Electron-emitting device and production process thereof

Details Electron-emitting device and production process thereof Electron-emitting device and production process thereof

2000-05-10

2004-07-13

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Discharge devices having three or more electrodes

C313S310000

Reexamination Certificate

active

06762541

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
This application claims benefit of priority under 35 U.S.C. 120 to Japanese Patent Application No. P11-134972 filed in the Japanese Patent Office on May 14, 1999, the entire contents of which are incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electron-emitting device to be applied to images units and electron beam exposure systems. The present invention relates also to a process for producing the same.
2. Description of the Related Art
Application of a high electric field (on the order of 10
7
V/cm) to the surface of a metal or a semiconductor causes electrons to be emitted into a vacuum through tunneling to the vacuum level. Electron emission of this kind is generally called field emission.
A field emission type cold cathode offers the advantage of emitting a larger number of electrons per unit area than emitted by a hot cathode. In other words, electron emission from cold cathodes can be as large as is 10
7
to 10
9
amperes per cm
2
, whereas that from a hot cathode is limited to tens of amperes per cm
2
. Therefore, a field emission type cold cathode is particularly useful for the miniaturization of vacuum electron devices.
An actual example of a miniaturized vacuum device, or vacuum microelectronic device, that employs a cold cathode was reported by Shoulders (Adv. Comput. 2 (1961) 135.) This publication discloses a process for producing a device of the size of 0.1 micron, and also covers a process for producing a minute diode of a field emission type by using the disclosed device.
Spindt et al (J. Appl. Phys. 39 (1968) 3504.) reported a process for producing by thin-film technology a large number of gated cold cathodes., or triodes, of micron size arranged in an array structure on a substrate. Since then, many reports have appeared in this field.
The cold cathode proposed by Spindt et al is designed such that an electric field is concentrated at the sharp tip of a minute pyramidal emitter of micron size, and the field emission of electrons is controlled by a gate electrode located nearby the tip.
The Spindt-type device is provided with a gate having an opening right above the emitter and an anode placed above the emitter. The number of electrons emitted toward this anode can be controlled by the gate-emitter voltage.
Many other electron emitting devices of similar structure have been reported. They are produced by etching of silicon or by molding and are of a “vertical structure,” in which the emitter and gate are arranged vertically with respect to the substrate.
By contrast, those of a so-called “horizontal structure” are also reported. These have a pair of electrodes arranged on a substrate, one functioning as the emitter and the other functioning as the gate.
The horizontal type device is inferior to that of a vertical type in electron emitting efficiency. However, the former offers the advantage of being produced easily especially in the case where a number of elements are arranged in a large area.
Incidentally, electron emitting efficiency is defined as the ratio of current reaching the anode to current flowing through all the elements. In other words, it is the quotient obtained by dividing the number of electrons leaving the emitter and reaching the anode by the total number of electrons entering the emitter. Electrons emitted by the emitter partly reach the anode, and partly are absorbed by the gate. The greater the value of electron emitting efficiency, the larger the number of electrons reaching the anode or the larger the amount of current.
One example of the horizontal type device shown in
FIG. 8
, reported in J. Vac. Sci. Technol. B13 (1995) 465, consists of a silicon substrate
701
, an insulating layer
702
of SiO
2
, and an H-shaped metal thin film
703
, arranged sequentially on top of each other. The metal thin film has a minute narrow gap
705
(about 2 &mgr;m wide) formed by a focused ion beam
704
. Thus, a pair of electrodes (an emitter
706
and a gate
707
) is formed, with the minute narrow gate gap
705
interposed between them. An anode is placed above the paired electrodes a certain distance away from and parallel to the silicon substrate
701
. Electrons are emitted by applying a potential difference across the emitter
706
and the gate
707
, and partly recovered at the anode.
One effective way to reduce the voltage for electron emission in the horizontal type device is to sharpen the tip of the electrode, or to narrow the gap between the electrodes down to a sub-micron level (below a micron) between the electrodes.
Many other devices having similar configurations have also been fabricated by a process called “electroforming.” In this process, an electric current is passed through the film, and the film suffers Joule heating which forms cracks that separate electrodes in the film. The electron-emitting device of this kind is sometimes called a “surface conduction type electron-emitting device.”
An example of such device is disclosed by M. I. Elinson, Radio Eng. Electron Phys., 10. 1290 (1965). The advantage of this device is the ease with which it can be produced. Elinson's devices in which the film is formed by vacuum deposition, however, suffers the disadvantage of being unstable in action and short in life. This disadvantage was overcome by the device shown in
FIG. 9
, which is disclosed in Japanese Patent No. 2-646235. This device, formed from a fine particle film, is greatly improved in reliability. It has electrodes
1101
and
1102
on a substrate and it also has a fine particle thin film between these electrodes
1101
and
1102
. A minute gap
1105
is formed by the electroforming. This minute gap
1105
separates the emitter
1103
and the gate
1104
from each other. Fine particles
1106
are partly exposed on the edges of the minute gap
1105
.
The horizontal type device as mentioned above is inferior to that of a vertical type in electron emitting efficiency. One reason for this is reported by A. Asai, SID 97 DIGEST, 127. The device described proposed by Asai et al. is shown in FIG.
10
. It has wiring
12204
formed on a substrate. The wiring is covered with a thin film, which has a minute gap
12201
formed at the center thereof. The emitter
12202
and the control electrode
12203
are formed, having this minute gap
12201
interposed between them. The anode
12206
is placed a certain distance away and above the emitter
11202
and the control electrode
12203
.
As the emitter
12202
emits electrons into a vacuum, the emitted electrons fly mostly toward the control electrode
12203
and partly toward the anode
12206
. Most of the emitted electrons reach the control electrode
12203
. Electrons are partly disturbed by the control electrode
12203
, and are caused to move toward the anode
12206
. Almost all of the electrons are recovered at the control electrode
12203
. On the upper side of the control electrode near the emitter is a region in which there exists an upward electric field. Electrons which enter this region are accelerated toward the control electrode. The problem with this is that most of the emitted electrons do not reach the anode
12206
. In addition, the flow of electrons from the emitter
12202
to the control electrode
12203
causes heat generation, and wastes electric power.
One known way to improve the device efficiency is to increase the number of electrons scattered or reflected by the control electrode. For example, Japanese Patent Laid-open No. 265899/1998 discloses a control electrode which is provided with an easily oxidizable material to increase the reflection of electrons.
Also, Japanese Laid-Open Patent No. 231674/1994 discloses a control electrode which is provided with electrically conductive ultra fine particles having a radius equal to the mean free path of electrons. The film of ultra fine particles readily changes the direction of the electrons colliding with it. The result is a reduction in number of electrons captured by the control electrode.
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