the target 26, there is formed an array of discrete PN junctions 38 in the wafer 34 separated by a web of insulating material 40 on the surface of the wafer between the junctions 38. Degenerately doped silicon contact pads 42 contact each junction 38 and overlap to some extent the insulating material 40. The contact pads 42 are scanned by the electron beams 28. The tube 10 is operated with voltages typical for the two sections 12, 14. For example, the anode cone 24 and the buffer layer 36 are preferably about 3,000 volts positive with respect to the photocathode 22 and about 10 volts positive with respect to the cathode of the electron gun.

In operation of the tube 10, light passing through the fiber optic faceplate l6 and to the photocathode 22 results in the emission of photoelectrons 43 in a pattern corresponding to the image input. The photoelectrons 43 are accelerated through the anode cone 24 and to the target input face 28. The trajectories of the electrons 43 cross over one another at the input opening of the anode cone and result in an inverted, focused electron image at the input face 28 of the target 26. Upon striking the chromium buffer layer 36 on the input face 28 of the target 26, the photoelectrons 43 give up a portion of their energy to the buffer layer 36 before entering the silicon bulk of the target wafer 34, where they result in the generation of a large number of secondary charge carriers. The positive secondary charge carriers are swept to the nearest of the PN junctions 38 on the output face 32 of the target 26, which are reverse-biased, where they result in a stored charge pattern corresponding to the input electron image. This stored charge pattern is read from the target by scanning the output face 32 with the electron beam 30 in a conventional maner, generally as described in the patent to M. H. Crowell et al. cited above.

EXAMPLE ll In another embodiment of the novel image intensifier camera tube 10, the input face 28 of the target 26 is provided with a first layer 44 of chromium about 20 nanometers thick and a second layer 46 of aluminum about 100 nanometers thick on top of the chromium layer 44 as shown in FIG. 3. We have found that it is desirable to use a layer 44 of chromium at least 7.5 nanometers thick to obtain uniformity, although it may be somewhat thicker. For example, the chromium layer 44 may be between 7.5 and about 20 nanometers thick and the aluminum layer 46 between about 80 and about 110 nanometers thick.

GENERAL CONSIDERATIONS Electron bombardment induced conductivity targets for vidicons, generally, are operated by establishing a potential difference between the input and output faces. This may be done by fixing the input face voltage at a target voltage of between about 5-50 volts positive with respect to ground. The output surface is scanned with an electron beam from a cathode at ground potential to charge the output surface to a ground equilibrium potential. Positive carriers, or holes, are generated locally in the target by elemental portions of the input electron image and diffuse to the output face, where they discharge to some extent the local equilibrium potential. Every l/30th second, the local area is again scanned by the beam to reestablish equilibrium potential there. The extent of recharging necessary to reestablish equilibrium potential determines the strength of the signal portion that is generated at the local area. Thus, where no input electrons are incident, the local area remains substantially at equilibrium potential and no signal results. Where the input image is most intense, in the highlight areas of the image, the local areas of the target are completely discharged within the 1/30 second, to result in maximum signal. At levels less than the highlight level, there are various lesser signal strengths resulting, these representing various shades of gray. When portions of the input image other than highlight portions result in the generation of so many holes that there is complete discharge of the local output areas within the l/30th of a second, these portions of the image will result in the maximum signal strength, just as well as do the highlights. Thus, areas which should be a shade of gray will appear white, so that there is a loss of gray-scale. A lowering of the target voltage can compensate to some extent for such a loss of gray-scale, but it also results in a decreased total signal strength and a lowered signal-to-noise ratio. A better approach is to decrease the average energy of the input electron image. This can be done by lowering the accelerating voltages of the image intensifier section. However, the design of electrostatic image intensifier sections most suitable for an image intensifier camera tube use is such that when the average energy to which the image electrons are accelerated falls below about l,000 electron volts, the electron image becomes poorly focused at the input face of the target. Yet, when the electrons are accelerated to average energies of a thousand or more electron volts in the intensifier section, even though there is a low light level input to the photocathode, the average energy of the electrons is still sufficient to result in a substantial loss of gray-scale in the tube.

