Image pick-up tube

Abstract: An image pick-up apparatus is disclosed which includes a target portion (101, 102, 103) having a photoconductive film (103) on a substrate (101) and a target electrode (102) and reads video information converted into an electric signal in the photoconductive film (103) by an electron beam. An insulating region (104) is provided for the target portion (101, 102, 103) such that carrier generated in an ineffective scanned region (a target region corresponding to an area not scanned by the electron beam) does not appear on a surface of the target portion (101, 102, 103).

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to an image pick-up tube and, if speaking in detail, to a structure of a target portion of an image pick-up tube of a photoconductive type for visible light or for x-rays. More particularly, the present invention relates to improvement of the target portion suitable for the image pick-up tube used with an increased target voltage. DESCRIPTION OF THE RELATED ART Generally, a photoconductive type image pick-up tube or an x-ray image pick-up tube (to be called an image pick-up tube as a general term hereinafter) includes a target portion for converting an incident light or x-ray image into a charge pattern to accumulate the charge, and an electron beam scanning section for reading the charge pattern to obtain a current signal. Immediately after the target portion is scanned by an electron beam, the image pick-up tube operates to balance a surface potential on the scanned side of the target portion with a cathode potential. Such an image pick-up tube is described in, for example, "Satuzou Kougaku (imaging engineering)", pp. 109 - 116, by Ninomiya et al., published by Corona-sha, 1975, IEEE Electron Device Letters, 1987, EDL - 8, No. 9, pp 392 - 394, or Preliminary Transactions for National Congress of the Institute of Television Engineers of Japan, 1982, pp. 81 - 82, by Kawamura, et al. With such an image pick-up tube, if secondary electrons are easily emitted from a surface of a target portion on a side scanned by an electron beam, the normal operation mentioned above of the image pick-up tube cannot be achieved. In order to decrease a secondary electron emission yield on the scanned side surface, a method is disclosed, in which an electron beam landing layer composed of porous Sb 2 S 3 is deposited on the scanned side surface of the target portion in inert gas by evaporation (JP-B-52-40809). In addition, in order to suppress a false signal generated when excess electrons are reflected by an electrode in the tube during scanning an electron beam and come into the target portion again so as to obtain an output signal of a high S/N ratio, some methods are disclosed; in one method of them, a new electrode is provided on an area of the scanned side surface of the target portion except for an area scanned by an electron beam (JP-A-61-131349) and in another method of them, a transparent conductive film of the target on the light or x-ray incident side is separated into two portions in correspondence with a portion scanned by an electron beam and the other portion and the two portions are connected to individual power supplies to control an operation of the tube (JP-A-63-72037). With the image pick-up tube according to the above conventional methods, it is necessary to make a photoconductive layer of target portion thick in order to increase its sensitivity and decrease capacitive after-image. In addition, it is necessary to make a voltage between a target electrode and a cathode electrode of the image pick-up tube (to be referred as a target voltage hereinafter) higher in order to cause avalanche multiplication in the photoconductive layer so as to realize higher sensitivity. However, if such an image pick-up tube is used with the higher target voltage, bad phenomena such as a phenomenon that image distortion or shading appears in a reproduced image on a monitor, a phenomenon that an abnormal pattern varying in a ripple manner is generated in the periphery of the reproduced image (to be referred to as a water fall phenomenon), and a phenomenon that the polarity of a portion of an output signal from the tube corresponding to the periphery of the reproduced image is inverted (to be referred to as a signal inversion phenomenon). Therefore, there is, in the conventional image pick-up tube, a problem in which it is difficult to stably obtain a better reproduced image. US-A-2 900 569 discloses an image pick-up tube which comprises a cathode electrode for emitting an electron beam, a substrate, a target section with a photoconductive layer for converting an electromagnetic wave into charge carriers as electrical signals and a target electrode provided on the substrate, and means for scanning the photoconductive layer by the electron beam to read the electrical signals. To avoid certain image-deteriorating phenomena that occur when portions of a photoconductive layer are not appropriately discharged by the scanning beam, the target electrode is provided in an area smaller than the scanning area of the electron beam. However, conductor strips are required to connect the target electrode to the periphery of the target section, and the phenomena still occur due to the fact that these connecting strips cross the photoconductive layer. PATENT ABSTRACTS OF JAPAN, Vol. 5, no. 205 (E-88) (877), 25th December 1981; & JP-A-56 126237 disclose an image pick-up tube in which both the photoconductive layer and the target electrode are rectangularly shaped and have almost the same size as the effective scanning zone, to improve the signal/noise ratio by reducing the stray capacity of the target electrode. US-A-3 872 344 describes an image pick-up tube in which a target electrode is provided only outside the effective scanning area in order to reduce the static capacity of the target and decrease the after-image