Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
2000-03-15
2003-05-13
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C313S496000, C313S509000
Reexamination Certificate
active
06563260
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electron emission element and a method of manufacturing the electron emission element, and also to a display device using the electron emission element and a method of manufacturing the display device. The present invention is applicable to an image display device, an electron beam exposing device, etc.
2. Description of the Related Art
Application of high electric field of about 10
7
(V/cm) level to the surface of metal or semiconductor induces such a phenomenon that electrons are emitted from the surface of metal or semiconductor into vacuum, and this phenomenon is called as “field emission”. The field emission is caused by tunneling of electrons in the vicinity of the Fermi energy level in metal or electrons excited up to the conduction electron band into the vacuum level. However, in the case of the semiconductor, electrons located in the valence band or various levels existing between the bands, such as the impurity levels, the surface levels, etc. may be emitted.
A field emission type cold cathode has such a merit that the electron emission current density can be set to a larger value as compared with that of a thermionic cathode. In the case of thermionic cathodes, the field emission density is limited to about several tens of amperes per one square centimeter at maximum. On the other hand, with cold cathodes, the electron emission current density of about 10
7
to 10
9
amperes per one square centimeter can be achieved. Therefore, use of the field emission type cold cathode is particularly effective to design micron-sized miniature vacuum electron devices.
An actual example of a vacuum micro-electric device using the cold cathode, was first reported by Shoulders in 1961(Adv.Comput.2(1961)135), and he reported a method of manufacturing a 0.1-micron size device and a minute field emission type diode by using the above device. Further, Spindt, et al. reported fabrication of an array structure in which a number of micron-size cold cathodes (triodes) having gates formed on a single substrate by a thin film technique (J. Appl. Phys. 39 (1968) 3504). Following this year, various reports have been submitted.
Various types of structures have been proposed for the vacuum micro-electronic device. According to the report of Spindt, et al., there is proposed a structure having a micron-size minute conical emitter having a sharp tip and a electron extracting electrode (gate) having an open portion located just above the emitter. An anode is provided above the emitter.
With such a structure, the electric field is concentrated on the tip portion of the emitter, and the current of electrons emitted from the emitter to the anode can be controlled by the voltage applied across the gate and the emitter.
As other examples of devices having the same structure, there have been reported various reports for manufacturing the devices having the same structure by using a method using anisotropic etching of Si (Grey's method), a mold method using a mold or the like. The common features of the conventional electron emission elements having the above structure resides in that each of these structures has an extremely sharp emitter tip portion, the radius of curvature of which is equal to about several nanometers, and that the electric field applied at the tip by the difference between the gate voltage and the emitter voltage is increased 100 times to about 1000 times as compared to the voltage difference divided by the gate-emitter distance, resulting from the effect of the concentration of the electric field at the sharply pointed tip of the emitter.
The diameter of the opening portion of the gate ranges from the micron order to the sub-micron order. An actual manufacturing process of these elements need to position the gate and the conical emitter inside the minute opening portion. It is technically and economically difficult to perform such a precise positioning work by using lithography. This difficulty can be avoided by using self-alignment techniques. However, use of such techniques rather causes lots of restrictions.
For example, a manufacturing process based on Spindt's method will be described.
First, after a gate opening is provided, a peeling layer is formed on the top surface of the gate while the film thereof is prevented from being deposited inside the gate. Subsequently, an emitter material is vapor-deposited from the vertical direction. At this time, the opening diameter of the gate becomes gradually smaller due to the increase of the emitter material adhering to the edge of the opening portion of the gate, so that a conical emitter is formed inside the gate opening. Thereafter, the emitter material adhering to the opening portion of the gate is removed by removing the peeling layer.
As reported in J. Vac. Sci. Technol. B13(1995) 487, a conical shape having an ideal ratio (diameter of bottom surface: height) (aspect ratio) can be formed when Mo is used; however, it cannot be formed when Ti or Zr is used. That is, the material usable for the emitter is limited to special materials not only in consideration of the physical properties which directly affect the field emission characteristics, but also in terms of the shaping of the elements. Accordingly, the emitter material is substantially limited to Mo due to a requirement for forming a conical body having an excellent shape in the vapor-deposition process.
Likewise, the emitter material is limited to Si in the gray method because the tip of the Si conical body is sharpened by thermal oxidation in the Gray's method.
These methods are too low in flexibility to reduce the cost by reconsidering the process and the material.
In order to widen the range of the materials usable for the emitter, it is required to loosen the restriction caused by the manufacturing process, and the following method is known to satisfy this requirement.
This method directs to such an approach that an emitter having a single emission point is not necessarily located at the center portion of the gate, and but a plurality of emission points are provided in the opening portion of the gate, thereby omitting the positioning work between the gate and emitter. Even when this approach is used, the electron emission amount is actually prevented from being remarkably lowered although the loss of the effective current due to withdrawal of electrons emitted from the emitter by the gate is increased.
In general, there are two factors influencing the intensity of electric field at the tip of the emitter. The one is the sharpness of the tip of the emitter and the other is the distance between the gate and the tip of the emitter. Since the electric-field intensity is more greatly dependent on the sharpness of the tip of the emitter, the above approach can be effectively used. Accordingly, this approach makes it easier technically and economically to form a large-area array of electron emission elements. Such an approach is classified into two types.
One type of approach relates to a method of providing an electric-field concentration structure. For example, Japanese Laid-open Patent Application No. Hei-8-329823 discloses such a structure that an infinite number of columnar crystals of beta type tungsten are grown in the opening portion of the gate and electrons are emitted from the pointed portions of the respective crystals.
The other type of approach uses materials having small work function or small electron affinity. This method enables electron emission from a film having no discrete pointed portions. In general, as the work function or the electron affinity is reduced, the field emission is more likely to occur. Semiconductor materials having a broad band gap of about 5 electron volts or more can be used as materials having especially excellent characteristics for such a film. For example, as these materials are known diamond, boron nitride of cubic or hexagonal system, lithium fluoride, calcium fluoride or the like which have extremely low electron affinities.
For
Asakawa Koji
Fukuda Yumi
Hara Yujiro
Hiraoka Toshiro
Itoh Goh
Kabushiki Kaisha Toshiba
Patel Vip
Quarterman Kevin
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