1. Field of the Invention
The present invention relates to a field emission type flat display apparatus and in particular to a flat display apparatus in which a plurality of small pointed cathodes are used as electron emission sources.
2. Description of the Prior Art
The use of a flat display apparatus which will be used in lieu of the currently and mainly used CRT for television receivers has been studied. The different types of flat display apparatus include a liquid crystal display (LCD), an electroluminescence display (ELD) and a plasma display panel (PDP). The field emission type of display has attracted attention in view of brightness.
The field emission type display apparatus will be briefly described herein. Conical cathodes of molybdenum having a diameter not larger than 1.0 .mu.m are formed as electron emission sources on a substrate by the semiconductor manufacturing process. A flat gate electrode having apertures for each of the cathodes is formed on the side where the pointed ends of the cathodes are located. The gate electrode is separated from the pointed ends of the cathodes. A high voltage is selectively applied across the gate electrode and the cathodes. An electrostatic field is thus induced to extract electrons from the cathodes. A given picture is displayed on a screen by irradiating with electron beams a light emitting layer (luminescence layer) disposed on the reverse side of an anode. Such a field emission type display apparatus is described in, for example, U.S. Pat. No. 3,665,241 and Japanese Unexamined Patent Publication No. Hei 1-294336.
FIG. 1 is a sectional view showing an example of a prior art field emission type display apparatus. A plurality of pointed cathodes 2 are formed on a substrate 1. A gate electrode 4 is formed on an insulating film 3 formed on the substrate 1. Electrons are liberated and extracted from the cathodes by a voltage applied across the gate electrode 4 and the cathodes 2. The gate electrode 4 has an aperture 4a above each of the cathodes 2. Electron beams from the cathodes 2 pass through the apertures 4a and collide with a flat anode 5 facing to the substrate 1 and to which a high voltage is applied. The electrons reach a light emitting layer 6 or the reverse side of the anode 5 so that the layer 6 emits light.
The dimensions of the display apparatus are as follows. The diameter of the gate is about 1 .mu.m. The curvature radius of the pointed ends of the cathodes is 50 .mu.m. Molybdenum or tungsten is used as a material for these components. The spacing between the cathodes and the anode is 200 .mu.m. A voltage of 300 volts is applied thereacross. The drive voltage of the gate is 40 volts.
In such a field emission type display apparatus, the beams of electrons emitted from the pointed cathodes tend to scatter. The intensity of the light emitted from the light emitting layer 6 is not enough.
The causes of scattering of the electron beams will be described with reference to FIG. 2, which is an enlarged FIG. 1. FIG. 2 shows the distribution of the potential between the substrate and the anode. When a desired voltage is applied to the gate 4, the equipotential surfaces E are curved toward the anode. This is referred to as an electrostatic field lens. The electrons e.sup.- are subjected to forces in a direction normal to the equipotential surfaces E. Therefore, the electrons are scattered. The electrons which have been scattered in such a manner are incident upon an anode 5 and reach at a light emitting layer 6 oh on the reverse side of the anode 5. Therefore, the intensity of the light emitted from the layer 6 is lowered.