phenomenon. An insulating layer is disposed between part of the photoconductive layer and an electrically conductive layer which serves to keep all portions of the photoconductive layer at the same potential. DE-A-1 464 377 teaches the use of Se for the photoconductive layer of an X-ray image pick-up tube. SUMMARY OF THE INVENTION It is an object of the invention to provide an image pick-up tube which includes a target capable of suppressing generation of the above bad phenomena even at a higher target voltage and stably obtaining the better reproduced image. This object is solved by the image pick-up tube set forth in claim 1. The subclaims are directed to preferred embodiments of the invention. One implementation of the present invention as claimed is an image pick-up tube having a target portion which includes a substrate for permeating electromagnetic wave, such as light or x-ray, having video information of a subject, a target electrode, a photoconductive region having a function (a photoelectric conversion function) for generating carriers by absorption of the electromagnetic wave, and an insulating thin film as means disposed in at least one portion of an ineffective scanned region, which is a region of the target portion except for an effective scanned region, which is a region corresponding to an area scanned by an electron beam, for blocking raise or fluctuation of a potential on the surface of the target portion on a side scanned by the electron beam, and an image pick-up apparatus employing the image pick-up tube. The means for blocking the raise or fluctuation of the potential is realized by an insulating thin film with a high resistance. "The at least one portion of the region except for the effective scanned region" is in that area of the target portion which is not scanned by the electron beam and may be in an arbitrary location between the target electrode and the scanned side surface in a direction perpendicular to the layers in the target portion. The insulating thin film is provided as a single layer or a plurality of layers. Typically, the higher the resistance of the film is, the more remarkable the effect becomes. However, the effect is great when the resistance is higher than the dark resistance of the photoconductive region or film and the effect is less when it is lower than the dark resistance. Therefore, the resistivity of the insulating thin film is preferably substantially higher than 10 12 Ω-cm. The insulating thin film is mainly composed of material with high resistance such as oxide, halogenide, nitride, carbide, compound of II-VI groups, or organic material. More specifically: an oxide or oxides of at lest one of Mg, Al, Si, Ti, Mn, Zn, Ge, Y, Nb, Sb, Ta and Bi; a fluoride or fluorides of at least one of Li, Na, Mg, Al, K, Ca, Ge, Sr, In and Ba; a nitride or nitrides of at least one of B, Al and Si; silicon carbide; zinc sulfide; or a polyimide insulator is effective as the material. The insulating thin film including a single layer which is mainly composed of at least one material selected from among the above materials, or a multiple film obtained by laminating the single layers of two or more types is available. Preferably, in order to decrease secondary electron emission from the surface area of the ineffective scanned region, the insulating thin film or the photoconductive film is made porous or a single or multiple porous film is provided on the photoconductive film. A compound of at least one selected from the group consisting of Zn, Cd, Ga, In, Si, Ge, Sn, As, Sb and Bi and at least one selected from the group consisting of S, Se and Te is used as the material of the porous film. The secondary electron emission yield is controlled on the basis of the thickness of the porous film, composed of the above compound, which is formed in inert gas by evaporation, and the pressure of the inert gas upon the evaporation. Another implementation of the present invention as claimed is an image pick-up tube including a substrate for permeating electromagnetic wave, a first electrode, a photoconductive structure for generating carriers by absorbing the electromagnetic wave, a second electrode for radiating an electron beam for obtaining an electric signal in accordance with distribution of the carriers generated in the photoconductive structure, and an insulating thin film as means for substantially balancing a potential on an area of the surface of the photoconductive structure except for an area of an effective scanned region on a side scanned by the electron beam, with a potential of the second electrode, and an image pick-up apparatus employing the tube. Such an image pick-up tube operates in a state having means for scanning the electron beam and an outer tube for sealing. The means for balancing the surface potential with the potential of the second electrode prevents the surface potential of the area corresponding to the ineffective scanned region from balancing with a potential of the first electrode. Therefore, the present invention as claimed in which such means is provided in or on the photoconductive structure, is specifically effective for the image pick-up tube having the photoconductive structure to which strong electric field is applied to cause it operate. Another implementation of the present invention as claimed is an image pick-up tube including a substrate, a structure having a photoconductive region for receiving incident video information of light of x-ray and generating carriers by photoelectric conversion or x-ray to electric conversion, the structure having a surface scanned by an electron beam, and an electrode provided between the substrate and the structure, and wherein a region of the structure corresponding to an area not scanned by the electron beam (an ineffective scanned region) includes an insulating thin film as means for preventing the carrier from running in the