US 3725711 A
An image pick-up tube with a diode array target having a chemically deposited silicon resistive film covering the diode array of the target and a target mounting assembly including a conductive ring having an inwardly extending flange adjacent one end of the ring. The flange has a window or face plate side in sealing engagement with a face plate and a target side which is grooved to receive the periphery of the diode array target. The inner wall of the ring coacts with the ring flange to form a notch for a spring type retaining ring. The retaining ring when properly positioned within the notch directs the periphery of the diode array target into the groove and retains the target firmly within the groove of the conductive ring. The target bearing side of the flange is channeled to prevent air entrapment between the target and face plate when vacuumizing the image pick-up tube. The end of the ring opposite the face plate surface forms a seat for the bulb of the image pick-up tube and a stop for a retaining ring positioning tool. The stop properly positions the target retaining ring when carried by the tool and prevents the retaining ring from contacting the fragile diode array target. When assembled with the target the conductive ring provides the electrical contact for biasing the target and taking off the video signal.
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Description (OCR text may contain errors)
United States Patent [191 Sadler [451 Apr. 3, 1973 [541 IMAGE PICK-UP TUBE SUPPORT STRUCTURE FOR SEMICONDUCTIVE TARGET  Inventor: Porter Sadler, Dallas, Tex.
 Assignee: Texas Instruments Dallas, Tex.
 Filed: June 1, 1971  Appl. No.: 148,762
 US. Cl. ..313/66, 313/288, 313/283  Int. Cl ..H0lj 29/02, l-IOlj 31/38, H0lj 29/45  Field of Search.....3l3/65 A, 65 AB, 65 T, 65 R,
Primary ExaminerRobert Segal Attorney-Harold Levine, James 0. Dixon, Andrew M. Hassell, Melvin Sharp, Gary C. Honeycutt and Alva H. Bandy  ABSTRACT An image pick-up tube with a diode array target having a chemically deposited silicon resistive film covering the diode array of the target and a target mounting assembly including a conductive ring having an inwardly extendingflange adjacent one end of the ring. The flange has a window or face plate side in sealing engagement with a face plate and a target side which is grooved to receive the periphery of the diode array target. The inner wall of the ring coacts with the ring flange to form a notch for a spring type retaining ring. The retaining ring when properly positioned within the notch directs the periphery of the diode array target into the groove and retains the target firmly within the groove of the conductive ring. The target bearing side of the flange is channeled to prevent air entrapment between the target and face plate when vacuumizing the image pick-up tube. The end of the ring opposite the face plate surface forms a seat for the bulb of the image pick-up tube and a stop for a retaining ring positioning tool. The stop properly positions the target retaining ring when carried by the tool and prevents the retaining ring from contacting the fragile diode array target. When assembled with the target the conductive ring provides the electrical contact for biasing the target and taking off the video signal.
6 Claims, 4 Drawing Figures PATEtm-imPRa 1915 sum 2 0r 2 PHOTONS DEPLETION REGION IMAGE PICK-UP TUBE SUPPORT STRUCTURE FOR SEMICONDUCTIVE TARGET BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an image pick-up tube and more particularly to an image pick-up tube having an improved diode array target and target supporting assembly.
2. Description of the Prior Art In the past, image pick-up tubes have had a hybrid type electron gun with magnetic deflection and electrostatic focus. This design was to allow the axis ofthe camera to be brought closer to the axis of the display tube, thereby reducing the parallax error. Electronic zoom has been accomplished simply by holding the deflection currents fixed and varying all electrode voltages except the accelerator voltage proportionally. The accelerating mesh potential was therefore varied between 400 to 1,000 volts to vary the raster area by ratio of 2.5 to 1. Operation of this tube revealed that this mode produced severe raster burn-in which manifests itself as changes in the tube dark current. An antimony trisulphide target, which behaves as a simple photoconductor, was used as the target for the electron gun. It has a relatively high dark current and a variable gamma (degree of contrast in the image) less than unity, both of which gives rise to poor low light level performance and dynamic range. The antimony trisulphide target is readily damaged during operation by raster burn, accidental photoconductor or photoemitter light burn, or thermionic cathode slump due to ion burn and other causes. Also because ofits instability at elevated temperatures above 120C the tube cannot be baked at the desirable high temperatures to reduce the dark current. Dark current is of importance to tube operation as it limits the low light level capability of the tube by introducing shading and noise. Antimony trisulphide was replaced by gallium arsenide but the target was found to have undesirable aging characteristic effects-It also has been the practice to pressure fit the target to a face plate of the image pickup tube using an indium ring. The indium ring was segmented to permit the evacuation of air which would otherwise be trapped between the target and face plate when the tube was air evacuated. However, a target mounted in this fashion was found to have unacceptable performance due to the harmonics generated in the target which was not held firmly by the segmented indium rings. In this configuration the face plate was attached to the bulb. In attaching the bulb, care had to be exercised that the bulb material when melted did not contact and damage the target. Further the tube output had to be taken from the target through the bulb; when connecting the output lead to the target the fragile target was often broken.
