A field-emission cathode in which fine field-emission cathodes each comprising a fine cone-shaped emitter, and a gate electrode formed in the immediate vicinity of the emitter to draw a current from the emitter and control the current are arranged in an arrayed pattern has been proposed on pages 3504 and 3505 of "Journal of Applied Physics, Vol. 39, No. 7, June 1968". This is defined a field-emission cathode of Spindt type, and has advantages in which a current density is obtained to be higher than that of a thermal cathode, and a velocity dispersion of emitted electrons is small. Further, the field-emission cathode has advantages in that current noise is small as compared to a single field-emission emitter, an operation voltage is as low as several tens to 200 V, and it operates even under a condition of relatively low vacuum degree.
A first conventional field-emission cathode which is Spindt type is described in the U.S. Pat. No. 4,940,916, a second conventional field-emission cathode is described in the Japanese Patent Kokai No. 4-249026, a third conventional field-emission cathode is described in the Japanese Patent Kokai No. 5-47296, and a fourth conventional field-emission cathode is described in the Japanese Patent Kokai No. 5-94760. The details of the first to fourth conventional field-emission cathodes will be explained later.
On the other hand, a conventional method for fabricating a field-emission cathode is described in the Japanese Patent Kokai No. 5-36345. The details of the conventional method for fabricating a field-emission cathode will be also explained later.
In general, the electron-emission property of a field-emission cathode depends largely on a structure. For instance, when a tip position of an emitter is displaced relative to a gate electrode by 1%, a current emitted from the emitter changes by approximately 5%. In order to unify currents emitted from a plurality of emitters, therefore, a height of an emitter, a curvature radius of an emitter tip, a thickness of an insulating layer, a thickness of a gate electrode, an aperture diameter of a gate electrode, etc. must be highly precise in the fabrication of a field-emission cathode. However, the providing the uniformity of current-emission makes the fabrication condition extremely severe and substantially lowers fabrication yield.
In order to overcome such disadvantages, it is proposed in the first to third conventional field-emission cathodes that a resistance layer or a non-linear device be positioned below an emitter to unify an emission current.
However, there are disadvantages in the first to fourth conventional field-emission cathodes and the conventional method for fabricating a field-emission cathode, as set out below.
In the first conventional field-emission cathode, interference among emitters occurs to make it difficult that an emission current is controlled individually for each emitter, when intervals among the emitters are made narrow to increase a density of the emitters, because a resistance layer is not independent for each emitter.
In the second conventional field-emission cathode, emission currents are dispersed dependent on the fluctuation of structures, before an emission current of each emitter reaches a level for operation of a current constant device. Therefore, the second conventional field-emission cathode is not suitable to be applied to the use in which all emission currents obtained by the cathode are changed or modulated. Further, it is necessary to align an exposing mask with positions of a current-constant device and a gate electrode aperture. Therefore, an exposure apparatus of high precision must be provided in fabricating a field-emission cathode with emitters of a high density.
In the third conventional field-emission cathode, the maximum resistance value is limited to approximately 200 K.OMEGA., because a resistance layer is composed of a composite material including a metal, and the resistance layer is a part of an emitter. For this structure, current-limitation is not sufficient for a small emission current.
In the fourth conventional field-emission cathode, interference among emitters occurs due to the structure in which emitters are provided on a common resistance layer of tantalum oxide.
In the conventional method for fabricating a field-emission cathode, it is impossible to select a material for an emitter which is most suitable for a current emission property, because the emitter material is limited to silicon. Further, it is difficult to make a gate aperture diameter and an emitter interval small due to the fabrication process.