ineffective scanned region. The preventing means prevents the carrier generated in the ineffective scanned region of the photoconductive region from running toward the scanned surface of the structure (due to electric field externally applied). The above bad phenomena are caused by the carrier in the ineffective scanned region. If such carrier appears on the scanned surface and changes the surface potential of the structure, it is difficult to stably take out a better video signal. That is, the inventors studied the above image distortion, shading, water fall phenomenon and signal inversion phenomenon in detail and understood that the bad phenomena are due to the following factors. Generally, the image pick-up tube of a photoconductive type is used under application of 200 to 2000 V to the mesh electrode and a few to a few hundreds V to the target electrode with respect to the cathode electrode. Electrons in the electron beam sequentially adhere on the surface of the effective scanned area of the target portion for every field period during an operation of the tube. Therefore, the surface potential of the effective scanned area substantially balances with the potential of the cathode immediately after the beam is scanned and excess electrons when the beam is scanned return to the cathode. These excess electrons are called a returning electron beam. Photocurrent occurs in the photoconductive layer when light is radiated and the surface potential is raised by a voltage determined on the basis of an amount of radiated light for the field period and capacitance of the photoconductive film. This raised voltage is at most a few to a few tens V in a normal operation and the surface potential restores to substantially the cathode potential until a next electron beam scan. On the other hand, the area corresponding to the ineffective scanned region is not directly scanned by the electron beam. Therefore, the surface potential of the area does not always become the cathode potential. It tends to balance with the potential of the target electrode rather than that of the cathode. It is because when difference between the potentials on the surfaces of the photoconductive film occurs in the ineffective scanned region, the dark current and the photocurrent generated due to incoming of stray light or scattered light into the tube flows in a direction eliminating the difference between the potentials. Therefore, the more the target voltage is increased, the higher the surface potential of the ineffective scanned area becomes, with the result that a great potential difference occurs between the effective and ineffective scanned regions. For this reason, the electron beam scanning nearby the boundary of the surface areas corresponding to the effective and ineffective scanned regions is deflected due to influence of the surface potential of the ineffective scanned region, so that it becomes difficult for the electron beam to vertically income to the target. As a result, the image distortion or shading phenomenon is generated in a reproduced image corresponding to neighborhood of the boundary of the effective and ineffective scanned region. In addition, when the surface potential in the ineffective scanned region is high, the surface potential acts on stray electrons such as the secondary electrons generated in the tube, electrons in the returning electron beam, and the scattered electrons resulting from reflected electrons by the electrodes, such that the ineffective scanned region absorbs the stray electrons to actively emit the secondary electrons as the surface potential becomes high. As a result, the surface potential in the ineffective scanned region varies complexly and becomes unstable, with the result that the water fall phenomenon is generated. When the secondary electron emission yield exceeds one, the surface potential of the ineffective scanned region acceleratingly is increased to exceed the potential of the target electrode, and finally the region of the high surface potential invades the effective scanned region, thereby resulting in the signal inversion phenomenon. As described above, the bad phenomena for a reproduced image appeared at the periphery of the reproduced image on a monitor such as image distortion, shading, water fall phenomenon and signal inversion phenomenon are caused by raise of the surface potential on the ineffective scanned region during the operation of the tube. However, in the present invention as claimed, an insulating thin film of high resistance is provided in at least one portion of the ineffective scanned region of the image pick-up tube target portion. Therefore, by use of a tube as claimed, the operation that the surface potential of the at least one portion rises to balance with the target potential can be suppressed. In addition, since there is remained only phenomenon that the surface potential of the at least one portion is lowered due to adhesion of stray electrons in the tube, the surface potential in the ineffective scanned region intends to balance with the cathode potential, thereby to suppress generation of the bad phenomena such as the image distortion, the shading, the water fall phenomenon and the signal inversion phenomenon. Further, when a porous thin film is provided for suppressing the secondary electron emission yield on the surface of the ineffective scanned region, the effect of the present invention can be achieved more effectively and stably. Furthermore, the present invention is disclosed, taking an example of the image pick-up tube of the photoconductive type. However, the present invention can be apparently applied to an image pick-up tube for x-rays using a Be or Ti thin plate with a high permeability for the x-ray as the substrate. Generally, in order to increase an amount of absorption of incident x-ray in the x-ray image pick-up tube, the thickness of an x-ray film (it is