1. Field of the Invention P The present invention relates to an electron beam apparatus using a field emission cold cathode device manufactured by, e.g., a thin film technology and, more particularly, to an electron beam apparatus using an electron beam which is focused using an electromagnetic field and modulated by a high-frequency signal.
2. Description of the Prior Art
A field emission cold cathode in which micro-cold cathodes, each constituted by a small conical emitter and an electron extraction electrode formed near the emitter and having a function of extracting a current from the emitter and a current control function, are arrayed has been proposed (Journal of Applied Physics, Vol. 39, No. 7, p. 3504, 1968). FIGS. 1A to 1D show the structure of this field emission cold cathode. Referring to FIGS. 1A and 1B, reference numeral 101 denotes a silicon substrate; and 102, an insulating layer of silicon oxide. An electron extraction electrode 103 is stacked on the insulating layer 102. Parts of the insulating layer 102 and electron extraction electrode 103 are removed, and emitters 104 each having a sharp distal end are formed in these removed surface portions on the silicon substrate 101. The emitter 104, the electron extraction electrode 103, and the cavity formed in the insulating layer 102 constitute a micro-cold cathode 107. A lot of micro-cold cathodes 107 are arrayed to form a cold cathode 108 having an electron emission region.
FIG. 1C is a sectional view showing one of the micro-cold cathodes 107 constituting the cold cathode 108. This cold cathode 108 can obtain a higher current density or smaller velocity distribution in the electron emission direction than that of a conventional hot cathode.
Application of such a field emission cold cathode to the electron sources of various electron beam apparatuses is proposed. When the field emission cold cathode is applied to a cathode ray tube or the like, the fluorescent screen or phosphor screen is spaced apart from the cold cathode by several tens of cm. An electron beam is emitted from the emitter toward the fluorescent screen, focused into a predetermined beam diameter or less through an electromagnetic lens system, and impinged onto the fluorescent screen to cause the phosphor to emit light, thereby displaying a desired image.
When the field emission cold cathode is used as the electron source of an electron beam apparatus, electrons are emitted from the emitter with a certain divergent angle. Therefore, in an application to a cathode ray tube or the like using a focused electron beam, no sufficiently small beam diameter can be obtained. Alternatively, a large-diameter electron lens capable of minimizing spherical aberration is required to obtain a sufficiently small beam diameter, resulting in a bulky apparatus. Various measures have been proposed to solve this problem.
According to measurements by the present inventors, electrons emitted from the field emission cold cathode has a divergent angle of 20.degree. to 30.degree. in terms of half angle with respect to the emission direction. The main reason for this is that the potential distribution near the distal end of the emitter 104 is greatly distorted by the sharpness of the emitter 104 to generate, in the electrons, horizontal velocity components perpendicular to the emission direction. This horizontal velocity component generated due to distortion of the potential distribution is unique to the field emission cold cathode in which electrons are emitted from an emitter with a sharp distal end. In the conventional hot cathode, electrons are emitted from a flat cathode and contain no such extreme horizontal velocity components. The electrons emitted from the hot cathode have thermal velocity components in random directions that are determined by the cathode temperature, although the magnitudes of these components do not pose any practical problem in an application to a cathode ray tube or the like.
Electrons having horizontal velocity components degrade the characteristics of equipment or a device using an electron beam. In an application to a flat display device, the phosphor of an adjacent pixel is caused to emit light, thus degrading the resolution or color purity. In an application to a camera tube, the electron beam cannot be sufficiently focused, so that high resolution cannot be obtained.
To solve these problems, an examination has been made to reduce the divergent angle of the electron beam with a structure using a deflection electrode or focusing electrode to repel the electrons.
FIG. 1D shows a prior art approach disclosed in Japanese Unexamined Patent Publication No. 5-242794 in which the field emission cold cathode is constituted by stacking an insulating layer 105 and a focusing electrode 106 on the electron extraction electrode 103 shown in FIG. 1C. Normally, a voltage lower than that of the electron extraction electrode 103 is applied to the focusing electrode 106 to decelerate the electrons, thereby suppressing the divergence of the electron beam. Alternatively, a voltage lower than that of the emitter 104, i.e., a negative voltage is applied to the focusing electrode 106 to focus the electron beam by an electrostatic repelling force.
Japanese Unexamined Patent Publication No. 5-343000 discloses a technique of forming a multi-stage ring-shaped electrode around the emitter group to surround the emitter region, as shown in FIG. 2. An electron gun 140 is formed on a insulating substrate 134 or Si or glass on a ceramic substrate 133. Insulating layers 136, electron extraction electrodes 137, emitter holes 138, and emitters 139 are formed on a cathode conductive member 135. A plate-like conductive member 142 is connected to the electron extraction electrode 137. The conductive member 142 is connected to a gate stem 143 extending through the ceramic substrate 133. An insulating layer 144 is formed on the conductive member 142. An electron beam focusing electrode 145 having a 0.5- to 0.6-mm hole 145a is formed on the insulating layer 144. A second electron beam focusing electrode 148 is formed on the electron beam focusing electrode 145 via a 0.1- to 0.2-mm ceramic insulating member 147.
Upon operation, the emitter 139 is grounded, a voltage of 30 to 150 V is applied to the electron extraction electrode, a voltage of 0 to 150 V is applied to the first electron beam focusing electrode 145, and a voltage of 200 to 500 V is applied to the second electron beam focusing electrode 148.