SUMMARY OF THE INVENTION It is an object of this invention to provide a rugged image pick-up tube which may be constructed using convenient fabrication procedures.
It is another object of this invention to provide an image pick-up tube having slow aging characteristics.
It is still another object of this invention to provide an image pick-up tube which is repairable.
It is a further object of this invention to provide an image pick-up tube constructed of materials compatible with high temperature baking procedures for improving tube performance.
It is yet another object of this invention to provide an image pick-up tube which has a low dark current, high light sensitivity in both visible and infrared spectrums and unity gamma.
In accordance with the invention, there is provided an image pick-up tube, suitable for use with an VIDICON type electron beam scan device such as, for example, an all magnetic, or an all electrostatic, or hybrid, or inverse hybrid type structure for directing a beam of electrons from an electron gun to a silicon diode array target having a chemically deposited silicon resistive sea capable of withstanding tube bake-out temperatures of up to 400C. The target is mounted in the tube using a retractable target mounting technique. The mount holds the thin silicon target in a manner that avoids strain and distortion which if present generate noise in the output signal of the tube, and also permits replacement of the target when its performance is unsatisfactory. In addition the target mount is in electrical contact with the target and the power supply and video signal take off circuit can be connected to the target mount thereby enhancing the ruggedness of the tube.
BRIEF DESCRIPTION OF TI-IE'DRAWINGS FIG. 1 is a cross-sectional view of the embodiment of this invention.
FIG. 2 is a partial cross-sectional view of the target and a schematic diagram of the target biasing circuit and video output signal circuit.
FIG. 3 is an exploded. isometric view of the image pick-up tube target supporting assembly.
FIG. 4 is an isometric view of the target electrode ring disclosing details of the target supporting side of the ring.
A detailed description of a preferred embodiment of this invention follows with reference being made to the drawings wherein like parts have been given like reference numerals for clarity and understanding of the elements and features of the invention.
Referring to FIG. 1, the image pick-up tube 10 construction of the present invention comprises a tubular housing or bulb 12 having a pin plug-in base 14 at one end and a target supporting assembly 16 at its other end. The pin plug-in base 14 may be, for example, any type base compatible with the plug-in socket used in a camera package and capable of withstanding tube bake out temperatures up to 400C. The bulb 12 houses a cathode 18 of an electron gun which when heated by heater 20 emits electrons through a beam forming grid 22. The electron beam is accelerated and focused as it passes respectively through an accelerator grid 24 and a focusing grid 26 on its way to a decelerating grid 28. The decelerating grid 28, often referred to as the decelerating mesh, is provided to decrease the velocity of the beam electrons reaching a target 30. The image pick-up tube utilizes magnetic focus and deflection of the electron beam which is provided by a horizontal and vertical deflection coil 32 and a focusing coil 34. The deflection coil 32 and focusing coil 34 together with a beam alignment coil 36 are positioned within a pre-aligned camera package (not shown) to provide a proper target 30 scanning beam in the tube when it is operated in the camera package.