called a photoconductive film as a general term without differing it from another photoconductive film hereinafter) is increased to operate the tube at a higher target voltage. As a result, the bad phenomena described above are easily caused. However, these are remarkably suppressed according to the present invention. Still more, the present invention is applied to an image pick-up tube of a carrier multiplication type which is used with so high target voltage that avalanche multiplication of charges occurs in the photoconductive film, the high sensitivity that quantum efficiency exceeds one can be obtained while the bad phenomena such as image distortion, shading, water fall phenomenon, and signal inversion phenomenon are suppressed. The present invention requires no limitation to the material of the photoconductive film of the image pick-up tube and can be applied to image pick-up tubes of various types. More particularly, when the present invention as claimed is applied to an image pick-up tube of a blocking type, at least one portion of whose photoconductive film is composed of Se or Si as a main component, remarkable effect can be obtained and in this case a reproduced image with high sensitivity, high resolution, and low after-image can be obtained. Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiment. BRIEF DESCRIPTION OF THE DRAWINGS The invention may take in various parts and arrangement of parts. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. Figs. 1A and 1B show a structure of an image pick-up tube according to an embodiment of the present invention. Figs. 2A, 3A, 4A and 5A are plan views showing surfaces of target portions of image pick-up tubes according to other embodiments of the present invention when the target portions are viewed from an electron-beam-scanned side, respectively. Figs. 2B, 3B, 4B and 5B show structural cross sections of the target portions in Figs. 2A, 3A, 4A and 5A, respectively. Figs. 6A, 6B, 6C and 6D show structural cross sections of the target portions according to further other embodiments of the present invention. Figs. 7A, 7B, 7C, 8A, 8B, 9A, 9B are plan views showing target portions according to still other embodiments of the present invention. Figs. 10A and 10B are cross sections of the target portions of the image pick-up tubes according to still another other embodiments of the present invention. Fig. 11 is a block diagram showing an arrangement of an embodiment when the image pick-up tube according to the present invention is applied to a three-tube system of color camera apparatus for a high definition TV. Fig. 12 is a block diagram showing an arrangement of a system when the image pick-up tube according to the present invention is used for x-ray image analysis. DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to Figs. 1A and 1B, one embodiment according to the present invention will be described. Fig. 1A is a plan view showing a target portion of an image pick-up tube when the target portion is viewed from the side scanned by an electron beam, and Fig. 1B shows a cross section showing a major portion of the image pick-up tube. The target portion of the image pick-up tube includes a target electrode 102, a photoconductive film 103, an insulating thin film 104 as an insulating means, and a surface layer 105 on the scanned side, which are in turn provided on a substrate 101. The substrate 101 is composed of material permeating image information, i.e., light or x-ray from a subject and transparent glass is available for the substrate. A conductive film is formed of indium oxide, tin oxide, or the like as a main component by sputtering or electron beam evaporation, to provide the transparent target electrode 102. A thin film of metal such as Be is also available for the conductive film. The photoconductive film 103, which is mainly composed of semiconductive material, e.g., an amorphous semiconductor having Se or a tetrahedral element as a main component, such as amorphous-Se, amorphous-Si:H, or amorphous-SiC:H, is formed on the target electrode by vacuum evaporation. The photoconductive film may be a single layer structure or a laminating structure having a plurality of layers composed of different semiconductive materials. More specifically, a two-layer or three-layer structure is available for the photoconductive film 103. In a case of the two-layer structure, the photoconductive film 103 includes a first photoconductive film which is formed on a target electrode side thereof and a second photoconductive film which is formed on an electron beam scanned side thereof. The first photoconductive film is mainly composed of material having a remarkably large absorption coefficient at a wavelength of incident light or x-ray, e.g., an amorphous semiconductor containing hydrogen and having Si, or Si and C as main components. The second photoconductive film is mainly composed of material suitable for carriers (holes), which are generated in the first photoconductive film, to run toward the scanned side, e.g., an amorphous semiconductor having Se as a main component. In a case of the above three-layer structure, the film 103 further includes a third photoconductive film, provided between the first and second photoconductive films, for the carriers smoothly running between them. A positive carrier injection blocking structure, e.g., a Schottky barrier (not shown) is provided between the target electrode 102 and the photoconductive film 103 to block injection of the positive carriers from the electrode 102 into the film 103, thereby to lower a dark current, though the structure is not shown in the figure. In order to enhance the blocking function, a special carrier injection blocking layer made of material such as n-type amorphous-SiC:H or cerium oxide may be provided. In this embodiment, the insulating film 104 is provided on the photoconductive film 103. The film 104 is