Cold cathode

Abstract: Disclosed is a cold cathode (5) which has an emitter (2) formed in the center portion (6) where ion bombardment is likely to occur, and a gate (4) formed partially thicker, or which has a metal layer with the same potential as the emitter (2) and a high melting point, formed in the center portion (6) of the cathode. Therefore, even bombardment of positive ions at the center portion (6) of the cold cathode (5) will not affect the electron emission, thereby ensuring an always stable cathode current or electron beam. Further, as the cold cathode (5) is designed to prevent the deterioration of the insulation between the emitter (2) and gate (4), abrupt stopping of the electron emission hardly occurs. Therefore, an electron tube using this cold cathode (5) has a stable function and high reliability.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a cold cathode, and, more particularly, to a cold cathode using a vacuum microelectronics technology. Description of the Related Art Fig. 1 presents a cross section exemplifying a conventional cold cathode, called the Spindt type, belonging to cold cathodes that use a vacuum microelectronics technology. As shown in Fig. 1, formed on a semiconductor substrate 1 are a cone-shaped emitter 2 that will serve as a minute cold cathode, an insulating layer 3, and a gate 4 formed of a metal layer. A positive voltage―positive with respect to the potential of the emitter that is the same as that of the semiconductor substrate 1―is applied to this gate 4. As the tip of the emitter 2 is very sharp, it is applied with a high electric field and thus emits electrons which correspond in an amount to the voltage applied to the gate 4. The amount of current to be emitted from a single emitter 2 is as much as 10 to 100 µA. For use as the cathode of an electron tube, a number of emitters 2 are arranged two-dimensionally (see, for example, Journal of Applied Physics, Vol. 47, 1976, No. 12, pp. 5248 to 5263). EP-A-0 434 330 describes a cold cathode device corresponding to the preambles of claims 1 and 6. In an electron tube such as a traveling-wave tube or a cathode ray tube, when gas remaining in the electron tube hits an electron beam, it is ionized and the positive ions are accelerated through the center of the electron beam in the opposite direction to that of this beam, finally hitting the center portion of the cathode. This bombardment of the positive ions at the center portion of the cathode may destroy the emitters 2 in this area, stopping electron emission therefrom. The reduction in electron emission from the center portion decreases the cathode current accordingly, deforming the shape of the electron beam. Such phenomenon definitely makes the function of the electron tube unstable as the time passes. Further, the ion bombardment may cause melting or sputtering of the material of the cathode and mechanical deformation of the emitter 2 and gate 4, resulting in the electric short circuit of the emitter 2 with the gate 4. This disables the application of a voltage between the emitter 2 and gate 4, preventing electron emission from the whole cathode. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a cold cathode designed to prevent the reduction in cathode current and deformation of the shape of the electron beam, thereby ensuring stable performance of an electron tube with passage of time. To achieve this object, there is provided a cold cathode in accordance with the present invention as defined in claims 1 and 6. Embodiments of the invention are set out in the dependent claims. According to the present invention, even if the center portion of the cold cathode (hereinafter simply referred to as cathode) is bombarded with ions, the emitter is hardly destroyed and the insulation between the emitter and gate will not be deteriorated. It is therefore possible to always provide a stable cathode current or electron beam, and to prevent the cathode current or electron beam from being abruptly stopped by even such ion bombardment. The present invention can thus contribute to accomplishing a stable and reliable electron tube. BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: Fig. 1 is a cross section illustrating one example of a conventional cold cathode, called the Spindt type, belonging to cold cathodes that use a vacuum microelectronics technology; Fig. 2 presents a plan view of a cathode according to a first embodiment of the present invention and a partially enlarged cross-sectional view of the center portion of that cathode; Fig. 3 is a cross section showing the center portion of a cathode according to a second embodiment of the present invention; Fig. 4 is a cross section showing the center portion of a cathode according to a third embodiment of the present invention; Fig. 5 is a cross section showing the center portion of a cathode according to a fourth embodiment of the present invention; Fig. 6 is a cross section showing the center portion of a cathode according to a fifth embodiment of the present invention; and Fig. 7 presents cross sections giving a step-by-step illustration of a method of fabricating the third embodiment shown in Fig. 