The target 30 is a diode array target which as shown in FIG. 2 is a silicon diode array target having, for example, an N conductivity type substrate 40. An N conductivity type layer 42 about 1,500 A thick is formed over the incident light side of the target 30 by phosphorous diffusion. This N -layer 42 enhances the collection efficiency of photoexcited holes, particularly at the short wave length end of the visible spectrum and thus enhances the blue sensitivity of the image pick-up tube 10. In addition the N -layer 42 provides a gettering action for some impurities which significantly enhances the dark current performance of the silicon diode array target 30. The side of the substrate 40 opposite the N -film side has a silicon oxide (SiO layer 44 which may be formed thereon by the thermal oxidation technique. A diode array having a density of about 620,000 diodes per square centimeter is formed through the silicon oxide layer 44. BY using wellknown photoresist techniques the diodes are formed by diffusing, for example, boron into the N-type doped silicon substrate 40 to form P-type islands 46. The diode bearing surface is then covered overall with a thin resistive film 48, commonly referred to as a resistive sea. This film 48 is essential to a silicon diode array target, because without it, the oxide potential can charge below cathode potential and act as a coplanar grid preventing the electron beam from landing on the diodes. The resistive sea 48 is a layer of a chemically deposited silicon with a thickness of about 1,000 A and a sheet resistivity of about 10 to 10 ohms per square. The method of forming this chemically deposited silicon layer is disclosed in co-pending application Ser. No. 139,068 filed Apr. 30, 1971 by Victor Harrap for an Improved Diode Array Vidicon. Image pick-up tubes having resistive seas formed in accordance with the teachings of this application will operate satisfactorily with up to 650 volts on the decelerating mesh 28.
The target 30, FIG. 1, is mounted in the target support assembly 16 sealed to the end of bulb 12 opposite the plug-in end 14. The target support assembly, FIGS. 1 and 3, comprises a conductive ring 50 having adjacent one end an inwardly extending flange member 52 adapted to receive on one side, hereinafter referred to as the window side, a window or face plate 54. The face plate 54 may be constructed of glass or other suitable radiation transmitting material. A suitable glass, for
example, for visible and near infrared wavelengths is a borosilicate glass having a composition of SiO 80 percent, B 0 14 percent, Na O 4 percent, and A1 0 02 percent. Suitable glass is sold by Corning as Corning No. 7052. In order to form a reliable seal under thermal and mechanical stress between the ring 50 and the hard glass face plate 54 it is necessary that the conductive ring 50 be made of a conductive materialhaving a thermal coefficient of expansion which matches that of the hard glass. An iron-nickel-cobalt alloy having a composition of iron-54 percent, nickel-29 percent, and cobalt-l7 percent sold under the trademark KOVAR has substantially the same coefficient of expansion and therefore matches the thermal coefficient of the hard glass, Corning No. 7052, sufficiently to form a reliable vacuum proof seal. However, any hard glass-metal seal should perform satisfactorily if the differential calibration between the thermal coefficients of expansions is less than 100 parts per million. The opposite side of the flange 52, hereinafter referred to as the target side, is provided with a groove 56 adjacent to the edge of the flange 52. The inner wall 58 on the target side of the conductive ring is sloped inwardly at an acute angle with the target side of flange 52 to form a notch for a spring type retaining ring 60. The retaining ring 60 may be made of any suitable material such as, for example, nickel or a nickel-chromium alloy sold under the trademark Nichrome which will not lose its elasticity as a result of electron bombardment and operating temperatures. With the target 30 positioned with its peripheries extending over the groove 56, the spring retaining ring 60 may be compressed for insertion to the conductive ring 50 where it is released to expand into the notch and force the periphery of the target 30 into the groove 56. To prevent air from being trapped between the face plate and the target, the conductive ring 50 is constructed with grooves 64 (FIG. 4) preferably four in number, across the target bearing side of the flange 52 so that the area between the face plate 54 and target 30 (FIG. 1) is in communication with the area of the bulb. With this target supporting assembly the target is held firmly to avoid strain and distortion within the target. The bulb 12 is sealed to a seat 62 formed on the end of the target side of the conductive ring 50. The seat 62 is sufficiently wide to accommodate the thickness of the glass tube 12 and provide an inwardly extending stop member for the purpose hereinafter described. The bulb 12 is preferably made of a hard glass material such as, for example, the Corning No. 7052 glass previously described which has a coefficient of expansion matching that of the KOVAR conductive ring 50 to which it is sealed.