formed of aluminum oxide, silicon oxide, Al 2 F 3 , yttrium oxide, Sb 2 S 3 , or the like by sputtering. The film 104 acts such that carriers, which are generated in an ineffective scanned region by the dark current, stray lights, or scattered lights, do not run under an applied electric field and do not appear on a surface area of the ineffective scanned region on the scanned side. When these carriers, i.e., charges unnecessary for a video signal move to the scanned surface of the target portion, it is considered that fluctuation of an electric surface potential of the target portion due to the carriers gives a bad influence to an effective scanned region, thereby resulting in the water fall phenomenon and the signal inversion phenomenon. The surface layer 105 acts such that an emission yield of secondary electrons to incident electrons does not exceed one, the secondary electron being generated due to impact of the incident electrons accelerated by the target voltage. A numeral 106 in Fig. 1A indicates a boundary of an area of the effective scanned region for the electron beam. The image pick-up tube includes, other than the above components, an outer tube 107, a sealing material composed of indium or the like for coupling the substrate 101 to the outer tube 107 and sealing the inside of the outer tube 107 in a vacuum state, a metallic ring 109, a mesh electrode 110 for accelerating the electron beam 111, a cathode electrode 112 for emitting the electron beam, and a coil 113 for deflecting and focusing the electron beam. An image pick-up tube having a scanning electron beam generating section of an electromagnetic deflection electromagnetic focusing system is shown in Fig. 1B as an example. However, the deflection focusing system of the electron beam is not limited to the above system and a system such as an electromagnetic deflection electrostatic focusing system, an electrostatic deflection electromagnetic focusing system or an electrostatic deflection electrostatic focusing system may be employed, as well known. In the embodiment, the insulating thin film 104 is provided on the photoconductive layer 103 and in an area of the target portion except for an effective scanned region. The surface layer 105 is provided on the insulating thin film 104 and the effective scanned region of the photoconductive film 103. Figs. 2A to 5B show structures of the target portions of the image pick-up tubes according to other embodiments of the present invention. Figs. 2A, 3A, 4A and 5A are plan views showing the surfaces of the target portions in the other embodiments when they are viewed from the scanned side, and Figs. 2B, 3B, 4B and 5B show structural cross sections of the target portions according to the other embodiments. The components indicated by numerals 101 to 110 are the same as those shown in Figs. 1A and 1B. Numeral 214 indicates a target electrode pin connected to the target electrode 102. Numeral 315 indicates a guard electrode which is provided in the ineffective scanned region and is separated and insulated from the target electrode. Numeral 516 indicates a guard electrode pin connected to the guard electrode. In any one of the other embodiments, the insulating thin film 104 is provided between the photoconductive film 103 and the surface layer 105 in the ineffective scanned region of the target portion. Therefore, each of these image pick-up tubes operates similarly to that shown in Figs. 1A and 1B. In addition, in each of the image pick-up tubes shown in Figs. 2A to 5B, the transparent conductive layer as the target electrode 102 has a minimum of area necessary for an output signal to be read from the target electrode pin 214 and hence a structure for decreasing stray capacitance as much as possible is employed. Therefore, in addition to the suppression of the above-mentioned bad phenomena for the reproduced image, a signal-to-noise (S/N) ratio can be increased, compared to the image pick-up tube shown in Figs. 1A and 1B. Particularly, in the image pick-up tubes shown in Figs. 3A, 3B, 5A and 5B, the metallic ring 109 or the guard electrode pin 516 is connected to a new different power supply to allow the guard electrode 315 to operate at a voltage lower than the target voltage. Therefore, even if a further higher voltage is applied to the target electrode and the mesh electrode to operate them, there is a remarkable advantage in that the above-mentioned bad phenomena for the reproduced image can be suppressed. In the embodiments shown in Figs. 1A to 5B, the image pick-up tubes according to the present invention wherein the insulating thin film is provided between the photoconductive film 103 and the surface layer 105 in the ineffective scanned region of the target portion of the image pick-up tube, are described. However, the insulating thin film 104 as claimed is not necessarily provided in the region shown in each figure. As shown in Figs. 6A to 6D, in the ineffective scanned region of the target portion, the film 104 may be provided between the photoconductive film 103 and the target electrode 102 (Fig. 6A) or inside of the photoconductive film 103 (Fig. 6B). In addition, a plurality of insulating thin films may be provided on the surface of or inside of the photoconductive film 103 (Fig. 6C). Furthermore, the entire ineffective scanned region of the photoconductive film 103 may be replaced by the insulating thin film 104, thereby obtaining the same effect (Fig. 6D). The insulating thin film as claimed is not necessarily provided on the entire ineffective scanned region of the target portion of the image pick-up tube. For example, as shown by double oblique lines in plan views of Figs. 7A, 7B and 7C, the insulating thin film 104 may be provided in a portion of the ineffective scanned region on the target portion. In these cases, the effect is achieved in its own way. The target portion of the image pick-up tube according