When a buffer layer is provided on the target, the average energy of the electrons becomes relatively noncritical, since the image intensifier section may now be operated at its optimum voltage. The particular choice of material for a buffer layer, as well as its thickness, determines how much energy is lost by an electron passing through it. Aluminum seems at first to be a good material, since it is already widely used as an electron permeable coating on the phosphor screens of image intensifier tubes and kinescopes. It has been found, however, that aluminum interacts chemically with a number of semiconductor target bulk materials, especially with silicon. The interaction changes the electrical characteristics of the input face portion of the target in a non-uniform manner, so that the output singal is degraded by shading, or mottling.

A buffer layer of chromium does not result in a degraded signal. It may be applied for example, by evaporation. Any change of electrical characteristics of the target due to the chromium is uniform, so that no shading is present in the signal. Chromium is not as reactive as is aluminum, and does not chemically interact noticeably with the semiconductor materials which are most suitable for electron bombardment induced conductivity targets. Moreover, the chromium is especially advantageous for silicon targets, since it is particularly compatible chemically with it, and since its coefficient of thermal expansion closely matches that of silicon.

It may be desirable to add one or more layers of other materials on the layer of chromium to form a composite buffer layer, as in example ll above. The chromium layer will prevent reaction of the other layers with the target wafer material. For composite buffer layers, the chromium layer should be at least 7.5 nanometers thick in order to provide a uniform coating. The thickness of the other layers is chosen to provide the desired energy loss for the image electrons. For the example ll above, it was found that the chromium layer should be between 7.5 and about 20 nanometers thick, and the aluminum layer on the chromium should be between about and about nanometers thick. When chromium alone is used for the buffer layer, as in example 1 above, the chromium should be between about 52 and about 65 nanometers thick when the average energy of the image electrons are between 2,000 and 4,000 electron volts. For average electron energies of 3,000 electron volts and a silicon diode array target, the preferred thickness of the chromium buffer layer is about 60 nanometers. Other electrostatically focused image intensifier sections achieve optimum performance when the image electrons are accelerated to average energies of about 12,000 electron volts or more. For such energies the buffer layer is made much thicker.

The target may be any type of charge storage target which exhibits electron bombardment induced conductivity and is otherwise compatible with a chromium buffer layer. Most materials presently contemplated for charge storage targets fall into this category. The storage mechanism may be an array of heterojunctions, a single junction, or simply a photoconductive layer without junction.

The chromium layer may be applied by methods other than evaporation. The particular method used should be one which will not result in damage to the target material and which provides a sufficiently uniform thickness of chromium metal.

We claim: 1. An image intensifier camera tube of the type having:

an evacuated envelope having a transparent faceplate; a photocathode on the inside surface of said faceplate;

means for accelerating electrons emitted from said photocathode to average energies greater than one thousand electron volts and for substantially focusing said accelerated electrons in a plane in said envelope;

a thin target disposed with one face substantially in said plane and having at its other face means for storing charge carriers generated in the bulk of said target, and

means for scanning said other face of said target with an electron beam, wherein the improvement comprises a continuous layer of chromium on said one face of said target.

2. The camera tube defined in claim 1 wherein said target comprises a substantially monocrystalline wafer of'semiconductor.

3. The camera tube defined in claim 2 wherein said layer has a thickness of between about 52 and about 65 nanometers.

4. The camera tube defined in claim 2 wherein said layer is a composite layer comprising a first layer of chromium having a thickness of between about 7.5 and about 20 nanometers on said semiconductor, and a second layer of aluminum having a thickness of between about and about 1 10 nanometers on said chromium layer.

5. The camera tube defined in claim 1 wherein said target comprises a substantially monocrystalline wafer of silicon having on said one face a layer of chromium and having at its other face an array of charge storage diodes, said layer of chromium having a thickness of between about 52 and about 65 nanometers.

Claims

2. The camera tube defined in claim 1 wherein said target comprises a substantially monocrystalline wafer of semiconductor.
3. The camera tube defined in claim 2 wherein said layer has a thickness of between about 52 and about 65 nanometers.
4. The camera tube defined in claim 2 wherein said layer is a composite layer comprising a first layer of chromium having a thickness of between about 7.5 and about 20 nanometers on said semiconductor, and a second layer of aluminum having a thickness of between about 80 and about 110 nanometers on said chromium layer.
5. The camera tube defined in claim 1 wherein said target comprises a substantially monocrystalline wafer of silicon having on said one face a layer of chromium and having at its other face an array of charge storage diodes, said layer of chromium having a thickness of between about 52 and about 65 nanometers.