There are techniques of controlling focusing of the electron beam by a circuit in a conventional picture tube. In one technique, a voltage synchronized with the deflection coil current is applied to a quadrupole lens electrode inserted between the focusing electrodes of the picture tube, thereby always maintaining an optimum focusing state independently of the beam position on the fluorescent screen.
Another technique optimally corrects the focusing state in accordance with changes in luminance. Japanese Unexamined Patent Publication No. 52-18547 discloses a technique of supplying a signal having a given voltage ratio to that applied to the cathode to a fifth grid 150 as one of electrodes constituting the main lens, as shown in FIG. 3. With this technique, variations in crossover position according to current modulation are converted into the power of the main lens to always obtain an optimum focused beam spot.
Japanese Unexamined Patent Publication No. 7-85812 discloses a technique of applying a voltage for modulating the electron beam current to a correction electrode inserted between the focusing electrodes of the picture tube, as shown in FIG. 4. Referring to FIG. 4, a correction electrode 151 is inserted to improve focusing of an electron beam and arranged between a first acceleration electrode 152 and a focusing electrode 153 while being spaced apart from these electrodes by an equal distance. A voltage for current modulation is amplified and applied to the correction electrode 151. Depending on the voltage applied to the correction electrode, the focusing condition of a main electrostatic lens 154 is optimally corrected in accordance with the amplitude of a luminance modulation signal to be supplied to a cathode 155.
In the technique disclosed in Japanese Unexamined Patent Publication No. 50-146264, an electrode and a sub (second) grid 156 to which a voltage changing in accordance with a change in electron beam current is applied are arranged between arbitrary electrodes of the picture tube (CRT) electron gun, thereby suppressing an increase in beam spot size due to an increase in electron beam current, as shown in FIG. 5.
In the driving technique disclosed in Japanese Unexamined Patent Publication No. 5-266806, a predetermined focusing characteristic is obtained independently of the magnitude of the voltage to be applied to the electron extraction electrode, i.e., independently of the magnitude of the electron beam to be emitted in a field emission cold cathode having a focusing electrode as shown in FIG. 1D.
When the conventional field emission cold cathode is applied to a cathode ray tube or the like, the following problems are posed.
Upon focusing of a divergent electron beam, in the prior art (FIG. 1D) in which two layers of electron extraction electrodes and focusing electrodes are stacked, electrons emitted from the distal end are bent by the second focusing electrode having a positive potential. Particularly, when the divergent angle increases, the electrons collide against the focusing electrode.
To solve this problem, the diameter of the upper focusing electrode is increased to prevent collision of the electrons, or the difference between the electron extraction electrode voltage and the focusing electrode voltage is reduced to minimize the bend of electrons. However, these measures degrade the focusing effect. In addition, since electrons passing near the upper focusing electrode travel while being strongly attracted by the gate, the electrons have a large divergent angle at the anode.
Furthermore, since the focusing electrode voltage has a value smaller than the electron extraction electrode voltage value, field concentration at the sharp distal end of the emitter is impeded. When a voltage of 70 V is applied to the electron extraction electrode, and a voltage of 20 V is applied to the focusing electrode spaced apart from the electron extraction electrode by 0.5 .mu.m, a current corresponding to only 15% to 20% of a current which is emitted with an electron extraction electrode voltage of 70 V in a structure having the electron extraction electrode alone is extracted.
With the focusing electrode structure shown in FIG. 1D, the divergent angle of the electron beam can only be reduced to about 15.degree.. When a sufficient focusing characteristic is to be obtained for an electron beam having such large divergent angle using an electron lens, the electron beam diameter increases due to the spherical aberration of the electron lens. To prevent a degradation in focusing characteristic due to the aberration of the electron lens, the diameter of the electron lens must be increased.
When ring-shaped focusing electrodes are arranged around the emitter region constituted by a plurality of emitters, electrons from the central portion of the emitter region are focused by the focusing electrodes. Near the outermost periphery of the emitter region, however, electrons emitted from an emitter near the outermost periphery are only bent in one direction and not sufficiently focused because the electric field points in one direction toward the center. Particularly, when the emitter region is large, the number of devices at the periphery becomes larger than that at the center in proportion to the length of the circumference of the emitter region. Therefore, all emission currents cannot be sufficiently focused only by the plurality of emitters and the focusing electrodes surrounding the emitters. That is, the focusing electrodes having this structure cannot reduce the divergent angle of all electrons emitted from the entire emitter region.
In addition to the above problems in reducing the divergence of the electron beam, a secondary problem is posed. That is, when the focusing electrodes having the above-described various structure are arranged, a relatively large electrostatic capacitance is formed between the emitter and the focusing electrode, so the electron beam emitted upon applying a high-frequency signal between the emitter and the electron extraction electrode can hardly be modulated by a high-frequency signal.
As an application to a cathode ray tube, in cathode ray tubes used for personal computer display devices which are rapidly becoming popular in recent years, the electron beam must be modulated by a signal having a frequency higher than 100 MHz at maximum. In such a case, the electrostatic capacitance between the emitter and the electron extraction electrode or between the emitter and the focusing electrode largely impedes the operation. In the cathode ray tube, normally, the electron extraction electrode is set at the ground potential, and the modulated signal is applied to the emitter, thereby preventing changes in focusing characteristic of the electron lens due to changes in electron beam current. For this reason, when a high-frequency signal is applied to the emitter, a very high power is consumed to charge/discharge the electrostatic capacitance between the emitter and the focusing electrode. In addition, the signal between the emitter and the electron extraction electrode is attenuated so the electron beam current cannot be modulated at a desired amplitude.