to the present invention as claimed will be described below, taking examples. Example 1 On a substrate with 26 mm Φ in diameter composed of transparent glass, a transparent conductive film having indium oxide as a main component is formed by sputtering or evaporation by electron beam. On the film, a hole injection blocking layer with 20 nm in thickness is formed of cerium oxide by vacuum evaporation and, on the layer, a photoconductive film with 1 to 10 µm in thickness is formed of an amorphous semiconductor having Se as a main component. On the photoconductive film, an insulating thin film with 0.5 to 5 µm in thickness is formed of Al 2 F 3 by vacuum evaporation. At this time, a metallic plate evaporation mask is used so as not to evaporate Al 2 F 3 on the effective scanned region of the photoconductive film. Then, antimony trisulfide is evaporated on the entire surface in an Ar gas with 0.3 mbar in pressure to form a porous surface layer with 0.1 µm in thickness, thereby obtaining the image pick-up tube target. Example 2 With reference to Fig. 8A, an example 2 will be explained. In the figure, a transparent conductive film having indium oxide as a main component is formed, as a target electrode 102, in an area surrounded by lines a-a' and b-b' on a substrate 101 with 18 mmΦ in diameter composed of transparent glass. In an oblique line portion of the figure, BiO 2 is deposited in a composite gas of argon and oxygen, with 0.4 mbar in pressure to form a porous insulating thin film with 1 to 5 µm. Then, in a dot-scattered area shown in the figure, a cerium oxide thin film, an amorphous semiconductive film having Se as a main component, and a porous antimony trisulfide layer are formed by a technique similar to that of the example 1, thereby obtaining the image pick-up tube target. Example 3 With reference to Fig. 8B, an example 3 will be described. In the figure, a transparent conductive film having tin oxide as a main component is formed, as the target electrode 102, on the surface of a substrate 101 composed of transparent glass by CVD. In an oblique line portion of the figure, an insulating thin film with 0.5 to 5 µm in thickness is formed of silicon oxide by sputtering. Then, in the oblique line portion and dot-scattered portion, the hole injection blocking layer with 10 nm in thickness is formed on n-type amorphous hydride SiC by glow discharge CVD, and, on the blocking layer, the photoconductive film with 0.1 to 1 µm is formed of amorphous hydride Si. Subsequently, on an area of the amorphous Si film corresponding to the oblique line portion, an insulating thin film with 0.5 to 5 µm in thickness is formed of silicon oxide. Then, in the oblique line portion and the dot-scattered portion, an amorphous semiconductive film with 1 to 5 µm having Se as a main component is formed by vacuum evaporation and, on the amorphous semiconductive film, Sb 2 S 3 is deposited in Ar gas with 0.3 mbar in pressure to form a porous surface layer with 0.1 µm in thickness, thereby providing the image pick-up tube target. Example 4 With reference to Fig. 9A, an example 4 will be described below. A hole is formed in the substrate 101 composed of transparent glass and the signal electrode pin 214 is welded thereto. A transparent conductive film composed of indium oxide as a main component is formed on an entire surface of the glass substrate 101. The transparent conductive film is processed in a shape shown by the oblique line portion in Fig. 9A by normal chemical etching to provide the target electrode 102 and the guard electrode 315. In an area inside of a circle 917 representing boundary of the photoconductive film except for the effective scanned region, the insulating thin film is applied with 0.5 to 5 µm in thickness composed of aluminum oxide by sputtering. On the entire area inside of the circle 917, the hole injection blocking layer with 20 nm in thickness composed of cerium oxide and the photoconductive film with 1 to 10 µm in thickness composed of an amorphous semiconductor having Se as a main component are formed by a method similar to that in the example 1. On the film, CdTe is deposited in nitrogen gas with 0.5 mbar in pressure to form a porous surface layer with 0.1 µm in thickness, thereby providing the image pick-up tube target. Example 5 With reference to Fig. 9B, an example 5 will be described below. Holes are formed in the substrate 101 composed of transparent glass and the signal electrode pin 214 and the guard electrode pin 516 are welded to them. The guard electrode 315 composed of Cr-Au is formed thereon by vacuum mask evaporation. A transparent conductive film composed of indium oxide as a main component is deposited on the entire surface. The transparent conductive film is processed in shape shown in the figure by chemical etching to provide the target electrode 102. The insulating thin film, the hole injection blocking layer and the photoconductive layer are formed thereon under the same condition as in the example 4. Sb 2 S 3 is deposited in Ar gas with 0.3 mbar in pressure to form a porous surface layer with 0.1 µm in thickness, thereby providing the image pick-up tube target. Example 6 Fig. 10A is a rough cross-section showing the target portion of the image pick-up tube according to an embodiment of the present invention. In the embodiment, a conductive Be thin plate is used as the substrate and also serves as the target electrode. As shown in Fig. 10A, an insulating multiple thin films, which have thickness of 0.5 to 5 µm and are mainly composed of yttrium oxide and silicon oxide, respectively, are formed in the area of the Be thin plate substrate 1018 outside of the effective scanned region. A hole injection blocking layer (not shown) with 20 nm in thickness composed of cerium oxide and an amorphous semiconductive film with 4 to 50 µm composed of Se as a main component are formed on the entire surface except for