A special tool (not shown) is used to insert or retract the spring retaining ring 60 (FIGS. 1 and 3) into the conductive ring notch without damaging or breaking the thin silicon target 30. This tool has a length longer than the bulb 12 and a cross section corresponding to the cross section of the bulb 12. It has a spring retaining ring carrying mechanism on one end adapted to engage the retaining ring 60 firmly and a manipulating handle on the other end for actuating the retaining ring carrying mechanism to compress the ends of the retaining ring 60 for insertion into or removal from the conductive ring 50 notch through the bulb 12. The outer periphery of the tool forms a flange area adapted to engage the stop of the conductive ring 50 to limit insertion of the tool and to properly position the retaining ring carrying mechanism for retaining the retaining ring or releasing it into the notch without breaking the target 30.
With the target support assembly 16 sealed to the bulb 12, the target 30 in place, and the grids and elec-.
tron gun electrically connected to the pins of the plugin base 14, the grids and electron gun are inserted into the bulb 12 and the base 14 sealed to the bulb 12. The image pick-up tube may then be baked to a temperature up to 400C and air evacuated from the bulb 12 to form a vacuum.
For operation the pin type plug-in base 14 is inserted into a corresponding plug-in socket of a camera package preferably having voltage-regulated supplies or filament transformers as no change in such power supplies is required. As the target 30 is in electrical contact with the conductive ring 50, a signal electrode is attached to the conductive ring 50 (FIG. 2) for taking the video signal off the target between the ring contact and a load resistor 66. The load resistor is also connected to a source of power 68 for biasing the target positive with respect to the cathode. In operation the target 30 receives the previously described low energy electron beam passing through the decelerating mesh 28 in a scanning mode. Since the substrate of the target is about volts positive relative to the cathode, the electron beam will deposit electrons at the diodes charging them to cathode potential. The diodes will remain charged until the depletion layer capacitance, which is a measure of the diode charge storing capability, is discharged by light created minority carriers, (holes) or by diode leakage. The holes generated at the light incident surface are swept across the depletion region to the diode P-type region and contribute to the leakage current. Recharging of the diode by the electron beam creates the desired video signal at the video amplifier load resistor 66. Since all the holes reaching a diode during a scan time contribute to the discharging of the diode, the signal is proportional to the integrated local photon flux.
Although a preferred embodiment of the invention has been described herein, it will be apparent to the person skilled in the art that various modifications to the details of construction shown and described may be made without departing from the scope of this invention.
What is claimed is:
1. An image pick-up tube comprising:
a. an evacuated housing:
b. an electron beam producing device in one end of the housing operative to produce a target scanning electron beam;
c. a substantially circular target comprising a semiconductive substrate mounted in the other end of the housing operative responsive to light falling on one side to forma charge pattern on another side; and
d. an electrically conductive target supporting means supporting the target in the path of the target scanning electron beam, said target supporting means including (i) a cylindrical support member having a portion of reduced diameter for receiving said other end of said housing to form a sealed longitudinal extension thereof, the support member further having an internally extending flange, (ii) a substantially circular face plate at least partially within said support member and sealingly engaging said flange, (iii) a groove adjacent the edge of the interior side of the flange receiving the perimeter portion of the target and making electrical contact therewith, and (iv) a target retaining spring in substantial contact with said groove thereby to secure the target to the electrical conductive member whereby the target is supported without strain or distortion and the electrically conductive target supporting member forms a target output and biasing contact. 2. An image pick-up tube according to claim 1 wherein said target is a silicon diode array target having a chemicallydeposited silicon resistive sea over all the diodes and 81110011 dioxide layer for preventing charge accumulation on the silicon dioxide layer and loss in resolution.
3. Am image pick-up tube according to claim 1 wherein said target retaining means is a retractable spring.
4. Am image pick-up tube according to claim 1 wherein said target includes a silicon substrate of one type conductivity, a more heavily doped film of the same type conductivity diffused on one side of said substrate for enhancing the collection efficiency of photoexcited holes, a plurality of islands of conductivity type opposite that of the substrate diffused within the opposite side of the substrate through a silicon oxide layer for forming an array of diodes in the silicon substrate, and a high resistivity film covering the diode bearing surface of the substrate for preventing charge accumu lation on the silicon dioxide layer and loss in resolution. 5. Am image pick-up tube according to claim 4 wherein the silicon substrate is of N conductivity type.
6. An image pick-up tube according to claim 1, wherein the target supporting side of the support member flange of the electrically conductive target supporting means includes a plurality of grooves extending across the target supporting side of said flange -to provide a plurality of passages in communication with the area formed between the face plate and target and the remaining area of the housing.