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described referring to the accompanying drawings. Fig. 2(a) is a plan view of a cathode according to a first embodiment of the present invention, and Fig. 2(b) a partially enlarged cross-sectional view of the center portion of that cathode. In those diagrams, numeral 1 denotes a semiconductor substrate, numeral 2 an emitter, numeral 3 an insulating layer, numeral 4 a gate, numeral 5 a cathode, and numeral 6 the center portion of that cathode. According to the first embodiment, positive ions hit the center portion 6 of the cathode where the semiconductor substrate 1 is exposed, as shown in Figs. 2(a) and 2(b). Since the emitter 2 and gate 4, which are sensitive to ion bombardment and are applied with a voltage with the insulating layer 3 therebetween, are not formed in the center portion 6, it is unlikely that the characteristics will change or the insulation will deteriorate. Even when positive ions hit that portion where the semiconductor substrate 1 is exposed, the ion bombardment will not mechanically or thermally affect the emitter 2, insulating layer 3 and gate 4 around the center portion 6 of the cathode. Fig. 3 presents a cross section showing the center portion of a cathode according to a second embodiment of the present invention. The second embodiment has the same structure as the first embodiment shown in Fig. 2B except that it has a gate ring 7 as shown in Fig. 3. The gate ring 7 is formed of the same material, and at the same time, as the insulating layer 3 and gate 4. The gate ring is electrically insulated from the gate 4 and surrounds the cathode center portion 6. Positive ions hit against the semiconductor substrate 1 inside the gate ring 7. Even if the material of the semiconductor substrate 1 or the like is sputtered by the bombardment of the positive ions, the possibility of the sputtered material sticking on the insulating layer 3 becomes smaller due to the presence of the gate ring 7 insulated from the gate 4. This can always ensure good insulation between the gate 4 and emitter 2 or between the gate 4 and semiconductor substrate 1. Fig. 4 is a cross section showing the center portion of a cathode according to a third embodiment of the present invention. As shown in Fig. 4, the third embodiment has a uniform lamination of two layers, the insulating layer 3 and gate 4, at the cathode center portion 6, where there are no emitters 2 or cavities for the emitter 2 formed in the insulating layer 3 and gate 4. A refractory metal layer 8 having a high melting point is formed on the gate 4 at the center portion 6. Since the metal layer 8 has a sufficient thickness besides the high-temperature resistance, even bombardment of positive ions there will not cause sputtering of the metal material or mechanical deformation of the metal layer 8, and insulation between the gate 4 and semiconductor substrate 1 will not be affected. Fig. 5 is a cross section showing the center portion of a cathode according to a fourth embodiment of the present invention. The fourth embodiment is the same as the second embodiment shown in Fig. 3 except for the presence of a metal layer (B) 9 as shown in Fig. 5. The metal layer (B) 9 is formed so as to cover the top of the gate ring 7 and the top of the semiconductor substrate 1 within the gate ring 7, and to electrically connect both. The metal layer 9 is electrically connected to the semiconductor substrate 1 and emitter 2. As the gate ring 7 is electrically insulated from the gate 4 as in the second embodiment, the metal layer (B) 9 is also insulated from the gate 4 electrically. This metal layer (B) 9 may be formed of the same material and at the same time as the emitter 2, or may be formed of a different material from that of the emitter 2. If the metal layer (B) 9 is formed of a refractary metal having a high melting point, it is possible to eliminate the influence of bombardment of positive ions as in the third embodiment. Fig. 6 is a cross section showing the center portion of a cathode according to a fifth embodiment of the present invention. The fifth embodiment is the same as the first embodiment shown in Fig. 2B except for the presence of an insulating layer (B) 10 and a metal layer (C) 11 as shown in Fig. 6. The metal layer (C) 11 is in contact with the semiconductor substrate 1 at the center of the cathode 5 and is insulated from the gate 4 via the intervening insulating layer (B) 10 around the center portion 6 of the cathode. Therefore, the metal layer (C) 11 is electrically connected to the semiconductor substrate 1 while being electrically insulated from the gate 4. If the metal layer (C) 11 is formed of a refractary metal having a high melting point, it is possible to eliminate the influence of bombardment of positive ions as in the fourth embodiment shown in Fig. 5. Further, since the insulating layer 3 between the gate 4 and semiconductor substrate 1, particularly, that portion which borders the cathode center portion 6, is sufficiently covered with the insulating layer 10 and metal layer 11, the insulating performance of this insulating layer 3 hardly deteriorates. In all the embodiments except the third one, the current of positive ions reaching the cathode flows out to an external circuit through a wire connected to the gate 4. The first embodiment shown in Figs. 2A and 2B and the second embodiment shown in Fig. 3 can be fabricated in quite the same process as the conventional cathode shown in Fig. 1. Figs. 7A through 7D present cross sections illustrating step by step a method of fabricating the third embodiment shown in Fig. 4. The fourth and fifth embodiments can be fabricated in almost the same process as illustrated in Figs. 7A through 7D. In the third embodiment, first the semiconductor (silicon) substrate 1 is thermally oxidized to form a silicon oxide film thereon as the insulating layer 3. Then, the gate metal layer 12 is deposited on the insulating layer 3 by vapor deposition or sputtering technology. Next, the top of the gate metal layer 12, excluding the cathode center portion 6, is coated with a resist 13 using a lithography technology. Then, metal for forming the metal layer 8 is deposited on the top of the resultant structure to form a metal layer 14 overlying the gate, thus yielding the structure as shown in Fig. 7A. Subsequently, the resist 13 and the overlying metal layer 14 are removed using a lift-off technology, forming the metal layer 8. Then, part of the gate metal layer 12 is removed to form the gate 4 using a lithography technology, and with the gate 4 as a mask, part of the insulating layer 3 is etched off, yielding the structure as shown in Fig. 7B. Then, the semiconductor substrate 1 is positioned obliquely in a vapor depositing device so that a material