an indium seal portion in the outer portion. Further, Sb 2 S 3 is deposited in Ar gas with 0.4 mbar in pressure to form a porous surface layer with 0.1 µm in thickness, thereby providing the image pick-up tube target for x-ray. Example 7 With reference to Fig. 10B, an example 7 will be described below. As shown in Fig. 10B, a conductive Be thin plate 1018 having a hole for allowing the target electrode pin 214 to pass therethrough, an insulating glass thin plate 1020, and the target electrode pin 214 are adhered to each other by an insulative adhesive 1019 to provide the substrate. Al with 0.02 to 0.1 µm in thickness is deposited on the entire surface thereof. The Al deposited film is processed by chemical etching and separated in the same electrode shape as that shown in Fig. 9A to provide the target electrode 102 and the guard electrode 315. The insulating thin film 104, the hole injection blocking layer, the semiconductive layer and the surface layer 105 are sequentially formed thereon, thereby providing the image pick-up tube target for x-ray. Example 8 Sb 2 S 3 is deposited in Ar gas with 0.5 mbar (0.4 Torr) in pressure on the surface layer 105 of the image pick-up tube target except for the effective scanned region to additionally form the porous surface layer with 0.2 µm in thickness, thereby providing the image pick-up tube target. Fig. 11 is a rough block diagram showing a main portion of a color camera apparatus of a three-tube system for high resolution television which employs the image pick-up tube according to the present invention. Symbols R, G and B indicate image pick-up tubes for R, G and B channels according to the present invention. Numeral 1121 indicates a power supply, 1122 video signal amplifier, 1123 power supply for controlling an electron beam, 1124 a view finder, 1125 a control panel, 1126 a video monitor, 1127 color resolution prism, and 1128 a lens. With the color camera, a voltage is applied to the target electrode of each image pick-up tube from the power supply 1121 such that it is positive with respect to the cathode electrode, so that the tube operates at such electric field as avalanche multiplication occurs in the photoconductive film. For example, in case that the image pick-up tube according to the present invention, which has the photoconductive film with 2 µm in thickness composed of amorphous Se as a main component, is used, the target voltage is 240 V, and the number of scanning lines are 1125, images reproduced by the very high sensitive high definition TV system can be obtained about ten times as good as a conventional camera, without defect in a reproduced image such as distortion, shading, water fall phenomenon and signal inversion phenomenon. Fig. 12 is a rough block diagram showing an x-ray image analysis system which employs the image pick-up tube according to the present invention. Numeral 1231 indicates the image pick-up tube according to the present invention, 1232 a subject, 1233 an x-ray source, 1234 radiated x-ray, 1235 a power supply for the target voltage, 1236 a video signal amplifier, 1237 a power supply for controlling an electron beam, 1238 a frame memory, 1239 an image processor, and 1240 a video monitor. Symbol R e indicates a load resistor. For example, in case that the image pick-up tube of the example 7, which has the photoconductive film with 10 µm in thickness containing As of 2 weight percent and composed of amorphous Se, is employed in the x-ray image analysis system shown in Fig. 12, the target voltage is 1000 V, and the mesh voltage is 2500 V, the avalanche multiplication can be caused to occur in the photoconductive film, without distortion, shading, water fall phenomenon and a signal inversion phenomenon and hence detection and analysis of x-ray images can be achieved with high sensitivity and high S/N ratio. When the image pick-up tube which uses the target according to one of the examples 1 to 8 is mounted in a TV camera, even if the target voltage is 300 V, no defect in reproduced images such as shading occurs in a case of using any target. More particularly, when in the image pick-up tube having the guard electrode the guard electrode voltage is lower than 50 V, the defect in the reproduced images is never observed at the more than 500 V target voltage. In the example 5, since the guard electrode is transparent, the effect above mentioned is remarkable. In the image pick-up tube having the guard electrode, an area of the target electrode can be a minimum, with the result that floating electrostatic capacity of the target electrode can be decreased, thereby obtaining a high signal-to-noise (S/N) ratio while the above defect can be suppressed. As described on the basis of the detailed examples, according to the present invention as claimed, the image pick-up tube can be obtained which can operate at increased target and mesh electrode voltages, without accompanying occurrence of image distortion, shading, water fall phenomenon and signal inversion phenomenon and hence characteristics of sensitivity, resolution or after-image of the image pick-up tube can be considerably improved, thereby realizing an image pick-up system of high quality. The image pick-up tube according to the present invention as claimed is suitable for a television camera necessary for high quality, e.g., a camera for high definition TV and if the x-ray image pick-up tube according to the present invention as claimed is applied to an x-ray image analysis system, the effect that the system can perform signal processing of high S/N ratio can be obtained. The invention as claimed has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to those of ordinary skill in the art upon reading and understanding the present specification. It is intended that the invention be construed as including all such alterations and modifications insofar as they cope with the scope of the appended claims.

Claims: An image pick-up tube comprising a cathode electrode (112) for emitting an electron beam (111), a substrate (101), a target section having a surface on the side to be scanned, a photoconductive film (103) for converting an electromagnetic wave into charge carriers as electrical signals, and a target electrode (102) provided between the substrate (101) and the photoconductive film (103), and means (113) for scanning a first area of said surface by the electron beam (111) to read the electrical signals, but not scanning another second area of said surface, characterized in that an insulating thin film (104) is located between the target electrode (102) and said second area, it viewed in a direction perpendicular to said surface, for suppressing the operation that the surface potential of the second area rises to balance with the potential of said target electrode. The image pick-up tube of claim 1, wherein the insulating thin film (104) is located between the photoconductive film (103) and the target electrode (102). The image pick-up tube of claim 1, wherein said surface of the target section is provided with a surface layer (105) and the insulating thin film (104) is located between the photoconductive film (103) and the surface layer (105). The image-pick-up tube of claim 1, wherein the insulating thin film (104) is located within the photoconductive film (103). The image pick-up tube of claim 1, wherein the photoconductive film (103) is located only in a portion of the target section corresponding to the scanned first area of the surface (105). The image pick-up tube of any one of claims 1 to 5, wherein the insulating thin film (104) is mainly composed of a material of a resistivity substantially higher than 10 12 Ω-cm. The image pick-up tube of claim 6, wherein the insulating thin film (104) includes a film mainly composed of at least one of the following components: (a) an oxide of Mg, Aℓ, Si, Ti, Mn, Zn, Ge, Y, Nb, Sb, Ta and/or Bi, (b) a fluoride of Li, Na, Mg, Aℓ, K, Ca, Ge, Sr, Ln, and/or Ba, (c) a nitride of Ba, Aℓ and/or Si, (d) silicon carbide, (e) zinc sulfide, and (f) a polyimide insulator. The image pick-up tube of any preceding claim, wherein at least part of the substrate (101) is formed of an insulative glass plate (1020) and the target electrode (102) is formed of a conductive film extending on the glass plate (1020). The image pick-up tube of claim 8, wherein the conductive film is divided into a first film portion corresponding to said scanned first area and a second film portion (305) corresponding to said non-scanned second area and being insulated from the first film portion. The image pick-up tube of claim 9, wherein the first film portion (315) is formed of a transparent conductive film. The image pick-up tube of claim 10, wherein the first and second film portions are connected to electrode pins (214, 516) which pass through the substrate (101). The image pick-up tube of any preceding claim, wherein the substrate is mainly composed of a material which allows incoming X-rays to permeate. The image pick-up tube of claim 12, wherein at least part of the substrate is composed of a thin plate (1018) made of Be or Ti. The image pick-up tube of any preceding claim, wherein at least part of the non-scanned second area of the surface (105) has a smaller secondary electron emission yield than the scanned first area. The image pick-up tube of any preceding claim, wherein at least part of the non-scanned second area of the surface (105) is formed of a porous layer on the side to be scanned of the surface. The image pick-up tube of claim 15, wherein at least part of the porous layer includes a film mainly composed of at least one component selected from (a) an oxide of Mg, Aℓ, Si, Ti, Mn, Zn, Ge, Y, Nb, Sb, Ta or Bi, (b) a fluoride of Li, Na, Mg, Aℓ, K, Ca, Ge, Sr, Ln, or Ba, (c) a nitride of Ba, Aℓ or Si, (d) silicon carbide, (e) zinc sulfide, (f) polyimide, and (g) a compound of at least one of the elements Zn, Cd, Ga, In, Si, Ge, Sn, As, Sb, Pb and Bi with at least one of the elements S, Se and Te. The image pick-up tube of any preceding claims, including a blocking layer for preventing charge injection from the target electrode (102) into the photoconductive film (103). The image pick-up tube of claim 17, wherein the blocking layer includes an n-type semiconductor, for preventing the injection of a plurality of holes from the target electrode (102) into the photoconductive film (103). The image pick-up tube of claim 18, wherein the insulating thin film (104) is located between the target electrode (102) and the blocking layer. The image pick-up tube of claim 17, wherein the photoconductive film (103) is capable of charge multiplication. The image pick-up tube of claim 20, wherein the photoconductive film (103) includes an amorphous semiconductor. The image pick-up tube of claim 21, wherein the amorphous semiconductor includes Se.

Inventors: HIRAI TADAAKI, HIRUMA EIKYU, KUBOTA MISAO, OGAWA HIROFUMI, SAMESHIMA KENJI, TAKASAKI YUKIO, TANIOKA KENKICHI, UNNAI TAKAAKI, YAMAZAKI JUNICHI, HIRAI, TADAAKI, HIRUMA, EIKYU, KUBOTA, MISAO, OGAWA, HIROFUMI, SAMESHIMA, KENJI, TAKASAKI, YUKIO, TANIOKA, KENKICHI, UNNAI, TAKAAKI, YAMAZAKI, JUNICHI
Applicants: HITACHI LTD, JAPAN BROADCASTING CORP, HITACHI, LTD., NIPPON HOSO KYOKAI
Application year: 1990
Main classification: H01J31
IPC codes: H01J31, H01J29