to be vapor-deposited is sputtered from the oblique direction of the semiconductor substrate 1, and the semiconductor substrate 1 is rotated, forming an obliquely vapor-deposited metal layer 15. The resultant structure is shown in Fig. 7C. Then, emitter metal is vapor-deposited on the semiconductor substrate 1 directly above, so that an emitter metal layer 16 is formed on the metal layer 15, and at the same time, emitters 2 which become narrower toward their tips are formed on the semiconductor substrate 1 through cavities formed in the metal layer 15. The resultant structure is shown in Fig. 7D. Finally, the metal layer 15 and emitter metal layer 16 are removed to provide the structure of the third embodiment as shown in Fig. 4. Although the foregoing description of the embodiments has been given with respect to cathodes that have metal emitters, it should be apparent to those skilled in the art that the first to fifth embodiments can also be applied to a cathode having emitters made of a semiconductor such as silicon. Further, the metal layer 8 on the gate 4 in the third embodiment should not necessarily be located at the center portion of the cathode, but may be formed all over the cathode, excluding the area around the cavities formed in the gate 4. In this case, reduction in the film resistance of the gate 4 is accomplished besides the original advantages of the third embodiment, thus preventing a voltage drop at the gate 4 from occurring due to the flow of the ion current. In short, even bombardment of positive ions at the center portion of the cathode of the present invention will not affect the electron emission, thereby ensuring an always stable cathode current or electron beam. Further, as the insulation between the emitter and gate will not be deteriorated, abrupt stopping of the electron emission hardly occurs. Therefore, an electron tube using the cathode of the present invention has a stable function and high reliability. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.

Claims: A cold cathode (5) comprising: a substrate (1) of either a conductive type or a semiconductor type; an insulating layer (3) deposited on said substrate (1); a gate (4) formed of a first metal layer deposited on said insulating layer (3); an emitter (2) of minute cold cathodes each with a pointed tip formed of an electrode of either a conductive type or a semiconductor type and provided on said substrate (1) in cavities formed in parts of said first metal layer (4) and said insulating layer (3), wherein said cavities and said minute cold cathodes are formed on a portion of said substrate other then a center portion (6) thereof and not on said center portion, characterized in that not any of said first metal layer, said insulating layer (3), said cavities or said emitters (2) are formed on said center portion (6) which corresponds to an area where ion bombardment from positive ions is likely to occur when in use, so that the influence of said ion bombardment on the electron emission characteristics of said cold cathode (5) is substantially eliminated. The cold cathode according to claim 1, wherein said center portion (6) is limited by a gate ring (7) which is formed of the same metal layer as said gate (4) and which is insulated from said gate (4). The cold cathode according to claim 2, wherein a second metal layer (9) is formed on said semiconductor substrate (1) at said center portion (6) and on the top of said gate ring (7) for electrically connecting said semiconductor substrate and said gate ring (7). The cold cathode according to claim 1, wherein a second metal layer (11) is in contact with said semiconductor substrate (1) at the center of said center portion (6) and is insulated from said gate (4) via an intervening second insulation layer (10) around said center of said center portion (6) so that said second metal layer (11) is electrically connected to said semiconductor substrate (1), while being electrically insulated from said gate (4); and the portion of the first insulating layer (3) between said gate (4) and said semiconductor substrate (1) which borders said center portion (6) is covered with said second insulating layer (10) and said second metal layer (11). The cold cathode according to claim 3 or 4, wherein said second metal layer (9, 11) is formed of a refracting metal having a high melting point. A cold cathode (5) comprising: a substrate (1) of either a conductive type or a semiconductive type; an insulating layer (3) deposited on said substrate (1); a gate (4) formed of a first metal layer deposited on said insulating layer (3); an emitter (2) of minute cold cathodes each with a pointed tip formed of an electrode of either a conductive type or a semiconductive type and provided on said substrate (1) in cavities formed in parts of said metal layer (4) and said insulating layer (3), wherein said cavities and said minute cold cathodes are formed on a portion of said substrate other than a center portion (6) thereof and not on said center portion characterized in that said center portion corresponding to an area where ion bombardment from positive ions is likely to occur when in use, said insulating layer (3) and said gate (4) are formed uniformly on said center portion (6), with a second metal layer (8) deposited on the first metal layer in said center portion, said second metal layer being formed of a different metal to that of the first metal layer and consisting of a refractory metal having a high melting point of such a sufficient thickness that the influence of said ion bombardment on the electron emission characteristics of said cold cathode (5) is substantially eliminated.

Inventors: IMURA HIRONORI, MAKISHIMA HIDEO, YAMADA KEIZO, IMURA, HIRONORI, MAKISHIMA, HIDEO, YAMADA, KEIZO, Imura, Hironori, c/o NEC Corporation, Makishima, Hideo, c/o NEC Corporation, Yamada, Keizo, c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1993
Main classification: H01J1
IPC codes: H04L12, H01J1